The RHIC Collider Fulvia Pilat Collider Workshop, JLAB,
February 24, 2009
Slide 2
RHIC Collider Complex 100x100 GeV/u ions 250x250 GeV polarized
p Chronology: 1996 commissioning AtR 1998 sextant test 1999
engineering runs 2000 first collisions 10 years of operations
Slide 3
Outline Introduction Collider design and evolution Optics,
interaction regions, correction systems Validation of design and
correction schemes Operational use of corrections systems (ex: IR
corrections) Commissioning operations: increasing machine
performance example: lower beta* Recent developments Stochastic
cooling of bunched beams Tune, coupling, orbit and chromaticity
feedback Lesson learned from RHIC
Slide 4
Optics for Run-10 Au-Au *=0.7m
Slide 5
Optics for Run-9 P-P*=0.7m Optics for Run-9 P-P*=0.7m
Slide 6
Optics: zoom into triplets in IR6 (STAR)
Slide 7
Interaction regions: layout and correction systems
Slide 8
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 corrections 2 families of sextupoles in the
arcs linear chromaticity 4 additional sextupole families in arcs
nonlinear chromaticity (added later) IR correction packages (each
triplet) 1 dipole H, 1 dipole V, 1 skew quadrupole 1 normal and 1
skew sextupole 2 octupoles 1 decapole 2 dodecapoles (skew octupole
and dodecapole layers exists but are not powered)
Slide 9
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 tracking Selected capabilities of the offline
model are part of the online machine model But, over the
operational life of the machine we ended relying mostly on beam
based corrections (orbit, coupling, IR corrections, nonlinear
chromaticity)
Slide 10
The magnetic errors in an accelerator magnet can be described
in terms of the multipole errors a n and b n 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 (Q) and the linear coupling term (c) for
different bump planes (H and V) and for different multipole errors
(normal, skew, even and odd orders) can be expressed as follows
(where c n is either the a n or b n 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(b 2 ),
vertical for Skew Sextupole (a 2 ), horizontal and vertical for
octupole (b 3 ) 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
Slide 11
Before Correction After Correction Example: normal sextupole IR
correction Schematics of IR bumps Tune shift vs. amplitude Before
correction Tune shift vs. amplitude Before correction Tune shift
vs. amplitude After sextupole correction Tune shift vs. amplitude
After sextupole correction
Slide 12
Bump power supply Before Correction After Correction Example:
skew quadrupole IR correction Beam decay evolution during the
correction Tune shift vs. amplitude Before correction Tune shift
vs. amplitude Before correction Tune shift vs. amplitude After skew
sextupole correction Tune shift vs. amplitude After skew sextupole
correction
Slide 13
Horizontal Tune Shift Before CorrectionVertical Tune Shift
Before Correction Tune Shifts After Correction Example: octupole IR
correction Before correction H After correction Before correction
V
Slide 14
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
Slide 15
Operational correction for IR decapole and dodecapole 15
Generic scanner scans magnet strength (for list of magnets)
observes beam loss rate minimizes beam loss rate with strength
Slide 16
Decapole/Dodecapole correction result 16 Tested effect of 10-
and 12-pole correctors on beam loss rate by switching off all
correctors 10998
Slide 17
Estimate of luminosity gain 17
Slide 18
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 2 nd order chromaticity minimized for
phase advance of (2n+1)* /2 between 2 equal IPs 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-bucketing Tried nonlinear chromaticity
corrections but measurements not reliable at small radial offsets
Had to step back beta* from 0.6m to 0.7m and shift the tunes for
momentum aperture
Slide 19
Outline Introduction Collider design and evolution Optics,
interaction regions, correction systems Validation of design and
correction schemes Operational use of corrections systems (ex: IR
corrections) Commissioning operations: increasing machine
performance example: lower beta* Recent developments Stochastic
cooling of bunched beams Tune, coupling, orbit and chromaticity
feedback Lesson learned from RHIC
Slide 20
Performance increase Performance increase Heavy ion
runsPolarized proton runs Integrated luminosity L [pb -1 ]
Integrated nucleon-pair luminosity L NN [pb -1 ] 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)
Slide 21
Beta* squeeze at RHIC GOALS: Increase of luminosity Preparation
for dynamic beta* squeeze with transverse stochastic cooling
HISTORY:
Slide 22
Beta* squeeze: methodology Before beam the optics matching to
lower * 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 * are typically in within 10-15% from nominal, and * 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 * in
operations. We then readjust non-linear corrections for the new
configuration, namely local IR triplet correction and possibly
non-linear chromaticity corrections.
Slide 23
Example results: d-Au Run-8 We first reduced * 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 3 rd 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 *=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 *=0.7m in both rings, gaining ~30% in integrated luminosity
increase for the run.
Slide 24
Outline Introduction Collider design and evolution Optics,
interaction regions, correction systems Validation of design and
correction schemes Operational use of corrections systems (ex: IR
corrections) Commissioning operations: increasing machine
performance example: lower beta* Recent developments Stochastic
cooling of bunched beams Tune, coupling, orbit and chromaticity
feedback Lesson learned from RHIC
Slide 25
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: * 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 * in the
experimental insertions IR6 and IR8.
Slide 26
2006: first test of longitudinal cooling 2007: longitudinal
cooling operational 2009: first transverse testsystem installed and
tested M. Blaskiewicz, J.M. Brennan signal suppression
demonstrated: feedback onfeedback off 2010: first test of
transverse cooling of ion beams 2012: 200 GeV, Au+Au, full
stochastic cooling RHIC luminosity upgrade (for ions): Au+Au, 200
GeV: 40 10 26 cm -2 s -1 (4) (with * = 0.5 m, 56 MHz rf cavity)
Stochastic cooling system
Slide 27
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 System Schematics
Slide 28
Mike Blaskiewicz C-AD 28 Predictions for longitudinal cooling
Run-10 Current 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)
Slide 29
Blue longitudinal cooling measurements Run-10 System status:
Yellow transverseoperational Yellow longitudinalbeing repaired Blue
transverseoperational Blue longitudinaloperational (surprise: ring
cross talk)
Slide 30
orbit tune coupling chromaticity 10 Hz dynamic reference orbit
control and feed-forward demonstrated ( 02/04/10) extensively
improved in Run-9 fully operational in Run-10 ready for test To
counteract 10 Hz orbit jitter from triplet vibrations Under
development RHIC Weekly Meeting, February 8, 2010 successful ramp
to store ( 02/04/10) Orbit, Tune & Coupling, Chromaticity, 10Hz
feedbacks
Slide 31
orbit feedback ON orbit rms in the blue ring orbit rms in the
yellow ring orbit feedback OFF APEX Meeting, February 5, 2010 N b 6
N ppb ~ 1E9 (Au) SVD tolerance 100 FB gain 10% / 10% Ramp
development with continuous orbit and tune/coupling feedback
Slide 32
RHIC Weekly Meeting, February 8, 2010 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 yy
xx yy QxQx QyQy QxQx QyQy Chromaticity measurements (prep for
feedback)
Slide 33
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)