Investigation of Energetic Ions in a 100-A Hollow Cathode

Abstract

The role of ion acoustic turbulence in the formation of high-energy ion tails in the plume of a 100-A LaB6 hollow cathode is experimentally and theoretically examined. At fixed flow rate and varying discharge current, single-point measurements of fluctuation intensity in the cathode plume are taken and compared to ion energy measurements. It is shown that for high discharge current the formation of energetic ions is correlated with the amplitude of the ion acoustic turbulence. Two-dimensional maps of background plasma parameters and wave turbulence are made at the highest discharge current investigated, 140 A. A simple, one-dimensional quasilinear model for the interaction of the ion energy distribution with the ion acoustic turbulence is employed, and it is shown that the energy in the measured wave turbulence is sufficiently large to explain the formation of ion tails in the cathode plume. Mitigation techniques for minimizing the amplitude of the turbulence are discussed.

Ion Heating Measurements on the Centerline of a High-Current Hollow Cathode Plume

“…Mikellides et al [10] claimed that IAT could exist in the cathode plume, caused by a high electron Mach number and electron-to-ion temperature ratio. Experimental work by Jorns et al [11][12][13] confirmed the existence of IAT in the plume, showed that the ion energy and temperature trends measured in the far plume correlated with the wave energy measurements obtained in the near plume, and found the IAT wave energy to increase with discharge current and decreasing flow rate. Theoretical work by Jorns et al [9,14] showed qualitative agreement with a 1D kinetic model estimating ion heating on the cathode centerline, and found the measured wave energy to be sufficient to cause the formation of a hot ion tail.…”

Hollow Cathode Simulations with a First-Principles Model of Ion-Acoustic Anomalous Resistivity

“…3 in [23]), the logarithm of the ratio of the upper-bound to lower-bound cutoff frequencies is order unity. Similarly, typical ion velocities in cathode plumes have been measured to be 2-5 km∕s [24,37] with electron temperatures of 2-5 eV [23,[38][39][40]. We therefore have…”

“…For cathode plasmas that we have investigated in previous work [15,23,40], we have computed the ratio of the electron drift and thermal velocity to be at most of order unity, and because the square root of the mass ratios is ∼480 and the maximum value of the exponential contribution T e ∕T i 3∕2 exp−T e ∕2T i ≈ 1.16, we therefore can neglect the anomalous heating compared to the frictional heating by the anomalous collision frequency.…”

Hybrid-PIC Simulation on Plasma Flow of Hollow Cathode

“…One of the most serious issues on the lifetime of cathodes is sputtering of a keeper electrode by energized ions. 2,3,[18][19][20][21][22] Most probable origins of the high-energy ions is the ion acoustic turbulence (IAT), first predicted to exist and to generate anomalous resistivity and high energy ions in cathodes by OrCa2D simulations [23][24][25] which utilizes fluid model coupled with collisionless neutral model in the plume region. 26) The propagation of the IAT and its damping which causes the high-energy ions is related with the kinetics of heavy particles (neutrals and ions).…”

“…26) The propagation of the IAT and its damping which causes the high-energy ions is related with the kinetics of heavy particles (neutrals and ions). [20][21][22] Another candidate of the origins of the energized ions is a potential hill formed downstream of the cathode orifice as a result of extensive electron-impact ionization, which accelerates ions created in the hill spherically through the potential fall that develops around the hill crest. 2,3,13) In either case, kinetics of heavy particles should be pursued at least in order to capture the phenomenon which averts the ion distribution function from Maxwellian.…”