VASIMR Research Articles

Exploration on the Possible Bump-on-Tail Instability in VASIMR

The bump-on-tail (BOT) instability is generally caused by a beam of energetic particles existing in relatively cold background plasma. The employment of second-stage wave-driven module in the variable specific impulse magnetoplasma rocket (VASIMR) yields the production of energetic ions, which could drive the BOT instability. The present work explores this possibility for the first time via numerical simulations based on the experimental data on the VASIMR, i.e., referring to VX-50. It is found that the BOT instability does exist even in the plume region away from antenna. The results indicate that velocity space diffusion provides a stabilizing effect on the nonlinear evolution of waves, while dissipation in the bulk plasma essentially impedes it. To show the practical values implied by these computations, the influences of this BOT instability on the power coupling and thrust are investigated particularly. These findings are valuable for VASIMR, as well as other plasma thrusters that yield energetic particles inside relatively cold background plasma, to suppress BOT instability and thus increase the power coupling efficiency and thrust performance.

A new configuration for high power propulsion plasmas driven by ion cyclotron resonance energization

The ion cyclotron resonance mechanism can play a crucial role in high-power electric propulsion of space vehicles. Compared with the existing high-power thruster design such as the single-pass configuration in Variable Specific Impulse Magnetoplasma Rocket, we propose a new ion cyclotron resonance configuration that is still efficient under high radio frequency power. A single-ion acceleration theory and particle-in-cell simulations reveal the mechanism of the new configuration by making the phases of the ion and the wave match to each other for more energy gain in the ion cyclotron resonance and a raise of the energy conversion efficiency. Thus, this new configuration can have important applications not only to electric propulsion but also to ion cyclotron resonance heating in nuclear fusion.

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-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> 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 (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ). There was no significant change observed to the applied magnetic field.

Improved Efficiency and Throttling Range of the VX-200 Magnetoplasma Thruster

Testing of the Variable Specific Impulse Magnetoplasma Rocket VX-200 engine was performed over a wide throttle range in a vacuum chamber with sufficient pumping to permit exhaust plume measurements at argon background pressures less than () during firings, ensuring charge-exchange mean free paths longer than the vacuum chamber. Measurements of plasma flux, radio frequency power, propellant flow rate, and ion kinetic energy were used to determine the ionization cost of argon and krypton in the helicon discharge. Experimental data on ionization cost, ion fraction, exhaust plume expansion angle, thruster efficiency, and thrust are presented that characterize the VX-200 engine performance over a throttling range from 15 to 200 kW radio frequency power. A semiempirical model of the thruster efficiency as a function of specific impulse indicates an ion cyclotron heating efficiency of . Operation at a total radio frequency coupled power level of 200 kW yields a thruster efficiency of at a specific impulse of with argon propellant. A high thrust-to-power operating mode was characterized over a wide parameter space with a maximum thrust-to-power ratio of at a specific impulse of for a ratio of ion cyclotron heating radio frequency power to helicon radio frequency power of .

Numerical Modeling of Electrodeless Electric Thruster by Ion Cyclotron Resonance/Ponderomotive Acceleration

We have studied ion cyclotron resonance/ponderomotive acceleration (ICR/PA) by making use of test particle simulations, and applied this concept to the next-generation electric thruster, as a part of the HEAT (Helicon Electrodeless Advanced Thrusters) project [1, 2]. The PA gives rise to the pure parallel acceleration of ions [3], while the ICR causes the perpendicular ion heating followed by the energy conversion from the perpendicular to the parallel directions in the presence of the divergent magnetic field. A well-known application of the ICR to the plasma thruster is the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) [4]. Here, we note that the PA and the ICR are inseparable, yet the former is preferred since it is less likely to be influenced by the ion-wall interactions (due to the smaller gyro radius). In this presentation, we focus on the ponderomotive effect on the ion acceleration. Based on a model that includes ion-neutral collisions and ion wall-loss, we estimate the exhaust velocity and the thrust. We discuss the thrust depending on various parameters such as propellant species, device size, magnetic field strength, and number densities of charged and neutral particles. Based on the numerical result, we develop the scaling laws of the exhaust velocity and the thrust. The results of our scaling laws will be compared with the experimental results of the VASIMR.

