1. October 14, 2004SPIN20041 The Bates South Hall Ring: A Unique Instrument for Studying Polarization D. Cheever, K.Dow, M. Farkhondeh, W. Franklin, D. Hasell, E. Ihloff, S. Krause, L.Longcoy, C. Tschalaer, E. Tsentalovich, J. van der Laan, F. Wang, A. Zolfaghari, T. Zwart SPIN 2004 Conference October 14, 2004 1.Facility Overview 2.Performance during BLAST Experiments 3.Polarization

2. October 14, 2004SPIN20042 Standard: Linac and recirculator provide intense polarized electron beams up to 1 GeV at 600 Hz, low duty cycle Pulse Stretcher (OOPS): Limited turns in South Hall Ring before gradual extraction to external target Storage (BLAST): Gradual stacking of electron pulses in South Hall Ring for long-lived CW beam MIT-Bates Linear Accelerator Center Three distinct modes of operation driven by needs of experiments

3. October 14, 2004SPIN20043 MIT-Bates South Hall Ring Stored beam operation at 850 MeV from 2002-2004 for BLAST Over 200 mA stored electron current achieved Racetrack design 190 m circumference 16 dipoles Single RF cavity (f=2.856 GHz) Full energy two-turn injection

4. October 14, 2004SPIN20044 BLAST Beam Requirements High average beam intensity High duty cycle Reliable operation Low experimental background Protection of internal target Good diagnostics High longitudinal polarization on target Program of experiments in South Hall Ring using internal targets and large accepetance toroidal spectrometer encircling beamline (V. Ziskin, C. Crawford, A. Maschinot, 10/15 Session 5) Bates South Hall Ring physics programs rely on one of world’s most intense stored polarized electron beams (I peak ~ 175 mA, P ~ 0.65)

5. October 14, 2004SPIN20045 Storage Mode for the South Hall Ring Injection: Stacking electron pulses at 10 Hz (2.3  s long, 2 mA peak current ) Efficient Fill to 175 mA in ~30 sec. Integrated with BLAST HV control Stacking Storage Storage Cycle Fully Automated (EPICS Control System) Storage: 15-20 minute period for DAQ Gradual decay of beam current Lifetime governed by target

6. October 14, 2004SPIN20046 Beam Diagnostics 32 sets RF pickups serve as Beam Position Monitors during storage Framegrabber digitizes synchrotron light images (beam profile info) Tune sweeper for beam dynamic properties at outset, end of fill

7. October 14, 2004SPIN20047  BLAST has open configuration, tune crucial  Beam and background very sensitive to stray magnetic field components (BLAST and target holding field)  Different optimal orbits for injection and storage (smooth transition)  Halo slits inserted each storage cycle Beam Quality  Halo monitors (plastic) give rapid feedback for tuning  BLAST wire chamber signals used for reduction of background from non-target related sources

8. October 14, 2004SPIN20048 P olarized target storage cell provides strict requirements for beam tune (15 mm aperture, 60 cm in length) Cell wall coating crucial for high target polarization Collimator shielding cell wall gives 10 mm aperture for beam No adverse effect on beam lifetime and maximum current stored. Successful in preserving target polarization after collimator added Empty target reconstructed events down significantly. Positron background introduced by collimator Target Collimator (Tungsten) Collimator Target cell

9. October 14, 2004SPIN20049 Good tune Bad tune DAQ Live time Stored current (mA) Beam Intensity Beam intensity limits set by experiment Linear dependence of live time on beam current Z-vertex (mm) Bad steering Triangular profile from target gas Now running at 175 mA Good steering

10. October 14, 2004SPIN200410 Daily accounting of charge delivered to BLAST experiments Continuing to improve efficiency

11. October 14, 2004SPIN200411 Over 1,000,000 Coulombs delivered

12. October 14, 2004SPIN200412 South Hall Ring Polarization Compton polarimeter data from Dec. 2003 – Sept.2004 Mean polarization of 66.3% during BLAST experiments

13. October 14, 2004SPIN200413 Polarization in the South Hall Ring Full Siberian Snake (Budker) restores longitudinal beam polarization at target Compton polarimeter Spin Flipper Monitoring beam polarization in the ring. Spin-flipping RF dipole allows dynamic spin reversal of stored beams (Michigan) Inject beam for peak longitudinal polarization at internal target (Wien filter in polarized source)

14. October 14, 2004SPIN200414 Siberian Snake Calibration Nominal current-based calibration corrected to Siberian Snake by 0.4%. Siberian Snake strength determined by electron energy, solenoidal field Spin flip resonant frequency provides sensitive measurement of spin tune as function of Siberian Snake current

15. October 14, 2004SPIN200415 Polarization Lifetime Long fills (empty target, 190 mA injection, slow decay to < 1 mA) Polarization lifetime (800 +/- 170 min.) >> beam lifetime (~45 min.) Depolarization mechanism uncertain (ASPIRIN prediction 1100 min) Search for radiative polarization offered no conclusive evidence of nonzero equilibrium polarization (prediction.03)

16. October 14, 2004SPIN200416 Polarization and Tune Spreading Initially, large losses of P for high I, restored by changing ring lattice. Effect linked to betatron tune shifts and spreading from trapped ions Practical solution: operate away from expected spin-orbit resonances, empirical hunt for max polarization Limited study of polarization as function of current, storage time, and tune Relevant issue for high luminosity devices (Electron-Ion Collider) y PLPL

17. October 14, 2004SPIN200417 Center for Accelerator Science and Technology at MIT being proposed for interdisciplinary research and education (FY06) Bates to play important role in CAST activities Possibility of ring operation for beam physics research and education South Hall Ring a good laboratory for investigation of polarization dynamics

18. October 14, 2004SPIN200418  MIT-Bates South Hall Ring features intense CW electron beams with high polarization for storage ring experiments with the Bates Large Acceptance Spectrometer Toroid.  Optimized tunes at 850 MeV have been achieved. Over one million Coulombs delivered to BLAST experiments in 2004.  Polarization dynamics have been studied in the South Hall Ring using a set of complementary components (Compton polarimeter, Siberian Snake, spin flipper). Summary