The Crystal Collimation System of the Relativistic Heavy Ion

Collider

Ray Fliller III

University of Stony Brook

Brookhaven National Laboratory

CollaboratorsBNL

• Angelika Drees• Dave Gassner• Lee Hammons• Gary McIntyre• Steve Peggs• Dejan Trbojevic

IHEP – Protvino• Valery Biryukov• Yuriy Chesnokov• Viktor Terekhov

Outline• Brief RHIC Overview

• Collimation

• Crystal Channeling

• RHIC Crystal Collimation System

• Channeling Results

• Crystal Collimation and Background Reduction

• Conclusion

Run SpeciesIntegrated Luminosity

Energy

2000 Au-Au 7.3 b-1 (PHENIX) 70 GeV/u

2001 Au-Au 92.6 b-1 (PHENIX) 100 GeV/u

2002Polarized protons

100 nb-1 (STAR) 100 GeV

2003 d-Au 27 nb-1 (PHENIX) 100 GeV/u

2003Polarized protons

2500 nb-1 (STAR) 100 GeV

2004 Au-Au 1368 b-1 (PHENIX) 100 GeV/u

2004Polarized protons

3200 nb-1 (STAR) 100 GeV

2005 Cu-Cu 100 GeV/u

Crystal Collimator

RHIC Capabilities• Two 3.8 km counter-propagating superconducting

rings BLUE (clockwise) and YELLOW (counterclockwise).

• Can accelerate anything from polarized protons (250 GeV) to fully stripped gold ions (100 GeV/u), possibility of colliding uneven species.

• Six IRs with four experiments (STAR, PHENIX, BRAHMS, PHOBOS).

• Typical store each ring contains 110 bunches of 109 gold ions or 1011 polarized protons.

Typical RHIC Parameters• 95 % norm. Emittance: =15 mm-mrad

• rms momentum spread: p = 0.13 %

• Bunch length: l = 0.19 m

• Energy: 100 GeV/u

• Store Length: 4 hours

• Beam size at collimator: 5.3mm (*PHENIX=1m)

Need for CollimationVarious processes cause particles to enter into unstable orbits with large betatron amplitudes, causing beam halo formation. These halo particles cause:

The job of the collimation system is to remove the halo and alleviate these problems. In addition, it should provide a well defined location for beam losses in case of equipment failure.

•Background in experiments•Excessive radiation in uncontrolled areas of the tunnel•Magnet quenches in superconducting machines•Equipment malfunction and damage

Naive Collimation

Naively, all particles that enter the collimator are stopped in the collimator.

However, that is usually not the case….

Collimator

Beam

Most particles hit near edge and scatter out of the collimator forming secondary halo!

Two Stage CollimationSince primary collimator acts as a scatterer, secondary collimators are necessary to increase energy loss and absorb secondary halo particles.

The number of secondary collimators grows quickly when background or machine protection requirements are strict and a high collimation efficiency is required (see LHC collimation system!).

A simpler way to collimate

Use a bent crystal to channel halo away from the beam core, intercept with a scraper downstream. Number of secondary collimators can be greatly reduced.

Crystal ChannelingIons properly aligned to the crystal planes are channeled….

…Particles with large incident angles scatter through the crystal

Interplanar PotentialIons “properly aligned” to the crystal planes see an average potential. This potential is skewed by the bending of the crystal.

Curvature shifts minimum

Large electron density – particles will get lost.

Particles with are not channeled.cmp Ex 2

2

dp

-xc xcxmax

Ec

Critical Angle c

cmp Ex 2

2

The channeling condition gives an angle c, above which a particle will not be channeled.

pv

Ecc

2

Using a Si crystal with 100 GeV/u Au or 250 GeV p , c=11 rad

To have a large channeling efficiency, the angular divergence of particles impacting crystal should be less than 2c.

For 100 GeV p, c=19 rad

Channeling EfficiencyThe integral of the incoming particle distribution over the channeling phase space is the channeling efficiency

cc

p

c

d

x 66.04

2

For a beam with uniform divergence: 2>2c

2

Dechanneling and Volume CaptureScattering from:Impurities

ElectronsLattice Defects

And sudden curvature changes all cause particles to dechannel. The same processes cause dechanneled particles to become channeled – volume capture.

