Kristi Wilson

Magnetically enhanced vacuum arc thruster

A hydrodynamic model of the vacuum arc thruster and its plume is described. Primarily an effect of the magnetic field on the plume expansion and plasma generation is considered. Two particular examples are investigated, namely the magnetically enhanced co-axial vacuum arc thruster (MVAT) and the vacuum arc thruster with ring electrodes (RVAT). It is found that the magnetic field significantly decreases the plasma plume radial expansion under typical conditions. Predicted plasma density profiles in the plume of the MVAT are compared with experimental profiles, and generally a good agreement is found. In the case of the RVAT the influence of the magnetic field leads to plasma jet deceleration, which explains the non-monotonic dependence of the ion current density, on an axial magnetic field observed experimentally.

Magnetically Enhanced Vacuum Arc Thruster (MVAT)

*The Magnetically Enhanced Vacuum Arc Thruster (MVAT) is a solid propellant microthruster that can produce precisely controlled impulse bits. The MVAT is a variant on an earlier thruster (Vacuum Arc Thruster (VAT)). The VAT is a low mass (~500g), high efficiency (~10% measured) and high specific impulse (Isp ~ 1000-3000s) electric propulsion system. By adapting the technology of the vacuum arc thruster and combining with it an external axial magnetic field on the order of 0.1 Tesla, we have developed a thruster that has been shown to provide up to 50% higher thrust-to-power ratio than a conventional vacuum arc thruster. Also, the applied magnetic field can control the plasma plume produced by the thruster and therefore decrease contamination of adjacent optics and sensors on the spacecraft. Such a thruster would be well suited to provide precision control for small, power limited spacecraft for guidance, navigation and control (GN&C) applications.

Continued development of a 12 cm diameter nozzle for argon Z-pinches

We report on the variation of K-shell X-ray output of an argon Z-pinch as a function of the radial gas distribution. The tests, conducted on the Double-EAGLE simulator at /spl sim/3.5-MA peak current, utilized a 12-cm-diameter double-shell nozzle that was designed for use with the 300-ns rise-time current pulse (/spl sim/6 MA) of the DECADE QUAD pulsed power machine. By varying the plena gas pressures of the inner and outer shells, the net radial distribution could be changed from one that was strongly concentrated near the axis to one more broadly distributed as a function of radius. Previous work has shown that a roughly uniform radial distribution gives higher X-ray output than shell-like flows for gas Z-pinches. The present work was focused on refining the optimum radial distribution and to establish benchmarks for modeling calculations. The present data show that the K-shell yield has a broad optimum (and the relative strength of the K continuum >4 keV systematically changes) as the mass distribution becomes more peaked near the axis. Very-high-quality K-emitting volumes (<5 ns pulse width from <1 mm diameters) were achieved over a significant fraction of the pinch length.

LONG-TERM PERFORMANCE OF VACUUM ARC THRUSTERS

Vacuum Arc Thrusters (VAT) have been shown to work over a wide range of operating parameters with a constant, overall system efficiency approaching 10%. The combination of vacuum arc thruster heads with an inductive energy storage (IES) power processing unit (PPU) offers system mass and reliability advantages due to the elimination of energy storage capacitors and high voltages. New geometries and a feed mechanism have been developed to utilize these characteristics for high delta-V missions with constant performance.

Demonstration of a “gas anode” current return for a z-pinch

We have developed and successfully operated a “gas anode” for use with z-pinch plasma radiation x-ray sources (PRS). The gas anode replaces the multiple solid anode-currentreturn rods of a standard PRS with a cylindrical shell of low atomic number gas. Beginning at the start of current flow, either via pre-ionization by UV flashboards or via high voltage breakdown, the anode gas conducts the return current flow of the pinch for the duration of the implosion and beyond.

Characterization of a 500J Dense Plasma Focus for Producing Soft x-rays

The scaling of x-ray output with (stored energy)2 and ∼10ns radiation pulse width of the dense plasma focus make it an interesting source of soft x-ray radiation for lithography, biological imaging, nano imaging or soft x-ray diagnostic calibration. AASC was funded by DTRA to explore soft x-ray diagnostic calibration*. In this follow-on study, we explore an alternative electrode configuration with Ar. The soft x-radiation (≫1 keV) yield, radiation pulse width and debris are characterized for our 500J dense plasma focus over 100’s of pulses fired at 0.2 Hz. The radiation yield is compared with (current)4 scaling. A soft x-ray spectrometer is installed to examine the soft x-ray spectrum.