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 calculat ion. The simulation results correlate with experimental data on the p lasma loading and predict higher ICRH performance for a higher density plasma target. These simulations assist in optimizing the ICRF anten na so as to achieve higher VASIMR efficiency

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

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140428/1/6.2014-1422.pd

Prediction of Damage to Structure resulting from Recirculation of Particles from a Magnetoplasma Spacecraft Engine

A magnetoplasma spacecraft engine, such as the Variable Area Specific Inpulse Magnetoplasma Rocket (VASIMR®), uses magnetic fields and a magnetic nozzle to constrict and accelerate plasma to produce thrust. Most of the ejected plasma particles are expected to detach from the magnetic field lines and escape to provide thrust but some particles may not and could impact the spacecraft structure resulting in surface erosion and electrical charging. The plasma plume for a magnetoplasma engine was modeled computationally and scaled to determine what percentage of particles remained in the magnetic field and the kinetic energy of all impacting particles. Factors such as average particle velocity at the engine exit, magnetic field strength, and plume density distribution (i.e. width) were varied in a full factorial experiment to ascertain the effects of each factor and the important inter-relationships. The results are presented for a generic magnetoplasma engine and for the specific VASIMR® case. Detachment was found to be occurring with 99.42% of particles escaping under the worst conditions and only 0.0172% of particles impacting structure. It was determined that three things led to an increase in the number of impacting particles on spacecraft structure: a stronger magnetic field, a lower exit velocity of particles into the plume, and a wider plume. In addition, there was an “erosion zone” where an increasing particle exit velocity led to more erosion until the number of impacting particles was negligible and erosion dropped significantly. For the specific case under nominal conditions, the erosion rate was 1.386 nm/month of engine operating time on aluminum and 0.611 nm/month on silicon. The electrical charging on spacecraft surfaces was found to be -27.85 V DC, which can be mitigated with current plasma contactor technology or some variant. Therefore, magnetoplasma spacecraft engines can be shown to cause minimal erosion and electrical charging and should be capable of operating safely with current technology by varying the three parameters previously mentioned

Characterization of the Magnetic Nozzle Region of High Powered Electric Propulsion Thrusters Using Numerical Simulation, RF Interferometry and Electrostatic Probes.

Experimental results are presented from the plume of a high powered (200 kW) DC plasma gun emitting into an applied magnetic nozzle. The plasma source operated on helium and hydrogen and was attached to a large (3 m x 5 m) vacuum chamber kept at low background pressure ( 1 in accordance with magnetic detachment theory. Unique accomplishments of this research include detailed measurements of propulsion-appropriate plasmas exiting a magnetic nozzle and transitioning from Beta 1. This region is of particular interest for magnetized plasma thrusters since inefficient magnetic detachment may result in a serious efficiency penalty for their use in proposed in-space propulsion systems. Nozzle efficiency estimates are provided based on simulated and measured experiment conditions. In particular, an optimized magnetic nozzle condition is found that theoretically improves nozzle efficiency by 10% over the standard magnetic dipole condition. Plasma diagnostics are utilized, including microwave interferometers and Langmuir triple probes. Diagnostic theory is reviewed for these tools, specifically for the conditions found in this experiment. Prior theory was sometimes found inapplicable to the experimental conditions, particularly in the case of a Langmuir triple probe in a flowing plasma. To make up for inadequacies in standard theory, numerical simulations were conducted to find calibration factors for the appropriate experimental conditions. In addition, a new measurement methodology is developed utilizing electrostatic probes and microwave interferometers in tandem. Detailed density profiles were collected using this method, and a comprehensive error analysis was conducted. The error in density measurements was determined to be much lower than the error in electrostatic probe measurements, and on the order of microwave interferometer uncertainty – as low as 10%.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60653/1/cdeline_1.pd