A review of the processing, composition, and temperature-dependent mechanical and thermal properties of dielectric technical ceramics

The current review uses the material requirements of a new space propulsion device, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR®) as a basis for presenting the temperature-dependent properties of a range of dielectric ceramics, but data presented could be used in the engineering design of any ceramic component with complementary material requirements. A material is required for the gas containment tube (GCT) of VASIMR® to allow it to operate at higher power levels. The GCT’s operating conditions place severe constraints on the choice of material. An electrically-insulating material is required with a high-thermal conductivity, low-dielectric loss factor, and high-thermal shock resistance. There is a lack of a representative set of temperature-dependent material property data for materials considered for this application and these are required for accurate thermo-structural modelling. This modelling would facilitate the selection of an optimum material for this component. The goal of this article is to determine the best material property data values for use in the materials selection and design of such components. A review of both experimentally and theoretically determined temperature-dependent and room temperature properties of several materials has been undertaken. Data extracted are presented by property. Properties reviewed are density, Young’s, bulk and shear moduli, Poisson’s ratio, tensile, flexural and compressive strength, thermal conductivity, specific heat capacity, thermal expansion coefficient, and the factors affecting maximum service temperature. Materials reviewed are alumina, aluminium nitride, beryllia, fused quartz, sialon, and silicon nitride.

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

H IGH-POWER electric propulsion thrusters can reduce propellant mass for heavy-payload orbit-raising missions and cargo missions to the moon and near-Earth asteroids, and they can reduce the trip time of robotic and piloted planetary missions [1–4]. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR®) VX-200 engine is an electric propulsion system capable of processing power densities on the order of 6 MW=m with a high specific impulse (4000 to 6000 s) and an inherent capability to vary the thrust and specific impulse at a constant power. The potential for a long lifetime is due primarily to the radial magnetic confinement of both ions and electrons in a quasi-neutral flowing plasma stream, which acts to significantly reduce the plasma impingement on the walls of the rocket core. High-temperature ceramic plasma-facing surfaces handle the thermal radiation: the principal heat transfer mechanism from the discharge. The rocket uses a helicon plasma source [5,6] for efficient plasma production in the first stage. This plasma is energized further by an ion cyclotron heating (ICH) RF stage that uses left-hand polarized slow-mode waves launched from the high field side of the ion cyclotron resonance. Useful thrust is produced as the plasma accelerates in an expanding magnetic field: a process described by conservation of the first adiabatic invariant as the magnetic field strength decreases in the exhaust region of the VASIMR [7–9]. End-to-end testing of the VX-200 engine has been undertaken with an optimum magnetic field and in a vacuum facility with sufficient volume and pumping to permit exhaust plumemeasurements at low background pressures. Experimental results are presented with the VX-200 engine installed in a 150 m vacuum chamber with an operating pressure below 1 10 2 Pa (1 10 4 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 argon 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 the argon plasma. A plasmamomentum flux sensor (PMFS)measures the force density as a function of radial and axial positions in the exhaust plume. New experimental data on ionization cost, exhaust plume expansion angle, thruster efficiency, and total force are presented that characterize the VX-200 engine performance above 100 kW. A semiempirical model of the thruster efficiency as a function of specific impulse has been developed to fit the experimental data, and an extrapolation to 200 kWdc input power yields a thruster efficiency of 61% at a specific impulse of 4800 s.

