Session TF13: Electric Propulsion

Technology Development for In-space Nuclear Electric Propulsion

Nuclear electric propulsion (NEP)-powered vehicles have been contemplated for human Mars

missions. The nuclear power system contemplates using a high temperature light-weight nuclear reactor

for the production of electrical power in the range of 2-5 MWe with a 3-10 year service life. A myriad of

technology options exist that could potentially achieve these mission objectives. Significant technology

development is required to narrow the field of design options that could be used for a NEP-powered

vehicle.

The NEP system is comprised of five major critical technology elements: the nuclear power source,

the system to convert thermal power into electrical power, a large radiator to serve as the cold-side of the

power conversion thermodynamic cycle, the electric propulsion system, and a power management and

distribution system that will deliver the power to the propulsion system and provide required electrical

isolation between different components. At the system level, each of these technology elements must

operate in concert. For example, system extracting heat from the nuclear reactor must be compatible

with the input required for the power conversion system. NASA’s Space Nuclear Propulsion (SNP)

project has conducted a number of technical interchange meetings to survey technology options for each

critical element.

In this presentation, we discuss the current state-of-the-art for each sub-system and the options that

the SNP NEP team has identified for potential parallel-path development and technology maturation. The

development path for each technology is distinct, and involves hardware development and testing

supported by significant modeling and simulation of each element.   

Cross-field electron transport due to coupling of drift-driven microinstabilities in the presence of singly, doubly, and triply charged ion streams

Cross-field electron transport across magnetic field lines plays an important role in fusion, astrophysical, and $\mathbf{E}\times \mathbf{B}$ discharge plasmas. Microturbulence developed due to nonlinear coupling between different linear instabilities has been proposed to be one of the mechanisms responsible for anomalous turbulent transport across the magnetic field. Nonlinear interaction between kinetic instabilities driven by electron drift in crossed electric and magnetic fields ($\mathbf{E}\times \mathbf{B}$ drift) in the presence of multiply-charged ion streams is investigated using kinetic simulations. Singly, doubly, and triply charged ions, streaming in the direction parallel to the applied electric field, interact with electrons drifting in the cross field direction. Microturbulence that develops due to the mode coupling between ion-ion two-stream instability and electron cyclotron drift instability is shown to suppress cross-field electron transport when triply charged ion stream is present. Furthermore, ion trapping in the nonlinear saturation stage of the instability is investigated.      

Enhancing Confinement and Acceleration of Cathodic Plasma Plume by Using Electro-magnetic Nozzle

The concept of the magnetic nozzle (MN) is used for accelerating the plasma plume produced from the resonance-excited plasma source such as ECR and Helicon source for propulsive application. [1-4] Yet, the evaluation from MHD theory indicates an upper limit for the conversion of plasma internal energy (IE) to the kinetic energy (KE) before the occurrence of plasma detachment. A new concept, a.k.a. the electromagnetic nozzle (EMN), is proposed to enhance the conversion of plasma IE. [5] The idea is to introduce a positively biased ring-electrode to regulate the electric potential around the MN exit. This can enhance the dissipation of electron KE to the ionization processes as well as reshaping the potential gradience for plasma acceleration.

The EMN is demonstrated with a gridded hollow cathode and with argon as working gas. [5] The measurement of near-field plasma is performed with a Langmuir probe and Faraday probe. The formation of virtual cathode on the axis of the plume is observed when the B field at the throat is larger than 50 G, which indicates effective electron confinement. Ion flux density is greatly enhanced when the positive bias is applied on ring-electrode. The results prove the concept of EMN on plasma confinement and acceleration for propulsive applications.   

Investigation of Hall effect thruster instabilities with axial-azimuthal PIC simulation using a collective Thomson scattering approach

The last decade witnessed a growing interest in Hall Effect Thrusters due to their efficiency. However, a full grasp of their operation is not yet available and one significant question remains to be the electron transport. Recent PIC simulations suggested that electron transport is strongly affected by small-scale fluctuations which can be investigated using Collective Thomson Scattering (CTS).

