The RHIC Collider
description
Transcript of The RHIC Collider
The RHIC ColliderFulvia Pilat
Collider Workshop, JLAB, February 24, 2009
RHIC Collider Complex
100x100 GeV/u ions250x250 GeV polarized p
Chronology:
1996 commissioning AtR1998 sextant test1999 engineering runs2000 first collisions10 years of operations
Outline Introduction Collider design and evolution
Optics, interaction regions, correction systemsValidation of design and correction schemesOperational use of corrections systems (ex: IR corrections)
Commissioning operations: increasing machine performanceexample: lower beta*
Recent developmentsStochastic cooling of bunched beamsTune, coupling, orbit and chromaticity feedback
Lesson learned from RHIC
Optics for Run-10 Au-Au b*=0.7m
Optics for Run-9 P-P b*=0.7m
Optics: zoom into triplets in IR6 (STAR)
Interaction regions: layout and correction systems
Corrections systems Orbit correction
BPM + dipole correctors Coupling correction
3 families of skew quads 120 deg in the arcs (2 wired up in software orthogonal system)1 skew quad/triplet for local compensation of the roll misalignment of the triplet quads(no experimental solenoid compensators – all done by the skew quad families)
Chromatic corrections2 families of sextupoles in the arcs linear chromaticity4 additional sextupole families in arcs nonlinear chromaticity (added later)
IR correction packages (each triplet) 1 dipole H, 1 dipole V, 1 skew quadrupole1 normal and 1 skew sextupole2 octupoles1 decapole2 dodecapoles(skew octupole and dodecapole layers exists but are not powered)
Validation of design and correction schemes
In the design phase we did extensive modeling and simulations to validate the design and the correction schemes
Built a offline machine model for extensive DA simulations, including: Optics configurations Measured magnetic errors in arc and IR magnets Measured misalignments and roll errors in cold masses and cryostats Beam-beam (weak-strong approximation)Other performance issues dealt with stand-alone codes: Intra-beam scattering Beam-beam (strong-strong) Electron cloud Polarization trackingSelected capabilities of the offline model are part of the online machine
model
But, over the operational life of the machine we ended relying mostly onbeam based corrections (orbit, coupling, IR corrections, nonlinear chromaticity)
The magnetic errors in an accelerator magnet can be described in terms of the multipole errors an and bn
defined as:
An excursion of the local orbit through a region having non-linear fields generates feed-down effects to lower order field harmonics
The most useful observable effects come from the feed-down to the beam closed orbit and betatron tunes It is possible in theory to infer local non-linear effects both from the measure of residual RMS orbit and of
tune shifts generated by a local orbit bump in the IR. Given existing limitations on the resolution of the orbit measurement and on the allowable bump amplitude at the triplets, in practice we have used so far almost exclusively the measurement of tune shift as a function of bump amplitude for non-linear correction
The measured tune shifts arise from either the feed-down to the normal gradient or from the repelling effect of linear coupling
The tune shift (ΔQ) and the linear coupling term (Δc) for different bump planes (H and V) and for different multipole errors (normal, skew, even and odd orders) can be expressed as follows (where cn is either the an
or bn and z is x or y):
:
This table implies that for reasonable measurement of a tune shift due to I.R. magnetic field errors, the following bump types should be used to identify the relevant multipole: horizontal for Sextupole(b2), vertical for Skew Sextupole (a2), horizontal and vertical for octupole (b3) etc. A diagonal bump for skew octupole is necessary
In order to simplify the identification of individual multipoles using the observed tune shifts, the conditions should be such that the tune shifts produced by coupling are negligible when compared with the tune shifts from the normal gradient change
Where the functions g and h are defined as:
IR correction method - theory
Before Correction After Correction
Example: normal sextupole IR correctionSchematics of IR bumps
Tune shift vs. amplitudeBefore correction
Tune shift vs. amplitudeAfter sextupole correction
Bump power supply
Before Correction After Correction
Example: skew quadrupole IR correction
Beam decay evolution during the correction
Tune shift vs. amplitudeBefore correction
Tune shift vs. amplitudeAfter skew sextupole correction
Horizontal Tune Shift Before Correction Vertical Tune Shift Before Correction
Tune Shifts After Correction
Example: octupole IR correction
Before correction H
After correction
Before correction V
“The tune modulation (10 Hz due to triplet vibration via feed-down effect; that is, tune modulation due to off-axis beam in sextupoles driven by off-axis beam in triplet quads) was observed to reduce by a
factor of 2-3 after non-linear corrections.
