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The field-reversed configuration heating experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magnetoinertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration and are formed via a reversed-field theta pinch on an already magnetized background plasma. To extend the duration and temporal uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on the FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Historically, parallel operation of spark gap switches in a crowbarring arrangement has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current. In a low-inductance design, triggering can be further complicated by the rapid collapse of what little voltage there is across the switches as soon as the first spark gap begins conduction. This paper reports on the efforts that were made to develop a low-inductance crowbar switch for the FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the low-inductance parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. posits that if such high $Z$ plasma could instead be composed of deuterium or a deuterium–tritium mixture, then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low-energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low-energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept.

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is ${\sim}{\rm 25}~\mu{\rm s}$ . Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ${\sim}{\rm 14}~\mu{\rm s}$ (and therefore, total lifetimes after formation are now approaching ${\sim}{\rm 19}~\mu{\rm s}$ ). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7–9 $\mu{\rm s}$ before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.

The Marxed Blumlein line is a concept that has been touted as a highly compact pulsed power system because it combines the functions of energy storage, voltage scaling, and pulse shaping into a single sub-system. Inherent voltage reversal is compounded in a Marxed arrangement. The voltage reversal and transients induced by the switch jitter in a Marxed configuration require significant derating of the dielectric strength. Consequently, the system must be operated much below the intrinsic dielectric energy density, compromising efforts to design a compact pulsed power system. This paper presents the conclusion that for all practical configurations, the Marxed transmission line is the more compact architecture.

The Shiva program at the Air Force Weapons Laboratory developed radiation sources for the purpose of assuring nuclear survivability for defense systems. The program pioneered the use of MJ-class pulsed power systems for simulation of the x-ray radiation environment from a nuclear detonation. The high energy pulsed power development arc culminated in the 9.4 MJ Shiva Star capacitor bank, commissioned in 1982. The reconfiguration drove advancements in capacitor energy density and high-current rail-gap switch design. Modestly updated capacitors and rail-gap switches were integrated into Atlas more than a decade later. Shiva Star enabled many firsts in megagauss high energy density science. With its microsecond rise-time, Shiva Star was designed to function as a laboratory stepping stone to facilitate load development for explosive pulsed power generators producing 50 MA or greater peak current. High-current and high pulse energy were seen as the key performance enablers for weapons simulation, while the machines currently employed in this task reflect recognition of the importance of the radiation pulse duration. High energy density plasma (HEDP) science on Shiva Star initiated with soft x-ray source development based on Z-pinches of gas puff loads and cylindrical foils. The output reached large fractions of the stored energy with pulse widths on the order of 1 μs. In an effort to compress the pulse supplied to the Z-pinch load and thereby reduce the x-ray pulse length, the wire-array plasma flow switch was developed. The plasma flow switch itself represents an unexploited approach to warm x-ray production. HEDP research continued on Shiva Star, exploring several varieties of magnetized target fusion. Initial efforts centered on the MARAUDER concept to produce a compact toroid for eventual compression. About the same time, the research team pioneered the implosion of solid spherical and cylindrical metal shells; material strength stabilized the liners against the onset of the magneto-Rayleigh-Taylor instability, enabling large compression ratios with negligible material blow-off from the inner surface. This work led to a partnership between AFRL and researchers engaged in field reversed configuration (FRC) plasma development at LANL. The venture concentrated on formation, translation, capture, and compression of the high-density, closed field line FRC. The FRC work culminated in the first-ever compressional heating of a high-density, closed field-line plasma target. Shiva Star occupies a unique place in high energy density capability and the components theoretically have significant shot life remaining.

Femtosecond laser filamentation in the atmosphere produces plasmas with densities on the order of 1016 cm−3 or less than 1% of air ionized [1]. Filamentation is a nonlinear process where a short laser pulse is propagating through a nonlinear medium while a dynamic balance between non-linear focusing, diffraction and plasma defocusing is established. The short lifetime and relative low temperature of the laser-generated plasma results in a high resistivity, making femtosecond filaments difficult to utilize for some applications including guiding high voltage electric discharges [2]. To achieve guided discharges with femtosecond filaments over several meters requires voltages larger than 100 kV between the electrodes. Alternatively nanosecond lasers have been used to create plasma sparks and trigger short discharges. The nanosecond laser plasma is denser and hotter in comparison to the femtosecond filament plasma. These properties result in a high conductivity and a fast trigger of the high voltage discharge [3]. However, the spatial extent of these plasmas is limited and only short discharges can be initiated. A recent experiment on picosecond laser filamentation [4] indicates the possibility of combining the properties of the two plasmas described above: an extended plasma channel with high conductivity. Here, we present experimental results that show the effect of picosecond laser filamentation on guiding high voltage discharges. The results indicate that higher conductivity plasma is produced in comparison to a femtosecond pulse. We achieved a reduction of threshold break down voltage by 20% with our experimental setup. The experiments are conducted using the COMET laser, at Jupiter Laser facility, Lawrence Livermore Laboratory.

