The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Electron Spin Precession at CEBAF

Douglas W. HiginbothamJefferson Lab

and

Marie-Isabelle HoldrinetUniversité Blaise Pascal

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Polarized Source

• Strained GaAs Photocathode• Ultra-High Vacuum (Ions Destroy Cathodes)

C. Hernandez-Garcia, P. G. O'Shea and M. L. Stutzman, Physics Today 61N2 (2008) 44.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Continuous Electron Beam AcceleratorPresently a 6 GeV machine, scheduled to be upgraded to a 12 GeV accelerator.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Spin Precession In Storage RingBeautiful demonstration of the electron’s anomalous magnetic moment and QED.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Spin Precession At CEBAF

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Example CEBAF SetupSelect a beam energy and Wien angle based on physics requirements of the Halls (63.5 MeV Injector Energy and 565 MeV Per Linac Per Pass)

• Machine setup with balanced or energy match linacs.• Hall C required polarization (GEP-III) and Hall’s A & B requested polarization.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Imbalanced Linacs

• With imbalanced the linacs, we were able to boost the polarization.• 10 MeV was removed from the North linac and 10 MeV was added to the South linac.

Needed to give Hall A & C high polarization without changing the beam energy for Hall B. (63.5 MeV Injector Energy and 555 MeV & 575 MeV Per Linacs)

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Future Conditions (2010 – 2012) A two year long parity experiment requires first pass and no transverse polarization.

• The machine needs to be setup to give all Halls high polarization.• Either with energy (as shown above) or deliberately running off crest.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Magic CEBAF Energy (2.12 GeV/pass)

A

B

C

South Linac

-37,5168°

37,5°

1st 2.12 GeV2nd 4.23 GeV3rd 6.35 GeV4th 8.46 GeV5th 10.6 GeV

1st Pass

-1

-0,5

0

0,5

1

-180 -130 -80 -30 20 70 120 170

Wien Angle (°)

Inje

cted

Lon

gitu

dina

l % o

f P

olar

izat

ion Hall A, 1

Hall B, 1

Hall C, 1

3rd Pass

-1

-0,5

0

0,5

1

-180 -130 -80 -30 20 70 120 170

Wien Angle (°)In

ject

ed L

ongi

tudi

nal %

of

Pol

ariz

atio

n Hall A, 3

Hall B, 3

Hall C, 3

4th Pass

-1

-0,5

0

0,5

1

-180 -130 -80 -30 20 70 120 170

Wien Angle (°)

Inje

cted

Lon

gitu

dina

l % o

f P

olar

izat

ion Hall A, 4

Hall B, 4

Hall C, 4

2nd Pass

-1

-0,5

0

0,5

1

-180 -130 -80 -30 20 70 120 170

Wien Angle (°)

Inje

cted

Lon

gitu

dina

l % o

f Po

lariz

atio

n Hall A, 2

Hall B, 2

Hall C, 2

5th Pass

-1

-0,5

0

0,5

1

-180 -130 -80 -30 20 70 120 170

Wien Angle (°)

Inje

cted

Lon

gitu

dina

l % o

f Po

lariz

atio

n Hall A, 5

Hall B, 5

Hall C, 5

With the 12 GeV upgrade, these energies become available.

The 18th International Symposium on Spin PhysicsD. W. Higinbotham, AIP Conf. Proc. 1149 (2009)

751.

Summary• Many Jefferson Lab experiments require polarized electrons.

• The source can produce 200uA of 85% polarized beam.

• Historically to maximize the long. polarization, the energy and Wien were fixed.

• Demonstrated a third parameter of imbalanced linacs.

• A future parameter could be deliberately running a pass off crest.

• As the figure of merit goes as P2, even small improvements can have a real impact.