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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Polarized Electron Sources for Future Electron Ion Colliders

M. Farkhondeh, Bill Franklin and E. Tsentalovich

MIT-Bates accelerator Center

Ilan Ben-Zvi , V. Litvinenko, Brookhaven National Laboratory

PST05 Workshop, Tokyo, Japan,

November 14-17, 2005

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

OUTLINE

Electron Ion Colliders (EIC) Current EIC designs (eRHIC and ELIC) Polarized source for eRHIC

ring-ring design linac-ring concept

Polarized RF gun Polarized source for ELIC Summary

PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

EIC Kinematics range, a unique region

EIC

Gluon momentum distribution measured

Nucleon spin structure studied

• High Ecm large range of x, Q2

• Lepton probe provides precision but requires high luminosity to be effective

Fixed target experiment

Collider

Electron Ion Collider

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

EIC in USA

Currently two designs are under considerations:

1. At BNL and MIT: eRHICan electron –Ion collider based on the existing RHIC accelerator at BNL. In 2004 produced a zeroth design report (ZDR). Electron ring design and polarized source by MIT-Bates. http://www.agsrhichome.bnl.gov/eRHIC/eRHIC_ZDR/ZDR_start.pdf

2. At Jefferson Lab: ELICan Electron Light Ion Collider based on a 3-7 GeV ERL linac electron linac, a new electron circulator ring and a new light ion ring.

http://casa.jlab.org/research/elic/elic.shtml

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

eRHIC

Polarized electron and positron sources for storage mode of modest peak intensities required

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Very high intensity polarized source is required, I>100 mA

Linac Ring Concept

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

One accelerating & one decelerating pass through CEBAF

ELIC Layout

Ion Linac and pre-booster

IR IR

Beam Dump

Snake

CEBAF with Energy Recovery

3-7 GeV electrons 30- 150 GeV light ions

Solenoid

Ion Linac and pre-booster

IR IR

Beam Dump

Snake

CEBAF with Energy Recovery

3-7 GeV electrons 30- 150 GeV light ions

Solenoid

Ion Linac and pre-booster

IR IR

Beam Dump

Snake

CEBAF with Energy Recovery

3 -7 GeV electrons 30 -150 GeV (light) ions

Solenoid

Electron Injector

Electron Cooling(A=1-40)

Operated by the Southeastern Universities Research Association for the U.S. Department Of Energy

Thomas Jefferson National Accelerator Facility

Electron circulator ring

Source requirements for ELIC less demanding with circulator ring.

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Macroscopic Time structure for eRHIC

120 Bunches

480 mA

Collider storage ring (eRHIC)

Ion ring

360 Bunches

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Quantity Value Unit

Collider Ring (stacking pulses)

Stored current Frequency

480 (20ncx120/4.3us)28.

mAMHz

Ring circumferenceNumber of bunches in the ringCharge per macroscopic bunch

4.3120

20

ms

nCstacking: pulse train rep. rate Duration Total pulse train from injector Charge per bunch e per bunch

251015,000 (25x10x60)

1.38x10+6

Hzminutes

pC

Photocathode

Bunch durationBunch chargePeak current

~701.320

pspCmA

Linac Microscopic duty cycle (within 4.3 us)Macroscopic duty cycle during fillMacropulse average currentAverage current during fill

2x10-3

1x10-4

404

mAnA

Electron beam parameters for the ring-ring eRHIC

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Peak current requirements for eRHIC

(ring-ring)

• 450 mA average current in the ring and 120 bunches

• 10 minutes fill time at 25 Hz injection 15000 pulse trains stacked bunches from the injector each 1.3 pC and ~70-100 ps wide (I=dQ/dt)

• 18-20 mA peak current in linac

(instantaneous current within each bunch )

With QE of 5x10-4 , =800nm, would need

≥ 50 Watts of peak laser power.

1240

)()(.)(

nmmWPQEmAI

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Polarized source options for eRHIC (ring-ring)

Option 1: Modelocked laser Option 2: High power diode array laser

Ring timing

• F_bunch~2-10, I_inj ~ 2-20 mA

• Need work on the 102 MHz bunching.

Time Bandwidth laser

J-lab G0 laser: Ppeak=150 W

Need > 50 W

Bates laser system >120 W peak

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Linac-Ring specification for the electron beam (based on an ERL-CW electron linac and RHIC)

Beam rep-rate [MHz] 28.15RMS normalized emittance [m] 5- 50 Bunch length at cathode [ps] 100-200Electrons per bunch 1-10x·1010

Charge per bunch [nC] 1.6 -16Average e-beam current [A] 0.45Peak current [A]135

( ) ( ) ( ) (%) /124laserI mA nm P W QE

Sample QE (%) Polarization (nm) Plaser(W) Ppeak(kW)

Bulk 2 40 780 > 40 12

Strained 0.05 75 810 > 1400 420

Superlattice 0.1 85 760 > 750 210

Linac-ring

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Polarized Source for eRHIC linac-ring design

Polarized source

Using scaling law from current J-lab charge/cm2 and an FEL laser, need to use a large area photocathode. No provision for positron source.

