D. Keefer

Experimental and computer simulation studies of a pulsed plasma accelerator

Summary form only given. The present abstract describes an on-going effort to provide detailed experimental diagnostics and advanced computational simulations of the behavior and performance of high power plasma thrusters for possible applications in nuclear electric propulsion systems. Electromagnetic acceleration of plasmas for propulsion has long been seen as a means to efficient high specific impulse systems. Our approach is to evaluate simulations results from advanced computational codes with data collected from a laboratory prototype thruster that is designed for accurate diagnostics. The thruster is a coaxial electrode design that discharges plasma through a section of vacuum chamber with flat quartz windows that are used for optical diagnostics. High speed photography has documented successful firing of the thruster, a heterodyne laser interferometer has been used to obtain line-of-site electron number densities near the thruster exit, and Rogowski coils have been utilized to monitor thruster current. Tests with an array of B-dot probes positioned to help characterize the thruster current sheet have commenced. The supporting simulations are being obtained from two computer codes, MACH2 and GEMS. MACH2 is a well-established MHD code based upon the ALE formulation. It has been applied to diverse problems, and its capabilities are well recognized. For the present problem, however, it has several limitations: it is a 2-D code; its grid capabilities are limited to quadrilaterals; and its architecture does not lend itself to parallel computation. Of primary importance in the present application is MACH2s restriction to the MHD approximation, disallowing magnetic field propagation by wave processes. Therefore, we are extending the electromagnetics capability of the three-dimensional general equations mesh solver (GEMS) code that has been previously used to simulate a steady-state MHD generator to provide a time accurate simulation capability that incorporates modern computational methods. This extension has focused on identifying methods for solving coupled electromagnetic/fluid dynamic problems in regions where the MHD approximation fails. We have developed a solution algorithm that does not depend upon the MHD approximation, but solves the complete Maxwell equations at resource levels that appear to be similar to those associated with the magnetic induction equation. Comparisons between results from the present tests and those obtained from MACH2 and GEMS are expected to be available for ICOPS

Experimental pulsed plasma thruster for code evaluation

Summary form only given. UTSI has an on-going project to develop advanced computational codes that can predict the detailed behavior and performance of high power plasma thrusters for possible applications in nuclear electric propulsion systems, such as envisioned in Project Prometheus. We have modified the MHD code, MACH2, to perform initial design studies for a small-scale laboratory experiment that can be used for code evaluation. The thruster has been fabricated and installed in a vacuum chamber fabricated from stainless steel tubing and plate. The plasma discharges through a section of vacuum chamber with flat quartz windows that will be used for interferometric, photographic, and spectroscopic measurements. Initial high speed photography has documented a successful firing of the thruster. Diagnostics to obtain electron number density downstream of the exit plane estimates exhaust velocity are planned. Details of the modeling effort and available experimental results is presented.

Numerical MHD simulations of the DECADE plasma opening switch using MACH2

Summary form only given. Numerical simulations of the DECADE opening switch have been performed using the MHD code MACH2. This code has multiblock grid capability which makes it possible to simulate the detailed geometry of the actual switch configuration. Modifications were made to MACH2 to permit input of time dependent density and velocity of the initial injection plasma based on experimental measurements of these quantities. The code was also modified to permit a determination of the inductive component of switch voltage when using a non-flowing upstream boundary condition. The simulations have shown that the plasma flow and magnetic field evolution in the switch is a strong function of the initial plasma density distribution. Opening of the switch was simulated by use of an artificially large resistive diffusion. At the approach of opening time in the switch, the Hall parameter becomes large, and the Hall effect may play an important role in opening behavior. An effort is underway to add new numerical algorithms to MACH2 which will permit inclusion of the Hall effect in the simulation.

Numerical modeling of a pulsed plasma thruster

Summary form only given, as follows. Numerical simulations of a pulsed plasma thruster (PPT) were conducted to provide insight into the plasmadynamics and to provide the initial conditions required for plume studies. The approach was to validate a numerical model by comparing computed results with experimental data. The MACH2 computer code was chosen for these studies and the ablation rate of the Teflon propellant was obtained from an optically thin radiation model developed at UTSI. The data used for model validation was the Burton and Bushman (1999) experiments made with a capillary configuration where a hollow cylinder of Teflon was clamped between a brass anode and a conical boron nitride nozzle. The mass loss and impulse bit were reported, as well as the capacitor energy and energy efficiency, electron number density, electron temperature, and Mach number in the exhaust plume. These experimental values were used to select radiation parameters used in the ablation model. A typical example of the numeric simulation five microseconds into the discharge gives a maximum calculated temperature of 10 eV in the capillary that decreases both axially and radially to about 2 eV at the nozzle exit. The maximum density occurs at the capillary wall in the relatively cool ablated Teflon products. There is a strong radial density gradient in the capillary corresponding to the temperature gradient, but throughout the nozzle expansion the gradient is primarily axial. The current pulse is essentially complete in ten microseconds, and during this time electromagnetic forces accelerate a small amount of plasma. However, most of the impulse is produced by expansion of the residual high-pressure plasma during the next 50 to 100 microseconds. The simulations indicate that approximately 90 per cent of the total impulse is thermal, which agrees well with the value reported by Burton.

