Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment ͑SSX͒ ͓M. R. Brown, Phys. Plasmas 6, 1717 ͑1999͔͒ are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT ͓J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications ͑2004͔͒ to yield the electron temperature in the plasma with 1 s time resolution. Average T e is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer ͑IDS͒ ͓C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 ͑2006͔͒, likewise rises during spheromak merging, peaking at ϳ22 eV, but a similar increase in T i is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, which are dominated by carbon, but include some oxygen and nitrogen. A burst of soft x-ray emission is seen during reconnection with a new four-channel detector ͑SXR͒. There is evidence for spectral changes in the soft x-ray emission as reconnection progresses, although our single-temperature equilibrium spectral models are not able to provide adequate fits to all the SXR data.

We present laser-induced fluorescence measurements of acceleration zone scaling with discharge voltage (Vd), magnetic field strength (B), and facility background pressure (PBG) in NASA’s 12.5 kW Hall Effect Rocket with Magnetic Shielding. At fixed discharge current, the plasma potential profiles at discharge voltages from 300 to 600 V approximately overlapped in the region with plasma potential less than 300 V; ion acceleration began further upstream at higher Vd because the region with a steep potential gradient was broader. The radial divergence of mean ion velocity vectors in the outer half of the channel and near plume increased with decreasing Vd. At fixed Vd, the acceleration zone was located further upstream at higher B and at higher PBG. Bimodal ion velocity distribution functions (IVDFs) were measured along the channel centerline in the acceleration zone at high discharge voltages; this effect was attributed to time-averaging over movement of the acceleration zone during large-amplitude discharge current oscillations. At lower discharge voltages, the broadening of the IVDFs in the near plume could not be fully explained by ionization within the acceleration region. These results have implications for understanding front pole erosion, which can be an important wear mechanism over the long lifetimes of magnetically shielded thrusters, and they provide baseline data for validating first principles models of cross-field electron transport.

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The results of 1-D azimuthal, and 2-D z-θ, particle-in-cell simulations of Hall thruster acceleration and near plume regions are reported. The 1-D simulations show that fast moving electron waves drive coherent, large amplitude, ion acoustic waves. The ion waves become azimuthally coherent too quickly for ion sound to be the cause; the coherence comes from early electron waves. 2-D simulations with only fast moving, main beam, ions result in ion acoustic wave amplitudes much lower than those in the 1-D calculations. The addition of a small amount of ionization in the domain, at a rate consistent with Hall thruster parameters, causes the potential profile to divide into two distinct regions. In the upstream portion of the domain, the potential profile is very steep; in the downstream portion, the potential profile is almost flat. Electron transport in the two regions is discussed.