Coupled plasma transport and electromagnetic wave simulation of an ECR thruster

Abstract

An electron-cyclotron resonance thruster (ECRT) prototype is simulated numerically, using two coupled models: a hybrid particle-in-cell/fluid model for the integration of the plasma transport and a frequency-domain full-wave finite-element model for the computation of the fast electromagnetic (EM) fields. The quasi-stationary plasma response, fast EM fields, power deposition, particle and energy fluxes to the walls, and thruster performance figures at the nominal operating point are discussed, showing good agreement with the available experimental data. The ECRT plasma discharge contains multiple EM field propagation/evanescence regimes that depend on the plasma density and applied magnetic field that determine the flow and absorption of power in the device. The power absorption is found to be mainly driven by radial fast electric fields at the electron-cyclotron resonance region, and specifically close to the inner rod. Large cross-field electron temperature gradients are observed, with maxima close to the inner rod. This, in turn, results in large localized particle and energy fluxes to this component.

Using electron fluid models to analyze plasma thruster discharges

“…Certain HET discharge models [13,27,34,37] apply the so-called Morozov's collisionless limit, µ e → ∞ , to Eqs. ( 30) and (35). This yields ∇ � = 0 and ∇ � T e = 0 , that is, magnetic lines are isothermal and the Boltzmann relation, n e ∝ exp(−eφ/T e ) , applies along them.…”

“…A certain consensus on α a (z) suggests it is smaller within the channel than in the near plume, and would present a minimum around the maximum of E z (z) [31][32][33][34]. In EPTs with a quasi-axial magnetic field, α a = const is being used [35], partly due to the limited experimental evidence on anomalous diffusion in these devices. In HETs with 2D magnetically-shielded (MS) topologies, the choice of α a (r) seems more delicate; however, axial functions α a (z) are used too, supported on the idea that anomalous transport is larger in the near plume than inside the MS chamber.…”

Simulations of ion heating due to ion-acoustic instabilities in the presheath

“…Observation of the instabilities may also be obscured by the fact that the thermal noise level is enhanced in PIC simulations by the particle weight 33,34 , as seen in Eq. (7). For example, if the instabilities saturate because of ion trapping, we can estimate the saturation energy density as θ Sat ≈ T e n e /36 36 .…”

“…This finding is particularly influential in the context of applications that use a plasma and sheath to create a high emittance ion source, such as in plasma etching of semiconductors or ion beam generation [2][3][4][5] . It may also be applicable to some electric propulsion systems, where ions are accelerated to higher velocities to generate thrust, usually in the presence of hot electrons 6,7 , and to energy transport controlled by ion-acoustic instabilities in other contexts. [8][9][10][11][12][13][14] The common expectation is that the ion temperature at the sheath edge is near room temperature at low neutral gas pressures, but can increase at higher pressures due to inelastic collisions (such as ionization and charge exchange) in the presheath 15,16 .…”

Fully kinetic model of plasma expansion in a magnetic nozzle

“…Both fluid and kinetic continuum approaches must further make assumptions regarding the VDF, one of the main impact parameters in magnetised plasma expansion [24]. Hence, numerical studies need to be extended to fully kinetic [26][27][28][29] or fluid-kinetic [30] approaches if the dynamics of a MN want to be treated self-consistently. The fully kinetic Particle-in-Cell (PIC) method represents the numerical strategy with the lowest level of assumptions.…”