Session EM2: Workshop II: Plasma Physics for Space Propulsion Technologies

Anatomy of cross-field electron transport by steady and unsteady plasma structures in Hall thrusters

Self-organizing plasma structures and their effects on the cross-field electron transport in Hall thrusters have been investigated. It is hypothesized that 1) an azimuthal plasma oscillation is induced by a gradient-drift instability (GDI), and 2) the azimuthal plasma structure enhances the axial electron transport by an E_θ×B_r drift. In this study, a hybrid particle-fluid model was developed to analyze the evolution process and characteristics of the GDI in the Hall thruster discharge. Our simulation results showed complex vortex-like structures with frequencies of 100 kHz to 1 MHz in the electron flow field. The observed oscillation exhibited a consistency with the perturbation theory of GDI. Further, to experimentally investigate the relationship between azimuthal plasma structures and cross-field electron transport, we operated a Hall thruster with artificial disturbances, such as azimuthally nonuniform propellant supply and nonuniform magnetic field. Because equilibrium plasma structures were attained in azimuth during the operations with artificial inhomogeneities, detailed probe diagnostics were performend to obtain the azimuthal plasma properties. The measurement results clearly showed that the electron density and plasma potential were azimuthally out of phase, which further indicated that the azimuthal plasma structure enhanced the cross-field electron transport toward the anode direction.   

The effects of collisions and oscillating fields on the thrust in electric propulsion

The study of electric propulsion involves interesting basic plasma physics issues. The dependece of the efficiency on the particle energy distribution and implications to air-breathing electric propulsion [1] will be discussed. A major figure of merit in electric propulsion is the thrust per unit of deposited power, the ratio of thrust over power. Thrust over power is important for air-breathing propulsion where propellant saving is not the major issue. As we have demonstrated [2],  for a fixed deposited power in the ions, the momentum delivered by the electric force is larger if the accelerated ions collide with neutrals during the acceleration. Operation in the collisional regime can be advantageous for air-breathing propulsion, and its practicality should be examined. In a magnetic nozzle it is the magetic pressure whch provides the thrust [3]. The relation between the magnetic field configuration and the height of the potential hill which delivers energy to the accelerated ions will also be discussed. In electrodeless electric thrusters, oscillating electromagnetic fields are used to deliver energy to either electrons of ions while momentum is delivered by the magnetic pressure. Mechanism where electromagnetic waves themselves deliver momentum to ions are interesting [4]. We have recently cosidered such a mechanism that relys on standing waves [5]. Unfortunately, It is diffcult to achieve a considerable acceleration by this mechanism, but the mechanism could be of interest for other applications.

[1] F. Romano et. al., Acta Astronautica 16, 476 (2020).

[2] G. Makrinich and A. Fruchtman, Phys. Plasmas 20, 043509 (2013).

[3] K. Takahashi et. al., Phys. Rev. Let. 107, 231001 (2011).

[4] S. Shinohara et. al., Plasma Phys. and Controlled Fusion 61, 014017 (2018).

[5] A. Fruchtman and G. Makrinich, 36th Intern. Electric Propulsion Conf., IEPC-2019-514, Vienna (2019).

 

In-Space Electric Propulsion System Enabling JAXA Commercial Removal of Debris Demonstration (CRD2): Challenges and Relevant Physics

Space debris is being recognized as an important environmental issue. The increase of space debris population poses potential danger on human spacecrafts including International Space Station as well as communication and navigation satellites.

 

To mitigate debris threat, both approaches of Post Mission Disposal (PMD), to reduce the manmade debris increasing rate; and Active Debris Removal (ADR), to selectively remove highly dangerous debris; are widely recognized as necessary.

 

Japan Aerospace Exploration Agency (JAXA) is taking its leadership in establishing the technological feasibility of PMD and ADR. In accordance with the Japanese government's Basic Plan on Space Policy, JAXA has started the Commercial Removal of Debris Demonstration (CRD2) program in March 2020, with the aim of establishing ADR as a new business field and developing a new market where the private sector can play an active role.

 

There are many technical challenges for debris removal, and a significant portion of them are closely related to plasma physics. In-space plasma devices such as electric propulsions and electro-dynamic tether are essentially important to provide mobility necessary for debris removal, plasma and spacecraft interference is critical to achieve safe and reliable debris capturing. A brief overview of the CRD2 program, the challenges and the details of the research activities are introduced.

    

Magnetically Expanding Plasmas for Space Propulsion

Plasma expansion along diverging magnetic fields is a fundamental process for a variety of low-temperature plasmas with applications to plasma processing and spacecraft electric propulsion. Expansion coincides with the conversion of plasma thermal energy into directed ion kinetic energy. For plasmas in which the electron temperature greatly exceeds the ion temperature, ion acceleration is driven mainly by the formation of an ambipolar electric field. The gradient in electron pressure provides the source of the ambipolar electric field in addition to transferring the force of the accelerated plasma onto the magnetic circuit via diamagnetic drift currents. This talk will focus on two important topics related to the behavior of electrons in magnetically expanding plasmas that are critical to the thermal-to-kinetic energy conversion efficiency. The first topic, electron cooling, describes how electron energy is transported along the magnetic field lines. Experimental and theoretical results will be presented that highlight the importance of field-aligned electron heat conduction on electron cooling and the resulting energy conversion process. The second topic, electron demagnetization, describes how the electrons detach from the expanding magnetic field. Both experiment and theory will again be used to argue that electron detachment can result from finite Larmor radius effects in regions of low magnetic field and high magnetic field curvature.   

