Introduction

Impact ionization experiments have had a distinguished role in helping to explore and better understand our Solar System. They rely on the impact of micron and sub-micron sized particles at high speed onto spacecraft. Because the typical relative speeds of spacecraft and dust particle are in the many km s"1 range the resulting impact is in the hypervelocity regime. Such impacts involve impact speeds above the speed of the resulting compression waves in both projectile and target. The result is an impact shock which develops extreme high pressures and temperatures in the projectile and the impact site in the target as the shock and subsequent release waves propagate. These temperatures and pressures are sufficient to not only melt but also vaporize the projectile and part of the target. The bulk target material also undergoes motion away from the impact site, excavating an impact crater. It is this crater that is the surviving legacy of the impact.

This impact process is perfectly natural in space. Due to the flux of small (dust) particles in space it occurs regularly on spacecraft. Measuring the dust flux throughout the Solar System has been a long term aim of space science since its earliest days. Initially, this was to determine the degree of damage to spacecraft, and it is now a science in its own right. This series of Dust Colloquia, stretching back many years, is an indication of the importance attached to these studies by the academic community. A good spacecraft deployed measurement system for dust is one that can generate a real time electronic signal characterizing the impact. This would determine the mass, velocity and composition of the particle. The information has to be real time, so a time dependent flux can be calculated, and electronic as the spacecraft may not be retrieved from space and will have to transmit the data to Earth. Fortunately, since the 1960's, it has been realized that the phenomena of impact ionization provides such a measurement opportunity.

During a hypervelocity impact as described earlier, the particle is vaporized and forms an ionic plasma. If a clean metal target is used, with a grid above it, with an electric field between target and grid, the presence of this impact generated plasma will allow a brief current flow and hence a pulse in external circuitry. Under the right conditions, the rise time of this pulse is related to the impact speed. Further, the magnitude of the ionization charge (/) measured can be related to the particle mass (m) and impact velocity (v) by a relationship of the form:

The determination of the relationship between rise time and impact speed and the measurement of the coefficients a and ft in eqn. (1) represents the laboratory calibration of a particular dust detector. Once these are known, then for an impact (where I and V are measured), the mass m can be determined. Many such detectors have been deployed in space and several are still operational. For example, one family of similar instruments is widely deployed; in Earth Orbit the Gorid detector [1] is on the Russian Express2 telecommunications spacecraft, the Ulysses spacecraft carries one [2] in a Solar polar orbit and Galileo carries one [3] on its mission to the Jovian system. All of these map the dust flux in various regions of the Solar System.

However, this does not assist in determining particle composition. To do this another feature of the ionic plasma can be exploited. If the ionization detector has an intense electric field for a short height above the target the ions will undergo a rapid acceleration. If they are singly ionized, then their resultant velocity is dependent on the ion mass. If a further, weak, field is then applied to guide the ions to a detector a distance away, then the ion arrival time at the detector is related to the square root of the ion mass. Thus, in effect, a time of flight (mass) spectrometer can be manufactured. Two versions of such detectors are currently deployed in space. The first is the Cosmic Dust Analyser (CDA) [4] on the Cassini spacecraft to Saturn. This is essentially as described, and is a modification of the instruments on board Ulysses, Galileo etc. The second type is CIDA [5], deployed onboard the Stardust spacecraft travelling to Comet-P-Wild-2. In this type of instrument during the drift to the detector, the ions are focussed by an electric field. This removes the spread on the arrival time of an ion species due to slight differences in initial energy in their initial production. The result is a better mass resolution. Just as before, both types of instruments have to be calibrated in the laboratory.

Laboratory calibrations are usually carried out using Van de Graaff (VdG) dust accelerators. These are traditional style VdG machines, into which a dust source is placed at the top terminal. These sources charge the dust particles (mass m) with charge q and eject them into the full potential difference (V) of the VdG. The beam lines are maintained at a vacuum of typically 10"6 mbar, and the result is that the charged particle is electrostatically accelerated to a velocity v given by:

The charge on each accelerated particle can be measured in flight by its passage through a conducting ring positioned along the axis of flight. The time difference in the signals from passage through two such rings separated along the line of flight by a known distance gives the velocity. The VdG voltage is monitored continually, so for each particle accelerated the mass m can be found from eqn (2). A typical such VdG is described in detail in [6].

The type of dust used in such VdG dust accelerators is limited to those that are easily charged. Iron has been the dust of choice for many years. However, iron is limited in its suitability as an analogue for Solar System dust. Attempts have been made to use other materials. In the 1970's use was made of an ion beam to charge particles and then accelerate them in a VdG [7], but this did not gain general use. Later the coating of particles with conducting metals was used [8], but again has not found general use. More recently a new material has been used in dust accelerators, namely conducting polymers. The first report [9] accelerated pure particles of the polymer polypyrrole to 5 km s"1, although no data for ionization resulting from such impacts was given. The technique was then developed further to use the conducting polymer as a thin coating on the outside of a core particle. This allowed the acceleration of materials which are non-conducting. This is reported in [10], where use of three different conducting polymers as coatings was demonstrated, polypyrrole (PPY), polyanilane (PANi) and poly(3,4-ethylenedioxythiphene) (PEDOT). The core particle in each case was polystyrene latex which could be chosen in the size range 0.1 to 5 microns diameter.

This represents a significant advance in laboratory impact ionization experiments. It is relatively straightforward to synthesize the materials involved and carry out the coating. The conducting polymers are low density, as is the latex, with typical coated particle densities of 1080 to 1160 kg m"3. Further, the size range of latex used allowed acceleration to different velocities, up to 20 km s"1 for the coated particles. In addition, the materials used are organic, allowing a controlled experiment where the resulting ionization signals and time of flight (mass) spectra can be studied. For the latter this is important, as the thermal decomposition of latex has been well studied in the past. Thus the effects of hypervelocity impact ionic plasma production can be compared to the properties of a well understood organic material.

In this present work we report on three further developments in this field, namely the acceleration of pure PPY particles to a velocity of 35 km s"1, the use of PPY coated latex where the core latex particle has been doped with bromine, and the acceleration of nano-composite particles where 20 nm grains of silica were mixed with PPY to form mixed particle.

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