Frequency Tuning in a Pasotron

Carleen Boyer, John Rodgers, and Dan LathropUniversity of Maryland

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

Experimental Pasotron System

High-Power RF Coupler

High-Current Plasma Cathode

E-BeamPlasma

- 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 1.45Frequency (GHz)

50

250

Tim

e (μ

s)

150

Missing Frequencies!

Tuning the Pasotron4

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 1.25 1.35Frequency (GHz)

1.15 1.25 1.35Frequency (GHz)

1.05

-4

De-Q these points

2000

4000

6000

Q-fa

ctor

Log(

|Z|)

(O

hms)

Hot Testing: Low Current

1 1.15 1.3 1.45 1 1.15 1.3 1.45Frequency (GHz) Frequency (GHz)

50

150

250

50

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Tim

e (μ

s)

Tim

e (μ

s)

UNTUNED TUNED

Relatively low current of around 30 A shows the excitable frequencies of the system

Hot Testing: Mid Current

v

vv

v v50

150

250

1 1.15 1.3 1.45Frequency (GHz)

1 1.15 1.3 1.45Frequency (GHz)

50

150

250

Tim

e (μ

s)

Tim

e (μ

s)

UNTUNED TUNED

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

v1.15 1.3 1.45Frequency (GHz)

1.15 1.3 1.45Frequency (GHz)

vv

50

150

250

1

50

150

250

1

v v

Tim

e (μ

s)

Tim

e (μ

s)

UNTUNED TUNED

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.