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Oak Ridge Lab’s SEOP R&D Efforts

Oak Ridge Lab’s SEOP R&D Efforts. Wai Tung Hal Lee, Xin Tong (Tony), Joshua Pierce, Mike Fleenor, Valeria Hanson*, Akbar Ismaili **, J. Lee Robertson. Instrument Development Group, Neutron Facilities Development Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA

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Oak Ridge Lab’s SEOP R&D Efforts

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  1. Oak Ridge Lab’s SEOP R&D Efforts Wai Tung Hal Lee, Xin Tong (Tony), Joshua Pierce, Mike Fleenor, Valeria Hanson*, Akbar Ismaili **, J. Lee Robertson. Instrument Development Group, Neutron Facilities Development Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA * Hamilton College ** University of Tennessee - Knoxville

  2. Polarized 3He neutron spin filter Setup of one of the first tests of using polarized 3He on neutorn scattering instrument: POSY 1 neutron reflectometer at the Intensed Pulsed Neutron Source, Argonne National Laboratory. This setup came from Mike Snow’s group at the Indiana University Cyclotron Facility.

  3. SNS Instruments that can benefit from polarized 3He polarizer/analyzer

  4. HFIR Instruments that can benefit from polarized 3He polarizer/analyzer

  5. The development focuses at Oak Ridge What we started with: Built & tested in-situ polarizer/analyzer with the 3He polarized on beam. Now it gets a bit more exciting: Developing a medium-capacity laboratory-based SEOP-based filling station to supply several instruments. And the fun continues (Tony): Compact instrument-based filling station that is located at the instrument and will automatically refill wide-angle analyzer with high-polarization gas every few hours.

  6. Detector Polarized 3He Neutron Spin Filter Polarized Neutrons Unpolarized Neutrons Sample (CoFe Analyzer) * Laser & optics In-situ spin-filter with stable polarization and spin-state switching • We worked with the polarized 3He community (Hamilton, NIST, LENS) to develop the use of polarized 3He in neutron scattering. Some highlights: • Put polarized neutrons on a pulsed source scattering instrument – Single Crystal Diffractometer, IPNS; • Online continuous polarizing to maintain the highest polarization that is stable for days during experiment; • Adiabatic-Fast-Passage technique to flip the 3He polarization to make a spin filter-flipper. • G.L. Jones, et. al., Physica B 356, 86-90 (2005). • G.L. Jones, et. al., Proceedings of ICANS-XVII, Vol. III, 838-843 (2006). 1 flip /10 min 1 filp/2 min

  7. Spin + Spin - Polarized Neutrons Sample magnet refrigerator Co Fe Analyzer to verify beam polarization Experiment at SCD, IPNS: Magnetic moments on Mn & Sb in Yb14MnSb11 Using spin+/spin- polarized neutrons produced by the polarized 3He polarizer, we measured many magnetic peaks. Maximum entropy magnetization density reconstruction shows possible presence of a magnetic moment on the Sb site with opposite sign with respect to the Mn moment. 9,3,2 9,3,2 _ _ 10,0,0 10,0,0 9,2,1 9,2,1

  8. Laser optics Coils & Shield Oven Neutron Beam 3He 3He analyzer for Magnetism Reflectometer • Operates at neutron wavelengths from 1.8 Å to 6 Å • Cell ID~12 cm to accommodate off-specular scattering. • Online continuous optical pumping during experiment to maximize and maintain a stable 3He polarization. • To use it with sample magnet, the analyzer is in a uniform holding field enclosed in m-metal magnetic shielding. • Use adiabatic-fast-passage for both NMR polarization monitoring and 3He polarization flipping. This will enable the system to analyze spin-up and spin-down neutrons with fast-switching from one mode to the other. • The system is located inside a laser-shielding housing. All operations will occur online. • A total of 4 cells were made by Wang Chun Chen and Tom Gentile at NIST. • 3He gas pressure = 1.52 - 1.92 bars at R.T. • Cell ID ~ 12 cm , cell length ~ 8 cm