Correction benefits: reduction of tune modulation
Operational correction for IR decapole and dodecapole
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Generic scanner
• scans magnet strength (for list of magnets)• observes beam loss rate• minimizes beam loss rate with strength
Decapole/Dodecapole correction result
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Tested effect of 10- and
12-pole correctors on
beam loss rate by switching off all
correctors
10998
Estimate of luminosity gain
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Nonlinear chromaticity – Run 10 experience Momentum aperture essential for re-bucketing at store (turn on 196
MHz RF system at store – on top of the accelerating 28 MHz RF system to get more beam in the experiment acceptance)
Nonlinear chromaticity reduces the available momentum aperture 2nd order chromaticity minimized for phase advance of (2n+1)*p/2
between 2 equal IP’s Running now with increased arc phase advance from 86 to 93deg/cell
(IBS reduction lattice, lower dispersion in the arcs) Also lower beta* (0.6m instead of 1m) reduced aperture in the triplets Insufficient momentum aperture for re-bucketingTried nonlinear chromaticity corrections but measurements not reliable at
small radial offsetsHad to step back beta* from 0.6m to 0.7m and shift the tunes for
momentum aperture
Outline Introduction Collider design and evolution
Optics, interaction regions, correction systemsValidation of design and correction schemesOperational use of corrections systems (ex: IR corrections)
Commissioning operations: increasing machine performanceexample: lower beta*
Recent developmentsStochastic cooling of bunched beamsTune, coupling, orbit and chromaticity feedback
Lesson learned from RHIC
Performance increaseHeavy ion runs Polarized proton runs
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Increase:Bunch intensity (limits: instabilities)Number of bunches (limits: electron cloud)Total intensity (limits: losses, beam-beam)
Decrease:Beta star (limits: aperture, lifetime)Emittance (via stochastic cooling and electron cooling at low energies)
Beta* squeeze at RHIC
GOALS: Increase of luminosity Preparation for dynamic beta* squeeze with transverse stochastic
coolingHISTORY:
Beta* squeeze: methodology• Before beam the optics matching to lower b* in IP6 and IP8 is turned into a ramp with ramp
application software The ramp, typically 300 s, is first tested without beam for power supply limits and the quench protection system.
• Ramp development Ramp development follows with 6-12 bunches/ ring. Care is taken to avoid
transverse emittance growth to avoid losses in the aperture limiting triplets. The ramps are done with tune & coupling feedback. Orbits are corrected to to 0.1-0.2 mm rms
• Store set-up We tune for lifetime at store (orbit, tunes, coupling, and chromaticity), then steer
for collisions, compare rates and test collimation. Optics measurements with the AC dipole follow. Measured b* are typically in within 10-15% from nominal, and b* is also verified with Vernier scans in operation.
• Test of physics ramp and store We test the new configuration with a physics store (56-109 bunches/ring for
ramp transmission, collimation, experimental backgrounds. If successful we can use the lower b* in operations. We then readjust non-linear corrections for the new configuration, namely local IR triplet correction and possibly non-linear chromaticity corrections.
Example results: d-Au Run-8 We first reduced b* in the yellow ring (gold), where we ran a lattice with higher phase
advance per arc cell to minimize intra beam scattering effects. After 2 attempts, the 3rd ramp with tune & coupling feedback brought the beam to store with good transmission
A 56x56 physics ramp allowed us to establish that the normalized collision rates ratios between the baseline (yellow at 1m) and the one with squeezed optics (yellow at 0.70m) yielded the expected 15% luminosity increase.
