Dodson C.A., Jorns B.A., Wirz R.E., “Measurements of ion velocity and wave propagation in a hollow cathode plume,” Plasma Source Science and Technology, 28, 065009, 2019, https://iopscience.iop.org/article/10.1088/1361-6595/ab1c48

The mechanism responsible for the production of energetic ions in the plume of hollow cathodes for electric propulsion is still an open issue. These ions are of concern to cathode and thruster lifetime, particularly for cathodes operating at high (>20 A) discharge currents. Recent theoretical and experimental investigations suggest that there is a correlation between ion energy gain and ion acoustic turbulence. In this paper we present measurements of the evolution of the ion velocity distribution function in the near plume of a 100 A-class hollow cathode, operated in a regime in which the dominant mode is ion acoustic turbulence. Ion flow and thermal properties were related to measurements of the background plasma, fluctuation spectra, and dispersion relations obtained from an array of Langmuir probes. We found ions to flow outward from the cathode and accelerate downstream, to supersonic speeds, approximately aligned with the acoustic wave group velocity vectors. The directions of the ion flow and wave propagation were similar for a range of discharge currents and mass flow rates in the jet region of the plume. One operating condition showed a significant temperature increase, also in the direction of acoustic wave propagation, corresponding to the highest wave energy condition. These results are interpreted in the context of ion acoustic turbulence as a contributing mechanism for ion energy gain.

Dodson C.A., Perez-Grande D., Jorns B.A., Goebel D.M., Wirz R.E., "Ion Heating Measurements on the Centerline of a High-Current Hollow Cathode Plume", Journal of Propulsion and Power, 2018, https://doi.org/10.2514/1.B36788

An experimental investigation into the correlation between ion acoustic turbulence (IAT) and anomalous ion heating in the plume of a 100 A-class LaB6 hollow cathode is presented. Laser-induced fluorescence is employed to measure the ion velocity distribution function, and a translating ion saturation probe is used to quantify the spatial dependence of the IAT wave energy. It is found that over a range of flow rates and operating currents both the ion temperature and IAT energy increase downstream of the cathode in qualitatively similar ways. Both parameters also are shown to be impacted by operating conditions: the IAT energy and ion temperature decrease at higher flow rates and lower discharge currents. It is shown that the ratio between ion temperature and wave energy is related by a scaling parameter that depends on the background plasma parameters, and this relation is examined in the context of previous analytical work on IAT-induced ion heating.

Jorns B.A., Dodson C., Goebel D.M., Wirz R.E., "Propagation of Ion Acoustic Wave Energy in the Plume of a High-Current LaB6 Hollow Cathode", Physical Review E, Vol. 96, 023208, Aug. 2017, https://doi.org/10.1103/PhysRevE.96.023208

A frequency-averaged quasilinear model is derived and experimentally validated for the evolution of ion acoustic turbulence (IAT) along the centerline of a 100-A class, LaB6 hollow cathode. Probe-based diagnostics and a laser induced fluorescence system are employed to measure the properties of both the turbulence and the background plasma parameters as they vary spatially in the cathode plume. It is shown that for the three discharge currents investigated, 100 A, 130 A, and 160 A, the spatial growth of the total energy density of the IAT in the near field of the cathode plume is exponential and agrees quantitatively with the predicted growth rates from the quasilinear formulation. However, in the downstream region of the cathode plume, the growth of IAT energy saturates at a level that is commensurate with the Sagdeev limit. The experimental validation of the quasilinear model for IAT growth and its limitations are discussed in the context of numerical efforts to describe self-consistently the plasma processes in the hollow cathode plume.