Christopher S. Olsen

Investigation of Plasma Detachment From a Magnetic Nozzle in the Plume of the VX-200 Magnetoplasma Thruster

Understanding the physics involved in plasma detachment from magnetic nozzles is well theorized, but lacking in large scale experimental support. We have undertaken an experiment using the 150-m3 variable specific impulse magnetoplasma rocket test facility and VX-200 thruster seeking evidence that detachment occurs and an understanding of the physical processes involved. It was found that the plasma jet in this experiment does indeed detach from the applied magnetic nozzle (peak field sim~2 T) in a two part process. The first part involves the ions beginning to deviate from the nozzle field 0.8-m downstream of the nozzle throat. This separation location is consistent with a loss of adiabaticity where the ratio of the ion Larmor radius to the magnetic field scale length (r_Li|∇ B|B ) becomes of order unity and conservation of the magnetic moment breaks down. Downstream of this separation region, the dynamics of the unmagnetized ions and magnetized electrons, along with the ion momentum, affect the plume trajectory. The second part of the process involves the formation of plasma turbulence in the form of high-frequency electric fields. The ion and electron responses to these electric fields depend upon ion momentum, magnetic field line curvature, magnetic field strength, angle between the particle trajectories, and the effective momentum transfer time. In stronger magnetic field regions of the nozzle, the detached ion trajectories are affected such that the unmagnetized ions begin to flare radially outward. Further downstream as the magnetic field weakens, for higher ion momentum and along the edge of the plume, the fluctuating electric field enables anomalous cross-field electron transport to become more dominant. This cross-field transport occurs until the electric fields dissipate 2-m downstream of the nozzle throat and the ion trajectories become ballistic. This transition to ballistic flow correlates well with the sub-to-super Alfvénic flow transition (βk ). There was no significant change observed to the applied magnetic field.

Parameterization of magnetic nozzle flow physics for an in-space propulsion application

The e ciency of a magnetic nozzle in converting thermal energy into directed axial energy is dependent on several factors, most notably on how and where the plasma separates or detaches from the nozzle. A detailed knowledge of the physics of this detachment process is presently lacking, but is necessary in order to optimize magnetic nozzle design for in-space applications. In an e ort toward studying detachment computationally, we wish to rst establish an appropriate mathematical model for the nozzle ow physics. In this work, we perform an order of magnitude analysis of the Boltzmann equation in terms of characteristic length scales for an Argon plasma. From this analysis, we create a parameter map of the relevant Argon physics in terms of temperature and density. We apply our analysis to examine the magnetic nozzle ow physics of the latest VASIMR R rocket, the VX-200. We conclude that a uid approximation is an appropriate mathematical model for the VX-200 system. This conclusion is valid if at least tensorial resistivity is included in the model near the exit plane of the rocket and further downstream local electric eld e ects are also included.

Using VASIMR ® for the Proposed Europa Mission

We explore the capability of a VASIMR ® reusable probe catapult concept to send a 4000-5000 kg spacecraft to Jupiter on a Hohmann-like transfer orbit, arriving in just 36 months elapsed time. The VASIMR ® performs a slingshot pass close to the Sun and uses the high level of available solar energy to produce a sustained burst of high thrust. Enough kinetic energy is provided to the probe to reach Jupiter orbit within 0.7-1.4 AU. The Catapult release the probe with enough speed to reach Jupiter in three years, and returns to Earth for another mission. This study identifies the important parameters in the probe ejector operation (power level, propellant mass, payload release point, distance of closest approach to the Sun), and scan these parameters to understand and optimize the capabilities of the proposed system. We assume that the Catapult and its payload begin at the Earths sphere of influence (SOI), and are coasting in the Earths orbit about the Sun. The VASIMR ® engines power rating must match the peak power available when the spacecraft is closest to the Sun. The solar array is assumed to be a planar array rather than a concentrator since it will have to operate near the Sun, where a concentrator would overheat photovoltaic cells. The feasibility of not releasing the payload and using the VASIMR ® to provide thrust for the duration of the transfer orbit will also be examined. In this scenario, the VASIMR ® RF generators could serve double duty as radar RF sources.

Adiabatic plasma expansion in a magnetic nozzle

Summary form only given. A mechanism for ambipolar ion acceleration in a magnetic nozzle is proposed. The plasma is adiabatic in the diverging section of a magnetic nozzle so any energy lost by the electrons must be transferred to the ions via the electric field. Fluid theory indicates that the change in average electron energy equals the change in plasma potential. These predictions were compared to measurements in the VX-200 experiment which has conditions conducive to ambipolar ion acceleration. A planar Langmuir probe was used to measure the plasma potential, electron density, and electron temperature for a range of mass flow rates and power levels. Axial profiles of those parameters were also measured, verifying the adiabatic ambipolar fluid theory.

ISS Space Plasma Laboratory: An ISS instrument package for investigating the opening/closing of solar and heliospheric magnetic fields

We describe a proposed laboratory-experiment research program that will answer the fundamental question: What is the role of reconnection in opening and closing the solar magnetic field? While attacking this question, we will also address the most important, longstanding questions on magnetic reconnection, in all contexts: What determines the rate of reconnection, and whether or not it is bursty? How is the released energy partitioned between thermal, kinetic, and particle? Of course, it seems completely contradictory to use a