# Upgrades of a Small Electrostatic Dust Accelerator at the University of Stuttgart
## 2 Materials and Methods
### Dust Detector Upgrade  
The dust detector is needed to monitor the particles launched by the dust source. It is based on charge induction and enables the non-contact measurement of individual dust particles down to the sub-micron range. Positively charged dust particles attract electrons and induce electron movement in metal electrodes. A charge sensitive amplifier (CSA) connected to the electrode transforms the induced charge to an equivalent voltage, which is evaluated using the detection system. In order to measure hypervelocity dust particles with surface charges down to 0.1 fC, [PERSON] [18] developed a low-noise dust detector. The dust detector employed a copper cylinder electrode with a diameter of 9 mm and a length (\(L\)) of 200 mm. The particle velocity (\(v\)) is determined by the flight time (\(T\)) through the electrode (Equation (3)). \[v=\frac{L}{T} \tag{3}\]  
Such a dust detector has a very high accuracy in the measurement of low- and high-speed particles up to 200 km/s. However, the integrated Amptek A250F/NF CSA in the design has a lower cutoff frequency of 2 kHz, which restricts its application in the measurement of dust particles with speeds below 100 m/s launched by small accelerators. In contrast, our upgraded dust detector consists of four short cylindrical tubes each 5 cm in length and 0.9 cm in inner diameter (see Figure 6). The electrodes are surrounded by two cylindrical Faraday caps to restrain any interference of environmental charges or electromagnetic fields. The electrodes are shielded from each other by grounded grids and insulated by Polytetrafluorethylen (PTFE) supports. The connection of two sensor electrodes to one single differential CSA further reduced the influence of common mode interference. The screening by the shielding cylinders is improved by attaching larger meshes. The electrodes and CSAs are mounted in a 100 mm vacuum pipe with standard CF 100 flange interfaces. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Lens** & **Focusing Voltage** & **Acceleration Voltage** & **Focal Length** \\ \hline \multirow{4}{*}{Einzel lens} & 10 kV & 15 kV & beyond beam (5 m) \\  & 10.745 kV & 15 kV & 3000 mm \\  & 10.94 kV & 15 kV & 1500 mm \\  & 11 kV & 15 kV & 1327 mm \\  & 12 kV & 15 kV & 467 mm \\ \hline \multirow{4}{*}{Single-cylinder} & 10 kV & 15 kV & beyond beam (5 m) \\  & 11 kV & 15 kV & beyond beam (5 m) \\ \cline{1-1}  & 11.15 kV & 15 kV & 3000 mm \\ \cline{1-1}  & 11.18 kV & 15 kV & 1500 mm \\ \cline{1-1}  & 12 kV & 15 kV & 250 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Lens effect of focusing system based on SIMION simulations. The design of the differential CSA is based on an improved version of the design presented in reference [19]. While traditional CSAs evaluate the influenced charge of a single electrode with reference to ground, the detector presented in this paper contains two electrodes and evaluates the charge difference between both electrodes. This results in a bipolar output pulse as given in Figure 7, which in subsequent designs will be filtered with a matched-filter system. The amplifier Application Specific Integrated Circuit (ASIC) is designed in a 350 nm technology. It has a 3 dB frequency range from 4 Hz to 1 MHz and therefore is able to accurately reproduce the influenced charges even of very low-speed particles. The low cutoff frequency of 4 Hz is achieved by replacing the feedback resistor \(R_{f}\) typically required for CSAs with a MOSFET in an off-state. This enables the design to reach an effective feedback resistance in the \(\mathrm{tera}\)-ohm range. To stop the amplifier from leaving the correct operating point due to leakage currents, a reset pulse is applied to the amplifier every few seconds. During this pulse the MOSFET in the feedback path is transferred to the linear region. Thereby, the feedback resistance is lowered significantly and the correct input voltage is reestablished. Additionally, materials with a very high specific electrical resistance are used for all parts with contact to the input of the amplifier.