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magnetic field
Pierre-Jean Nacher, Emmanuel Courtade, Marie Abboud, Alice Sinatra, Geneviève Tastevin, Tomasz Dohnalik
To cite this version:
Pierre-Jean Nacher, Emmanuel Courtade, Marie Abboud, Alice Sinatra, Geneviève Tastevin, et al..
Optical pumping of helium-3 at high pressure and magnetic field. 2002, pp.2225-2236. �hal-00002223�
P.-J. Nacher, E. Courtade,M. Abboud, A. Sinatra, G. Tastevin
LaboratoireKastlerBrossel,24rueLhomond,75231Pariscedex05,France.
and
T. Dohnalik
InstituteofPhysics,JagiellonianUniversity,ul. Reymonta4,30-059Krakow,
Poland.
At lowmagnetic eld,the eÆciency ofmetastability-exchange optical
pumpingofhelium-3isknowntobeoptimalforpressuresaround1mbar.
Wedemonstrateonseveral examples(up to 32mbar)that operatingin a
higher magnetic eld (here 0.12T) can signicantly increase the nuclear
polarisationsachievedathigherpressures. Sincepolarisationmeasurements
cannot be made with the standard technique, we use a general optical
method based on absorption measurements at 1083 nm to measure the
polarisationof theatomsin thegroundstate.
PACSnumbers:32.80.BxLevelcrossingandopticalpumping32.70.-nIntensities
andshapesofatomicspectrallines
1. Introduction
Highly polarised 3
He gas is used in various domains, for instance to
preparepolarisedtargetsfornuclearphysicsexperiments[1 ],to obtainspin
lters for cold neutrons [2 , 3], or to perform magnetic resonance imaging
(MRI) of air spacesinhuman lungs[4 , 5]. Allthese applicationsrequire a
veryhighnuclearpolarisation,alsocalledhyperpolarisationsinceitisorders
of magnitudeabove the Boltzmannequilibrium value (of order 10 5
/Tesla
at roomtemperature).
A very eÆcient and widely used polarisation method relies on optical
pumpingofthe2 3
Smetastablestate ofheliumwith1083 nmresonant light
PresentedatPAAT2002,Krakow,Poland,May31-June2,2002
[6 , 7]. Transfer of nuclear polarisationto atoms in theground state is en-
suredbymetastabilityexchangecollisions. Opticalpumping(OP)isusually
performedwithalowappliedmagneticeld(upto afewmT). Thiseldis
required onlyto prevent fast magnetic relaxation of theopticallyprepared
orientationandhasnegligibleeectonthestructureoftheatomicstates. In
particular, all Zeeman splittings aremuch smaller than theDoppler width
oftheatomictransitionsandthepumpinglightmustbecircularlypolarised
to selectively depopulate sublevels and deposit angular momentum in the
gas.
OPcanprovideahighnuclearpolarisation,above80%foroptimalcon-
ditions [8 ], but operates eÆciently only at low pressure (of order 1 mbar)
[9 ]. Productionof dense polarisedgas is a key issueforsome applications.
Polarisation-preservingmechanical compressionof theheliumgasafterOP
atlowpressureisperformedbyseveralresearchgroupsusingdierentmeth-
ods[10 ,11 ,12 ], butitisademandingtechniqueandnocommercialappara-
tuscancurrentlybeused. ImprovingtheeÆciencyofOPathigherpressures
could facilitate thiscompression bysignicantlyreducing therequirements
oncompressionratioandpumpingspeed. Itisalsoawayto directlyobtain
largermagnetisation densities. Thishasbeenused toperformlungMRIin
humans,simplyaddinganeutralbuergastothepolarisedheliumtoreach
atmosphericpressureand allowinhalation[13 ].