Capacitor energy density for submicrosecond discharge applications was investigated for capacitors based on the following: 1) polymer-ceramic nanocomposite (PCNC); 2) antiferroelectric (AFE); and 3) paraelectric (PE) ceramic dielectrics. The developmental PCNC dielectric enabled design, fabrication, and testing iterations to be completed relatively rapidly. The PCNC capacitors were nominally 4 nF and were tested to dc potentials of at least 75 kV. The capacitors were then charged from 20 to 48 kV with a dc high-voltage power supply and discharged into a nearly critically damped test circuit of up to 5 pulses/s (pps) repetition rate for lifetime testing. The discharge time was 65 ns. Shot life as a function of the charge voltage was compared for three design iterations. Changes in the manufacturing of the PCNC capacitors have yielded up to 100× improvements in pulse discharge life. The 1-2-kV prototype, nonlinear (antiferroelectric and paraelectric) multilayer ceramic capacitors had zero-voltage capacitance ratings of between 60 and 300 nF. They were charged to their operating voltage and discharged into a nearly critically damped load in 2-6 μs, depending on their capacitance, at repetition rates of up to 75 pps. Their operating voltage for fast, repetitive discharge was determined for lifetimes consistently over 105 shots. Discharge energy densities of 0.27-1.80 J/cc and energy losses of 7.9-36.8% were obtained for the packaged multilayer capacitors with different formulations of nonlinear dielectrics. Increased field-induced strain was correlated with increased permittivity and contributed to the limitations on the operating voltage. Multilayer ceramic capacitors fabricated from AFE and PE ceramic dielectrics have the potential to achieve high energy density owing to their high relative permittivities that vary with applied electric field, assuming they can be scaled up to sufficiently high voltages.

Experimental and analytical investigations were conducted to predict the wear of the discharge cathode keeper orifice in the NASA Evolutionary Xenon Thruster. The ion current to the keeper was found to be highly dependent on the beam current, and the average keeper ion current density was nearly identical to that of the NASA Solar Electric Propulsion Technology and Applications Readiness (NSTAR) thruster for comparable beam current density. The ion current distribution was highly peaked toward the keeper orifice. The wear assessment predicted keeper orifice erosion to the same diameter as the cathode tube after processing 250 or 460 kg of xenon, depending on whether warm or cold ions, respectively, were assumed. Although the presented simple wear analysis does not predict failure, comparison with the NSTAR extended life test results suggests that the discharge cathode assembly will continue to operate beyond the qualification goal of processing 405 kg of propellant. Probabilistic wear analysis showed that the ion energy at the keeper surface and the sputter yield contributed most to the uncertainty in the wear assessment. It is recommended that fundamental experimental and modeling efforts focus on accurately describing the plasma potential, ion temperature, and the sputtering yield.

An experimental investigation of the operating characteristics of 3.2-mm diameter orificed hollow cathodes was conducted to examine low current and low flow rate operation. Cathode power was minimized with an orifice aspect ratio of approximately one and the use of an enclosed keeper. Cathode flow rate requirements were proportional to orifice diameter and the inverse of the orifice length. The minimum power consumption in diode mode was 10-W, and the minimum mass flow rate required for spot-mode emission was approximately 0.08-mg/s. Cathode temperature profiles were obtained using an imaging radiometer and conduction was found to be the dominant heat transfer mechanism from the cathode tube. Orifice plate temperatures were found to be weakly dependent upon the flow rate and strongly dependent upon the current.

