S. J. Stephanakis

Theoretical modeling and experimental characterization of a rod-pinch diode

The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even f...

Plasma Erosion Opening Switch Research at NRL

This paper is a review of plasma erosion opening switch (PEOS) research performed at the Naval Research Laboratory (NRL). Several experimental and theoretical results are described to illustrate the present level of understanding and the best switching results obtained to date. Significant power multiplication has been achieved on the Gamble II generator, producing 3.5 TW with less than 10-ns rise time. Switching after nearly 1-¿s conduction time has been demonstrated on Pawn, producing a 0.2-TW 100-ns pulse. Scaling the switch to higher current, power, and conduction time should be possible based on theoretical analysis and the favorable results of scaling experiments performed thus far.

X‐ray ’’light pipes’’

A simple theory and experimental results are presented which describe the efficient transport of soft x radiation through hollow glass tubing by means of multiple grazing‐incidence internal reflection. It is shown that a large number of such tubes can be used to remotely image weak plasma sources of soft x radiation. The efficiency of transport through bent tubes and the use of light pipes as low‐pass filters for x radiation are discussed.

X radiation from high‐energy‐density exploded‐wire discharges

Exploded‐wire discharges of tungsten and titanium driven by a high‐power pulse generator have been used to produce intense x‐ray continuum and line radiation. A calibrated LiF crystal spectrograph recorded the radiation spectrum in the 3‐ to 25‐keV range. More than 20 J of x radiation are emitted in this photon energy band by tungsten plasmas in less than 50 nsec. The source of emission is less than 1 mm in diameter and about 3.5 cm long.

Ultra-high electron beam power and energy densities using a plasma-filled rod-pinch diode

The plasma-filled rod-pinch diode is a new technique to concentrate an intense electron beam to high power and energy density. Current from a pulsed power generator (typically ∼MV, MA, 100 ns pulse duration) flows through the injected plasma, which short-circuits the diode for 10–70 ns, then the impedance increases and a large fraction of the ∼MeV electron-beam energy is deposited at the tip of a 1 mm diameter, tapered rod anode, producing a small (sub-mm diameter), intense x-ray source. The current and voltage parameters imply 20–150 μm effective anode-cathode gaps at the time of maximum radiation, much smaller gaps than can be used between metal electrodes without premature shorting. Interferometric diagnostics indicate that the current initially sweeps up plasma in a snowplow-like manner, convecting current toward the rod tip. The density distribution is more diffuse at the time of beam formation with a low-density region near the rod surface where gap formation could occur. Particle simulations of the...

Experimental evaluation of a megavolt rod-pinch diode as a radiography source

The rod-pinch diode is a cylindrical pinched-beam diode that provides an intense pulsed small-diameter bremsstrahlung source for radiography. For this work, the diode consists of a 1- to 6.4-mm-diameter anode rod that extends through the hole of an annular cathode. After exiting the cathode, wider anodes taper down to a 1 mm diameter. All of the anode rods then have a 1-mm-diameter tungsten tip that is usually tapered to a point. Rod-pinch diodes with anode rods of different materials, lengths, and diameters were powered by the Gamble II generator at peak voltages of 1.0 to 1.8 MV and peak currents of 30 to 60 kA. The radiation was characterized with temporally and spatially resolved X-ray diagnostics. Pinhole-camera images and time-resolved pin-diode measurements indicate that the radiation is emitted primarily from the vicinity of the rod tip. The dose measured with thermoluminescent detectors through a plexiglass transmission window ranges from 0.6 to 2.8 R at 1 m from the rod tip and the dose/charge scales faster than linearly with the diode voltage. The full-width at half-maximum (FWHM) of the radiation pulse is 30 to 50 ns. The size of the radiation source-is determined by measuring its edge spread function. The source diameter, defined here as the FWHM of the derivative of the edge spread function, decreases from 2 mm for a 6.4-mm-diameter rod to 1 mm or less for a 1-mm-diameter rod. Analysis suggests that the central portion of the radiation distribution at the source can be approximated by a uniformly radiating circular disc.

Effect of pulse sharpening on imploding neon Z‐pinch plasmas

The radial implosion of hollow, cylindrical neon gas columns, driven by currents of up to 1.45 MA, produces a linear Z pinch with over 70% of the radiation in neon K lines. A plasma erosion opening switch (PEOS) is used to eliminate prepulse and to reduce the current rise time from ∼60 to ∼20 ns. Incorporation of the PEOS improves the uniformity of the Z pinch and increases the radiation yield.

Neutron and energetic ion production in exploded polyethylene fibers

Neutron production in exploded‐fiber z‐pinch plasmas containing hydrogen or deuterium is reported. Yields in excess of 1010 neutrons have been measured with deuterated fibers. The character of the neutron emission changes from that consistent with a thermal‐fusion source for large fiber diameters (100 μm) to one primarily due to energetic ion collisions for small fiber diameters (<25 μm). In the latter case, more than 1013 ions of multi‐MeV energies have been observed. This transition in the character of neutron emission is correlated with a fundamental change in the nature of the plasma as evidenced by resistivity measurements.

Laser wavefront analyzer for imploding plasma density and current profile measurements

The laser wavefront analyzer (LWA) consists of a polarized laser beam pulse that traverses an imploding z-pinch, and a microlens array that focuses the laser beam into a large number (104) of very tiny spots. LWA image analysis determines the refractive bending angles (due to density gradients) and Faraday rotation angles (due to the magnetic field-density integral) throughout the plasma cross section. Electron density and current distributions are derived from LWA data in an imploding gas-puff z-pinch plasma.

Intense ion‐beam‐transport experiments using a z‐discharge plasma channel

A z‐discharge plasma channel is used to confine and transport an intense proton beam. A pinch‐reflex ion diode on the NRL Gamble II accelerator focuses a proton beam onto the entrance aperture of a 2.5 cm diam, 1.2 m long z‐discharge transport system. The beam ions are charge and current neutralized in the discharge plasma, and execute betatronlike orbits in the magnetic field of the discharge. Ion beam diagnostics include shadowbox imaging and prompt‐γ radiation measurements from LiF targets. Under appropriate conditions, 95% particle transport and 90% energy transport are observed, with the only energy loss attributed to classical stopping in the channel gas. The transverse phase‐space distribution of the beam measured by the shadowbox is consistent with full charge and current neutralization of the transported beam.