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Control and Feedback for RF Linacs

Control and Feedback for RF Linacs. 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. Marc Ross. Typical controls and feedback loops.

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Control and Feedback for RF Linacs

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  1. Control and Feedback for RF Linacs 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 Marc Ross

  2. 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 & sz …and protection systems for high power linacs

  3. Phase and amplitude tolerances – NLC example 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. Phase and amplitude tolerances - NLC example (2) • X-band: • 11.424 GHz • = 26.3 mm • /360 = 73 um • /360*0.66 = 50 um (0.66 for plastic cable signal speed)

  5. 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. Linac RF block diagram • 3 basic loops: • Long baseline distribution • High power amplifier (Klystron) • Beam – based 1 2 1 3

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

  9. RF long baseline distribution system specifications – NLC • 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 DL/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. ‘Common mode’ effects are worst

  10. NLC RF Distribution System

  11. NLC RF Distribution test - 2001

  12. NLC RF Distribution test - 2001

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

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

  16. Matched Detector Diodes from Agilent Volts out Power in Showing deviation from square law at moderate power – Low signaloutput is proportional to the square of the incoming RF AC voltage (V out µ P)

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

  20. Phase measurement • Mixer output – if both RF / LO ports are basically the same frequency: ARF*ALO* cos(q) • neglect usual mixer issues (intermod, compression) • worry about others – offsets/diode matching • Phase ambiguity and offsets: • Nulling + dither to measure sign of derivative • Wobbler (+/- 180°) • active synchronized wobbling to monitor offset • I/Q • calibrated ‘double channel’ • 5 parameters – two gains/offsets and 1 angle

  21. ‘PAD’ phase detector (and shifter) circuit Nulling + Wobbler

  22. Controls Architecture Phase and Amplitude Detector SLAC - 1983

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

  24. What the long term feedback is doing… motorized phase controller – 1 klystron Single klystron – environmental (e.g. leakage through insulation) temp temp manual drive line length Common mode error – either injection or distribution system

  25. “Precision” microwave 2002… Modern RF controls • 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

  26. 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 • Home-made • outgrowth of DSP based multi-bunch storage ring feedback systems • TTF & SNS (DSP/FPGA based) • 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. Synchronous Digital Sampling – Direct down conversion sampling clock effectively LO - importance of sampling clock stability Digital RF  How it really works

  29. 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 SNS Low level ‘within the pulse’ feedback Gate Array program schematic (LINAC 2002 proceedings)

  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 Uses ~ 20% of the $20 FPGA • I / Q gain scaling and recombination ‘KCM’ • System calibration input • DAC driver • Integrator loop for fine error zeroing and feedforward input

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

  35. Commercial Digital I/Q receiver Integrated by Echotek

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

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

  38. NLC Linac LLRFMeasurement Requirements

  39. DLDS Waveforms with Beam Loading Compensation 8 7 6 5 Amplitude at each DLDS Output 4 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 3 2 1 Time (ns)

  40. 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. Transverse deflection Old idea – 1965 ‘LOLA IV’ Testing in linac sector 29 Brute force Calibrated Expensive Excellent resolution SLAC LCLS – Krejcik/Emma (EPAC 02) SLAC/DESY TTF2

  42. Krejcik / Emma EPAC 2002

  43. Krejcik / Emma EPAC 2002

  44. Krejcik / Emma EPAC 2002

  45. 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

  46. Response of Cavity BPMto Point Charge S. Smith – SLAC, Snowmass 2001 Q d

  47. Response of BPM to Tilted BunchCentered in Cavity q q/2 Treat as pair of macroparticles: d/2 d/2 st q/2

  48. Point charge offset by d Centered, extended bunch tilted at slope d/st Tilt signal is in quadrature to displacement The amplitude due to a tilt of d/s is down by a factor of: with respect to that of a displacement of d (~bunch length / Cavity Period ) Tilted bunch

  49. Bunch length st = 200 mm/c = 0.67 ps Tilt tolerance d = 200 nm Cavity Frequency F = 11.424 GHz Ratio of tilt to position sensitivity ½ft = 0.012 A bunch tilt of 200 nm / 200 mm (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? Example Need 1 mrad tilt sensitivity for linac tuning

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