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. Maintaining performance with high beam current. Improving polarimetry

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.

Top view of magnet concept

Bigbite and target/magnet on beamline

Bigbite and target/magnet

Cutaway view showing some elements of the target

Close-up on some elements of the target

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.

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

Laser system

View showing laser system together with oven and coils

Laser system

Overview of medical polarizer

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.

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.

Kentucky NMR apparatus

Average of 50 water sweeps at Kentucky

Average of 500 water sweeps at Kentucky

Single helium sweep at Kentucky

Rb spin destruction is a major issue when polarizing 3 He Slow spin exchange necessitates very high Rb number densities: 2-10 x 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.

Hybrid (K and Rb) spin-exchange optical pumping Data showing dramatically higher spin-exchange efficiency for K (Baranga et al., PRL 80, Pg (1998)) 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.

What does it all mean? The spin-exchange rate of K with 3 He is about the same as Rb with 3 He, 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!

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.

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