“Pump up your luminosity:” High pressure 3 He target with an ex situ SEOP polarizer

1. “Pump up your luminosity:” High pressure 3He target with an ex situ SEOP polarizer Bill Hersman1,2,Iulian C. Ruset2,1, Jan Distelbrink2, and David Watt2 1University of New Hampshire 2Xemed LLC Disclosure: The author/speaker has a financial interest in Xemed LLC 1

2. Talk outline • Optimization of an ex situ JLab polarized 3He target • Xemed’s large volume helium polarizer design • Predicted performance • Data from prototype v2.0 • Outlook towards v3.0

3. Intrinsic limitations of In situ pumping • Pumping cell pressure (optimal) • Target cell pressure (not optimal, limits luminosity) • Pumping cell material (optimal) • Target cell material (not optimal, limits luminosity) • Target cell geometry (optimal) • Pumping cell geometry (not optimal, must fit in beam line, uniform field) Linkage between pumping cell and target cell limits performance 1

4. Beam depolarization • From Hall A target: Loss rate of (622 hours)-1 per microamp • Scales linearly with target cell length, inversely with affected volume • Causes ~ 6% drop in polarization • Limits figure of merit p2L at high luminosity • In practice beam current is presently limited by target cell material An optimized high luminosity target will have losses dominated by beam depolarization, where beam-related polarization losses are balanced by very high production rates of polarized 3He 1

5. Pressurized target with ex situ polarizer • High pressure titanium target cell allows increased luminosity • Large volume polarizer allows high net spin exchange to maintain high polarization in the target. • Ex situ configuration allows polarizer to sit outside the beam line, relaxes geometry, materials, radiation shielding, size constraints.

6. Polarization performance of an ex situ target Target Length Correction External Volume Correction Target Volume Correction

7. An ex situ high pressure target • Continuous SEOP within a large volume vessel • Compress polarized 3He by 20:1 pressure ratio and deliver to titanium target cell at 1 scfm • Requires compression ratio ~20, immersion in magnetic field, rubidium-free gas leaving polarizer, <3% polarization loss • Throttle polarized gas back into the polarizer, de Laval nozzle 15 Bar 238 Bar Recirculating at 1.0 scfm 1 cm x 40 cm titanium target cell Requires two ports, entrance and exit 4

8. System integration

9. Diaphragm pump • Compression is accomplished by displacing an elastic (titanium) diaphragm • System is inherently sealed • Hydraulic plumbing allows remote drive motor • Pressure Products Industries (Warminster,PA) has submitted a quotation to fabricate a 3-stage all-titanium compressor with 20:1 compression Intake/Exhaust Valves Diaphragm Hydraulic Fluid Reciprocating Piston

10. Large scale 3He polarizer – v2.0 design • Xemed’s system is based on SEOP using hybrid alkali mix (K-Rb) [1]. • Large cylindrical cell (10 cm dia, 125 cm length, ~11L volume, S/V=0.45) pressurized up to 6 atm cold (initial target 50L polarized 3He per batch). • Equalize pressure inside and outside glass optical pumping cell by surrounding the cell inside with a pressure vessel • Cell temperature is stabilized by a clam-shell heat exchanger with silicone oil as thermal agent (cold top, hot bottom) • High power laser diodes (1.4kW pumping) allow for short pump-up times (4h). • Hardware limits operating temperature to under 250oC. • Final asymptotic polarization depends greatly on the cell lifetime and the X-factor. [1] E. Babcock et al. Phys Rev Lett, 91:123003 (2003). [2] E. Babcock et al. Phys Rev Lett, 96:083003 (2006). Cross sections of the polarizer.

11. Numerical calculations - SEOP • Theoretical framework includes spectral dependence of laser absorption, variable alkali ratio, temperature, pressure variables • Program calculates alkali polarization as function of alkali thickness, obtains 3He spin-up rate and polarization (1D). • Can optimize a defined figure-of-merit function of specified operating parameters (laser power, temperature, alkali ratio). Program determines optimal operating temperature and minimum laser power required. • K/Rb vapor density ratio of 4.4 (liquid ratio 10) chosen “low” to maintain high alkali polarization, yet allow operation at ~250oC (~4h pump-up time). What we hope to achieve Where we are 2

12. 3He polarizer: version 2 prototype • Vertical tower (2 meters height) ~23 gauss • Large 40cm dia and 120cm long solenoid assures magnetic field uniformity for central NMR • Pressure vessel encases pumping cell. Vessel and feed-throughs tested at 160psi • Multiple zone thermal bath regulated by flowing silicone oil, electrical heaters, and copper heat spreaders. Broad spectrum laser with 1.4kW power. Cartridge with cell and oven, to be loaded in pressure vessel. Prototype polarizer in operating state.

