Bulletin of the American Physical Society
75th Annual Gaseous Electronics Conference
Volume 67, Number 9
Monday–Friday, October 3–7, 2022;
Sendai International Center, Sendai, Japan
The session times in this program are intended for Japan Standard Time zone in Tokyo, Japan (GMT+9)
Session DF3: Plasma Propulsion III |
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Chair: Justin Little, University of Washington Room: Sendai International Center Tachibana |
Friday, October 7, 2022 1:30PM - 1:45PM |
DF3.00001: Electrostatic instabilities in ExB discharges: comparison of the linear theory dispersion relation with the reconstructed power spectrum Federico Petronio, Alejandro Alvarez Laguna, Anne Bourdon, Pascal Chabert In the last decades, the ExB community has shown a great interest in plasma instabilities, from theoretical, numerical and experimental point of view. Although it has been discussed for quite a long time how these instabilities determine the anomalous electron mobility coefficient, and consequently the efficiency of an Hall Thruster (HT), some work still needs to be accomplished to fully understand the origin of instabilities and their consequences. In our work, we show that it is possible to retrieve the dispersion relations (DRs) of the most known ExB instabilities (i.e. IAW, MTSI and ITTI) from the same 3D fluid electrostatic DR. This procedure allows us to consider not only the purely 1D DRs, that are commonly studied, but also the instabilities with a minor component developing along a secondary axis. The comparison of the analytic DRs with the results of PIC simulations, in conditions similar to the ones of a HT, gives important insights about the origin and the development of these instabilities. The Two-Points Power Spectral Density reconstruction method allows to investigate the evolution of the spectrum along the thruster axis, in particular, it permits to make and challenge some hypotheses about the instabilities wavefronts bending and about the temporal evolution of their amplitude. A complete discussion about the instabilities development and a comparison with the analytic DRs are carried out to unveil the bi-dimensional nature of these instabilities. |
Friday, October 7, 2022 1:45PM - 2:00PM |
DF3.00002: Observation of Instability driven propagating localized patterns in E×B discharges in 2D-axial azimuthal PIC-MCC simulations Bhaskar Chaudhury, Teja V Reddy, Durgesh Mishra, Miral Shah, Mainak Bandyopadhyay In low temperature E x B plasma discharges, Electron Cyclotron Drift Instability (ECDI) is a possible candidate to explain the anomalous electron transport across the magnetic field. Recently, a 2D axial azimuthal Particle-In-Cell benchmark study has been carried out using seven independently developed PIC codes with same specified geometry and simulation conditions (Charoy et al 2019 PSST. 28). In this work, we validate our independently developed parallel 2D PIC code with the benchmark case using the same geometry by applying a potential between anode and cathode, and a magnetic field perpendicular to the simulation domain. The simulations have been carried out with unscaled plasma density, real collisions are not considered, and the ionization profile is given as an input parameter as mentioned in the benchmark case. The constant ionization rate, calculated from maximum ion density, is provided at each time step. The state-of-the-art supercomputing facility has been used to accelerate the simulations involving hundreds of particles per cell while satisfying the stringent numerical PIC conditions. The FFT has shown two kinds of instabilities. The instabilities observed near the magnetic field region evolves with time and moves in the E x B direction, possibly ECDI. We also observe localized high-density patterns, believed to be induced by instabilities, moving across the magnetic field downstream along the axial direction between anode and cathode with a slightly higher velocity than ion acoustic velocity. The generation of localized high-density patterns is observed near the edge of the given ionization profile. |
Friday, October 7, 2022 2:00PM - 2:30PM |
DF3.00003: Plasma flow and acceleration in the magnetic nozzle Invited Speaker: Andrei Smolyakov Plasma flow and acceleration in the magnetic nozzle with converging-diverging configuration are important for applications in electric propulsion as well as in fusion systems such as open mirrors. We report on some novel features of plasma acceleration in the magnetic nozzle that have been revealed in recent analytical and computational studies. We show that the non-monotonic magnetic field with a local maximum of the magnetic field is a necessary condition for the formation of the quasineutral accelerating potential structure resulting in a unique velocity profile fully determined by the magnetic field. The explicit form of the solution can be obtained in the form of the Lambert function. The fluid model has been further extended to include the effects of warm ions with anisotropic ion pressure. It is shown that the perpendicular ion pressure enhances plasma acceleration due to the mirror force. The fluid model predicts that the accelerating solution has a unique value v0 at the entrance to the magnetic nozzle. Subsonic and supersonic solutions are possible below and above some critical values, v0 < va and v0>vb, respectively. The solutions in the gap region va < v0 < vb are multivalued and therefore do not exist in fluid theory. Kinetic effects are investigated using the quasineutral hybrid model with kinetic ions and isothermal Boltzmann electrons. It is shown that for cold ions the velocity profile agrees well with the analytical theory. A new mechanism of the instabilities is revealed due to the wave breaking, particle trapping, and reflections that occur for particles in the gap region va < v0 < vb . Stationary turbulent fluctuations reflect a fraction of ions. Nevertheless, the velocity profile for passing particles generally follows the accelerating velocity profile which emerges as a constraint on the plasma velocity in the magnetic nozzle. The role of this constraint in the matching of the magnetic nozzle with the plasma source is discussed. |