CATCH Simulation

CATCH by Valery Biryukov

Important Considerations for Crystal Collimation

• Crystal alignment to beam halo.

• Angular divergence of beam halo hitting crystal.

How to we predict these??

Crystal Collimator Geometry

Model of Beam Hitting CrystalAssuming a Gaussian beam distribution of:

2

2

2expexp

2

1),(

pp

JJ

• J = J(x, x’, ) is the particle amplitude• is the rms unnormalized emittance• is the fractional momentum deviation• p is the rms fractional momentum spread

By transforming from {J, } to {x, x’, } and integrating over momentum: )',(),( xxJ

Angular AlignmentAssuming the distribution extends over the entire crystal face, the angle between the beam orbit and particles striking the crystal is

2'

2022

2

20 )('

)('x

xxx

p

pxp x

D

DDxx

•x0 is the distance between crystal and beam center• x is width of crystal face• , , D, D’ are lattice functions at crystal

The crystal planes need to be at this angle relative to the beam orbit!

This is proper alignment!

Angular DivergenceThe equation for angular divergence, x’(x0), is not very illuminating. However, it depends strongly on:

For those who REALLY want to see the equation, read my thesis!

• D’– large values increase x’(x0) • p – large values increase x’(x0) • , D – large values decrease x’(x0) • x – large values increase x’(x0) (assuming particles hit

whole crystal face)By optimizing these parameters, the angular spread of beam across the crystal face is minimized.

Phase Space at Crystal

When crystal is moved into beam, it needs to be realigned

And the angular spread increases!

x6

Angular Width – Model Optics

*PHENIX = 2 m model

Critical Angle

measured (FY2001)

and D affect ellipse orientation and shape

Critical Angle

*PHENIX = 2 m

Angular Width – Measured Optics

Caveat Emptor!There are a few holes in the model:

1. Particle distribution – Gaussian in the tails??2. Assumption that particles strike across the whole face of crystal.3. Does not take into account multiple turns.4. Not useful for volume capture predictions.

However, this model gives us a starting place….

Placement of the CrystalCrystal should be placed at a location that has low and D’ and a maximun of so that:•xp’ is independent of x0

• x’(x0) is reduced•Channeling efficiency is increased•Operation of crystal collimator is easier

However, in RHIC all warm spaces have large !

RHIC Collimation System

Upstream PIN DiodesDownstream PIN Diodes

STAR

Scraper can move horizontally, vertically and rotate in horizontal plane

Hodoscope courtesy of Y. Chesnokov and V.Terekhov

Changed after FY2003

Vessel Cutaway

Crystal

Inchworm

Moveable Stage

Pivot

Crystal Vessel

Crystal

Crystal Motion

Beam

Crystal

Measuring Crystal Angle

By measuring the deflection of the laser beam, the crystal angle is measured

•Crystal can rotate approx: 6 mrad•Measurement Resolution: 20 rad•Angular Step Size: 30 nrad

*PHENIX = 2 m FY2003

Crystal Collimator

PHENIXScraper

Lattice Functions

Synopsis of Data

Run Species *PHENIX Stores Scans

FY2001 Au 5 m 8 27

FY2001 Au 2 m 4 24

FY2001 Au 1 m 12 109

FY2002 p 3 m 11 119

FY2003 Au 2 m 4 20

Volume Capture

Crystal Aligned

Crystal Channeling

November 12, 2001 Au beam at store.

“Typical” Crystal Scan

x’p

x’(x0)

b

A

Hodoscope Signal

Very noisy compared to PIN diodes. Coincidence rate is almost useless. Limited use in analysis.

Comparison to Simulation

Simulation used CATCH and one turn matrix.

Model Optics:•Location wrong•dip width too narrow•efficiency too large

Design optics do not agree well with data. However, measured optics agrees better.

Comparison to Simulation

Volume capture region strongly affected by number of turns in simulation.