We present results of an experimental investigation of laser pulsewidth dependence of filamentation-guided high voltage breakdown in air. The experiments are conducted at laser peak power levels of 1 TW and pulse durations of 0.7 to 10 ps with a discharge gap separation of 10 cm. Synchronized electrical and optical diagnostic techniques were used to determine the pulsewidth dependence on the breakdown mechanism, threshold levels, time delays and associated jitter. The results indicate that longer pulses provide greater than 30% reduction in breakdown threshold voltage.

A compact pulse forming line (CPFL) concept based on a folded transmission line and high-breakdown strength dielectric was explored through an effort combining proof-of-principle experiments with electromagnetic modeling. A small-scale folded CPFL was fabricated using surface-mount ceramic multilayer capacitors. The line consisted of 150 capacitors close-packed in parallel and delivered a 300 ns flat-top pulse. The concept was carried to a 10 kV class device using a polymer-ceramic nanocomposite dielectric with a permittivity of 37.6. The line was designed for a 161 ns FWHM length pulse into a matched load. The line delivered a 110 ns FWHM pulse, and the pulse peak amplitude exceeded the matched load ideal. Transient electromagnetic analysis using the particle-in-cell code ICEPIC was conducted to examine the nature of the unexpected pulse shortening and distortion. Two-dimensional analysis failed to capture the anomalous behavior. Three-dimensional analysis replicated the pulse shape and revealed that the bends were largely responsible for the pulse shortening. The bends not only create the expected reflection of the incident TEM wave but also produce a non-zero component of the Poynting vector perpendicular to the propagation direction of the dominant electromagnetic wave, resulting in power flow largely external to the PFL. This analysis explains both the pulse shortening and the amplitude of the pulse.

Laser triggered switching has been studied and used since the early 1960's. [1, 2] Despite the long development history of laser triggered switching, electrical triggering remains far more common in compact pulsed power systems. Laser triggered switching offers performance and operational advantages over electrical triggering provided its limitations can be overcome or mitigated. This paper reviews laser triggered switch technology development. Early work focused on the demonstration of the concept of laser triggering. [1] By the 1980's, diagnostic and modeling tools had matured sufficiently to support detailed analysis of the evolution of current carrying channels from a seed electron population. [3] Laser triggering also began to be implemented on systems requiring low jitter. [4] More recently, studies have explored the possibility of using laser triggered switching with compact pulsed power systems. [5] Advances in laser technologies have also opened up new options for the implementation of laser triggering systems. The paper discusses some of the progress made based on laser technology.

A plasma environment closely approximating the ionosphere can be generated by a hollow cathode assembly in a large vacuum chamber. This capability allows examinations of ionospheric-plasma phenomena in a controlled setting without, in most cases, relying on scaling techniques. The hollow cathode provides a low-temperature, lowdensity, fairly uniform plasma in its far e eld, and the large chamber provides ample room such that the effects of plasmacone nementarereducedtoaminimum.Mostpreviousstudiesofhollowcathodeshaveconcentratedontheir plasma environment in thenear e eld, i.e., within a few tens of centimeters. Thiswork, however, examines their fare eldplasmaenvironment,i.e.,1 ‐2m.Thischaracterizationshowsthatthehollowcathodeprovides,inthefare eld,a fairly uniform ionospheric-level plasma environment. Thehollow cathodewasoperated at ninedifferent operating conditions consisting of threedifferent gases (argon, krypton, and xenon )each at three different e ow rates. Results from these nine operating conditions are summarized, and the corresponding far-e eld plasma environments are analyzed. Comparison of the hollow cathode’ s plasma environment parameters with typical ionospheric values is presented. Uses of the facility, such as full-scale scientie c instrument verie cation, are presented.

Laser-induced fluorescence (LIF) was used to measure the mean and variance of the velocity distribution of xenon ions emitted from two hollow cathode assemblies operating at low power. High-energy ions detected in plume-mode and spot-mode operation are consistent with the potential-hill model of high-energy ion production. The distributions of velocities were modeled, yielding temperatures on the order of a few electronvolts in plume mode and 1 eV in spot mode. The LIF of neutral xenon atoms in the plumes indicated thermal temperatures significantly less than the temperatures associated with the ion velocity distributions.

