High Power RF Measurements

1. High Power RF Measurements Ben Woolley, Amos Dexter, Igor Syratchev. Jan Kovermann, Joseph Tagg HG2013, Trieste June 2013

2. Outline Phase Stabilisation of CLIC crab cavities RF distribution to the cavities. Accurate phase measurements and stabilisation system. LLRF for future/current X-Band test stands Production of vector modulated signals for type II SLED pulse compressor. Phase and power measurements of RF signals. Working example using TWT and SLED II pulse compressor. Easily transferable to other test stands (e.g. TERA C-band test)

3. CLIC SynchronisationRequirement Cavity to Cavity Phase synchronisation requirement (excluding bunch attraction) Estimate RF to beam synchronisation ~ 100 fs (0.43 degrees)

4. Integration 1)Use over-moded waveguide from klystron to the tee 2)Two klystrons with low-level/optical signal distribution. 1)35m of waveguide from the Tee to the cavities, Scale: 20.3m from cavities to IP

5. RF Distribution Option 2: A klystron for each cavity synchronised using LLRF/optical distribution. • Femtosecond level stabilized optical distribution systems have been demonstrated (XFELs). • Requires klystron output with integrated phase jitter <4.4 fs. Option 1: A single klystron with high level RF distribution to the two cavities. • Klystron phase jitter gets sent to both cavities for identical path length. Δφ=0. • Will require RF path lengths to be stabilised to within 1 micron over 40m.

6. Expected Klystron Stability • For the Scandinova modulator with measured voltage stability of 10-4phase and amplitude stability should be 0.12° and 0.013%. • Active feed-forward or feedback would be needed to gain the required stability of 0.02°. From kinematic model of klystron, phase and amplitude stability depend on gun voltage, V:

7. WaveguideChoice Rectangular invar is the best choice as it offers much better temperature stability-> Expands 2.3 microns for 35 m of waveguide per 0.1 °C.

8. RF path length measurement RF path length is continuously measured and adjusted 4kW5ms pulsed 11.8 GHz Klystron/TWT repetition 5kHz Cavity coupler 0dB or -40dB Cavity coupler 0dB or -40dB Waveguide path length phase and amplitude measurement and control Forward power main pulse 12 MW Single moded copper plated Invar waveguide losses over 35m ~ 3dB -30 dB coupler -30 dB coupler Expansion joint Expansion joint LLRF Magic Tee LLRF Reflected power main pulse ~ 600 W Reflected power main pulse ~ 500 W Phase shifter trombone Phase shifter trombone (High power joint has been tested at SLAC) Waveguide from high power Klystron to magic tee can be over moded Phase Shifter Main beam outward pick up Main beam outward pick up From oscillator 48MW200ns pulsed 11.994 GHz Klystron repetition 50Hz Vector modulation 12 GHz Oscillator Control

9. LLRF Hardware Layout (Low BW) • Fast phase measurements during the pulse (20-30 ns). • Full scale linear phase measurements to centre mixers and for calibration. • High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz). • DSP control of phase shifters. Linear Phase Detector (1MHz BW) Amp LPF 10.7GHz Oscillator DBM DBM ADC Amp + LPF ADC DBM Power Meter To DSP DSP DAC2 Wilkinson splitters DAC1 From DAC2 Calibration Stage To Phase Shifter -30 dB coupler -30 dB coupler Magic Tee To Cavity To Cavity Piezoelectric phase shifter Piezoelectric Phase Shifter

10. Board Development and CW tests Front end electronics to enable phase to be measure during the short pulses to an accuracy of 2 milli-degrees has been prototyped and dedicated boards are being developed. MCU PLL controller 10.7 GHz VCO Wilkinson splitter Digital phase detector 400 ns span: RMS: 1.8 mdeg Pk-Pk: 8.5 mdeg DBMs 90 s span: Drift rate : 8.7 mdeg/10s Total drift: 80 mdeg Power Meters Inputs

11. Crab cavity stabilisation: Next steps LLRF board revision: Some problems with the PLL and tolerances on the Wilkinson splitters. (Non equal power splitting). Investigation into pulsed power operation, effect of amplitude instabilities and higher order mixing products (not visible in CW tests). RA joining Lancaster in September to increase/broaden the effort on this project.

12. Future LLRF Generation and Acquisition for X-band test stands 12GHz vector modulated signal to DUT 2.4GHz vector modulated signal 12 GHz BPF IF RF Vector Modulator RFout LOinLOout 2.4 GHz Oscillator LO 9.6GHz BPF X4 freq. Amp 12GHz CW reference signal LO 3dB hybrid 12 GHz BPF RF IF 2.4GHz CW reference signal Oscillators should be phase locked IF LO RF Input 1 IF LO 1.6 GSPS 12-bit ADCs RF Input 2 400 MHzLPFs IF Amps 11.6 GHz BPF X4 freq. IF Amp 2.9 GHz Oscillator LO RF Input 3 Digital IQ demodulation IF 12GHz CW reference signal LO RF_Referance

13. System Testing: SLED II PC RF Input 1 RF Input 2 RF Input 3 20 dB attenuator 20 dB attenuator -40 dB coupler -40 dB coupler Phase modulated pulse input TWT LOAD SLED II pulse compressor

