electron cooling for rhic dong wang collider-accelerator department brookhaven national laboratory...

Electron Cooling for RHIC

Dong WangCollider-Accelerator Department

Brookhaven National Laboratory

February 26th, 2003

MIT-Bates

Feb. 26, 2003, MIT Bates 2

Outline

• RHIC Luminosity Upgrade• Electron Cooling Simulations• Overall Design Parameters• Photo-injector: c.w. RF-gun• Superconducting Linac Cavity• Transport of Intense, Magnetized Beam • Summary


RHIC complex

Sketch of RHIC e-cooling layout RF gun Beam dump

4 x 700MHz 5-cell cavities

RHIC ring

Cooler solenoid (a few sections)

Stretcher Compressor

Electron coolingis likely around “4 o’clock”


RHIC: Relativistic Heavy Ion Collider

2*

2

4fN

L

Circumference 3834mBeam Energy (Au ion) 100 GeV/c (proton) 250 GeV/cNumber of IPs 6Beta at IP(H/V) 1~2 mLum. Lifetime ~10 hours *N of Bunches 60~120Bunch Length 30~150 cmEmittance(95%) 15~40 mm mrad * present operation phase

High luminosity is vital for physics experiments.


Luminosity and Intra-Beam Scattering

• Ions at RHIC energy have little synchrotron radiation• Ions interact each other via Coulomb force(IBS)• Overall consequence is emittance growth

Comparison of RHIC IBS Calculations by JW-IBS, BETACOOL and

SIMCOOL(scaled from FWHM)Tran. Emittance vs. Time

Time(hour)

0 1 2 3 4 5 6

Em

ittan

ce o

f Io

n(95

%,

mm

.mra

d)

14

16

18

20

22

24

26

28

30

JW-IBSSIMCOOLBETACOOL(Martini)

RHIC Luminosity and beam currentCourtesy: W. Fischer


RHIC Luminosity Upgrade Plan

RDM RDM+ RHIC II

Initial emittance(95%)

Final emittance(95%)

IP beta function

Number of bunches

Bunch population

B-B parameter

Peak luminosity

Ave. luminosity

m

m

m

109

1027cm-2s-1

1027cm-2s-1

15

40

2

60

1.0

0.0016

0.8

0.2

15

40

1

120

1.0

0.0016

3.2

0.8

15

<6

1

120

1.0

0.004

8.3

7

RHIC II emittance: Cooling is assumed. RHIC II ave. luminosity: 5 hours luminosity time(instead of 10 hours)


Expected scenario with cooling

RHIC with electron cooling

Time(s)

0 10000 20000 30000 40000N

of i

ons,

Bea

m s

ize(

FW

HM

), L

um. a

nd L

um R

atio0

1

2

3

4

5

Change in number of ions(N/N0)Change in beam size(FWHM)Instant luminosity(L/L0)Ratio of int. lum(cooling/no cooling)

Beam dimensions need to be reduced: cooling cooling

0.2 0.1 0 0.1 0.20

500

1000

1500

20001.59 10

3

0

Bi 1

Bi 2

Bi 3

Bi 10

0.250.25 Bi 0

Ion transverse distribution


Electron cooling project at BNL

2000-2001

RHIC e-cooling discussions, inspired by progress in:

1, high current Energy Recovery Linac experiment at Jlab-FEL

2, principle of transport of magnetized beam(Derbenev et al.)

initial calculations were done by BINP.

2001 fall Electron Cooling group(2 persons) at Collider-Accelerator Dept.,

feasibility study, evaluation of cooling, design of e beam facility

2002 fall to (2005?)

more support from C-AD and lab in manpower and funding.

R&D on cathode, beam dump, gun, lianc cavity, solenoid, etc.

some experiments(gun,RF) planned in BLDG 939.


Ions can be cooled by cold electrons

• Proved technique• Good for intense ion beam

(compare to stochastic cooling)

• at low energy so far, up to ~500 keV e- energy.(Fermi Lab: 5 MeV e-, installed). Much more difficult at high energy

1 2 3 4

5 6 7 8

9 10 11 12 13 14 15 16 17 18 19

20 21 22 23 24

25 26 27 28 29

Ion ring

Electron gun Beam dump

2/322222/1)(

)2(8

/3

cm

Tk

cm

Tk

cLZrn

mmt

t

TT

dt

dT

i

eB

e

eB

ee

eieq

eq

iei

‘cool’ electrons mix with ‘warm’ ions

‘Temperature’ of beams:degree of random motion, i.e., emittance, energy spread, etc.