Electrodeless plasma thrusters and magnetized plasma expansions for space propulsion

This invited contribution presents the latest advancements in experiments and modeling of electrodeless plasma thrusters (EPTs), including the helicon plasma thruster, the electron-cyclotron plasma thruster, and the novel magnetic arch thruster. In contrast to more mature technologies like the Hall effect thruster and the gridded ion engine, EPTs do not feature any naked electrodes exposed to the plasma nor require a neutralizer, advantages that can lead to extended lifetimes and system simplicity. Several aspects of their physics are discussed, in particular: (1) the different geometries and magnetic topologies, (2) the mechanisms for power delivering to the plasma, across many wave regimes, resonances, and cutoffs, (3) the internal transport physics, (4) the plasma expansion and electron kinetics in the near-collisionless magnetic nozzle, and (5) the effect of the plasma-induced magnetic field.

Experimental measurements with plasma probes are used to illustrate the typical profiles of plasma density, electron temperature, ion current, and electrostatic potential in the different devices at the Plasmas and Space Propulsion Team (EP2) in Madrid. Then, hybrid PIC/fluid codes, electromagnetic codes, and kinetic codes are used to investigate the various mechanisms involved in power absorption, ionization, internal transport, and the expansion in the magnetic nozzle. Special focus is placed on the external magnetic topology of the novel magnetic arch thruster, which features closed lines and in which the plasma-induced magnetic field plays a dominant role, even for low-moderate beta plasmas.   

Electron thermodynamics and ion transport in the magnetic nozzle of electrodeless electric thrusters

Electric Propulsion (EP), an efficient way of moving a spacecraft, relies upon the ejection of ions to generate a net momentum. The large velocities of ions makes EP much more efficient than chemical propulsion from a propellant consumption viewpoint. Contrary to the well-established gridded ion engine and Hall-effect thruster technologies, electrodeless thrusters have no electrode in direct contact with the plasma discharge. In addition, such thrusters expel a current free quasi-neutral plasma to generate thrust, which prevents the use of an external neutralizer. Electrodeless thrusters, which encompass RF, helicon, electron cyclotron resonance and VASIMR thrusters, therefore offer attractive features in terms of reliability, lifetime and propellant options. One of the main element of an electrodeless thruster is the magnetic nozzle. A magnetic nozzle (MN) acts similarly to the solid nozzle of a chemical rocket: It guides particles and efficiently converts thermal energy into kinetic energy. In most devices the thermal energy reservoir is the electron temperature. In that case the energy transfer between hot electrons and cold ions is quite complicated and mostly originates in the creation of an ambipolar electric field. After a brief introduction to EP fundamentals, electron properties and transport of ions in the magnetic nozzle of various electrodeless devices are discussed in light of recent experimental results. All devices have been operated with xenon and krypton as propellant. This study especially focuses on the impact of the magnetic field strength, the MN geometry (shape, throat location), the propellant flow rate and the power level on the electron thermodynamics, described by means of the isentropic exponent, the ion acceleration, flow velocity and detachment.   

The Blue Core Paradigm

The blue core mode in magnetized rf plasmas has been the topic of recurring debates since it was first observed in 1968 in an argon helicon wave dominated discharge. Blue cores are typically seen with plasma densities in excess of 1012 cm−3. Because high degrees of ionization are desirable in a variety of applications, the study of the blue core mode has been a dynamic research topic. Observing a blue core is often taken as a sign of the existence of electron-heating powered by helicon waves. However, many clues indicate that a blue core is neither a sufficient nor a necessary condition of wave heated regimes. Direct measurements of helicon waves at the correct phase velocities to energise electrons have been made in discharges without the observation of blue cores, while studies report "helicon blue core mode" without direct proof of helicon waves contributing to the ionization.

Two new experiments have looked at the generation of blue cores without any evidence of helicon waves existing in the discharge. Both experiments share the same physical dimensions and magnetic field topologies but differ in the type of antenna employed and the driving frequency; a double-saddle driven at 13.56 MHz on one-hand, a single loop at 27.12 MHz on the other. In each, a Helmholtz pair of solenoids can be progressively moved away from the antenna to observe the effect on the discharge. Strong ArII emission and densities ≥ 1012 cm−3 are obtained for Prf ≥ 200 W and applied magnetic fields ≥ 600 G, when the Helmholtz pair is placed 30 cm away from the antenna. EEPFs measured along the magnetic field lines crossing the skin-depth region under the loop antenna are bi-Maxwellians with hot electrons in the inelastic energy range up to 50 cm away from the antenna. Plasma potential mappings show a strong ion radial confinement and an axial trapping of the electrons, matching with the location of maximum plasma density and of the blue core.