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\chapter{EXPERIMENT DESCRIPTION}
\section{Experiment Setup}
The experiment consists of the sensor head, its readout electronics and the power supply. The electronics are housed in a separate \ac{E-Box}. Both units are surrounded by a pressure housing and attached to a base plate \change{(see section \ref{sec:pressure_housing})}. A pressure sensor is placed inside this housing and another one outside. \change[3]{There are two further thermistors placed inside the pressure housing, one on the base plate and one on the sensor head.} The whole experiment is \replaced{encased by an insulating polystyrene thermobox.}{planned to be packed in an insulating expanded polystyrene thermobox.} A schematic can be seen in figure \ref{fig:design_schematic}, while figure \ref{fig:cad_model} shows a \ac{CAD} model of the experiment design. Figures \ref{fig:chaos_integrated} and \ref{fig:chaos_integrated2} show the integrated instrument.
\begin{figure}[!h]
\centering
{\includegraphics[width=0.9\textwidth]{images/experiment_description/CHAOS_housing_v03.pdf}}
\caption{Schematic of the proposed experimental design.}
\label{fig:design_schematic}
\end{figure}
\begin{figure}
\centering
\includegraphics[width=\textwidth]{images/experiment_description/CAD_drawings/chaos_iso.jpeg}
\caption{\change{\ac{CAD} model of the proposed experiment design.}}
\label{fig:cad_model}
\end{figure}
\begin{figure}\centering
\subfigure[Close-up of the \ac{CHAOS} instrument.]{\includegraphics[width=0.49\textwidth]{images/experiment_description/chaos_integrated.jpg}\label{fig:chaos_integrated}}
\subfigure[Close-up of the \ac{CHAOS} instrument.]{\includegraphics[width=0.49\textwidth]{images/experiment_description/chaos_integrated2.jpg}\label{fig:chaos_integrated2}}
\caption{\change[3]{The integrated \ac{CHAOS} instrument.}}
\end{figure}
\begin{figure}\centering
\subfigure[\ac{CHAOS} with closed insulation box.]{\includegraphics[width=0.49\linewidth]{images/experiment_description/chaos_fully_integrated.jpg}\label{fig:chaos_fully_integrated_box}}
\subfigure[Top-view of the \ac{CHAOS} instrument in the insulation box.]{\includegraphics[width=0.367\linewidth,angle=90]{images/experiment_description/chaos_top_view_box.jpg}\label{fig:chaos_top_view_box}}
\caption{\change[4]{The \ac{CHAOS} instrument in its insulation box.}}
\end{figure}
The \ac{E-Box} contains the preamp interface board, the \acf{IRENA} board, the power board and the \acf{HV} interface board, as well as the \ac{HV} \replaced{driver}{daughter} board. \replaced{The electronics design is}{These are} described in further detail in section \ref{sec:electronics_design}.
\change{A more detailed description of the sensor head consisting of multiple \aclp{SSD}, a \acl{BGO} scintillator and the Cherenkov detector can be found in section \ref{sec:sensor_head}. A \ac{CAD} model of the experiment is provided on the \ac{RX/BX} team site under \url{https://rexusbexus.zarm.uni-bremen.de/share/page/} in the folder \texttt{CHAOS/CAD/}.}
\clearpage
\section{Experiment Interfaces}
\subsection{Mechanical Interface}
% VORGABEN VOM USER MANUAL:
% ESCARGO is made of 45x45mm Bosch profiles and it is possible to attach rails everywhere
% To mount experiments on ESCARGO the following parts are needed:
% • M6x25mm T-bolts
% • M6 nuts
% • 30x30mm brackets
% • R28x28x38mm brackets
% • Rubber bumper (recommendation)
Two Rexroth Bosch 20\,x\,11 aluminum mounting rails are mounted on to the ESCARGO gondola using M6\,x\,25 Hammerhead screws. The insulation box is placed on top of the mounting rails. \replaced[3]{\replaced[4]{For each rail two M6 Hammerhead screws are used going directly through the insulation box. The interface plate made out of \ac{POM} is then screwed on to the bottom of the insulation box using these screws. This setup presses both the interface plate and the mounting rails against the insulation box.}{For each rail two M6 Hammerhead screws are connected to a sandwich female-female rubber buffer, which goes directly through the insulation box. The interface plate made out of \ac{POM} is then screwed on to the bottom of the styrofoam box into the rubber bumpers using a hexagonal M6 screw.} Thus the insulation box is held in place and the occurring vibrations as well as shocks are mostly absorbed by the sytrofoam. \deleted[4]{In addition to that the rubber bumpers will ensure that there is a tension between the rails and the insulation box after the slight deformation of the box after the landing due to the shock absorption.}}{A threaded bolt establishes a static connection between the rail and the styrofoam box and everything is fastened by nuts and washers. For suspension sandwich female-female rubber buffers are screwed on top of the threaded bolts and thus allow a connection to the interface plate made out of \ac{POM} which is also fastened with nuts and washers.} Since \ac{CHAOS} will not fit into the \change[3]{styrofoam} box from the previous projects, a new box \replaced{was}{needs to be} purchased. The dimensions \replaced{are}{will be} about 495\,mm\,x\,395\,mm\,x\,370\,mm \change{without the rails and 600\,mm\,x\,395\,mm\,x\,381\,mm including the rails.}\\
In terms of placement on the gondola, the experiment preferably requires a free line of sight towards the top \deleted{as well as the bottom with the top being more important}.
\begin{figure}[H]\centering
\subfigure[\change{Exploded \acs{CAD} drawing of the mechanical interface.}]{\includegraphics[width=0.49\textwidth]{images/experiment_description/CAD_drawings/exploded_interface.png}\label{fig:interface_exploded}}
\subfigure[\change{Close-up of the mechanical interface from the side.}]{\includegraphics[width=0.48\textwidth]{images/experiment_description/CAD_drawings/frontview_interface.png}\label{fig:interface_sideview}}
\subfigure[\change{Close-up of the mechanical interface with the interface plate in black.}]{\includegraphics[width=0.49\textwidth]{images/experiment_description/CAD_drawings/sideview_interface.png}\label{fig:mechanical_mounting}}
\subfigure[\change{\acs{CAD} drawing of the experiment mounted onto the gondola.}]{\includegraphics[width=0.49\textwidth]{images/experiment_description/CAD_drawings/chaos_gondola_iso.png}\label{fig:chaos_on_gondola}}
\caption{\change[3]{Mechanical mounting.}}
\end{figure}
\subsection{Electrical Interface}
% The two telemetry systems used are E-Link and EMPIRE.
% • E-Link is used by experimenters to transfer data to and from ground.
% • EMPIRE is only used by SSC for piloting and housekeeping data.
% E-Link telemetry system
% Esrange Airborne Data Link (E-Link) is a telemetry system that offers a simplified interface
% to experiments with a standard Ethernet protocol. Only the Ethernet interface is provided for
% BEXUS Experiments.
% 4.1.1 E-Link System Overview
% The E-Link system consists of a ground station and an airborne unit. The ground station
% consists of an antenna, an antenna controller and a Monitor & Control Unit. The airborne
% system includes the main unit, an antenna, a battery, and an RF interface unit. One
% connection is available to each experimenter.
% The experimenter is allowed to implement an additional internal Ethernet switch, in case
% there is more than one connection required.
% The main features of the system are:
% • Ethernet 10/100 Base-T Protocol
% • MIL-C-26482-MS3116F-12-10P connectors (as seen in Figure 4-1)
% • High data bandwidth, 2 Mbps duplex nominal
% • All electrical parts are approved by FCC and ETSI (standards)
% • Fixed IP address allocations
% The experimenters should be aware of the bitrate decline as a function of BEXUS distance
% as indicated by figure 5-2.
% N.B. The E-Link system and all the experiments share the available bandwidth, there is no
% prioritisation of the E-Link. Consequently, if too much data is sent at the same time, the
% communication can be lost temporarily. Downlink coordination is essential to ensure that
% the communication with the experiments is maintained during the flight.
% 4.1.2 Technical Specification of the E-Link Airborne Unit
% Antenna: Vertical polarised omni
% Operating frequency: S-band
% Max output power: Peak 10 watt
% Modulation: DSSS
% Channel bandwidth: Nominal ±11 MHz
% Maximum range at LOS: 500 km at 30 km altitude
% Data bandwidth: 2 Mbps duplex nominal, decreasing with range
% User interfaces: 2 Ethernet 10/100 Base
% Power supply: 20 to 38 volt DC
% Operation time: Nominal > 11 hours
% Weight: Nominal ~20 kg, including batteries
% 4.1.3 Technical Specification of the E-Link Ground Unit
% Antenna: 1.8 meter parabolic dish
% Operating frequency: S-band
% Max output power: Peak 10 Watt
% Modulation: DSSS
% Channel bandwidth: Nominal ± 11 MHz
% Maximum range at LOS: 500 km at 30 km altitude
% Data bandwidth: 2 Mbps duplex nominal, decreasing with range
% User interfaces: Ethernet 10/100 Base-T
% Placed on the outside of the experiment structure/housing, the experiment must have a 4 pin,
% male, box mount receptacle MIL – C-26482P series 1 connector with an 8-4 insert
% arrangement (MS3112E8-4P) (Figure 5-11).
