Fast Neutral Lithium Beam for Density and
its Flutuation Measurements at the
Boundary Regions of ETE Tokamak
R.M. Oliveira, M. Ueda, and W.A. Vilela
INPE-InstitutoNaionalde PesquisasEspaiais,
P.OBox515,12227-010, S~aoJose dosCampos,SP,Brazil
rogerioplasma.inpe.br,fax(12)39456710,phone(12)39456692
Reeivedon26June,2001
AFastNeutralLithiumBeamProbe(FNLBP)isbeingdeveloped,inorder,toperformmeasurement
of the boundary plasma density in disharges of the Spherial Tokamak ETE, reently built at
LAP/INPE.Thisplasmadiagnostimethodisadequateforuseinfusiondeviesbeauseitdoesnot
perturbtheplasmaanditprovidesdatawithhighspaeandtimeresolutionfortheentiredisharge
lifetime.Toobtainreliablemeasurements,however,FNLBprobingdependsonhighsignal-to-noise
ratio(S/N)duringbeamemissionspetrosopyoftheLibeaminjetedintotheplasma,speiallyif
densityutuationmeasurementsaresought. Hene,oureortisbeenfousedintheahievement
ofhigh intensity Li 0
output. Reently, probingofa lowtemperature(T 5eV)and low density
target plasma (n 10 10
m 3
) produed in a plasma immersion ion implantation experiment
(PIIIE)wasperformedbya2-5keVFNLBP.Themethodusedforthedensitydeterminationofthis
glowdishargeplasmawasbasedontheomparisonoftheuxesof6708
A photonsemittedfrom
Li 0
beaminjetedintonitrogengasthatlledthePIIIEhamberandfromthesamebeaminjeted
intotheplasma disharge. For thisase, theattenuationof thebeamwasnegleted. Theplasma
density measuredwas ne =8:510 10
m 3
for pressureof operation ofp=7:010 4
mbar and
ne=7:210 10
m 3
forp=4:610 4
mbar. TheseresultsareingoodagreementwithLangmuir
probedata. ThesuessofthismeasurementwasonlypossibleafteratenfoldinreaseintheLi +
outputandtheinreaseofthelifetimeofLisourebyvitriationofthe-euryptitesoure,besides
theoptimizationofthe optialdetetion systemand theneutralizationofthebeam. Presently,a
new10keVFNLBP is beendeveloped to probe ETE plasma. For this ase, where the Li beam
will bestrongly attenuatedby thehigh density plasma (n
e 10
13
m 3
), the methodof density
reonstrutionfrom the whole photonux prolewill be used. Allthe improvementsperformed
duringtheoperationoftheoldFNLBPdevie,willbeimplementedonthisnewFNLBP.
I Introdution
Presently,theresearhinfusionareahasbeen
on-siderablyfousedinsomesmallspeishemes,mainly
due to thelow ostinvolved in their onstrution
be-auseofitsmodestdimensionsforagivenplasma
ur-rent. ThisistheaseofETE -ExperimentoTokamak
Esferio - a spherialtokamak that has the following
harateristis : major radius R = 0:3m,aspet ratio
A=1:5,toroidal magnetield B
=0:4 0:8T,and
projetedplasmaurrentI
p
=220 440kA.
Oneof themain features of spherialtorus is that
it anoperatewitha highratio ofplasma pressureto
magneti eld pressure,due to the inreasein plasma
stability,aonsequeneofthelargetwistingofthe
mag-netieldlinesintheentralregion[1℄. Thisadvantage,
that only beome apparent forA <2, makes relevant
velopmentoffutureompatfusionreators. However,
the suess of all the investigation programme to be
arriedoutin this experimentwill beloselylinkedto
the developmentof spei diagnostisystemsneeded
to measure the partiular harateristis of ETE
dis-hargeparameters. Fromthispointofview,speial
at-tentionwill bedevotedto theinvestigationofthe
on-nement properties in ETE in whih the edge region
playsa ruial role, asit wasobservedin manyother
fusiondevies [2-4℄. This paperdesribesthe
develop-ment of a 10keV FNLBP that will allow the density
and its utuationmeasurementsat the boundary
re-gions of ETE tokamak. The desriptionof theFNLB
diagnostiispresentedinSetion II.SetionIIIshows
theseveraloptimizationsperformedinordertoahieve
theneutralizationsystem. Setion IVpresentsthe
ex-perimental results of the probing of a target plasma
bya3keVFNLBP.InsetionVitisdesribedthe
im-provementsimplementedinanew10keVFNLBPwhih
willbedediatedtoprobeETEdisharges. Finally,the
summary ispresentedinsetionVI.