Characterization of Ion Cyclotron Resonance Acceleration for Electric Propulsion with Interferometery

This paper describes the experimental characterization of single-pass ion cyclotron resonance heating as applied to acceleration of ions for electric propulsion. A millimeter-wave interferometer system has shown to be a clear and simple method of quantifying ion acceleration due to ion cyclotron resonance heating. The experimental work was done on the VX-10 experiment of the variable specific impulse magnetoplasma rocket concept. The perpendicular velocity of the ions generated by ion cyclotron resonance heating was converted into axial velocity by the decreasing gradientoftheaxialmagnetic fieldattheexhaustofthepropulsionsystemfromconservationofthemagnetmoment. Thisincreaseinaxialvelocityispredictedtocauseadecreaseindensityduetoconservationofcurrentintheplasma. InterferometerdensitymeasurementsweretakenatthreedifferentlocationsontheVX-10experimentupstreamand downstream of the ion acceleration zone. A clear measurement of a 25% density drop for helium and a 40% density drop for deuterium was measured downstream of the ion resonance zone characteristic of ion acceleration.

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 × 1020 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 104 to 105 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 1015 m−3 and 2 × 10−5 Torr, respectively, in a 150 m3 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.

Magnet Configuration and Experimental Analysis of Helicon Source for Space Magnetoplasma Propulsion

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <?Pub Dtl=""?>The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is one of the high performance electric propulsion devices for future interplanetary applications. Helicon sources can produce high-density plasmas with high-ionization efficiency suitable for VASIMR, which usually use two electromagnets to produce the required magnetic field configuration. How to optimal design the solenoids and how to create the requisite B-field are the key factors to affect the characteristics and performance of the helicon plasma. In this paper, the magnets were designed and fabricated, and an experimental helicon source was presented. Two solenoids situated around the cylindrical quartz tube were used to create an expanding magnetic field of about 250 G in the center decreasing to a few tens of Gauss in the downstream diffusion region. The antenna was powered by a radio-frequency (RF) system of 13.56 MHz, which maximum changeable power is 500 W. Our helicon device was installed with vacuum chamber that is pumped down to a base pressure of about 0.001 Pa using cryopumps. Langmuir probe is mounted on to measure the plasma parameters. It is found that, when increasing the radio-frequency power or varying the magnetic field, both the plasma density and operating mode of helicon-wave ion are also changed. Permanent magnets (PM) and High temperature superconducting (HTS) magnets are also considered to use in the helicon source, and relative analyses are discussed. The details of further research are to investigate the relationship that the applied magnets affect on the efficiency and performance of the helicon plasma. </para>

Observations of single-pass ion cyclotron heating in a trans-sonic flowing plasma

The VAriable Specific Impulse Magnetoplasma Rocket (VASIMR®) is a high power electric spacecraft propulsion system, capable of Isp/thrust modulation at constant power [F. R. Chang Díaz et al., Proceedings of the 39th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 8–11 Jan. 2001]. The VASIMR® uses a helicon discharge to generate plasma. This plasma is energized by an rf booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. In the experiments reported in this paper, the booster uses 2–4 MHz waves with up to 50 kW of power. This process is similar to the ion cyclotron heating (ICH) in tokamaks, but in the VASIMR® the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The single-pass ICH produced a substantial increase in ion velocity. Pitch angle distribution studies showed that this increase took place in the resonance region where the ion cyclotron frequency was roughly equal to the frequency on the injected rf waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR®. This paper will review all of the single-pass ICH ion acceleration data obtained using deuterium in the first VASIMR® physics demonstrator machine, the VX-50. During these experiments, the available power to the helicon ionization stage increased from 3 to 20+ kW. The increased plasma density produced increased plasma loading of the ICH coupler. Starting with an initial demonstration of single-pass ion cyclotron acceleration, the experiments demonstrate significant improvements in coupler efficiency and in ion heating efficiency. In deuterium plasma, ≥80% efficient absorption of 20 kW of ICH input power was achieved. No clear evidence for power limiting instabilities in the exhaust beam has been observed.

Rockets for the Red Planet

The paper describes VASIMR (variable specific impulse magnetoplasma rocket), nuclear-electric rocket engine - a fission reactor with a plasma thruster that could potentially push people to Mars and back using a fraction of the propellant and time needed for a chemical rocket.