Experimental Characterization of Plasma Detachment from Magnetic Nozzles

Magnetic nozzles, like Laval nozzles, are observed in several natural systems and have application in areas such as electric propulsion and plasma processing. Plasma flowing through these nozzles is inherently tied to the field lines and must separate for momentum redirection or particle transport to occur. Plasma detachment and associated mechanisms from a magnetic nozzle are investigated. Experimental results are presented from the plume of the VASIMR® VX-200 device flowing along an axisymmetric magnetic nozzle and operated at two ion energies to explore momentum dependent detachment. The argon plume expanded into a 150m3 vacuum chamber where the background pressure was low enough that charge-exchange mean-free-paths were longer than experiment scale lengths. This magnetic nozzle system is demonstrated to hydrodynamically scale up to astrophysical plasmas, particularly the solar chromosphere, implying general relevance to all systems. Plasma parameters were mapped over a large spatial range using measurements from multiple plasma diagnostics. The data show that the plume does not follow the magnetic field lines. A mapped integration of the ion flux shows the plume may be divided into three regions where 1) the plume briefly follows the magnetic flux, 2) diverges quadratically before 3) expanding with linear trajectories. Transitioning from region 1→2, the ion flux departs from the magnetic flux suggesting ion detachment. An instability forms in region 2 driving an oscillating electric field that causes ions to expand before enhancing electron cross-field transport through anomalous resistivity. Transitioning from region 2→3 the electric field dissipates, the trajectories linearize, and the plume effectively detaches. A delineation of sub-to-super Alfvénic flow aligns well with the inflection points of the linearization without a change in magnetic topology. The detachment process is best described as a two part process: First, ions detach by a breakdown of the magnetic moment when the quantity |v/fcLB| becomes of order unity. Second, the turbulent electric field enhances electron transport up to a factor of 4±1 above collisional diffusion; electron cross-field velocities approximate that of the ions and depart on more centralized field lines. Electrons are believed to detach by breakdown of magnetic moment further downstream in the weaker magnetic field

Physics of RF heating systems on proto-MPEX

Realizing controlled fusion as a commercial energy source is faced with many challenges. One of the main challenges being the development of Plasma Facing Components (PFC) that can survive the extreme environment encountered in a fusion reactor. To expedite the testing and development of PFCs Oak Ridge National Laboratory (ORNL) is building the Materials Plasma Exposure eXperiment (MPEX), which is a linear device purposed specifically for studying Plasma Material Interactions (PMI). Current linear devices cannot produce plasmas with fusion divertor relevant electron and ion temperatures and instead rely on electrostatic biasing of the target to simulate the relevant ion energies. This methodology inhibits studying the interaction of the eroded material and recycled neutral gas with a fusion relevant divertor plasma and does not properly simulate the angular energy distribution of the ion fluxes, therefore, PMI studies on these linear devices omit a vast amount of rich physics important to PFC development. MPEX will enable the study of fusion relevant PMI by producing fusion divertor relevant plasma conditions in front of a target station using RF technology. Proto-MPEX is the device that is currently operating at ORNL, where the viability of this RF technology is being demonstrated. The electron density, electron temperature, and ion temperature of the target plasma will be controlled independently with separate RF heating systems. This thesis focuses on the electron density production system and the ion heating systems on Proto-MPEX and their viability for MPEX. The electron density production on Proto-MPEX is accomplished by a helicon plasma source. Helicon plasma sources have been shown to efficiently produce high-density plasmas for a relatively low amount of RF power. Efficient electron density production of helicon plasma sources in light