Polarean 9800 Xenon Polarizer Performance Optimization

Transcription

1 Polarean 9800 Xenon Polarizer Performance Optimization

2 Questions to Address Understand the key equations for Rb polarization (the Standard Model) The effects of temperature The effects of cryogenic accumulation Broad vs narrow spectrum lasers Optimizing polarization for system Why do we run such lean mixtures? What s wrong with the standard model?

3 Our Old Friend the 129 Xe Polarizer N 2 Xe N 2 He Flow Meter Cell Out Flow Isolate Pressure Monitor Flow Shut Off Flow Control General Isolate Xe Out Purge Outlet Evacuate Oultet

4 129 Xe Polarizer Essentials 795 nm Laser Continuous Flow Cryogenic xenon extraction Lean Mix 1% Xe 89% He, 10% N 2 B. Driehuys et al., Appl Phys Lett 69, 1668 (1996).

5 Modeling Polarizer Performance Length Xe N 2 795nm laser Rb He Diameter Incident Laser Spectrum Rb Absorption Cross-section Transmitted Laser Light

6 The key optical pumping equations Alkali Polarization P A (z) = Γ OP (z) Γ OP (z) + Γ SD Optical pumping rate spin destruction rate Optical pumping rate Γ OP ( z) = Φ(ν, z)σ(ν)dν 0 Photon flux D1 cross section Loss of Photons dφ( ν, z) dz ( ) = [ A]σ ( ν)φ( ν, z) 1 P A ( z) Alkali density Alkali polarization

7 Light Propagation and Alkali Polarization 290 ml cell, r=2.71cm, l=12.7cm 90 W laser, 2.5nm FWHM, 50% absorption [He]=5amg, [N 2 ]=0.5amg, [Xe]=0.05amg PRb =48%

8 Adding in Spin Exchange Rb 129 Xe α I S Rb 129 Xe S I

9 129 Xe Polarization vs Time ( t) P ( Rb 1 e t τ ) SE 1 τ SE = k SE [ Rb]

10 Rb Density Controls Spin Exchange Time

11 Optimization Dilemma Low Temperatures (<100C) P Rb is high [Rb] is low T SE is SLOW High Temperatures (>100C) P Rb starts to decrease [Rb] is high T SE is FASTER

12 Why Not Run at High [Rb]? Photon Flux Loss dφ( ν, z) dz ( ) = [ A]σ ( ν)φ( ν, z) 1 P ( A z) Alkali density Alkali polarization

13 Alkali Polarization vs Absorption 10% laser absorption T SU =165 sec 50% laser absorption T SU =29 sec 90% laser absorption T SU =5 sec

14 Why Absorption; Not Temperature? Absorbing many Watts of laser light heats the gas in the cell and cell surface This increases [Rb] relative to what we might expect from oven air temperature [Rb] also varies somewhat with cell age Although temperature is important Absorption is a more robust indicator of our polarizer operating point

15 Peak 129 Xe Polarization vs Absorption 1 τ ( t) = P SE Rb 1 τ SE +1 T 1 1 e t 1 τ SE +1 T 1 ( ( ) ) T 1 =240min T 1 =2min

16 129 Xe Polarization vs Flow Rate ( t) P ( Rb 1 e t τ ) SU t = V cell F [ G] [ ] τ SU F ( ) ( F) P Rb 1 e V cell G F crit = V cell ( F) P ( Rb 1 e F crit F ) τ SU [ G] when F = F crit t = τ SU

17 129 Xe Polarization Flowing Out of the Cell ( F) P Rb 1 e F crit F ( ) (Model) 290 ml cell, r=2.71cm, l=12.7cm 90 W laser, 2.5nm FWHM [He]=5amg, [N 2 ]=0.5amg, [Xe]=0.05amg 50% laser absorption =48%, F crit =3.0 SLM

