Warner C. Meeks

On the delayed gas breakdown in a ringing theta-pinch with bias magnetic field

A single particle model and particle-in-cell simulations are used to elucidate the breakdown physics in a ringing theta-pinch with a bias magnetic field. Previous experimental results show that gas breakdown occurs when the bias magnetic field is nullified by the theta-pinch magnetic field. The analyses presented here agree with the experimental results and show that electron kinetic energy does not exceed the ionization threshold of deuterium until the net magnetic field is approximately zero. Despite the presence of a strong electric field, the gyromotion of electrons within the bias magnetic field prevents them from gaining energy necessary to ionize the gas. Parametric analysis of the peak electron energy as a function of the bias and pre-ionization magnetic fields reveals that: (1) when the bias magnetic field is ≈97% of the pre-ionization magnetic field, peak electron energies are highly erratic resulting in poor overall ionization, and (2) full ionization with repeatable behavior requires a pre-ion...

Magnetic Field Mapping of a Field Reversed Configuration Test Article

Devices that form and accelerate field reversed configuration plasma may potentially be applied to spacecraft propulsion. Propulsion applications require heavy-gas plasma and the fundamental processes for heavy-gas field reversed configuration formation are still not well understood. Pre-ionization plasma properties are known to influence the success and final properties of field reversed configuration formation. In the following study the magnetic field of the pre-ionization stage of a heavy-gas field reversed configuration test article is presented. Initial results show discharge frequencies increase in the presence of plasma from 440 kHz in atmosphere discharges with no plasma to 472 kHz in 33 mTorr of air with plasma, both at an initial charge of 15 kV. Calibration of a three-axis magnetic field probe is completed using EMC Studio. Calibration values for the axial and azimuthal components of the probe are 4.66x10 and 9.45x10 G/V, respectfully. Magnetic field measurements at 15 and 20 kV are presented. The 15 and 20 kV discharges produce a peak current of 38 and 50 kA, respectfully. EMC simulations using these peak current values produce a maximum axial magnetic field of 632 and 819 G, respectfully. Measured axial magnetic field strengths of MPX at 15 and 20 kV using the B-dot probe yield 640 and 885 G, respectfully.

Investigation of Pre-Ionization Characteristics in Heavy Gas Pulsed Inductive Plasmas via Numerical Modeling

A globally-averaged, pulsed inductive plasma model is reproduced and utilized to investigate pre-ionization conditions for a pulsed inductive plasma accelerator. Attention is given to better quantifying the formation and energy conversion/loss processes associated with the pre-ionization stage. Simulations are completed for different power input pulse duration, seed plasma density, and total input energy. Results are analyzed based on the ion energy fraction and peak ion density. Ion energy fraction is the percentage of total input energy contained in ionization. Analysis shows that reducing pulse duration from 10-6 to 10-7 seconds increases ion energy fraction by 16.5%. Reducing pulse duration further to 10 seconds increases ion energy fraction only another 2.5%. The optimum pulse duration from these simulations is 200 ns because this duration maximizes both ion energy fraction and peak ion density. Results show that a low seed plasma density, less than 10 14 m-3, yields the highest ion energy fraction of 40%. Increasing seed plasma density above 10 m increases peak ion density but causes a corresponding decrease in ion energy fraction. Increasing total energy deposition from 5 to 160 mJ increases ion energy fraction from 33 to 58% at a 200 ns pulse duration. However this increase is not linear, but has a diminishing return with ion energy fraction plateau estimated to be 65%.