ERL-FEL to produce KW of IR laser for Polarized source.

•A conceptual consideration by BNL

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

• With 0.5-2 kW laser power illuminating a 3 cm2 surface.

k= 0.75 W/cm.C for GaAs., 0.1 cm thick.,

T = 20-80. Too much for an NEA surface with mono layer of Cs atoms.

• With a molybdenum cathode stock, L=30 cm, S=0.5 cm2,

T will be too high across the stock without active cooling.

• Must have active cooling (flowing liquid or cold gas) to remove heat from photocathode and the cathode stock.

Photocathode heat dissipation:(For Linac –ring photocathodes)

R&D for heat removal from cathode:

• Design and construct actively cooled cathode

• Test cathode assembly with cooling using high power diode lasers while monitoring the UHV conditions.

1 mm

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Super conducting RF Gun, BNL and AES

Cathode installation cart

Cathode installed location

Cathode retracted location

Vacuum vessel

Linear rail system

Rail system adjustment

• Under construction at AES for the RHIC electron cooling.

• GaAs based photocathode tests in this SRF gun may begin in 2 years (BNL, AES and MIT-Bates).

Ilan Ben-Zvi, V. Litvinenko BNL

• Also considered for ILC polarized source (FermiLab meeting, Nov 7, 2005.

• Main issues:

1. base vacuum: UHV?

2. electron back- bombardment in an RF field

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Continuing Trend Towards Higher Average Beam Current

ELIC with circulator ring

JLab FEL program with unpolarized

beam

Year

Ave

. Bea

m C

urre

nt (

mA

)

First polarized beam from GaAs photogun

First low polarization, then

high polarization at CEBAF

Source requirements for ELIC less demanding with circulator ring. Few mA’s versus >> 100 mA for required for linac-ring.M. Poelker, EIC2

Workshop, 2004

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Summary EIC is required within a decade to maintain progress in the study of the fundamental structure of matter

partonic basis of atomic nuclei spin structure of nucleon

An eRHIC ring-ring accelerator design has been developed based on realistic considerations with luminosity close to 1033 cm-2 s-1 . Storage ring reduces polarized source requirement for this option. Some R&D for polarized source is required and will be pursued at MIT-Bates. Polarized positrons based on self polarization in the ring is considered.

A more futuristic linac-ring eRHIC concept is also under consideration that requires very high intensity CW polarized source > 100 mA. An FEL based laser system is envisioned. Also, a superconducting polarized Rf gun may be considered for long term.

An ELIC design at J-Lab is under consideration based on a CW ERL-linac and a circulating ring. The circulating ring reduces the current requirement compared to a linac-ring design but complicates the time structure requirements. R&D in laser systems for this design at J-Lab is need.

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Bates polarized Source facility

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Additional slides

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Gun Issues for ELIC

• Need 80% polarized e-beam.

• Use SVT superlattice photocathode. 1% QE at 780 nm; • ~ 1 W provides 1/e operation at 2.5 mA (if CW)

• Commercial Ti-Sapp lasers with CW rep rates to 500 MHz provide 0.5 W. Homemade lasers provide ~ 2W.• Injector micropulse/macropulse time structure demands laser R&D. • 25 mA operation requires more laser power and/or QE.• Charge Limit? Yes, at 1.6 nC/bunch and low QE wafers. • Lifetime? Can benefit from further vacuum improvement• Gun HV ~ 500 kV to mitigate emittance growth.

• Must limit field emission.

M. Poelker, EIC2004 Workshop

R&D needed on all of the above areas.

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Option 2:

bunchingcap

linacinj F

II

F_bunch~2-10, _cap~0.5 at Linac frequency I_inj ~ 2-20 mA

Need work on the 102 MHz bunching.

Ppeak=150 W CW-1 kHz

Linac frequency

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

Option 1:

Ppeak=50 W

For ring Iav =450 mA, need Ipeak=18 mA from injector

• Seems there is enough laser power. But need some R&D to test such lasers for this application. Surface charge limit and lifetime.

Time Bandwidth laser

J-lab G0 laser: Ppeak=150 W

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PST05 Workshop, Nov 14-17, 2005 M. Farkhondeh

MIT-Bates 60 keV test beam setup

Source R&D at Bates using test beam setup

• R&D on the eRHIC polarized source at Bates

Laser R&D for the two concepts of eRHIC injector