% Pin A: +
% Pin B: -, do not connect to chassis or ground
% Pin C: empty
% Pin D: empty
% A panel mounted connector for the E-Link is to be used. This connector (Amphenol RJF21B)
% can be mounted to the front or side panel of the experiment. Insert CODE A should be used
% for BEXUS. The inside of the connector requires a standard RJ45 (Ethernet) connector.
% Connector and drilling pattern are depicted below.
A modified connector plate designed by previous \ac{BEXUS} teams from \ac{CAU} Kiel such as BX14\_MONSTA and BX19\_\acs{ADAM} (see figures \ref{fig:connector_plate_new} and \ref{fig:connector_plate}) is reused. It is located outside of the insulation box and provides easily accessible connections. This plate consists of one Amphenol PT02E8-4P power connector and one Amphenol RJF21B E-Link connector. \change{An additional LED will be added to indicate \acf{HV} operation. If the \ac{HV} is off, the LED flashes green, and if the \ac{HV} is on, the LED flashes red.} \replaced{A hermetic D-Sub-15-connector (XAVAC15M/SI.0/AA) is used for internal cable routing through the pressure housing}{Those are wired internally to one Sub-D 15 connector}. For connecting the power and Ethernet cable it is not necessary to open the insulation box. The experiment uses the \ac{BEXUS} battery pack. The instrument will be switched on by simply plugging in the power cable into the power connector on the connector plate. There is no extra switch intended.
\begin{figure}[H]
\subfigure[Picture of the electrical interface.]{\includegraphics[width=0.36\linewidth]{images/experiment_description/electrical interface.jpg}\label{fig:connector_plate_new}}
\subfigure[Schematic of the connector plate.]{\includegraphics[width=0.55\linewidth]{images/experiment_description/connector_plate.png}\label{fig:connector_plate}}
\caption{\change[3]{Electrical interface.}}
\end{figure}
\begin{longtable}{|c|c|}
\caption{\change{Usage of the E-Link}} \label{data_interface}\\
\hline
data rate of uplink & 4.8 kbit/s \\%%%%STEPHAN
\hline
data rate of downlink (maximum) & 200 kbit/s (throttled) \\%ausrechnen!
\hline
protocol & UDP\\
\hline
connector type & Amphenol RJF21B\\
\hline
\end{longtable}
\pagebreak
\begin{longtable}{|c|c|}
\caption{\change{Usage of BEXUS batteries}} \label{battery_interface}\\
\hline
connector type & Amphenol PT02E8-4P\\
\hline
expected current & average and maximum: 0.2\,A\deleted[4]{; max: 0.3\,A}\\
\hline
protection & safety precautions for humans and experiments\\
\hline
grounding & \change[4]{to the gondola via a grounding strip and the instrument's rails}\\
& \deleted[4]{to minus pole of the \ac{BEXUS} battery pack via power cable.}\change[5]{(the grounding strip was removed by request of \acs{SSC} during launch campaign)} \\
\hline
\end{longtable}
Once the power connection is established, \ac{CHAOS} can take measurements autonomously and has sufficient SD card storage space. An E-Link is required to connect to the ground station and should be used to send down and store measurement data, as well as to retrieve status reports and send control sequences to \ac{CHAOS}. A variable downlink rate with a maximum of \replaced[4]{200}{100} kbit/s is expected. The uplink is only intended to be used in case some changing commands need to be sent to \ac{CHAOS}. For example, the downlink limit could be throttled and adapted to the possible downlink rate during the flight. The uplink should not exceed 4.8\,kbit/s.
\change[4]{The experiment is grounded to the gondola rails using a grounding strip bridging the interface plate made out of \ac{POM} (see figure \ref{fig:Groundingstrip}). The grounding strip connects the pressure housing with a screw connected to one of the rails of the instrument. The instrument's rails are themselves connected to the gondola rails. The +28\,V taken from the gondola battery are returned to the minus pole of the battery. There is no direct connection of the minus pole to the gondola via the electronics. A detailed grounding scheme can be found in figure \ref{fig:GroundingScheme}.}\deleted[4]{The grounding of our instrument is connected to the minus pole of the \ac{BEXUS} battery pack via the power cable. If the battery itself is not grounded via the case of the gondola, we will connect a grounding cable to our case and a mounting screw.} \change[3]{\deleted[4]{An example can be seen in Fig. \ref{fig:Groundingstrip}, where our aluminum base plate will be between the two nuts at the top of the left screw.}}The Ethernet connection cable is on a floating potential. \change[5]{The grounding strip was removed during launch campaign by request of \ac{SSC} to make the instrument floating. \ac{SSC} measured the +28\,V of the battery on the gondola rails and did not want that.}
\begin{figure}[h!]
\centering
\includegraphics[width=0.4\linewidth,angle=-90]{images/experiment_description/grounding_strip.jpg}
\caption{\change[4]{Grounding strip.}}
\label{fig:Groundingstrip}
\end{figure}
\begin{figure}[h!]
\centering
\includegraphics[width=0.9\linewidth]{images/experiment_description/Grounding Scheme.png}
\caption{\change[4]{Grounding scheme of the \ac{CHAOS} instrument.}}
\label{fig:GroundingScheme}
\end{figure}
\subsection{Radio Frequencies}
\ac{CHAOS} has no radio transmitter.
\subsection{Thermal Interface}
The experiment has neither active cooling nor heating. The only heat source is the power dissipation of about \replaced[3]{$\un{5.0}{W}$}{$\un{4.1}{W}$} by the electronics. Due to a sufficient insulation and the use of \change[4]{only four screws for the connection with the gondola rails.}\deleted[4]{rubber buffers \replaced{within the mechanical interface}{for the connection with the gondola}} The experiment has negligible heat transfer to the gondola and does not disturb other experiments.
\clearpage
\section{Experiment Components}\label{sec:experiment_components}
\newcommand{\MassAndDimensions}{\color{black}
\begin{longtable}[H]{|m{6.6cm}|m{6.5cm}|}
\caption{Experiment mass and dimensions}\\
\hline
Expected experiment mass (in kg): &
\begin{tabular}{r|l}
Insulation box & $1.063$\\
pressure housing & $3.6$\\
base plate & $1.23$\\
interface plate & $1.35$\\
cherenkov detector & $1.33$\\
scintillation detector & $1.35$\\
BGO bracket & $0.2$\\
sensor head bracket & $0.44$\\
\change{rails} & $0.36$\\
\change{interface} & $0.5$\\
\change{U-profiles} & $0.2$\\
electronic box & $0.5$\\
wires & $< 0.2$\\
screws & $0.3$\\
\hline
\textbf{Total} & \replaced{12.63}{11.13}\\
\end{tabular}\\
\hline
\change[4]{Measured experiment mass (in kg):} & \change[4]{11.9} \\
\hline
Expected experiment dimensions without rails ($L{\times}W{\times}H$~in~m): & $0.495\times0.395\times0.370$\\
\hline
\change[4]{Measured experiment dimensions without rails ($L{\times}W{\times}H$~in~m):} & \change[4]{$0.493\times0.393\times0.367$} \\
\hline
Expected experiment dimensions with rails ($L{\times}W{\times}H$~in~m): & $0.600\times0.395\times0.381$\\
\hline
\change[4]{Measured experiment dimensions with rails ($L{\times}W{\times}H$~in~m):} & \change[4]{$0.600\times0.393\times0.374$} \\
\hline
Expected center of gravity ($X{\times}Y{\times}Z$~in~mm): & \change[3]{$281,3{\times}-82,538{\times}169,472$} \\
\hline
\end{longtable}
}
\MassAndDimensions
\change[3]{The center of gravity is determined relative to the interface screw closest to the electrical interface with an offset in the xz-plane of 10,9\,mm so that the plane aligns with the bottom of the insulation box. The center of gravity can be seen in yellow and the origin lies within the purple and the whites planes (see figure \ref{fig:CoG})}
\begin{figure}[H]
\centering
\includegraphics[width=0.5\linewidth]{images/experiment_description/CAD_drawings/CoG.PNG}
\caption{\change[3]{Center of gravity relative to origin}}
\label{fig:CoG}
\end{figure}
\newpage
\section{Sensor Head}\label{sec:sensor_head}
The sensor head is made up of the Cherenkov detector consisting of an aerogel block and a \acf{PMT}, the scintillation detector consisting of a \acf{BGO} crystal and \replaced{six}{two} photodiodes, six identical \acfp{SSD}, part of the \ac{HV} supply electronics for the \ac{PMT} and the mounting mechanism.\\
The names of the detectors are \replaced{labeled}{listed} alphabetically according to the order in which particles entering from above pass through them. The first \ac{SSD} is therefore referred to as A, the following Cherenkov detector as B, the next \ac{SSD} as C and the scintillation detector as D. Together, the last four \acp{SSD} form detector E. The individual \acp{SSD} of detector E are labeled E$_0$ to E$_3$. A schematic of this detector arrangement can be seen in figure \ref{fig:arrangement_detectors}.
\begin{figure}[h!]
\centering
\includegraphics[width=0.8\textwidth]{images/experiment_description/sketch_sensorhead.pdf}
\caption{\change{Schematic arrangement of the detectors with possible particle trajectories shown as dashed arrows.}}
\label{fig:arrangement_detectors}
\end{figure}
\subsection{\acfp{SSD} (A, C, E)}
In total, six identical silicon \acp{SSD} \deleted{(detectors A, C and E)}, each with a thickness of $300\,\mathrm{\mu m}$, are used to measure the energy loss of passing particles. \change{The detectors used are spare parts from the Solar Orbiter EPD-HET mission by the Department for Extraterrestrial Physics at \acs{CAU} Kiel.