Time-Resolved Electron Temperature in an Argon Theta Pinch by Line Ratio Methods

Pulsed inductive argon plasma in an 80-J, 15-kV, 490-kHz theta pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained by coupling line intensity ratios with steady-state corona and collisional-radiative (CR) models. Neutral excited state argon transitions from the 2p to 1s subshells were utilized. The backfill pressures of 50 and 100 mtorr are of primary focus. Time-resolved electron temperature estimates are presented ranging from 1 to 11.1 eV for the corona model up to an excess of 80 eV for the CR model. Near-IR spectral emission is seen to increase rapidly near the first zero crossing of the oscillating discharge current while electron temperature increase lags by roughly one full discharge cycle later near the third zero crossing. An analysis of long exposures provides an account of substantial second-order diffracted spectra. Weak spectral signal quality for the short exposures of 0.25-μs yielded time-resolved spectral intensity trend lines of a low accuracy with an average percent difference of 69% between raw data and trend lines.

Time-resolved argon theta-pinch plasma properties by line ratio method with collisional-radiative model

Summary form only given. Spectral data of a cylindrical theta-pinch pulsed inductive plasma are compared to collisional-radiative (CR) model simulations to determine time-averaged and time-resolved plasma properties. Argon data are analyzed using a CR model that includes the 14 lowest excited neutral states of argon. The model also takes into account a measure of opacity of the plasma to its own radiation and predicts spectral line intensity ratios in the near-IR band. Because of the greater level of detail in which argon has been studied and availability of argon excited state collisional information, an extensive ratio cross-point method1 is permissible yielding both electron temperature and electron density estimates as well.During an inductive discharge, circuit energy is converted into plasma energy modes via a large number of plasma processes. Dependent upon the application, only a few of these processes produce species or phenomena of use to the end-goal of the application. Though pulsed inductive plasma devices such as the theta-pinch have been studied extensively in recent decades, minimal information is available quantifying the energy conversion and loss mechanisms involved in initial or subsequent (if the device is of an oscillatory nature) plasma breakdown. Parallel to this is the ever-expanding utilization of pulsed inductive plasmas, which have highlighted the need for better understanding of the plasma thermodynamics. The main objective of this research then is to shed light on these early time energy modes and their dependence on operating pressure and gas species. Time-averaged and time-resolved electron temperatures and electron densities will be estimated for four different operating pressures of 10, 30, 50, and 100 mTorr and correlated with coil current profiles. Comparison of argon results with previously reported xenon data as well as to literature data of similar devices will be provided along with error analysis. The experimental test article is a 0.76 m long by 15.5 cm diameter copper theta-pinch coil that discharges at 460 kHz in vacuum at 15 kV with a total energy of 80 Joules. Spectral data are acquired with a Princeton Instruments ST-133 spectrometer with PI-MAX iCCD camera.

Time-resolved temperature in an argon theta-pinch by line ratio methods

Pulsed inductive argon plasma in an 80 J, 15 kV, 490 kHz theta-pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained with line intensity ratios coupled with corona and collisional-radiative models. Neutral excited state argon transitions from the 2p to 1s sub-shells were utilized. Backfill pressures of 50 and 100 mTorr are of primary focus here. Time-resolved electron temperature estimates range from 2.4 to 8.6 eV for the steady-state corona model analysis while the collisional-radiative analysis predicts temperature in excess of 80 eV. Electron temperature can be seen to ramp up between approximately the first and third zero-crossings of the oscillating discharge current. Analysis of long exposures reveal substantial second order diffracted spectra. Low spectral signal quality for short exposures of 0.25 μs yield poor time-resolved spectra and ultimately elucidate the fundamental time-resolution limitation of this experimental test article.

PPPS-2013: Optical emission spectroscopy of initial plasma formation in a heavy gas theta pinch coil

