The Bates South Hall Ring: A Unique Instrument for Studying Polarization

1. 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 • Facility Overview • Performance during BLAST Experiments • Polarization SPIN2004

2. MIT-Bates Linear Accelerator Center • 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 Three distinct modes of operation driven by needs of experiments SPIN2004

3. MIT-Bates South Hall Ring • Racetrack design • 190 m circumference • 16 dipoles • Single RF cavity (f=2.856 GHz) • Full energy two-turn injection • Stored beam operation at 850 MeV from 2002-2004 for BLAST • Over 200 mA stored electron current achieved SPIN2004

4. BLAST Beam Requirements 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) • High average beam intensity • High duty cycle • Reliable operation • Low experimental background • Protection of internal target • Good diagnostics • High longitudinal polarization on target Bates South Hall Ring physics programs rely on one of world’s most intense stored polarized electron beams (Ipeak ~ 175 mA, P ~ 0.65) SPIN2004

5. Storage Mode for the South Hall Ring Stacking Storage Storage Cycle Fully Automated (EPICS Control System) • 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 • Storage: • 15-20 minute period for DAQ • Gradual decay of beam current • Lifetime governed by target SPIN2004

6. 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 SPIN2004

7. Beam Quality • 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 • Halo monitors (plastic) give rapid feedback for tuning • BLAST wire chamber signals used for reduction of background from non-target related sources SPIN2004

8. Target Collimator (Tungsten) Target cell Collimator • Polarized 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 SPIN2004

9. Beam Intensity Good tune Bad tune • Beam intensity limits set by experiment • Linear dependence of live time on beam current DAQ Live time Good steering Stored current (mA) • Triangular profile from target gas • Now running at 175 mA Bad steering Z-vertex (mm) SPIN2004

10. Daily accounting of charge delivered to BLAST experiments • Continuing to improve efficiency SPIN2004

11. Over 1,000,000 Coulombs delivered SPIN2004

12. South Hall Ring Polarization • Compton polarimeter data from Dec. 2003 – Sept.2004 • Mean polarization of 66.3% during BLAST experiments SPIN2004

13. Polarization in the South Hall Ring Monitoring beam polarization in the ring. Compton polarimeter • Inject beam for peak longitudinal polarization at internal target (Wien filter in polarized source) Full Siberian Snake (Budker) restores longitudinal beam polarization at target Spin Flipper • Spin-flipping RF dipole allows dynamic spin reversal of stored beams (Michigan) SPIN2004

14. Siberian Snake Calibration • 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 • Nominal current-based calibration corrected to Siberian Snake by 0.4%. SPIN2004

15. 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) SPIN2004

16. Polarization and Tune Spreading PL ny • 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) SPIN2004

17. 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 SPIN2004

18. Summary • 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). SPIN2004