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“Slow” Feedback Requirements: Deflections and Luminosity

“Slow” Feedback Requirements: Deflections and Luminosity. Linda Hendrickson IPBI Meeting, SLAC June 26, 2002.

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“Slow” Feedback Requirements: Deflections and Luminosity

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  1. “Slow” Feedback Requirements: Deflections and Luminosity Linda Hendrickson IPBI Meeting, SLAC June 26, 2002

  2. Overview:1. Deflection feedback and ground motion simulations: keeping the beams in collision, train-train. (~timescale: 120 Hz)2. Luminosity optimization: Maximizing luminosity and stabilizing higher-order aberrations. (~timescale: 30 minutes)

  3. Feedback timescales: NLC simulations (Andrei Seryi et al, 2002) With SLC-style IP deflection feedback Uncorrected

  4. Feedback timescales: NLC simulations (Andrei Seryi, PAC 2001)

  5. Instrumentation for “Slow” Feedback, Needed by Control System: IP Beam position monitors (2 incoming+2 outgoing, X&Y, * 2 beams) Good resolution (< 1 um per S. Smith ) Low noise, not subject to erroneous results from beam spray Slow/low offset drift (offsets calibratable with luminosity or defl. scan) Low latency (<<< 1/120 sec) Luminosity monitor(s) Good resolution (~10%, comparable to real luminosity jitter?) Low latency (< 1/120 sec) Multiple monitor options are desirable Returns maximum signal for maximum luminosity! (no systematics) Other instrumentation(?): Intensity monitor (defl and lum normalization, consistent timescale) Beam timing monitor, ala SLC? Crab cavity phase monitor? Detector background signals, needed in realtime!

  6. Actuators for “Slow” Feedback, Needed by Control System: IP Correctors or kickers (X&Y, * 2 beams) Fast response (<< 1/120 sec) Slower correctors with larger range needed for longer-term drifts FF sextupole orbit feedback correctors (X&Y, 2 phases?, 2 beams) Fast response (<~ 1/120 sec) Luminosity Optimization controls X and Y sextupole offsets; skew quadrupole strengths (per Y. Nosochkov) Prefer fast response (< 1/10 sec ala SLC) Prefer equal speeds in a multiknob (less susceptible to systematics) Minimal hysteresis (reproducibility of actuator settings)

  7. Feedback timescales: NLC vs SLC feedback design response: (It helps to assume a faster control system: low-latency BPMs, fast IP kickers/correctors)

  8. Feedback timescales for Luminosity Optimization: SLC experience (Nan Phinney, Pantaleo Raimondi, and the SLC team): A.F.A.R.A! (As Fast As Reasonably Achievable) • Fast response to upstream tuning, supports higher order optimizations. • Want < 30 minutes to optimize all, from untuned state (10-30 minutes typical SLC running) • Typical SLC optimization scenario: optimize 10 parameters every 2 hours, plus on user request: 2 beams: X&Y waist; X&Y eta; coupling Estimated < 2% luminosity loss due to dithering • Possible NLC optimization scenario: optimize 10-13 parameters every 2 hours, plus on user request: 2 beams: X&Y waist; X&Y eta; coupling; crab cavity phase(?) 1 beam: compressor phase(?)

  9. Dither Method: Maximize luminosity while moving multiknob up and down by small amounts, average 1000’s of pulses Original Scan method: Minimize beam width-squared from deflection scans (subject to meas error ~20-40% luminosity) Luminosity Optimization in the SLC: Bhabha BSM

  10. Luminosity Optimization in the SLC: Comparative Resolution of Scan Method vs Dither Method Dither Scan

  11. SLC Optimization : typical feedback command changes over 3 days. June, 1998

  12. SLC Optimization : typical old-scan-method command changes over 3 days. June, 1997

  13. Normalized luminosity during dither cycle (arb units) SLC Optimization : Typical optimization cycle over 12 hours; June, 1998

  14. And now… “The Movies”! courtesy of Andrei Seryi and the NLC accelerator physics group Damping Ring >> IP << Damping Ring • Consistent ground motion simulations(2 beams, one continuous ground, with P(ω,k) spectrum (elastic waves, slow ATL, systematic motion, technical noises) • SLC-style IP deflection feedback • MATLAB(simulation driver, feedback calculations, display&analysis) • LIAR(linac tracking with structures, wakefield calculations: beam slice representation) • DIMAD(tracking engine run within LIAR; for bunch compressors, bends, sextupole,octupole tracking; particle representation) • Guinea Pig(beam-beam code; interfaced to LIAR-DIMAD via MATLAB; gives us the deflection and luminosity “measurements”)

  15. Ground motion models • Based on data, build modeling P(w,k) spectrum of ground motion which includes: • Elastic waves • Slow ATL motion • Systematic motion • Technical noises at specific locations, e.g. FD) Example of integrated spectra of absolute (solid lines) and relative motion for 50m separation obtained from the models

  16. P(w,k) is then used to generate x(t,s) and y(t,s) and beams GO Example of Mat-LIAR modeling

  17. Intermediate ground motion

  18. Zoom into beginning of e- linac … Transition from linac to transfer line

  19. Noisy ground motion

  20. Quiet ground motion

  21. Beam-beam collisions calculated by Guinea-Pig [Daniel Schulte]

  22. calculated by Guinea-Pig [Daniel Schulte] Pulse #100, Z-Y

  23. calculated by Guinea-Pig [Daniel Schulte] Pulse #100, Z-X

  24. calculated by Guinea-Pig [Daniel Schulte] Pulse #100, X-Y

  25. CONCLUSIONS? Controlling deflections and luminosity optimization will be at least as difficult for NLC as for SLC. Need tools that are at least as good! (i.e. fast, reliable, low-latency instrumentation and controls). Future work for NLC: Optimization of 120-Hz deflection feedback response for expected ground motion. More complete simulations of NLC tuning: sextupole orbit correction, optimization with luminosity jitter, realistic imperfections, upstream tuning; IP angle feedback? Etc…

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