TheeÆciencyof OPcan beimprovedat highpressures byoperationin
a higher magnetic eld than is commonly used. High eld OPin 3
He had
been previously reported at 0.1 T [14 ] and 0.6 T [15 ], but the worthwhile
use ofa high eld(0.1 T) forOPat highpressures (tens of mbar)had not
beenhighlighteduntilrecently[16 ,17 ]. Asystematicinvestigationofvarious
processesrelevantforOPinnonstandardconditions(higheldand/orhigh
pressure)hasbeeninitiated,andthereisexperimentalevidenceofmolecular
formation(metastable He
2
molecules)and of increasedrelaxation when an
intense OP laser light is used in a high pressure plasma [18 , 19 ]. In this
article we rst discussvarious OPsituations, with emphasis on the eects
ofahighmagneticeldontheOPprocessinhelium,thengiveexperimental
demonstrationoftheOPimprovement obtainedusinga0.12Teldinhigh
pressuresituations.
2. Known eects of pressure and eld on OP in helium
2.1. Standard OP conditions
Inordertopopulatethe2 3
SmetastablestateandperformOP,aplasma
discharge is sustained in the helium gas. This constantly produces highly
endsintothe2 3
Smetastablestate,whichinpracticemayonlydecaythrough
a collisionprocess:
i. diusionto thecellwall andlossof excitation,
ii. ionising(Penning) collisions[20 ]:
He
+ He
! He + He
+
+ e ; (1)
iii. 3-bodycollisionswithconversion into a metastableheliummolecule
He
+ 2He ! He
2
+ He; (2)
iv. excitationquenchingbygasimpurities(non-heliumatomsormolecules).
Inequations (1) and (2), He
refers to either the2 3
S or2 3
P state 1
. In
steadystate, thereplacement ofHe
atomshavingdecayed byprocesses(i)
to (iv) resultsin an angularmomentum loss(e.g. throughemission of cir-
cularlypolariseduorescencelight orthroughdepolarisingcollisionsinthe
highly excited states[21]). Onthe one hand,this losscan be characterised
by the nuclear magnetisation decay time T
1
, which is found to decrease
for increasing plasma intensities. On the other hand, the steady state 2 3
S
metastable state density increases with the plasma intensity, and so does
theOPlightabsorptionandangularmomentumdepositionrateinthegas.
The most favourable plasma conditions, which lead to the highest steady
statepolarisations,areusuallyfoundforweakdischarges(forwhichprocess
(ii) isreduced) ina very pure heliumgas ( 3
He or heliumisotopic mixture)
forwhichprocess (iv) isnegligible.
Theratesand relativeimportanceof processes (i)and (iii)stronglyde-
pendon theOP celldimensionsand on thegas pressure. When thetrans-
verse cell dimension (which governs the lifetimes of all metastable species
due to process (i))is of order a few cm, an optimal pressure of order 0.5-
1 mbar is experimentally found to be mostsuitable (see gure 1). Indeed,
the actualoptimalplasma and pressure conditionsalso dependon OPcell
shape and size (e.g. due to radiation trapping), and on OP laser power
and spectral characteristics [22 ]. This will not be discussed in the follow-
ing, where we shall only consider the consequences of operation at higher
pressureormagnetic eld thanusual.
1
Infact, inthepresenceofanintenseOPlight,theexcited2 3
P statecanbealmost
as populated as the2 3
Smetastable state and mayplay animportantrole inthese
2.2.Pressuredependence of OP
At high gas pressure P (above a few mbar) the proportion of atoms
in the metastable 2 3
S state is reduced since their number density tends
to be limited by the non linear process (ii). In addition, the rate of cre-
ation of metastable molecularstates byprocess (iii)isenhanced witha P 2
dependence (equation(2)) and thediusionlifetimeof these moleculeslin-
earlyincreaseswithP;whichresultsinahigherdensityofmolecularstates.
Thesefactorstendto stronglyreduce theeÆciencyofOPat highpressure.
