The polarized target for GEn

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

• Snuggling up close to Bigbites fringe fields.
• Magnetic shielding
• No room for the laser hut.
• More compact laser system.
• Maintaining performance with high beam current.
• Improving polarimetry

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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.

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Top view of magnet concept
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Bigbite and target/magnet on beamline
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Bigbite and target/magnet
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Cutaway view showing some elements of the target
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Close-up on some elements of the target
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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 (43)x214 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.

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Similar laser system on noble-gas polarirzer used
for medical imaging
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Laser system
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View showing laser system together with oven and
coils
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Laser system
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Overview of medical polarizer
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Performance summary

• Maximum input power without excessive heating
  around 3x40120 Watts.
• Power on oven 93 Watts.
• Homogeneity ok, but not great.
• Diffuser made by Newport Corp. may be good
  alternative to Coherents new homogenizer.
• 200 Watt input power not yet achieved.

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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.

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Kentucky NMR apparatus
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Average of 50 water sweeps at Kentucky
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Average of 500 water sweeps at Kentucky
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Single helium sweep at Kentucky
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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.

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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))
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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!

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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.

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Status and responsibilities

• Magnet components will arrive soon.
• Assembly and testing everyone?
• Most design work remains on targets 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