1. Beam Position Stabilization Using an Active Collimator in Hall D at Jefferson Lab Richard Jones, University of Connecticut, for the GlueX collaboration American Physical Society Div. of Nuclear Physics Meeting October 23-26 2008, Oakland, CA.

2. APS DNP Meeting, October 23-26, 2008, Oakland, CA 2 Tungsten pin-cushion detector – used on SLAC coherent bremsstrahlung beam line since 1970’s – SLAC team developed the technology through several iterations – reference: Miller and Walz, NIM 117 (1974) 33-37 active device primary collimator (tungsten) Active collimator design

3. APS DNP Meeting, October 23-26, 2008, Oakland, CA 3 12 cm5 cm beam Active collimator design

4. APS DNP Meeting, October 23-26, 2008, Oakland, CA 4 x (mm) y (mm) current asymmetry vs. beam offset 20% 40% 60% Active collimator simulation

5. APS DNP Meeting, October 23-26, 2008, Oakland, CA 5 inner ring of pin-cushion plates outer ring of pin-cushion plates 10 -4 radiator I e = 1  A Simulated detector response Monte Carlo

6. APS DNP Meeting, October 23-26, 2008, Oakland, CA 6 using inner ring only for fine-centering ±200  m of motion of beam centroid on photon detector corresponds to ±5% change in the left/right current balance in the inner ring Position sensitivity Monte Carlo

7. APS DNP Meeting, October 23-26, 2008, Oakland, CA 7 Position sensitivity intensity inside aperturepolarization inside aperture

8. APS DNP Meeting, October 23-26, 2008, Oakland, CA 8 Prototype construction Pin cushions fabricated using EDM machine in FSU Physics shop pure tungsten too brittle machinable tungsten OK finding the right material is crucial

9. APS DNP Meeting, October 23-26, 2008, Oakland, CA 9 Prototype construction partitioned anode (Al) insulating cathode holder (CaB6)

10. APS DNP Meeting, October 23-26, 2008, Oakland, CA 10 Prototype construction

11. APS DNP Meeting, October 23-26, 2008, Oakland, CA 11 Installed in Hall B alcove direction of scans Notice: cable on one side hangs into the beam. Only one channel pair can be read out at a time

12. APS DNP Meeting, October 23-26, 2008, Oakland, CA 12 Results with Hall B photon beam beam energy 5.053 GeV radiator 50 microns thick (3  10 -4 rad.len.) beam current 9.0 nA (1% of max Hall D flux) radiator-collimator distance ~65 m beam collimated upstream of CLAS to 0.43 m/E – beam transverse size truncated

13. APS DNP Meeting, October 23-26, 2008, Oakland, CA 13 Results with Hall B photon beam inner wedges, raw data outer wedges, raw data inner cable outer current (arb. units)

14. APS DNP Meeting, October 23-26, 2008, Oakland, CA 14 Results with Hall B photon beam fit the blue, overlay on green fit the green, overlay on blue current (arb. units)

15. APS DNP Meeting, October 23-26, 2008, Oakland, CA 15 Future plans New beam test without narrow upstream collimator to enable a more quantitative test of simulations. New beam test without narrow upstream collimator to enable a more quantitative test of simulations. Instrument all four wedges along one axis and demonstrate ability to reproduce beam position. Instrument all four wedges along one axis and demonstrate ability to reproduce beam position. Record currents at frequency 1KHz and measure 60Hz beam motion, and higher harmonics. Record currents at frequency 1KHz and measure 60Hz beam motion, and higher harmonics.