These are \acp{PIPS} with round detector faces provided by \textit{MIRION} former \textit{CANBERRA}.} There are three segments in each of the detectors that can be read out (see figure \ref{fig:ssd_dim}).
The first carrier is mounted above the Cherenkov detector. The second one is placed between the \ac{BGO} and the Cherenkov detector. The area covered by the \acl{SSD} \ac{SSD} below the \ac{BGO} should be as large as possible. This is achieved by stacking four of the \acp{SSD}. The first is placed in the center and the others as shown in figures \ref{fig:SSDE_iso} and \ref{fig:SSDE_top}.\\
\begin{figure}[!ht]
\centering
\subfigure[Dimensions of a \ac{SSD}.]{\includegraphics[width=0.4\textwidth]{images/experiment_description/HETB_dimensions.png}\label{fig:ssd_dim}}
\hfill
\subfigure[Picture of a \ac{SSD}.]{\includegraphics[width=0.4\textwidth]{images/experiment_description/hetB_photo.png}\label{fig:ssd_pic}}
\hfill
\subfigure[\change{Side view of \acp{SSD} E$_1$ to E$_3$.}]{\includegraphics[width=0.4\textwidth]{images/experiment_description/CAD_drawings/SSDs_iso.png}\label{fig:SSDE_iso}}
\hfill
\subfigure[\change{Top view of \acp{SSD} E$_1$ to E$_3$.}]{\includegraphics[width=0.4\textwidth]{images/experiment_description/CAD_drawings/SSDs_top.png}\label{fig:SSDE_top}}
\caption{\aclp{SSD}.}
\label{fig:ssds}
\end{figure}
\subsection{Cherenkov Detector (B)}
The Cherenkov detector \deleted{B} uses silica aerogel. It is comprised of three-dimensional silica (SiO$_2$) networks with air-filled pores of nanometer-scale. Due to the fine structure of the silica networks, the aerogel is transparent and highly porous. The aerogel blocks used were supplied by Japan Aerogels \citep{TABATA2016183}. The refractive index is approx. $1.05$. \replaced{One block with dimensions of $40\,\mathrm{mm}\times62\,\mathrm{mm}\times62\,\mathrm{mm}$ is used (see figure \ref{fig:aerogel}).}{Two blocks with dimensions of $20\,\mathrm{mm}\times40\,\mathrm{mm}\times40\,\mathrm{mm}$ are stacked to give a cube with a side length of $40\,\mathrm{mm}$ (see figure \ref{fig:aerogel})}. Furthermore, the \replaced{block is}{blocks are} wrapped in highly reflective material. The Cherenkov detector is shielded to be light-tight but vented.
A charged particle passing through the Cherenkov detector with a speed \replaced{larger}{higher} than the speed of light in the aerogel will cause Cherenkov radiation. \replaced{This radiation is measured by a \ac{PMT} because of the limited number of photons created. The \acf{PMT} makes the use of \acfp{HV} necessary.}{This radiation is measured by the \ac{PMT}.}
\change{Four different \ac{PMT} types supplied by Hamamatsu Photonics are tested as part of \justus's bachelor's thesis. The one with the best resolution for photons in the Cherenkov radiation spectrum will be selected for the experiment. All possible \acp{PMT} types conform with the experiment design.} \change[5]{In the end, a \ac{PMT} of type R1924A-700 was chosen \citep{PMT2022}}.
\begin{figure}[!ht]
\begin{center}
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=\textwidth]{images/experiment_description/dimensions_PMT.png}
\caption{Dimensions of a possible \acs{PMT}.}
\label{fig:pmt_dim}
\end{minipage}
\hfill
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=\textwidth]{images/experiment_description/Picture_pmt.png}
\caption{Picture of a possible \acs{PMT}.}
\label{fig:pmt_pic}
\end{minipage}
\end{center}
\end{figure}
\subsection{\acf{BGO} Scintillator (D)}
The main part of the scintillation detector \deleted{D} is a $20$\,mm thick hexagonal \ac{BGO} crystal (see figures \ref{bgo_pic} and \ref{bgo_dim}). With a density of $7.13\,\mathrm{g/cm}^{3}$, it weighs about $1$\,kg. To detect the photons produced by ionizing particles in the \ac{BGO}, \replaced{six instead of the originally two planned Si-PIN photodiodes (Hamamatsu S3590-09) with sensitive areas of $10\,$mm\,$\times\,10\,$mm instead of $10\,$mm\,$\times\,20\,$mm are glued to all sides of the \ac{BGO}. This change was motivated by \tom's bachelor's thesis (see \citep{ruge.2024}). While analyzing the photon signals in the \ac{BGO} using two photodiodes, a strong anisotropy was found depending on the position where the detected particles passed the \ac{BGO}. The use of photodiodes on all six sides of the crystal should prevent this. Since the number of channels on the \ac{IRENA} board is limited, the six diodes are connected to two preamplifiers. This is possible because only the sum of the photodiode signals is needed for the data evaluation. To avoid two adjacent diodes being connected to the same preamplifier, one diode is always skipped and connected to the second preamplifier. The change in size of the photodiodes is due to the fact that with six instead of two diodes, the total detection area and the total capacitance of the diodes increases. To limit the latter, diodes with a smaller detection area and capacitance have been selected.}{two photodiodes with sensitive areas of $10\,$mm\,$\times\,20\,$mm are glued to two opposite sides of the \ac{BGO}. It is to be determined whether the \acp{CSA} for the two diodes will be soldered directly onto the diodes or placed on the preamp board.} \\
The number of photons produced can be calibrated to the energy loss of the ionizing particles passing through the detector \change{using calibration measurements with muons and comparing the results with simulations}. \\
\\
\change{Dow Corning 93-500 Space Grade Encapsulant is used for gluing the photodiodes onto the \ac{BGO} crystal. It consists of two components mixed in a 10:1 ratio. Curing takes place in a vacuum chamber to keep the encapsulant free of entrapped air. The first step is to fill the diodes with the encapsulant. After a waiting period of 3 days, the filled diode is glued to the \ac{BGO} crystal with a new layer of glue. A small gap is left between the ceramic package of the photodiode and the \ac{BGO} crystal. This gap allows the adhesive to "flow" as the temperature changes. In previous applications without this gap, different expansion coefficients caused the encapsulant to contract and peel off, severely affecting the readout signals. The complete curing process takes seven days. After this time, all exposed sides of the \ac{BGO} are being covered with 3 layers of Millipor (highly reflective material) and wrapped with Teflon tape.}
\begin{figure}[!ht]
\centering
\subfigure[\change{Picture of an aerogel block.}]{\includegraphics[width=0.45\textwidth]{images/experiment_description/picture_aerogel.jpg}\label{fig:aerogel}}
\subfigure[\change{Dimensions of a Si-PIN photodiode.}]{\includegraphics[width=0.45\textwidth]{images/experiment_description/photodiode_small_dimensions.png}\label{diode_dim}}
\subfigure[Picture of a \ac{BGO} while wrapping.]{\includegraphics[width=0.45\textwidth]{images/experiment_description/BGO_picture.jpg}\label{bgo_pic}}
\subfigure[Dimensions of the \ac{BGO} crystal.]{\includegraphics[width=0.45
\textwidth]{images/experiment_description/BGO_dimensions.png}\label{bgo_dim}}
\caption{Images and dimensions of detector components.}
\end{figure}
\clearpage
\section{Mechanical Design}
The whole experiment setup will be located inside an insulation box and can be divided into two main compartments: the sensor head and the \ac{E-Box}, \change{both of which are enclosed in a pressure housing. A \ac{CAD} model of the experiment is provided on the \ac{RX/BX} team site under \url{https://rexusbexus.zarm.uni-bremen.de/share/page/} in the folder \texttt{CHAOS/CAD/}.}
\change[3]{Technical drawings are also provided in the folder \texttt{CHAOS/Technical Drawings/}} \change[4]{and in chapter \ref{sec:add_tech_info} of this document.}
\subsection{Pressure Housing}\label{sec:pressure_housing}
\begin{figure}[h]
\centering
\subfigure[\ac{CAD} model of the pressure housing.]{
\includegraphics[width=0.45\textwidth]{images/experiment_description/CAD_drawings/pressure_housing.png}
}
\subfigure[U-profile fixture]{
\includegraphics[width=0.45\textwidth]{images/experiment_description/CAD_drawings/U-profiles_exploded-1.png}
}
\label{fig:cad_pressure_housing}
\end{figure}
Since a \acf{PMT} is needed for the readout of the Cherenkov detector, there is also a need for \acf{HV} to operate such a device. This and the low pressure environments during the \ac{BEXUS} flight lead to substantial risk of corona discharges, which could endanger the whole experiment. Therefore, a pressure housing encases the whole experiment and keeps the pressure at 1\,bar, so there is no possibility of corona discharges.\\
For the main pressure vessel, an aluminum box manufactured by Multi-Box (MBA 332318) is used and combined with an aluminum base plate that was built in-house by the machine shop at \ac{CAU} Kiel.
