F.C. Young

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...

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...

Evaluation of self-magnetically pinched diodes up to 10 MV as high-resolution flash X-ray sources

The merits of several high-resolution, pulsed-power-driven, flash X-ray sources are examined with numerical simulation for voltages up to 10 MV. The charged particle dynamics in these self-magnetically pinched diodes (SMPDs), as well as electron scattering and energy loss in the high-atomic-number target, are treated with the partic by coupling the output from LSP with the two-dimensional component of the integrated tiger series of Monte Carlo electron/photon transport codes, CYLTRAN. The LSP/CYLTRAN model agrees well with angular dose-rate measurements from positive-polarity rod-pinch-diode experiments, where peak voltages ranged from 5.2-6.3 MV. This analysis indicates that, in this voltage range, the dose increases with angle and is a maximum in the direction headed back into the generator. This suggests that high-voltage rod-pinch experiments should be performed in negative polarity to maximize the extracted dose. The benchmarked LSP/CYLTRAN model is then used to examine three attractive negative-polarity diode geometry concepts as possible high-resolution radiography sources for voltages up to 10 MV. For a 2-mm-diameter reentrant rod-pinch diode (RPD), a forward-directed dose of 740 rad(LiF) at 1 m in a 50-ns full-width at half-maximum radiation pulse is predicted. For a 2-mm-diameter nonreentrant RPD, a forward-directed dose of 1270 rad(LiF) is predicted. For both RPDs, the on-axis X-ray spot size is comparable to the rod diameter. A self-similar hydrodynamic model for rod expansion indicates that spot-size growth from hydrodynamic effects should be minimal. For the planar SMPD, a forward-directed dose of 1370 rad(LiF) and a similar X-ray spot size are predicted. These results show that the nonreentrant RPD and the planar SMPD are very attractive candidates for negative-polarity high-resolution X-ray sources for voltages of up to 10 MV.

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.

High‐voltage, high‐power operation of the plasma erosion opening switch

A plasma erosion opening switch (PEOS) is used as the opening switch for a vacuum inductive storage system driven by a 1.8‐MV, 1.6‐TW pulsed power generator. A 135‐nH vacuum inductor is current charged to ∼750 kA in 50 ns through the closed PEOS which then opens in <10 ns into an inverse ion diode load. Electrical diagnostics and nuclear activations from ions accelerated in the diode yield a peak load voltage (4.25 MV) and peak load power (2.8 TW) that are 2.4 and 1.8 times greater than ideal matched load values for the same generator values.

Development of a sodium-pump/neo-lasant photopumped soft X-ray laser

An intense source of sodium pump-line radiation has been created and used to photopump a neon plasma for application to a pulsed-power driven sodium/neo X-ray laser. Properties of the sodium-pump plasma and the neon-lasant plasma required to optimize fluorescence and lasing are determined. The implosion of a sodium-bearing plasma with a megampere pulsed-power driver (Gamble II) is used to produce a linear Z-pinch with up to 25 GW of sodium-pump-line radiation. A separate neon plasma, driven by part of the return current from the imploding sodium plasma, is created parallel to the sodium line source at a distance of 5 cm. Evidence for population inversion is indicated by fluorescence enhancement of the 11-AA resonance line from the n=4 level of neon when pumped by sodium. >

Rod-pinch diode operation at 2 to 4 MV for high resolution pulsed radiography

The rod-pinch diode is operated successfully at peak voltages of 2.4–4.4 MV for peak electrical currents of 55–135 kA delivered to the diode. At 4 MV, tungsten anode rods of 1 or 2 mm diam produce on-axis doses at 1 m of 16 rad(Si) or 20 rad(Si), respectively. The on-axis source diameter based on the full width at half-maximum (FWHM) of the line-spread function (LSF) is 0.9±0.1 mm for a 1 mm diam rod and 1.4±0.1 mm for a 2 mm diam rod, independent of voltage. The LANL source diameter, determined from the modulation transfer function of the LSF, is nearly twice the FWHM. The measured rod-pinch current is reproduced with a diode model that includes ions and accounts for anode and cathode plasma expansion.The rod-pinch diode is operated successfully at peak voltages of 2.4–4.4 MV for peak electrical currents of 55–135 kA delivered to the diode. At 4 MV, tungsten anode rods of 1 or 2 mm diam produce on-axis doses at 1 m of 16 rad(Si) or 20 rad(Si), respectively. The on-axis source diameter based on the full width at half-maximum (FWHM) of the line-spread function (LSF) is 0.9±0.1 mm for a 1 mm diam rod and 1.4±0.1 mm for a 2 mm diam rod, independent of voltage. The LANL source diameter, determined from the modulation transfer function of the LSF, is nearly twice the FWHM. The measured rod-pinch current is reproduced with a diode model that includes ions and accounts for anode and cathode plasma expansion.

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.

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.