Figure 1 displays data already obtained at moderate (0.3 W [9 ]) or high
(several W[23 ])laserpower. Forpure 3
He;thepolarisationishalvedwhen
0.1 1 10
0.0 0.2 0.4 0.6 0.8 1.0
0.3 W, pure 3He 3 W, pure 3He 1.5 W, 3He-4He mixture
Nuclear polarisation M
Pressure (mbar)
Fig.1. Steady-state polarisationsobtained by OP at roomtemperature and low
magneticeld areplottedasafunction ofthegaspressure. OPwasperformedat
roomtemperaturein5cm(diameter)5cm(length)cylindricalsealedcellslled
witheitherpure 3
He,withtheOPlasertunedontheC
8 orC
9
transition(themost
eÆcientone ischosen,dependingongaspressureandlaserpower),ora 3
He 4
He
mixture (25%
3
He)with the OPperformed on the 4
He D
0
line. The lowpower
data(squares)arefromreference[9],theotheronesfromreference[23].
thepressureisincreasedfrom0.5to4mbar. Forthetestedisotopicmixture
(25%
3
He ) the pressure dependence is much weaker, and the plotted data
actually appear to only dependon the 3
He partial pressure. This remains
tobefullyveried,butitmayindicatethatonlypartofthemetastableHe
2
molecules (those includinga 3
He atom) contribute to nuclearrelaxation in
theplasma.
pressures,aswillbedescribed and discussedinsection 3.3.
2.3.Field dependenceof OP
Animportanteect ofahighenoughmagneticeldistostronglyreduce
theinuenceof hypernecouplinginthestructures ofthedierentexcited
levels of helium. In the variousatomic and molecular excited states which
arepopulatedintheplasma,hyperneinteractiontransfersnuclearorienta-
tion to electronicspinandorbitalorientations. Thistransfer oforientation
hasan adverse eect ontheOP eÆciencybyinducinga netlossof nuclear
polarisationinthegas. Thedecouplingeectofanappliedeldreducesthis
polarisation loss and may thus signicantly improve the OP performance,
especially insituations of limited eÆciency, such as low temperatures (be-
low) orhigh pressures(section 3.3).
At lowtemperature, areducedmetastabilityexchangecrosssectionsets
a tight bottleneck and strongly limits the eÆciency of OP 2
[24 , 25 , 26 ].
Since the plasma-induced relaxation rate is found to be much faster than
thereducedmetastabilityexchangerate,even astrongOPandhighpolari-
sationofthe2 3
Sstate resultina limitednuclearpolarisationoftheground
state : fromafewpercentsat1K[27 ] to15-20%at 4.2K[28]. Inthelatter
situation,aeldincreaseupto40mTwasfoundtoprovideasignicantim-
provementinnuclearpolarisation,asshowningure2. Boththerelaxation
timeT
1
andthe steadystate polarisationincreasewiththeoperating eld.
In thislow temperature regime wherethe orientations in theground state
and the metastable state are only weakly coupled, the observed polarisa-
tion increase(proportionaltoT
1
) canbedirectly attributedto thereduced
relaxation assuming that OP and exchange processes are not signicantly
aected by the eld increase(a reasonable assumption for these moderate
eld values[17 ]).
The OP conditions at high pressure are actually quite dierent, since
very frequentmetastabilityexchangecollisions stronglycoupletheorienta-
tion in the 2 3
S state to that of theground state. Still, it is not surprising
that a signicantimprovement isobtained by suppressingrelaxation chan-
nelsinhigheld[16 ],even ifthedetailsoftheinvolved relaxationprocesses
remain to befullyelucidated.
2
This was extensively studied [26 , 28 ]in anattemptto directlyobtain highpolari-
sations ina quantumuid (a heliumvapouror liquid, at low enough temperature
for quantum statistics to play an essential part in thermodynamic and transport
6 8 10 12 0.15
0.20 0.25 0.30
1 mT
10 mT
38 mT
nuclear polarisation M
T1 (minutes)
Fig.2. Steady-statepolarisationsobtainedbyOPat4.2Kareplottedasafunction
ofthemeasuredrelaxationtimeT
1
forthreevaluesoftheeldB:OPisperformed
in a 5 cm (diameter) 3.5 cm (length) sealed cell, lled at room temperature
with 1.33mbarof 3
Heand 6mbarofH
2
(toform asolidH
2
coatingandprevent
wallrelaxation). TheOPlaser(100mW) istunedon theC
5
transition,themost
eÆcientone inthese conditions(datafrom[28]).