\change[3]{The pressure housing lies on a silicon O-ring which is inside a groove in the base plate \change[4]{which presses the aluminum box into the O-ring} (see \ref{fig:pressure_sealing}). The O-ring can endure temperatures from -50\,$^\circ$C to up to 200\,$^\circ$C, which \replaced[4]{was}{will be} tested at a range of temperatures that are common for the flight environment. The base plate and aluminum box are connected with four M6 screws in the corners of the base plate.}
%\change[3]{The aluminum box lies on a silicon O-ring which is inside a groove in the base plate (see \ref{fig:pressure_sealing}). The O-ring can endure temperatures from -50\,$^\circ$C to up to 200\,$^\circ$C, which tested at a range of temperatures that are common for the flight environment.}
After some testing we encountered pressure losses at small pressure differences (0.3\,bar) between the inside and outside of the pressure housing due to the bending of the base plate (see section \ref{sec:test1}). In order to mitigate the bending, a cross of aluminum U-profiles is placed on top of the pressure housing and is then connected to the base plate using M4 threaded rods. Through the placement of the attachment points the bending force is transferred on to the U-profiles. \change[4]{There are no torques specified for the threaded rods. The nuts are fastened until there is tension on the rods. A sufficient sealing is verified by the vacuum tests performed after closing the pressure housing. After the tests the instrument will not be opened again. The transition between the aluminum box and base plate is covered with aluminum tape for grounding purposes. Furthermore, black duct tape is used as a light seal.} With this design, the pressure housing can hold pressure differences larger than $\un{1}{bar}$.
\begin{figure}
\centering
\includegraphics[width=0.6\textwidth]{images/experiment_description/CAD_drawings/pressure_sealing.pdf}
\caption{\change[3]{Close-up of the pressure sealing}}
\label{fig:pressure_sealing}
\end{figure}\\
In order to be able to check the pressure during flight and thus ensuring that no corona discharges occur, one pressure sensor (MS5534C) is placed inside the pressure housing on the \ac{FPGA} which itself is inside the \ac{E-Box} and another one outside on the bottom of the base plate. The cable for the pressure sensor outside goes through a hermetic D-Sub-15-connector (XAVAC15M/SI.0/AA) that is placed on the base plate. All signals are routed through this single feedthrough.\\
\change[4]{The pressure housing is purged with nitrogen (N$_2$) as safety measure against condensation. Furthermore, some silica beads were fixated inside the housing using Kapton tape.}
\change[4]{Fastening process of the pressure housing:
\begin{itemize}
\item Position aluminum box upside down and place the base plate with the instrument on top. Ensure that the aluminum box lies on the O-ring.
\item Place the pressure housing inside a plastic bag. Lift the base plate and leave a small gap between base plate and aluminum box to purge the pressure housing with nitrogen (N$_2$) with a small hose.
\item Remove the hose and close the pressure housing. Take the pressure housing out of the plastic bag and fasten the four screws in the corners.
\item Place the pressure housing on wooden blocks so that the threaded rods can be countered from below while fastening the nuts.
\item Position the U-profiles and threaded rods on the pressure housing and fasten the nuts.
\end{itemize}}
\subsection{Sensor Head Fixtures}
\begin{figure}[H]
\centering
\includegraphics[width=0.7\linewidth]{images/experiment_description/CAD_drawings/sensor_head_beschr.pdf}
\caption{\change[3]{\ac{CAD} model of the sensor head.}}
\label{fig:cad_sensor_head}
\end{figure}
The sensor head is composed of multiple \acfp{SSD}, as well as the Cherenkov detector and a \acf{BGO} scintillation detector. \change[3]{Most of the detector head, except the lower three \acp{SSD} , are screwed on an aluminum L-bracket, which stands in the middle of the base plate.} The exact alignment can be seen in figure \ref{fig:cad_sensor_head}. \deleted{The \acp{SSD} will be used for coincidence purposes as well as energy measurements.} The alignment of the \acp{SSD} determines the field of view of the instrument, especially above and below the Cherenkov detector.
\begin{figure}[H]
\centering
\includegraphics[width=0.7\linewidth]{images/experiment_description/CAD_drawings/sensor_head_expl_beschr-1.png}
\caption{\change[3]{\ac{CAD} model of the different detector groups.}}
\label{fig:cad_sensor_head2}
\end{figure}
\deleted{The Cherenkov detector consists of an aerogel block and a \acf{PMT} which requires \acf{HV}. In figure \ref{fig:cad_sensor_head} the \ac{PMT} is colored black and the aerogel yellow. Due to the possibility of corona discharges, when using high voltages at low pressures, it was decided to enclose the whole setup in a pressurized housing to nullify these chances. Furthermore, the \ac{HV} supply is soldered directly onto the \ac{PMT} to reduce cabling issues. The pressure housing encapsulates the electronics as well to reduce the amount of lead-throughs.} \change{Figure \ref{fig:cad_sensor_head} shows the aerogel block in yellow and the \acf{PMT} in green.} Because the aerogel is made up of about 99\,\% air, it is very brittle. Therefore, a system of \replaced[3]{3-D printed}{different} springs holds the aerogel block in place (\ref{fig:chkv_iso}). \change[3]{There are multiple holders with holes inside of them, where springs can be slipped in (\ref{fig:chkv_springs}). The number of springs can be varied to ensure an optimal fixation of the aerogel block.} \change[4]{The aerogel in its housing can be seen in figure \ref{fig:aerogel_housing}.} \replaced[3]{}{This is also why the \ac{PMT} cannot be glued onto the aerogel itself.} The structure holding the \ac{PMT} along with its power supply will be screwed onto the housing of the aerogel until the the \ac{PMT} firmly touches the aerogel \change[3]{to avoid gluing the \ac{PMT} to the aerogel. The \ac{PMT} is held inside an aluminum shell and is wrapped in Teflon tape to ensure a good fixation.} A rubber ring around the \ac{PMT} prevents photons from escaping the setup.\\
Lastly, the scintillation detector contains a hexagonal shaped \acf{BGO} crystal (purple in figure \ref{fig:cad_sensor_head}) with photodiodes glued onto \replaced{all six}{two opposite} sides. The detector is encased in a circular housing with screw-able blocks that hold the \ac{BGO} in place and ensure that the whole sensor head is held in place on the aluminum base plate. For installation purposes a second base plate, the interface plate made out of \ac{POM}, is \replaced[4]{used.}{screwed on top of the rubber buffers.} \deleted{The base plate contains a leak-proof lead-through (XAVAC15M/SI.0/AA) in order to connect the setup with the gondola as well as the pressure sensor beneath the base plate with the electronics inside.}
\begin{figure}[H]
\centering
\subfigure[Cherenkov housing without the aerogel block]{\includegraphics[width=0.45\textwidth]{images/experiment_description/CAD_drawings/chkv_springs_iso.jpeg}\label{fig:chkv_iso}}
\subfigure[Exploded Cherenkov housing]{\includegraphics[width=0.45\textwidth]{images/experiment_description/CAD_drawings/chkv_springs_beschr.png}\label{fig:chkv_springs}}
\caption{\change[3]{Cherenkov housing in detail}}
\end{figure}
\begin{figure}
\centering
\includegraphics[width=0.5\linewidth]{images/experiment_description/aerogel.jpg}
\caption{Aerogel block wrapped in reflective material inside its housing. The springs are not visible in this picture.}
\label{fig:aerogel_housing}
\end{figure}
\subsection{\acl{E-Box}}
\change{The second compartment is a stack of \acp{PCB} housed in two aluminum cases that are stacked on top of each other and screwed onto the base plate.} \change[3]{The four full-size PCBs are each individually mounted to the aluminum chassis with 10 M2 screws, while the smaller \ac{HV} driver board is mounted with spacers to the same frame as the \ac{HV} interface board.
The boards are interconnected by mezzanine and pin header connectors.} \change[4]{An example for the fixation of the electronics boards inside the \ac{E-Box} can be found in figure \ref{fig:electronics_board}.}
Any further information regarding the electronics compartment can be found in section \ref{sec:electronics_design}.
\begin{figure}
\centering
\includegraphics[width=0.4\linewidth,angle=90]{images/experiment_description/preamp_board.jpg}
\caption{Example for the fixation of the electronics boards inside the \ac{E-Box}. The screws are visible at the side of the board.}
\label{fig:electronics_board}
\end{figure}
\clearpage
\section{Electronics Design}\label{sec:electronics_design}
The general schematic electrical design of \ac{CHAOS} is shown in figure \ref{fig:flowchart_electronics}. In the following the individual sections will be described. Both the electronics design of the \acf{IRENA} board and the preamp board were already successfully used for other experiments built by the Department for Extraterrestrial Physics at \ac{CAU} Kiel. New is the electronics design for the \acf{HV} supply that is needed by the \acf{PMT} while the \acfp{SSD} require $\un{-40}{V}$ to operate.
The power board is modified under the supervision of \stephan. An \ac{HV} interface board is designed by our team under supervision of \replaced{\stephan\:}{as well as \bjoern}\:to communicate with the \ac{HV} \replaced{driver}{daughter} board. \change{Normally, an adapter board is attached to the power board, which contains the Ethernet chip and the power supply connector. To save space, this adapter board has been integrated into the HV interface board.}
The final \ac{HV} supply and connection to the \ac{PMT} is \deleted{also} designed under the supervision of \bjoern.
