# Upgrades of a Small Electrostatic Dust Accelerator at the University of Stuttgart
## 2 Materials and Methods
### Dust Source Upgrade  
The charging of dust particles is crucial for electrostatic acceleration. Particles may be charged with UV irradiation [16], electron and ion beams, or contact with a charged surface. The latter method, which results in an adequate particle charge-to-mass ratio (\(q/m\)), was utilized to develop the dust source in the electrostatic accelerator. The newly upgraded dust source is based on our previous design described in [17], which mainly consists of one vacuum housing, one dust reservoir, one needle electrode and one collimating system (see Figure 4). The dust reservoir has a cylindrical shape with a length of 25 mm and a diameter of 12 mm. The 1 mm tungsten needle is very sharp and has a tip diameter in the range of 1-4 \(\upmu\)m, which is centered in the cylinder axis of the dust reservoir and aligned to the accelerator beam line. The dust reservoir itself lies on the same electric potential as the needle and is pulsed down frequently to blow up the dust powder. Conductive dust particles are filled into the reservoir. The electric field will induce a charge on the surfaces of single particles and lift them up. Whenever these levitated particles hit the needle tip, they obtain higher electric charges and are ejected. The accelerated dust particles have to pass through a small hole (0.8 mm in diameter) in the extraction plate and after a collimation system they enter the beam line.  
Conductive dust particles can obtain surface charges by contact with highly charged surfaces [16]. The surface charge of a spherical particle with radius \(r\) under vacuum permittivity \(\epsilon_{0}\) and surface potential \(\Phi\) is given by:  
\[q=\Phi\cdot 4\pi\epsilon_{0}r \tag{2}\]  
All particles obtain their final speeds from the dust source in a dust accelerator, shown in Figure 1. For a dust particle of a given radius and density, its final speed is only dependent on the high voltage added in the dust source. As a result, the high-voltage interface used in the dust source has been modified and redesigned. Our new design allows input voltages up to 40 kV through two ceramic feedthroughs (CeramTec) (see the right side of Figure 4). Furthermore, the mechanical designs of the dust reservoir and isolator are simplified to reduce the number of assembled parts and optimise the evacuation by the vacuum pump. The simpler and lighter structure of the new dust source allows an easier handling and a faster dust refilling. The parameters of our upgraded dust source are summarized in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Previous Setup** & **New Setup** \\ \hline Total length & 1.5 m & Up to 4.5 m \\ Focusing system & Single cylinder & Einzel lens \\ Acceleration voltage & Up to 20 kV & Up to 30 kV (+120 kV) \\ Particle speed detection range & Above 100 m/s & 10 m/s to 10 km/s \\ Test chamber & \(\diameter\) 100 mm & \(\diameter\) 100 mm and \(\diameter\) 300 mm \\ Operation mode & Continuous mode & Continuous and single mode \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of previous small accelerator and upgraded setup.