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A new dust source for the Heidelberg dust accelerator M. Stübig3, G. Schäfer3, T.-M. Hoa, R. Sramaa and E. Grüna aMax-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany

The Max-Planck-Institut für Kernphysik in Heidelberg owns a dust accelerator, which is able to provide micron and submicron sized particles with speeds between 1 and 80 km The type of dust materials and the speed and mass range of the accelerated particles is dependent on the design of the dust reservoir at the front end of the accelerator beam line. In order to improve the capabilities of this reservoir, a new dust source was developed. A simplification in the source design allows a more efficient particle extraction. Also new sorts of dust can now be accelerated at the Heidelberg facilities: Iron, silver, copper, carbon and latex. This paper describes the dust source and the properties of the first accelerated dust samples. The experiments were performed at both, the 20 kV test bench and the 2 MV accelerator.

1. INTRODUCTION

Nearly 40 years ago, the first experiments with dust accelerators started. The aim of these experiments were studies of hypervelocity impact physics. The first dust source was developed by Shelton in 1960 [1]. It was reconstructed from an ion source for ion beam accelerators. Micrometer sized particles are electrically charged with the dust source. The dust source is described in [2]. It runs best with a powder of spherical iron particles from 200 nm to 2 fim in diameter. After leaving the source, a high electrostatic potential will accelerate the particles to their final velocity. The accelerator works with a 2 MV Van-de-Graaff generator (Figure 1), which was developed by Friichtenicht [3]. Velocities from 2 to 10 km s"1 can be achieved. With particles less than 100 nm, even higher velocities were detected (80 km s-1, [4]). A Particle Selection Unit (PSU) allows to select particles of a defined speed and charge. Particles not meeting these constraints are electrostatically deflected out of the beam line. Our dust accelerator is mainly used for studies of impact ionization, micro crater formation, calibration of impact detectors and time-of-flight mass spectrometers (TOF-MS). These detectors are used for cosmic dust space research [5-7]. In order to extend material types and the mass and speed range of the dust particles, a new dust source (Figure 2) was developed [8].

2. FUNCTION PRINCIPLE

The requirements of the source design are high: Potential differences higher than 20 kV over a few millimeters and a pressure difference of 18 bar between the interior and the environment. The most important change of the source from the previous system is the

Figure 1. Schematic view of the Heidelberg 2 MV dust accelerator. On the left side the pressure tank with the belt generator and dust source. In the middle section of the beamline: Particle Selection Unit for preselection of desired particle parameters. On the right side: Vacuum chamber for experimental setups.

Figure 1. Schematic view of the Heidelberg 2 MV dust accelerator. On the left side the pressure tank with the belt generator and dust source. In the middle section of the beamline: Particle Selection Unit for preselection of desired particle parameters. On the right side: Vacuum chamber for experimental setups.

reservoir and the charging system [8]. The new reservoir is of cylindrical shape, rounded at the outlet side. A tungsten needle is centered in the reservoir and aligned to the accelerator beam line (Figure 2). In order to save weight, the housing of the new source is completely made of antimagnetic titan. The properties of the dust source are collected in Table 1. The dust reservoir itself lies on the same electric potential as the needle and is pulsed down frequently to blow up the dust powder. Electrically conductive particles carry surface charges and follow the time varying field conditions and swirl around the reservoir. If a particle hits the tip of the needle it gets its final charge and is accelerated by the field of the extraction plate. It has to pass through a small hollow in the extraction plate and a collimation system before it enters the acceleration section of 2 MV.

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