Hypervelocity impact research with an electrostatic dust accelerator

Due to the their ubiquity and the high impact energy leading to extremely high temperatures and pressures in the affected materials, the physical processes caused by HVIs play an important role in a variety of fields such as the investigation of matter at extreme pressures and temperature, shock waves in solid bodies or even Solar System research, planetology, cosmic dust research and space engineering:

• Cratering phenomena throughout the Solar System :The first systematic investigation of HVIs of micro-meteoroites was dedicated to the understanding of micro-cratering on lunar rock samples. The size and morphology of resulting micro-craters was investigated as a function of particle size and impact speed.
• Planetology – Characterization, development and calibration of dust sensors measuring the composition, size and trajectory information of micrometeoroids aboard interplanetary spacecrafts.
• Astrobiology – Simulation of hyper-velocity impacts of organic micron-sized projectiles and mass spectrometric analysis of impact plasmas containing complex organic molecules; simulation of micrometeoroid impacts onto water ice surfaces.
• Space weathering: Alteration of bombarded surfaces
• Cosmic Dust research: A major part of what we know today of HVIs of micro-meteoroites was obtained in the process of developing, calibrating and operation of in situ instruments for the investigation of dust in the Solar System. Thereby induced physical processes generate measurable signals which are then transmitted to Earth and can be analyzed afterwards.There are a variety of methods for in situ dust measurements such as the detection of thin foil penetration, the particle charge, the emerging impact flash or ions generated upon impact, revealing the particles’ velocity, trajectory, mass and even chemical composition. Of all these methods, the generation of charge during impacts provides one of the most sensitive methods for the detection and and the most comprehensible characterization of dust particles in space. The characterization of the dynamical and even chemical properties of dust particles in the Solar System allows us to investigate the origin of cosmic dust and its role in the formation of the Solar System and even its role in the origin of life.
• Impact physics/Materials under extreme conditions – Investigation of plasma and material conditions of projectile-surface interactions under hyper-velocity impact conditions.

Electrostatic dust accelerators 

To calibrate in-situ dust  instruments and to get a deeper understanding of the processes involved, hypervelocity impact measurements under similar and well defined conditions are required. For this purpose, a Van-de-Graaff type ion accelerator was modified at the MPI-K/HD in the late 1960ies. The accelerator was equipped with a dust source capable of charging and accelerating dust particles (Fig. 1).

The accelerator covers a large portion of the speed and size ranges needed for most cosmic applications with velocities between 1 to about 80 km s⁻¹ (Fig. 2).

After being located for over 5 decades at the MPI-K, the dust accelerator has been moved to the IRS/UniS. This relocation gives us the opportunity to optimize the set up and the also the whole dust research laboratory in its entirety.

Particle properties and beam monitoring

The dust beam originates from the dust source within the high voltage terminal of the accelerator.(Fig.1). After exiting the source, the dust particles are accelerated in the electrostatic field towards the experimental set-up. Before reaching the experiment chamber, the particles are registered, characterized, and eventually selected while passing the beam line detectors of the Particle Selection Unit (PSU). For this, the particles are been detected by a chain of detectors measuring the particle's primary surface charge using an induction tube and a charge-sensitive amplifier (CSA). The PSU determines the grain speed and mass in order to select individual dust grains on the basis of a speed and mass window given by the experimentator. 

Cosmological relevant materials 

Dust Materials: For the above described method of acceleration to work, the particles must therefore be capable of carrying charge and hence the range of materials used has been restricted to those which are either wholly conductive or those with a conductive coating. In the last few years two techniques of coating underwent significant improvements, opening up a whole new range of material types to investigate.

Target Materials: Due to the bean geometry and the vacuum conditions , there are a variety of constraints fr the target mounting and properties. Solid metal and silicate target, of terrestrial and meteoritic origin,  can be easily used and have been investigated numerous times in the past. Experiments with icy targets are planned for the near future. 

Investigation of impact ionization with A linear TOF mass spectrometer

The characteristics of the impact plasma, such as the velocity distribution of the ions and the ion appearance in the mass spectra, can be analyzed with a linear TOF mass spectrometer. Here, the combination of velocity and angular distributions of the ions results  in a broadening of the mass lines, determining their shapes. To study the distribution of the ion velocities alone, we developed an optimized narrow aperture mass spectrometer (Fig.3). 

The simple set up and the almost homogenous fields allow to calculate the flight times due to the known response function of the instrument. The measured mass line profile can be inverted for the distribution of initial velocity and subsequently the initial kinetic energies of the ions as shown in Fig.4.

In addition to TOF other important measurements will address: cratering, secondary  ejecta, neutral production, optical spectroscopy of the of the impact flash, and the characterization of the EM waves.The combination of future theoretical studies of the impact processes and the subsequent expansion of the impact produce plasma with this expanded set of measurements will be a powerful tool to investigate the state of the hot compressed matter.