Benjamin W. Longmier

New low-power plasma thruster for nanosatellites

The CubeSat Ambipolar Thruster is a new, electrodeless, permanent magnet, helicon thruster specically designed for the constraints of a nanosatellite. The design processes is outlined, indicating expected thrust, Isp, and eciency of 1 mN, 1000 s, and 20%, respectively. The major components of the thruster|quartz plasma liner, helical half-twist antenna, permanent magnets, and Faraday shield|are described. Finite element magnetic eld simulations were compared to magnetometer measurements of the prototype device and showed agreement to within 5%. Initial testing on xenon demonstrated ignition at < 50 W. The electrodeless design enables the use of a wide variety of propellants and a discussion of iodine’s potential as a propellant is included.

Azimuthal Spoke Propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall effect thrusters (HETs) that travel in the E x B direction. The mechanisms for spoke formation are unknown, but their presence has been associated with improved thruster performance in some thrusters motivating a detailed investigation. The propagation of azimuthal spokes are investigated in a 6 kW HET by using high-speed imaging and azimuthally spaced probes. The spoke velocity is determined from high-speed image analysis using three methods with similar results. The spoke velocity for three discharge voltages (300, 400, and 450 V) and three anode mass flow rates (14.7, 19.5, and 25.2 mg/s) are between 1500 and 2200 m/s across a range of magnetic field settings. The spoke velocity is inversely dependent on magnetic field strength for lower B-fields and asymptotes at higher B-fields. Spoke velocities calculated from the probes are consistently higher by 30% or more. An empirically approximated dispersion relation of ω^α = v_ch^αk_θ^α - ω_ch^α where α ≥ 1 yields a characteristic velocity that matches the ion acoustic speed for N5 eV electrons which exist in the near-anode and near-field plume regions of the discharge.

Mode Transitions in Hall Effect Thrusters

Mode transitions have been commonly observed in Hall Effect Thruster (HET) operation where a small change in a thruster operating parameter such as discharge voltage, magnetic field or mass flow rate causes the thruster discharge current mean value and oscillation amplitude to increase significantly. Mode transitions in a 6-kW-class HET called the H6 are induced by varying the magnetic field intensity while holding all other operating parameters constant and measurements are acquired with ion saturation probes and ultra-fast imaging. Global and local oscillation modes are identified. In the global mode, the entire discharge channel oscillates in unison and azimuthal perturbations (spokes) are either absent or negligible. Downstream azimuthally spaced probes show no signal delay between each other and are very well correlated to the discharge current signal. In the local mode, signals from the azimuthally spaced probes exhibit a clear delay indicating the passage of spokes and are not well correlated to the discharge current. These spokes are localized oscillations propagating in the E×B direction that are typically 10-20% of the mean value. In contrast, the oscillations in the global mode can be 100% of the mean value. The transition between global and local modes occurs at higher relative magnetic field strengths for higher mass flow rates or higher discharge voltages. The thrust is constant through mode transition but the thrust-to-power decreased by 25% due to increasing discharge current. The plume shows significant differences between modes with the global mode significantly brighter in the channel and the near-field plasma plume as well as exhibiting a luminous spike on thruster centerline. Mode transitions provide valuable insight to thruster operation and suggest improved methods for thruster performance characterization.

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.

Initial operation of the CubeSat Ambipolar Thruster

Summary form only given. Operation and characterization of the CubeSat Ambipolar Thruster (CAT), a miniature helicon electric propulsion device, is presented. Its small plasma volume (~10 cm3) and low power requirements (<;100 W) make it ideal for propelling nanosatellites (<;10 kg). Permanent magnets generated a magnetic nozzle with a maximum field strength of 600 G. This field decreased to 0.5 G, the strength of earths magnetic field, within 50 cm allowing the entire exhaust plume to develop in the vacuum chamber without being affected by the chamber walls. Low gas flow rates (~10 sccm) and high pumping speeds (~10,000 l/s) were used to more closely approximate the conditions of space. A parametric study of the thruster operational parameters was performed to determine its capabilities as both a thruster and as a plasma source for magnetic nozzle experiments. The plasma density, electron temperature, and plasma potential in the plume were measured with Langmuir probes, double probes, and emissive probes. These measurements characterized the ion acceleration mechanism which produced thrust.

