Plasma-Filled Diode for High Dose-Rate Bremsstrahlung

Abstract

A plasma-filled diode (PFD) technique is described for producing high dose-rate bremsstrahlung over small areas. A PFD was developed for this purpose using the Gamble II generator. For maximum dose-rate conditions, the short-circuit phase duration was 40 ns, after which the voltage increased to 1.8 MV and the total current in the diode was 0.5 MA. The X-ray pulsewidth was 8 ns, much less than the typical 50 ns pulsewidth with a vacuum diode. This PFD was adapted to the higher-current Saturn generator at 6-8 MA by making a 10-cm diameter circular array of 6, 9, or 12 isolated PFDs. Higher dose rates were obtained using fewer PFDs; about 1 10 12 rad/s with the 12-PFD array, 2 10 12 with nine PFDs and 4 10 12 with 6 PFDs. Analyses based on electrical and radiation diagnostics indicate that 40%-77% of the electrical current produces radiation at the time of maximum dose rate. The X-ray pulsewidth was typically 13-16 ns, less than the pulsewidth for standard (vacuum) bremsstrahlung diodes on Saturn. This system, with improvements in reproducibility, could provide a high dose-rate, small-area bremsstrahlung capability for high current generators.Index Terms-Bremsstrahlung, dose rate, plasma-filled diode (PFD), pulsed power.

Propagation speed, linear stability, and ion acceleration in radially imploding Hall-driven electron-magnetohydrodynamic shocks

“…In this paper, we demonstrate that the previous results also hold for a cylindrical shock wave, such as might be encountered in the geometry of z-pinch, imploding liner, or plasma-filled diode (PFD) experiments. [6][7][8] EMHD physics has been hypothesized to be applicable to PFD experiments 1 , and future research could compare the instability reported in this paper to experimental results. The main difference between a cylindrical and a planar EMHD magnetic shock is that the planar shock moves at constant speed, while the speed of the cylindrical shock depends on its radial location.…”

“…In particular, the magnetic shock front can be unstable to a Kelvin-Helmholtz (KH) like mode, which will break the shock front apart into vortices that propagate faster than the shock front itself is predicted to move. This effect was examined for the cylindrical model by first doing linear stability calculations of a cylindrical shock front, and then computing solutions to the full 2D equations ( 6) and (7).…”

Propagation speed, linear stability, and ion acceleration in radially imploding Hall-driven electron-magnetohydrodynamic shocks

“…In this paper, we demonstrate that the previous results also hold for a cylindrical shock wave, such as might be encountered in the geometry of z-pinch, imploding liner, or plasma-filled diode (PFD) experiments. [6][7][8] EMHD physics has been hypothesized to be applicable to PFD experiments 1 , and future research could compare the instability reported in this paper to experimental results. The main difference between a cylindrical and a planar EMHD magnetic shock is that the planar shock moves at constant speed, while the speed of the cylindrical shock depends on its radial location.…”

“…In particular, the magnetic shock front can be unstable to a Kelvin-Helmholtz (KH) like mode, which will break the shock front apart into vortices that propagate faster than the shock front itself is predicted to move. This effect was examined for the cylindrical model by first doing linear stability calculations of a cylindrical shock front, and then computing solutions to the full 2D equations ( 6) and (7).…”

Power Increase of the Electron Source Based on the Plasma-Filled Diode

“…Experiments on the Gamble II generator with a planar diode for the negative polarity of the central conductor [9] and with a coaxial diode with a rod-type central electrode of the positive polarity [10] confirm the possibility of increasing the current density and, hence, the power of bremsstrahlung. The Gamble II generator is based on an intermediate storage line with a liquid dielectric, which ensures the short current rise time (∼50 ns) in the low-resistance phase of the diode.…”

Plasma-filled diode based on the coaxial gun

“…1). Plasma (4) is formed as a result of the discharge in the dielectric channel (6) between the central electrode of the gun (7) and the diode cathode (2). The length of the channel in the dielectric is 25 mm.…”

Propagation speed, linear stability, and ion acceleration in radially imploding Hall-driven electron-magnetohydrodynamic shocks

“…In this paper, we demonstrate that the previous results also hold for a cylindrical shock wave, such as might be encountered in the geometry of z-pinch, imploding liner, or plasma-filled diode (PFD) experiments. [6][7][8] EMHD physics has been hypothesized to be applicable to PFD experiments 1 , and future research could compare the instability reported in this paper to experimental results. The main difference between a cylindrical and a planar EMHD magnetic shock is that the planar shock moves at constant speed, while the speed of the cylindrical shock depends on its radial location.…”

“…In particular, the magnetic shock front can be unstable to a Kelvin-Helmholtz (KH) like mode, which will break the shock front apart into vortices that propagate faster than the shock front itself is predicted to move. This effect was examined for the cylindrical model by first doing linear stability calculations of a cylindrical shock front, and then computing solutions to the full 2D equations ( 6) and (7).…”

Power Increase of the Electron Source Based on the Plasma-Filled Diode

“…Experiments on the Gamble II generator with a planar diode for the negative polarity of the central conductor [9] and with a coaxial diode with a rod-type central electrode of the positive polarity [10] confirm the possibility of increasing the current density and, hence, the power of bremsstrahlung. The Gamble II generator is based on an intermediate storage line with a liquid dielectric, which ensures the short current rise time (∼50 ns) in the low-resistance phase of the diode.…”

Plasma-filled diode based on the coaxial gun

“…1). Plasma (4) is formed as a result of the discharge in the dielectric channel (6) between the central electrode of the gun (7) and the diode cathode (2). The length of the channel in the dielectric is 25 mm.…”

Propagation speed, linear stability, and ion acceleration in radially imploding Hall-driven electron-magnetohydrodynamic shocks

“…In this paper, we demonstrate that the previous results also hold for a cylindrical shock wave, such as might be encountered in the geometry of z-pinch, imploding liner, or plasma-filled diode (PFD) experiments. [6][7][8] EMHD physics has been hypothesized to be applicable to PFD experiments 1 , and future research could compare the instability reported in this paper to experimental results. The main difference between a cylindrical and a planar EMHD magnetic shock is that the planar shock moves at constant speed, while the speed of the cylindrical shock depends on its radial location.…”

“…In particular, the magnetic shock front can be unstable to a Kelvin-Helmholtz (KH) like mode, which will break the shock front apart into vortices that propagate faster than the shock front itself is predicted to move. This effect was examined for the cylindrical model by first doing linear stability calculations of a cylindrical shock front, and then computing solutions to the full 2D equations ( 6) and (7).…”

Power Increase of the Electron Source Based on the Plasma-Filled Diode

“…Experiments on the Gamble II generator with a planar diode for the negative polarity of the central conductor [9] and with a coaxial diode with a rod-type central electrode of the positive polarity [10] confirm the possibility of increasing the current density and, hence, the power of bremsstrahlung. The Gamble II generator is based on an intermediate storage line with a liquid dielectric, which ensures the short current rise time (∼50 ns) in the low-resistance phase of the diode.…”

Plasma-filled diode based on the coaxial gun

“…1). Plasma (4) is formed as a result of the discharge in the dielectric channel (6) between the central electrode of the gun (7) and the diode cathode (2). The length of the channel in the dielectric is 25 mm.…”