3. New OP results at high pressure and high eld
AsademonstrationoftheimprovementofOPobtainedathighpressure
whenoperatinginahighmagneticeld,wereportmeasurementsperformed
in similar conditions at 1 mT and 0.115 T, both in pure 3
He and in an
isotopicmixture.
3.1.Experimental setup
The experiment arrangement is sketched in gure 3. The helium is
enclosed insealed cylindricalPyrex glasscells, 5 cmin diameterand 5 cm
in length. Results presented here have been recorded in 3 cells lled with
8mbaror32mbarofpure 3
He;or32mbarofheliummixture(25%
3
He ,75%
4
He). AweakRFdischarge(<1Wat3MHz)sustainedbymeansofexternal
electrodes isused to populatethe 2 3
S state inthe cell. The magnetic eld
B isproducedbyanaircoreresistivemagnetofsuÆcienthomogeneityover
the total cellvolumeto inducenegligible magnetic relaxation in these OP
experiments[17 ].
Theprobelasersourceisa50 mWlaserdiode(6702-H1, formerlyman-
ufactured by Spectra Diode Laboratories). Its output is collimated into
PUMP
beam laser diode 0.5 W amplifier+
RF discharge Q.W.
P.D.
P.C.
Probe laser diode P.D. 5
P.D. 8
P.C.
Tilted
P.C. probe
beam
B
Fig.3. The main elementsof theOP experiment are shown (not to scale). The
pump beam,parallel tothemagneticeldB; iscircularlypolarisedusingalinear
polarising cube (P.C.) and aquarter-waveplate (Q.W.). The absorption of the
pump beamismonitoredbyaphotodiode(P.D.) afteradouble passthroughthe
cell. Thetransverseprobebeamispreparedwithandpolarisationcomponents
whichareseparatedafter crossingthecellandsimultaneouslyrecorded.
attenuated to provide a weak probe beam. It passes across the cell per-
pendicularto B withlinearpolarisationsuchthatthe and polarisation
components are equal. The absorption of the probe beam components is
measured usinga modulation technique. The discharge intensityis modu-
lated ata lowenoughfrequency(100Hz)forthedensityoftheabsorbing
atoms 2 3
S to follow the modulation, and the and intensities are anal-
ysedusing lock-inampliers. The average valuesof the transmitted probe
intensitiesarealsorecorded,andusedtonormalisetheabsorptionmeasure-
ments. This eliminates errors due to laser intensity changes and strongly
reduces theeectsof optical thickness ofthe gas[18 ].
TheOPlaserusedfortheseexperimentsisasecondlaserdiodeamplied
using a 0.5 W bre amplier [33 ] (YAM-1083-500, manufactured by IPG
Photonics). The pump beam is expanded (diameter 3 cm) to match the
plasma distribution inthe cells,and back-reected after a rst pass inthe
3.2.Optical detection method
Inthestandardopticaldetectiontechnique[8 ,29 ,30 ],thecircularpolar-
isationof a chosenheliumspectral lineemittedbytheplasma is measured
and thenuclearpolarisationM ofthegroundstate of 3
He isinferred. This
technique relieson hyperne couplingto transfer angular momentum from
nuclear to electronic spins in the excited state which emits the monitored
spectral line. The decoupling eect of an applied magnetic eld unfortu-
nately reduces the eÆciency of this angular momentum transfer, which is
also sensitive to depolarising collisions. This technique must then be used
at lowelds(.10 mT), lowgas pressures(.5 mbar) and limited 4
He con-
centrations (.50%
4
He ) to avoid a signicant sensitivityloss (2 for each
of thequotedlimits).