The \ac{CHAOS} team is responsible for soldering the connection cables and \acfp{PCB} as well as adjusting the gains of the \acfp{CSA} and shapers on them. All the boards have to be tested one by one. In addition, the sensor head has to be built and all detectors checked for noise. The whole experiment will be calibrated with cosmic muons.\\
All electronics are placed inside the pressure housing and connected to the outside via a hermetic D-Sub-15-connector \change[4]{(XAVAC15M/SI.0/AA)} in the aluminum base plate. The pressure sensor located outside the pressure housing is also fed through the Sub-D-15 and evaluated on the \ac{IRENA} board.\\
\change{The individual \acp{PCB} are described in more detail in the following sections. The relevant schematics and assembly diagrams can be found in the chapter \ref{sec:add_tech_info}.}
\begin{figure}[H]
\centering
\includegraphics[width=\textwidth]{images/experiment_description/electronics_flowchart_v3.3.pdf}
\caption{\change[3]{Flowchart of the electronics.}}
\label{fig:flowchart_electronics}
\end{figure}
\subsection{Signal Readout Channels}
The \acl{PMT} of the Cherenkov detector is read with a single channel, denoted as B, which requires a \acf{CSA} and a shaper. The \ac{BGO} has \replaced{six photodiodes wich are connected to two preamps}{two photodiodes}, each of which is read with dual gain (\ac{H} and \ac{L}), denoted D1H, D1L, D2H and D2L. This makes two further \acp{CSA} and four shapers necessary. Then there are the six identical \acp{SSD}. For both detectors \acp{SSD} A and C the two inner segments combined and the outer segment are read with dual gain channels called A1H, A1L, A2H, A2L, C1H, C1L, C2H, C2L. So four further \acp{CSA} and eight shapers are required. For the four \acp{SSD} forming E, all three segments are read as a single segment. The central detector is read with dual gain, so there is E0H and E0L, while the three lower ones are read with single gain E1, E2, E3. These four detectors require four \acp{CSA} and five shapers. In total, the electronics consists of 11 \acp{CSA} and 18 shapers.
\subsection{Preamp Board}\label{sec:preampboard}
The preamp board uses \acfp{CSA} to amplify the signals coming from the detectors before being processed in the data acquisition system. \change{The overall design is reused from the former \ac{BEXUS} experiment \acs{ADAM} which also came from \ac{CAU} Kiel.} It supports 18 channels, was designed at the Department for Extraterrestrial Physics and adapted for our sensor head by our team.
The preamp board consists of \acp{LDO} and 18 \acp{CSA} \replaced{of which eleven are needed for the detectors.}{of which only nine are needed because two preamplifiers for the photodiodes on the BGO are soldered directly to the diodes}.
%For the detectors B, C, D and E0 we want to have a maximum gain with 1 pF CSAs, which leads to a gain of ${\frac{1.602 \cdot 10^{-19}\mathrm{C}}{3.6\mathrm{eV}}} / (1 \mathrm{pF}) = 44.5 \frac{\mathrm{mV}}{\mathrm{MeV}}$.
%In order to be able to measure more energetic particles with the detectors A and E1,E2,E3, we want a larger energy range and use 3.3 pF CSAs with an amplification of ${\frac{1.602 \cdot 10^{-19}\mathrm{C}}{3.6\mathrm{eV}}} / ( 3.3 \mathrm{pF}) = 13.5 \frac{\mathrm{mV}}{\mathrm{MeV}}$.}
\subsection{\acs{IRENA} Board}\label{sec:irena}
The \acf{IRENA} board also comes from the Department for Extraterrestrial Physics and was already used for numerous missions to handle the data processing and storage. It is connected via Mezzanine connectors with upper and lower \acfp{PCB}.
There are 18 shapers on the \ac{IRENA}, all of which are needed. The eight high gain shapers \change[3]{(A1H, A2H, B, C1H, C2H, D1H, D2H, E0H)} amplify the signal by a factor of 15, the seven low gain shapers \change[3]{(A1L, A2L, C1L, C2L, D1L, D2L, E0L)} by a factor of 1. In addition, there are three medium gain shapers for the signals coming from E1, E2 and E3 which amplify the signal by a factor of 2.2.
%The 8 high gain signals from the preamp board are each time stretched with a shaper for the ADC and amplified 15 times again to obtain a maximum resolving particle energy of $3 \mathrm{V} / ( 15 \cdot 44.5 \frac{\mathrm{mV}}{\mathrm{MeV}} ) = 5\mathrm{MeV}$.
%The low gain signals are fed in parallel into 2 shapers. One amplifies with 15 and the other with 1. So they can dissolve a particle with the maximum energy of $3 \mathrm{V} / ( 1 \cdot 44.5 \frac{\mathrm{mV}}{\mathrm{MeV}} ) \approx 200\mathrm{MeV}$
The shapers are connected to the \acfp{ADC}. \change[3]{The final gains for the \acp{ADC} are currently under determination}.\\
A \acf{FPGA} reads the \ac{ADC} signals and determines the pulse height of the incoming signals. There are two pressure sensors on the \ac{IRENA} board. One will monitor the pressure inside the pressure housing and one will be wired outside the case to monitor the outside pressure. \\
A \change{32 bit ARM based} microprocessor \change{(LPC2148)} stores the pulse height, time stamp and pressure data on a supplied SD\:memory\:card and can communicate via Ethernet. \change{This microcontroller also contains the main software for the experiment. There is a 16 bit \ac{SSP} connection to the \ac{FPGA} \change[3]{with the \ac{IRENA} microcontroller as the master} where the signal data processing is done. In addition, the microcontroller communicates with four clients via a 8 bit \ac{SPI} \change[3]{with being the master here as well}. One of the clients goes to the Ethernet chip to send measurement data and receive commands from the \ac{CHAOS} ground station. Another sends the desired \ac{HV} value to the power board microcontroller at least once a minute. The next client is connected to the SD\:card to store data. The last connection is to the flash chip.}
% | n | Id | Name | pa | sh |
% |----+-----+-----------+-------+-----|
% | 0 | A1L | SSD A | - | 1 |
% | 1 | A1H | SSD A | 3pF | 15 |
% | 2 | E1 | SSD E1 | 1pF | 2.2 | Waren ursprünglich 3pF beim preamp
% | 3 | A2L | SSD A | - | 1 |
% | 4 | A2H | SSD A | 3pF | 15 |
% | 5 | E2 | SSD E2 | 1pF | 2.2 | Waren ursprünglich 3pF beim preamp
% | 6 | C1L | SSD C | - | 1 |
% | 7 | C1H | SSD C | 3pF | 15 |
% | 8 | E3 | SSD E3 | 1pF | 2.2 | Waren ursprünglich 3pF beim preamp
% | 9 | C2L | SSD C | - | 1 |
% | 10 | C2H | SSD C | 3pF | 15 |
% | 11 | D1L | BGO | - | 1 |
% | 12 | EL | SSD E0 | - | 1 |
% | 13 | EH | SSD E0 | 3pF | 15 |
% | 14 | D1H | BGO | 1pF | 15 |
% | 15 | D2L | BGO | - | 1 |
% | 16 | D2H | BGO | 1pF | 15 |
% | 17 | B | Cherenkov | 10pF | 2.2 | ALT!!!!!!!
% | 17 | B | Cherenkov | 100pF | 15 | NEU!!!!!!!
% Der Cherenkov müsste jetzt 100 pF beim preamp haben und eine Shaper-Verstärkung von 15
% Preamp müsste mit 1/pa verstärken
\subsection{Power Board}\label{sec:powerboard}
The power board transforms the $\un{28}{V}$ from the \ac{BEXUS} battery pack for the detector bias voltage and all other needed supply voltages.
\change{The basic electronics have been developed and already used extensively at the Department for Extraterrestrial Physics and by former \ac{BEXUS} projects from \ac{CAU} Kiel. \change[3]{To generate the various low voltages required by the electronics of the instrument, a regulator of type LT8580 is used, along with hand-wound transformers.} Some modifications have been made for \acs{CHAOS} and the design has been adapted to fit the \acs{HV} interface and driver board.}\\
\deleted{\:Additionally, it houses the AVR microcontroller which is responsible for managing the \acf{ADC}/\ac{DAC} connection to the \ac{HV} daughter board via the interface board.}
\change{The board contains an AVR microcontroller (ATmega32M1), which communicates via \acf{SPI} with the \ac{IRENA} microcontroller, which sends the desired \acf{HV} value at least once a minute. This routine is controlled by a watchdog. If no information is received from the \ac{IRENA}, an interrupt command is executed and the \ac{HV} is switched off.\\
This microcontroller also controls the LED located outside the insulation box. The light flashes red as long as a \ac{HV} is generated. When the \ac{HV} is off, the light flashes green. The LED will light up in the color of the last information sent if the last communication was a long time ago, e.g. if the system crashed.\\
The \ac{SPI} master is responsible for managing the \acf{ADC}/\ac{DAC} connection to the \ac{HV} driver board via the \ac{HV} interface board.}\\
\change[3]{A linear regulator of type LM317 was added to the board during testing and integration of the instrument to reduce noise on the detector signals. This regulator transforms the 28\,V from the input to 22\,V which are then transformed to the required voltages. This lead to changes in the power consumption of the instrument (see section \ref{sec:power}).}
\subsection{\ac{HV} Interface Board}\label{sec:HV-interface}
The \acf{HV} interface board establishes the connection to the Ethernet and power for the electronics stack. Also, it serves as \change{an} interface between the \ac{HV} \replaced{driver}{daughter board} and the power board. Furthermore, there are an \ac{USB} port and a reset button for technical support. \change{The main components of the \ac{HV} interface board are comprised of an 8-channel \ac{ADC} chip (ADS8688) and a single-channel \ac{DAC} chip (LTC1655).
