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Frequency Tuning in a Pasotron
Carleen Boyer, John Rodgers, and Dan LathropUniversity of Maryland
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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
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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!
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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)
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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
150
250
Tim
e (μ
s)
Tim
e (μ
s)
UNTUNED TUNED
Relatively low current of around 30 A shows the excitable frequencies of the system
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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.
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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.
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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
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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.