HOM Transfer Function Measurements and other topics Larry Doolittle, LBNL ICFA Workshop on High Order Modes in Superconducting Cavities 2014-07-15
Applicability • any linac with photoinjector (context is LCLS-II) HOMSC14, July 13-15, 2014 2
LCLS-II Numbers Game of subharmonics • L-band RF is f 0 = 1300 MHz • Gun cavity is f 0 /7 = 185.714 MHz • APEX construction uses 1 laser pulse per 5 gun cavity cycles, 37.143 MHz Most short-pulse lasers have rep-rate between 30 MHz and 100 MHz 99% of the documentation for the planned LCLS-II talks about the photon pulses at the photocathode, and by derivation the electron bunches, being regularly spaced at ‘1 MHz’, sometimes with the footnote that this is really 928.6 kHz. • One nominal bunch every 40 laser pulses HOMSC14, July 13-15, 2014 3
What’s in a laser? “A chain of 4 pre-amplifiers gets the seed from the oscillator. After the first two pre-amplifiers, the repetition rate is decreased to 1 MHz by a acousto- optic kicker (AOM), driven by a 2 W 100 MHz pulsed rf signal synchronous with the oscillator. The AOM’s rise and fall time are at the 20-30 ns level.” • The Photocathode Laser System for the APEX High Repetition Rate Photoinjector D. Filippetto et al. , Proceedings of FEL2013 Flexible! There are many limits to how often the AOM can be fired, but especially at low bunch charge, it is pretty much pulse-on-demand subject to the 26.9 ns granularity, and keeping approximately 1 pulse per microsecond or less over tens of microseconds. HOMSC14, July 13-15, 2014 4
Normal Usual spectrum of ‘1 MHz’ beam, with TE 111 mode impedances shown. 40 35 30 Strength (dB) 25 20 15 10 5 0 2442 2444 2446 2448 2450 2452 2454 f (MHz) HOMSC14, July 13-15, 2014 5
Abnormal Same conditions, but jiggle each pulse’s timing ± 13 ns to hit 2449.9 MHz 40 35 30 Strength (dB) 25 20 15 10 5 0 2442 2444 2446 2448 2450 2452 2454 f (MHz) HOMSC14, July 13-15, 2014 6
Normal Usual spectrum of ‘1 MHz’ beam, with TE 111 mode impedances shown. 40 30 20 Strength (dB) 10 0 -10 -20 -30 -40 2380 2390 2400 2410 2420 2430 2440 2450 2460 f (MHz) HOMSC14, July 13-15, 2014 7
White phase noise added Random jitter modulated 40 30 20 Strength (dB) 10 0 -10 -20 -30 -40 2380 2390 2400 2410 2420 2430 2440 2450 2460 f (MHz) HOMSC14, July 13-15, 2014 8
White amplitude noise added Random 5% pulse dropouts 40 30 20 Strength (dB) 10 0 -10 -20 -30 -40 2380 2390 2400 2410 2420 2430 2440 2450 2460 f (MHz) HOMSC14, July 13-15, 2014 9
Transfer function • Excite from photoinjector • Couple to monopole cavity mode • Synchronous RF measurement on HOM probe (or fundamental probe) Timing system planned for LCLS-II is flexible enough to synchronize this stimulus/response over whole accelerator site HOMSC14, July 13-15, 2014 10
Alternative Modulating the bunch charge, as described by Daniel Hall of Cornell, is perhaps simpler to think about. But a full linac’s longitudinal bunch propagation is sensitive to bunch charge due to short-range wakes. Nothing in the system besides HOMs (and presumably the end-station photon experiments) is sensitive to ± 13 ns changes in bunch arrival time. HOMSC14, July 13-15, 2014 11
Conclusions • Photoinjectors don’t have to be operated only at their nominal setting • Pulse-on-demand feature can be used as a programmable frequency source to excite (or avoid exciting) HOMs at any frequency Thanks for listening! HOMSC14, July 13-15, 2014 12
Familiar Block Diagram housekeeping and custom functions Signal Signal ADC DAC Conditioning Conditioning FPGA Signal Signal ADC DAC Conditioning Conditioning Host I/F Host CPU or PHY Network HOMSC14, July 13-15, 2014 13
Familiar Block Diagram housekeeping and custom functions Clock ADC DAC FPGA ADC DAC Host I/F LO Host CPU or PHY Network HOMSC14, July 13-15, 2014 14
Data Acquisition 1.7 GHz Antenna ADC LO 1.7 GHz 20 MHz Antenna ADC LO1 LO2 2.8 GHz 3.9 GHz 20 MHz Antenna ADC HOMSC14, July 13-15, 2014 15
Data Acquisition - DESY/Bou Habib 1.3 GHz 1.7 GHz 2.4 GHz 0.5 0.4 0.3 0.2 0.1 0 0.45 0.5 0.55 0.6 0.65 0.7 f s (GHz) HOMSC14, July 13-15, 2014 16
Data Acquisition - LCLS-II LLRF compatible 2-board set, plus 3960 MHz HOM second LO off-board 3954 MHz 1320.0 MHz downconversion LO x3 cavity filters (20 MHz BW) 3940 MHz 20 MHz HOM Antenna 20 MHz IF 1200-2800 MHz? 3940 MHz 20 MHz HOM Antenna 20 MHz IF 1200-2850 MHz ÷ 16 75-177 MHz freq. monitor VCO 5100-6800 MHz HMC586? 188.57 MHz reference PLL ADF4106? HOMSC14, July 13-15, 2014 17
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