Simplest case: 2-component plasma


Electron cooling calculations

Basically:high chargestrong, high quality solenoidlow emittance and energy spreadmatched beam size

Numerical simulations• Calculation is complicated with

cooler solenoid.• No precise analytical approach. • Semi-phenomenological model is

used(V. Parkhomchuk) • New codes being developed.

050

100

1st

Qt

r

Eas t

West

North

Friction force vs. Solenoid strength, 0 and 1.0T


Electron cooling simulations

Cooling vs. e- bunch charge(Solenoid error level=8E-6)

Time(hour)

0 1 2 3 4 5 6

FW

HM

of I

on(t

rans

vers

e)(m

m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0Charge=2.5ncCharge=3.75ncCharge=5ncCharge=7.5ncCharge=10ncCharge=15ncCharge=20nc

Cooling vs. different solenoid field errors(Bunch charge is 10 nc)

Time(hour)

0 1 2 3 4 5 6

FW

HM

of

Ion(t

ransv

ers

e)(

mm

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Error Level: 2E-5Error Level: 1.3E-5Error Level: 8E-6Error Level: 5E-6Error Level: 6E-6Error Level: 4E-6

Very high charge and tiny solenoid errors are required. 5~10 nc/bunch ~8E-6 error level(B_tran/B)


Electron beam parameters

Energy 55 MeV Particles per bunch 6 x1010 Charge per bunch 10 nc

Ratio of cooler/circumference 0.0078 Average current 94 mA

Beta function at cooler ~5 meter Transverse temperature ~330 eV

Energy spread 10-4 Bunch length ~30 cm

RHIC e-cool: electron beam parameters

Most challenging issues1, high average current(record: 5mA in Jlab FEL)2, transport of high-charge, magnetized beam


E-cooling facility design

• Photo-cathode RF-gun:

produce intense and

high quality electron beam• Superconducting cavity:

for high current beam • Energy recovery for main linac:

save tremendous power (5 MW)• Multi-function arcs:

stretch and compress beam, magnetization matching, beam separation and combination.

Sketch of e-cooling facilityCourtesy: J. Kewisch


Photo-cathode RF-gun

• Cathode&laser:under study(T. Rao, BI)

• Gun: 2 ½ -cell,

1.3 GHz to 700 MHz

Major issue for gun: high dissipated power Field (MV/m) 15 20 25 Diss. power (kw) 773 1373 2140 Ave. power den. (w/cm2) 293 520 810 Max. power den. (w/cm2) 359 638 937 120oC operating temperature.

Courtesy: AES

700 MHz gun: ~9 MV/m at cathode Low field at cathode is bad for beam quality


Gun simulations

Optimization of beam quality:balanced transverse and longitudinal parameters

Major parameters Unit Exit of RF-gun

Entrance of linac

Beam energy MeV 2.35 2.35 Trans. emittance mm.mrad 35 15 Long. emittance KeV.deg 32.3 72.1 Energy spread % 2.2 4.3

Gun geometry and field(SF)


Beam combination

Low energy beam: 2.5 MeVHigh energy beam: 55 MeV

Avoid a large bending angle for low energy beam (space charge effect makes matching difficult)

Septum magnet is chosen.Magnet design is underway.

Larger bending angle+achromat compensation, being explored

Layout of beam merging scheme


Superconducting Cavity

2hom QkfP lossbeam

Initial choice:

TESLA 9-cell 1.3 GHz cavity

Recently we decide to develop a

new cavity with

fewer cells

lower frequency TESLA 9-cell L-band sc cavity

Major Issues high current operations:

• high average current means huge HOM power high bunch charge makes situation even worse

• Multibunch effects driven by high-Q sc cavities

• Single bunch effects


New sc cavity: fewer cells

G 240

R/Q 710

Qbcs 4.9 1010

Ep/Ea 2.1

Hp/Ea 5.94 mT/MV/m

• there are fewer trapped modes in a structure with fewer cells. • fewer cells per structure makes coupling of HOMs easier

1.4 1.6 1.8 2 2.20

5

1010

0.224

R t 0.703( )