The plasma properties of the very-near-e eld (10‐50 mm) plume of the D55 anode layer thruster (TAL) were measured. The D55 is the 1.35-kW TAL counterpart to the SPT-100 and was made by the Central Scientie c Research Institute of Machine Building of Kaliningrad, Russia. The thruster was tested in the 6 m diameter £ 9 m longvacuumchamberattheUniversityofMichigan’ sPlasmadynamicsandElectricPropulsionLaboratory,andthe diagnosticprobes werepositioned using a three-axis translation tablesystem. Water-cooled Hall probes, a Faraday probe, emissive probes, and langmuir probes were used to examine the near-e eld plasma properties. Water-cooled Hall probes were employed to exploretheeffectof the closed-drift current on theradial magnetic e eld. The change in the magnetic e eld during thruster operation was found to be less than 5% over the region examined, which indicated that the Hall current was limited to several tens of amperes. Evidence also indicated that the closed-drift current extended between 5 and 10 mm downstream of the anode. Ion current density proe les showed that the annular beam focuses within 40 mm of the thruster exit plane. Plasma potential measurements indicated that ion acceleration occurred primarily within 10 mm of the anode. The highest electron temperature measured in this investigation occurred immediately downstream of the anode, and the temperature decreased with axial distance from the thruster. The low-energy electrons were cone ned to the high-density core of the plasma beam.

This Special Issue on Megagauss Magnetic Fields serves as a focal point for the documentation of recent research achievements and advances from around the world in high magnetic field generation and utilization to produce extreme states of matter. The Megagauss Conferences began in 1965 and have provided an international venue for communication of scientific advances related to the production and use of very high magnetic fields. The most recent Megagauss Conference was held in October 2012, in Maui, Hawaii, and this Special Issue represents a selection of the topics presented. We hope you will find this Special Issue both interesting and informative.

Experiments on FRC formation and translation into the interior of a metal shell or liner have been conducted at AFRL. Flux exclusion, collimated light, and interferometer data on magnetized plasma injection will be presented. These are a pre-requisite for FRC compression by liner implosion, experiment progress on which will also be presented. FRC translation, capture, and compression experiments all use primarily axial ∼ 2 Tesla guide and mirror fields established inside the liner, using ∼ 5 millisecond rise time discharges into an array of pulsed magnet coils surrounding the liner implosion portion of the device. A 12 MA, 4.5 MJ axial discharge drives the liner implosion for compression experiments. The FRC capture experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 Megamp) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, He Ne laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations.

Arrays of Si PIN diodes are fielded on radiation producing experiments on the Shiva Star pulsed power generator at AFRL's Directed Energy Directorate. This work is a continuation of earlier work done on Shiva Star1. Each diode is filtered to respond to a part of the band of interest and the signals are then deconvolved to generate the radiation spectrum as a function of time. The plasma temperature may be inferred from this method, assuming Bremsstrahlung radiative processes, and impurities may be detected by a departure from the mean temperature.

Summary form only given. The stacked Blumlein line is a concept that has been touted as a highly compact pulsed power system because of its combination of the functions of energy storage, voltage scaling, and pulse shaping into a single sub-system. In reality, the voltage reversal and transients induced by the operation of the stacked lines and the switch jitter require significant derating of the dielectric strength. Consequently, the system must be operated much below the intrinsic dielectric energy density, compromising efforts to design a compact pulsed power system. Nevertheless, composite dielectrics have renewed interest in stacked Blumlein lines for compact pulsed power. This paper discusses the limitations on the energy density of stacked Blumlein lines as well as efforts to develop stacked Blumlein lines using a ceramic loaded polymer composite dielectric. Experiments have been conducted to evaluate electrically triggered spark-gap switches for use in testing stacked lines. Additionally, small capacitors have been fabricated using the same composite dielectric used for the Blumlein lines. The capacitors have been tested to evaluate the material and thereby inform testing of multiple-stage Blumlein lines.

The EFF is used at Los Alamos as the primary opening switch for high current applications. It has interrupted currents from ~10 kA to 25 MA, thus diverting the current into low inductance loads. To understand and optimize the performance of full-scale experiments, many parameters were studied in a series of small-scale experiments, including: electrical conduction through the explosive products; current density; explosive initiation; insulator type; conductor thickness; conductor metal; metal temper; and on. The results show a marked inverse correlation of peak EFF resistance with current density. In this paper we postulate and refute a simple extrusion mechanism of EFF operation; demonstrate that the EFF switch has a near-ideal profile for producing flat-topped voltage profiles; and explore possible mechanisms for the degradation of small scale switch performance.

