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Electron Cooling at RHIC

Enhancement of Average Luminosity for Heavy Ion Collisions at RHIC

R&D Plans and Simulation Studies

8th ICFA SeminarKyungpook Natioanl University

Daegu, Korea, September 29, 2005

Satoshi Ozaki for the RHIC e-Cool TeamBrookhaven National Laboratory

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RHIC Operations and Plans

• Run 1: FY 2000 28 wks Au-Au (130 GeV/A)• Run 2:FY 2001-02 40 wks Au-Au (200 GeV/A) p↑-p↑ (200 GeV)• Run 3:FY 2003 29 wks d-Au (200 GeV/A) p↑-p↑ (200 GeV) ~30% Pol.• Run 4:FY 2004 27 wks Au-Au (200, 62 GeV/A) p↑-p↑ (200 GeV)• Run 5:FY 2005 32 wks Cu-Cu (200, 62 GeV/A) p↑-p↑ (200 GeV) ~50% Pol

Near term improvements in progress• Superconducting helical snakes in the AGS for higher polarization for FY 2006 Runs• Development of EBIS ion source for flexibility of ion operation

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First Five Years of RHIC Experiments

• The luminosity performance of RHIC for Au-Au & Cu-Cu collisions exceeded the design values.

• We observed creation of a new state of matter in Au-Au collisions at 200 GeV/A collision energy: – hot, dense and strongly coupled, – behaving like perfect fluid.

• Next stage of the program:– Study properties of the new state of matter– Study of rare processes Requires much higher

average/integrated luminosity

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Typical Au-Au Operation on Feb. 23, 2004

Au Beam Intensity vs. Time

Au-Au Luminosity vs. Time

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Control of Emittance Growth: Cooling

• The Au-Au luminosity life-time is only a few hours• Strong intra-beam scatterings cause emittance growth:

– Longitudinal: loss of ions from colliding buckets– Transverse: larger crossing beam spot size

• Cooling of ion beams: the key to a longer luminosity life-time: i.e., a higher average luminosity

• Cooling:– Stochastic cooling: more effective for hot beam

• Difficult for bunched Proton beams but it appears that it can work for heavy ion beams in RHIC

• Longitudinal cooling test in preparation– Electron cooling: more effective for cool beam

• It has been successful at lower energies but has not been demonstrated at high energy like RHIC

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The Objectives of RHIC e-Cooling and Challenges

• ~10 times Increase of RHIC average luminosity for Au-Au at 100 GeV/A

• Reduce background due to beam loss

• Keep short collision diamond by maintaining short bunch length to match detector’s acceptance

• Cooling rate slows in proportion to 7/2.

• Energy of electrons needed (54 MeV) is well above DC accelerators.

• Requires bunched e beam.

• Need exceptionally high electron bunch charge and low emittance.

• Need ERL to provide low emittance e-beam while maintaining a reasonable power demand.

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R&D: Theory Issues

• We must understand cooling physics in a new regime:

– understanding IBS, recombination, disintegration– binary collision simulations for benchmarking– experimental benchmarking of the magnetized cooling

efficiency issues

• Cooling dynamics simulations with precision

• A good estimate of the luminosity gain is essential.

• Simulations show that:10X increase in the average luminosity can be

achieved(from 7x1026 to ~7x1027 cm-2s-1)

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Parameters for of RHIC Magnetized e-cooling

Key e-beam parameters:– Bunch charge: q = 20nC– E-Beam Energy = 54MeV E/E < 3x10-4 – Emittance: 50m-rad– Magnetization: 380mm.mr

Energy Recovery Linac– fSRF: 703.5 MHz– Repetition rate: 9.4 MHz

Cooling solenoids:

2 x 40m long

B = 5T, B/B < 10-5

Collider operation:

Collisions at 3 IPs,

*=0.5m,

112 bunches

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Ne/bunch=3*1011

w/ cooling. *=0.5m

w/o cooling, *=1m

Simulation for Au-Au at 100 GeV/A

Luminosities per IP in cm-2sec-1 vs. time in seconds

The luminosity gain may be limited either by the collision beam burn out or the beam-beam parameter

X, Y, Z Distribution ()

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R&D: ERL and Cooling Hardware Issues

– Development of a high current low emittance RF Gun:– photocathode, laser, etc.