II Desription of FNLB
diagnos-ti system
TheexperimentalsetupneededfortheLibeam
di-agnostionsistsbasiallyof: aniongun,a
neutraliza-tion hamberandadetetion systemto theolletion
ofthebeamemissionlight.
In the ion soure, Li ions are emitted due to
the heating of a lithium -euryptite ompound
(Li
2 O.Al
2 O
3 .2SiO
2
) that is plaed in a self-heated
molybdenumruiblewithafrontreeptaleof0.6mm
diameterand2mmdepth. Theiongun,aNagoya-type
injetor [5℄ that usesaPiere geometry [6℄for the
ex-trationofLi +
,hasatotalof3ylindrialeletrostati
lensesthatarepolarizedusingtheEinzelonguration
[7℄,fortheaelerationandfousingofthebeam.
The neutralization apparatus plays an important
role in this diagnosti system sine target plasmas to
beprobedareusuallyproduedusingeletri/magneti
eldsthatwouldinterfereonthetrajetoryofthebeam
if it was not neutralized. Basially, small piees of
metalli sodiumarevaporizedbytheheatingofa
neu-tralizer ell that is sealed by a pneumati valve. The
sodium vapourllsasmallpassthroughhamberand
when needed, the valveis opened and the Libeamis
neutralized byhargeexhangeproess.
Thedetetionsystemongurationvariesaording
to thedevie to be probedbut usually is omposedof
optial bers, spetrometer and photomultiplier, as it
is the experimental set-up of the detetion system of
theurrentexperiment.
III Optimization of the beam
output
Ahievement of reliable measurements with high
signaltonoiseratio(S/N)duringBeamEmission
Spe-trosopy(BES)oftheLibeaminjetedintotheplasma
depends essentially of a well neutralized beam, with
high intensity andlow divergene. Manyoptimization
tasks,desribedbelow,wereperformedina3keVFNLB
devie, looking for a Li 0
beam with adequate
hara-teristis for the probing of plasmas in whih reliable
measurementsofdensityandin additionofitsdensity
III.1 High intensity beam output
High intensity beam output was obtained by the
transformationof-euryptiteompoundfromthe
tra-ditionally used pasty state, that is the powder of
-euryptite mixed with amyl aetate solution, to the
glassystate,thatisobtainedbythemeltingofthepaste
with its subsequent slow ooling [8℄. Fig. 1 shows a
fatorof 8 10 inreasein theurrentdensity, in
fa-vor of the glassy load for two values of temperature,
dependingontheusedaelerationvoltage. Moreover,
theonrmationof thesuperiorqualitiesoftheglassy
soure ompared with the pasty one an be seen by
theresults presented in Fig. 2. Infat, the glassy
-euryptite soure exhibited better beam emission
his-tory with slow redution of the beam output during
the rst 60 min to about 75% of its initial emission,
and then the output remained almost onstant for a
longperiod (190min). Inontrast,thebeamoutput
inthepastyasefallsdramatiallyto40%ofitsinitial
valuein the rst 60min of operation, falling down to
only20%ofitsinitialemissionin150minofoperation.
III.2 Neutralization eÆieny
The neutralization eÆieny was measured using
2 deetor plates submitted to strong eletri eld
(200V/m)andplaedjustaftertheneutralizationell.