The performance test results of three NEXT ion engines are presented. These ion engines exhibited peak specific impulse and thrust efficiency ranges of 4060 4090 s and 0.68 0.69, respectively, at the full power point of the NEXT throttle table. The performance of the ion engines satisfied all project requirements. Beam flatness parameters were significantly improved over the NSTAR ion engine, which is expected to improve accelerator grid service life. The results of engine inlet pressure and temperature measurements are also presented. Maximum main plenum, cathode, and neutralizer pressures were 12,000 Pa, 3110 Pa, and 8540 Pa, respectively, at the full power point of the NEXT throttle table. Main plenum and cathode inlet pressures required about 6 hours to increase to steady-state, while the neutralizer required only about 0.5 hour. Steady-state engine operating temperature ranges throughout the power throttling range examined were 179 303 C for the discharge chamber magnet rings and 132 213 C for the ion optics mounting ring.

NASA’s Glenn Research Center (GRC) is leading the development of NASA’s Evolutionary Xenon Thruster (NEXT) system. The central feature of the NEXT system is an electric propulsion thruster that inherits the knowledge gained through the NSTAR thruster that successfully propelled the Deep Space 1 to asteroid Braille and comet Borrelly, while significantly increasing the thruster power level and making improvements in performance parameters associated with NSTAR. The thruster under development has a 40 cm beam diameter, twice the effective area of the Deep-Space 1 thruster. It incorporates mechanical features and operating conditions to maximize the design heritage established by the flight NSTAR 30 cm thruster, while incorporating new technology where warranted to extend the power and throughput capability. To date three engineering-model thrusters have been manufactured at NASA GRC, and performance, wear, vibration, and integration testing of these thrusters is underway.

Summary form only given. Flight qualification of ion thrusters typically requires testing on the order of 10,000 hours. Extensive knowledge of wear mechanisms and rates is necessary to establish design confidence prior to long duration tests. Consequently, real-time erosion rate measurements offer the potential both to reduce development costs and to enhance knowledge of the dependency of component wear on operating conditions. Several previous studies have used laser-induced fluorescence (LIEF) to measure real-time, in situ erosion rates of ion thruster accelerator grids. Those studies provided only relative measurements of the erosion rate. In the present investigation, a molybdenum tube was resistively heated such that the evaporation rate yielded densities within the tube on the order of those expected from accelerator grid erosion. A pulsed UV laser was used to pump the ground state molybdenum at 345.64-nm, and the non-resonant fluorescence at 550-nm was collected using a bandpass filter and a photomultiplier tube or intensified CCD array. The sensitivity of the fluorescence was evaluated to determine the limitations of the calibration technique. The suitability of the diagnostic calibration technique was assessed for application to ion engine erosion rate measurements.

In order to measure sputtered atoms and plasma properties which lead to erosion in ion thrusters, laser diagnostics are being incorporated in the NASA Glenn Research Center’s ion propulsion development program. Laser diagnostic capabilities include UV and IR laser -induced fluorescence and laser interferometry. Objectives of the diagnostics, issues associated with their implementation, and paths to reach the objectives are presented.

A particle and energy balance model of orificed hollow cathodes was developed to assist in cathode design. The model presented here is an ensemble of original work by the author and previous work by others. The processes in the orifice region are considered to be one of the primary drivers in determining cathode performance, since the current density was greatest in this volume (up to 1.6 x 10(exp 8) A/m2). The orifice model contains comparatively few free parameters, and its results are used to bound the free parameters for the insert model. Next, the insert region model is presented. The sensitivity of the results to the free parameters is assessed, and variation of the free parameters in the orifice dominates the calculated power consumption and plasma properties. The model predictions are compared to data from a low-current orificed hollow cathode. The predicted power consumption exceeds the experimental results. Estimates of the plasma properties in the insert region overlap Langmuir probe data, and the predicted orifice plasma suggests the presence of one or more double layers. Finally, the model is used to examine the operation of higher current cathodes.

