Upload
fleta
View
17
Download
0
Embed Size (px)
DESCRIPTION
“Pump up your luminosity:” High pressure 3 He target with an ex situ SEOP polarizer. Bill Hersman 1,2 ,Iulian C. Ruset 2,1 , Jan Distelbrink 2 , and David Watt 2. 1 University of New Hampshire 2 Xemed LLC. Disclosure: The author/speaker has a financial interest in Xemed LLC. 1. - PowerPoint PPT Presentation
Citation preview
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
“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 Hampshire2Xemed LLC
Disclosure: The author/speaker has a financial interest in Xemed LLCJune 17,2010
1
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
June 17,2010
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
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
June 17,2010
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
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
June 17,2010
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
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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.
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
Polarization performance of an ex situ target
CellV
CellP
TargetV
TargetP
TargetV
TargetP
TargetV
TargetP
cm 40-atm 10
cm 40-atm 10
TargetL
TargetP
iA Hall
target theby tiondepolariza effective the is i) situex
CycleP
CompressorD
VCompressor
AA Hall
cycle ncompressio per loss onpolarizati the is cycleP
nt,displaceme compressor effective the isCompressorD
rate, flow volume the is V
Compressori
situexDipoleCellSDx
SE
SEAlkali
PHe
P
(
1
622
1
)(,
)1(3
Target Length Correction
Target Volume Correction
External Volume Correction
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
June 17,2010
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 scfm1 cm x 40 cm titanium target cell
Requires two ports, entrance and exit4
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
System integration
June 17,2010
3He
4He
N2
MFC
RGA UHV Rough Pump
Gas Bottles
External Evacuation Port to Vacuum Bakeout, Distiller, etc.
High Pressure Argon for Pressure Vessel
Controller for Pressure Vessel
Rough Pump
Vent
3-Stage Compressor
Cell
Pressure Vessel
High Pressure Target
3He Storage
Laval Nozzle
Bidirectional Recovery
Pump
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
June 17,2010
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
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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.June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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).
June 17,2010
Where we are
What we hope to achieve
2
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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.
Cartridge with cell and oven, to be loaded in pressure vessel.
Broad spectrum laser with 1.4kW power.
Prototype polarizer in operating state.June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
Polarizer is tilted to create stable flowIn vertical orientation, buoyancy causes alkali to accumulate at the topTilting 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)
Velocity Fieldnear top window
Temperature Distribution
Potassium VaporDistribution
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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)
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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.
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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.
June 17,2010
Monolithic blown laser windowwith lensing correctable externally
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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
1400 W narrowed (square)
2200 W narrowed (round)
Output of one laser bar, corrected and uncorrected
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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
June 17,2010
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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
June 17,2010
1
UNIVERSITY of NEW HAMPSHIRE
LLCmedXe
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 EB007439June 17,2010
1