“slow” feedback requirements: deflections and luminosity

NLC - The Next Linear Collider Project

“Slow” Feedback Requirements: Deflections

and LuminosityLinda Hendrickson

IPBI Meeting, SLAC

June 26, 2002

Next Linear ColliderNext Linear Collider

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)


Feedback timescales: NLC simulations (Andrei Seryi et al, 2002)

Uncorrected With SLC-style IP deflection feedback


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


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 spraySlow/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 desirableReturns 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!


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 controlsX 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)


Feedback timescales: NLC vs SLC feedback design response:

(It helps to assume a faster control system: low-latency BPMs, fast IP kickers/correctors)


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; couplingEstimated < 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(?)


Luminosity Optimization in the SLC:

Original Scan method: Minimize beam width-squared from deflection scans

(subject to meas error ~20-40% luminosity)

Dither Method: Maximize luminosity while moving

multiknob up and down by small amounts, average 1000’s

of pulses

Bhabha

BSM


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

Dither

Scan


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


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


SLC Optimization : Typical optimization cycle over 12 hours; June, 1998

Normalized luminosity during dither cycle (arb units)


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”)

Next Linear ColliderNext Linear ColliderGround motion models

• Based on data, build modeling P(,k) spectrum of ground motion which includes:

– Elastic waves– Slow ATL motion– Systematic motion– Technical noises at

specific locations, e.g. FD)

1E-4 1E-3 0.01 0.1 1 10 100

0.1

1

10

100

"Model A"

"Model C"

"Model B"

Inte

grat

ed r

ms

mot

ion,

nm

Frequency, Hz

Example of integrated spectra of absolute (solid lines) and relative motion for 50m separation obtained from the models

Next Linear ColliderNext Linear ColliderP(,k) is then used to generate x(t,s) and y(t,s) and beams GO

Example of Mat-LIAR modeling

Next Linear ColliderNext Linear ColliderIntermediate ground motion

Next Linear ColliderNext Linear ColliderZoom into beginning of e- linac …

Transition from linac to transfer line

Next Linear ColliderNext Linear ColliderNoisy ground motion

Next Linear ColliderNext Linear ColliderQuiet ground motion

Next Linear ColliderNext Linear ColliderBeam-beam collisions calculated by Guinea-

Pig [Daniel Schulte]


Pulse #100, Z-Y

calculated by Guinea-Pig

[Daniel Schulte]


Pulse #100, Z-X


[Daniel Schulte]


Pulse #100, X-Y


[Daniel Schulte]


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…

“slow” feedback requirements: deflections and luminosity

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