NASA’s Glenn Research Center has been selected to lead development of NASA’s Evolutionary Xenon Thruster (NEXT) system. The central feature of the NEXT system is an electric propulsion thruster (EPT) that inherits the knowledge gained through the NSTAR thruster that successfully propelled Deep Space 1 to asteroid Braille and comet Borrelly, while significantly increasing the thruster power level and making improvements in performance parameters associated with NSTAR. The EPT concept under development has a 40 cm beam diameter, twice the effective area of the Deep-Space 1 thruster, while maintaining a relatively-small volume. It incorporates mechanical features and operating conditions to maximize the design heritage established by the flight NSTAR 30 cm engine, while incorporating new technology where warranted to extend the power and throughput capability. The NASA Hall thruster program currently supports a number of tasks related to high power thruster development for a number of customers including the Energetics Program (formerly called the Space-based Program), the Space Solar Power Program, and the In-space Propulsion Program. In program year 2002, two tasks were central to the NASA Hall thruster program: 1.) the development of a laboratory Hall thruster capable of providing high thrust at high power; 2.) investigations into operation of Hall thrusters at high specific impulse. In addition to these two primary thruster development activities, there are a number of other on-going activities supported by the NASA Hall thruster program. These additional activities are related to issues such as thruster lifetime and spacecraft integration.

The NASA Glenn Research Center (GRC) ion propulsion program addresses the need for high specific impulse ion propulsion systems and technology across a broad range of mission applications and power levels. Development areas include high-throughput NSTAR derivative engine and power processing technology, lightweight high-efficiency sub-kilowatt ion propulsion, micro-ion propulsion concepts, engine and component technologies for highpower (30 kW class) ion engines, and fundamentals. NASA GRC is also involved in two highly focussed activities: development of 5/10-kW class next-generation ion propulsion system technology, and development of high-specific impulse (> 10,000 seconds) ion propulsion technology applicable to deep-space and interstellar-precursor missions.

A preliminary thermal characterization of a newly-fabricated NSTAR-derived test-bed thruster has recently been performed. The temperature behavior of the rare-earth magnets are reported because of their critical impact on thruster operation. The results obtained to date showed that the magnet temperatures did not exceed the stabilization Emit during thruster operation up to 4.6 kW. Magnet temperature data were also obtained for two earlier NSTAR Engineering Model Thrusters and are discussed in this report. Comparison between these thrusters suggests that the test-bed engine in its present condition is able to operate safely at higher power because of the lower discharge losses over the entire operating power range of this engine. However, because of the 'burn-in' behavior of the NSTAR thruster, magnet temperatures are expected to increase as discharge losses increase with accumulated thruster operation. Consequently, a new engineering solution may be required to achieve 5-kW operation with acceptable margin.

The success of the ion thruster on the Deep Space One mission has opened the gate to the use of primary ion propulsion. Many of the projected planetary missions require throughput and specific impulse beyond those qualified to date. Spectroscopic, real-time ion thruster grid erosion-rate measurements are currently in development at the NASA Glenn Research Center. A preliminary investigation of the emission spectra from an NSTAR derivative thruster with titanium grid was conducted. Some titanium lines were observed in the discharge chamber; however, the signals were too weak to estimate the erosion of the screen grid. Nevertheless, this technique appears to be the only non-intrusive real-time means to evaluate screen grid erosion, and improvement of the collection optics is proposed. Direct examination of the erosion species using laser-induced fluorescence (LIF) was determined to be the best method for a real-time accelerator grid erosion diagnostic. An approach for a quantitative LIF diagnostic was presented.