  9. 0.690 0.672 0.654 3He Polarization 0.636 0.618 Relaxation: T1=315 hours 0.600 0.582 10 0 20 30 40 Time (Hour) 3He analyzer for the SNS Magnetism Reflectometer 73% polarization reached • The system was installed and the on-beam tests were being done. • Neutron measurements showed 73% 3He polarization reached. • NMR measurement of the pump-up time constant ~ 5 hours. • Adiabatic fast passage worked to flip the 3He polarization. Loss/flip<0.03%. • T1=315 hours at R.T. • In a previous test, we measured the 4 spin-dependent cross-sections of off-specular scattering at lower 3He polarization. • Next steps: • Make new cell with larger opacity to match the BL4A setup. • Side-pumping setup

  10. Test: Reflectivity and Off-Specular Scattering f Off-Specular Incident Specular f=i Off-Specular i f Horizon Time-Of-Flight Illustration of the specular and off-specular scattering [57Fe/Cr]x12/Al2O3 multilayer with an anti-ferromagnetic inter-layer coupling and with an in-plane magnetic domain structure. The polarized neutron reflection experiment was performed in an external magnetic field of 30 mT applied along the in-plane easy axis (001) after a saturation field of 0.5 Tesla. The 2D pattern ofspecular reflection and off-specular scattering was measured with polarized neutrons in the wavelength band 2 Å< l <4.75 Å and a polarization of 0.97. The polarization analysis measurement was performed at 2 incident angles in order to obtain the range of momentum transfer Qz from 0.008 to 0.06 Å-1. f Time-Of-Flight

  11. Test of Non-Inductive Electric Heater to Heat a Cell Electrical Heating: Conventionally, hot air oven is used in SEOP. Flowing >200ºC hot air through a system, however, presents a host of technical and safety problems. Alternatively, we can use electric heaters. The main concerns are the magnetic interference on 3He polarization and on using adiabatic fast passage method to flip the 3He polarization and heating uniformity. Our tests showed none of them are a problem. AFP: Fractional 3He polarization loss per flip Heater ON: 0.061% +/- 0.002% Heater OFF: 0.055% +/- 0.002% Pump-up time constant=33.1+/- 0.8 hours Highest T1 tested on cell ~ 90 hours

  12. Laser l/2 Reflection Grating Polarizing Beam Splitter Cube l/4 Magnetic field 3He cell High-Power Bandwidth-Narrowed Laser Commercial system using laser stack with external cavity(XeMed) 12-to-24-bar system from XeMed 24-bar: 1000 W; 12-bar: 500 W 0.4 nm-width 2x1mrad divergence 140x140mm beam cross-section Turn-key system with chillers, power supplies, safety interlocks and User Interface Laser Stack with volume Bragg grating(LaserTel) Volume Bragg grating feedback narrows the bandwidth to 0.5-0.7 nm FWHM. We just received and tested a 150 W 3-bar stack that centered on 794.7 nm (Rb D1). It shows ~ 1.7 x the performance obtain from narrowing a 100 W laser using Littrow external cavity (running at 75 W with 25 W feedback). High power lasers tuned to 770.1 nm (K D1) has arrived last week. Modified Littrow cavity A polarizing beam splitter cube separates the feedback and output beams, reducing the heating of the grating and allowing better optical arrangement for the feedback.

  13. Lab-based SEOP Filling Station (More details in Tony’s Talk) • Gas-supply system • 15-bar gas pressure • Supply gas to polarizing system while preparing 2 sealed cells. • Status: Working. Filled second cell. Automatic gas pressure control accurate to +/-1 torr. • 4.3 bar-liter optical-pumping cell • Material: GE180 • ID 84 mm x 130 mm (nominal) • 6 mm thick wall = 12 bar limit • T=300ºC, maximum 6 bar at RT • Production rate for this cell • Assume a relaxed 8-hour cycle • Prod. rate = 13 bar-liter/day • Status: • 2 cells made.

