Ryan A. Pahl

Pre-Ionization Plasma in an FRC Test Article

Pulsed plasma systems, specifically field reversed configuration devices, show great potential for future space propulsion systems. The fundamental formation process of heavygas plasma necessary for propulsion application is not well understood. The following study is focused on characterizing the pre-ionization stage of a field reversed configuration test article. Specifically, flux loop and B-dot probe data are presented and used calculate the magnetic flux and magnetic field strengths within MPX. Finally, plasma images collected with a high speed camera are presented. Tests are conducted at 15 and 20 kV using both air and argon over a pressure range of 2 – 70 mTorr and at atmosphere. Discharges with no plasma present have a frequency of 464 kHz while a discharge that produces plasma resonates at approximately 490 kHz. Details of probe construction and calibration are also presented. A maximum magnetic field of 882 G is observed for a 20 kV discharge with fill pressure of 45 mTorr of air near the edge of the theta coil. A maximum magnetic flux of 2.17 mWb is observed for a 20 kV discharge with a fill pressure of eight mTorr. The largest amount of energy absorbed by the plasma is 30.3 J and occurs during a 20 kV discharge in air at 60 mTorr. Peak magnetic fields, magnetic flux, and formation times as functions of gas species, pressure, and voltage are also presented. Maximum energy absorption into plasma occurs at 65 mTorr of air at a discharge voltage of 20 kV.

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.

Calibration of magnetic field probes at relevant magnitudes

Difficulty driving large currents through an inductive load at high frequency typically results in field magnitudes of a few microTesla or less. The calibration factor is then necessarily assumed linear, even though the magnetic field of the primary experiment is several orders of magnitude larger than the field magnitude used to calibrate the probe. In this work calibration factors of two differential configuration magnetic field probes are presented as functions of frequency and field magnitude. Calibration factors are determined experimentally using a 80.4 mm radius Helmholtz coil in two separate configurations. A conventional low-magnitude calibration using a network analyzer with field magnitude of 158 nT yielded calibration factors of 15,107±233 and 4,899±180 T/V-s at 457 kHz for the surface mounted inductor and hand-wound probe, respectively. A relevant-magnitude calibration using a pulsed-power setup with field magnitude of 43.5 to 83.0 mT yielded calibration factors of 14,541±41.8 and 4,484±15.8 T/V-s at 457 kHz for the surface mounted inductor and hand-wound probe, respectively. The uncertainty reported is one standard deviation of the repeated calibration measurement. Low-magnitude calibration always resulted in a larger calibration factor, with a maximum difference of 18.5%. Comparison of the pulsed-power Helmholtz coil current waveform with the magnetic field waveform measured with the magnetic probes showed differences of 1.4% and 0.7% in the waveform extrema at 457 kHz for the surface mounted inductor and hand-wound probe, respectively.

Energy Analysis of a Pulsed Inductive Plasma Through Circuit Simulation

Current profiles of a cylindrical ringing thetapinch are compared with SPICE simulations of an established circuit model and a least squares estimate is performed to determine plasma resistance and inductance for argon, hydrogen, and xenon plasmas with prefill pressures ranging from 10 to 100 mTorr. Plasma resistance is found to vary from 25.8 to 51.6 mQ with the lowest resistance occurring at 10 mTorr. Argon and xenon follow a similar trend with the xenon resistance averaging 4.2-mΩ (12.3%) larger than argon from 40 to 100 mTorr. Hydrogen resistance is found to increase rapidly as prefill pressure increases above 40 mTorr. Calculated plasma resistivity of 214-429 Ω-μm agrees with established literature. Plasma inductance varies from 41.3 to 47 nH and is minimized at 30 mTorr for argon and hydrogen, whereas xenon inductance is minimized at 20 mTorr. Hydrogen yields the highest inductance, averaging 1.9 nH (4.5%) more than argon over the pressure range tested. Temporal evolution of the energy partitioning into capacitive, inductive, and resistive loads is presented. Plasma inductive energy is found to be maximized when discharge current reaches its peak negative value of -23.5 kA. Xenon shows the greatest amount of inductive energy storage with a peak of 6.4 J (8.1%) of the initial 79.2 ± 0.1 J while argon dissipates the least energy through ohmic losses at most pressures. Hydrogen has the least inductive energy storage at all pressures and greatest ohmic losses above 60 mTorr. Xenon presents the largest ohmic losses over the 10-60-mTorr range.