– Design of a high current & very low emittance ERL

– Development of beam diagnostics

– Beam dynamics studies

– Further refinements of simulation codes

– Development of high field solenoid with B/B<105

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Laser Photocathode S/C RF Gun: Key to performance

1 ½ cell gun designed for cooler

½ cell gun prototype: Under construction

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Diamond Amplified Photocathode

0

100

200

300

400

500

600

0 5 10

Gradient (MV/m)

Ele

ctro

n tr

ansm

issi

on g

ain

5keV

4keV

3keV

2keV

Electron Amplifying Diamond Window•Less demanding on laser power•Longer cathode life•Protect SC cavities from contamination

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Schematics for Magnetized Beam ERL Lattice

←Compressor Stretcher→

Gun Z-bend merger

Cooling solenoids in RHIC ring

ERL Beam Dump

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The Possibility of Non-magnetized Electron Cooling

• Handling of magnetized beams is not easy, and the system is complex and expensive.

• At high , achievable solenoid error limits the cooling speed of the magnetized cooling.

Another way is the non-magnetized e-cooling:• A study showed that sufficient cooling rates can be achieved

with non-magnetized cooling.• Recombination beam loss is a concern but can be managed

to be small enough to assure a long luminosity life-time– By reduced bunch charge– By larger beam size

• Helical undulator can further reduce recombination**Suggested by Derbenev, and independently by Litvinenko

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Non-magnetized Cooling: Parameters Beam Parameters:• Rms momentum spread of electrons =10-3

• Rms normalized emittance: 2.5 µmrad• Rms radius of electron beam in cooling section: 2.5 mm• Rms bunch length: 5 cm• Charge per bunch: 5nC (cf. 20nC for magnetized case)• Cooling sections: 2x30 m• Large ion beam in the cooling section: β* = 200 m

All ERL technology developments for mag-cool applies here but• without complex magnetized electron beam gun, • without bunch stretcher and compressor, and• without complex beam optics to preserve magnetization

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Non-magnetized Cooling: SimulationAu-Au at 100 GeV/A

Magnetized CoolingNon-magnetized Cooling: Luminosity

Non-magnetized Cooling: Emittance

Non-magnetized Cooling: Bunch Length

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Beam Loss Comparison: Simulation

Recombination: OffUndulators: Off

Recombination: ONUndulators: OFF

Recombination: ONUndulators: ON

Undulator parameters:50 Gauss, 5 cm period,Radius of rotation 1.7 m

Beam Intensity

Time (sec)

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R&D ERL Under Construction

To study the issues of high-brightness, high-current electron beams as needed for RHIC II and eRHIC.

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SRF Cavity for High Current (Ampere Class) ERL

703.5 MHz 5 Cell Cavity with Beam Tune HOM Damping:

Built by Advanced Energy Systems Inc. of Long Island

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RHIC e-Cooling Project Milestones & Collaborations

• 2005 Dec: Electron cooling simulation completed• 2006 Jan: Decision on the cooling method• 2006 Feb: High power rf system for the gun in place• 2006 Apr: 5-cell superconducting cavity delivered• 2006: Beam dynamics simulation• 2006: Cost and Schedule of e-cooling system for CD0• 2007 Mar: Begin testing S/C gun,

hopefully with the diamond cathode• 2008: Hope to begin testing of ERL hardware

• The Milestones subject to the future funding level

• Collaborators: BINP, JINR, Celsius, GSI. US Jefferson Lab, Fermilab, Indiana Univ., and industry (AES and Tech-X)

• Supported by: the U.S. DOE, Division of Nuclear Physics, and partially bythe U.S. DOD HE Laser Joint Tech Office and ONR