This work tries to benefit from the wave vector selectivity of the CTS and the resolution of the PIC results to investigate these fluctuations. This is achieved by analyzing the signal that a CTS virtual diagnostic observes from PIC. The PIC data are from a simplified benchmark by T.Charoy et al., where both the radial magnetic field and the ionization profile are fixed so that to remove the breathing mode oscillation. The collective scattering signal is calculated from the electron density maps using the expression of the scattered field.

An analysis along the channel axis highlights changes in the direction and intensity of the propagating modes between the ionization and the acceleration zones. An azimuthal dominant mode at k = 3 rad.mm-1 is enhanced throughout the channel while modes at larger k are damped. The dispersion relations are also computed as well as the fluctuation rate.   

Recent performance improvement of a magnetic nozzle rf plasma thruster

  A magnetic nozzle rf plasma thruster is an electrodeless system for space propulsion, while the thruster efficiency is relatively low compared with other mature electric propulsion devices. The high density plasma produced in the source is transported along the magnetic field lines toward the source exit, is expanded along the magnetic nozzle, where spontaneous acceleration and momentum conversion processes occur. 

 A simple thruster model have shown that the imparted thrust and the thruster efficiency can be enhanced by increasing the magnetic field strength and by enlarging the diameter of the source tube. Furthermore, the experiments have shown that the neutral injector also affects the density profile and the imparted thrust.

 Here two different rf antennas exciting m = 0 and +1 modes are tested in about 10-cm-diameter rf plasma thruster. A better rf power coupling is obtained fom m=+1 mode case, which would be in helicon mode, while the larger thrust can be obtained for m=0 mode antenna. The difference between these two cases will be discussed in the presentation. Furthermore, the larger thrust can be obtained by injecting the neutral argon gas near the thruster exit; the thruster efficiency approaching twenty percent is successfully obtained for the rf power above a few kW, the propellant flow rate of 2 mg/s,  and the peak magnetic field strength. of about several hundreds of Gauss.      

Fully kinetic simulations of a magnetic nozzle radiofrequency plasma thruster using an open axial boundary condition

Fully kinetic simulations of a magnetic nozzle radiofrequency (rf) plasma thruster are conducted to investigate the plasma expansion in the magnetic nozzle. The magnetic nozzle accelerates the plasma through a divergent magnetic field and imparts the net momentum to the plasma. Our previous simulations assumed the Dirichlet boundary condition at a plasma exit surface, resulting in the sheath generation near the boundary. The simulated plasma expansion toward the sheath successfully validates the experimental results, because the experiment also involves the sheath near the chamber wall. However, the plasma expansion toward the sheath is not suitable for evaluating the space operation, in which the plasma does not involve the sheath and expands infinitely. In addition, it was reported that the plasma dynamics such as a polytropic index depended on the sheath existence. In this study, a Neumann boundary condition and low-energy electron reflection are applied to eliminate the sheath in the magnetic nozzle acceleration region as an open boundary. The simulated plasmas show different polytropic indexes depending on the boundary conditions.   

Study of the instabilities in radial-azimuthal and axial-azimuthal 2D PIC simulations of a Hall Thruster

Plasma propulsions systems such as Hall Thrusters (HTs) use an electric field perpendicular to the magnetic field to produce the thrust. The differential drift between electrons and ions in the ExB direction can origin several instabilities, among which the Electron Cyclotron Drift Instability (ECDI) in the direction perpendicular to the magnetic field and the Modified Two-Streams Instability (MTSI), with also a component along the magnetic field. We have derived and validated a stability condition for the appearance of the MTSI modes in 2D Particle-In-Cell (PIC) simulations of the radial-azimuthal plane of a HT. Furthermore, we have investigated their effects on the main discharge parameters.The plasma instabilities play a very important role also in the axial-azimuthal plane of a HT. In particular, we are addressing, using PIC simulations, the Ion Transit Time Instability (ITTI) and how it can be related to the electron emission temperature at the cathode and to the presence of a virtual third dimension in the radial direction. This third dimension may strongly affect the discharge behaviour, especially the Breathing Mode characteristics.