Friday, October 7, 2022 2:30PM - 2:45PM |
DF3.00004: Assessment of cross-field electron transport in a magnetic nozzle Kazunori Takahashi, Christine Charles, Roderick W Boswell A magnetic nozzle is a key element for a high-power electric propulsion device called a helicon plasma thruster. Since the magnetic field lines are closed and turn back to the thruster, the plasma has to be detached from the field lines. The plasma detachment has been a challenging problem, especially when electrons have a gyro-radius smaller than the system's scale. Here we experimentally demonstrate that a cross-field transport of the electrons toward the main nozzle axis is induced by the spontaneously excited wave having the frequency considerably higher than the ion cyclotron frequency and driving an ExB drift that only effects the electrons. The density and electric field fluctuations are simultaneously measured, and the direction and the magnitude of the cross-field electron flux are assessed and compared with the cross-field ion flux induced by the electrostatic field. Wave-induced transport and loss have been one of many important issues in plasma physics over the past several decades. Conversely, the presently observed electron inward transport has a beneficial effect on the detachment by neutralizing the ions deviating from the field lines. |
Friday, October 7, 2022 2:45PM - 3:00PM |
DF3.00005: Numerical investigation on plasma expansion and particle energy in a magnetic nozzle Kazuma Emoto, Kazunori Takahashi, Yoshinori Takao A magnetic nozzle is applied to plasma propulsion and has been studied worldwide. In the magnetic nozzle, the plasma expands along the magnetic field lines, and the plasma density decreases in the downstream direction. However, the polytropic process in the magnetic nozzle has not been fully understood yet, and its physics has been discussed in many experiments and simulations. In this study, the plasma expanding through the magnetic nozzle is numerically analyzed using two-dimensional fully kinetic simulations, where particle-in-cell and Monte Carlo collision techniques are employed. Ion and electron motions are kinetically simulated with the electromagnetic field, and the plasma expansion process is investigated depending on the particle energy. Preliminary numerical results show that low- and high-energy electrons indicate the isothermal and adiabatic expansions, respectively, implying that the particle energy has an important role in the magnetic nozzle plasma expansion. In the conference, the plasma expansion and the polytropic process in the magnetic nozzle will be discussed with plasma profiles in detail. |
Friday, October 7, 2022 3:00PM - 3:15PM |
DF3.00006: Identification of plasma fluctuations and energy flow in hall thruster Kouki Teshima, Naoji Yamamoto, Daisuke Kuwabara In order to increase the thrust to power ratio in the Hall thruster,decrease in power consumption is a good strategy, and for this,suppression of electron transport toward the anode is crucial. Electron transport can be suppressed by increasing the magnetic field strength in the acceleration channel up to critical magnetic field strength. A sudden increase in electron transport across the magnetic field beyond critical value, however, can shift the system from the classical electron diffusion regime to anomalous electron diffusion. A wide variety of theories has been developed to describe anomalous electron transport in Hall thrusters, but the problem remains unresolved. As a first step to understanding anomalous electron transport, the nature of instabilities observed in Hall thrusters and the relationship between anomalous transport and these oscillations should be investigated. Therefore, we observed plasma instabilities inside the acceleration channel of a Hall thruster using a 76 GHz microwave interferometer, which is a non-intrusive measurement technique and it offers wide bandwidth up to a few MHz. The electron number densities in four different azimuthal position were measured. The phase difference is almost constant, about zero in the 10 kHz range oscillation. However, the 100 kHz oscillation, on the other hand, the phase difference in the 100 kHz range is not constant, this result shows that the 100 kHz range oscillation is not propagatinin the azimuthal direction. |
Friday, October 7, 2022 3:15PM - 3:30PM |
DF3.00007: Characterization of a 2 MHz magnetically expanding RF plasma source for thruster development Thanatith Nakul, Kazunori Takahashi A 2 MHz band RF plasma source with a 14 cm diameter is developed to improve the performance of the helicon plasma thruster (HPT). The previous study has demonstrated that the performance of the helicon plasma thruster can be improved in terms of the thruster efficiency, by enlarging the source tube diameter. Here the source tube diameter is enlarged up to 14 cm, while the 9.5 cm diameter tube has been used previously. When a high-density plasma is produced inside the source, it is difficult for RF field to penetrate into the central plasma due to a skin effect. Therefore, a 2 MHz RF generator is chosen as the power supply. In the present study, the plasma density, the RF power transfer efficiency, and the ion energy distribution function, are measured. The results demonstrate that the high-density plasma above 1012 cm-3 is generated, and that the electrons are heated in the core, implying the penetration of the RF field into the plasma core. In the presently developed source, the RF power transfer efficiency above 90% is successfully obtained. Downstream of the open source exit, the supersonic ion beam is observed, implying that the RF frequency does not impact the ion acceleration process. |
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