The \ac{DAC} receives a digital value from the AVR microcontroller which it translates into an analogue voltage to control the \ac{HV} driver. The \ac{ADC} is capable of measuring eight analogue inputs, namely: current consumption of the instrument, the \ac{HV} output (through a voltage divider), the \ac{HV} regulator output (through a voltage divider), an onboard thermistor, the filtered $\un{+28}{V}$ \ac{BEXUS} battery pack supply voltage, the $\un{+5}{V}$ voltage for onboard components, the bias current of the \ac{PMT} and the \ac{DAC} output.}
\subsection{\ac{HV} Driver Board}\label{sec:HV-driver}
The \ac{HV} \replaced{driver}{daughter} board \replaced{generates an AC voltage of up to $\un{50}{V}$ peak-to-peak, which is used to drive the Cockcroft-Walton multiplier located inside the \ac{PMT} enclosure.}{produces and controls the bias voltage of the \ac{PMT}.} \change{This voltage is generated by a resonant circuit, whose amplitude is controlled via the \ac{DAC} on the \ac{HV} interface board.} Instead of being directly attached to the electronics enclosure, the \ac{HV} driver board is mounted via spacers to the frame of the \ac{HV} interface board above.
\subsection{\ac{HV} Multiplier board}
\change{The \ac{HV} multiplier board is located inside the sensor head next to the \acf{PMT} and supplies the required voltages to the \ac{PMT} dynodes. The board uses the AC voltage generated by the sinusoidal voltage generator on the \ac{HV} driver board. \deleted[3]{The peak-to-peak voltage is the measure of the individual \ac{PMT} dynode voltages.} The built-in voltage multiplier circuit is one of the standard electronic circuits in the \ac{HV} range and operates on the Greinacher doubler or Cockcroft-Walton principle to generate high DC voltages \change[3]{using a total of 13 stages.} \change[3]{The target value of the \ac{HV} can be controlled with a maximum value of around 1000\,V. To achieve an optimal amplification of the \ac{PMT} signal a \ac{HV} of around 800\,V will be used. This value can still be adjusted if necessary to achieve the scientific goals. The voltage drops over the dynodes of the \ac{PMT} are identical except for the first dynode which has a voltage drop three times as large as for the other dynodes.} Feedback to the AC voltage regulator is provided by a high impedance voltage divider.} \change[3]{The \ac{HV} Multiplier board is coated with Mapsil.}
\subsection{Safety}
\change[3]{The \ac{HV} will be only generated inside the \ac{PMT} enclosure. The maximum voltage (AC) generated on the \ac{HV}-driver board in the E-box will not exceed $\un{50}{V_{pp}}$. The whole experiment is placed inside the pressure housing which itself is placed inside a styrofoam box. This way, no direct contact is possible.}
\change{The \ac{HV} generator can drive a current of about $\un{20}{\mu A}$, loads a capacitance of \replaced[3]{$C=\frac{100\,\text{nF}}{13}\approx\un{7.7}{nF}$}{$\un{2}{nF}$} and is fully enclosed in a metal housing with the \ac{PMT} as part of the sensor head. \change[3]{The maximum voltage which can be produced by the experiment is approximately $U=1000$\,V. This corresponds to a stored energy of $E=\frac{CU^2}{2}\approx4\,\text{mJ}$ and poses no serious health hazard.} When power is turned off, the \ac{HV} discharges via a $R=\un{10}{G\Omega}$ resistor with a corresponding time constant of \replaced[3]{$\tau=RC\approx77\,\text{seconds}$.}{20 seconds.}} \change[3]{Measurements showed that the \ac{HV} has discharged after 10 minutes. A LED indicates \ac{HV} operation. A green flashing LED means the \ac{HV} is off and a red flashing light means it is active.}
% \an{Laut Stephan sind die Kondensatoren um den Faktor 10 größer!!!\\
%Wo genau liegen die ungefährlichen 4\,mJ an??? Ist das noch korrekt??? Was ist mit den 20 microamps???}
\subsection{Cable Routing}
\subsubsection{Outside the pressure housing}
\change[3]{Connected to the aluminum base plate is a hermetic Sub-D-15-connector which is connected to the LED as well as the power and ethernet connectors on the connector plate of the Styrofoam box. Furthermore, the D-15 is connected to the pressure sensor inside the Styrofoam box. A diagram can be seen in figure \ref{fig:sub-d-15}. The power receptacle is connected in accordance with the Bexus User Manual section 5.2 "Electrical Power" \citep{BEXUS_UserManual}, pin A bearing +28\,V and pin B being the return. The connector for the pressure sensor for this experiment has been soldered by the team. Also connected to the Sub-D-15-connector are four cables for Ethernet communication in accordance with handbook section 5.3 "Interface Description for E-Link Experiment Channels".
\begin{figure}[!h]
\centering
\includegraphics[width=0.75\linewidth]{images//experiment_description/chaos_harness_D-15.png}
\caption{\change[3]{Diagram of the Sub-D-15-connector.}}
\label{fig:sub-d-15}
\end{figure}
}
\subsubsection{Inside the pressure housing}
\change[3]{Inside the pressure housing, the pins of the male Sub-D-15 connector in the base plate are connected to pins of a male as well as a female Sub-D-9 connector. The cables for Ethernet, LED and power are bundled in a cable harness and routed to the male Sub-D-9 connector. The cables for the pressure sensors are routed in a cable harness to the female Sub-D-9 connector in the E-box. The cables for two temperature sensors (NTCs) are also routed to the female D-9. A diagram is shown in Figure \ref{fig:harnesses_inside_ph}.\\
All sensor head cables, with the exception of the \ac{PMT} cables, are routed to a sub-D-37 connector in the \ac{E-Box}. Monitoring, ground and driver of the \ac{PMT} \ac{HV} and the signal of the \ac{PMT} are routed directly from the E-box via four RG 178 cables to the housing of the \ac{PMT}.}
\begin{figure}[!h]
\centering
\includegraphics[width=0.75\linewidth]{images//experiment_description/Harnes_inside_PH.png}
\caption{\change[3]{Diagram of the cable harnesses inside the pressure housing.}}
\label{fig:harnesses_inside_ph}
\end{figure}
% \subsection{Grounding}
% \change[3]{All of our circuits are grounded to our experiments aluminum housing. To ensure proper grounding we want to electrically connect our housing to the gondola chassis via a grounding strap. The strap will be connected(see Fig. \ref{}) to one of the screws connecting the aluminum base plate to the plastic interface plate and to a screw connecting a rail to a rubber bumper. This means there will be a galvanic connection between our VRet, which is connected to ???, and the gondola chassis.}
% \an{Wo geht VRet hin ??}
% \an{Caption so verständlich? @Hannes}
\clearpage
\section{Thermal Design}\label{sec:thermal_design}
\change{\ac{CHAOS} will use a passive thermal design.} For insulation purposes, the experiment is encased in an expanded polystyrene thermobox, \change{also known as a "Fisherman's box".} This design \change{concept} has been successfully used by the previous \ac{BEXUS} teams from \ac{CAU} Kiel BX14\_MONSTA, BX19\_ADAM and BX29\_TANOS. In terms of heat dissipation, \ac{CHAOS} is similar to these. \deleted[3]{with a power of approximately $\un{4.1}{\text{W}}$ by the electronics.} \change[3]{A first approximation lead to a power consumption of 4.1\,W while Test T-07 showed a power consumption of 5.0\,W.} \replaced{Because \ac{CHAOS} does not fit in the old box used by previous missions, a new box was purchased (see figure \ref{fig:thermo_box_new}). The analysis performed in section \ref{sec:analysis1} shows that this design should lead to a temperature behaviour similar to the \acs{ADAM} experiment. \ac{CHAOS} will stay in the survivable temperature range based on table \ref{tab:thermal_range} for the duration of the flight.}{In figure 4.9(b) one can see that the temperature during the real flight of the \acs{ADAM} experiment was in a moderate temperature range between $35^\circ$\,C and $5^\circ$\,C before and during the flight. Both the electronics and sensor head of \ac{CHAOS} are designed for these temperatures. The flight of \ac{CHAOS}junior (see section \ref{sec:chaosjr}) has already allowed us to test some \acp{SSD} under real atmospheric conditions. Based on this, the passive thermal control provided by the insulation box should be sufficient. An additional thermal test will be conducted prior to launch.} \change[3]{This can be seen in figure \ref{fig:boundary_conditions_extreme_cases4}, where the thermal behaviour for a hot and a cold extreme case are shown. The corresponding boundary conditions were chosen based on data from previous \acs{BEXUS} flights and the pre-flight heating of the \acs{ADAM} experiment. The lower survivable temperature is determined by the datasheet of the photodiodes. The real lower boundary is most likely even lower. Temperature calibrations with these diodes were done at the Department for Extraterrestrial Physics for temperatures below $\un{-50}{^\circ\text{C}}$. More information on the thermal considerations can be found in section \ref{sec:analysis1} or on the \ac{RX/BX} team site under \url{https://rexusbexus.zarm.uni-bremen.de/share/page/} in the folder \texttt{CHAOS/Verification/}.\\The data sheet provided by the manufacturer states an operating temperature from -50\,$^\circ$ to 200\,$^\circ$ for the pressure housing's O-ring.} \change[5]{During launch it was decided to cover \ac{CHAOS} in rescue foil to further protect the instrument from cooling.}