As part of an effort to develop efficient hollow cathodes for the International Space Station and low power electric propulsion systems, the effects of orifice length-to-diameter aspect-ratio on the performance of 0.5 to 2 A hollow cathodes were examined to compare with previous experiments and a recently developed model. Cathodes were constructed with nominally identical orifice diameters and lengths that varied from 1 to 6 diameters. The performance of the cathodes was evaluated at the beginning of life and after at least 50 hours of operation. The data generally followed the trends predicted by the model with a few exceptions which appear to be related to the thermal environment of the cathode. Power consumption scaled with the orifice aspect-ratio, while the minimum spot mode flow rate was inversely related to the length-to-diameter ratio. At currents below 1.0 A, the cathode operation became more complex than that which is assumed in the orifice model; low operating temperature caused instabilities in several of the devices. Nomenclature AR = aspect-ratio of the orifice c = speed of light, 3 x 10 m/s h = Planck's Constant, 6.63 x 10' J-s k = Boltzmann's Constant, 1.38 x 10' J/K n = index of refraction T = temperature, K e = emissivity X = wavelength, m Introduction A parametric investigation has been conducted at the NASA Lewis Research Center in order to evaluate the effect of orifice aspect-ratio on the performance of low current hollow cathodes. The aspect-ratio is defined as the orifice length divided by its diameter. The NSTAR derivative ion thruster for the Deep Space 4 mission, the plasma contactor for the International Space Station, and the NASA Lewis 8 cm ion thruster program all have requirements for low flow rate, low power cathodes.' Experimental data' exist both to support and to refute the hypothesis that cathode performance is inversely proportional to the orifice aspect-ratio. A recent hollow cathode model developed by Mandell and Katz describes the physics leading to the conclusion that cathode performance scales inversely with orifice aspect-ratio. Ions entering a finite length orifice will undergo collisions as they traverse the channel. Ions lost to collisions with the walls must be balanced by ions created in the orifice in order to satisfy the condition of a quasineutral plasma at the exit of the orifice. As orifice aspect-ratio increases, the number of ionizations occurring increases, and the conductivity of the plasma in the orifice decreases. The energy transport within the orifice also varies inversely with the aspect-ratio, and modeling the orifice processes becomes increasingly important to making an accurate prediction about the performance of the cathode. The physical arguments for the importance of orifice aspect-ratio to cathode performance are sufficiently compelling to warrant a new evaluation of the experimentally observed effects, particularly when trying to minimize the power consumption during low current operation. This report summarizes the experimental findings concerning the impact of * Graduate Student Researcher, Student Member AIAA 1 Associate Professor, Senior Member AIAA * Research Engineer, Member AIAA Copyright © 1998 by Matthew T. Domonkos. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics orifice aspect-ratio on the power consumption of low current hollow cathodes. Although the current density in the orifice far exceeds that found anywhere else in a plasma contactor, ion thruster, or Hall thruster, previous modeling efforts' have ignored the effects of the orifice processes on the overall performance. Mandell and Katz have proposed a model which conserves both energy and ions necessary for quasineutrality. The model assumes the plasma properties at the entrance to the orifice are known. The ion loss to the walls of the orifice is balanced by electron-neutral collisional ionization. The Ohmic heating in the plasma is balanced by the ionization, radiative, and convective losses. With the orifice region taken as a control volume, the flux of ions incident at the entrance will either collide with the walls or be transmitted through the orifice. Since ion-wall collisions represent a loss in the energy balance, optimization of power consumption requires minimization of the orifice aspect-ratio. Typically in orificed hollow cathodes, the maximum required current determines the orifice diameter. While the theoretical picture indicates that orifice length should then be minimized, practical considerations force a finite length orifice. The machining and welding methods currently employed to fabricate the cathode tubes require a certain minimum overall thickness of the orifice plate. Rawlin and Kerslake9 were among the first investigators to observe the erosion of the orifice plates on hollow cathodes, and increasing the thickness of the orifice plate would, to first order, mitigate the problem of erosion. The goal of the investigation reported here was to provide experimental data for comparison with Mandell and Katz's5 model while meeting the practical considerations for cathode design. This paper presents the results of a parametric experimental investigation of the effects of the orifice aspect-ratio on the power consumption of 3.2 mm diameter orificed hollow cathodes. The experimental apparatus and techniques are described first. Next, the performance data are presented. The results are then analyzed with an emphasis on the performance, and the implications for cathode lifetime are also discussed. Finally, the major conclusions of this investigation are summarized. Experimental Apparatus Four 3.2 mm outer-diameter hollow cathodes with different orifice aspect-ratios were assembled as shown in Figure 1. The construction of the hollow cathodes closely followed the methods developed previously at the NASA Lewis Research Center.10-11 The orifice plates were machined on a lathe. Figure 2 illustrates the variation in the aspect-ratios for three of the cathodes tested. Although some dimensions of this schematic were exaggerated for clarity, the orifice plate dimensions, the cathode-keeper gap (Lck), and the keeper orifice diameter (dj are all drawn to scale. For convenience, the cathodes are referred to in this paper as AR1, AR3, and AR6 for the orifices with aspect-ratios of 1, 3, and 6, respectively. The fourth cathode had an orifice diameter forty percent smaller than the other cathodes and an aspect-ratio of approximately 3; it will be referred to as AR3' here. The orifice plates were electron beam welded to a refractory metal alloy tube with a 3.2 mm outer diameter. The electron emitter was a cylinder of sintered porous tungsten impregnated with 4BaOCaO-Al2O3 and was located at the downstream end of the tube. The emitter outer diameter was nominally 0.38 mm smaller than the inner diameter of the tube. Cathode Tube with Insert Propellant Isolator Keeper Thermocouple — Heater Electrical Stand-Offs Mounting Bracket a) Schematic for the Hollow Cathode Assemblies b) IPhotbgraph Figure 1 The Hollow Cathode Assembly Refractory metal electrical leads attached to the upstream end of the emitter were swaged into electrical contact the with cathode tube. This also fixed the position of the emitter within the cathode American Institute of Aeronautics and Astronautics tube. A sheathed heater was wound and friction fit to the end of the cathode tube. Metal foil was wrapped tightly around the outside of the heater to function as a radiation shield.

