Download - Polarized Proton Acceleration in J-PARC
4-6-2008 J-PARC Spin Physics Workshop 1
Polarized Proton Acceleration in J-PARC
M. Bai
Brookhaven National Laboratory
4-6-2008 J-PARC Spin Physics Workshop 2
Outline
Introduction Challenges in accelerating polarized
protons in circular accelerator Polarized proton acceleration in J-
PARC LINAC RCS Main Ring
Setup for preserving polarization Conclusion
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Spin motion in a circular accelerator
In a perfect accelerator, spin vector precesses around the bending dipole field direction: vertical
Spin tune Qs: number of precessions in one orbital revolution. In general,
SBGBGm
eS
dt
Sd
])1([ //
Spin vector in particle’s rest frame
B
beam
GγQs
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Come from the horizontal magnetic field which kicks the spin vector away from its vertical direction
Spin depolarizing resonance : coherent build-up of perturbations on the spin vector when the spin vector gets kicked at the same frequency as its precession frequency
xB
x
y
z
beam
Initial
xB
x
y
z
beam
1st full betatron Oscillation period
xB
x
y
z
beam
2nd full betatron Oscillation period
Depolarizing mechanism in circular accelerator
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Spin depolarizing resonance
Imperfection resonance
Source: dipole errors, quadrupole mis-alignments
Resonance location:
G = k k is an integer
Intrinsic resonance Source: horizontal
focusing field from betatron oscillation
Resonance location:
G = kP±Qy,P is the periodicity of the
accelerator, Qy is the vertical
betatron tune
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Layout of J-PARC
Pol. H- Source
180/400 MeV Polarimeter
Rf Dipole
25-30% Helical Partial Siberian Snakes
pC CNI Polarimeter
Extracted BeamPolarimeter
50 GeV polarized protons for slow extracted beam primary fixed target experimentsLow intensity (~ 1012 ppp), low emittance (10 mm mrad) beams
Optically Pumped Polarized Ion Source: 1012 H- per 0.5 ms pulse and > 5 Hz rep. rate, 85% polarization, emittance: ~ 5 mm-mrad and 0.3 eVs for 2 x 1011 protons.
Courtesy of T. Roser
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Harmonic correction
J-PARC accelerators for pp LINAC: polarization transparent
RCS: Energy: 180 MeV – 3 GeV (G: 2.2 -- 7.5) Periodicity 3, Working point: Qx=6.735, Qy=6.356 5 imperfection resonances:
With the RCS acceleration rate, keep the rms orbit distortion better than 0.38mm
Harmonic orbit correction should also help to reserve the polarization
Provided by Hikaru Sato
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Intrinsic Spin Resonance at RCS
• emittance: 10 mm-mrad, 95%• repetition rate 25Hz• sinusoidal ramping• kinetic energy: 180MeV – 3GeV
• intrinsic resonance strength for a particle at an emittance of 10 mm-mrad Full spin flip by a rf dipole
=2.33x10-5
=6.18x10-5
=7.63x10-5 =6.60x10-5Fast tune jump?
G = 2.65(9- Qy), 3.35(-3+ Qy), 5.65(12- Qy), 6.35(0+ Qy)
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AC dipole for RCS
Magnet gap: 95mm Beta function at the ac dipole: 24 m Maximum coherent amplitude: 10
Coherence amp [mm]
Freq [kHz]
BGauss
m
Current 4- turn magnet
1.872 50.3 25.465 13 65.9
3.148 36.6 28.563 18 90.5
3.545 34.3 28.900 19 96.6
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Alternative: Tune jump
Advantage: can handle the first weak resonance
However, tune needs to get jumped by about 0.06 at the second resonance, this can cause emittance blowup
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Accelerating polarized protons in Main Ring
Beam energy: 3 GeV ~ 50 GeV (G = 7.5 -- 97.5) Design working point: Qx = 22.339, Qy = 20.270 Many imperfection resonances Strong intrinsic resonance
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Spin tracking: A. Luccio
Spin tracking of one particle at the nominal tune of the lattice. =10 mm.mrad. No snakes. The polarization is lost at the resonances, located at G = 3N +-
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Full Snake in Main Ring?
Needs two snakes to maintain vertical stable spin direction
Limited space
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Main ring pp setup – dual snake: T. Roser
Vertical component of stable spin
Fractional part ofspin tune
Injection Intrinsic resonance
G
1
0.
preaxis OT gg( )( )T
2
sptune OT gg( )( )
13.57.5 gg8 9 10 11 12 13
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Qy = 20.96
Qx = 20.12
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Possible locations of partial snakes in MR
First 30% snake Second 30% snake
Courtesy of T. Roser
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Spin tracking with dual snake setup: A. Luccio
Single particle at 4 mm-mrad
Working point: Qx = 20.128 Qy = 20.960
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Conclusion
Scenarios for preserving polarization through the J-PARC accelerator complex are explored. We should be able to accelerate polarized protons to 50 GeV
RCS: imperfection resonance: harmonic correctors intrinsic resonance: ac dipole
50 GeV Main Ring: a pair of AGS type 30% partial snakes operate at working point Qx=20.12, Qy=20.96
The design requires the polarized proton beam size of 10 mm-mrad. The smaller beam size, the less polarization loss.
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Remaining issues
RCS Can we raise the injection energy higher than the
first intrinsic resonance? A moderate fast quad to jump through the 1st
intrinsic resonance
To achieve > 80% polarization at 50 GeV
Source pol
Transmission efficiency
RCS
mini trans efficiency MR
Ignore 1st resonance
0.85 0.95 0.99
Correct/ inj above
1st resonance0.85 1.0 0.95
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Remaining issues
Main ring Optics design for dual snake setup
Correction quadrupoles on either side of each partial snake are necessary to compensate the optics distortion due to the strong focusing field from the snake
The effect goes down with energy and is strongest at injection. It is very possible that both horizontal and vertical tune have to stay farther away from integer at injection to allow stable operation.
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Betatron tune path: A. Luccio
• additional polarization losses at •-9+Qy • 6 horizontal resonances
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Remaining issues
Main ring Optics design for dual snake setup
What’s the best tune path Keep Qy high at low energy and ramp Qx up to 0.12
between injection and gamma=10
Can the slow extraction be done with near integer tunes?
Spin matching between RCS and Main Ring