Initial experiments of a new permanent magnet helicon thruster

Summary form only given. A new design for a permanent magnet helicon thruster is presented. Its small plasma volume (~10 cm3) and low power requirements (<;100 W) make it ideal for propelling nanosatellites (<;10 kg). The magnetic field reached a maximum of 600 G in the throat of a converging-diverging nozzle and decreased to 0.5 G, the strength of earths magnetic field, within 50 cm allowing the entire exhaust plume to develop in the vacuum chamber without being affected by the chamber walls. Low gas flow rates (~10 sccm) and high pumping speeds (~10,000 l/s) were used to more closely approximate the conditions of space. A parametric study of the thruster operational parameters was performed to determine its capabilities as both a thruster and as a plasma source for magnetic nozzle experiments. The plasma density, electron temperature, and plasma potential in the plume were measured with Langmuir probes, double probes, and emissive probes. These measurements characterized the ion acceleration mechanism which produces thrust. Thrust measurements were made with an innovative micronewton thrust stand. Measurements were compared to predictions made with fluid theory and particle-in-cell simulations.

VASIMR ® : Deep Space Transportation for the 21 st Century

®) VX-200 engine, a 200 kW flight-technology prototype. Results from high power Helicon only and Helicon with ICH experiments are presented from the VX-200 using argon propellant. Total VX-200 system efficiencies are presented from recent results with 200 kW of RF power. A two-axis translation stage has been used to survey the spatial structure of plasma parameters, momentum flux and magnetic perturbations in the VX-200 exhaust plume. These recent measurements of axial plasma density and ambipolar potential profiles, magnetic field-line shaping, charge exchange, and force measurements were made within a new 150 cubic meter cryo-pumped vacuum chamber and are presented in the context of plasma detachment. A semi-empirical model of the thruster efficiency as a function of specific impulse was developed to fit the experimental data, and reveals an ICH RF power coupling efficiency of 89%. The thruster performance at 200 kW is 72 ± 9%, the ratio of effective jet power to input RF power, with an Isp = 4900 ± 300 seconds. The thrust increases steadily with power to 5.8 ± 0.4 N until the power is maximized and there is no indication of saturation. Comparisons of the plasma flux to magnetic flux in the plume show evidence that the plasma flow does not follow the magnetic field at distances downstream on the order of 2 m. The plume is more directed when the ions are significantly accelerated. The planned ISS flight test of the VASIMR ® VF-200 Aurora experiment is discussed.

Mode Transitions in Magnetically Shielded Hall Effect Thrusters

A mode transition study is conducted in magnetically shielded thrusters where the magnetic field magnitude is varied to induce mode transitions. Three different oscillatory modes are identified with the 20-kW NASA-300MS-2 and the 6-kW H6MS: Mode 1) global mode similar to unshielded thrusters at low magnetic fields, Mode 2) cathode oscillations at nominal magnetic fields, and Mode 3) combined spoke, cathode and breathing mode oscillations at high magnetic fields. Mode 1 exhibits large amplitude, low frequency (1-10 kHz), breathing mode type oscillations where discharge current mean value and oscillation amplitude peak. The mean discharge current is minimized while thrust-to-power and anode efficiency are maximized in Mode 2, where higher frequency (50-90 kHz), low amplitude, cathode oscillations dominate. Thrust is maximized in Mode 3 and decreases by 5-6% with decreasing magnetic field strength. The presence or absence of spokes and strong cathode oscillations do not affect each other or discharge current. Similar to unshielded thrusters, mode transitions and plasma oscillations affect magnetically shielded thruster performance and should be characterized during system development.

High Power Electric Propulsion Using VASIMR (TM): Results from Flight Prototypes

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR™) is a high power magnetoplasma rocket, capable of Isp/thrust modulation at constant power. The plasma is produced by a helicon discharge. The bulk of the energy is added by ion cyclotron region heating (ICRH.) Axial momentum is obtained by adiabatic expansion of the plasma in a magnetic nozzle. Thrust/specific impulse ratio control in the VASIMR™ is primarily achieved by the partitioning of the RF power to the helicon and ICRH systems, with the proper adjustment of the propellant flow. Ion dynamics in the exhaust were studied using probes, gridded energy analyzers (RPA’s), microwave interferometry and optical techniques. This paper will summarize results from high power ICRH experiments performed on the VX-100 using argon plasma during 2007, and technology demonstration from the VX-200. An overview of the way forward will be touched on briefly, with some emphasis on the fact that VASIMR™ is now being developed by private enterprise. The new VX-200 machine is described.

Quasi-one-dimensional code for particle-in-cell simulation of magnetic nozzle expansion

The formulation and validation of a novel quasi-one-dimensional particle-in-cell code for the simulation of magnetic nozzles is presented. Quasi-one-dimensional effects are included through virtual displacements of magnetized particles from the axis of symmetry and cross-sectional area variation according to preservation of magnetic flux. A modified, semi-implicit Boris algorithm is developed for capturing the Lorentz force effects in quasi1D. Validation problems are selected to test the components of the code required to model the important physics of magnetic nozzles. Simulations are performed of two stream instabilities, Landau damping, source and collector sheaths, and magnetic mirrors. Results from the validation simulations show that the code produces physically accurate results when compared with both theory and other simulations.