Once we established the feasibility of operations with yellow at b*=0.7m, we repeated the development for the blue ring, running deuterons. The entire development took an integrated beam time of ~24h, over a few days. We ran the reminder of the d-Au run with b*=0.7m in both rings, gaining ~30% in integrated luminosity increase for the run.
Outline Introduction Collider design and evolution
Optics, interaction regions, correction systemsValidation of design and correction schemesOperational use of corrections systems (ex: IR corrections)
Commissioning operations: increasing machine performanceexample: lower beta*
Recent developmentsStochastic cooling of bunched beamsTune, coupling, orbit and chromaticity feedback
Lesson learned from RHIC
Dynamic beta* squeeze– Motivation Run10: longitudinal and vertical Stochastic Cooling (SC) are
operational => potential for luminosity increase improve luminosity by a ~factor 2
The goal is to have an application similar to the one used for orbit correction at store: β* as a function of time should follow the change in emittance as achieved by Stochastic Cooling.
To help reaching higher peak luminosity, an application is being developed using the RHIC online model to further push the squeeze of β* in the experimental insertions IR6 and IR8.
2006: first test of longitudinal cooling2007: longitudinal cooling operational 2009: first transverse testsystem installed and tested
M. Blaskiewicz, J.M. Brennan
“signal suppression” demonstrated:
feedback on
feedback off
2010: first test of transverse cooling of ion beams2012: 200 GeV, Au+Au, full stochastic cooling RHIC luminosity upgrade (for ions):
Au+Au, 200 GeV: 40 × 1026 cm-2s-1 (×4)
(with b* = 0.5 m, 56 MHz rf cavity)
Stochastic cooling system
1. New pickup, Blue longitudinal
2. New pickup, Yellow longitudinal
3. Microwave link, upgraded kicker (9 GHz), new low-level enclosure
4. New pickup, Blue vertical (from 1.)
5. New kicker, Blue vertical
Transversepickups, FO
Transversekicker
Transversepickups, FO
Transversekicker
1
2
3 5
4
SystemSchematics
Mike Blaskiewicz C-AD
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Predictions for longitudinal cooling Run-10Current profile at 0, 2.5 and 5 hours without burn-off. 4 MV on storage system, IBS suppression lattice(Vertical cooling only dQmin=0.01, dQbare=0)
Blue longitudinal cooling – measurements Run-10
System status:Yellow transverse operationalYellow longitudinalbeing repairedBlue transverse operationalBlue longitudinal operational(surprise: ring cross talk)
orbit
tune
coupling
chromaticity
“10 Hz”
dynamic reference orbit control andfeed-forward demonstrated (02/04/10)
extensively improved in Run-9fully operational in Run-10
ready for test
To counteract 10 Hz orbit jitter from triplet vibrationsUnder development
RHIC Weekly Meeting, February 8, 2010
successful ramp to store (02/04/10)
Orbit, Tune & Coupling, Chromaticity, 10Hz feedbacks
orbit feedback ONorbit rms in the blue ring orbit rms in the yellow ring
orbit feedback OFF
APEX Meeting, February 5, 2010
Nb 6Nppb ~ 1E9 (Au)SVD tolerance 100FB gain 10% / 10%
Ramp development with continuous orbit and tune/coupling feedback
RHIC Weekly Meeting, February 8, 2010
01/08/10 01/08/10
Chromaticity measurement algorithm improved: extracts chromaticity from ‘wiggled’ tunes
blue ring, ramp chromaticity measurement
yellow ring, ramp chromaticity measurement
ready for chromaticity feedback test
xx xyxx xy
Qx
Qy
Qx
Qy
Chromaticity measurements (prep for feedback)
conclusions
Disclaimer: RHIC in operations for 10 years – thousands of design and operational issues not covered in this talk
Lessons learned: Flexibility in the design pays off in operations and performance
(example beta*) Beam-based corrections play a critical role in a SC hadron
collider Stochastic cooling of bunched beams is a reality – although a
very specialized one. Could/Should become part of the design of new hadron colliders at high energies.
Feedback systems enhance performance and operability of a hadron collider
(Available to discuss topics not covered here)
APEX
STARPolarity