13. Polarizer is tilted to create stable flow • In vertical orientation, buoyancy causes alkali to accumulate at the top • Tilting the cell creates steady asymmetric temperature, velocity, and alkali distributions. • Shear layer promotes heat and mass transfer between up-and-downward streams. • Circulating flow creates alkali-depleted region near top window. • Fluent simulation of cell tilted 45o (1200 W, 5.8 amagat) Potassium Vapor Distribution Temperature Distribution Velocity Field near top window

14. Cell development • Cells are 1200 x 100 mm cylinders • Thin optical windows (<3 mm) to minimize laser absorption • Thin walls maximize heat exchange, thermal stability • Cells are baked over 400oC for 3-5 days under UHV. • Cells are charged with 25-80 g of 10:1 K:Rb mixture. • Alkali is distilled into mixing retort, and then distilled into the cell. • Both Pyrex and aluminosilicate cells have been fabricated and tested Loading distiller with K and Rb K:Rb puddle in cell bottom (after removal from polarizer)

15. Experimental results • T1 has improved to 13 hours over the last 24 months • Actual polarizations > 30% have been achieved. • Polarizers trending towards 50% with spin up times ~2.5 hours. • Spin exchange rates ~20 %-hr-1 at 1000 W laser have been measured. • Operated at high power (1400 W) for short periods. • Spin up rates vary with run-time, wall temperatures, helicity of laser light.

16. Prototype design-version 3 • Spectrally-narrowed lasers,12 bar stacks 4ea, up to 2200 W. • Pressure vessel design completed, certified to 300 psi. • More robust thermal control system with extruded channels. • Monolithic blown aluminosilicate cell mated with custom corrector • Two port cell design (entrance + exit) to allow continuous 3He flow. • Software control to allow autonomous operation and remote monitoring • Polarizer commissioning November 2010. Monolithic blown laser window with lensing correctable externally

17. High-power spectrally narrowed lasers • High power diode bar laser stacks • ( 2 or 4 arrays of 12 bars each) • External-cavity with beam shaping optics. • Precision optics for low divergence square beam (< 2mrad). • 1.4kW and 2.8 kW spectrally-narrowed. • “Smile” correction of each bar for superior spectral uniformity Output of one laser bar, corrected and uncorrected 1400 W narrowed (square) 2200 W narrowed (round)

18. Polarizer Layout v3.0 • Rigid hot/cold oven simplifies assembly and provides better temperature control • System has two modules: polarizer and support systems • Allows tilt angles up to 90o

19. 3He polarizer plan - v3.0 • Implement developed technology of spectrally narrowed lasers (four 12 bar with expected useful power of 2.2kW) • Redesign assembly into a compact, portable, versatile system • Develop software to fully automate process control and archive all diagnostics data • Obtain first long-life cell and achieve 70% polarization in prototype system • Establish optimum operating regime • Develop understanding of materials and methods for recirculating the gas without polarization losses, for neutron analyzers and electron beam use • Specifications for titanium electron target and high pressure compressor (200 atm) would be experiment-specific 1

20. Summary – 3He ex situ SEOP e-beam target • By decoupling the optical pumping system from the electron beam target, ex situ SEOP polarization • Reduces or eliminates space limitations and target geometry requirements • Reduces or eliminates sensitivity to magnetic fields from spectrometer quadrupole • Reduces or eliminates radiation damage to optical pumping components • Reduces or eliminates cell breakage in the beam • Reduces luminosity of non 3He scattering centers • Allows scalable spin-up rate by varying the pressure in the pumping cell (to 20 amag.) • Allows scalable luminosity by selecting the pressure in the target cell (to 200 amag.) • Allows scalable polarization by adding additional polarizers in series • Polarization system is working now at ~47% asymptotic polarization with ~11 hour cell and broadband laser pumping • New polarization system should work much better • Recirculation components are custom, but commercially available Funding: DOE grant DE-FG02-08ER86369 NIBIB grant EB007439 1