Description

The dust accelerator facility of the Max-Planck-Institut für Kernphysik consists of a dust particle source, an acceleration path, a drift tube with particle selection unit and an experiment chamber.

The dust source is made of titanium and contains a dust reservoir, a tungsten needle as charging electrode inside the reservoir, an extraction plate and a beam collimator. The dust powder inside the reservoir has to be electrically conductive. The reservoir housing and the tungsten needle have a potential of approx. 20 kV. While the potential on the needle remains fixed, the reservoir potential is frequently pulsed with amplitudes about 10 kV. A charge is induced on the particles inside. They start to swirl around due to coulomb repulsion forces. Hitting the tip of the needle, a particle receives its final electric charge and is pulled out of the reservoir by the field of a grounded extraction plate. After passing a collimator, the particle enters the acceleration unit. A Van-de-Graaff belt generator provides the acceleration voltage of 2 MV. Equipotential rings generate an electric field gradient along the acceleration line. The potential energy difference of the particle from 2 MV to ground is converted into kinetic energy:

½ mv² = qU

Depending on their charge/mass-ratio, the particles reach speeds up to 100 km/s. The smaller the particles, the higher the achievable charge/mass-ratio. Thus the highest speeds are only reached by the smallest particles.

The accelerated particles pass focussing and steering electrodes. Their speed and charge is measured with induction tubes, connected to charge sensitive amplifiers. From energy conservation result the masses and radii (from assuming spheres) of the particles. A particle selection unit compares the measured charge and speed values with manually adjustable constraints. Particles not meeting the given values will be deflected by capacitor plates. Three operation modes are available:

Vacuum chamber at the dust accelerator facility. The chamber has a diameter of 1.4 m and can be cooled and heated.

The experiment chamber has a mounting table for the experimental setup which can be moved vertically to the beam line by a total of 25 cm. The chamber has a total diameter of 1.4 m (1.1 m with thermal shroud). A thermal shroud allows heating and cooling of the chamber from +100 °C to 100 °C, eg. for thermal vacuum tests of spacecraft instruments. The chamber is connected by a CF100-flange to the vacuum tube of the beam line. Alternatively, a smaller chamber (0.4 m) is available. Small targets/experiments (< 0.1 m) can directly be mounted on a CF100-flange.

The vacuum (< 10-4 Pa / 10-6 mbar) is generated by magnetically driven turbomolecular pumps, cryo pumps and ion getter pumps. Thus, the vacuum is not contaminated by organic compounds. This allows reliable results for impact ionisation mass spectroscopy as performed by cosmic dust detectors like CDA (onboard Cassini) and CIDA (onboard Stardust).