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Measurement and Control of Charged Particle Beams in the Relativistic Heavy Ion Collider

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Measurement and Control of Charged Particle Beams in the

Relativistic Heavy Ion Collider

Michiko Minty

Instrumentation Systems Group Leader

Collider-Accelerator Department

Brookhaven National Laboratory

ESS/AD seminar - April 16th , 2014

OUTLINE

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

High Energy Colliders in the US

RHIC: versatile collider in terms of species (p, d, Cu, Au, U,…) and beam energies (maximum of

100 GeV/n for ions, 250 GeV for protons); the only high energy polarized proton collider

Stanford Linear Collider, e+ e- (1989 – 1998)

2 miles

0.75 miles

1.2 miles

TeVatron, p+p (1987 – 2011)

RHIC (>2001)

Relativistic Heavy Ion Collider (RHIC)

RHIC

COLLISIONS

INJECTION

ACCELERATION

PHENIX

STAR

RHIC consists of 2 separate superconducting accelerators, 2.4 miles (3.8 km) long

LINAC

Booster

EBIS

AGS

Tandems

RHIC beams: 110 bunches, each bunch contains ~1E9 ions or 1E11 protons

RHIC bunches are guided and focused using ~ 1750 superconducting magnets

RHIC bunches are very small (~100 mm at interaction points)

RHIC bunches circulate ~ 80,000 times per second

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

Maximizing the scientific output of RHIC

RHIC performance (ions or protons) is characterized by the rate at which particles collide, the

N1

N2

Luminosity

~

f

Ncol

Sx

Sy

N

N

is the number of

colliding bunches

Ncol

Sy

Sx

is the collision

frequency

f

Maximizing the scientific output of RHIC

RHIC performance with protons is also characterized by the beam’s polarization

spin

Uhlenbeck and Goudsmit (1926):

protons possess a spin angular momentum

the spin of a proton responds like a magnetic dipole; it precesses in magnetic fields

at RHIC we preserve the average orientation

of all the proton’s spins, the polarization

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

Challenges - orbits

bunches should collide head-on to maximize collision probability

offsets between the bunches

degrade luminosity

40% loss if

Dx = 100 mm

(Dy = 0)

zoom

L / L0 = e-(Dx/4Sx)2e-(Dy/4Sy)2

Dx

correction factor

for position errors

beam’s orbits (positions and angles) must be controlled

Challenges

The stability of beams in a circular accelerator depends on the so-called “tune” of the accelerator

oscillations about the ideal trajectory

9

the tune, Q

10

8

7

11

6

equals the number of

oscillations made by a bunch in one revolution

around the accelerator

12

5

13

4

3

0

1

2

we monitor and control

the fractional tune

13.5

Q

in this sketch, the vertical (y) tune is Qy = 13.5 and Q = 0.5

Challenges

Resonances! These characterize the tendency of a system to oscillate at a greater amplitude at certain frequencies of excitation

improperly timed pushes …

… will not rock the chair

properly timed periodic forces …

… will rock the chair

if the forces are too large …

……………………

In accelerators, resonances must be avoided

Challenges

In an accelerator, resonances can occur if perturbations act on a bunch in synchronism with its oscillatory motion.

The errors arise from imperfections (or misalignments) of the magnets

resonance condition:

m Qx + n Qy = p

order 0 - driven by dipole magnets

order 0 - driven by dipole magnets

order 0 - driven by dipole magnets

resonance diagram

resonance diagram

resonance diagram

resonance diagram

resonance diagram

resonance diagram

(m, n, and p are integers)

first order

Qy

Qy

Qy

Qy

Qy

second order

third order

the (fractional) tunes

should be irrational

Qx

Qx

Qx

Qx

Qx

Challenges

“working point” in RHIC (protons, at full energy)

zoom

beam 1

beam 2

bounded by strong 3rd and 4th order resonances

for polarized proton operation, the resonance at 0.70 is critical during acceleration

the operating point is therefore moved during acceleration

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

yrms during acceleration, run-9

WHY AT RHIC

time

of day

23:50

10:20

14:30

17:40

22:55

magnetic field errors - including power

supply variations, bit limitations, response

time and magnet alignment errors

correct

start

acceleration

(unavoidable) persistent currents and

hysteresis effects

thermal effects

end

time (s)

