electron cooling for rhic dong wang collider-accelerator department brookhaven national laboratory...
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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
Feb. 26, 2003, MIT Bates 3
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”
Feb. 26, 2003, MIT Bates 4
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.
Feb. 26, 2003, MIT Bates 5
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
Feb. 26, 2003, MIT Bates 6
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)
Feb. 26, 2003, MIT Bates 7
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
Feb. 26, 2003, MIT Bates 8
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.
Feb. 26, 2003, MIT Bates 9
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
Feb. 26, 2003, MIT Bates 10
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
Feb. 26, 2003, MIT Bates 11
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)
Feb. 26, 2003, MIT Bates 12
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
Feb. 26, 2003, MIT Bates 13
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
Feb. 26, 2003, MIT Bates 14
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
Feb. 26, 2003, MIT Bates 15
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)
Feb. 26, 2003, MIT Bates 16
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
Feb. 26, 2003, MIT Bates 17
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
Feb. 26, 2003, MIT Bates 18
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
Feb. 26, 2003, MIT Bates 19
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
Feb. 26, 2003, MIT Bates 20
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
Feb. 26, 2003, MIT Bates 21
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
Feb. 26, 2003, MIT Bates 22
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
Feb. 26, 2003, MIT Bates 23
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
Feb. 26, 2003, MIT Bates 24
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
Feb. 26, 2003, MIT Bates 25
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
Feb. 26, 2003, MIT Bates 26
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
Feb. 26, 2003, MIT Bates 27
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.
Feb. 26, 2003, MIT Bates 28
Particle tracking: envelope
PRAMELA: tracking along beam line, cathode to cooler
Feb. 26, 2003, MIT Bates 29
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
Feb. 26, 2003, MIT Bates 30
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
Feb. 26, 2003, MIT Bates 31
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
Feb. 26, 2003, MIT Bates 32
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.