Andrew V. Ilin

Ambipolar ion acceleration in an expanding magnetic nozzle

The helicon plasma stage in the Variable Specific Impulse Magnetoplasma Rocket (VASIMR ® ) VX-200i device was used to characterize an axial plasma potential profile within an expanding magnetic nozzle region of the laboratory based device. The ion acceleration mechanism is identified as an ambipolar electric field produced by an electron pressure gradient, resulting in a local axial ion speed of Mach 4 downstream of the magnetic nozzle. A 20 eV argon ion kinetic energy was measured in the helicon source, which had a peak magnetic field strength of 0.17 T. The helicon plasma source was operated with 25 mg s −1 argon propellant and 30 kW of RF power. The maximum measured values of plasma density and electron temperature within the exhaust plume were 1 × 10 20 m −3 and 9 eV, respectively. The measured plasma density is nearly an order of magnitude larger than previously reported steady-state helicon plasma sources. The exhaust plume also exhibits a 95% to 100% ionization fraction. The size scale and spatial location of the plasma potential structure in the expanding magnetic nozzle region appear to follow the size scale and spatial location of the expanding magnetic field. The thickness of the potential structure was found to be 10 4 to 10 5 λDe depending on the local electron temperature in the magnetic nozzle, many orders of magnitude larger than typical laboratory double layer structures. The background plasma density and neutral argon pressure were 10 15 m −3 and 2 × 10 −5 Torr, respectively, in a 150 m 3 vacuum chamber during operation of the helicon plasma source. The agreement between the measured plasma potential and plasma potential that was calculated from an ambipolar ion acceleration analysis over the bulk of the axial distance where the potential drop was located is a strong confirmation of the ambipolar acceleration process. (Some figures in this article are in colour only in the electronic version)

VX-200 Magnetoplasma Thruster Performance Results Exceeding Fifty-Percent Thruster Efficiency

IGH-POWER electric propulsion thrusters can reducepropellant mass for heavy-payload orbit-raising missions andcargo missions to the moon and near-Earth asteroids, and they canreduce the trip time of robotic and piloted planetary missions [1–4].TheVariableSpecificImpulseMagnetoplasmaRocket(VASIMR®)VX-200 engine is an electric propulsion system capable ofprocessing power densities on the order of 6MW

Temperature gradients due to adiabatic plasma expansion in a magnetic nozzle

A mechanism for ambipolar ion acceleration in a magnetic nozzle is proposed. The plasma is adiabatic (i.e., does not exchange energy with its surroundings) 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 plasma potential is proportional to the change in average electron energy. 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, showing consistency with the adiabatic ambipolar fluid theory.

Fast Transits to Mars Using Electric Propulsion

The use of electric propulsion for Mars has been explored since the 1970’s when men looked to travel beyond the moon. The use of electric propulsion has been recommended in several studies as a low-risk, lower cost approach to the robotic Mars sample return missions. Electric propulsion has been evaluated for delivery of Mars cargo using power systems order of magnitude beyond state-of-the-art. Electric propulsion has also been considered for fast transits to Mars supporting manned exploration activities. Results of generalized electric propulsion transits form Earth to Mars are presented. Trades are presented as a generalized assessment based on spacecraft mass-to-power ratio, trip time, and propulsion system performance including variable and constant specific impulse, and efficiency.

PARTICLE SIMULATIONS OF PLASMA HEATING IN VASIMR

An important motivation for particle simulation in a Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is plasma heating by Radio Frequency (RF) electromagnetic waves. Mathematical simulation helps with the design of an Ion-Cyclotron Radio Frequency (ICRF) antenna by showing where adjustments can maximize the power coupling and control the absorption profile of RF power into the plasma in the resonance area. Not only should the ions gain high energy from the ICRF waves, but the heating must also be accompanied by a high antenna loading to reduce power loss in the RF circuit.

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.

VASIMR ® VX-200 Improved Throttling Range

Testing of the VX-200 engine was performed over a wide throttle range in a vacuum facility with sufficient volume and pumping to permit exhaust plume measurements at low background pressures and sufficiently large charge exchange mean free paths. Experimental results are presented with the VX-200 engine installed in a 150 m 3 vacuum chamber with an operating pressure below 1x10 -3 Pa (1x10 -5 Torr), and with an exhaust plume diagnostic measurement range of 5 m in the axial direction and 1 m in the radial directions. Measurements of plasma flux, RF power, and neutral gas flow rate, combined with knowledge of the kinetic energy of the ions leaving the VX-200 engine, are used to determine the ionization cost of argon and krypton in the first stage helicon plasma discharges. Experimental data on ionization cost, ion fraction, exhaust plume expansion angle, thruster efficiency, and total force are presented that characterize the VX-200 engine performance over a wide throttling range from 15 kW to 200 kW total RF power. A semi-empirical model of the thruster efficiency as a function of specific impulse was developed to fit the experimental data, and reveals a second stage Ion Cyclotron Heating (ICH) RF power coupling efficiency of 85%. Operation at an RF power level of 200 kW yields a thruster efficiency of 72%±9% at a specific impulse of 4900±300 s. A high thrust-to-power operating mode was characterized over a wide parameter space with a maximum thrust to power ratio of 51 mN/kW at a specific impulse of 1660 s for a ratio of ICH RF power to helicon RF power of 0.7:1.

Error control and mesh optimization for high-order finite element approximation of incompressible viscous flow

Abstract We discuss the use of a posteriori error estimates for high-order finite element methods during simulation of the flow of incompressible viscous fluids. The correlation between the error estimator and actual error is used as a criterion for the error analysis efficiency. We show how to use the error estimator for mesh optimization which improves computational efficiency for both steady-state and unsteady flows. The method is applied to two-dimensional problems with known analytical solutions (Jeffrey-Hamel flow) and more complex flows around a body, both in a channel and in an open domain.

Ambipolar Ion Acceleration in the Expanding Magnetic Nozzle of the VASIMR ® VX-200i

An observed 20 eV argon ion energy is attributed to a measured axial plasma potential profile within the expanding magnetic nozzle region of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR ® ) VX-200i device, a 10% field version of the VX-200 prototype. The ion acceleration mechanism is identified as an ambipolar flow caused by expanding plasma that follows an idealized electron Boltzmann relation, resulting in a maximum axial speed of S c 1 . 4 ~ . The VX-200i prototype was operated with 25 mg/s argon

Plasma Heating Simulation in the VASIMR System

The paper describes the recent development in the simulation of the ion-cyclotron acceleration of the plasma in the VASIMR experiment. The modeling is done using an improved EMIR code for RF field calculation together with particle trajectory code for plasma transport calculation. The simulation results correlate with experimental data on the plasma loading and predict higher ICRH performance for a higher density plasma target. These simulations assist in optimizing the ICRF antenna so as to achieve higher VASIMR efficiency.