1. NLC - The Next Linear Collider Project Control and Feedback for RF Linacs Marc Ross RF Control and Monitoring Feedback Like most modern ‘plants’ there are loops within loops and feedforward Special issues: precision ‘handling’ of microwave and ~high bandwidth

2. Author Name Date Slide # 2 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Typical controls and feedback loops Accelerating vector – phase and amplitude –Low Level (long distance) Distribution –Source –High Power distribution –Structure  beam loading & thermal… Feedback –Environment Transverse –Position –Emittance Longitudinal –Energy –Energy spread &  z …and protection systems for high power linacs

3. Author Name Date Slide # 3 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Phase and amplitude tolerances – NLC example Config # 1E1 (GeV) 2E2 (GeV) 3 7 18  30 55 335 -30  8 20  30 77 320 -30  9 22  30 99 300 -30  A parameter that characterizes the strength of the wakefield relative to the focusing is the BNS energy spread needed for autophasing: Autophasing is the condition where the chromatic growth of a beam performing a coherent betatron oscillation exactly cancels the wakefield growth and thus the beam oscillates as a rigid body. BNS phase offsets imply tight phase stability tolerances (+ extra gradient)

4. Author Name Date Slide # 4 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Phase and amplitude tolerances - NLC example (2) ParameterAccuracyStability& Resolution Units Energy profile 0.50.1% voltage Energy gain knowledge 50.1% voltage Phase readback 10.1degree Phase stability N/A0.1degree X-band: 11.424 GHz = 26.3 mm /360 = 73 um /360*0.66 = 50 um (0.66 for plastic cable signal speed)

5. Author Name Date Slide # 5 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA RF stabilization speeds Two kinds of linacs: –Pulse width is long compared to the transit times  ‘within the pulse’ feedback is necessary Superconducting –Stabilize microphonics Warm proton linacs –Pulse width is short compared to transit times  ‘within the pulse’ feedback is not possible Warm electron linacs Interpulse feedback is required –Stabilize thermal effects Beam loading

6. Long Pulse RF Control (Proton linacs and cold electron linacs) S. Simrock - DESY

7. Author Name Date Slide # 7 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Linac RF block diagram 3 basic loops: 1.Long baseline distribution 2.High power amplifier (Klystron) 3.Beam – based 1 1 2 3

8. SLAC RF Distribution schematic - 1985 PAD = Phase and Amplitude Detector

9. Author Name Date Slide # 9 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA RF phase stability at the 1 degree X-band (0.2 picosecond) level over the ~30 kilometer length of the machine. The RF timing requirement corresponds to a  L/L stability of <2.5  10-9, which would be impractical without feedback. ( the timing distribution system will use the same hardware as the RF distribution system.) It is assumed that RF phase measurements relative to the electron beam will be used to obtain long term stability. The RF distribution system needs to maintain the RF phase to within 20 degrees X-band (<5  10-8) for long periods of time when the beam is not running. RF long baseline distribution system specifications – NLC ‘Common mode’ effects are worst

10. NLC RF Distribution System

11. NLC RF Distribution test - 2001

13. Author Name Date Slide # 13 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA High Power Phase and Amplitude Detection and Control High power couplers (40-70 dB) Cables Diode/Mixer detectors Attenuators Phase shifter Phase measurement Control system architecture

14. High Power X-band Waveguide coupler – 60 dB High Power S- band Bethe hole Waveguide coupler

15. RF input Cutaway detector diode – showing failed connection RF Amplitude: Diode Detectors ‘Video’ out Diode junction Simple illustration of RF detector diode operation Output matching network

16. Author Name Date Slide # 16 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Matched Detector Diodes from Agilent Power in Volts out Showing deviation from square law at moderate power – Low signal output is proportional to the square of the incoming RF AC voltage (V out  P)

17. Author Name Date Slide # 17 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Non-’Square Law’ detector: Thermionic Diode Used in original SLAC phasing system to extend dynamic range Closer to linear Very radiation hard

18. Structure Phasing system - SLAC (1965)

19. High Power RF attenuator RF Phase shifter In Out Capacitance change in ‘varactor’ diode moves effective reflection point Conductance change in ‘PIN’ diode changes reflection coefficient