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

Precision beam position measurements

“stripline” beam position monitor (BPM)

v

vacuum

chamber

23 cm length

added digital equivalent of a single-pole, low pass filter (IIR filter) to effectively average out predominantly ~ 10 Hz variations in the closed orbit

yold

ynew

Xnew

Xold

precision of average orbit measurements improved by > factor 10

4 km full scale

at RHIC we use 600 BPMs

(150 /plane) to measure the

orbits along accelerator

zoom

400 m full scale

+/- 60 microns full scale

precision of measurement now ~ 5 mm

(smaller than the diameter of 1 red blood cell)

BPM data delivery

before Run-10

acquisition rate: nominally 0.5 Hz

nondeterministic

After Run-10

acquisition rate: 1 Hz

deterministic

Orbit Feedback

measurement

based on existing beam position monitors

using new and improved algorithm for measuring average orbit

using original survey (e.g. offset) data

deterministic data delivery

feedback design

orbit correction algorithm (“singular valued decomposition”)

extended to application at 1 Hz rate during energy ramp

reference orbits specified in terms of BPM data (not corrector strengths)

Mij

x = M q

x

x = vector of ~ 320 BPM

measurements

M = matrix of transfer functions

- = vector of angular deflection
- of ~ 230 correctors

Mij

is the transfer matrix between the

ith BPM and jth corrector dipole

Dq = M-1Dx

proof-of-principle for orbit feedback using existing infrastructure (2010)

energy feedback principle improved (2011)

constrain average horizontal corrector strengths

use all arc BPMs for energy offset determination

implementation of orbit and energy feedback on all ramps (2011)

~400 mm

no feedback

acceleration

start

250 GeV

collisions

with feedback

~20 mm

orbits well controlled, reproducible, and well below the 200 mm tolerance

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

Precision tune measurements

apply (using a “kicker”) a broadband excitation

near the beam’s natural frequency

the beam responds at it’s

natural resonant frequency,

fres

the fractional tune, Q, is

Q = fres / frev

frev = (known) revolution frequency

“kicker”

BPM

signal processing

frequency generator

fres

measurement precision: 2E-5

Precision coupling measurements

resonance-free region has

Qx ~ Qy … precisely where coupling effects are strongest

with Qx ~ Qy and nonzero

coupling (C = 0) beam control in one plane affects the other and produces unexpected results

/

C

run-08

run-11

> factor 10 improvement in measurement resolution

Tune and Coupling Feedback

measurement

based on direct-diode detection (BBQ = base-band tune) for precision

measurements - M. Gasior , R. Jones (2005)

feedback design

uses methodology of coupling angle measurement – Y. Luo (2004)

distinguishes between eigenmodes - R. Jones, P. Cameron, Y. Luo (2005)

history

demonstrated at RHIC in 2006 - P. Cameron et al (2006)

successfully applied for all ramp developments in 2009

used regularly by operations for ramp development in 2010

used together with orbit and energy feedback for all ramps in 2011

before:

8 periods

8 periods

………………………………... (repeat) ……………………………..…….………

1 (possibly corrupted) period

used BBQ/BTF

1 period used

for BBQ/BTF

1 period used

for BBTF

1 in 16 periods of data (AFE output, I/Q demodulator input) used for BBQ/BTF

intermittent corruption of this data due to CPU-limits and

data overwrites with BBTF (ADOs removed)

after:

…………………………………….………………..... (repeat) ……………………………..…………………….………

average of 8 periods used

for BBQ/BTF

multiple super-

imposed ramps

C

tunes and coupling well controlled, reproducibility is excellent

The Relativistic Heavy Ion Collider (RHIC)

Maximizing the scientific output of RHIC

Accelerator physics challenges

Feedback-based beam control

orbits

tunes and coupling

Impact on RHIC performance

Summary

Impact on RHIC performance

(1) Accelerator availability

Time required to successfully accelerate beams to full energy reduced from > 3 days to 2 hours

~ $ 100k savings for initial beam setup

~ $ 100k per operational mode change - particle species, energies or optics (3-4 per fiscal year)

~ $ 100k eliminated need for dedicated

re-optimization efforts

at least 1 extra week for physics operation with electrical costs at 25 MW at $60/MW-hr

(2) Operation under extreme conditions: near-resonance acceleration

end of

acceleration

2/3 resonance

DQy= 0.006

With routine orbit, energy, tune, and coupling feedback on every acceleration of protons to high energies, the vertical tune could be lowered towards dangerous 2/3 orbital resonance (and away from spin resonance at 7/10).

since run-9

25 % increase in relative polarization of each beam

equivalent to 14 additional weeks of RHIC operations ($3.5 M) for same level of statistical uncertainty for physics program

