1. Frequency Tuning in a Pasotron Carleen Boyer, John Rodgers, and Dan Lathrop University of Maryland

2. Motivation: IEDs and Microwaves? -Microwaves can be used to safely detonate or defeat IEDs HOWEVER Current systems of generating microwaves exhibit two major issues: -Systems are not powerful enough -Frequency output is difficult to control Wikipedia Commons

3. Experimental Pasotron System -Plasma generated from heating helium gas -Grid creates a voltage potential, accelerating electrons -Electron beam couples with the field of the helix J.C. Rodgers 1 1.15 1.3 1.45 Frequency (GHz) 50 250 Time (μs) 150 Missing Frequencies!

4. Tuning the Pasotron 4 0 1.05 Frequency plot of the resonances in the Pasotron -Shows the spectrum of frequencies in the tuned Pasotron Q-factor shows which resonances will be excited in device -Wanted to shift frequency range 1.15 1.251.35 Frequency (GHz) 1.15 1.251.35 Frequency (GHz) 1.05 -4 De-Q these points 2000 4000 6000 Q-factor Log(|Z|) (Ohms)

5. Hot Testing: Low Current 1 1.15 1.3 1.45 1 1.15 1.3 1.45 Frequency (GHz) 50 150 250 50 150 250 Time (μs) UNTUNEDTUNED Relatively low current of around 30 A shows the excitable frequencies of the system

6. Hot Testing: Mid Current v v v v v 50 150 250 1 1.15 1.3 1.45 Frequency (GHz) 1 1.15 1.3 1.45 Frequency (GHz) 50 150 250 Time (μs) UNTUNEDTUNED Mid-range current of around 110 A shows frequency spread in the untuned system, and smaller excited band in the tuned system.

7. Hot Testing: High Current v 1.15 1.3 1.45 Frequency (GHz) 1.15 1.3 1.45 Frequency (GHz) v v 50 150 250 1 50 150 250 1 v v Time (μs) UNTUNEDTUNED High current of around 170 A results in chaotic excitation of the frequencies, and the phenomena of mode hopping.

8. 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

9. 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.