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%.

Effects of DC Preionization Voltage and Radial Location on Pulsed Inductive Plasma Formation

The effects of dc preionization (PI) voltage and radial location on plasma formation repeatability are presented for argon prefills of 20-200 mtorr with a discharge energy of 79.5 J at 15 kV. Current profiles of a ringing theta-pinch are compared with circuit simulation in SPICE to estimate plasma resistance and inductance and quantify plasma formation uncertainty. Plasma thickness is calculated using axial imaging and experimental geometry and used to determine the mutual inductance coupling of the plasma and the theta-pinch coil. At all pressures tested, plasma formation failed to occur in the absence of dc PI. At pressures less than 100 mtorr, PI voltage has a significant impact on plasma formation, repeatability, and energy coupling into the plasma. At 20 mtorr, 0.20 W of dc power is sufficient to stabilize plasma formation at the first zero-crossing of the current. With 1.5 W, an additional 39% of inductive energy is coupled into the plasma. Increasing pressure also increased plasma repeatability and resulted in a convergence of plasma circuit parameters.

Effect of a DC preionization source on energy deposition in a pulsed inductive plasma

Summary form only given. Understanding of the early time formation of inductively coupled pulsed plasmas is a necessary step in producing high power pulsed plasmoid propulsion systems, namely field reversed configuration devices1. Several circuit models exist for estimating plasma properties but neglect the effect that a preionization source will have on the plasma properties2,3. Other authors have studies preionization sources but have not performed a detailed parametric study4. This paper studies the effect that position and bias voltage of a DC preionization source have on energy coupling to a pulse inductive plasma formed using a theta pinch. Five radial positions and five bias voltages are applied to Argon prefills of 10-1,000 mTorr. Using SPICE5 software and established plasma models, an iterative method is used to determine the effective plasma resistance and inductance and are used to determine the energy present in the plasma as a function of time. The greatest amount of energy transferred to the plasma was TBD J occurred with at TBD mTorr with the DC preionization source located at TBD cm with a bias voltage of TBD V. It is observed that energy coupling into plasma increased TBD% as the probe neared the wall of the quartz insulator. Increasing the bias voltage increased plasma energy by TBD%. Increasing Argon prefill pressure past TBD mTorr resulted in reduced plasma energy.

PPPS-2013: Magnetic field probe calibration at relevant field magnitude and frequency

Summary form only given. Magnetic field probes are invaluable diagnostics for pulsed inductive plasmoid devices where field magnitudes on the order of several kG are common.1 The most common method of calibrating magnetic field probes is to use a Helmholtz coil to generate a large region of uniform magnetic field.2,3 Difficulty driving large currents through an inductive load at high frequency typically results in field magnitudes of a few Gauss or less. The calibration factor is then necessarily assumed linear, even though the magnetic field of the primary experiment is several orders of magnitude larger than the field magnitudes used to calibrate the probe.2,4,5 Calibration factors of two differential configuration magnetic field probes are presented as functions of frequency and field magnitude. Calibration values are determined experimentally using a 7.74 cm radius Helmholtz coil in three separate configurations. A broadband frequency sweep is performed with a network analyzer for frequency domain measurements while a function generator provides time domain measurements. These methods produce calibration factors over a large frequency range (100 kHz-1 MHz) with field magnitudes less than one Gauss. Large field magnitudes are achieved with a pulsed-power setup using multiple capacitor banks charged from 13-25 kV producing magnetic fields of at least 0.1 kG up to 3.8 kG. Tests are conducted at five frequencies in the range of interest. The calibration factors are calculated and presented as a function of field magnitude and frequency.

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.