20. Author Name Date Slide # 20 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Phase measurement Mixer output – if both RF / LO ports are basically the same frequency: A RF *A LO * cos(  ) –neglect usual mixer issues (intermod, compression) –worry about others – offsets/diode matching Phase ambiguity and offsets: 1.Nulling + dither to measure sign of derivative Wobbler (+/- 180  ) active synchronized wobbling to monitor offset 2.I/Q calibrated ‘double channel’ 5 parameters – two gains/offsets and 1 angle

21. Author Name Date Slide # 21 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA ‘PAD’ phase detector (and shifter) circuit Nulling + Wobbler

22. Author Name Date Slide # 22 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Controls Architecture Phase and Amplitude Detector SLAC - 1983

23. Sampled RF waveforms One point digitized/pulse (120 Hz) with 30 MHz bandwidth Cal. RF amplitude- Fitted for energy gain estimate - lattice feedforward RF amplitude- vs klystron drive atten. Klystron saturation Beam Volts Modulator timing SLAC - 1985

24. Author Name Date Slide # 24 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA motorized phase controller – 1 klystron temp manual drive line length What the long term feedback is doing… Common mode error – either injection or distribution system Single klystron – environmental (e.g. leakage through insulation)

25. Author Name Date Slide # 25 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA “Precision” microwave High power RF controls and monitoring + beam position monitors + beam phase monitors –SC Cavity tuning at TTF; lorentz force compensation + coupling control –Bunch length and Beam ‘tilt’ monitors programmed phase control –NLC ‘Delay Line Distribution System’ –Beam loading feedforward for short pulse linacs 2002… Modern RF controls

26. Author Name Date Slide # 26 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Digital Low Level RF Precision I/Q determination –Phase/amplitude calculations with very low (bit) noise –Use of complex math linearizes v/v amplitude and phase 1.Home-made –outgrowth of DSP based multi-bunch storage ring feedback systems –TTF & SNS (DSP/FPGA based) 2.Commercial –Echotek ‘Digital Down-Conversion’ (Digital receiver) (Within the last 10 years) Biggest challenge  integration with the control system & diagnostics

27. Cold Linac LLRF – TESLA / TTF Simrock, DESY System Block Diagrams  Analog (CEBAF ~1994) Digital (TTF ~ 1998)

28. Author Name Date Slide # 28 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Synchronous Digital Sampling – Direct down conversion sampling clock effectively LO - importance of sampling clock stability Digital RF  How it really works

29. Author Name Date Slide # 29 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA TTF LLRF Drive Controls S. Simrock DESY Also have tuners, coupling etc.

30. TTF (DESY) DSP based I/Q controller (Simrock)

31. Larry Doolittle – LBNL SNS Low Level RF Digital Feedback FPGA (LINAC 2002 proceedings) SNS Low level ‘within the pulse’ feedback Gate Array program schematic

32. Larry Doolittle – LBNL SNS Low Level RF Digital Feedback -1 Input Sampler Diagnostic buffer Averaging Set point subtraction

33. Larry Doolittle – LBNL SNS Low Level RF Digital Feedback -2 I / Q gain scaling and recombination ‘KCM’ System calibration input DAC driver Integrator loop for fine error zeroing and feedforward input Uses ~ 20% of the $20 FPGA

34. FPGA DAC ADC SNS Digital LLRF prototype circuit - LBNL Small, Simple hardware, ~ simple software (EPICs) Easily tested

35. Commercial Digital I/Q receiver Integrated by Echotek

36. S. Smith, SLAC Programmed phases/amplitudes used to switch outputs and compensate for beam loading Delay Line Distribution System

37. Linac LLRF Drive NLC RF compression system control using Direct Digital Synthesis – waveform memory

38. NLC Linac LLRF Measurement Requirements ParameterValueDetails Bandwidth> 100 MHz at -3 dB Rise time< 5ns10% to 90% Phase resolution1 degreeAt 11.424 GHz Dynamic Range> 20 dB Amplitude Resolution10 -3 of full scale Beam phase wrt RF1 degreeAt 11.424 GHz Beam signal / RF-40 dB(!) Reflected power detector max input< 100 mWPeak Reflected power detector rise time< 10 ns

