Progress on the Design of a High Power S-Band MTM Microwave Source

Transcription

1 Progress on the Design of a High Power S-Band MTM Microwave Source CST PIC Simulations MURI TELECON September 6 th, 2013 Jason S. Hummelt

2 #2 Introduction Outline Metamaterial (MTM) circuit design CST PIC Simulations Conclusions

3 #3 Introduction MIT part of the FY12 MURI Experimental and theoretical research on innovative use of MTMs in continuing, controlling and radiating intense microwave pulses Near term MIT goals as part of MURI: MTMBWO device Experimental demonstration of S-Band (2-4 GHz) BWO using complementary split ring resonators (CSRRs) Utilize 500 kv, 80 A electron beam, 1 μs pulse Efficiency 10% (4 MW output Power)

4 #4 MTMBWO Design Considerations Utilize HRC 500 kv, 80 A electron gun 1 μs flat top pulse Electrostatically focused, spacecharge limited Use of existing magnet system (solenoid and magnetic lens) as well as collector Property of MTM microwave devices: low group velocity, long turn on time Turn on time depends strongly on group velocity Motivate design switch (from QuadMTM-discussed April 30 th, 2013) to MTMBWO

5 #5 Negative index waveguide MTMBWO Design Concept MTM plates machined with CSRRs put in standard waveguide Very simple design/fabrication Tuning/optimization of structure by varying iwg, th, etc.

6 #6 HFSS MTMBWO Dispersion Relation Obtain field profile and Dispersion Relation Dispersion Relation Positive Index HE Group velocity at operating point: v g =0.075c Negative Index HE Light Operating point Field Profile from HFSS Negative Index EH E-Beam Simulation upper ½ structure (symmetry); 1 period p Beam line dispersion relation: 2ππ k z v beam =0

7 #7 Introduction Outline Metamaterial (MTM) circuit design CST PIC Simulations Conclusions

8 #8 Electron Beam/Magnetic Field Have existing magnet system to focus beam from electrostatically focused gun Space charge dominated beam focused to r b =3.75 mm by the gun Use beam envelope equation for space charge dominated beam to get beam profile after focal spot CST simulation of a similar Pierce type gun (electrostatically focused) r b =3.75 mm SC term r = k r α2 r B field term Beam expands due to space charge k = m 1 α = qq 2m e ccc

9 #9 Electron Beam/Magnetic Field Numerically integrate envelope equation to get beam profile B field from measured values r from r = k r α2 r Solenoid Field Use CST PIC solver to simulate beam transport through magnetic field Agrees with numerical result 1.4 kg solenoid field used for PIC simulations of MTMBWO Mag. Lens Field E-beam Direction Focal spot of gun Beam Trajectory-Free Space (CST PIC particle trajectory)

10 CST PIC Model Setup #10 Front View Port Definition E-beam Direction Interaction Length =44 cm E-beam Direction Rear View Closeup MTM surface

11 #11 CST Results: Electron Bunching CST simulation of beam through MTMBWO t=400 ns Electron beam bunches at λ z =9 cm 850 kev Electron Energy 150 kev E-beam Direction

12 #12 CST Results: Fields CST Fields indicate growing backward wave, with power going out ports (back) along MTM plates Power Flow (direction indicated by arrows) Power out E-beam Direction E t=400 ns E-beam Direction Perspective: fields along midplane E-beam Direction

13 #13 CST Results: Output Power Power out of ports 1&2 Includes ohmic (copper structure), coupling losses Stationary power level: 5.75 MW Turn on Time (260 ns) Stationary Power (5.75 MW) FFT of Stationary Output

14 #14 Output Efficiency Efficiency investigated for two different beam radii Highest efficiency 14.5% Theoretical max efficiency for BWO: 20% r beam =2.2 mm r beam =8.5 mm

15 Automodulation As the length is increased, the efficiency goes down and sidebands are observed in Fourier Transform of output signal Consistent with BWO theory FFT of Stationary Output L=434 mm Modulating Output FFT of Stationary Output L=469 mm L=469 mm r beam =2.2 mm for both #15

16 #16 Turn on Time Obstacle in pulsed MTM devices: slow start up Filling time long for small v g, t fill ~L/v g =20 ns, r beam =2.2 mm r beam =8.5 mm

17 #17 Future Work Cold test MTMBWO concept with VNA at MIT 2 cold test designs are currently being made at MIT machine shop Finalize designs of MTMBWO Waiting on DURIP funds to initiate further experiments Install structure at MIT Hot test with 80 A, 500 kv beam Application of effective medium theory for this structure publication in progress

18 #18 Conclusions Observed in simulation self start-up of 2.6 GHz negative index mode in MTMBWO Verified excitation of backward wave observed in HFSS with CST PIC solver HFSS and CST frequencies agree Investigated different regimes of operation for MTMBWO with a 500 kv, 80 A electron beam Efficiency: peak of 14.5% (5.7 MW output power) Turn on Time: ~ ns for ~45 cm structure

19 #19 MURI collaborators LSU UNM Ohio State UC-Irvine MIT WAB staff and postdocs Rick Temkin Ivan Mastovsky Michael Shapiro Bill Guss Paul Woskov Sudheer Jawla Acknowledgements MIT WAB-students Sergey Arsenyev Elizabeth Kowalski Samantha Lewis Xueying Lu Brian Munroe Alexander Soane Sam Schaub Haoran Xu JieXi Zhang Visiting Scientists Zhaoyun Duan

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