\pagebreak
\begin{figure}
\centering
\subfigure[\change{Expanded polystyrene thermobox for insulation purposes.}]{\includegraphics[width=0.47\textwidth]{images/experiment_description/thermo_new_cut.jpg}\label{fig:thermo_box_new}}
\subfigure[Thermal model applied to different boundary conditions using the standard parameters ($P_\text{heat}=\un{4.1}{W}$).]{\includegraphics[width=0.51\textwidth]{images/verification/boundary_conditions_extreme_cases_4W.pdf}\label{fig:boundary_conditions_extreme_cases4}}
\caption{Thermal behaviour of \acs{CHAOS}.}
\end{figure}
\begin{figure}
\centering
\includegraphics[width=0.5\linewidth]{images/experiment_description/rescue_foil.jpg}
\caption{\ac{CHAOS} covered in rescue foil.}
\label{fig:rescue_foil}
\end{figure}
% \begin{figure}[H]\centering
% \subfigure[\ac{ADAM} experiment inside the insulation box.]{\includegraphics[width=0.39\textwidth]{images/experiment_description/ADAM_insulation.png}\label{fig:ADAM_insulation}}
% \subfigure[Thermal behavior of the \ac{ADAM} experiment and pressure during the flight.]{\includegraphics[width=0.59\textwidth]{images/experiment_description/ADAM_temp_press2.png}\label{fig:ADAM_temp_press}}
% \caption{Thermal properties of BX19 \ac{ADAM}}
% \end{figure}
\clearpage
\newcommand{\ThermalRange}{
\color{black}
\begin{longtable}{|p{2.3cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|p{1.2cm}|p{1.3cm}|p{1cm}|p{1.8cm}|}
\caption{Thermal ranges of individual components.}\label{tab:thermal_range}\\
\hline
\rowcolor{lightgrey} \textbf{Component} & \multicolumn{2}{l|}{\textbf{Operating T}} & \multicolumn{2}{l|}{\textbf{Survivable T}} & \multicolumn{2}{l|}{\textbf{Power Dissipated}} & \textbf{Comments} \\
\rowcolor{lightgrey} & \multicolumn{2}{l|}{\textbf{[$^\circ$C]}} & \multicolumn{2}{l|}{\textbf{[$^\circ$C]}} & \multicolumn{2}{l|}{\textbf{[W]}} & \\
\hline
& Min & Max & Min & Max & Average & Max & \\
\hline
MBA 332318 (pressure housing) & -40\,$^\circ$C & +100\,$^\circ$C & -50\,$^\circ$C & +140\,$^\circ$C & & & \\
\hline
O-ring (for pressure housing) & -50\,$^\circ$C & +200\,$^\circ$C & & & & & \\
\hline
MS5534C Miniature Barometer Module & -40\,$^\circ$C & +125\,$^\circ$C & & & & & \\
\hline
XAVAC15M & -40\,$^\circ$C & +85\,$^\circ$C & & & & & \\
/SI.0/AA &&&&&&&\\
\hline
CD-HET1000-300EB/C (\aclp{SSD}) & -20\,$^\circ$C & +35\,$^\circ$C & -35\,$^\circ$C & +100\,$^\circ$C & & & \\
\hline
R1924A-700 (possible \acs{PMT}) & -30\,$^\circ$C & +50\,$^\circ$C & -80\,$^\circ$C & +50\,$^\circ$C & & & \\
\hline
R1924P-700 (possible \acs{PMT}) & -30\,$^\circ$C & +50\,$^\circ$C & -30\,$^\circ$C & +50\,$^\circ$C & & & \\
\hline
R3550A-600 (possible \acs{PMT}) & -30\,$^\circ$C & +70\,$^\circ$C & -80\,$^\circ$C & +70\,$^\circ$C & & & \\
\hline
R3550P-600 (possible \acs{PMT}) & -30\,$^\circ$C & +70\,$^\circ$C & -30\,$^\circ$C & +70\,$^\circ$C & & & \\
\hline
S3590-09 (photodiodes) & -20\,$^\circ$C & +60\,$^\circ$C & -20\,$^\circ$C & +80\,$^\circ$C & & & \\
\hline
\end{longtable}
\color{black}
}
\ThermalRange
\clearpage
\section{Power System}\label{sec:power}
\ac{CHAOS} needs access to the $\un{+28}{V}$ of the gondola battery pack. \replaced{\replaced[3]{During test T-07 it was measured that the experiment draws approximately 178\,mA at a voltage of 28\,V with all systems turned on. This leads to a total power of around 5.0\,W. If the battery provides 360\,Wh, the experiment can run for around 72 hours.}{All boards of the electronics excluding the preamp board were tested with $\un{+28}{V}$ and the power consumption was determined to be $\un{3.7}{W}$. Based on previous experience the eleven preamps are assumed to need $\un{400}{mW}$ leading to an estimated power consumption of $\un{4.1}{W}$. There are no further power consumers. If the experiment runs for 10 hours, this results in an energy consumption of $\un{41}{Wh}$.}\\The power consumption of the experiment does not change during the different flight phases. We therefore do not provide a detailed table or I\,vs.\,t graph, even though it is encouraged in the \ac{SED} guidelines.}{The estimated power consumption is $\un{5}{W}$. If the whole procedure lasts 10 hours, this results in an energy consumption of $\un{50}{Wh}$. This is less than the estimated 9\,W of former \ac{BEXUS} experiment \ac{FaNS}. The exact power consumption of the individual parts cannot be determined at the moment.}
\change[3]{Some unwanted noise was detected on the measured signals of each detector channel. This lead to the addition of a linear regulator to the power board (see section \ref{sec:powerboard}) and results in a higher power consumption of the instrument. Therefore, the power consumption rose to 5.0\,W instead of the earlier estimated 4.1\,W. Again, the power consumption does not change during the different flight phases.}
\change[4]{During the integration of \ac{CHAOS}, the instrument was also operated at voltages different to the 28\,V. Furthermore, the instrument underwent multiple power cycles.}
\newcommand{\PowerRange}{
\begin{longtable}{|l|l|l|l|l|l|l|l|}
\caption{List of all power consumers.}\label{tab:power_consumers}\\
\hline
\rowcolor{lightgrey} \textbf{Phase} & \textbf{Duration} & \textbf{Component} & \textbf{Voltage} & \textbf{Current} & \textbf{Duty} & \textbf{Power} & \textbf{Energy}\\
\rowcolor{lightgrey} & \textbf{[h]} & & \textbf{[V]} & \textbf{[A]} & \textbf{Cycle} & \textbf{[W]} & \textbf{[Wh]} \\\hline
\hline
Before&2& Sensor head &36 & && $\approx0$&$\approx0$ \\\cline{3-8}
Launch&& Preamp board &5 & && $\approx0$&$\approx0$ \\\cline{3-8}
&& IRENA board &5 & && & \\\cline{3-8}
&& Power board &36 & && & \\\cline{3-8}
&& Ethernet chip &3.3 & && & \\\cline{3-8}
&& \textbf{Sub Total}& & &&$\approx5$&$\approx10$ \\\hline
\hline
During&4& Sensor head &36 & && $\approx0$&$\approx0$ \\\cline{3-8}
Flight&& Preamp board &5 & && $\approx0$&$\approx0$ \\\cline{3-8}
&& IRENA board &5 & && & \\\cline{3-8}
&& Power board &36 & && & \\\cline{3-8}
&& Ethernet chip &3.3 & && & \\\cline{3-8}
&& \textbf{Sub Total}& & &0.15&100 $\approx4.2$&$17$ \\\hline
\hline
After&& Sensor head &0 & && $0$&$0$ \\\cline{3-8}
Landing&& Preamp board &0 & && $0$&$0$ \\\cline{3-8}
&& IRENA board &5 & && & \\\cline{3-8}
&& Power board &36 & && & \\\cline{3-8}
&& Ethernet chip &3.3 & && & \\\cline{3-8}
&& \textbf{Sub Total} & & && & \\\hline
\hline
\textbf{TOTAL}&10&All &28 & $\approx0.15$ && $\approx5$ & $<50$ \\\hline
\end{longtable}
}
%%%%%%% Power Table from TANOS SED: %%%%%%%
% \newcommand{\PowerRange}{
% \begin{longtable}{|l|l|l|l|l|l|l|l|}
% \caption{List of all power consumers during the different phases.}\label{tab:power_consumers}\\
% \hline
% \rowcolor{lightgrey} \textbf{Phase} & \textbf{Duration} & \textbf{Component} & \textbf{Voltage} & \textbf{Current} & \textbf{Duty} & \textbf{Power} & \textbf{Energy}\\
% \rowcolor{lightgrey} & \textbf{[h]} & & \textbf{[V]} & \textbf{[A]} & \textbf{Cycle} & \textbf{[W]} & \textbf{[Wh]} \\\hline
% \hline
% Before&2& Sensor head &36 & && $\approx0$&$\approx0$ \\\cline{3-8}
% Launch&& Preamp board &5 & && $\approx0$&$\approx0$ \\\cline{3-8}
% && IRENA board &5 & && & \\\cline{3-8}
% && Power board &36 & && & \\\cline{3-8}
% && Ethernet chip &3.3 & && & \\\cline{3-8}
% && \textbf{Sub Total}&28 & &&$\approx5$&$\approx10$ \\\hline
% \hline
% During&4& Sensor head &36 & && $\approx0$&$\approx0$ \\\cline{3-8}
% Flight&& Preamp board &5 & && $\approx0$&$\approx0$ \\\cline{3-8}
% && IRENA board &5 & && & \\\cline{3-8}
% && Power board &36 & && & \\\cline{3-8}
% && Ethernet chip &3.3 & && & \\\cline{3-8}
% && \textbf{Sub Total}&28 &28&0.15&100 $\approx4.2$&$17$ \\\hline
% \hline
% After&& Sensor head &0 & && $0$&$0$ \\\cline{3-8}
% Landing&& Preamp board &0 & && $0$&$0$ \\\cline{3-8}
% && IRENA board &5 & && & \\\cline{3-8}
% && Power board &36 & && & \\\cline{3-8}
% && Ethernet chip &3.3 & && & \\\cline{3-8}
% && \textbf{Sub Total} &28 & && & \\\hline
% \hline
% \textbf{TOTAL}&10&All &28 & $\approx0.15$ && $\approx5$ & $<50$ \\\hline
% \end{longtable}
% }
% \PowerRange
\clearpage
\section{Software Design}\label{sec:software_design}
We use the data acquisition system \ac{IRENA} which was mainly developed by our supervisor \stephan. Figure \ref{fig:software} shows a schematic of the data flow.
