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


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


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


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


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


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


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


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)


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


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


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


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 +-


Full Snake in Main Ring?

Needs two snakes to maintain vertical stable spin direction

Limited space


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


Possible locations of partial snakes in MR

First 30% snake Second 30% snake

Courtesy of T. Roser


Spin tracking with dual snake setup: A. Luccio

Single particle at 4 mm-mrad

Working point: Qx = 20.128 Qy = 20.960


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.


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


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.


Betatron tune path: A. Luccio

• additional polarization losses at •-9+Qy • 6 horizontal resonances


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

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Polarized Proton Acceleration in J-PARC

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