Anode plasma dynamics in the self-magnetic-pinch diode

Bruner, N.; Welch, D. R.; Hahn, Kelly; Oliver, B.V.

doi:10.1103/physrevstab.14.024401

Cited by 32 publications

(23 citation statements)

References 21 publications

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“…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.…”

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confidence: 99%

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

et al. 2013

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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...

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confidence: 99%

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

et al. 2013

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“…18. Namely, these shots have the nominal diode current for some portion of the pulse and then an abrupt rise.…”

Section: Operation Of the Smp Diodementioning

confidence: 97%

“…[16][17][18][19] For example, the impedance is observed to decrease $0.33 X/ns during a nominal shot on RITS-6. (The nominal values for cathode diameter (d c ) and anode-cathode (AK) gap length (g) increase with voltage, as seen in Refs.…”

Section: Introductionmentioning

confidence: 97%

The impact of plasma dynamics on the self-magnetic-pinch diode impedance

et al. 2015

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The self-magnetic-pinch diode is being developed as an intense electron beam source for pulsedpower-driven x-ray radiography. The basic operation of this diode has long been understood in the context of pinched diodes, including the dynamic effect that the diode impedance decreases during the pulse due to electrode plasma formation and expansion. Experiments being conducted at Sandia National Laboratories' RITS-6 accelerator are helping to characterize these plasmas using timeresolved and time-integrated camera systems in the x-ray and visible. These diagnostics are analyzed in conjunction with particle-in-cell simulations of anode plasma formation and evolution. The results confirm the long-standing theory of critical-current operation with the addition of a time-dependent anode-cathode gap length. The results may suggest that anomalous impedance collapse is driven by increased plasma radial drift, leading to larger-than-average ion v r Â B h acceleration into the gap. V C 2015 AIP Publishing LLC. [http://dx.

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“…(1) includes bipolar flow, but does not include the effects of evolving plasmas as described in Ref. [10]. Two plasmarelated effects were identified which decrease the diode impedance during operation.…”

Section: Diode Current Comparison To Theorymentioning

confidence: 99%

“…In normal operation, this diode has a decreasing impedance related to anode plasma expansion [10]. Occasionally this diode experiences a more rapid impedance drop (> 0.5 Ω=ns) resulting in a radiographically poor shot.…”

Section: Introductionmentioning

confidence: 99%

Shot reproducibility of the self-magnetic-pinch diode at 4.5 MV

Bennett

Crain

Droemer

et al. 2014

Phys. Rev. ST Accel. Beams

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In experiments conducted at Sandia National Laboratories' RITS-6 accelerator, the self-magnetic-pinch diode exhibits significant shot-to-shot variability. Specifically, for identical hardware operated at the same voltage, some shots exhibit a catastrophic drop in diode impedance. A study is underway to identify sources of shot-to-shot variations which correlate with diode impedance collapse. The scope of this report is limited to data collected at 4.5-MV peak voltage and sources of variability which occur away from the diode, such as sheath electron emission and trajectories, variations in pulsed power, load and transmission line alignment, and different field shapers. We find no changes in the transmission line hardware, alignment, or hardware preparation methods which correlate with impedance collapse. However, in classifying good versus poor shots, we find that there is not a continuous spectrum of diode impedance behavior but that the good and poor shots can be grouped into two distinct impedance profiles. In poor shots, the sheath current in the load region falls from 16%-30% of the total current to less than 10%. This result will form the basis of a follow-up study focusing on the variability resulting from diode physics.

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Anode plasma dynamics in the self-magnetic-pinch diode

Cited by 32 publications

References 21 publications

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

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

The impact of plasma dynamics on the self-magnetic-pinch diode impedance

Shot reproducibility of the self-magnetic-pinch diode at 4.5 MV

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