High Power Ion Heating in Helium and Hydrogen Plasmas for Advanced Plasma Thrusters

High power ion cyclotron resonance heating is performed in a fast-flowing plasma operated with hydrogen and helium gases. Ion heating is clearly observed in hydrogen plasma as well as in helium plasma. The resonance region of magnetic field is broader and wave absorption efficiency is higher in hydrogen plasma than those in helium plasma. The thermal energy of the heated ions is converted to the kinetic energy of the exhaust plume by passing through a diverging magnetic nozzle set in a downstream region. In the magnetic nozzle energy conversion occurred as keeping the magnetic moment constant, but some discrepancy was observed in larger gradient of magnetic field. The kinetic energy of the exhaust plume is successfully controlled by an input power of radio-frequency wave, which is one of the key technologies for the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) type plasma thruster.

Electromagnetic ion cyclotron resonance heating in the VASIMR

Plasma physics has found an increasing range of practical industrial applications, including the development of electric spacecraft propulsion systems. One of these systems, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, both applies several important physical processes occurring in the magnetosphere. These processes include the mechanisms involved in the ion acceleration and heating that occur in the Birkeland currents of an auroral arc system. Auroral current region processes that are simulated in VASIMR include lower hybrid heating, parallel electric field acceleration and ion cyclotron acceleration. This paper will focus on using a physics demonstration model VASIMR to study ion cyclotron resonance heating (ICRH). The major purpose is to provide a VASIMR status report to the COSPAR community. The VASIMR uses a helicon antenna with up to 20 kW of power to generate plasma. This plasma is energized by an RF booster stage that uses left hand polarized slow mode waves launched from the high field side of the ion cyclotron resonance. The present setup for the booster uses 2–4 MHz waves with up to 20 kW of power. This process is similar to the ion cyclotron heating in tokamaks, but in the VASIMR the ions only pass through the resonance region once. The rapid absorption of ion cyclotron waves has been predicted in recent theoretical studies. These theoretical predictions have been supported with several independent measurements in this paper. The ICRH produced a substantial increase in ion velocity. Pitch angle distribution studies show that this increase takes place in the resonance region where the ion cyclotron frequency is equal to the frequency on the injected RF waves. Downstream of the resonance region the perpendicular velocity boost should be converted to axial flow velocity through the conservation of the first adiabatic invariant as the magnetic field decreases in the exhaust region of the VASIMR. In deuterium plasma, 80% efficient absorption of 20 kW of ICRH input power has been achieved. No evidence for power limiting instabilities in the exhaust beam has been observed.

Behavior of a 14 cm Bore Solenoid With Multifilament ${\rm MgB}_{2}$ Tape

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The properties of ${\rm MgB}_{2}$ have the potential to make this material a viable solution for applications in which temperature, costs or weight are considered relevant constraints. In order to realize large scale applications, it is important to investigate the material, but also the winding process for ${\rm MgB}_{2}$ wires and tapes. In the literature small coils have already demonstrated central magnetic flux density above 2 Tesla, overcoming common winding problems related to ${\rm MgB}_{2}$ wires. In this current research, efforts are being made in order to improve the performance of solenoid coils, which are of particular interest for many applications, e.g. for space propulsion systems such as the VASIMR engine. A number of coils with ${\rm MgB}_{2}$ tapes are being built. In this paper we present results of the test of a 14 cm bore solenoid wound with 400 meters of multifilament, copper stabilized tape. The magnet was tested in a cryocooled vacuum chamber and it reached 175 A at 16 K with a central ${\rm B}_{0}$ of 1 Tesla. </para>

