1. 7.8GHz Dielectric Loaded High Power Generation And Extraction F. Gao, M. E. Conde, W. Gai, C. Jing, R. Konecny, W. Liu, J. G. Power, T. Wong and Z. Yusof Advanced Acceleration Concept Workshop, Santa Cruz, 2008

2. 2 Outline Introduction Design of the 7.8GHz power extractor Cold test Beam tests: i. Single bunch tests ii. Bunch train tests Summary

3. 3 Dielectric loaded two beam acceleration Power: >100MW Frequency: up to 30GHz RF pulse length: a few nanoseconds to a few tens of nanoseconds lower group velocity for RF pulse compression

4. 4 The power extractor using a circular DL waveguide dielectric vacuum Dielectric loaded waveguide and the power extractor transverse view e-field of the TM 01 mode (v p = c) E z of the TM 01 mode (v p = c) wakefield

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6. 6 Design of the power extractor: the deceleration waveguide DL deceleration waveguide TM 01 -TE 10 coupler rf output port Frequency: 7.8GHz Inner diameter: 12.04mm Outer diameter: 22.34mm Deceleration section length: 266mm Group velocity: 0.23c Generated power (Gaussian bunch length = 2mm): Single bunch: 79MW @100nC per bunch Bunch train (T b = 769ps): 100MW @30nC per bunch 280MW @ 50nC per bunch 1.1GW @ 100nC per bunch

7. 7 Bunch Length Measurements Experimental Results* *Preliminary Results old AWA gun new AWA gun

8. 8 mode launcher inside Design of the power extractor: the RF output coupler S 21 = -0.41dB @7.8GHz Power coupling efficiency: 91%.

9. 9 Experimental setup

10. 10 Single bunch test Power P s - MW q - nC t - ns voltage - Volt measured t - ns voltage arbitrary unit simulated f - GHz voltage spectrum measured

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12. 12 Bunch train test – UV laser micropulse train generation A B

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14. 14 Bunch train test – electron bunch train generation

15. 15 Charge Phase Scan 50 degree

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17. 17 Check UV laser bunch train with photodiode & energy meter t - ns v - Volt train A train B voltage signals from photodiode bunch charge comparison with energy meter index peak voltage - mV voltage - mV t - ms

18. 18 10ns long RF pulse generation Beam for ~10ns RF pulse generation 769ps

19. 19 2.4MW, 10ns long RF pulse generation 2.4MW generated, 2.2MW extracted. t - ns v - Volt f - GHz voltage spectrum

20. 20 22ns long RF pulse generation Beam for 22ns RF pulse generation 1.538ns

21. 21 4-bunch test for high power generation Simulation shows the power reach “flat-top” saturation level when the drive bunch contains 4 or more consecutive bunches spaced by 769ps. To maximize this power level the UV laser bunch was only split into 4 bunches. 44MW generated 40MW extracted 26.5nC per bunch f - GHz t - ns 769ps the 4 th bunch

22. 22 Summary Dielectric loaded power extraction has been demonstrated. 30MW of power has been generated in single bunch tests and 44MW in bunch train tests. 10ns and 22ns RF pulses have been observed. Currently the limitation for higher power generation is the beam current, which is further limited by the QE of the magnesium photocathode (~10 -4 ). A new cesium telluride photocathode with much higher QE (~10 -2 ) has been developed, yet to be installed and tested in a new gun (AWA G3). 280MW of output power are expected to be generated by electron charge of 50nC per bunch. For much higher charge with the new photocathode, space charge effect will be much stronger. Thus beam confinement with quadruple magnets may be needed.