ions is hypothesized to be enabled by strong core power deposition when the plasma conditions allow for the formation of helicon normal modes. Experimental evidence supporting this hypothesis is presented in the form of B-dot probe and IR camera measurements, showing the increase of on-axis RF magnetic field strength and the formation of eigenmode structure concurrently with an increase in core power deposition at the expense of power deposition in the periphery of the plasma column. An RF full-wave model of the helicon region is made, which predicts the formation of cavity-like structures of the RF magnetic field when core power deposition is increased. Next, the problem that Proto-MPEX’s helicon source has been shown to not operate efficiently at higher magnetic field strengths is addressed. The hypothesis that the power balance does not allow enough density production for the helicon antenna to sustain a mode of operation that enhances core power deposition is tested by coupling a 2D axisymmetric full-wave simulation of the helicon antenna to a volume integrated 0D power/particle balance. This model is compared to experimental measurements of electron density and shows that there is a decrease in electron density production due to a decrease in core power coupling in the region where electron density decreases on Proto-MPEX. The model shows that if the Proto-MPEX helicon plasma source is operated at even higher magnetic fields strengths than efficient electron density production is recovered. Finally, the performance of Proto-MPEX is compared with other experimental devices by calculating the ionization cost of the plasma source, which shows that improvements to efficiency can practically be achieved to match the ionization cost of other plasma sources. Experimental improvements to the helicon source region are suggested and quantified with the couple RF and particle/power balance model. The ion heating on Proto-MPEX is accomplished by ion cyclotron heating via the beach heating technique. This technique is expected to increase ion temperatures on Proto-MPEX to values of Ti = 20 eV or more. The beach heating technique has been successfully demonstrated on previous devices, however, these devices were operating at much lower electron densities than Proto-MPEX. A theoretical route to core ion heating is first explored. At magnetic field strengths near the ion cyclotron resonance, the higher electron density in Proto-MPEX brings the existence of the Alfv ́en resonance into the Proto-MPEX plasma. This layer acts to cut-off the cold plasma slow wave, called the inertial Alfv ́en wave for the high-density core plasma. On the high-density side of the Alfv ́en resonance, the kinetic Alfv ́en wave can propagate when the electron temperature allows. In Proto-MPEX this wave is thought to be responsible for the core heat- ing of ions in the device. A simplified kinetic plasma tensor is implemented in COMSOL to simulate the propagation of this wave and to show that at Proto-MPEX relevant conditions this wave is responsible for core heating of the ions. Next, experimental evidence for core ion heating is presented in the form of ion temperature and target heat flux measurements. However, the core heating is shown to transiently cool, which is proposed to be due to the charge exchange with neutral gas born from plasma recombination at the target. When the target material is changed from carbon to stainless steel, the heat flux at the target reaches an increased steady state and higher ion temperatures are measured throughout the plasma column. The axial peak of the ion temperature is also located closer to the target for the case of the stainless steel target. These phenomena are hypothesized to be due to the increased reflection coefficient of the target material, and a model for quantifying this hypothesis shows that the flux of energetic neutral particles born from the reflection of sheath accelerated ions could explain these observations. Finally, experimental optimization of the magnetic field in the ICH region is shown with COMSOL simulations showing good agreement with these experimental results. The numerical simulations are then used to explore the parameter space of driving frequency, antenna length, and distance of the antenna to the ion cyclotron resonance on the predictions of heating in the ion cyclotron region

