Polarized 3 He target based on an ex situ polarizer

1. Polarized 3He target based on an ex situ polarizer Bill Hersman University of New Hampshire and Xemed LLC David W. Watt Xemed LLC FCOI Disclosure: Prof. Hersman has a financial interest in Xemed LLC

2. Overview • Rationale and goals • reaching high p2L • Our approach • Remote large-scale polarizer, high density target, mechanical gas transfer • Zeppelin-3 implementation • Measured performance • Plans for Zeppelin-4 • Recommendation

3. Rationale: Paradigm shift: ex situ pumping • Provide high luminosity by exposing a very dense 3He target to the full beam current. • p2L is maximized when beam adds a depolarization factor of 50%. • Provide large spin-up rates by exposing a huge number of 3He atoms to polarized alkali atoms. Requires laser power. • Provide high transfer rates between pump cell and target cell using a mechanical pump • Also achieves isolation from radiation and field gradients • Originally proposed in 2008 • Expectation of p2L enhancement factor of ~50

4. Our approach • Place a large 8.5L thin-walled aluminosilicate pumping cell, multi-zone thermal bath, and NMR coil inside a pressure vessel. • Pressurize to up to 10-20atm • Immerse in a 24g solenoidal magnetic field • Illuminate 5:1 hybrid alkali mix with a 2.5kW-4kW high-power wavelength-locked spectrally-narrowed external cavity laser Zeppelin-3 is our third generation 3He polarizer built with this technology • Transfer gas at up to 22 slpm with an industrial-scale diaphragm pump with non-ferrous wetted surfaces • Polarized 3He would interact with the beam in a pressurized, cryogenically cooled, vertical (transverse) flow, thin-windowed, aluminum target cell

5. Pumping cell: optical windows • Hand blown: • Prior studies found that cells with the best polarization had fully blown surfaces • Examination of blown windows reveals most thickness variations are azimuthally symmetric • We mapped deflections of hundreds of point beams, inverted, fit to twelfth-order polynomial • Custom lens corrected dominant aberrations • Machine polished: • Using a melt shop, we produced a custom-melt 4” diameter GE180 ingot • Sliced into 1.5mm thicknesses; polished surfaces • Selected wafers based on minimum bubbles • Blown into a conformal dome Hand-blown window with illumination pattern GE180 ingot from Nor-Cal Inc.

6. Pumping cell: fabrication • Cylindrical barrel pumping cell measures 9.5cm inner diameter by 120cm long, 8.5 liters interior volume. • Valved capillary for gas transfer (currently only one is implemented for neutron analyzer filling) • Blown port for distillation of alkali into the cell, permanently sealed. Mike Souza, Princeton Glassblowing prepares a GE180 barrel end with GE180 domed window Aaron Kirchoff of NIST produced “Titan” fully from GE180

7. Pumping cell: preparations and filling • Distillation #1: 1-5 g of rubidium and 5-25 g potassium are distilled separately in two stages into charging ampule at low temperature (200oC for Rb, 280oC for K). • Cell Bakeout: Pyrex retort is attached to cell, sealed with a Teflon cap, baked with a retort temperature of 350oC and a cell temperature of 450oC under UHV for ~ 1 week. • Alkali charging: Purified alkali mixture driven into the retort under flow of UHP N2. The mixture is then distilled into the cell at 260oC. 1-5 g Rb 5-25 g K Charging Ampule Breakseal

8. Thermal Bath • Cell operates at an angle to drive buoyant convection • Flowing silicone oil delivers heat during warm up, removes heat during operation • Provides independent control over three thermal zones • Lower 72% of the barrel is the hot-polarization zone, most of the laser power is absorbed here • Upper 28% of the barrel is maintained at a lower temperature, less alkali vapor, less laser absorption. • Bottom window zone (and perimeter) is controlled electrically, establishes local alkali density All polarizer services are fed through the top flange • An opening is maintained for NMR coil and RF flux return Cell in encased in custom aluminum extrusion that serves as a dual zone thermal bath

9. Laser Specifications: Power 2.7kW (2.12kW circular aperture) Wavelength 794.8nm Locking efficiency 75% Spectral width 0.6nm Beam divergence 3x6mrad A: Lasers, total of 4 B: External cavity C: Step mirrors D: Grating E: Beam shaping optics F: Combining prism G: Diffuser/ waveplate H: Main collimator E: Exit optic

10. Initial laser performance Four 12 bar lasers (foreground) combining their outputs into a single 10 cm diameter beam (center). Less-than-optimal components decommissioned from other projects cause beam inhomogeneities 48 Bar exit beam, and 1m downstream with diffuser. Divergence ~3x6 mrad (hor x vert.)