Summary form only given. Pulsed inductive plasma devices such as the common theta-pinch have become a standard high energy plasma source in research and industry. Recent pulsed inductive plasmas currently being investigated by the fusion and space propulsion communities utilize deuterium and xenon, respectively, and have provided promising new results. However, little has been done to better understand the energy conversion processes during early plasma formation times (i.e., during the initial inductive coupling). The broad efforts of this research are to elucidate the electric-to-particle energy conversion processes during initial plasma formation over time scales of 10-8 to 10-6 seconds. In this work an analysis of spectral emission data is performed on a theta pinch test article intended for use in field reversed configuration (FRC) studies. Testing is performed on a pulsed xenon plasma at energies of around 80 joules, neutral back-fill pressures of 10-2 Torr, and an RLC discharge frequency of 500 kHz. Efforts are paralleled by magnetic field studies (B-dot probes, flux loops) of the same experiment. Using a collisional-radiative model previously developed for analysis on xenon Hall effect thrusters, line emission intensity ratios are used to approximate electron temperatures independent of plasma density. A Princeton Instruments SP2300i spectrometer with PI-MAX 1024×1024 pixel iCCD camera is used with gate times of 10-9 to 10-8 seconds and variable delay to allow for time-resolved spectral data.

Optical emission spectroscopy of plasma formation in a xenon theta pinch

Analyses of xenon spectral emission data in the IR range from excited neutral xenon transitions and estimations of electron temperature are performed on a theta-pinch test article. Estimations are based on a collisional-radiative model originally written for Hall-effect thrusters utilizing apparent collisional cross-sections. Tests performed on a pulsed xenon plasma at an energy of 80 J, neutral back-fill pressures of 10-100 mtorr, and vacuum discharge frequency of 462 kHz yield time-averaged electron temperatures of 6.4-11.2 eV for spectra integrated over the entire 20 μs. Time-resolved Te estimations are done using charge coupled device gate widths of 0.25 μs and yield estimates of up to 68 eV during peak spectral activity. Results show that back-fill pressures of 30 and 50 mtorr appear to generate plasma earlier and remain cooler than 10 and 100 mtorr. Poor signal-to-noise ratios produce substantial fluctuation in time-resolved intensities and thus estimation errors, while not quantified here, are assumed high for the time-resolved studies. Additionally, spectra acquired in the UV band verify: 1) the presence of second-order diffraction in the near-IR band from singly ionized xenon transitions and 2) the absence of air (contaminant) spectra.

Preliminary findings from efforts to model pulsed inductive theta-pinch plasmas via Particle-In-Cell

A method is pursued to approximately model the electron energy distribution of pulsed inductive plasma devices with Particle-In-Cell code to elucidate formation physics during early times (t < 1 μs). Specifically, reported results from AFRL-Kirtlands pulsed inductive device, FRCHX, are used as a test case to validate results. An r-z slab approximation is outlined and gyro-frequency, Larmor radius, and E×B guiding center drift are verified against theory to within 1% difference. The analyses presented, using both single electron and Particle-In-Cell modeling, agree with FRCHX reported results by showing that average electron kinetic energy does not exceed the ionization threshold of 15.47 eV for gaseous deuterium until after the first ¼ cycle of the ringing pre-ionization stage (when net magnetic field is approximately nullified). These results provide supportive evidence for the concept that bias field actually inhibits ionization if improperly implemented.

Numerical and experimental efforts to explain delayed gas breakdown in 0-pinch devices with bias magnetic field

A single particle model and particle-in-cell simulations have been used to elucidate the breakdown physics in a ringing theta-pinch with a bias magnetic eld. Previous experimental results show that gas breakdown occurs when the bias magnetic eld is nulli ed by the theta-pinch magnetic eld. The analyses presented here agree with the experimental results and show that electron kinetic energy does not exceed the ionization threshold of deuterium until the net magnetic eld is approximately zero. Despite the presence of a strong electric eld, the gyromotion of electrons within the bias magnetic eld prevents them from gaining energy necessary to ionize the gas. Parametric analysis of the peak electron energy as a function of the bias and pre-ionization magnetic elds reveals that: (1) when the bias magnetic eld is 97% of the pre-ionization magnetic eld, peak electron energies are highly erratic resulting in poor overall ionization, and (2) full ionization with repeatable behavior requires a pre-ionization to bias magnetic eld ratio of approximately 2 to 1 or higher. E orts to better characterize this phenomena experimentally are ongoing. However some preliminary ndings using a dual-probe cancellation technique are presented.