RPA and Langmuir probe measurements were made in the near-field plume on the axis of a hollow cathode assembly (HCA). High-energy ions were detected at low-powers consistent with the detection of such ions at high powers in previous investigations. The absence of a potential hill in the testing conditions suggests that an additional source of high energy ions may be charge exchange ions in the luminous region of the cathode discharge. The peaks in the RPA data tend to support this theory. In addition, it was established that the conditions in the near-field of the HCA expellent would be condusive to laser-induced fluorescence (LIF) measurement.

The performance of a D-100 thruster with anode layer, designed to operate on xenon at a nominal operating power of 4.5 kW, was investigated during operation on krypton. This evaluation was preliminary to a more thorough investigation of D-100 operation on alternative propellants, such as mixtures of krypton and xenon. Several operating regimes were evaluated ranging in power from 0.9 to 4.5kW. Two of these regimes were explored extensively using Faraday, RPA and emissive probes, with the second regime being of particular importance for orbit raising missions. Direct thrust measurements were taken with an inverted ballistic pendulum thrust stand. Thrust was also calculated using average velocity and ion density measurements, and this method yielded results in agreement with the thrust stand data. Thruster performance was compared to operation on Xe and showed a 20% efficiency loss at the same specific impulse during operation on krypton. ' Graduate Student. Aerospace Eng. Dept. fGraduate Student , Aerospace Eng. Dept.. Member AIAA ¥ Assistant Professor, Aerospace Eng. Dept.. Member AIAA j. •••Executive Director "Member of the Technical Staff. Member AIAA

A thrust stand was modified, and a primary calibration technique was developed to evaluate the performance of applied-field pulsed magnetoplasmadynamic thrusters (MPDTs) for 10 kW class solar electric orbit transfer vehicle (SEOTV) missions. The NASA Lewis Research Center (LeRC) 30 kW thrust stand was modified to accept high current pulses delivered to the MPDT. A pendulum system was developed to deliver an in situ primary impulse calibration of the thrust stand. An MPDT was built to accommodate future testing of an externally heated cathode and to allow rapid diffusion of the applied magnetic field. The thruster was driven by a pulse forming network to anode-cathode powers of 350 kW. The thrust stand was calibrated for impulses ranging from 4.8 to 12.2 mN-s, and thruster discharges generated impulses ranging from 2.6 to 4.5 mN-s after subtracting the cold gas flow impulse.

Pulsed magnetoplasmadynamic (MPD) thruster systems were analyzed for application to solar-electric orbit transfer vehicles at power levels ranging from 10 to 40 kW. Potential system level benefits of pulsed propulsion technology include ease of power scaling without thruster performance changes, improved transportability from low power flight experiments to operational systems, and reduced ground qualification costs. Required pulsed propulsion system components include a pulsed applied-field MPD thruster, a pulse-forming network, a charge control unit, a cathode heater supply, and high speed valves. Mass estimates were obtained for each propulsion subsystem and spacecraft component using off-the-shelf technology whenever possible. Results indicate that for payloads of 1000 and 2000 kg pulsed MPD thrusters can reduce launch mass by between 1000 and 2500 kg over those achievable with hydrogen arcjets, which can be used to reduce launch vehicle class and the associated launch cost. While the achievable mass savings depends on the trip time allowed for the mission, cases are shown in which the launch vehicle required for a mission is decreased from an Atlas IIAS to an Atlas I or Delta 7920.

In this community white paper, we describe an approach to achieving fusion which employs a hybrid of elements from the traditional magnetic and inertial fusion concepts, called magneto-inertial fusion (MIF). The status of MIF research in North America at multiple institutions is summarized including recent progress, research opportunities, and future plans.