Langmuir Probe Studies of Magnetic Confinement in an Ion Thruster Discharge Plasma

Abstract

Magnetic confinement studies on a 30-cm-diameter ion thruster were performed to determine the dependence of plasma confinement and uniformity on the ring-cusp magnetic field. Four primary cases were investigated to determine the effects of an additional magnetic cusp, increasing the strength of the highest value closed magnetic contour line, and varying the magnetic field free volume. A laboratory model NASA Solar Electric Propulsion Technology Application Readiness engine was modified to investigate three-and four-ring-cusp geometries. Electrical parameters and langmuir probe sweeps in the discharge chamber were used to measure the performance of each configuration. Increasing the strength of the closed magnetic contour line reduces ion loss to the anode, resulting in a reduction in discharge power for a given beam current. Similarly, increasing the magnetic field free volume in the near-grid region improves plasma uniformity, removing the on-axis current density peak responsible for accelerator grid erosion. The enhanced magnetic circuit geometries investigated resulted in a 20% reduction in discharge loss and discharge current at the TH15 (2.3 kW) throttle point, with similar gains over the full throttle range. The reduction in discharge power and peak current density can significantly increase the total throughout per engine by limiting wear mechanisms that limit thruster life. NomenclatureA p = probe area e = electron charge I sati;e = saturation current (ion, electron) J B = beam current J D = discharge current j i = ion current density k = Boltzmann constant m e = electron mass n e = electron density T e = electron temperature V B = beam voltage V D = discharge voltage V f = floating potential V p = plasma potential " B = discharge loss tot = total efficiency

Electron temperature measurement in Maxwellian non-isothermal beam plasma of an ion thruster

“…[15][16][17] The probe I-V characteristic is created by plotting the measured current versus the applied voltage, from which the floating potential V f is determined by finding out the voltage where the net current is zero, and the plasma potential V s is determined by finding out the voltage where the derivative of the I-V characteristic is maximum. The transition region of the I-V characteristic should be a straight line when the probe current is plotted semi-logarithmically versus the probe voltage V p under the assumption that the electrons being measured follow the Maxwellian distribution.…”

Modeling Grid Erosion in the NEXT Ion Thruster Using the CEX2D and CEX3D Codes

“…Nominal voltage from throttle table [33] Nominal voltage from throttle table [38] Nominal voltage from throttle table [39] Nominal voltage from throttle table [12] Nominal voltage from throttle table [12] Table A1 Sources of data used to define CEX2D and CEX3D code input parameters (continued) 5 eV for all cases (consistent with Langmuir probe measurements on LM thruster) [40] 5 eV for all cases (consistent with Langmuir probe measurements on LM thruster) [41] 5 eV for all cases (consistent with Langmuir probe measurements on LM thruster) [41] 5 eV for all cases (consistent with Langmuir probe measurements on LM thruster) [41]…”

“…The upstream plasma potential was assumed to be equal to the sum of the beam voltage and the discharge potential. Emissive probe measurements in NSTAR and NEXT thrusters showed that the plasma potential near the grids was within ±1 V of the discharge voltage [40,41]. The upstream electron temperature was assumed to be 5 eV for all simulations.…”

“…The upstream electron temperature was assumed to be 5 eV for all simulations. Langmuir probe measurements in an NSTAR LM thruster yielded temperatures of 3 -5 eV [40] and 4 -6 eV in a NEXT LM thruster [41]. Sensitivity tests showed that the code results were not particularly sensitive to the upstream temperature.…”