1 / 26

NLC Linac BPMs

Next Linear Collider Beam Position Monitors Steve Smith SLAC Interaction Point Beam Instrumentation Study June 26, 2002. NLC Linac BPMs. “Quad” BPM (QBPM) In every quadrupole (Quantity ~3000) Function: align quads to straight line Measures average position of bunch train

ajay
Download Presentation

NLC Linac BPMs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Next Linear ColliderBeam Position MonitorsSteve SmithSLACInteraction PointBeam Instrumentation StudyJune 26, 2002

  2. NLC Linac BPMs • “Quad” BPM (QBPM) • In every quadrupole (Quantity ~3000) • Function: align quads to straight line • Measures average position of bunch train • Resolution required: 300 nm rms in a single shot • Structure Position Monitor (SPM) • Measure phase and amplitude of HOMs in accelerating cavities • Minimize transverse wakefields • Align each RF structure to the beam • Few 104 devices in the two linacs • Not used in Interaction Region • “Multi-Bunch” BPM (MBBPM) • Measure bunch-to-bunch transverse displacement • Compensate residual wakefields • Measure every bunch, 1.4 ns apart • Requires high bandwidth (300 MHz), high resolution (300 nm) • Line up entire bunch train by steering, compensating kickers • Intra-Train Feedback BPM

  3. QBPM Requirements

  4. BPM Cavitywith TM110 Couplers • Dipole frequency: 11.424 GHz • Dipole mode: TM11 • Coupling to waveguide: magnetic • Beam x-offset couple to “y” port • Sensitivity: 1.6mV/nC/m • (1.6109V/C/mm) • Couple to dipole (TM11) only • Does not couple to TM01 • Compact • Low wakefields Port to coax Zenghai Li

  5. Parameter Value Comments Cavity BPM Parameters Dipole frequency 11.4 GHz Monopole frequency 7.66 GHz Cavity Radius 16 mm Wall Q ~4000 Ignoring beam duct, etc Cavity coupling  = 3 Loaded Q 1000 Bandwidth 11 MHz Beam aperture radius 6 mm Sensitivity 7 mV/nC/mm  (too much signal!) Bunch charge 0.7 x 1010 e- Per bunch Signal power @ 1mm - 29 dBm Peak power Decay time 28 ns Required resolution  = 200 nm Required Noise Figure 57 dB For  = 100 nm, thermal only Wakefield Kick 0.3 volt/pC/mm Long range Structure wakefield kick ~2 volt/pC/mm Per structure Short-range wakefield ~1/200th of structure

  6. Antenna Scan

  7. Antenna Scan - Residuals

  8. Multi-Bunch BPMs • Stripline pickups • Report position of every bunch in bunch train • Used to program broadband kickers to straighten out bunch train

  9. ATF Bunch Current

  10. Energy from BPMs • Dispersion: • How energy deviation translates into transverse beam motion x  pp  • Where  is the dispersion •  ~ 20 – 80 mm • BPM stability of ~ 1 m  energy measurement of ~ 10-5 • But BPM measures mean position of charge of bunch • Transverse tails? • Energy tails? • 1% of charge off energy by 1%  error of 10-4. • Need luminosity weighted energy • Get charge-weighted energy

  11. Luminosity from BPMs • BPMs measure beam-beam deflection angle  • Deflection angle depends on beam size, charge, beam offsets, angles, bunch length,… • But luminosity is given by sx, sy: L = • Can be difficult to extract true L from deflection. • Really need luminosity monitor that measures collision processes. • Deflection measurement is crucial for optimization, feedback, spot size measurement, etc.

  12. Beam-Beam Deflection(electron microscope!)

  13. Beam-Beam Deflection “Guinea Pig” simulation provided by A. Seryi Slope at Large Offset Initial Slope

  14. Beam-Beam Deflection Slope 40 mm focal length!

  15. Power of Deflection Measurement • Beams see 40 mm focal length lens (at < 1 sy offset) • Image distance 4 meters • Magnification of 4m/40 mm = 105 • BPM resolution of 1 mm  • beam offset resolution of 1 mm / 105 = 10 pm (!) • Confounded by • Tails • Angle jitter • Rotation • Dilution of 100  beam–beam offset resolution ~ 1nm

  16. Beam-Beam Scans • Important diagnostic • Spots sizes • Alignment • Waists • Etc. etc. etc… • Some Limitations: • Takes lots of time at one measurement per pulse train • Sensitive to drifts over length of scan • i.e. low frequency noise, ground motion, damping ring jitter. • Can we do a scan in one bunch train? • Increase utility of diagnostic by increasing speed • Reduce sensitivity to drift and low frequency noise • Yes • Assuming existence of Intra-train IP feedback.

  17. IP Intra-Train Feedback BPM • Interaction Point Intra-Train Feedback BPM • IP only • Much like Multibunch BPM • Critical difference: • Very short propagation delay • Close loop in ~ 40 ns < 250 ns train passage time. • Other differences: • Half the bandwidth • Don’t care about bunch-bunch motion • feedback only makes bunch-bunch motion worse!

  18. Intra-Train Feedback • Vertical spot size comparable to ground motion • Small beam size  luminosity sensitive to beam jitter • We can measure extremely small inter-beam offsets via beam-beam deflection • Can we make a feedback fast enough to correct beam-beam mis-alignment within the 265 ns bunch-crossing time? • System consists of a fast position monitor, kicker, and feedback regulator. • Old-fashioned analog feedback.

  19. Intra-Pulse Feedback 14 ns 14 ns

  20. Intra-Train Feedback • Fix interaction point jitter within the crossing time of a single bunch train (265 ns) • BPM measures beam-beam deflection on outgoing beam • Fast (few ns rise time) • Precise (micron resolution) • Close (~4 meters from IP?) • Kicker steers incoming beam • Close to IP (~4 meters) • Close to BPM (minimal cable delay) • Fast rise-time amplifier • Feedback algorithm is complicated by: • round-trip propagation delay to interaction point in the feedback loop • Response non-linearity of beam-beam defelction for flat beams

  21. Limits to Beam-Beam Feedback • Must close loop fast • Propagation delays are painful • Beam-Beam deflection is non-linear • Feedback gain drops like 1/d for large offsets • Feedback converges too slowly beyond ~ 30 s to make a recover luminosity • Can fix misalignments of 100 nm with modest kicker power

  22. Intra-Train Feedback • Setup critical: • IT Feedback holds outgoing beam to a position setpoint • Other hardware required to establish/maintain proper setpoint • Multibunch BPM readout of IT feedback, symmetric pickups • Higher bandwidth • Better resolution • Better stability • (SLOW) • Response of beam to FB drive contains diagnostic information • Need deflection vs. bunch digitized and logged

  23. Single-Pulse Beam-Beam Scan • Fast BPM and kicker needed for interaction point stabilization • Open the loop and program kicker to sweep beam • Digitize fast BPM analog output • Acquire beam-beam deflection curve in a single machine pulse • Eliminates inter-pulse jitter from the beam-beam scan. • Noise is all at low frequency • Use to • Establish collisions • Measure IP spot size • Waist scans • What else?

  24. Intra-Pulse Feedback(with Beam-Beam Scan & Diagnostics)

  25. Beam-Beam Scan Beam bunches at IP: blue points BPM analog response: green line

  26. IP Beam Position Monitors • Stabilize collisions • Measure spot sizes • Diagnostics

More Related