Frequency Tuning in a Pasotron Carleen Boyer, John Rodgers, and Dan Lathrop University of Maryland Motivation: IEDs and Microwaves? - Microwaves can be used to safely detonate or defeat IEDs HOWEVER Wikipedia Commons Current systems of generating microwaves exhibit two major issues: - Systems are not powerful enough - Frequency output is difficult to control Experimental Pasotron System Missing Frequencies! Time (μs) 250 150 Plasma E-Beam High-Current Plasma Cathode High-Power RF Coupler J.C. Rodgers - Plasma generated from heating helium gas - Grid creates a voltage potential, accelerating electrons - Electron beam couples with the field of the helix 50 1 1.15 1.3 Frequency (GHz) 1.45 Tuning the Pasotron Log(|Z|) (Ohms) 4 Frequency plot of the resonances in the Pasotron - Shows the spectrum of frequencies in the tuned Pasotron 0 -4 1.05 1.15 1.25 Frequency (GHz) 1.35 Q-factor 6000 Q-factor shows which resonances will be excited in device - Wanted to shift frequency range 4000 2000 1.05 De-Q these points 1.15 1.25 Frequency (GHz) 1.35 Hot Testing: Low Current UNTUNED TUNED Time (μs) 250 Time (μs) 250 150 150 50 1 50 1.15 1.3 Frequency (GHz) 1.45 1 1.15 1.3 Frequency (GHz) 1.45 Relatively low current of around 30 A shows the excitable frequencies of the system Hot Testing: Mid Current UNTUNED Time (μs) 250 150 150 v v Time (μs) v v 250 TUNED 50 1 50 1.15 1.3 Frequency (GHz) 1.45 v 1 1.15 1.3 Frequency (GHz) 1.45 Mid-range current of around 110 A shows frequency spread in the untuned system, and smaller excited band in the tuned system. Hot Testing: High Current UNTUNED Time (μs) 250 150 150 v v Time (μs) v v 250 TUNED 50 1 50 1.15 1.3 Frequency (GHz) 1.45 v 1 1.15 1.3 Frequency (GHz) 1.45 High current of around 170 A results in chaotic excitation of the frequencies, and the phenomena of mode hopping. Conclusions • Wanted high spectral density and wide bandwidth • Adjusted the cold Q-factors and showed that hot operation was affected by these adjustments • Increased spectral density • Eliminated mode competition, allowed for mode hopping operation Acknowledgments - Advisers Dan Lathrop and John Rodgers - Special thanks to Dave Meichle and Freja Nordseik - NSF Award Number PHY1156454 Y.R. Bliokh, G.S. Nusinovich, J.C. Rodgers, et al., “Ion noise in the plasma-assisted slow wave osillator”, IEEE Trans. Electronic Devices, vol. 52, pp.845-857, 2005. G.S. Nusinovich, O.V. Sinitsyn, J.C. Rodgers, A.G. Shkvarunets, Y. Carmel, “Phase locking in backward-wave oscillators with strong end reflections”, Physics of Plasmas, vol. 14, 2007.