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

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)

CLIC Synchronisation Requirement Cavity to Cavity Phase synchronisation requirement (excluding bunch attraction) Target max. luminosity loss fraction S f (GHz)  x (nm)  c (rads)  rms (deg)  t (fs) Pulse Length (  s) Estimate RF to beam synchronisation ~ 100 fs (0.43 degrees)

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

RF Distribution 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. 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.

Expected Klystron Stability From kinematic model of klystron, phase and amplitude stability depend on gun voltage, V: For the Scandinova modulator with measured voltage stability of phase 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°.

Waveguide Choice 7 Waveguide type 35 meters COPPER Expansion = 17 ppm/K ModeTransmissionTiming error/0.3°C Width Timing error/0.3°C length N o of modes WR90(22.86x10.16mm)TE1045.4%210.5 fs498.9 fs1 Large Rectangular (25x14.5mm) TE1057.9%189.3 fs507.8 fs2 Cylindrical r =18mmTE0166.9%804.9 fs315.9 fs7 Cylindrical r =25mmTE0190.4%279.6 fs471.4 fs17 Copper coated extra pure INVAR 35 meters Expansion = 0.65 ppm/K ModeTransmissionTiming error/0.3°C Width Timing error/0.3°C length N o of modes WR90(22.86x10.16mm)TE1045.4%8.13 fs19.04 fs1 Large Rectangular (25x14.5mm) TE1057.9%6.57 fs19.69 fs2 Cylindrical r =18mmTE0166.9%30.8 fs12.1 fs7 Cylindrical r =25mmTE0190.4%10.7 fs18.02 fs17 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.

RF path length measurement 48MW 200ns pulsed GHz Klystron repetition 50Hz Vector modulation Control Phase Shifter 12 GHz Oscillator Main beam outward pick up From oscillator Phase shifter trombone (High power joint has been tested at SLAC) Magic Tee Waveguide path length phase and amplitude measurement and control 4kW 5  s pulsed 11.8 GHz Klystron/TWT repetition 5kHz LLRF Phase shifter trombone LLRF Cavity coupler 0dB or -40dB Expansion joint Single moded copper plated Invar waveguide losses over 35m ~ 3dB -30 dB coupler Forward power main pulse 12 MW Reflected power main pulse ~ 600 W Reflected power main pulse ~ 500 W Waveguide from high power Klystron to magic tee can be over moded Expansion joint RF path length is continuously measured and adjusted

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) DSP ADC Magic Tee To Cavity Wilkinson splitters -30 dB coupler DBM 10.7GHz Oscillator -30 dB coupler Piezoelectric Phase Shifter Piezoelectric phase shifter DAC1 Amp + LPF LPF Amp Power Meter To DSP DAC2 From DAC2 To Phase Shifter Calibration Stage

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. PLL controller MCU 10.7 GHz VCO Digital phase detector DBMs Power Meters Wilkinson splitter Inputs 400 ns span: RMS: 1.8 mdeg Pk-Pk: 8.5 mdeg 90 s span: Drift rate : 8.7 mdeg/10s Total drift: 80 mdeg

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.

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

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

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 TWT: 3kW 10-12GHz SLED II Pulse compressor Igor Syratchev RF Load Power Meter

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

LLRF System Test: Dynamic Range PhasePower 400MHz raw signals 40 dB below TWT saturation!

Pulse compressor Detuning PhasePower 400MHz raw signals

LLRF system Accuracy I Power accuracy: 0.4%rmsPhase accuracy: 0.9°rms 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

LLRF system Accuracy II 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. Input Output Calibrated Power Meter Output: Gain 2.9 Input

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

Screenshots from ‘CBOX1’

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. 22

Thank you for your attention!

Extra Slides

Observed Klystron Stability 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. Amplitude jitter reduced to ~1-2%. Phase measurements will be performed in the coming weeks.

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. To Cavity Wilkinson splitter DBM 12 GHz Oscillator -57 dB coupler phase shifter Klystron TWT Φ PIN diode switch Oscilloscope/ADC for FFT analysis RF LO IF Waveguide Measurement requires good amplitude stability as any AM will be present in the IF.

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

Planned CLIC crab high power tests 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 Travelling wave GHz phase advance 2  /3 TM110h mode Input power ~ 14 MW

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. 29

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. 1.6 GSPS 12- bit ADC’s DSP ADC Magic Tee To Cavity Wilkinson splitters -30 dB coupler DBM 11.6 GHz Oscillator -30 dB coupler Piezoelectric Phase Shifter Piezoelectric phase shifter DAC1 Amp + LPF 400 MHz LPF Amp Power Meter To DSP DAC2 From DAC2 To Phase Shifter Calibration Stage