Status and Prospects for the Existing Polarized Target at JLab. Josh Pierce Newport News 3/12/14

Transcription

1 Status and Prospects for the Existing Polarized Target at JLab Josh Pierce Newport News 3/12/14

2 Dynamic Nuclear Polarization Material is prepared with free electron spins Through irradiation or chemical doping Electrons spins polarize in an environment with J Maxwell high magnetic field and low temperature 2.5 T / 300 mk 5 T / 1 K Microwave radiation is used to drive transitions in the hyperfine states Effectively transfers polarization from electron system to proton system Polarization Measured with NMR

3 Polarized Target requirements High tensor polarization Good reliability Good dilution No acceptance problems Much of this has already been done for g2p J Maxwell [3] Jefferson Lab experiment E07-003

4 G2p and GEp Experimental Overview g2p Measurement of spin structure function g 2 of the proton at very low Q 2 [1] Low beam energy Very forward scattering angle (achieved with normal conduction septum magnet) Transverse target polarization GEp Measurement of the ratio µg E /G M for the proton for 0.01 <Q 2 <0.7 (GeV/c) 2 [2] Polarization at a set angle from the beam (6 degrees) [1] Jefferson Lab experiment E [2] Jefferson Lab experiment E08-007

5 New Refrigerator Refrigerator rebuilt Welded joints and metal seals where possible Designed and constructed to Code Fridge performance 1.1 K with 3W of microwave power New pump tube CF aluminum to stainless seal Nose is made of AL 7075

6 New Refrigerator Baffles Separator Run Valve Heat Exchangers

7 The Superconducting magnet The initial plan was to use the magnet from the Uva/SLAC/Hall C target Failed irreparably during tests at Jefferson Lab in late 2011 Magnet suffered arcing during a quench while the target was being tested in preparation for installation On careful inspection, it was determined that repair of the burnout would be impossible

8 eg1 Magnet The superconducting magnet from the eg1 Hall B polarized target was identified as a possible replacement eg1 cryostat would not have been suitable Magnet was removed from the eg1 cryostat and installed in the g2p cryostat Required the design and fabrication of new helium service plumbing, relief plumping, and mechanical supports Required a modification of the GEp experiment due to more restricted open angles (20º became 6º)

9 Hall B Magnet in situ View of magnet looking upstream Downstream opening angle ±50 Upstream opening angle around ±6 Hanger/Alignment System Conflat Seals replace indium

10 Hall B Magnet in situ Side View of Magnet Transverse opening angle ±17 Helium supply and electrical services Hanger system Hall B magnet

11 New Insert Target material must remain cold (<100K) Insert is loaded in LN2 bath Insert has carbon fiber shaft for high rigidity Lots of extra space Insert was designed to accommodate cold NMR circuit Four SMA feedthroughs for two targets

12 New Insert and Motion System New Insert: Over 2m long Easy to handle Target Cells 2 Ammonia 1 Carbon 1 Dummy 1Empty 1 Polyethylene Microwave horn Room temperature alignment using pinned KF flange Carbon fiber shaft with internal services

13 Target Performance Material changes can happen more quickly Rotation system saves many hours and requires fewer personnel No crane or manlift required No manipulation of large tubing components End result is safer, faster, and more reliable Polarizations ~25% at 2.5T / ~90% at 5T In beam averages of ~15% at 2.5T and ~70% at 5T for g2p and ~80% at 5T for GEp 5T performance limited by beam instrumentation >2W microwave power at 1.1K

14 Historic Deuteron Performance Current really matters GEn 01 ran in Hall C. High current 15ND 3 Target Average in-beam deuteron polarization ~24% High current creates high overhead Frequent need for anneals and target changes has a detrimental effect on either the polarization or the total beam time

15 Equilibrium Tensor Polarization No new techniques Results are easy to interpret Using in-beam results from GEn 01 it looks grim P z = 0.24 P zz = For a P zz = 0.2 you would require P z = 0.50 For a P zz = 0.3 you would require P z = 0.60 Very challenging Especially as an in beam average for ammonia Different materials may work better

16 Enhancing Tensor Polarization RF hole burning requires the most likely candidate Polarization measurement will be the biggest challenge Modest improvements in tensor polarization may give better overall results after the error is taken into account Vector polarization must be high Careful with the run plan and overhead. G2p/GEp shows that the best experiment isn t necessarily the one with the highest current

17 But you really, really want high luminosity Radiation damage on the target is the primary cause of depolarization Longer target with lower current New magnet, fridge, insert, and cryostat Larger diameter target New magnet, fridge, insert, and cryostat Higher field New magnet, cryostat, and mucrowaves More rad-hard materials Just live with the overhead. Frequent anneals and target changes

18 Why doesn t anyone ever want photons? A frozen spin target will not work in the JLAB electron beam Fine for photons and worked well in Hall B Experiments could in principle run in Hall B or D Generally lower overhead (no target changes, no anneals) All of the techniques described in the previous talk work much better in frozen spin conditions Not that you would need them

19 Why doesn t anyone ever want photons? Deuterated propanediol was vector polarized to -87% in Hall B P z = P zz = 0.68 Not optimized, first test.

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