# Presentation: 123902

## The Crystal Collimation System of the Relativistic Heavy Ion Collider

- Ray Fliller III
- University of Stony Brook
- Brookhaven National Laboratory

## Collaborators

- BNL
- 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

## Slide 4

| Run | Species | Integrated Luminosity | Energy |
| --- | --- | --- | --- |
| 2000 | Au-Au | 7.3 b-1 (PHENIX) | 70 GeV/u |
| 2001 | Au-Au | 92.6 b-1 (PHENIX) | 100 GeV/u |
| 2002 | Polarized protons | 100 nb-1  (STAR) | 100 GeV |
| 2003 | d-Au | 27 nb-1 (PHENIX) | 100 GeV/u |
| 2003 | Polarized protons | 2500 nb-1  (STAR) | 100 GeV |
| 2004 | Au-Au | 1368 b-1  (PHENIX) | 100 GeV/u |
| 2004 | Polarized protons | 3200 nb-1  (STAR) | 100 GeV |
| 2005 | Cu-Cu |  | 100 GeV/u |

## 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 Collimation

- Various 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 Collimation

- Since 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 Channeling

- Ions properly aligned to the crystal planes are channeled....

- ...Particles with large incident angles scatter through the crystal

## Interplanar Potential

*Interplanar Potential*

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

- Curvature shifts minimum

- dp

## Critical Angle  c

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

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

- For  100 GeV p, c=19 rad

## Channeling Efficiency

- The integral of the incoming particle distribution over the channeling phase space is the channeling efficiency

- For a beam with uniform divergence: 2>2c

## Dechanneling and Volume Capture

*Dechanneling and 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.

## Crystal Collimator Geometry

## Model of Beam Hitting Crystal

- Assuming a Gaussian beam distribution of:

-  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:

## Angular Alignment

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

- 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 Divergence

- The equation for angular divergence, *x*’(*x**0*), 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*’(*x**0*)
-  *p        *– large values increase *x*’(*x**0*)
* ****, D  *– large values *decrease* *x*’(*x**0*)
-  *x     *– large values increase *x*’(*x**0*) (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

## Angular Width – Model Optics

- *PHENIX = 2 m

- model

- Critical Angle

## Angular Width – Measured Optics

- 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:

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

- However, this model gives us a starting place....

## Placement of the Crystal

- Crystal should be placed at a location that has low  and D’ and a maximun of  so that:
*x**p**’* is independent of *x**0*
-  *x*’(*x**0*) 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 Diodes

- Downstream 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

## Lattice Functions

*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 |

## “Typical” Crystal Scan

- Volume
- Capture

- Crystal
- Aligned

- Crystal Channeling

- November 12, 2001 Au beam at store.

*“**Typical” Crystal Scan*

*x’**p*

- *x’**(x**0**)*

## Hodoscope Signal

*Hodoscope Signal*

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

## Comparison to Simulation

*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

*Channeling Angle vs. Position*

- *=1m at PHENIX

- Design:                  rad/mm

- Measured
- Optics:                       rad/mm

- Data:                      rad/mm

## Beam Divergence

| Run | *PHENIX | x’(x0) [rad] |  |  |  |
| --- | --- | --- | --- | --- | --- |
|  |  | Design optics | Measured optics | Simulation | Channeling 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 | Simulation | Measured width | Channeling 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 Background

- 4 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

*Crystal Collimation vs. Raw Background*

- Scraper moves closer to beam

- Crystal Collimation reduces
- Background 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

*Single Stage Collimation*

- Horizontal Collimator

- Vertical Collimator

- Closer to beam

- Partially retracting the vertical collimator increases backgrounds

- Fill 03094 d-Au run

- During 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 Results

- Fill 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.