Figure 1. Experimental setup for the microwave power source.
Figure 2. MagNet 6 B-field profile along the centerline of the GDM. For 2.45 GHz microwave, ECR occurs at 875 G. Zero axial location marks the beginning of the electromagnetic coils, and for this specific field configuration, the ECR zone is at ~5.3 inches downstream.
The gasdynamic mirror (GDM) is a magnetic confinement device that can function as an effective plasma thruster by accelerating its propellant without the endurance limitations imposed by electrodes.
The magnetic mirror is one of the oldest thermonuclear plasma confinement concepts.
Although the GDM was initially proposed as a fusion propulsion concept, the fact that the GDM is nothing more than a plasma confinement and acceleration device means that the concept is very versatile and can be scaled to satisfy near and long term applications depending on available technologies.
In particular, with an external power source, the GDM can function as an electrode-less plasma thruster.
Our goal is to study the plasma dynamics inside the GDM through modeling and experiments.
The geometry of the GDM is that of a simple magnetic mirror, with a magnetic field configuration resembling that of a meridional nozzle where the fluid flow velocity is everywhere parallel to the magnetic field lines.
The magnetic field strength is stronger at the ends, called mirrors, than at the center, producing a turning force that helps confine the plasma ions long enough for heating before being ejected through one of the mirrors that serves as a magnetic nozzle.
In order to achieve better confinement and to provide plasma stability, the system is designed with a large aspect ratio (length >> plasma radius). Unlike a 'collisionless' mirror system, the requirement of a high density inside the GDM ensures that the ion-ion collision mean free path is much smaller than a characteristic dimension of the system, typically its length, which underlies the confinement principle of the GDM. Under these conditions, the plasma behaves like a fluid, and its escape from the system is analogous to the flow of a gas into vacuum from a vessel with a hole. Due to their small mass, the electrons initially escape rapidly through the mirror, leaving behind an excess of positive charge resulting in an ambipolar potential that generates an electric field. This self-generated electric field then accelerates the ions while slowing down the electrons until both species escape at the same rate, resulting in a charge-neutral propellant beam that produces thrust.
We have provided a mathematical formulation that models the plasma dynamics inside an asymmetric GDM (see ref. 1), where the two mirrors have different magnetic field strengths to bias the flow of ions to one end in order to produce thrust.
The model addresses the effect of diffusion due to collisions and the contribution of the electric field in the ion and electron fluxes.
The physics model allows us to predict plasma characteristics such as the plasma length, particle energy, and the magnitude of the ambipolar potential.
The ambipolar potential dictates plasma behavior and is central to the operation of the GDM thruster concept as it underlies the main acceleration mechanism.
Results from the model suggest that the ambipolar potential is on the same order of magnitude as the ion escape energy (see ref. 2).
This represents a substantial increase in the ion velocity and hence the performance of the system.
In order to validate the physics model and the GDM concept, we are building a scaled-down version of the GDM at PEPL to experimentally study its plasma characteristics.
Figure 1 illustrates our experimental setup.
A 2.45 GHz microwave discharge source will supply power to the system, with the microwave power ranging from 1 – 2 kW.
Due to the strong resonant interaction with free electrons, electron cyclotron resonance heating (ECRH) is able to produce a high density plasma on the order of 1018 – 1019 m-3 with an electron temperature of several eV's.
An ECR plasma source is particularly suited for the GDM since the magnetic coils required for ECRH is already part of the GDM configuration, and the ionization zone will be in the GDM chamber where the electron cyclotron frequency matches the microwave frequency.
In order for the microwave to be absorbed by the plasma, the wave must be launched from the high magnetic field side, i.e. region with decreasing field gradient.
This condition can be readily satisfied by the GDM since it has higher magnetic field at the mirrors than at the center.
The microwave will be launched axially through the non-thrusting mirror and absorbed downstream.
Figure 2 shows an example of the centerline magnetic field profile of our GDM given by MagNet 6 simulation.
Our GDM consists of 12 electromagnetic coils, and the B-field profile can be precisely shaped.
The goal for our experiments is to characterize our ECR plasma, such as plasma density and temperature profiles, as well as plume velocity measurement, using various diagnostics such as the Langmuir probe, micro-RPA, and the HARP.
Figure 3. Conceptual drawing of an ECR-GDM thruster. Courtesy of Reisz Engineers
Selected Relevant Publications
Kammash, T. and Tang, R., "Propulsive Capability of an Asymmetric GDM Propulsion System,"
AIAA-2006-4391, 42nd Joint Propulsion Conference, Sacramento, CA, July 9-12, 2006.
Tang, R., Kammash, T., and Gallimore, A., "Field Asymmetry and Thrust Control in the GDM Fusion Propulsion System,"
43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Cincinnati, OH, July 8-11, 2007
R. Tang, A. Gallimore, and T. Kammash, "Design of an ECR Gasdynamic Mirror Thruster,"
IEPC-2009-210, 31st International Electric Propulsion Conference, Ann Arbor, MI, USA, September 20-24, 2009.