EIC Users Meeting, SBU, 6/27/14

Polarized Electron Beams in the MEIC at JLab

Fanglei Lin for MEIC Study Group

EIC Users Meeting, Stony Brook University, June 27, 2014

EIC Users Meeting, SBU, 6/27/14

MEIC layout and electron polarization requirements

MEIC electron polarization design̶) Overview of strategies̶) Initial Injection bunch pattern̶) Polarization collision pattern and measurement̶) Optimization of average polarization̶) Continuous injection option

Conclusions & Outlook

Outline

2

EIC Users Meeting, SBU, 6/27/14

MEIC & e- Polarization Requirements

3

Three compact rings:•3 to 12 GeV electron•Up to 25 GeV/c proton (warm)•Up to 100 GeV/c proton (cold)

Warm large booster(up to 25 GeV/c)

Warm 3-12 GeV electron collider ring Medium-energy IPs with

horizontal beam crossing

Injector

12 GeV CEBAF

Pre-boosterSRF linac Ion

source

Cold 25-100 GeV/cproton collider ring

Three Figure-8 rings stacked vertically

Electron cooling

Cross sections of tunnels for MEIC

Ion SourcePre-booster

Linac

12 GeV CEBAF

12 GeV

11 GeV

IP IPMEIC

collider rings

Full Energy EIC Collider rings

Electron cooling

Electron polarization of 70% or aboveLongitudinal electron polarization at collision pointsSpin flipping

EIC Users Meeting, SBU, 6/27/14

Highly vertically polarized electron beams are injected from CEBAF– avoid spin decoherence, simplify spin transport from CEBAF to MEIC, alleviate the detector background

Polarization is designed to be vertical in the arc to avoid spin diffusion and longitudinal at collision points using spin rotators Universal spin rotator rotates the electron polarization from 3 to 12GeV Desired spin flipping is implemented by changing the source polarizationCompton polarimeter is considered to measure the electron polarization

– Two long opposite polarized bunch trains (instead of alternate polarization between bunches) simplify the Compton polarimetry

Polarization configuration with figure-8 geometry removes electron spin tune energy dependenceContinuous injection of electron bunch trains from the CEBAF is considered to

– preserve and/or replenish the electron polarization, especially at higher energies, and – maintain a constant beam current in the collider ring

Spin matching in some key regions is considered if it is necessary

Overview of e− Polarization Strategies

4

… …… …

Empty buckets Empty buckets1.33ns

748.5MHz

Polarization (Up) Polarization (Down)

bunch train & polarization pattern

EIC Users Meeting, SBU, 6/27/14

Initial Injection Bunch Pattern

5

……

200 us Iave = 82.6 nA

Macro bunch train

100 ms (~21740 turns)

Beam energy Gev 3 5 6 7 9 12

Beam current A 3 3 2.0 1.1 0.4 0.13

Injection time min 2.8 2.8 1.9 1.0 0.4 0.12

Such injection bunch pattern needs no upgrade of CEBAF beyond 12 GeV upgrade

……

1.33 ns, 748.5 MHz 2.4 pC

2.3μs, ~1700 bunches (Iave= 1.8 mA)

half ring injection

2.4 pC1.33 ns, 748.5 MHz

2.3μs, ~1700 bunches

……

half ring injection

EIC Users Meeting, SBU, 6/27/14

HERA:– Each ion bunch only sees the same electron bunch

MEIC:– Each ion bunch sees all electron bunches

– No non-colliding bunches

Polarization Collision Pattern

6

j

jelejionjelejion PPqqFOM 2,

2,,,

n

nelenelem

mionmion PqPqFOM 2,,

2,,

ion

electron

ion

electron

Therefore, in the MEIC– Bunch-to-bunch variation does not contribute to the uncertainty– One can measure average polarization of each macro bunch train

EIC Users Meeting, SBU, 6/27/14

Polarization Measurement

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Compton polarimetry: (next talk given by Alexandre Camsonne)– same polarization at laser as at IP due to zero net bend

Spin dancing (using spin rotators):– Experimentally optimize (calibrate) longitudinal polarization at IP

c

Laser + Fabry Perot cavity

e- beam

Quasi real photon tagger

Quasi real photon tagger

Electrontracking detector

Photon calorimeter

IPforward ion detectionforward e-

detection

final focusing elements

e-

ions

IP

Arc

S S

Schematic drawing of USR

Illustration of spin rotation by a USR

EIC Users Meeting, SBU, 6/27/14

Optimization of Average Polarization

8

measinj

t

i

T

measdepol dteP

T

dttPPFOM

0

2

22

2)(

inj

meas

Average Pol.

depolinj

measiP : Initial polarization : Injection time

: Depolarization time : Measurement time

Energy (GeV) inj (min) meas (min) (Pave/Pi)max *

5 2.8 12 0.82

7 1 5 0.83

9 0.4 2.3 0.85

12 0.12 0.6 0.85

)(2

)1(

2

depol

meas

depol

inji

avedepol

meas

ePP

measinjT

* Includes duty factor

EIC Users Meeting, SBU, 6/27/14

Continuous Injection Option

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Lost or Extracted

P0 (>Pt)Pt

10 )1(

injdk

ringrevequ I

ITPP

……

1.33 ns, 748.5 MHz 2.4 pC

2.3μs, ~1700 bunches

2.4 pC1.33 ns, 748.5 MHz

2.3μs, ~1700 bunches

……

……

200 us Iave = 33 nA 250 ms Pave /P0= 92%

Continuous injection principle

Low injected current preserves high polarizationOne possible injection bunch pattern– Damping time at energy > 5 GeV << 250ms– No beam dump needed

EIC Users Meeting, SBU, 6/27/14

Electron polarization schemes have been developed – Initial injection bunch pattern– Polarization collision pattern and measurement– Optimization of average polarization– Continuous injection option

Outlook– Scheme or technique optimization– Spin matching to improve polarization lifetime– Spin tracking with realistic errors

Acknowledgements– Y.S. Derbenev, J. Grames, J. Guo, L. Harwood, V.S. Morozov, P. Nadel-

Turonski, F. Pilat, R. Rimmer, M. Poelker, R. Suleiman, H. Wang, S. Wang, Y. Zhang, – JLab

– D. P. Barber – DESY/Liverpool/Cockcroft– M. Sullivan SLAC

Conclusion and Outlook

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EIC Users Meeting, SBU, 6/27/14

Thank You for Your Attention

11

EIC Users Meeting, SBU, 6/27/14

Injection Bunch Pattern

12

……

200 us Iave = 82.6 nA

Macro bunch train

duty factor 4.3e-4

……

1.33 ns, 748.5 MHz 2.4 pC

2.3μs, ~1700 bunches (Iave= 1.8 mA)

2.4 pC1.33 ns, 748.5 MHz

2.3μs, ~1700 bunches

……

100 ms (~21740 turns)

Beam energy Gev 3 5 6 7 9 11

vertical damping time ms 167.2 36.2 20.8 13.2 6.2 3.4

Beam energy Gev 3 5 6 7 9 11

Beam current A 3 3 2.0 1.1 0.4 0.18

Injection time min 2.8 2.8 1.9 1.0 0.4 0.2