A slug-mode operated coaxial plasma gun as a source of low energy neutral particles



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The ion and neutral particles fluxes derived from a slug-mode operated coaxial plasma gun were studied. Using a time-of-flight technique the significant ion parameters were determined: the ion temperature, the plasma flow velocity, the ion flux normalization constant, and the time of the release of the plasma from the gun relative to the discharge initiation. Ion fluxes from discharges in hydrogen, helium, neon, and argon Were studied. Using the measured ion parameters, it is shown that the operation of the plasma gun can be described with a slug mode acceleration model, slug acceleration being defined as that mode in which only Joule heating contributes significantly to the thermal energy of the plasma. This verification is accomplished by showing that a linear relationship exists between the ion temperature, T, and the modified flow velocity, V/, as required by the slug acceleration model ( is a geometric correction factor arising from the azimuthal asymmetry of the accelerating plasma). This relationship also leads to values for the average plasma resistance which compare favorable with the values obtained using the signals from a discharge current probe mounted in the gun (internal probe). This favorable comparison is taken as another indication of slug-mode operation. The range of flow energies measured are 20-250 ev for hydrogen, 300-1400 ev for helium, 1200-7000 eV for neon and 3500-10,000 ev for argon. For hydrogen the energy dispersion, i.e., the thermal energy,was found In all cases to be less than 10% of the flow energy and In 65% of the discharges the dispersion was less than 1%. For helium, the maximum observed energy dispersion was 35% off the flow energy. However, 70% of the helium discharges had thermal energies of less than 10% of the flow energy. All off the neon and argon plasmas had energy dispersions off less than 10% off the flow energy. These small energy dispersions constitute a further Indication that the gun does operate In the slug mode. The charged components of the hydrogen plasmas were removed using a grounded grid device, and the remaining neutral hydrogen flux was observed with av. BeCu secondary electron emission detector. It is shown that the values of the ion temperature and flow velocity obtained from the ion analysis can also be applied to the neutral plasma component. Using measured and extrapolated values for the secondary electron emission coefficients, the number of neutrals N[lowered o], in the hydrogen plasmas was observed. For energies < 50 ev the measured number of neutrals and the number predicted using the Saha equation disagree too much to be reconciled. It is concluded that the secondary electron emission coefficients are much smaller than indicated by the extrapolation. Finally, it was found that for the hydrogen discharges one may expect [approximately equal] 10[raised 10] neutral particles for each analyzable plasma.