39. DLDS Waveforms with Beam Loading Compensation Amplitude at each DLDS Output Time (ns) 1 2 3 4 5 6 7 8 Each of 8 klystrons is programmed and combined to give independent outputs for each of 8 structure groups Klystron  3.2 us Structure  400 ns

40. Author Name Date Slide # 40 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Bunch length Streak cameras –resolution limited to ~ 1mm –space charge, calibration Coherent radiation –stronger signal with shorter beams –asymmetry difficult (use power spectrum – phase info lost) Deflecting RF structures –promising  Broadband microwave emission –cheap, relative – a given accurate monitor critical for short wave FEL Microwave based beam diagnostics

41. Author Name Date Slide # 41 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Transverse deflection Brute force Calibrated Expensive Excellent resolution SLAC LCLS – Krejcik/Emma (EPAC 02) SLAC/DESY TTF2 Old idea – 1965 ‘LOLA IV’ Testing in linac sector 29

43. Krejcik / Emma EPAC 2002

46. Author Name Date Slide # 46 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Beyond Bunch length  Correlations E – z y – z x – y Proposed use of simple microwave single cell cavities to estimate correlations Most phase space distortions start with a linear correlation –a monitor simple, cheap and accurate compared to a profile monitor can be more widely distributed and used to pinpoint errors Microwave based beam diagnostics

47. Author Name Date Slide # 47 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Response of Cavity BPM to Point Charge  Q S. Smith – SLAC, Snowmass 2001

48. Response of BPM to Tilted Bunch Centered in Cavity q Treat as pair of macroparticles:   tt q/ 2  

49. Tilted bunch Point charge offset by  Centered, extended bunch tilted at slope  t Tilt signal is in quadrature to displacement The amplitude due to a tilt of  is down by a factor of: with respect to that of a displacement of  (~bunch length / Cavity Period )

50. Example Bunch length  t = 200  m/c = 0.67 ps Tilt toleranced = 200 nm Cavity FrequencyF = 11.424 GHz Ratio of tilt to position sensitivity½  f  t = 0.012 A bunch tilt of 200 nm / 200  m (1 mrad) yields as much signal as a beam offset of 0.012 * 200 nm = 2.4nm Need BPM resolution of ~ 2 nm to measure this tilt Challenging ! –Getting resolution –Separating tilt from position Use higher cavity frequency? Need 1 mrad tilt sensitivity for linac tuning

51. Angled trajectories A trajectory that is not parallel to the cavity axis also introduces a quadrature signal (in phase with ‘tilt’ signal) Projected ‘dipole’ sensitivity is increased by  z /cavity length –~ 50  ATF  z ~ 8mm gives expected tilt resolution ~ 0.1mrad  y res /  y ~ 5%  y’ res /  y’ ~ 10x Relative normalized precision Beam position/beam traj angle

52. Very good resolution possible – 25 nm achieved in FFTB few nm possible by limiting spatial dynamic range  Wave cavity BPM X-band 12 mm bore Naito/Li

53. ATF extraction line C-band cavity L = 12mm, Radius = 26mm, f = 6426MHz, =46.6mm Movers – x, y, pitch (y-z) ATF Cavity BPM – V. Vogel / H. Hayano

54. Author Name Date Slide # 54 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Tilt monitor electronics J. Frisch

55. Raw ‘mixed – down’ scope data from cavity BPM Phase and amplitude wrt ref are extracted (I and Q) I Q response as the cavity is moved vertically using mover The angle is arbitrary (phase offset between ref and BPM cavity) A ‘monopole’ beam with an axial trajectory should give a (0,0) response at some point Use the cavity ‘tilter’ to observe response to tilted trajectories (Beam ‘tilter’ was not ready during this test – May 2002) Compare 35 urad with 26 in table estimate

56. Author Name Date Slide # 56 Joint Accelerator School - 2002 Marc Ross – SLAC November 14, 2002 CLIC J/NLC TESLA Concluding remarks Digital LLRF field attractive and exciting –Wide variety of application from controls to instrumentation Next few years will see the application and success of critical new techniques, opening up new paths to higher brightness, low emittance transport, more stability…