Development of Electromagnetic Codes to Analyze and Optimize Satellite Propulsion Systems and Communication Antennas

Satellite communication systems have demonstrated their essential role providing timely services for disaster management in a variety of distress situations. Their effectiveness requires high mapping and pointing accuracy in terms of displacement capability, and high gain, high bandwidth, directional, and reconfigurable antennas in terms of communication capability. A Helicon plasma thruster, and an enhanced communication system meet the aforementioned requirements. The former is an electric plasma-based propulsion system that provides an high accuracy attitude control, while the latter could be either an optimized state-of-the-art antenna or an innovative concept based on plasma antennas. In this research work, several computationally efficient codes have been developed to analyze, design and optimize the helicon plasma thruster, and the antenna for an enhanced communication system. The present work progresses starting from the definition of the requisites, and continues to describe the innovative numerical methods: the SPIREs finite-difference frequency-domain electromagnetic solver for magnetized plasma cylinders; the WAVEQM equilibrium condition solver for radiofrequency heated plasmas; the PARTYWAVE particle in cell code for cylindrical geometries, and the Moment Method for antenna design. Their numerical accuracy has been verified, and they have been validated against physical cases

The impact of magnetic geometry on wave modes in cylindrical plasmas

Both space and laboratory plasmas can be associated with static magnetic field, and the field geometry varies from uniform to non-uniform. This thesis investigates the impact of magnetic geometry on wave modes in cylindrical plasmas. The cylindrical configuration is chosen so as to explore this impact in a tractable but experimentally realisable configuration. Three magnetic geometries are considered: uniform, focused and rippled. For a uniform magnetic field, wave oscillations in a plasma cylinder with axial flow and azimuthal rotation are modelled through a two-fluid flowing plasma model. The model provides a qualitatively consistent description of the plasma configuration on a Radio Frequency (RF) generated linear magnetised plasma (WOMBAT, Waves On Magnetised Beams And Turbulence [Boswell and Porteous, Appl. Phys. Lett. 50, 1130 (1987)]), and yields agreement between measured and predicted dependences of the wave oscillation frequency with axial field strength. The radial profile of the density perturbation predicted by this model is consistent with the data. Parameter scans show that the dispersion curve is sensitive to the axial field strength and the electron temperature, and the dependence of the oscillation frequency with electron temperature matches the experiment. These results consolidate earlier claims that the density and floating potential oscillations are a resistive drift mode, driven! by the density gradient. This, to our knowledge, is the first detailed physics modelling of plasma flows in the diffusion region away from the RF source. For a focused magnetic field, wave propagations in a pinched plasma (MAGPIE, MAGnetised Plasma Interaction Experiment [Blackwell et al., Plasma Sources Sci. Technol. 21, 055033 (2012)]) are modelled through an ElectroMagnetic Solver (EMS) based on Maxwell's equations and a cold plasma dielectric tensor. [Chen et. al., Phys. Plasmas 13, 123507 (2006)] The solver produces axial and radial profiles of wave magnitude and phase that are consistent with measurements, for an enhancement factor of 9.5 to the electron-ion Coulomb collision frequency and a 12% reduction in the antenna radius. It is found that helicon waves have weaker attenuation away from the antenna in a focused field compared to a uniform field. This may be consistent with observations of increased ionisation efficiency and plasma production in a non-uniform field. The relationship between plasma density, static magnetic field strength and axial wavelength agrees well with a simple theory developed previously. More! over, the wave amplitude is lowered and the power deposited into the core plasma decreases as the enhancement factor to the electron-ion Coulomb collision frequency increases, possibly due to the stronger edge heating for higher collision frequencies. For a rippled magnetic field, the spectra of radially localised helicon (RLH) waves [Breizman and Arefiev, Phys. Rev. Lett. 84, 3863 (2000)] and shear Alfvén waves (SAW) in a cold plasma cylinder are investigated. A gap-mode analysis of the RLH waves is first derived and then generalised to ion cyclotron range of frequencies for SAW. The EMS is employed to model the spectral gap and gap eigenmode. For both the RLH waves and SAW, it is demonstrated that the computed gap frequency and gap width agree well with the theoretical analysis, and a discrete eigenmode is formed inside the gap by introducing a defect to the system's periodicity. The axial wavelength of the gap eigenmode is close to twice the system's periodicity, which is consistent with Bragg's law, and the decay length agrees well with the analytical estimate. Experimental realisation of a gap eigenmode on a linear plasma device such as the LArge Plasma Device (LAPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] may be possible by introducing a symmetry-breaking defect to the system's periodicity. Such basic science studies could provide the possibility to accelerate the science of gap mode formation and mode drive in toroidal fusion plasmas, where gap modes are introduced by symmetry-breaking due to toroidicity, plasma ellipticity and higher order shaping effects. These studies suggest suppressing drift waves in a uniformly magnetised plasma by increasing the field strength, enhancing the efficiency of helicon wave production of plasma by using a focused magnetic field, and forming a gap eigenmode on a linear plasma device by introducing a local defect to the system's periodicity, which is useful for understanding the gap-mode formation and interaction with energetic particles in fusion plasmas

Two-Photon Absorption Laser Induced Fluorescence Measurements of Neutral Density in Helicon Plasma

Neutral particles play a critical role in plasma experiments. They simultaneously act as a source of particles and a sink of momentum and energy. Also, they act as an intermediary between the plasma and surrounding material walls. However, few methods exist to make localized, direct neutral density measurements. A new diagnostic, based on two-photon absorption laser induced fluorescence (TALIF), capable of making direct ground state measurement is developed. A high intensity (5 MW/cm2), narrow bandwidth (0.1 cm -1) laser is used to directly probe the ground state of neutral hydrogen, deuterium and krypton. The diagnostic represents an improvement over traditional laser induced fluorescence in that it can be used to measure the spatial profile from a single porthole, and it can be operated in a Doppler-free configuration. The system is tested and calibrated in a low temperature helicon plasma source. Calculations show the system has sufficient performance to achieve an acceptable signal-to-noise ratio in a high temperature fusion plasma. Development of the diagnostic is presented in this work along with both steady-state and time-resolved TALIF measurements in helicon plasma