Cutaway view of a first-generation half-scale prototype plasma jet accelerator module which uses tailored electrode profiles to suppress the occurrence of the blow-by instability. Two separate HV circuits power the electrothermal injectors and the main EM accelerator.
Illustration of blow-by in a straight coax accelerator driven by a HV capacitor. The larger magnetic field and higher current density at smaller radii causes an imbalance in the nominally axially directed Lorentz force, resulting in a faster acceleration of plasma near the inner electrode, which runs away from the bulk of the plasma.
Baseline plasma jet accelerator geometry for simulations. The right angle turn in this drawing turns out to be too large, and an angle closer to 45° is much better. See Figs. 4 and 5 for examples of simulated plasma flow in this geometry.
MACH2 density and joule heating contours at 11.5 and for baseline Wasp profile run hyperv37. Highest density is the inner red contour, lowest the outer dark blue contour. Velocity plots (not shown) indicate the plasma rapidly accelerating around the corner, leaving a low density pocket there which prevents significant current flow.
MACH2 kinetic energy density contours for a , 200 km/s case for a full-scale “wasp” profile. The total gun length is 75 cm, muzzle diameter is 15 cm, and the diameter at the angled injection point is .
MACH2 simulations show performance capability of coax gun for (a) low mass and (b) high mass armatures. Curves peak and then drop off in (a) as plasma leaves computational zone at muzzle exit.
Gun testing facility with the main accelerating PFN in foreground and target vacuum tank in background. The two large boxes in the center contain the capacitors and switches for the 32 capillary injectors.
Schematic of accelerator system showing two separate HV circuits for the capillary injectors and the main accelerator, respectively.
Detailed view of internal structure of capillary used in main accelerator. Except for the tungsten nozzle which requires machining, all components are commercial-off-the-shelf items simply cut to length. Internal vacuum seals are achieved using acrylic adhesives.
Closeup view of capillary HV connections extending out the back of the gun from their insulating carrier bolts. Current enters through the center tungsten electrode and returns via the coaxial brass tube. An external coaxial braid (not shown) keeps the current coaxial to reduce noise generation in diagnostics and other circuitry. See also Figs. 9 and 11.
The capillaries are mounted in a plastic Kynar ring with individual polyethylene nozzles.
Left: fast PImaximage of capillaries firing. Gate is 25 ns wide triggered at . Right: gate of . Capillary discharge fully established. Both images looking back up through the muzzle. Dark central circle is the center electrode.
Sparkgap gun configuration with 112 tungsten electrodes and a center electrode with a smooth circular arc profile instead of the “wasp” profile of the first prototype gun shown in Fig. 1.
Rear view of sparkgap gun with main bank disconnected to show only the 112 sparkgap injectors firing.
Left: arrangement of the original TwoPi test fixture showing copper bus plates, ballast resistors, capacitors, switches, and vacuum vessel. Right: Nikon open shutter photograph of a test shot in original TwoPi test fixture.
Main accelerating current rings due to impedance mismatch.
Nikon open shutter photograph of bright jet plume in vacuum tank. One arm of the interferometer and the interferometer probe beam are visible at the top of the image. Plasma jet travels toward the bottom where it stagnates on one of the 8 side port windows.
PIMax image of plasma stream incident on the pressure probe. This image coincides with an increase in the measured pressure. The bright spot at the probe tip and the dim arc upstream of it are reproducible features of the bow shock.
Relative line intensities for carbon ions. (a) Viewed radially from the gun, implying an electron temperature in the blob from ionization balance of . (b) Viewed axially inside the gun, implying an electron temperature internal to the gun from ionization balance of , higher than in the expanding blob.
Velocity for ions from the Doppler shift in two C II spectral lines viewed along the axis compared to the unshifted radial view. A shift in 0.18 nm here is equivalent to a velocity of .
Stark broadened Balmer line of neutral hydrogen with a Lorentzian fit at an electron density of . The lack of a central dip is attributed to integration along the radial line of sight.
The upgraded TwoPi test fixture has all tungsten electrodes, twice the capacitance per capillary of the original, as well as lower base pressure and better diagnostic access. Test firings exhibit much better symmetry as indicated by Nikon open shutter photograph.
Well defined imploding plasma liners produced by the merging of 64 discrete plasma jets are observed. (a) A highly symmetric implosion (but with small mass) at 80 km/s produces a small well defined central sphere of higher density plasma (Table I). (b) The slower more massive shell follows much later, and finally collapses in (c). Pictures are contrast enhanced to show detail.
Precursor jet implosion data.
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