MACH2 simulations of an explosively formed fuse

Summary form only given. MACH2 is a general purpose MHD code that has been used to simulate the evolution of an explosively formed fuse consisting of a thin cylindrical aluminum shell that is driven into a Teflon die by an explosion. The simulations included the initiation and propagation of the explosive detonation wave and tracked the extrusion of the fuse into the Teflon die. The code predicted that the fuse resistance increased by approximately three orders of magnitude due to the lengthening and thinning of the fuse material and the rise of temperature caused by the Ohmic heating. Comparisons with recent experimental measurements performed at Los Alamos National Laboratory indicate that the predicted resistance is approximately one order of magnitude too small and suggest that the fuse must undergo a transition to a plasma in order to reach the observed switch voltage. Additional simulations of a short fuse segment were performed to investigate this transition and to evaluate the effect of spent explosive resistivity on the maximum electric field obtained.

Experimental study of the DECADE cableguns

Summary form only given. The cableguns used to initiate the operation of the DECADE plasma opening switch (POS) consist of sections of Teflon insulated copper coaxial cable machined to a conical nozzle shape. An arc formed across this nozzle ablates the Teflon insulator and copper center conductor and injects the resulting plasma into the POS. Single cableguns have been tested in a laboratory facility to obtain measurements of the average mass ablated and to obtain optical emission spectra, laser interferometer measurements and time resolved photographs. The cableguns were weighed and measured before and after a sequence of 300 shots to determine the average mass of Teflon and copper lost per shot. It was found that the average mass loss of 277 micrograms per shot consisted of 61% copper and 39% Teflon when a capacitor voltage of 25 kV was used. Time integrated spectral emission data revealed mostly spectral lines of singly ionized fluorine and carbon with no copper visible. A semi-quantitative analysis of the spectrum suggests a plasma temperature of approximately 2.5 eV.

MACH2 simulations of the DECADE plasma opening switch

Summary form only given. The DECADE nuclear effects simulator at the Arnold Engineering Development Center (AEDC) uses a plasma opening switch (POS) to create a high voltage pulse used for a bremsstrahlung X-ray diode. AEDC has supported a modeling effort for the past several years at the University of Tennessee Space Institute (UTSI) in which the MACH2 computer code has been used to make numerical simulations of the POS and the cableguns used to initiate the switch. It was found that the Hall effect played an important role in the opening of the switch, but simulations predicted that current progressed along the cathode prior to the anode, in contrast to measurements obtained with B-dot probes. Some interferometer measurements presented at a DECADE review meeting in 1998 indicated that a higher density plasma layer approximately one millimeter thick formed on the electrode surfaces.

MACH2 simulations of an ablative pulsed plasma thruster

Summary form only given, as follows. MACH2 is a general purpose MHD code developed and maintained at the Air Force Research Laboratory, Phillips Site. It has been successfully used in the past to simulate a wide range of pulsed plasma devices. We have adopted this code to simulate several different electric thrusters, as well as cableguns that are used in plasma opening switches, in an attempt to better understand the complicated plasma flow in these devices. The code normally uses tabular thermodynamic data and transport properties from the SESAME tables developed at Los Alamos National. Laboratory. These tables are sparse in the density and temperature range of interest for the Teflon material commonly used for ablative thrusters and for cableguns. We have calculated new tables for Teflon and for polyethylene for use in MACH2 simulations. Simulations have been run for a research thruster at UTSI and a pulsed plasma thruster under development at the University of Illinois. Simulations have also been run for the cablegun used for the DECADE opening switch. Detailed predictions of the plasma flow and electromagnetic fields have been obtained for these devices and simulation results will be compared with experimental measurements where available.

MACH2 simulations of some DECADE plasma opening switch experiments

Summary form only given, as follows. The DECADE Facility at the Arnold Engineering Development Center (AEDC) is a short pulse high power X-ray generator used for nuclear effects simulation. It uses a plasma opening switch (POS) to produce the high voltage pulse required by the X-ray source. Over the past several years, we at the University of Tennessee Space Institute have used the MACH2 computer code to simulate several experimental POS configurations proposed for the DECADE Facility. It was found that the Hall effect must be included in an MHD code in order to simulate the opening physics. When the Hall effect is correctly implemented in the code, the simulations predict time-delay and voltage characteristics which compare favorably with the experimental results. Cableguns are used for initial plasma injection into the DECADE POS. The current MACH2 simulations use an empirical model based on experimental measurements of cablegun velocity and density to simulate this injection process. Our current efforts to model the cablegun plasma source are described. The methodology and limitations of the newly developed Hall algorithm are described and the simulation results are compared with experimental measurements for several POS configurations.

Cylindrical Langmuir probe measurements in an ion thruster plume

Summary form only given. Cylindrical Langmuir probe measurements have been made in the exhaust plume of a 15 cm diameter three-grid ion thruster. Xenon gas was used as a propellant. The experiments were performed in a large vacuum chamber with background pressure values of 2/spl times/10/sup -4/ Torr and 3/spl times/10/sup -6/ Torr. A 250 um diameter tungsten probe 2.78 mm long was used in the experiments. The axis of the probe was kept approximately parallel to the ion flow direction in order to minimize the probe surface exposed to the ion jet. The probe was referenced to a 1 m/spl times/1 m graphite plate placed 3 m downstream of the thruster and perpendicular to the plume. The graphite plate provided sufficient ion collection for reliable measurement in the electron saturation pan of the probe characteristic curve. Both the Langmuir probe and the reference electrode were electrically isolated with respect to the test chamber and the ion thruster. The probe and the thruster were positioned on perpendicular translation stages which permitted axial and radial displacement of the probe relative to the thruster. The data collected for radial and axial scans were analyzed to provide measurements of electron temperature, plasma potential and electron density. A method of data analysis was developed which includes an estimation of the collected ion current. This method will be compared with a numerical particle simulation model for calculation of the ion current component.