(3) acceleration/deceleration

A dedicated study was performed to confirm the degree of residual polarization loss during acceleration

executed with complete suite of feedbacks demonstrating fully automated beam control, enabled an otherwise impossible

experiment

Summary

The resolution of all measurements (beam position, energy deviation, tune, coupling, and chromaticity) has been improved by more than a factor of 10 and is nearing the limitations of the instrumentation

Control of the parameters affecting beam properties during acceleration in RHIC has transitioned from being pre program-med to based on measurements of the beam’s properties

Feedback-based beam control is now the norm: all beams in RHIC are now established using orbit, tune, coupling and energy feedback. Precision control of these parameters has expanded the parameter space accessible during acceleration. This allows for more extreme operating conditions and is now essential for polarized proton operation.

Acknowledgements

BPM support

R. Hulsart,

P. Cerniglia, A. Marusic, K. Mernick, ,

T. Satogata, P. Thieberger

R. Michnoff

Orbit feedback

T. D’Ottavio,

A. Marusic, V. Ptitsyn, G. Robert-Demolaize

Tune/coupling feedback

A. DellaPenna, M. Gasior (CERN), L. Hoff, R. Jones (CERN),

, C. Schultheiss, C.Y. Tan (FNAL), S. Tepikian

P. Cameron,

Y. Luo, A. Marusic

A. Curcio, C. Dawson, C. Degen, Y. Luo, G. Marr, B. Martin,

P. Oddo, T. Russo, V. Schoefer,

A. Marusic,

K. Mernick,

M. Wilinski

Chromaticity feedback

A. Marusic,

S. Tepikian

Energy feedback

A. Marusic,

K. Smith

10 Hz feedback

P. Cerniglia, A. Curcio, L. DeSanto, C. Folz, C. Ho, L. Hoff, , C. Liu, Y. Luo,

W.W. MacKay, G. Mahler, W. Meng, , C. Montag, R.H. Olsen,

P. Popken, V. Ptitsyn, G. Robert-Demolaize, and many others

R. Hulsart

K. Mernick, R. Michnoff

P. Thieberger

Run coordinators

M. Bai, K. Brown, H. Huang, G. Marr, C. Montag, V. Schoefer

Operations

G. Marr, V. Schoefer

, R. Smith, J. Ziegler

Management

W. Fischer, T. Roser

Accelerator Reproducibility

parameter

no feedback

Feedback

to automate well-defined processes to reduce sensitivities to external influences

WHY

compare measurement with desired value

apply correction

HOW

cruise control

apply required

change in

gas

difference

desired

speed

cruise

measured speed

maintain steady conditions

GOAL

tune/coupling feedback at RHIC

QF

QD

kicker

BPM

signal processing

frequency generator

conversion to currents

model

phase lock

loop

Qx

Qy

Qx

desired

desired

Qy

Qx

10 Hz feedback at RHIC

reduce orbit changes due to triplet magnet vibrations

collision point

triplet

triplet

x (mm)

time

time

10 Hz feedback at RHIC

history

< run-9 feedback on relative beam positions

run-10 new 10 Hz feedback, proof-of-principle with

new correctors

high speed daughter cards for BPMs

dedicated networking

digital signal processing

run-11 routine application

higher integrated luminosity

essential for (possible future) RHIC operation with near-integer tunes

Chromaticity Feedback

The chromaticities, xx and xy represent the coupling of transverse and longitudinal

motion and may be defined as

xx,y = DQx,y/ (Dp/p)

where DQ = the spread in betatron tunes within the bunch of momentum spread Dp/p

Equivalently the chromaticity may be expressed as

xx,y = - (a - 1/g2) DQx,y/ (Dfrf/frf) or xx,y ~ DQx,y/ (Dfrf/frf)

where DQ is the change in betatron tune with change in accelerating frequency frf .

We use this form for measurement of the chromaticity: we measure the change in

tune with applied change in accelerating frequency. Corrections are applied to the

sextupoles (no skew sextupoles to date).

Chromaticity Feedback: WHY

- initially thought to be a requirement for operational tune feedback (tune peak too
- broad and flat if x too large)
- beam stability requires x < 0 below transition energy and x > 0 above
- dynamic aperture issues if x too large

Chromaticity Feedback: HOW

tune/coupling feedback

chromaticity feedback

Measurement:

Vary rf frequency

(specifically, add

a frequency

modulation of

amplitude Dfrf

with periodicity

fD) and measure

tunes

With feedback

(and T/C

feedback too),

the corrections

sent to the

quadrupoles

(the “filtered

tunes”) are used

as input to the

measurement

algorithm