  14. Fast Pump-Up – Temperature Requirement Spin-exchange rate = kse [al.] kse,K = 5.5×10-20 cm3 s-1 kse,Rb = 6.76×10-20 cm3 s-1 … by increasing the temperature To increase the spin-exchange rate, we need to increase the alkali density [al.] Example: Compare to the spin-exchange rate of a Potassium-based cell at 225ºC (1.5 day to reach 95% of equilibrium), the spin-exchange rate has a 10 x increase at 292ºC (4 hours) and 20 x increase (2 hours) at 316ºC due to the increase in alkali density.

  15. Fast Pump-Up – Laser Power Requirement The spin-destruction comes from collision between polarized alkali atoms with other alkalis, nitrogen, and 3He Example: Raising the temperature from 225ºC to 292ºC increases the spin-destruction rate by 6 x; and at 316ºC by 11 x. The optical pumping rate gopt ~ F(v0)s(v0)is typically 400 s-1 per mW/cm2 • = the light density, s(n) = optical absorption cross-section Photon efficiency ~ 10% A 10 bar 3He, 50 torr N2 , 5cm diameter cell will absorb 52 W at 292ºC , 100 W at 316ºC Need:200-400W at 292ºC, 400-1kW at 316ºC.

  16. Electron Paramagnetic Resonance Frequency Shift(Gordon Jones, Valarie Hanson, Xin Tong) We tested EPR-shift to measure the absorb 3He polarization. When placed in a magnetic field the electron spin states of an atom split with an energy difference proportional to that field. In optical pumping experiments pump light polarizes the alkali by exciting only the electrons in e.g. the 5S-1/2 state, such that eventually there becomes a net surplus in the 5S+1/2 state, indicating the gas is essentially polarized. When incoming photons are driven at the Zeeman frequency corresponding to the splitting due to the magnetic field, electrons in the 5S+1/2 state are recycled back to the 5S-1/2 state. This creates an increase in fluorescence emitted from excited electrons decaying down to ground state, as well as an increase in pump laser absorption. By locating the frequency where the fluorescence is maximized, the Zeeman splitting of the two states can be determined precisely. In the vicinity of polarized 3He, the B-field produced by the 3He shifted the Zeeman splitting frequencies. Measuring this shift gives us the absorb 3He polarization. Static magnetic field Cell of Rb, 3He, N2 V V Laser Photodiode RF field Test setup: 50% polarization

  17. Faraday Rotation (Valarie Hanson, Xin Tong) We tested Faraday rotation to measure the alkali density. Beam Splitter Photodiode Probe Laser Photodiode • Assuming 100% rubidium polarization we could then calculate the density. • We measured the λ/2 angle change when reversing the pump laser from σ+ to σ-. Experimentally Derived Densities [Rb] at 165˚C = 1.78x1014 cm-3 [Rb] at 175˚C = 3.18x1014 cm-3 Calculated Densities Based on Temperature [Rb] at 165˚C = 1.76x1014 cm-3 [Rb] at 175˚C = 2.77x1014 cm-3

  18. Acknowledgement IUCF Indiana Univ. Helmut Kaiser David Baxter Christopher Lavelle W. Mike Snow Hai Yan Yan Peter Chenyang Jiang NIST Tom Gentile WangChun Chen Changbo Fu ORNL Valeria Lauter (Mag. Refl.) Hailemariam Ambaye Andre Parizzi Rick Goyette Kevin Shaw Mark Hagen (HYSPEC) Bill Leonhardt David Anderson Bryan Chakoumakos (HB3A SCD) Kenneth Litttrell (CG2 GPSANS) Christina Hoffmann (TOPAZ) Jack Thomison Mark Lumsden (HB3 triple-axis) Hamilton College Gordon L. Jones Valerie Hanson Freddie Dias Brian Collett Jonathan Wexler JCNS-FRM-2 Earl Babcock ISIS Steve Parnell Stephen Boag Chris Frost Univ. of New Hampshire Bill Hersman IPNS Paula M. B. Piccoli Martha E. Miller Art Schultz Suzanne te Vultuis ILL Ken Andersen Eddy Lelievre-Berna David Jullien Pascal Mouveau Alexander Petukove ANSTO Frank Klose

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