R t 1.3( )

3

1

2.21.5 t

BCS resistance vs. temp.Courtesy: I. Ben-Zvi


New sc cavity: lower frequency

• Lower frequency features:

large aperture(19 cm radius),

low loss factor

ABCI 9.4, Spectrum of Loss Factor704 MHz 5-cell Cavity

Frequency(GHz)

0 1 2 3 4 5 6 7

(V/p

C/G

Hz)

-2

0

2

4

6

8

10

12

14

16

ABCI 9.4, Loss Factor, integrated

Frequency(GHz)

0 1 2 3 4 5 6 7

(V/p

c)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cavity (single) TESLA 1.3 GHz 0.7 GHz

Kl (V/pC) 7.8 1.2

Power (kW) 39.6 6.6

Energy spread 30x10-4 5x10-4


Damping HOMs with Ferrite Absorber

Ferrite absorber in B-Factory

One of the worst higher modes

Mode Frequency (GHz)

R/Q (ohms)

Q (copper)

1 7.487009656E-01 0.074 36276 2 7.709806766E-01 0.0092 37132 3 8.034016928E-01 0.97 37597 4 8.397425598E-01 0.68 36309 5 8.787407073E-01 5.1 34786 6 8.835137215E-01 5.3 32782 7 9.523257700E-01 0.24 39326 8 9.567389139E-01 4.1 41466 9 9.639754102E-01 15.3 45403 10 9.709659748E-01 5.8 58026 11 1.052351432E+00 4.9 40865 12 1.053215490E+00 0.012 41182 13 1.189297561E+00 0.5 67051 14 1.224878540E+00 0.003 62710 15 1.268784332E+00 2.3 63299 16 1.322983822E+00 3.0 68243 17 1.383433085E+00 0.8 72849 18 1.441151012E+00 4.1 74000 19 1.559390294E+00 6.3 71363 20 1.566383880E+00 7.1 66539

Waveguide coupler: J. Setukowicz


HOMs with absorbers

Mode Frequency(Re)

(MHz)

Frequency(Im)

(MHz)

Q

1 672.8 1.2E-9 5.61E11

2 680.4 4.9E-9 1.39E11

3 690.0 1.07E-8 6.4E10

4 698.2 1.66E-8 4.2E10

5 701.4 9.59E-9 7.3E10

6 1101 34.1 32

7 1101 34.2 32

8 1231 66.2 19

9 1275 15.13 84

10 1276 0.384 3323

Improving HOM damping of specific modes by changing positions and shapes of absorbers

Dipole modes with ferrites absorbers

Eps=10.0,-1.0 Mu=2.0,/-0.5 (Selected modes are with high R/Q)

Mode Frequency(Re)

(MHz) Frequency(Im)

(MHz) Q

Ferrite only A 877.3 0.00585 1.6E5 B 882.2 0.00779 1.2E5 C 956.7 0.0113 8.5E4 D 963.8 0.0409 2.4E4 E 971.2 0.0424 2.3E4 F 1016 3.66 294

Material: TT2, Ferrite-50N of absorbers: 2/cavity

Monopole modes with ferrite

Local fields around an absorber, the worst mode


Beam Break-Up(BBU)

• Multi-bunch instability• Double-pass in ERL case• Beam energy: 2.5 ~ 55 MeV

Sketch of RHIC e-cooling layout RF gun Beam dump

4 x 700MHz 5-cell cavities

RHIC ring

Cooler solenoid (a few sections)

Stretcher Compressor

Cures: Reduce Q(HOM) High injection energy(expensive) Low R/Q Proper optics

rmmm

r

MkQQRe

cp

12th )/(

2I

Simplest case


Beam Break-Up simulations

Beam-Break-Up simulationsRHIC e-cool, 4x700MHz 5-cell s.c. cavity

With HOM ferrite absorbers,I_th>500mA by TDBBU

Bunch number

0.0 2.0e+4 4.0e+4 6.0e+4 8.0e+4 1.0e+5 1.2e+5 1.4e+5 1.6e+5

Ho

rizon

tal A

mpl

itude

(cm

)

-0.023

-0.022

-0.021

-0.020

-0.019

-0.018

-0.017

Bunch number

0.0 2.0e+4 4.0e+4 6.0e+4 8.0e+4 1.0e+5 1.2e+5 1.4e+5 1.6e+5

Ver

tical

Am

plitu

de(c

m)