Channeling Angle vs. Position*=1m at PHENIX

Design: rad/mm222'

x

xx

MeasuredOptics: rad/mm3232

' x

xx

Data: rad/mm2382'

x

xx

xx’/x2 is independent of *

PHENIX. Measurements during other runs indicate 36 2 rad/mm. Other

datasets agree with this number as well.

Beam Divergence

Run *PHENIX

x’(x0) [rad]

Design optics

Measured optics

SimulationChanneling

data

FY2001 5 12.3 39 4

FY2001 2 9.98 20 1 78 4

FY2001 1 8.91 9 1 11 1 38 3

FY2002 3 10.8 58 3

FY2003 2 9.98 14 1 16 1 28 2

Even using the correct optics, the predicted angular spread is too small.

Multiple turns are not in the theory!Assumed Gaussian halo distribution!

Channeling Efficiency

Run *PHENIX

Channeling Efficiency

Design optics

Measured optics

SimulationMeasured

widthChanneling

data

FY2001 5 59 % 19 2 % 24 3 %

FY2001 2 71 % 39 2 % 9 1 % 28 3 %

FY2001 1 74 % 75 1 % 56 3 % 20 2 % 19 3 %

FY2002 3 79 % 21 2 % 26 3 %

FY2003 2 71 % 52 2 % 50 1 % 26 2 % 26 3 %

Channeling Efficiency does not match predictions from the theory. This is because the beam divergence on the crystal does not match theory. Using the measured beam divergence (from x’(x0) ) the efficiency agrees well for most cases.

Channeling Results• RHIC optics did not match model, so initial predictions

overestimated crystal performance• Simple theory overestimates channeling efficiency –

lacking multiple turns, model of halo distribution too simple.

• Simulation agrees with data well.• Channeling efficiency is understood once optics and

beam halo distribution are understood.• Accurate knowledge of lattice functions and halo

distribution VERY IMPORTANT! Will low channeling efficiency result in too much scattering and hurt collimation?

STAR Background4 crystal scans with different scraper positions - xs

Crystal not moved.

Other Experiment Backgrounds

Only BRAHMS see significant effect

Placing the Scraper

Scattering from scraper

Scattering from crystal

By using both sets of PIN diodes, we can know when the scraper becomes

the primary aperture!

STAR Background Reduction

“Raw” Background

Scraper only

Crystal collimation does not do better than scraper alone!

Crystal Collimation vs. Raw Background

Scraper moves closer to beam

Crystal Collimation reducesBackground to uncollimated rate

Au beam, d-Au run, crystal collimation not always effective in reducing background.

Crystal Collimation Results• Crystal can cause background in experiments.• Scraper position very important.• Because of low channeling efficiency, crystal

collimation was not successful.• Scraper alone collimated the best.• Crystal Collimator removed from RHIC.

Traditional two stage collimation system installed for FY2004 run.

Summary• Bent Crystals were used for collimation in RHIC

• Crystal Channeling worked as expected once lattice functions and halo distribution were understood.

• Collimation was unsuccessful because lattice was not optimized in area of collimator.

• Crystal caused background.

• Tevatron is going to install our vessel (and I’ll be following it there!) Questions??

Single Stage Collimation

Horizontal Collimator

Vertical CollimatorCloser to beam

Partially retracting the vertical collimator increases backgrounds

Fill 03094 d-Au runDuring d-Au run, backgrounds were reduced by as much as a factor of 5.

Upgraded Collimation System

PIN Diodes downstream of V1 and H1 collimators are not shown for clarity

•Crystal Collimator removed•Primary is the same collimator as previous runs, moved to location reserved for the Crystal Collimator•Secondary collimators are based on design of primary•Controls software upgraded to include manual/automatic control of collimators

Upgraded Collimation ResultsFill 04436 Au-Au run

Collimators move simultaneously.

Backgrounds reduced by factor of 11,2x the pervious run!

PHENIX

STAR

Summary• Single stage collimation was adequate during lower

luminosity runs.

• Two stage collimation was successful during the FY2004 Au-Au run.

• Two more vertical collimators are installed for the FY2005 Cu-Cu run.