Electrode-plasma-driven radiation cutoff in long-pulse, high-power microwave devices

Abstract

The impact of electrode plasma dynamics on the radiation production in a high power microwave device is examined using particle-in-cell simulations. Using the design of a compact 2.4 GHz magnetically insulated line oscillator (MILO) as the basis for numerical simulations, we characterize the time-dependent device power and radiation output over a range of cathode plasma formation rates. These numerical simulations can self-consistently produce radiation characteristics that are similar to measured signals in long pulse duration MILOs. This modeling capability should result in improved assessment of existing high-power microwave devices and lead to new designs for increased radiation pulse durations. V C 2013 American Institute of Physics.[http://dx.doi.org/10.1063/1.4794955] High-power microwave (HPM) devices are capable of producing directed, sustained radiation pulses. 1 The formation of electrode plasmas in these devices has been identified along with a shortening of the radiation pulse and other effects. [2][3][4][5][6][7][8] Through the use of electromagnetic (EM) particle-in-cell (PIC) simulation codes, device designs have been optimized for efficient power output 1 although these simulations have generally not been able to model measured impedance collapse or radiation cutoff. In the case of magnetically insulated line oscillators 9,10 (MILOs), long pulse operation (typically greater than $100 ns) is hampered, reducing the potential peak radiated energy from these devices. 11,12 A MILO is essentially a coaxial transmission line that relies on the selfgenerated magnetic field due the current flowing in the center conductor to insulate an electron flow as it passes through a slow wave structure. The electron flow only becomes insulated from this anode structure once the current reaches a critical value. Therefore relatively fast rising power pulses are required to achieve insulation before electron bombardment of the anode leads to anode plasma formation and ion currents. In a MILO, the cutoff of the radiation pulse is often associated with a partial impedance collapse. A number of theoretical investigations have been carried out to understand the radiation cutoff in MILOs including simulating the impact of backscattered electrons, ion flows, and finite background gas pressures. 13-15 While these studies have motivated design changes resulting in measurable improvements, the radiation pulse duration in MILOs and other HPM devices is still much less than typical input power-pulse durations, particularly at higher operating voltages.Recent computational modeling developments have been utilized to examine the impedance collapse in a number of highpower devices, including charged-particle-beam diodes 16,17 and high-power transmission lines. 18 These models for electrode plasma generation are applied here to study the impedance evolution in a MILO and the associated radiation cutoff. The results of this study suggest that modest plasma formation rates (and modest monolayer contaminant levels) can strongly influen...

Computational analysis of current-loss mechanisms in a post-hole convolute driven by magnetically insulated transmission lines

“…This observation is consistent with other simulation results that model electrode plasma formation [32][33][34][35][36][37]. Here the underlying mechanism is associated with the highly nonuniform sheath current that occurs in the variable impedance MITLs; electron vortices at the edge of the plasma layer generate electric field reversals that pull plasma ions out into the MITL gap.…”

Simulations of the generation and transport of a 5 MV end-point x-ray beam on a pulsed power generator

“…Because of these large gaps, the plasmas that form on the surfaces do not have time to expand significantly into the gap before the pulse is over, which limits the effect that surface plasmas have on the diode impedance. This is helpful from a simulation standpoint, because impedance collapse caused by surface plasmas is notoriously hard to simulate in high-power diodes with small gap spacings (although some progress is being made [3][4][5][6][7][8]).…”