NLC - The Next Linear Collider Project Next Linear Collider Beam Position Monitors Steve Smith SLAC Interaction Point Beam Instrumentation Study June 26, 2002
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NLC - The Next Linear Collider Project Next Linear Collider Beam Position Monitors Steve Smith SLAC Interaction Point Beam Instrumentation Study June 26, 2002 NLC Linac BPMs • “Quad” BPM (QBPM) – – – – Next Linear Collider 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 Steve Smith - 6/26/02 Author Name Date Slide # QBPM Requirements Next Linear Collider Parameter Value Conditions Resolution Position Stability 300 nm rms @ 1010 e- single bunch 1 m over 24 hours (!) Position Accuracy 200 m With respect to quad magnetic center Position Range 2 mm Charge Range 0.5109 to 7.5109 eper bunch Number of bunches Bunch spacing 1 - 190 1.4 ns Author Name Steve Smith - 6/26/02 Date Slide # BPM Cavity with TM110 Couplers • • • • Dipole frequency: 11.424 GHz Dipole mode: TM11 Coupling to waveguide: magnetic Beam x-offset couple to “y” port Next Linear Collider Port to coax • Sensitivity: 1.6mV/nC/m (1.6109V/C/mm) • • • • Couple to dipole (TM11) only Does not couple to TM01 Compact Low wakefields Author Name Zenghai Li Steve Smith - 6/26/02 Date Slide # Cavity BPM Parameters Next Linear Collider Parameter Value Comments Dipole frequency 11.4 GHz Monopole frequency 7.66 GHz Cavity Radius 16 mm Wall Q ~4000 Cavity coupling =3 Loaded Q 1000 Bandwidth 11 MHz Beam aperture radius 6 mm Sensitivity 7 mV/nC/m (too much signal!) Bunch charge 0.7 x 1010 e- Per bunch Signal power @ 1m - 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 Ignoring beam duct, etc Steve Smith - 6/26/02 Author Name Date Slide # Antenna Scan Next Linear Collider Author Name Steve Smith - 6/26/02 Date Slide # Antenna Scan - Residuals Next Linear Collider Author Name Steve Smith - 6/26/02 Date Slide # Multi-Bunch BPMs Next Linear Collider • Stripline pickups • Report position of every bunch in bunch train • Used to program broadband kickers to straighten out bunch train Parameter Value Conditions & Comments Resolution for bunch-bunch diplacement frequencies below 300 MHz Position Range 300 nm rms At 0.6 x 1010 e- / bunch 2 mm Bunch spacing 1.4 ns Number of Bunches 1 - 190 @ 1.4 ns Beam current dynamic range 1109 to 1.4 1010 Particles / bunch Number of BPMs 278 Author Name Steve Smith - 6/26/02 Date Slide # ATF Bunch Current Next Linear Collider Author Name Steve Smith - 6/26/02 Date Slide # Energy from BPMs Next Linear Collider • Dispersion: – How energy deviation translates into transverse beam motion x pp – 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 Author Name Steve Smith - 6/26/02 Date Slide # Luminosity from BPMs Next Linear Collider • BPMs measure beam-beam deflection angle • Deflection angle depends on beam size, charge, beam offsets, angles, bunch length,… • But luminosity is given by x, y: L= 2H D N Pbeam 4 Ecm x y • 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. Author Name Steve Smith - 6/26/02 Date Slide # Beam-Beam Deflection (electron microscope!) e- d Next Linear Collider e+ q Deflection coefficient (for small offsets) 25 radian / nm At Interaction Point Displacement 100 m / nm At Position Monitor Author Name Steve Smith - 6/26/02 Date Slide # Beam-Beam Deflection “Guinea Pig” simulation provided by A. Seryi Next Linear Collider Author Name Steve Smith - 6/26/02 Date Slide # Beam-Beam Deflection Slope Next Linear Collider 40 m focal length! Author Name Steve Smith - 6/26/02 Date Slide # Power of Deflection Measurement Next Linear Collider Beams see 40 m focal length lens (at < 1 y offset) Image distance 4 meters Magnification of 4m/40 m = 105 BPM resolution of 1 m – beam offset resolution of 1 m / 105 = 10 pm (!) • Confounded by • • • • – Tails – Angle jitter – Rotation • Dilution of 100 beam–beam offset resolution ~ 1nm Author Name Steve Smith - 6/26/02 Date Slide # Beam-Beam Scans Next Linear Collider • 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. Steve Smith - 6/26/02 Author Name Date Slide # IP Intra-Train Feedback BPM Next Linear Collider • 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! Author Name Steve Smith - 6/26/02 Date Slide # Intra-Train Feedback Next Linear Collider • 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. Author Name Steve Smith - 6/26/02 Date Slide # Intra-Pulse Feedback ke Kic Next Linear Collider r Amp Amp IP + Round Trip Delay BPM Processor BPM Author Name Steve Smith - 6/26/02 Date Slide # Intra-Train Feedback Next Linear Collider • 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 Author Name Steve Smith - 6/26/02 Date Slide # Limits to Beam-Beam Feedback Next Linear Collider • 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 to make a recover luminosity • Can fix misalignments of 100 nm with modest kicker power Author Name Steve Smith - 6/26/02 Date Slide # Intra-Train Feedback Next Linear Collider • 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 Author Name Steve Smith - 6/26/02 Date Slide # Single-Pulse Beam-Beam Scan Next Linear Collider • 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? Author Name Steve Smith - 6/26/02 Date Slide # Intra-Pulse Feedback (with Beam-Beam Scan & Diagnostics) Next Linear Collider ker Kic Amp Amp IP + BPM Processor BPM Round Trip Delay Beam-Beam Scan & Diagnostics Ramp Digitizer Author Name Steve Smith - 6/26/02 Date Slide # Beam-Beam Scan Beam bunches at IP: blue points BPM analog response: green line Steve Smith - 6/26/02 Next Linear Collider Author Name Date Slide # IP Beam Position Monitors Next Linear Collider • Stabilize collisions • Measure spot sizes • Diagnostics Author Name Steve Smith - 6/26/02 Date Slide #