George Tynan

Experimentally Determined Neutral Density and Plasma Parameters in a 30cm Ion Engine

range of 1 to 7.9 eV and an electron density range of 4e10 to 1e13 cm-3, throughout the discharge chamber, consistent with the results in the literature [1-2]. Plasma potential estimates, computed from the first derivative of the probe characteristic, indicate potential from 0.5V to 11V above the discharge voltage along the thruster centerline. These values are believed to be excessively high due to poor saturation of the probe IV characteristic in the low-density regions of the plasma. Relative neutral density profiles are also obtained with a fiber optic probe sampling photon flux from the 823.1 nm excited to ground state transition. Plasma parameter measurements and neutral density profiles will be presented as a function of probe location and engine discharge conditions. A discussion of the measured electron energy distribution function will also be presented, with regards to variation from pure maxwellian. It has been found that there is a distinct primary population along the thruster centerline, which causes estimates of electron temperature, electron density, and plasma potential, to err on the high side, due this energetic population. Computation of the energy distribution function of the plasma clearly indicates the presence of primaries, whose presence become less obvious with radial distance from the main discharge plume.

Erosion and redeposition of graphite by hydrogen plasmas

The results of erosion and redeposition studies of graphite by hydrogen plasma bombardment in the PISCES facility are reviewed. The total erosion yields of several types of graphites have been measured during plasma exposure with ion fluxes of up to 2 x 10/sup 18/ cm/sup -2/ . S/sup -1/, ion energies of 50 to 200 eV, and sample temperatures of 50 to 950/sup 0/C. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, and pyrolytic graphites, and a four-directional carbon-carbon (C-C) composite weave. The erosion rates of all the graphites tested are about equal, suggesting that surface damage by the ion bombardment results in similar erosion yields. The C-C composite weave material showed an increased weight loss during initial exposure, and then equal or lower erosion yields compared to the other graphites.

Molybdenum and Carbon Cluster Angular Sputtering Distributions Under Low Energy Xenon Ion Bombardment

Molybdenum and carbon cluster (C 2 and C 3) angular sputtering distributions are measured during xenon ion bombardment from a plasma, with incident ion energy EXe ranging between 50 and 225 eV. A quadrupole mass spectrometer (QMS) is used to detect the fraction of sputtered neutrals that is ionized in the plasma, and to obtain the angular distribution by changing the angle between the target and the QMS aperture. The angular sputteri ng distribution for molybdenum presents a maximum at 60°, and this maximum becomes less pronounced as the incident ion energy increases. The dependence of the total sputtering yield on incident ion energy is in good agreement with previous experiments. The re is a large increase of about two orders of magnitude in the sputtering yield from EXe = 50 to 125 eV, and a more moderate increase for higher energies. Sputtered C 2 and C 3 clusters exhibit a similar angular sputtering distribution with a maximum at appr oximately 45 -60°; however, this maximum becomes more pronounced for higher incident energies, in contrast to the molybdenum case. The angular distribution of the sputtered clusters depends on the energy with which they are ejected. The low energy populatio n of sputtered particles has a broad maximum at 45°, while the high energy population has a sharp maximum at 60°. The cluster sputtering yield monotonically increases by less than one order of magnitude from EXe = 50 to 225 eV for all measured sputtering a ngles except for normal sputtering, which has a maximum yield at EXe� 100 eV.

Lower hybrid wave coupling in PBX-M

The coupling of the waves launched from a 4.6 GHz lower hybrid (LH) system into PBX-M plasmas has been studied for both L mode and H mode plasmas. The characteristics of the plasma in front of the LH coupler have been measured with a fast Langmuir probe. The reflected power of the coupler has been measured across the transition to the H mode as a function of phase and the distance between the coupler and the separatrix. A transient rise in the LH reflection coefficient was observed near the L-H transition under some conditions. Coupling depends primarily on the electron density in the vicinity of the coupler, and proper positioning of the coupler can compensate for changes in the plasma edge due to H mode transitions. Good coupling can be maintained throughout the H mode