Twoseondaryeletrondetetors(SED)wereinstalled,
therst one (SED1) being positioned in front of the
trajetoryoftheneutralizedbeamwhiletheseondone
(SED2)was10m shiftedupward, olletingthe
non-neutralized partiles of Li ions being deviated by the
eletrieldtotheSED2. InFig. 3itispossibletosee
themeasurementoftheeÆienyof theneutralization
with the variation of the temperature of the metalli
sodium element intheneutralizationell,where 100%
of neutralization was obtainedfor temperature of the
sodium of280 0
C.
Figure1. AomparisonoftheextratedLi +
urrentdensity
temper-Figure2. Thelithium-emissionhistoryforpastyandglassy
-euryptitesoures.
Figure3. Measurementof theneutralizationeÆieny
a-ordingtotheheatingofthemetallisodiumelement.
III.3 Other optimization tasks
Otheroptimizationtasksrelatedtothepolarization
of theeletrostati lenses,whih allowedextration of
highbeamoutputandsmallbeamdiameter,werealso
performed. Bestresultswere reahed withthe
follow-ingonditions: thediereneof potentialbetweenthe
ion gun (V
GUN
= 3kV) and the rst lens responsible
for the extration of the beam (V
1
= 1:7kV) was of
V =V
GUN V
1
=1:3kV; theseond lens, used to
fo-us thebeam, was submittedto a potential of 2.3kV;
nally,thethirdlens,usedtoaeleratethebeam,was
onnetedto theground. With suhonditionsof
po-larization the urrent density measured at the end of
theighttube,1.5mfarfromtheionsourewasabout
30A,andthebeamdiameter wasabout1.0m.
IV Experimental results
This setion presents the experimental results of
the probing of atarget plasma, a glowdisharge
pro-dued in aplasma immersion ion implantation
exper-iment (PIIIE), by a 3keV FNLB devie. Here, the
methodusedfortheplasmadensitydeterminationwas
based onthe omparison ofthe uxes of 6708
A
pho-tonsemittedfromLi 0
beaminjetedintoargongasthat
lledthePIIIEhamber(usedforalibrationpurposes)
andfromthebeaminjetedintotheplasmadisharge.
For this low density ase (n
e 10
10
m 3
), the
at-tenuation of the beam was negleted and the photon
uxresultingfromexitationofbeamatomsbyimpat
witheletronsoftheplasmaisN
vp =K
1 n
e n
b Q
p ,where
n
b
is the beam neutral atom density, Q
p
the eetive
exitation ross setion, n
e
the eletron density, and
K
1
isaonstantof proportionalitywhih inludes
op-tis geometry and detetor sensitivity fators [9℄. On
theotherhand,photonemissioninthegasisgivenby
N
vg =K
2 n
g n
b Q
g
,where n
g
isthegas density,Q
g the
exitationrosssetionforollisionofbeamatomswith
neutralatomsinthegas,andK
1 =K
2
,sinethesame
geometryof observationandthesamedetetion
meth-ods were used during alibration and plasma density
measurement.
IV.1 Density determination
Fig. 4 shows the 6708
A Li I photon ux,
mea-suredfordierentvaluesofp,thepressureofargongas.
Then, the loal plasma density was diretly obtained
aording to equation 1, sine ross setion data was
available [10℄, and n
g
and the ratio N
vp (r)=N
vg were
measured. Here itmustbetakenintoaountthatthe
absolute valueof thephotonuxin theplasmahasto
bedisountedfromthephotonuxin thegas.
n
e (r)=
N
vp(r)
N
vg Q
g
Q
p n
g
(1)
Figure4. Evolutionof6708
ALiIphotonuxprolewith
thepressureofthedisharge.
