1. Diamond LLRF Activities Pengda Gu For DLS RF Group

2. The Diamond RF Group Chris Christou Alek Bogusz Pengda Gu Matt Maddock Peter Marten Shivaji Pande Adam Rankin David Spink Alun Watkins

3. Agenda Introduction of DLS RF System Probe Blip and analogue gain control Non-IQ phase mesurement Direct sampling of 500MHz DLLRF requirement for DLS

4. 4 RF Systems Linac: 3GHz 2 x 5.2m Cu DESY structures 2 x 35MW klystrons bunchers Booster: 500MHz 1 x 5 cell Cu PETRA cavity 1 x 60kW IOT Storage ring: 500MHz 3 x 1 cell Nb Cornell cavities 3 x 300kW IOT combined amplifier

5. General SR RF Plant Phase and Gain Waveguide combiner, circulator and load Superconducting cavity DAs and IOTs Liquid Helium Refrigerator System Two cavities in operation, each with its own RF amplifier, LLRF and control system

6. SR RF Systems Superconducting Cavities HV PSU RFTF Circulator and load IOTs and Combiner Cryogenic Plant LLRF, DAs and Aux Supplies

7. Performance of the RF System

8. Dual cavity Vcav 1 = 1.5 MV Vcav 2 = 1.55 MV RF Instability LLRF phase error causes excessive synchrotron motion Cavity voltage and power oscillation

9. Probe problem Cavity Signal Spare pickup Another spare pickup 1.Cavities suffer from probe blips. 2. Probe blips happen with and without beam. 3. Probe blips happen on the main pickup which isn’t copper plated. 4. Probes don’t have blips at the same time. 5. Probe blips don’t always trip the beam. 6. Very high voltage. Cavity Probe RF Signal ‘Blips’ on individual probes

10. Signal plot from SSRF FBT Near Bottom Test of SSRF Cavity DLS and SSRF Cavity pickups are the same. RF cabling is different. Suspected to be caused by charging up of the ceramic insulator on the pickup. Photo of pickup with and without centre antennae

11. A close up of the PM below shows a loss of field for ~2 µs after which the signal returns but the amplifier has already tripped on reflected power Power ramp due to ‘loss’ of cavity voltage Trip due to RF Probe Blip Problem: fast rise-time spikes causing loss of RF signal on cavity HF pickup (and spare probes). Up to 4 blips/day/probe. LLRF interprets this as reduction in cavity volts and pushes up the forward power to compensate, causing a beam trip in some circumstances.

12. Actions taken: Addition of filtering in the LLRF to reduce the bandwidth from 1MHz to 50 kHz. This has been successful at preventing trips at low loop gains. No probe blip trips since January 2014. Trial of a additional circuitry to detect the blip and reduce the loop gain during the period of the blip (~2 to 6us typically). Simplified schematic of probe blip detection circuit Cavity Probe Blips

13. Tests using cavity simulator and blip simulated by blanking probe signal for 10us: V Cavity V Fwd pwr Trigger Blip Detector o/p Circuit enabled Circuit disabled Probe blip detector PCB mounted in LLRF module Signal break-out connector Cavity Probe Blips

14. Storage ring tests @ 300mA with LLRF#3 modified to include blip circuit: LIBERA box, Cavity, PFwd and beam (X) motion signals: Residual beam kick from blip ±50 µm Probe blip -> Attack and decay time- constants of gain reduction need optimisation to minimise beam disturbance. Cavity Probe Blips

15. Phase Measurement Unit Two units finished and installed in RF hall. Xilinx ML605 Development Board FMC150 module uses Texas Instruments ADS62P49 dual 14-bit, 250 MSPS ADC ADC clock generation constructed using ADS9912 DDS Direct RF sampling Phase advance between two samples Then calculate I and Q

16. Algorithm Implementation Test Results

17. A deliberate 0.06 deg phase modulation is clearly seen from - 3dBm. Phase Sample number No averaging, smoothing etc Sampling at ~ 241 MHz Phase measurement ~ 16 MHz

18. IQ Plot of Demodulated Signal from -10 to 13dBm with 287degree Phase Modulation Phase measurement at discrete power levels. Minimal distortion over dynamic range Advantages of this method: No imbalance between I and Q channels often observed using IQ demodulators No DC offset errors

19. Results from cavity 3 probe signal Phase variation and the single bunch can be clearly recognized. Phase data Close up shows clear phase shift across bunch train Bunch train Hybrid bunch Bunch gap ~ 1 deg

20. Direct Sampling and Demodulation of 500MHz Signal TI ADC12D1800 sampling at 2.0 GSPS. Sample by sample demodulation

21. Test in the Lab

22. Test during normal Operation Transient of RF field can be seen clearly. Phase variation can clearly be observed. Beam loading can clearly be observed. The single bunch in the gap can clearly be observed.

23. Signal from the Probe at FBT Different from the LLRF probe due to relative amplitude of RF and beam signal.

24. LLRF basic Functionali ties  Vector control of cavity accelerating field 0.15 degree and 0.05% of amplitude (RMS value).  Cavity resonance control The cavity will be tuned to minimize the total forward power needed.  Reduce the effective cavity impedance apparent to the beam Increase the Robinson instability threshold. Limited by the loop delay and the closed loop gain.  Reduce noise from different sources in the system Switching noise of IOT high voltage power supply.  Ramping of cavity field  Interlocks and equipment protection  Probe blip blockage

25. LLRF desirable functionalities  Post-mortem function or triggered data output  Testing and self-calibration  Baseband network analyzer  Conditioning mode  Feed forward algorithm, comb filter (one-turn delay feedback)…..  Amplifier linearization

26. Summary DLS LLRF system has been in operation for more than 9years. No major problems experienced. Digital LLRF is planned for SRF cavity and NC cavities.