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
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
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
Our approach
• Place a large 8.5L thin-walled
aluminosilicate pumping cell, multizone 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 wavelengthlocked 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
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
Hand-blown window
with illumination pattern
• 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
GE180 ingot from NorCal Inc.
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
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
Breakseal
5-25 g K
Charging
Ampule
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
• 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
All polarizer services are fed through
the top flange
Laser
Specifications:
Power
2.7kW
Wavelength
Locking efficiency
Spectral width
Beam divergence
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
(2.12kW circular aperture)
794.8nm
75%
0.6nm
3x6mrad
Initial laser performance
Spectral Intensity (arb. units)
Wavelength locked beam @2.7kW
Four 12 bar lasers
(foreground) combining their
outputs into a single 10 cm
diameter beam (center).
power ()W)
Less-than-optimal
components decommissioned
from other projects cause
3000
beam inhomogeneities
48 Bar exit beam, and 1m downstream with diffuser.
Divergence ~3x6 mrad (hor x vert.)
16
14
12
10
8
6
4
2
0
792 793 794 795 796 797 798
Wavelength (nm)
Power 0.6 nm narrowed laser (max = 110A)
2500
2000
1500
1000
500
0
0
20
40
60
current (A)
80
100
3He
Polarizer:
Zeppelin-3
•
8.5L Cell cartridge
•
•
•
•
•
•
pressure vessel,
laser path,
thermal bath,
electronics
control system
diagnostics
•
Assembly/Operation
Performance: cell testing
Name
Materials
Test
Dates
Alkali
Ratio
Optimal
Temp (oC)
Density
(amagat)
T1 ,
Min
(hrs)
T1,
Max
(hrs)
Remarks
Souza 1
Corning
1720
JanApril
2012
10:1
(10 g
total)
220/200
1
3
8
Cell was refurbished due to
O2 contamination.
Souza 1
4
Souza 2
GE180
11/133/14
5:1
(5 g
total)
210/180
Kirchoff
GE 180
5/146/14
5:1
(10 g
total)
205/180
1
Souza 3
GE 180
7/148/14
5:1
(10 g
total)
205/180
Souza 3
8
1.8
Cell was refurbished due to
O2 contamination
4
7
Switched B0 orientation
1
*4
**10.
5
*T1 changed during
solenoid loss of Power.
**After intensive
degaussing
2
--
12.5
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
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
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
High Pressure
other impurities.
Compressor
Inlet to Analyzer
c.
Rubidium Trap
LN2 Trap
Plan for Zeppelin-4: laser
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
Output beams
50% 33% 25% 20%
4000
intensity Nlight 1 bar laser narrowed x38
cavity (~center 5 emitters)
3500
3000
intensity
2500
2000
1500
~10*4.4*0.353=16 pm
1000
500
0
pixel #; 4.4 microns/pixel; 0.353
1 21 41 61 81
101 121 141 161 181 201 221
pm/micron
Linewidth as low as 0.016nm
demonstrated for few emitters
Lasers contribute equal power to a single external cavity,
draw equal portions back for wavelength locking
to/from
external
cavity
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
Motorized rig for rotating glassware to
distribute sol-gel uniformly over inner surface
Sol-gel coated borosilicate cell 10cm in diameter with
surface coil attached for measuring polarization lifetime
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
Heat
Spreader
Graphite
Heater
Insulation
Cooling Plate
existing
eva
laser power (W)
3He gas density (Pump)
dipole relaxation (Pump)
3He gas temperature (target)
3He gas pressure (Target)
3He gas density (target)
Loschmidt (cm-3)
thickness (cm-2)
dipole relaxation (Target)
beam current (uA)
e
beam number density
Luminosity
depolarization const (loss/dilution)
pump cell T1
pump cell X-factor
spin up rate
pump volume
pump 3He (STP cc)
number of 3He pump atoms
moles 3He pump atoms
3He polarized per second
3He polarized moles per hour
target radius
target length
target volume
target 3He volume (STP cc)
total volume
3He volume (STP cc)
3He atoms (pump+target)
pressure related T1
polarization (bench)
beam related T1 (entire volume)
new
existing polarizer
180
3000
7.66
12
176.4 psia
97.1279373
62
297
170 K
10
69
1014.3 psia
10.0
120.5 1772.04 psia
2.69E+19
2.69E+19
1.07E+22
1.30E+23
12.05
74.4
6.17 hrs
15
65
1.60E-19
1.60E-19
9.38E+13
4.06E+14 s-1
1.01E+36
5.26E+37
0.0025
7.68E-04
1.292
33
11
0.5
0.3
7.1
5
1.42
230
8500 102000
1760.63 102000.00 STP cc
4.73E+22
2.74E+24
57.9
0.0786
4.5527
1.85E+18
1.52E+20
82.27
0.0111
0.9105
0.93
0.8
40
40
108.69
80.42 cc
1086.87
9694.97 STP cc
338.53
8580.42
2847.50 111694.97 39.2257
7.65E+22
3.00E+24
77.40
6.42 hrs
55.9%
54.5%
26.6667
20.0244 1.33171
beam related T1 (target cell volume)
beam depolarization rate (atoms)
polarization (in beam)
luminosity
figure of merit
ratio
10.1784
5.31E+16
48.7%
60
14.21
1.7381
2.78E+18
48.0%
3134.2235
721.05
50.73
104.286 minutes
p2L
2.39E+35
1.21E+37
50.7 ratio
52.2371
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
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
Recommendations
1.
2.
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.