A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research

Abstract

Investigating the dynamical and physical properties of cosmic dust can reveal a great deal of information about both the dust and its many sources. Over recent years, several spacecraft (e.g., Cassini, Stardust, Galileo, and Ulysses) have successfully characterised interstellar, interplanetary, and circumplanetary dust using a variety of techniques, including in situ analyses and sample return. Charge, mass, and velocity measurements of the dust are performed either directly (induced charge signals) or indirectly (mass and velocity from impact ionisation signals or crater morphology) and constrain the dynamical parameters of the dust grains. Dust compositional information may be obtained via either time-of-flight mass spectrometry of the impact plasma or direct sample return. The accurate and reliable interpretation of collected spacecraft data requires a comprehensive programme of terrestrial instrument calibration. This process involves accelerating suitable solar system analogue dust particles to hypervelocity speeds in the laboratory, an activity performed at the Max Planck Institut für Kernphysik in Heidelberg, Germany. Here, a 2 MV Van de Graaff accelerator electrostatically accelerates charged micron and submicron-sized dust particles to speeds up to 80 km s(-1). Recent advances in dust production and processing have allowed solar system analogue dust particles (silicates and other minerals) to be coated with a thin conductive shell, enabling them to be charged and accelerated. Refinements and upgrades to the beam line instrumentation and electronics now allow for the reliable selection of particles at velocities of 1-80 km s(-1) and with diameters of between 0.05 μm and 5 μm. This ability to select particles for subsequent impact studies based on their charges, masses, or velocities is provided by a particle selection unit (PSU). The PSU contains a field programmable gate array, capable of monitoring in real time the particles' speeds and charges, and is controlled remotely by a custom, platform independent, software package. The new control instrumentation and electronics, together with the wide range of accelerable particle types, allow the controlled investigation of hypervelocity impact phenomena across a hitherto unobtainable range of impact parameters.

The aerosol impact spectrometer: a versatile platform for studying the velocity dependence of nanoparticle-surface impact phenomena

“…An area of continued interest for an understanding of problems ranging from atmospheric chemistry to astrophysical phenomena to industrial applications pertains to the impact phenomena of nanoparticles. Theoretical and experimental studies of nanoparticle-surface collisions at both low and high velocity ranges continue to be reported [2][3][4][5][6][7]. There is a large body of work on hypervelocity impact phenomena dating back to the early 1960's motivated by the need to understand the effect of cosmic dust and micrometeoroid impacts on space vehicles.…”

Estimation of hypervelocity impact parameters from measurements of optical flash

“…The experimental facility is described in detail by Mocker et al [26]. At this facility, dust particles are charged using a Van de Graaff generator and then accelerated from a 2 MV terminal into a vacuum chamber.…”

Calibration of relative sensitivity factors for impact ionization detectors with high-velocity silicate microparticles

“…For our experiments we used the modified 2 MV Van de Graaff accelerator (Mocker et al, 2011), operated by the University of Stuttgart and located at the Max-Planck-Institut für Kernphysik. Here, positively charged dust grains, with sizes from 50 nm up to 5 lm, are accelerated via an electrostatic field of $2 MV to hypervelocity speeds between 1 and 100 km s À1 .…”