1. Sub-10 fs RF Regulation at REGAE Matthias Hoffmann for the LLRF team Low Level Radio Frequency Workshop 2015 Shanghai, 06.11.2015

2. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 2 Overview. > Very short: What is REGAE? > Short: Overview of the LLRF system at REGAE and the requirements > System identification and controller design for REGAE > RF stability measurements at REGAE  Feedback performance  Characterization of the klystron/actuator chain > Outlook/developments and plans for the future

3. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 3 What is REGAE? > Time resolved electron diffraction experiments (MPI) and test facility for laser driven plasma- wakefield acceleration (LAOLA) > Generation of sub-10 fs electron bunches  Timing stability: <10 fs at the target  Very low charge: 150..300 fC > RF parameter of the REGAE accelerator:  NRF gun (1.5-cell) and buncher cavity (4-cell)  2.998 GHz S-band structures  6 μs pulse length and up to 50 Hz repetition rate (up to now only 12.5 Hz)  Driven by one klystron, with motorized waveguide phase shifter Laser driven plasma wakefield Fast plasma dynamics on Cu mesh MPI for the Structure and Dynamics of Matter

4. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 4 The REGAE LLRF System. > MicroTCA.4 based LLRF system  Running since Nov. 2011  Single cavity controller since 2014 > Stability requirements:  dA/A < 0.01%  dφ < 0.01° (9.3 fs @ 3.0 GHz) > One klystron for two cavities  Coupling of cavities  2 input/4 output system  Buncher phase = RF gun phase + 90° Gun phase Δt = 100 fs/deg ΔE = -14 keV/deg Gun gradient Δt = -938 fs∙m/MV ΔE = 22 keV∙m/MV Buncher phase Δt = -1286 fs/deg ΔE = 5.3 keV/deg Buncher gradient Δt = 25 fs∙m/MV ΔE = 1.5 keV∙m/MV

5. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 5 System Identification at REGAE. System Identification : > Project work by Ayla Nawaz (Student from Technical University Hamburg- Harburg) > Couplings are not negligible! > Feedback controller should be designed for both systems > Problem: only 2 actuators for 4 signals!

6. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 6 Controller design for REGAE. > Feedback on either RF gun or buncher  Idea of new controller structure for dual feedback > MIMO with 2 nd order filter for the main diagonal  Lead-lag element, higher gain at lower frequency (up to 50 dB)  System latency of 680 ns (controller output to input)  Controller latency of 870 ns  Optimal feedback gain is 2 (minimal jitter on the probe signal) > Learning feed forward for slow pulse shape adaptation (algorithm from FLASH/XFEL) Latency Budget at REGAE Multi CavitySingle Cavity VM Output 464 ns360 ns RF Gun Probe 736 ns608 ns Controller Input 1040 ns684 ns

7. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 7 Example of Data Analysis. > Time trace of 750 (3750) subsequent pulses, 1 min (5 min) at 12.5 Hz repetition rate > Bunch position at 6.5 us > Region of interest: 6..7 us (125 samples, marked in red) => 1 MHz bandwidth > Cavity bandwidth w 12 ~ 125 kHz (QL ~ 12000 @ 2.998 GHz) 750 pulses (light blue/red) Mean (bold blue/red line) 1 chosen RF pulse (black) Average over ROI gives 1 point in time plot

8. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 8 Fast Feedback Performance on RF Gun. > Open loop stability:  Amplitude: >3e-4  Phase: >50 mdeg > Feedback on RF gun  Amplitude: 2.7e-4  Phase: 20.4 mdeg > Controller with FB gain of 2 and MIMO > RF reference measurement:  Only 7.8 mdeg  Expected: 3 mdeg (from Lab tests)  factor of 2 is missing > Buncher is affected by gun feedback as well

9. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 9 Feedback Performance on Buncher. > Feedback on buncher  Amplitude: 2e-4  Phase: 24.7 mdeg > Controller with FB gain of 2, and MIMO > Drifts on RF gun signals, caused by temperature change on the buncher

10. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 10 Modulator/Klystron and Vector Modulator Performance. > working point (850 V): @ 20 ppm (expected modulator stability)  dA/A= 1.24e-4 rms  dφ= 34 mdeg > Residual phase noise (in vs. out) > Contribution from the vector modulator:  [10 MHz..100 MHz]: 6.3 mdeg, (5.8 fs)  [10 Hz..10 MHz]: 6.7 mdeg, (6.2 fs)  [10 Hz..1 MHz]: 3.0 mdeg, (2.8 fs) Phase slope: 1.96°/V Ampl. slope: 0.438 1/V Klystron high voltage scan Vector modulator characterization

11. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 11 Characterization of the Klystron Chain (I). > Measured in open loop mode over 3750 pulses, ~5 min @ 12.5 Hz  REF: 8.5 mdeg, (7.9 fs)  VM out: 14.0 mdeg, (13.0 fs)  PA in: 14.3 mdeg, (13.2 fs)  PA out: 41.4 mdeg, (38.3 fs)  (KLY forw: 52.7 mdeg, (48.8 fs)) > Additive rms phase jitter:  VM: 11.1 mdeg  PA: 38.9 mdeg  KLY: 30.6 mdeg Reference PreAmp output Klystron forward Klystron reflected VM output PreAmp input 20150629T144548

12. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 12 Exchange of Pre-Amplifier. > Foreseen for XFEL-TDS > Characterization in the lab:  REF: 0.73e-4, 6.2 mdeg (5.7 fs)  VM: 0.96e-4, 6.9 mdeg (6.4 fs)  PPA: 0.88e-4, 7.8 mdeg (7.2 fs) > Additive rms phase jitter:  VM: 3.1 mdeg, (2.9 fs)  PA: 3.7 mdeg, (3.4 fs)

13. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 13 Reference PreAmp output Klystron forward Klystron reflected VM output PreAmp input Characterization of the Klystron Chain (II). > Installed XFEL TDS pre-amplifier > Optimized signal levels (adjust attenuators)  REF: 9.1 mdeg  VM: 11.0 mdeg  PA in: 17.1 mdeg (*)  PA out: 12.4 mdeg  KLY forw: 19.4 mdeg > Additive rms phase jitter  VM: 6.2 mdeg  PA: 5.7 mdeg  KLY:14.9 mdeg (*): low signal level at ADC

14. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 14 Feedback Performance on RF Gun. > Feedback on RF gun with XFEL-TDS PA > Out of loop setup:  Split of RF signals  Additional LO box, down converter, and ADC > RMS phase stability:  In-loop: 9.0 mdeg (8.3 fs)  Out-of-loop: 14.3 mdeg (13.2 fs) > Some drifts remain  needs to be investigated > Buncher is still uncontrolled > Performance check with beam is missing Out of loop

15. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 15 Summary. > Commissioned finally the single cavity controller in 2015 at REGAE > Performed system identification, suitable for the controller design (used for MIMO parameter and LFF matrices) > Investigated jitter contributions from different stations in the actuator chain  Exchanged (broken?) pre-amplifier > Achieved 10 fs (14 fs out of loop) rms phase jitter  Only on the RF gun  Buncher still uncontrolled  Validation of beam stability

16. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 16 Reorganization of the LLRF System Installation. > Move the LLRF system from the laser room into the tunnel  One temp. stabilized rack only for the LLRF system  Reduce cable length (reduce drift and latency)  Reduce disturbances (EMI rack) > Move master oscillator to the laser synch rack  One temp. stab. rack only for LSynch and MO  Reduce cable length  Reduce disturbances > RF reference distribution by an RF interferometer [K. Czuba]  Additional need for ANGUS laser synchronization system (high power laser system for plasma acceleration experiments at REGAE)

17. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 17 First Positive Effects of Movement on the LLRF System. > Reduce rms phase jitter on the MO from 600 fs down to 60 fs  Less disturbances, better shielding (EMI racks) > Improved jitter on the reference measurement down to 4 mdeg

18. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 18 Outlook. > Try special vector sum of RF gun and buncher  RF gun amplitude/inphase and buncher phase/quadrature  Tested already, but without beam to verify real performance improvement > Check FB performance with the LLRF system at the new location > Further FPGA/firmware developments:  Up to now, only 30% of FPGA resources are used  MIMO selector (vector sum of RF gun amplitude/inphase and buncher phase/quadrature)  Smith predictor (to overcome latency) > Installation of waveguide attenuator in the buncher waveguide arm  Reduce coupling between RF gun and buncher  Simplifies bunching operation

19. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 19 Future needs/plans. > Installation of a drift compensation module (like for XFEL [F. Ludwig]) to achieve long term stability > Implementation of the RF Interferometer (based on XFEL developments [K. Czuba]) > Second modulator for the buncher (depends on funding, 2 nd LLRF system needed) > Installation of a transverse deflecting cavity for beam diagnostic (3 rd LLRF system needed) > Development of a beam arrival cavity and corresponding readout electronics

20. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 20 Thank you for your attention!

21. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 21 Temperature Estimation based on RF Signals. > Water regulation system for RF gun and buncher cavity > To keep cavities on resonance > Use RF signals to estimate detuning => temperature > Estimated temperature gives fast read back value > Useable for feedback (e.g. pulse width modulation [S. Pfeiffer]

22. Matthias Hoffmann | Sub-10 fs RF Regulation at REGAE | 06.11.2015 | Page 22 New fast Water Controller. > Development by MPSD colleagues Mini Heater (Friedjof Tellkamp, MPSD)