Artem Smirnov

Parametric investigation of miniaturized cylindrical and annular Hall thrusters

Conventional annular Hall thrusters become inefficient when scaled to low power. An alternative approach, a 2.6 cm miniaturized cylindrical Hall thruster with a cusp-type magnetic field distribution, was developed and studied. Its performance was compared to that of a conventional annular thruster of the same dimensions. The cylindrical thruster exhibits discharge characteristics similar to those of the annular thruster, but it has a much higher propellant ionization efficiency. Significantly, a large fraction of multicharged xenon ions might be present in the outgoing ion flux generated by the cylindrical thruster. The operation of the cylindrical thruster is quieter than that of the annular thruster. The characteristic peak in the discharge current fluctuation spectrum at 50–60 kHz appears to be due to ionization instabilities. In the power range 50–300 W, the cylindrical and annular thrusters have comparable efficiencies (15%–32%) and thrusts (2.5–12 mN). For the annular configuration, a voltage less t...

Space charge saturated sheath regime and electron temperature saturation in Hall thrusters

Existing electron-wall interaction models predict that secondary electron emission in Hall thrusters is significant and that the near-wall sheaths are space charge saturated. The experimental electron-wall collision frequency is computed using plasma parameters measured in a laboratory Hall thruster. In spite of qualitative similarities between the measured and predicted dependencies of the maximum electron temperature on the discharge voltage, the deduced electron-wall collision frequency for high discharge voltages is much lower than the theoretical value obtained for space charge saturated sheath regime, but larger than the wall recombination frequency. The observed electron temperature saturation appears to be directly associated with a decrease of the Joule heating rather than with the enhancement of the electron energy loss at the walls due to a strong secondary electron emission. Another interesting experimental result is related to the near-field plasma plume, where electron energy balance appears to be independent on the magnetic field.

Electron cross-field transport in a low power cylindrical Hall thruster

Conventional annular Hall thrusters become inefficient when scaled to low power. Cylindrical Hall thrusters, which have lower surface-to-volume ratio, are therefore more promising for scaling down. They presently exhibit performance comparable with conventional annular Hall thrusters. Electron cross-field transport in a 2.6 cm miniaturized cylindrical Hall thruster (100 W power level) has been studied through the analysis of experimental data and Monte Carlo simulations of electron dynamics in the thruster channel. The numerical model takes into account elastic and inelastic electron collisions with atoms, electron-wall collisions, including secondary electron emission, and Bohm diffusion. It is shown that in order to explain the observed discharge current, the electron anomalous collision frequency νB has to be on the order of the Bohm value, νB≈ωc/16. The contribution of electron-wall collisions to cross-field transport is found to be insignificant.

Plasma measurements in a 100 W cylindrical Hall thruster

Conventional annular Hall thrusters become inefficient when scaled to low power. Their lifetime decreases significantly due to the channel wall erosion. Cylindrical Hall thrusters, which have lower surface-to-volume ratio and, thus, seem to be more promising for scaling down, exhibit performance comparable with conventional annular Hall thrusters of the similar size. Plasma potential, ion density, and electron temperature profiles were measured inside the 2.6 cm cylindrical Hall thruster with the use of stationary and slow movable emissive and biased Langmuir probes. Potential drop in the 2.6 cm cylindrical Hall thruster is localized mainly in the cylindrical part of the channel and in the plume, which suggests that the thruster should suffer lower erosion of the channel walls due to fast ion bombardment. Plasma density has a maximum of about (2.6–3.8)×1012 cm−3 at the thruster axis. At the discharge voltage of 300 V, the maximum electron temperature is about 21 eV, which is not enough to produce multiple...

Enhanced ionization in the cylindrical Hall thruster

Conventional annular Hall thrusters do not scale efficiently to low power. An alternative approach, a cylindrical Hall thruster with a cusp-type magnetic field distribution, has been investigated. A relatively large 9-cm-diam version of a cylindrical thruster, operated in 300–1000 W power range, and the 2.6 cm miniaturized cylindrical Hall thruster, operated in the power range 50–300 W, exhibited performance comparable with conventional annular Hall thrusters of the similar size. The cylindrical thrusters have unusually high propellant utilization, compared to conventional Hall thrusters. Numerical simulations, performed within the framework of a quasi-one-dimensional stationary thruster model, show that the increase in the propellant utilization does not appear to be quantitatively explained by a reduction of plasma wall losses. A more complete theoretical model, likely including kinetic effects, will be necessary to explain the observed propellant utilization effect.

A new high performance field reversed configuration operating regime in the C-2 devicea)

Large field reversed configurations (FRCs) are produced in the C-2 device by combining dynamic formation and merging processes. The good confinement of these FRCs must be further improved to achieve sustainment with neutral beam (NB) injection and pellet fuelling. A plasma gun is installed at one end of the C-2 device to attempt electric field control of the FRC edge layer. The gun inward radial electric field counters the usual FRC spin-up and mitigates the n = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The combined effects of the plasma gun and of neutral beam injection lead to the high performance FRC operating regime, with FRC lifetimes up to 3 ms and with FRC confinement times improved by factors 2 to 4.

Performance of a low-power cylindrical hall thruster

Recent mission studies have shown that a Hall thruster which operates at relatively constant thrust efficiency (45-55%) over a broad power range (300W - 3kW) is enabling for deep space science missions when compared with slate-of-the-art ion thrusters. While conventional (annular) Hall thrusters can operate at high thrust efficiency at kW power levels, it is difficult to construct one that operates over a broad power envelope down to 0 (100 W) while maintaining relatively high efficiency. In this note we report the measured performance (I(sub sp), thrust and efficiency) of a cylindrical Hall thruster operating at 0 (100 W) input power.

Cylindrical Hall Thrusters

The cylindrical Hall thruster, proposed and studied at the PPPL features high ionization efficiency, quiet operation, ion acceleration in a large volume-to-surface ratio channel, and performance comparable with the state-of-the-art Hall thrusters. These characteristics were demonstrated in low and medium power ranges. For a miniaturized 100 W cylindrical thruster, we achieved performance improvements, including a 30-40% plume narrowing, reliable discharge initiation, and stable operation in the discharge voltage range of 50-600 V. -

Controlling the Plasma Flow in the Miniaturized Cylindrical Hall Thruster

A substantial narrowing of the plume of the cylindrical Hall thruster (CHT) was observed upon the enhancement of the electron emission from the hollow-cathode discharge, which implies the possibility for the thruster efficiency increase due to the ion-beam focusing. It is demonstrated that the miniaturized CHT can be operated in the non-self-sustained regime, with the discharge current being controlled by the cathode electron emission. The thruster operation in this mode greatly expands the range of the plasma and discharge parameters normally accessible for the CHT. The observed variation of the plasma potential, electron temperature, and plasma density with the cathode current in the non-self-sustained regime points to the fact that the cathode discharge can affect the electron cross-field transport in the CHT plasma.

Maximizing ion current by space-charge neutralization using negative ions and dust particles

Ion current extracted from an ion source (ion thruster) can be increased above the Child–Langmuir limit if the ion space charge is neutralized. Similarly, the limiting kinetic energy density of the plasma flow in a Hall thruster might be exceeded if additional mechanisms of space-charge neutralization are introduced. Space-charge neutralization with high-mass negative ions or negatively charged dust particles seems, in principle, promising for the development of a high current or high energy density source of positive light ions. Several space-charge neutralization schemes that employ heavy negatively charged particles are considered. It is shown that the proposed neutralization schemes can lead, at best, only to a moderate but nonetheless possibly important increase of the ion current in the ion thruster and the thrust density in the Hall thruster.