11. 3He Polarizer: Zeppelin-3 • 8.5L Cell cartridge • pressure vessel, • laser path, • thermal bath, • electronics • control system • diagnostics • Assembly/Operation

12. Performance: cell testing

13. Performance: cell spinup Polarizer Testing Run times of up to 60 hours Studied effect of tilt angle, wall temperature Laser power up to 1800 W Internal NMR Measurement Probe: 30 mm dia multilayer surface coil Calibrated with in situ water phantom Corrected for changes in coil Q with temperature External NMR Measurement Short cylindrical coil surrounding sample bulb Calibrated with water sample in identical bulb Good SNR in calibration He-3 removed from polarizer through wire-wound PFA transfer line and PFA gas manifold. Polarization agreed with in situ measurement within 2 % (external measurement was slightly higher). Results Polarizations up to 59 % with 6 hr rise time. Best results with polarizer tilt ~15o from horizontal Average gas temperature ~230 oC. 1800 W laser power, ~1 nm spectral width

14. Non-ferrous Diaphragm Pump • Piston-driven hydraulic compression • Nominal 30 cps • Compression ratio ~6.5 • Two pumps ordered • Low pressure: 50 torr to 150 psi • 150psi to 1000 psi @ 22 SLM • PEEK valves • Titanium head 6AL4V • Three-layer diaphragm • Phosphor-bronze wetted • Delivered February, 2012

15. Gas Circulation Hardware • Two non-ferrous diaphragm compressors • Low pressure system : 50 10,000 torr • High Pressure (10  70 bar) • Low pressure system showed <2% polarization loss per cycle in continuous circulation • Transfer lines: Wire-wound PFA tubing with internal field ~4 G. • Polarized Gas Handling Manifold-PFA block with Pneumatic PTFE valves. • Return gas purifier-Rb metal followed by LN2 trap to capture residual O2, H2O, and other impurities. High Pressure Compressor

16. Plan for Zeppelin-4: laser Output beams 33% 25% 50% 20% to/fromexternal cavity Change feedback scheme from: • Low-efficiency grating feeds back small portion, each emitter occupies grating area To: • High efficiency grating feeds back all power, emitters share grating area • Increase magnification factor ~10 to reduce laser linewidth <50pm • Increase power capability to 3-4 kW Lasers contribute equal power to a single external cavity, draw equal portions back for wavelength locking Linewidth as low as 0.016nm demonstrated for few emitters

17. Plan for Zeppelin-4: cell • Cell trials to include: • New monolithic aluminosilicate cells, • sol-gel coating existing short-lifetime aluminosilicate cells, and • sol-gel coating new borosilicate cells Sol-gel coated borosilicate cell 10cm in diameter with surface coil attached for measuring polarization lifetime Motorized rig for rotating glassware to distribute sol-gel uniformly over inner surface

18. Heat Spreader Insulation Cooling Plate Plan for Zeppelin-4: infrastructure • Change pressure vessel from: • Aluminum pressure vessel (unrated) inside a solenoid surrounded by flux return steel to: • Engineer-stamped and rated steel pressure vessel • Steel vessel also serves as magnetic flux return • Solenoid relocated to fit inside pressure vessel • Greater isolation from ambient fields, including earth’s field • Steel e • Change thermal system from: • Two heat/cool oil systems, two electrical window heaters to: • Three zone direct electrical heat, oil systems provide for heat removal only Graphite Heater

19. Implications for a polarized 3He target • Compares new paradigm with old performance (not projected improvements) • Likely cell design is aluminum, transverse flow, cryogen cooled • Beam current and target density insufficient to maximize p2L • Roughly 7% beam-related 3He depolarization • Assumptions can be adjusted (cell, target; temperature and pressure) • Approximate improvement factor of 50

20. Funding and timeline • Polarizer funding renewed April 2015 by the DOE SBIR program to develop a filling station for large-angle neutron spin filters • Two year timeline with $500K/yr, total budget of $1M • Zeppelin-4 will be demonstrated on-site at Oak Ridge in January 2017 • Could also visit Jlab But… • Single capillary for gas entry/exit; not suitable for flow-through circulating Jlab target testing

22. Recommendations • Borrow Zeppelin-3 for tests. • Commission assembly of a copy of Zeppelin-4 with two capillaries, inlet and exit, for flow through operation. Delivery May 2017.