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The polarized target for G E n

The polarized target for G E n. Gordon D. Cates, Jr. University of Virginia Professor of Physics and Radiology. G E n , - October 24, 2003. Target issues. Snuggling up close to Bigbite’s fringe fields. Magnetic shielding No room for the laser hut. More compact laser system.

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The polarized target for G E n

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  1. The polarized target for GEn Gordon D. Cates, Jr. University of Virginia Professor of Physics and Radiology GEn, - October 24, 2003

  2. Target issues • Snuggling up close to Bigbite’s fringe fields. • Magnetic shielding • No room for the laser hut. • More compact laser system. • Maintaining performance with high beam current. • Improving polarimetry

  3. The solutions • Field clamp around Bigbite • Magnet based around an iron box • Laser system based around a “5-to-1” combiner. • Cell design with larger pumping chamber. • Perhaps utilize K/Rb hybrid pumping • Electromagnetic shielding (from the iron box) and vibration insulation.

  4. Top view of magnet concept

  5. Bigbite and target/magnet on beamline

  6. Bigbite and target/magnet

  7. Cutaway view showing some elements of the target

  8. Close-up on some elements of the target

  9. The laser system • Existing system • Provides both longitudinal and transverse pumping. • Four fiber array packages (FAPS) for longitudinal pumping. • Three FAPS for transverse pumping. • Total of (4+3)x2=14 beam lines. • Optics in laser hut. • New system • Only one pumping direction required. • System based on “5-to-1” combiner. • Only two beam lines, with all optics housed on the target itself.

  10. Similar laser system on noble-gas polarirzer used for medical imaging

  11. Laser system

  12. View showing laser system together with oven and coils

  13. Laser system

  14. Overview of medical polarizer

  15. Performance summary • Maximum input power without excessive heating: around 3x40=120 Watts. • Power on oven: 93 Watts. • Homogeneity: ok, but not great. • “Diffuser” made by Newport Corp. may be good alternative to Coherent’s new homogenizer. • 200 Watt input power not yet achieved.

  16. Polarimetry issues • Field in iron-box magnet will not be easy to scan. • AFP scans will need to be done scanning the rf frequency and not the field. • This means, among other things, that the lock-in will need to track a changing frequency. • Kentucky system seems to work well, but line-shape is somewhat asymmetric. • Need to convince ourselves we understand the signal if we want to trust it at the 2-3% level.

  17. Kentucky NMR apparatus

  18. Average of 50 water sweeps at Kentucky

  19. Average of 500 water sweeps at Kentucky

  20. Single helium sweep at Kentucky

  21. Rb spin destruction is a major issue when polarizing 3He • Slow spin exchange necessitates very high Rb number densities: 2-10 x 1014 cm-3. • Spin destruction rates mean high laser power. • Laser power itself is less of an issue. • High laser power brings its own problems • Significant heating. • Chemical effects? • Other problems? • New direction: take advantage of slower spin-destruction rates for K, which hence has higher spin-exchange efficiency.

  22. Hybrid (K and Rb) spin-exchange optical pumping • Potassium (K) is about ten times more efficient than rubidium (Rb) at transferring angular momentum from photons to 3He nuclei. • Cells containing a mixture of mostly K and a little Rb greatly improve the efficiency with which the light is used. Data showing dramatically higher spin-exchange efficiency for K (Baranga et al., PRL 80, Pg. 2801 (1998))

  23. What does it all mean? • The spin-exchange rate of K with 3He is about the same as Rb with 3He, but….. • For a given amount of laser power, you can polarize many more alkali-metal atoms using hybrid pumping. • It is unclear how much better performance will be when other effects are taken into account. • But it looks promising!

  24. Where hybrid pumping stands • 1998: Romalis clearly showed indirectly that K efficiency is much better than Rb efficieny. • 2003: Walker and co-workers showed directly K efficiency is much better than Rb efficiency. • A practical target has yet to be demonstrated.

  25. Status and responsibilities • Magnet components will arrive soon. • Assembly and testing: everyone? • Most design work remains on target’s “internal” components, ladder, movement mechanism, etc. • Al Gavalya working with Gordon and Bogdan • Laser system, some questions remain, ready to order some components. • Gordon, Jian-Ping, Bogdan • Polarimetry: many tests remain. • Todd, Wolfgang • Cells: work just beginning. • Jaideep, Gordon

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