\begin{figure}[h]
\includegraphics[width=1\linewidth]{images/experiment_description/flowchart_software.png}
\caption{Schematic of the data flow and existing software with the programming languages used. The \ac{HV} power supply is excluded.}
\label{fig:software}
\end{figure}
The microcontroller on the \ac{IRENA} board has an additional flash memory where the configuration and setting of the \acf{FPGA} \change{and setup of the microcontroller} is stored. As soon as the power is connected, it boots the \ac{FPGA} with these settings.
The \ac{FPGA} samples the \acfp{ADC} with 3\,MHz and stores the last 64 \ac{ADC} values. After each sampling, the \ac{FPGA} performs a \acf{PHA} for the stored \ac{ADC} values by comparing them with a given pulse shape.
%For this, 16 defined ADC values are multiplied with 16 defined values of the given pulse shape. The sum of these 16 multiplications is stored after every sample. If this sum has a maximum and a certain height (trigger level) the signal of the ADC is detected as a pulse and a trigger is set. In case of a trigger signal the PHA (Pulse Height Analysis) words for High and Low Gain of all 9 detector segments are sent to the Micro Controller.
If a pulse is above a defined trigger level \change{and has a maximum}, the signal of the \ac{ADC} is flagged as pulse. \deleted{In this case the \ac{PHA} words are sent to the microcontroller (ARM processor).}
\change{In case such an event was triggered the data and timestamp of the event will be saved in a \ac{FIFO} buffer on the \ac{FPGA}. This buffer will be read out by the microcontroller via a interrupt service routine, then stored on the SD\:card and sent to the ground station via \acf{UDP}. Housekeeping, pressure and event data are stored in different \ac{FIFO}s and read out with the following priority:}
\change{
\begin{itemize}
\item -1: Synchronous Serial Port Buffer (all data goes through here)
\item 0: Houskeeping data sent by the microcontroller
\item 1: Data generated by a request (e.g. pressure)
\item 2: Data generated by an event
\item 3: Sample data
\end{itemize}
}
\replaced{If a complete packet has been registered, the packet is sent through \ac{FIFO} -1 to the microcontroller which distributes it to the SD\:card and Ethernet.
The microcontroller sends housekeeping data of the voltages at the \acp{ADC} every \change[3]{10 seconds} to \ac{FIFO} 0 which is then sent via downlink and stored on the SD\:card.
The microcontroller also checks whether pressure data is in the buffer once per packet and checks with that data whether to shut down the data stream and the \acf{HV} according to Fig. \ref{DruckFlowChart}. The pressure data is requested automatically while alternating between the two sensors but can also be sent down on request.}{It stores the \ac{PHA} words and time information on the onboard SD memory card and sends them in \acf{UDP} packets via Ethernet downlink to the ground. Housekeeping data with temperatures and pressure will also be stored and send periodically to the ground station.}\\ % alle 10 s meine ich -> 1 sek ?
Every particle event generates a downstream of 96 bytes = 768 bits. %%%stimmen 96byte??
We assume a particle count rate of \replaced[3]{200}{100}\,counts/s. This multiplies to a downlink rate of \replaced[3]{approximately 154}{less than 100} kbit/s.
To make sure we do not use too much bandwidth, the maximum downlink rate is limited to 200\,kbit/s in the program code.
The limit can be throttled via uplink and adjusted to the possible downlink rate during the flight. \change[4]{The data rate of the uplink is around 4.8\,kbit/s based on analysis A-06.}% \change{via the command var/set spi\_min\_qtime}.\\
The measured data itself is stored in 512\,byte blocks on the SD memory card in binary form. There is no file system. \change{Instead data will be identified by their first two bytes. These bytes are unique for each type of data. After a conversion from binary data to plain text the data can be identified by their first column: H for housekeeping, HV for high voltage housekeeping, EI for detector/event data and P for data from the pressure sensors. The pressure sensors can be differentiated using their respective calibration words.} Furthermore, the data will be down-streamed in the form it is stored on the SD memory card. There \deleted{are no actions}\change{are no transformations of the data} in between.\\
When \ac{CHAOS} receives its power, it will \change{boot and setup automatically from the flash drive.} \change[3]{Although, the \ac{HV} will need to be started via Ethernet separately.} %\deleted{store and send data automatically}. No \change{external} shell commands are needed for that. \deleted{Only the destiny port, static IP, and MAC must be programmed.} %sind im Flash gespeichert, falls vor Ort überschreiben möglich (z.B. in flash_TANOS/ETH.RC)
%\deleted{In case of problems with \ac{CHAOS}, we can communicate with the experiment via Netcat. Because Netcat is a standard application of LINUX, all commands can be looked up by typing "\$ nc -h" \; in the shell. Thus no further example commands will be shown here.}
The \ac{HV} is regulated by the AVR controller via a 16\,bit \acf{DAC}/\acf{ADC} connection which also provides data regarding the \ac{DAC}-output, the oscillator amplitude, the \ac{HV} readout, the primary voltage ($\un{+28}{V}$), and possibly the temperature of the \ac{HV} cascade. This data is stored every minute.% Müsste HK data sein oder? -> neues HK Packet
The \ac{IRENA} \replaced[3]{will}{must} send a command to the AVR controller every \change[3]{10} second\change[3]{s} to reset the watchdog-timer. Otherwise, the \ac{HV} will be ramped down \change[3]{after two minutes} as a safety measure.
If the \ac{FPGA} measures a pressure of more than 800\,mbar \replaced[3]{10}{for several} times after it has already measured a pressure of less than 200\,mbar during runtime, the \ac{IRENA} issues a command to ramp down the \ac{HV} and suspends writing to the SD card to prevent data damage upon impact as shown in figure \ref{DruckFlowChart}. These settings ensure that it can do its job after switching on the power without any uplink commands in the case of disconnection. It is also possible to request housekeeping data.
\begin{figure}[h]
\centering
\includegraphics[width=0.7\linewidth]{images/experiment_description/Pressure logic.drawio.png}
\caption{\change{Logic behind the pressure monitoring with p\_i being the pressure inside and p\_o for the pressure outside of the pressure housing. The variable ascent stands for whether the balloon has already reached its floating altitude. The loop will be done once for every pressure measurement.}}
\label{DruckFlowChart}
\end{figure}
\clearpage
\section{Ground Support Equipment}
The data received by the downlink will be stored on a prepared notebook. The downlinked data will be delivered in binary format. On the notebook there are pre-installed \texttt{python}-scripts for communicating via Ethernet with the \ac{IRENA} (the used data acquisition system), as well as saving, converting and evaluating the received data.
\change{The uplink will be done through sending commands via netcat or a python script. Because Netcat is a standard application of LINUX, all commands can be looked up by typing "\$ nc -h" \; in the shell. Thus no further example commands will be shown here. The python script is a wrapper for the microcontroller commands and can send these directly via \acs{UDP}, \acs{USB} or \acs{UART}.}
Important commands in the python script are:
\change{\begin{itemize}
\item init\_irena() : initializes the connection to the \ac{IRENA}
\item irena\_exit(): stops the connection to the \ac{IRENA}
\item start() : opens a data stream
\item stop() : closes the data stream
\item pause(): pauses the data stream
\item enable() : starts the data stream from the \ac{FPGA}
\item disable() : stops the data stream from the \ac{FPGA}
\item reset() : resets the \ac{IRENA} board
\item housekeeping()/HK(): sends \ac{ADC} and \ac{DAC} voltages
\item status(): sends status of the data stream
\item Var(name,value): reads the named variable or writes it in case a value is given (can be used for example to limit the data rate)
\item thres(channel,value): set trigger level
\item Script(fname): executes a script from the flash drive (can be used for example to execute setup or flight routine)
\item \change[4]{Keep\_Alive() : pings the instrument every 30 seconds. Returns current temperature and pressure values. Needed to avoid a reset of the ethernet connection.}
\item \change[4]{set\_clock() : sets timestamps of recorded data to correct time.}
\end{itemize}
}
Very similar scripts have been used successfully for years at the Department of Extraterrestrial Physics.
\change[4]{A second laptop will be used to plot the most important housekeeping data.}
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