The Crystal Collimation System

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15 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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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

Run

Species

Integrated
Luminosity

Energy

2000

Au
-
Au

7.3
m
b
-
1
(PHENIX)

70 GeV/u

2001

Au
-
Au

92.6
m
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
m
b
-
1

(PHENIX)

100 GeV/u

2004

Polarized
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 10
9

gold ions or 10
11

polarized protons.

Typical RHIC Parameters


95 % norm. Emittance:
e
=15
p

mm
-
mrad


rms momentum spread:
s
p

= 0.13 %


Bunch length:
s
l

= 0.19 m


Energy: 100 GeV/u


Store Length: 4 hours


Beam size at collimator: 5.3mm (
b
*
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

Ions “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.

c
m
p
E
x

2
2
d
p

-
x
c

x
c

x
max

E
c

Critical Angle
q
c

c
m
p
E
x

2
2
The channeling condition gives an angle
q
c
, above which
a particle will not be channeled.

pv
E
c
c
2

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

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

For 100 GeV p,
q
c
=19
m
rad

Channeling Efficiency

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





c
c
p
c
d
x
q
q
p
e
66
.
0
4
2
For a beam with uniform divergence: 2

>2
q
c

2


Dechanneling and Volume Capture

Scattering from:

Impurities

Electrons

Lattice 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 Crystal

Assuming a Gaussian beam distribution of:


















2
2
2
exp
exp
2
1
)
,
(
p
p
J
J
s

e
e
ps




J = J(x, x’,

) is the particle amplitude



e

is the rms unnormalized emittance





is the fractional momentum deviation



s
p
is the rms fractional momentum spread

By transforming from {J,

} to {x, x’,

} and

integrating over momentum:

)
'
,
(
)
,
(
x
x
J




Angular Alignment

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

2
'
2
0
2
2
2
2
0
)
(
'
)
(
'
x
xx
x
p
p
x
p
x
D
DD
x
x
s
s
s
be
s
e










x
0

is the distance between crystal and beam center




x is width of crystal face




,
b
, 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,
s
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

s
x

(
x
0
)




s
p


large values increase

s
x

(
x
0
)




b
, D


large values
decrease

s
x

(
x
0
)





x


large values increase

s
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

And the angular spread increases!

x
s
6

Angular Width


Model Optics

b
*
PHENIX

= 2 m

model

Critical Angle

measured (FY2001)

b

and D affect ellipse

orientation and shape

Critical Angle

b
*
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 Crystal

Crystal should be placed at a location that has low


and D’
and a maximun of
b

so that:


x
p


is independent of
x
0



s
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
m
rad


Angular Step Size: 30 nrad



b
*
PHENIX

= 2 m FY2003

Crystal Collimator

PHENIX

Scraper

Lattice Functions

Synopsis of Data

Run

Species

b
*
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

s
x’
(x
0
)

q
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

b
*
=1m at PHENIX

Design:
m
rad/mm

22
2
'

x
xx
s
s
Measured

Optics:
m
rad/mm

3
23
2
'


x
xx
s
s
Data:
m
rad/mm

2
38
2
'


x
xx
s
s
s
xx’
/
s
x
2

is independent of
b
*
PHENIX
. Measurements
during other runs indicate 36 2
m
rad/mm. Other
datasets agree with this number as well.


Beam Divergence

Run

b
*
PHENIX

s
x’
(x
0
) [
m
rad]

Design
optics

Measured
optics

Simulation

Channeling
data

FY2001

5

12.3

39 4

FY2001

2

9.98

19 1

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

b
*
PHENIX

Channeling Efficiency

Design
optics

Measured
optics

Simulation

Measured
width

Channeling
data

FY2001

5

59 %

19 2 %

24 3 %

FY2001

2

71 %

39 2 %

37 1 %

9†ㄠ%

㈸†%

䙙F〰1

1

㜴%

㜵†%

㔶†%

20†%

ㄹ†%

䙙F〰2

3

㜹%

21†%

㈶†%

䙙F〰3

2

㜱%

㔲†%

㔰†%

26†%

㈶†%

















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
s
x’
(x
0
)
) 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
-

x
s

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 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

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.