18 Accounting for 129 Xe Relaxation in the Solid State Relaxation of Solid 129 Xe ( t) P 0 e t T 1s T 1xe =150 min ( 77K) Each subsequent 129 Xe fraction spends less time as solid ( t ) a = t a 0 t t a P 0 e t T 1s dt = P 0 T 1s t a 1 e t a T ( 1s )

19 Accounting for 129 Xe Relaxation in the Solid State Relaxation of Solid 129 Xe Each subsequent 129 Xe fraction spends less time as solid ( t) P 0 e t T 1s Faster flow means shorter accumulation time t a = f XeV Xe F ( ) = tp 0 e t a T 1s dt t a t a 0 = P 0 T 1s t a 1 e t a T ( 1s ) F ( F) P ( 0 1 e F s F ) F F s = f V Xe Xe s T 1s

20 Converting from Time to Flow Again T ( t ) = P 1s a 0 t a ( 1 e t a T 1s ) Accumulation time is inversely related to flow rate t a = V Xe f Xe F When F = F s it takes time T 1s to make V Xe F ( F) P ( 0 1 e F s F ) F F s = V Xe f s Xe T 1s

21 Solid 129 Xe Relaxation Sanity Check F ( F) = P ( 0 1 e F s F ) F s F 0 ( t a ) e F s F 0 ( 0) 0 F t a 0 ( ) e F s F 1 F S F ( 0) P 0

22 Combining Flow and Accumulation ( F) = P ( Rb 1 e F crit F ) P ( Xe F) = P ( 0 1 e F s F ) ( F) P ( Rb 1 e F crit F )( 1 e F F s )

23 Predicted Effect of Solid 129 Xe Relaxation ( F) P ( Rb 1 e F crit F )( 1 e F F s )

24 Predicted Performance Now Including Reality

25 Predicted Performance More Reality

26 Making it Fit. Parameters P 0 =20% F crit =3 SLM T 1s =30 min

27 Apparent Inventory of Problems Xe Polarization out of cell appears to be more than 2x lower than theory Apparent solid Xe T1 appears to be 5x shorter than measured for pure solid Xe Production rate may be on track, but hard to accurately deconvolve from solid Xe decay Need to separately investigate the optical pumping piece and the solid Xe piece. Let s start with the optical pumping

28 129 Xe Polarization vs. Flow Testing

29 Flow Testing in the Clinic

30 129 Xe Polarization vs. Flow P 0 =27.8% F crit =2.3 SLM V cell =290 ml (r=2.71cm, l=12.7cm) A cell =23cm 2 90 W laser, 2.5nm FWHM [He]=5amg, [N 2 ]=0.5amg, [Xe]=0.05amg 50% laser absorption

31 Comparison to Theory P 0 =48% F crit =3.0 SLM P 0 =27.8% F crit =2.3 SLM Not great, but not terrible Tempting to wave hands to resolve difference.

32 Production Rate Photon Efficiency: The Forgotten Concept d dt ( V P ) = ηδi Xe Xe Absorbed Photon Current η = σ SE σ SE + σ SD Photon Efficiency η( Rb 129 Xe) 4.6% dv Xe λ=795nm photons/s dt Polarean Efficiency = 25ml /W / hr Competitor Efficiency 1.5% 0.5% Bhaskar, N. D., (1982). "Efficiency of Spin Exchange between Rubidium Spins and 129Xe Nuclei in a Gas." Physical Review Letters 49(1):

33 Absorbing more light

34 Peak Polarization and Production Rate vs. %Laser Absorption 129 Xe Polarization vs absorption 129 Xe Production Rate vs Absorption 129 Xe polarization prefers less absorption 129 Xe production prefers more absorption

35 Anything to Optimize? 129 Xe Polarization x Critical Flow Rate Polarization Squared x F crit P x F crit product is linear with absorption Illustrates standard model photon efficiency that is independent of laser power absorbed However, P 2 x rate has a rough optimum (kind of neat)