Electron dynamics in the bremsstrahlung reflex triode

Summary form only given. The reflex triode is a versatile bremsstrahlung source for Nuclear Weapons Effects simulation. The reflex triode employs a rangethin foil anode between two identical face-to-face hollow cathodes. Electrons undergo multiple reflexes through the anode foil converter as they lose their energy and are focused radially. The reflex triode can operate in the high total dose mode; the high dose rate mode; or the "high fidelity" mode, which maximizes the production of warm (5 keV < E < 50 keV) X-ray photons. When compared to bremsstrahlung production devices employing a range-thick anode converter, the use of a range-thin anode converter not only enhances the escape of warm X-rays, but also mitigates converter debris. Despite these advantages, key aspects of the physics of the operation of the reflex triode need to be better understood to optimize its radiation output. One of these unresolved aspects is the detailed motion of the electrons as they reflex through the anode foil. Modeling of the reflex triode operation with 2-1/2 D PIC codes predicts an unexpectedly strong fluctuating electric field adjacent to the anode, having frequencies -/spl square/pe and magnitudes roughly equal to the average electric field. This stochastic electric field causes the average number of reflexes to scale much less strongly that the inverse of the thickness of the anode foil. Results of analytic modeling and 2-1/2 D Particle-in-Cell simulations will be presented which explore the dynamics of reflexing electrons and the accompanying fluctuating electric field in the reflex triode. To isolate this reflexing from other aspects of reflex triode operation, such as radial focusing, these efforts have centered on a simplified, I-D version of the device, which assumes the presence of a strong axial magnetic field. Analytic modeling for the steady state of a relativistic I-D reflex triode suggest that the strong stochastic electric field arises to disrupt a runaway feedback- between the space charge limited ion current and the electron density at the anode. Indeed, this analytic modeling predicts that a quiescent steady state exist for operating voltages of interest (0.25 MV - 1.5 MV) only for foils thicker than -1/4 of the electron range in the anode foil material (assumed to be Ta). For thinner anode foils (which includes all anode foils of interest for bremsstrahlung reflex triode operation) the above feedback mechanism produces a runaway electron and ion current. Equilibrium is restored only by the stochastic electric field, which disrupts this feedback. Work is in progress to confirm this hypothesis by comparing the onset of the fluctuating electric field in the PIC simulations with the predictions of the 1-D analytic modeling. Also underway is analytic modeling of a non-quiescent steady state of the 1-D relativistic reflex triode, which includes the effects of a stochastic electric field on the electron dynamics.