-0.0049

-0.0048

-0.0047

-0.0046

-0.0045

-0.0044

-0.0043

-0.0042

TDBBU codeThreshold: > 500 mA~1A with some frequency Spread(0.001(f_hom-f_o))

L-band: ~ 120mA

ERL circulating length: 107.42 mDistance between cavities: ~ 2.0 m


Transport of Magnetized Beam‘magnetizedmagnetized’ or‘ angular momentum angular momentum

dominateddominated’’ beambeamElectrons get angular momentum while they experience the radial field. Troubles in cooler: coherent motions.

0

Cause: cooler solenoid

])([)(2 2 cathode

e

ssrm

e

Busch’s theorem:

Other e cooling facilities: continual solenoid, no such trouble.

RHIC e cooling: discrete elements. certain optical matching is a must. Linear theory: Burov, Derbenev, et al. PRST, 2001. 1, beam must be magnetized at cathode, 2, global matching is needed


Simulating magnetized beamDescription of the magnetized beam:

Angular momentum is the fundamental thing.Beam: E = 55 MeV, emit = 30 mm mrad, beta = 5 m

Angular Momentum vs. r

r

0.0 0.1 0.2 0.3 0.4 0.5 0.6

M

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Electrons

Angular momentum of an e- bunch after experiencing end-field of 1T solenoidat different positions.

Angular Momentum vs. r

r

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

M

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Electrons


Compare different phase spaces

Angular Speed vs. r

R

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Angu

lar S

peed

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Electrons

Angular Speed

r

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Angu

lar S

peed

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Electrons

Magnetized BeamX-Y'

X

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Y'

-4

-3

-2

-1

0

1

2

3

4

Electrons

X-Y'

X

-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20

Y'

-4

-3

-2

-1

0

1

2

3

4

Electrons

Angular speed vs. r

A good measure(linear correlation) New PARMELA

(x,y’) or (y, x’)

Non-Invariant, but maybe useful insome cases


ARC: stretcher and compressor

Function of arcs:

Stretch(compress) e- bunch by a factor of 10~30(M56=30) 2 cavities are used to manipulate longitudinal phase space

Lattice functions of arc with MAD.


Particle tracking: envelope

PRAMELA: tracking along beam line, cathode to cooler


Particle tracking (2)

PARMELA, Evolution of beam emittance and energy spread

RHIC e-cool, magnetized beam transport(photo-injector, linac, stretcher)

Transverse emittance preservation

Longitudinal position(cm)

0 500 1000 1500 2000 2500 3000 3500

RM

S E

mitt

ance

(cm

.mra

d)

0

50

100

150

200

250

300

Horizontal EmittanceVertical Emittance

RHIC e-cool, magnetized beam transport optimization of beam energy spread

Longitudinal position from cathode to cooler(0~30meters)

RM

S e

nreg

y sp

read

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

Exit of photo-injector: ~5E-2Exit of linac: 1.5E-3Exit of stretcher: 0.8E-4


CAM preservation

Preservation of angular momentums is seen though not perfect It is feasible. Improving matching. Simulations with errors, etc.

At exit of linac At end of the first arc


Cooler solenoid (s.c.)

Main field: 1.0TTotal length:

~30 mN of sections:

TBDField error:

<8e-6(trans. field/main field)

Challenging!Challenging! Correctors: h/v

M. Harrison, A. Jain of Magnet Division

Courtesy: Magnet Division


Summary

• Feasibility of electron cooling in RHIC has been explored.

• Electron cooling simulation shows that a high performance cw e- beam facility is needed.

• Beam quality in RF-gun is good but somewhat limited by power issue.

• 700 MHz linac cavity is new choice to address HOM issues. Ferrite absorbers are effective.

• Magnetized beam simulations are exploited. Start-to-end tracking shows that transport line works properly. CAM can be mostly preserved with matching.

• Still a lot of work, solenoid, cathode, error effects, etc.

electron cooling for rhic dong wang collider-accelerator department brookhaven national laboratory...

Documents

mitbates slide

magnetized beam summary

mit bates4 rhic

cooler slide

mit bates26

mit bates25

mit bates5 luminosity

beam line