Otheroptical methods, which rely on absorptionmeasurements on the
2 3
S-2 3
Ptransition,havebeensuccessfullyusedtoquantitativelydetermine
thenuclearpolarisationof 3
He[6 ,8,31 ,32 ]. Theyprovideinformationboth
on the total number density of atoms in the 2 3
S state and on the relative
populations of the probed sublevels. In usual situations 3
, the population
distribution inthe 2 3
S state is strongly coupledby metastabilityexchange
collisions to that inthe ground state. Thesepopulationswouldexactly be
ruledbyaspintemperaturedistributionintheabsenceofOPorrelaxation
processes, bothina low[7 ] and a high[17 ] magnetic eld.
When two absorption measurements directly probe two populations of
atomsinthe2 3
Sstate, thederivation ofM isa straightforwardprocedure.
This is for instance the case at low eld when the line C
8 or D
0
is probed
with
+
and circular polarisations, or at high enough magnetic elds
for the Zeeman shifts to remove all level degeneracies (B &50 mT [17 ]).
When transitions simultaneously probe several sublevels (e.g. in low eld
with polarisation on any line, with any polarisation on line C
9
, etc...),
themeasurementsoftwoindependentcombinationsofpopulationscan still
be used to infer the nuclear polarisation M, but specic calculations are
thenrequired[8]. Similarresultsareindeedobtainedinanisotopicmixture
when 4
Heatomsareprobedtomeasurerelativepopulationsamongthethree
sublevels inthe2 3
S state [17 ].
In thisexperiment, absorption spectra of the probebeam are recorded
forandpolarisationsovertheC
8 -C
9
transitions(forpure 3
He)ortheD
0
transition (formixtures), bothforM=0and insteadystate OPsituations.
Recording both polarisation channels is required to infer the value of M
only in low eld situations. The high eld spectra provide a redundant
3
This would nothold at very low pressures, nor inthe lowtemperatureconditions
discussed insection 2.3,butisexpectedto beverywellveriedfor pressuresabove
determinationofpopulationratios,whichisusedtocheckfortheconsistency
of themeasurements.
3.3. Experimental results
Figure4displaysanexampleofabsorptionspectraobtainedatB =0.115T
inthe32 mbarcelllledwithpure 3
He gas,forthe-polarisedprobebeam
component. As the laser frequency is tuned over the 2 3
S - 2 3
P
0
transi-
mF= -1/2 mF= -1/2
mF= +1/2
mF= +1/2
4 GHz
M=0 M=13%
Fig.4. Absorptionspectraofthecomponentofthetransverseprobe,tunedtothe
C
8 andC
9
resonancelinesof 3
He ,inthecelllledwith32mbarofpure 3
Hegas.
Theapplied magneticeld isB=0.115T,andoptical pumpingisperformedwith
a C
9
pump beam and circular polarisation at moderate discharge intensity.
The nuclear polarisationM is deduced from the measured the peak amplitudes
at null polarisation(solid line) and steady-state polarisation (dotted line). The
m
F
=+1=2statesareheredepleted, andtheresulting 3
Henuclearorientationis
-0.13[17].
tion, four resonance lines (two for C
8
and two for C
9
) are recorded, which
correspond to optical transitions between hyperne sublevels of identical
angular momentum projections m
F
along theeld axis. Allthe peak am-
plitudesarepreciselymeasured. Theprobebeamintensityisheretoo weak
to opticallypump themetastable state. Thedata redundancycan be used
to check that the OP beam introduces no spuriouspopulation dierences
between pumped and unpumped metastable sublevels (i.e. no local over-
polarisationof the2 3
Sstate ascompared to the groundstate). Thiseect
of OP on the metastable populations has been discussed both at low and
highelds[8 ,17 ], andisindeednotexpectedtobesignicantinthepresent
magnetic eld and pressure ranges. The population ratio of two adjacent
