Theory, simulation, and experiments on a magnetically insulated line oscillator (MILO) at 10 kA, 240 kV near Hull cutoff condition

The magnetically insulated line oscillator (MILO) is a high power microwave source that has received increased attention recently because it does not require an external magnetic field. Self-magnetic insulation typically requires operation at high currents, similar to 50 kA in previous experiments (at similar to 10 Omega). This paper reports the first MILO experiment operating at moderate current, less than 10 kA, at a lower voltage of 240 kV, driven by the Michigan Electron Long Beam Accelerator. The viability of this lower current operation was predicted by our recently developed theory on Brillouin flow, which also led to the rigorous derivation, for the first time, of the Buneman-Hartree condition for the cylindrical MILO using both the Brillouin flow and single particle model. The experiments show that more than 90% of shots operate at a magnetic field less than 1.3 times the Hull-cutoff magnetic field, and this magnetic field is significantly lower than the magnetic field required at the Buneman-Hartree condition. These experiments also oscillated at less current than the Hull cutoff condition on over 80% of shots, suggesting that MILOs might operate at a current lower than that expected at exactly Hull cutoff; this peculiar feature was also predicted by the theory. Particle-in-cell simulations from the improved concurrent electromagnetic particle-in-cell (ICEPIC) and CST codes are detailed, which corroborate MILO operation at lower currents than the Hull cutoff condition. The maximum efficiency achieved in these experiments is 1%, at a resonant frequency of 1 GHz. An initial comparison of the newly developed theory against prior MILO experiments is presented.(c) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).