14. System Testing: SLED II PC • National Instruments PXI crate containing: • 2 CW generators for the LOs. • Vector modulator (up to 6.6GHz) with 200 MSPS I/Q generator • 5Chs 1.6GSPS 12-bit and 4Chs 250MSPS 14-bit ADC each connected to FPGAs. • 200 MHz digital I/O board for interlock and triggering signals. Up/down-mixing components and cabling Power Meter TWT: 3kW 10-12GHz RF Load SLED II Pulse compressor Igor Syratchev

15. LLRF System Test: SLED II PC Power Phase 400MHz raw signals Trans. Refl. Inc.

16. LLRF System Test: Dynamic Range Power Phase 400MHz raw signals 40 dB below TWT saturation!

17. Pulse compressor Detuning Power Phase 400MHz raw signals

18. LLRF system Accuracy I Possible sources of error • Problem with locking between the two CW generators and the 10MHz system reference caused the signal to beat.  This affects acquisition AND vector generation. Proposed solution: Clock ADC’s with 400MHz reference. • Bit-noise on the ADCs limit accuracy to 0.1% (9.5ENOB). • All harmonics generated by the multipliers and/or mixers will be mixed down to 400MHz and be indistinguishable from the signal of interest. 400MHz Reference Power accuracy: 0.4%rmsPhase accuracy: 0.9°rms

19. LLRF system Accuracy II Calibrated Power Meter Output: Gain 2.9 Input Input Comparison with calibrated power meter • Transients at the start of the main pulse and phase flip are observed (input). • Transients reduced when passed through the detuned system. Hybrid is narrow band  extra filtering of harmonics gives cleaner signal when mixing down. Output

20. TERA C-band Cavity Testing PMT and faraday cup Cavity under test PXI crate Directional coupler Circulator RF detector diodes, PMT and faraday cup inputs. Timing board/ trigger generation 5.7GHz 4MWMagnetron Pictures: Alberto Degiovanni

21. Screenshots from ‘CBOX1’

22. X-band LLRF: Future Developments Characterisation of errors/transients in the system. Both on the generation and acquisition side. Miniaturisation of LLRF system  components into crate. Software: logging, interlock control, timing and triggers etc. Integration into XBOX-2 test stand. Scaling up and integration towards XBOX-3.

23. Thank you for your attention!

24. Extra Slides

25. Observed Klystron Stability • Amplitude jitter reduced to ~1-2%. • Phase measurements will be performed in the coming weeks. Observed ~5% amplitude jitter on the output of the klystron.  This was due to a mismatch in the pulse forming diode in the LLRF network and a triggering error in the ADC firmware.

26. Klystron phase noise • Can use a standard method to measure the phase stability of the klystron. • Reference source is split such that its phase noise is correlated out by the mixer for both channels. • Phase shifter adjusted as to bring RF and LO inputs into quadrature. • Digital scope or ADC and FPGA/DSP preforms FFT analysis, to obtain phase noise curve. • Experiment can also be repeated for different lengths of waveguide to ascertain the effects of waveguide dispersion. Waveguide Klystron TWT To Cavity PIN diode switch -57 dB coupler 12 GHz Oscillator RF Oscilloscope/ADC for FFT analysis LO Φ IF phase shifter Wilkinson splitter DBM Measurement requires good amplitude stability as any AM will be present in the IF.

27. Waveguide Stability Model Use ANSYS to find “dangerous” modes of vibration for a 1 m length of waveguide fixed at both ends. Fundamental mode 65.4 Hz

28. Planned CLIC crab high power tests Travelling wave 11.9942 GHz phase advance 2p/3 TM110h mode Input power ~ 14 MW Test 1: Middle Cell Testing – Low field coupler, symmetrical cells. Develop UK manufacturing. Test 2: Coupler and cavity test – Final coupler design, polarised cells, no dampers. Made with CERN to use proven techniques. Test 3: Damped Cell Testing – Full system prototype

29. Future Continued investigations into the phase stability of the 50 MW X-band klystron. Development of feed-forward and/or feedback system to stabilise the klystron’s output. Continued characterisation of electronics to obtain stand alone phase measurement/correction system. Design/procurement of the waveguide components needed. Demonstration RF distribution system, with phase stability measurements. Perform phase stability measurements during the CTF dog-leg experiments. Measure phase across the prototype cavity during a high power test.

30. LLRF Hardware Layout (High BW) • Fast phase measurements during the pulse (50MHz). • 400MHz direct sampling to centre mixers and for calibration. • High accuracy differential phase measurements of RF path length difference (5 μs, 5 kHz). • DSP control of phase shifters. 400MHzLPF Amp 11.6 GHz Oscillator DBM DBM 1.6 GSPS 12-bit ADC’s Amp + LPF ADC DBM Power Meter To DSP DAC2 DSP Wilkinson splitters DAC1 From DAC2 Calibration Stage To Phase Shifter -30 dB coupler -30 dB coupler Magic Tee To Cavity To Cavity Piezoelectric phase shifter Piezoelectric Phase Shifter