Jared Philip Squire

Comparing experiments with modeling for light ion helicon plasma sources

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, results from a coupled rf and transport model are compared with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility [Goulding et al., Proceedings of the International Conference on Electromagnetics in Advanced Applications (ICEAA 99), Torino, Italy, 1999 (Litografia Geda, Torino, 1999), p. 107] (Mini-RFTF). Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution rf calculation including the rf parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the rf coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. The effects of absorption near the lower hybrid resonance in the near-field region of the antenna are discussed. Ponderomotive effects are also examined and found to be significant only in very low density and edge regions of the Mini-RFTF discharge.

Validating a Plasma Momentum Flux Sensor to an Inverted Pendulum Thrust Stand

The accuracy of a plasma impact force sensor was compared with that of the more commonly used inverted pendulum thrust stand using a 5 kW Xe Hall effect thruster. An improved plasma momentum flux sensor was designed and constructed based on a previous design. Real-time force measurements were made with both the plasma momentum flux sensor and the inverted pendulum thrust stand. The plasma momentum flux sensor measured the force exerted onto it by the Hall effect thruster exhaust plume with a resolution of 0.1 mN and an average discrepancy of 2 % compared with thrust stand measurements. Experiments were completed using a 9 m by 6 m cylindrical vacuum chamber. The total force from the Hall effect thruster was modulated from 34 to 356 mN by varying both the anode voltage, from 150 to 500 V, and the neutral Xe gas flow rate, from 5 to 15 mg/s.

Helicon Plasma Injector and Ion Cyclotron Acceleration Development in the VASIMR Experiment

In the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) radio frequency (rf) waves both produce the plasma and then accelerate the ions. The plasma production is done by action of helicon waves. These waves are circular polarized waves in the direction of the electron gyromotion. The ion acceleration is performed by ion cyclotron resonant frequency (ICRF) acceleration. The Advanced Space Propulsion Laboratory (ASPL) is actively developing efficient helicon plasma production and ICRF acceleration. The VASIMR experimental device at the ASPL is called VX-10. It is configured to demonstrate the plasma production and acceleration at the 10kW level to support a space flight demonstration design. The VX-10 consists of three electromagnets integrated into a vacuum chamber that produce magnetic fields up to 0.5 Tesla. Magnetic field shaping is achieved by independent magnet current control and placement of the magnets. We have generated both helium and hydrogen high density (>10(exp 18) cu m) discharges with the helicon source. ICRF experiments are underway. This paper describes the VX-10 device, presents recent results and discusses future plans.

Experimental evidence of parametric decay processes in the variable specific impulse magnetoplasma rocket (VASIMR) helicon plasma source

This project was proudly supported by the International nScience Linkages programme established under the Australian nGovernment’s innovation statement Backing Australia’s nAbility.

Ion Cyclotron Heating Results in the VASIMR VX-10

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine concept is an RF-powered thruster scaleable to multi-megawatt power levels. The prototype at the Johnson Space Center consists of a helicon plasma source, an ion cyclotron resonant heating system, and a magnetic nozzle. Theoretical and computational predictions of the ion cyclotron heating systems effectiveness have been validated by the experimental results presented here. At modest power levels, the prototype thruster has demonstrated a continuously variable specific impulse spanning the range of 5,000 - 12,000 seconds. An upgrade to 50 kW thruster operation is in progress.

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.

Advances in Duration Testing of the VASIMR® VX-200SS™ System

The VX-200SS program will complete design verification, testing and lifetime estimates of a VASIMR prototype engine, operating under thermal steady-state at power levels greater than 100 kW for at least 100 continuous hours. The work will bring the integrated VX-200SS VASIMR prototype to Technology Readiness Level of 5 (TRL-5), including RF Power Processing Units (PPUs), superconducting magnet, propellant management system, internal thermal management subsystems and the rocket core. The rocket core element resides down the bore of the magnet and is designed to utilize the RF power in conjunction with the high magnetic field to create and accelerate a high power-density plasma stream and manage any waste heat from the process. The accumulation of significant operating time will allow a measurement of the wear of plasma-facing components with sufficient accuracy to evaluate their projected lifetime. The VX-200SS program builds on the successful VX-200TM program and involves manufacturing of a new rocket core and significant upgrades to the RF subsystems, vacuum chamber, computer control, and performance measurement diagnostics. This paper describes the three-year multi-phase VX-200SS program that has been underway for one year.

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.

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.

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.