Long Pulse Operation of a High Power Microwave Source with a Metamaterial Loaded Waveguide

Transcription

1 MURI Grad Student Teleseminar Long Pulse Operation of a High Power Microwave Source with a Metamaterial Loaded Waveguide Xueying Lu MIT 02/03/2016

2 Outline Review of Stage I experiment Jason Hummelt thesis research Design and simulation of Stage II structure Reversed metamaterial design (MTM-R) MTM-R experimental results Sample high power pulses Operation space exploration Tuning of frequency and power Improved operation with steering coils Conclusions #2

3 Experimental setup Electron Beam Lens current Solenoid current 3 tunable operation parameters: Beam voltage Lens current Solenoid current #3

4 MTM structure design by J. Hummelt Complementary split ring resonators Beam travels between two identical MTM plates in a waveguide mm Beam #4

5 Experimental results Low magnetic field (< 450 G), antisymmetric mode, Cherenkov-cyclotron interaction Power level up to 5 MW High power pulses always observed with short pulse lengths below 300 ns #5

6 Reversed (MTM-R) circuit change Asymmetry introduced by reversing one of the MTM plates Built a complete new structure Two modes with negative group velocity Two types of interaction: Cherenkov, Cherenkov cyclotron. #6

7 Eigenmode simulation of field in a unit cell Electric field symmetry is changed Both modes are hybrid and deflecting in the MTM-R design MTM-R design Beam direction Mode 1 Mode 2 Plotted field: one period on the middle cutting plane #7

8 CST PIC simulation Uneven power in two arms CST predicts Cherenkov-cyclotron mode dominates at low magnetic field Output power in two arms at 900 G Spiraling beam Arm 1 Arm 2 #8

9 Sample pulse 1- long microwave pulse V = 420 kv, I = 60 A, B z = 437 Gauss Total output power 2.5 MW, efficiency 10%, pulse length 1 μs Electron beam interception as expected Both Mode 1 and Mode 2 are deflecting modes Coherent radiation at a single frequency of 2.35 GHz Normalized FFT amplitude #10

10 Sample pulse 2- high peak power V = 475 kv, I = 79 A, B z = 450 Gauss Peak total power: 8 MW Multiple frequencies #11

11 Operation space of high power MW-level power is found in a band of the solenoid-lens space The role of lens field Not changing magnetic field in the structure Affecting beam profile Fixed voltage: 490 kv Low power > MW power level Low power #12

12 Starting voltage measurement Starting voltage depends on both the lens field and the solenoid field MW level power when operating voltage is above starting voltage #13

13 Power variation with solenoid field Peak power experimental data taken at: A fixed lens field of 787 Gauss A fixed voltage of 460 kv Power (MW) Solenoid field (Gauss) #14

14 Frequency tuning with solenoid field Dots: experiment Fixed voltage 460 kv, fixed lens field 725 G Lines: calculation from uncoupled dispersion curves High power (> MW) Low Power (<1 kw) Mode 1, Cherenkov, Downshifted 5 MHz? Mode 1, cyclotron Mode 2, cyclotron, Upshifted 25 MHz #15

15 Frequency tuning with voltage: low power In the low power regime, excitation of different modes at different solenoid field values is verified. Solenoid field 648 Gauss Solenoid field 1511 Gauss *Theory line upshifted 25 MHz *Theory line downshifted 5 MHz Both terms increase with a higher beam voltage => Bigger slope Only one term increases With a higher beam voltage: => Smaller slope #16

16 Frequency tuning with voltage: the new type New type of interaction: not tunable with voltage. Not the Cherenkov type or the cyclotron type interaction Solenoid field 421 G, lens field 725 G #17

17 Steering coil installation Transverse kick on the beam to help with the cyclotron motion Combined effect of longitudinal focusing and transverse deflecting magnetic field B s v s v 0 Lens Steering oil position #18

18 Improved pulse with the steering coil on Steering coil current 0.2 A, beam deflecting angle in free space 13 mrad Beam voltage 450 kv, full beam current 73 A Lens 725 G, solenoid 466 G Improved microwave pulse envelope with a higher total power of 2.9 MW Central: GHz BW: 1 MHz #19

19 Conclusions The MTM-R structure has been built and tested. The design mode is a hybrid deflecting mode with a negative group velocity with the Cherenkov type and the cyclotron type interaction. A full width 1 μs long microwave pulse was generated with a 420 kv, 60 A beam. The total power in the two output arms was 2.5 MW and the efficiency was 10%. Frequency tuning measurement shows different types of interaction (Cherenkov, cyclotron and a new type) at varied operation conditions. A pair of steering coils was installed and helped increase the output power to 2.9 MW with a better flattop. #20

20 Acknowledgement Faculty and staff Sudheer Jawla Ivan Mastovsky Guy Rosenzweig Michael Shapiro Jacob Stephens Richard Temkin Paul Woskov Graduate students Hannah Hoffmann Julian Picard Samuel Schaub Alexander Soane Haoran Xu #21