Electron Cooling for RHIC

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Electron Cooling for RHIC

• Dong Wang
• Collider-Accelerator Department
• Brookhaven National Laboratory
• February 26th, 2003
• MIT-Bates

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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

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RHIC complex
Electron cooling is likely around 4 oclock
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RHIC Relativistic Heavy Ion Collider
Circumference 3834m Beam Energy (Au
ion) 100 GeV/c (proton) 250
GeV/c Number of IPs 6 Beta at IP(H/V) 12
m Lum. Lifetime 10 hours N of
Bunches 60120 Bunch Length 30150
cm Emittance(95) 1540 ? mm mrad present
operation phase
High luminosity is vital for physics experiments.
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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

RHIC Luminosity and beam current Courtesy W.
Fischer
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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 lt6 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)
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Expected scenario with cooling
Ion transverse distribution
Beam dimensions need to be reduced cooling
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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.
• RD on cathode, beam dump, gun, lianc cavity,
  solenoid, etc.
• some experiments(gun,RF) planned in BLDG 939.
  

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Ions can be cooled by cold electrons
Electron gun
Beam dump
cool electrons mix with warm ions
Temperature of beams degree of random motion,
i.e., emittance, energy spread, etc.
Ion ring

• 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

Simplest case 2-component plasma
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Electron cooling calculations

• Basically
• high charge
• strong, high quality solenoid
• low emittance and energy spread
• matched 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.

Friction force vs. Solenoid strength, 0 and 1.0T
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Electron cooling simulations
Very high charge and tiny solenoid errors are
required. 510 nc/bunch
8E-6 error level(B_tran/B)
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Electron beam parameters
RHIC e-cool electron beam parameters
Most challenging issues 1, high average
current(record 5mA in Jlab FEL) 2, transport of
high-charge, magnetized beam
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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 facility Courtesy J. Kewisch
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Photo-cathode RF-gun

• Cathodelaser
• under study(T. Rao, BI)
• Gun 2 ½ -cell,
• 1.3 GHz to 700 MHz

Courtesy AES
700 MHz gun 9 MV/m at cathode Low field at
cathode is bad for beam quality
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Gun simulations
Gun geometry and field(SF)

• Optimization of beam quality
• balanced transverse and longitudinal parameters

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Beam combination
Low energy beam 2.5 MeV High 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
angleachromat compensation, being explored
Layout of beam merging scheme
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Superconducting 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

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
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New sc cavity fewer cells

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

G 240 ?
R/Q 710 ?
Qbcs 4.9 1010
Ep/Ea 2.1
Hp/Ea 5.94 mT/MV/m
BCS resistance vs. temp. Courtesy I. Ben-Zvi
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New sc cavity lower frequency
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

• Lower frequency features
• large aperture(19 cm radius),
• low loss factor

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Damping HOMs with Ferrite Absorber
Ferrite absorber in B-Factory
Waveguide coupler J. Setukowicz
One of the worst higher modes
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HOMs with absorbers
Local fields around an absorber, the worst mode
Material TT2, Ferrite-50 N of absorbers
2/cavity
Monopole modes with ferrite
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
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Beam Break-Up(BBU)

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

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

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Beam Break-Up simulations
ERL circulating length 107.42 m Distance between
cavities 2.0 m
TDBBU code Threshold gt 500 mA 1A with some
frequency Spread(0.001(f_hom-f_o)) L-band
120mA
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Transport of Magnetized Beam

• magnetized or angular momentum dominated
  beam
• Electrons get angular momentum
• while they experience the radial field.
• Troubles in cooler coherent motions.

Cause cooler solenoid
Buschs 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
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Simulating magnetized beam
Description of the magnetized beam Angular
momentum is the fundamental thing. Beam E 55
MeV, emit 30 mm mrad, beta 5 m
Angular momentum of an e- bunch after
experiencing end-field of 1T solenoid at
different positions.
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Compare different phase spaces
Angular speed vs. r A good measure (linear
correlation) New PARMELA
(x,y) or (y, x) Non-Invariant, but maybe
useful in some cases
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ARC stretcher and compressor
Function of arcs Stretch(compress) e- bunch by
a factor of 1030(M5630) 2 cavities are used
to manipulate longitudinal phase space
Lattice functions of arc with MAD.
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Particle tracking envelope
PRAMELA tracking along beam line, cathode to
cooler
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Particle tracking (2)
PARMELA, Evolution of beam emittance and energy
spread
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CAM preservation
Preservation of angular momentums is seen though
not perfect It is feasible. Improving
matching. Simulations with errors, etc.
At end of the first arc
At exit of linac
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Cooler solenoid (s.c.)
Main field 1.0T Total length 30 m N of
sections TBD Field error lt8e-6 (trans.
field/main field) Challenging! Correctors
h/v M. Harrison, A. Jain of Magnet Division
Courtesy Magnet Division
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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.