Development of Two Propulsion Systems with Helicon Plasma

Two space propulsion systems, which are called K2H (KBSI-KAIST-Hanyang University) and DiPS (Diversified Plasma Simulator) devices, are being developed in parallel to explore the space propulsion parameters and optimal helicon operation conditions with the concept of VASIMR (Variable Specific Impulse Magnetoplasma Rocket). Both devices utilize the open-ended magnetic configuration. K2H has three regions such as helicon source, ion cyclotron resonance heating (ICRH), magnetic nozzle and expansion regions. DiPS is the space plasma simulator and composed of three major sections: helicon plasma source, extraction region and space simulation region. Helicon plasmas are generated for both devices by 13.56 MHz rf power using M=+1 right-helical antenna at pressure of several mTorr. Initial plasma parameters such as density, temperature, and drift velocity were measured by a laser induced fluorescence (LIF) system and a fast scanning electric probe system with an rf-compensated Langmuir probe and a Mach probe at ICRH and magnetic nozzle region. The results are given as follows: plasma density n = 1011 - 1013 cm-3 (K2H) and 1012 - 1013 cm-3 (DiPS), electron temperature Te = 3 - 9eV (K2H) and 2 - 4 eV (DiPS), ion temperature Ti = 0.144 - 0.164 eV (K2H), and drift velocity vd = 0.8 - 1.55 km/s (K2H). A simple analysis of the results is provided.

Mission design for Human Outer Planet Exploration (HOPE) using a magnetoplasma spacecraft

To send humans beyond Mars, a Human Outer Planet Exploration (HOPE) mission has been studied for new spacecraft concepts and technologies. In this paper, an interplanetary trajectory and a preliminary spacecraft design are presented for the HOPE visit to Callisto, one of Jupiter's moons. To design a round-trip trajectory for the mission, the characteristics of the spacecraft and its trajectories are analyzed. A detailed optimization approach is formulated to utilize a Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine with capabilities of variable specific impulse, variable engine efficiency, and engine on–off control. It is mainly illustrated that a 30 MW powered spacecraft can make the mission possible in a 5-year round trip constraint around the year 2045. Trajectories with different power and reactor options are also discussed. The results obtained in this study can be used for formulating an overall concept for the mission.

Momentum and Heat Flux Measurements Using an Impact Target in Flowing Plasma

An impact target plate was used to determine the momentum and heat flux quantities from a helicon plasma source. The plasma source is the first stage of the Variable Specific Impulse Magnetoplasma Rocket, which uses radio frequency waves to create and energize a flowing plasma. The momentum and heat flux quantities are determined separately and agree favorably. They also agree with expected flux quantities calculated from the plasma parameters determined by other probes. The momentum and heat flux quantities were obtained at two axial locations in the exhaust or magnetic nozzle region of the device using helium and argon propellant. Only data from the plasma source are used, which shows that the plasma source does produce a flowing plasma with significant momentum. The flux data at the two axial locations verify the increase in momentum as the plasma flows through the diverging magnetic field in the magnetic nozzle region. Although the axial location for maximum momentum (or thrust) was not determined, forces of several millinewtons were measured for a combined neutral particle and plasma flow. The ionization fraction for these data was approximately 22 %. Neutral particles entrained in the flow provided a significant part of the flow momentum thereby contributing to the potential thrust. The technique of measuring the momentum flux and heat flux in a flowing plasma is described.

Plasma propulsion for interplanetary flight

The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high-power propulsion concept using radio waves to create and accelerate plasma in a magnetic nozzle. Important features are its high and variable exhaust velocity, which greatly enhances performance. A NASA-led, research team is developing this technology in the U.S. Recent advances include demonstration of efficient propellant utilization in its helicon plasma source and experimental verification of high ion acceleration through ion cyclotron wave resonance absorption, as predicted by theory. This paper describes the physics and engineering of VASIMR, reviews recent progress and discusses its application in a piloted Mars mission architecture.

Focusing magnetic field contribution for helicon plasma on Mini-RFTF

The Mini-Radio Frequency Test Facility (Mini-RFTF) has contributed to helicon plasma source optimization for the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). In this paper, we report how the source, using hydrogen and helium, can be optimized for non-uniform geometries that are constrained to be suitable for VASIMR. The behavior of the density transition into high-density helicon operation was found to significantly depend on the magnetic field geometry. Operation indicates that a strongly focused magnetic geometry produces a continuously increasing density with increasing net power. For helium operation, mirror confinement effects possibly explain the observed plasma sustainment on upstream and downstream for several magnetic field geometries.