Evaluation of Self-Magnetically Pinched Diodes up to 10 MV as High-Resolution Flash X-Ray Sources

Abstract

The merits of several high-resolution, pulsed-powerdriven, 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.Index Terms-Bremsstrahlung, coupled electron-photon transport, electron beams, flash X-radiography, high-power diodes, ion beams, Monte Carlo, particle-in-cell.

Anode plasma dynamics in the self-magnetic-pinch diode

“…The self-magnetic-pinch (SMP) diode [1][2][3][4] is being developed as an intense electron beam source for pulsedpower-driven x-ray radiography. The diode is composed of a thin cylindrical cathode with a hollow, rounded tip, as illustrated in Fig.…”

“…A new equilibrium is established with these counterstreaming currents, resulting in a larger diode current than in monopolar flow [5]. The diode operates as a current-limited device with the equilibrium current determined by a critical value of [2,6] I crit ¼ 8:5 r C g ð 2 À 1Þ 1=2 ½kA ;…”

Advances in High Intensity e-Beam Diode Development for Flash X-Ray Radiography

“…The self-magnetic pinch (SMP) [2,3] is a bipolar space--charge limited diode consisting of an anode and hollow cathode separated by a small A-K gap typically on the order of 1 cm (see Fig. 1b).…”

“…The ion space charge provides nearly complete charge neutralization of the electron beam resulting in both an increased diode current and a pinching of the electron beam due to the self-magnetic field. The diode is conjectured to be a critically current limited system [2] such that the operating current can be described by…”

“…where α is an empirical scaling factor ≈ 2 and is associated with the increased current due to the presence of ion space-charge [2,4,5] and is dependent on geometry and the beam and ion space-charge profile (assuming a radially uniform space-charge limited distribution of ions results in α ≈ 1.6 [5]). A typical diode current and voltage profile is given in Fig.…”