Hollow cathode modeling: II. Physical analysis and parametric study

Abstract

A numerical emissive hollow cathode model which couples plasma and thermal aspects of the NASA NSTAR cathode has been presented in a companion paper and simulation results obtained using the plasma model were compared to experimental data. We now compare simulation results with measurements using the full coupled model. Inside the cathode, the simulated plasma density profile agrees with the experimental data up to the ±50 % experimental uncertainty while the simulated emitter temperature differs from measurements by at most 5. We then proceed to an analysis of the cathode discharge both inside the cathode where electron emission is dominant and outside in the near plume where electron transport instabilities are important. As observed previously in the literature, the total emitted electron current is much larger (34) than the set discharge current collected at the anode (13) while ionization plays a negligible role. Extracted electrons are emitted from a region much shorter than the full emitter (0.9 vs. 2.5). The influence of an applied axial magnetic field in the plume is also assessed and we observe that it leads to a 10-fold increase of the plasma density 1 cm downstream of the orifice entrance while the simulated discharge potential at the anode is increased from 10 up to 35.5. Lastly, we perform a parametric study on both the operating point (discharge current, mass flow rate) and design (inner radius) of the cathode. The simulated useful operating envelope is shown to be limited at low discharge current mostly because of the probable ion sputtering of the emitter and at high discharge current because of emitter evaporation, plasma oscillations and sputtering of the keeper electrode. The behavior of the cathode is also analyzed w.r.t. its internal radius and simulation results show that the useful emitter length scales linearly with the cathode radius.

Propagation of ion acoustic wave energy in the plume of a high-current LaB6 hollow cathode

“…From a practical perspective, the experimental validation of a first-principles theory for IAT growth has direct implications for on-going efforts to develop predictive and self-consistent models of this region. [8,10] Indeed, two dominant effects in the cathode plume, enhanced electron resistivity and the onset of energetic ions, both have been shown to depend on the total energy density of the IAT. [7,15] In order to evaluate self-consistently the role of these processes in the plume, there is a need to understand not only how the IAT drives these mechanisms, but more fundamentally, how the IAT itself propagates in this region.…”

“…Assuming that this mode with maximum growth dominates the IAT spectrum, then an appropriate substitution may be ω o ∝ ω pi . This is the approach that Sary et al [9,10] have applied in their numerical models for the IAT. We note, however, that as Fig.…”

“…The need to reduce risk in the design and qualification of these systems for flight has driven the demand for increasingly high-fidelity predictive codes. But while the level of sophistication of models is improving, it has been recognized [8][9][10] that in * bjorns@umich.edu order to fully self-consistently model these devices, it is necessary to account for the propagation of the IAT in the plume.…”

“…Sary et al recently have explored a similar hypothesis in modeling a lowcurrent discharge. [9,10] Our purpose here is to revisit this quasilinear model and attempt to evaluate its validity against experimental measurements. With this in mind, we have organized this paper in the following way.…”

Correlation of ion acoustic turbulence with self-organization in a low-temperature plasma

“…Indeed, for lowtemperature plasmas (LTPs) in particular, self-organization has been recognized as a critical research frontier in developing a better understanding of this class of plasmas. 11 While several experiments and numerical studies have investigated this process in LTPs, [8][9][10]12 the mechanisms underlying self-organization in many cases remain unclear. An intriguing theory that has recently gained renewed interest is the possibility that these self-organized structures are the product of a unique, inverse energy transfer across lengthscales.…”