Studies of plasma production at hypervelocity microparticle impact

Abstract

This paper is concerned with an investigation of the plasma generated during the impact of hypervelocity microparticles with a metal target. A laboratory source of hypervelocity micron-sized iron particles is first described which utilizes a 2 MV Van de Graaff generator, followed by a discussion of the techniques used to determine the mass and velocity of the particles. On impact of the iron projectiles on a slatted molybdenum target, charge is generated, extracted from the region of impact by suitably biasing a gridded electrode, and subsequently detected using a conventional wideband electronic amplifier or an electron multiplier. It is shown statistically that during impact equal numbers of electrons and positive ions are produced - indicating plasma generation - and that the total charge released (Q) may be described empirically in terms of the mass (m) and velocity (v) of the particle by a simple power law relationship of the kind Qαmαvβ, with β = 3·2 ± 0·1 over the complete velocity range investigated (0·05 to 10 km s−1) and with α = 0·85 for v>1 km s−1 and 1·33 for v<1 km s−1. It is also shown that the charge extracted from the plasma reaches a maximum a few microseconds after impact and subsequently decays exponentially with a time constant of several micro-seconds.A crude mass analysis of the positive ions generated during the impact using a simple time-of-flight mass spectrometer has indicated that, over the velocity range investigated, the dominant ions in the spectrum are characteristic of the projectile and not the target material. From considerations of the relative abundance of the metal ions in the spectra, together with the known relative metal atom concentrations in the iron projectiles, the temperatures of the plasmas have been estimated (from Saha equilibrium considerations) to be of the order of several thousands of degrees Kelvin.In a detailed discussion, attempts are made to consider how the kinetic energy of the projectile is dissipated - eventually producing plasma - in terms of available theoretical models of hypervelocity impact. It is shown that the most satisfactory model of this very complex interaction appears to be one in which the relative importance of shock-wave propagation into the projectile and target materials is considered. Finally, a discussion of the properties and the temporal behaviour of the impact-produced plasma is presented in order to assess the relative importance of diffusion and recombination processes, thus to assist in the interpretation of the experimental results obtained in this study.