Post on 27-Dec-2015
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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