For example, for p = 7:010 4
mbar, the urve
ThroughtheknownrosssetionvaluesofQ g =4:2 10 16 m 2
[10℄and Q
p
=3:010 14
m 2
[10℄,and the
measurementofn
g
,theloaldensityvalueobtainedwas
ofn
e
=8:510 10
m 3
. Usingforexampleotherpoint
oftheurveplotinFig.4,withp=4:610 4
mbar,the
plasma densityobtained wasof n
e
=7:210 10
m 3
.
These results are in good agreement with Langmuir
probedata[11℄ forsimilar dishargeonditions.
IV.2 Determination of the real energy of
the Li 0
beam
The observations of the6708
A LiI resonaneline
throughtwowindows,therstonepositioned in front
ofthepropagationofthebeamandtheseondone
posi-tionedin aperpendiulardiretion tothepropagation
of the beam, allowed to infer the real energy of the
beam. As an be seen, the plot shown in Fig. 5 is
Doppler shifted, with a displaement of = 5:5
A.
Using the relation E = h = h=, we obtained
E=2:2keVfortheenergyoftheLibeamatoms.
Figure 5. Photon uxof 6708
A Li I aptured by the
de-tetionsysteminstalledinaparallelandinaperpendiular
diretiontothepropagationofthebeam.
V Development of FNLB
diag-nostis for ETE tokamak
The development of this new FNLB system that
will be used to probe the edge plasmaregion of ETE
is in ourse, with several important modiations
in-luded. The ion gun was redesigned in order to
a-ommodate ion souresof 3dierentdiameters (
1 = 0:6m, 2 =1:0m, 3
=1:7m),allowingthe
ahieve-ment of higherionurrentdensitiesafter beam
fous-be used to inrease extration urrent and better
fo-usingand aelerationofthe beam. Speial arehas
beentakenhereto avoidany inueneof FNLB
oper-ation on the tokamak operation and vie-versa. The
shemati drawingof the FNLB set-upin the ETE is
showninFig. 6.
Thepollution freeonditionoftheextremely lean
vauumrequiredforthetokamakoperationis
guaran-teed by the negligible eet of the Li 0
beam (small
quantity and deliberately low atomi number). To
avoid that Na vapour enter the tokamak hamber, a
dierentialpumpingwillbeusedwithtwostrategially
loatedturbomoleularpumps. TheLi +
soureregion
will be shielded, to avoid the eets of the spurious
toroidalmagnetieldswhihwoulddeviatethebeam
before itsneutralization. Theoptialdetetionsystem
will be of a multihannel type, allowingsimultaneous
multipointmeasurementsofthephotonemissiononthe
beampropagationdiretionintotheplasma. The
typ-ial high density plasmas produed in tokamaks
(n10 12
10 13
m 3
evenin theedgeregion),require
the use of the method of density reonstrution [12℄
from thewhole photonux prole,for the
determina-tion of the loal densities, represented here by
equa-tion2. n e (r)= A h em i eff N p (r) R r 0 h i;x i t hemieff N p (r 0 )dr 0 (2) Here A
is the veloity of the monoenergeti Li 0
beamatoms,N
p
(r)thephotonux,h
em i
eff the
ef-fetiveemission rateoeÆient andh
i;x i
t
thetotal
rosssetion of ionization andhargeexhange of the
beamatoms.
VI Summary
Sine the edge region plays an important role in
the onnement properties of the whole plasma[2-4℄,
theFNLBPtehniquewill beavaluablediagnostisin
theinvestigationoftheplasmatobeproduedinETE.
Moreover,severaladvantagesouldbeexploredbythe
useofFNLBPasthenonperturbationandthelow
pol-lutionoftheplasma, thehighaurayreahedin the
measurementsand thespatial and time resolved
mea-surementsduringtheentiredishargelifetime. InETE
itisexpetedatimeresolutionoftheorderof
nanose-onds,givenbythetimeresponseofthephotomultiplier,
anda spatial resolutionof theorder of 0.2m, limited
basially by the optial system used for the
observa-tion of the photon ux resulting from the interation
oftheLi 0
beamwith theeletronsoftheplasma. The
FNLBwasdesignedto attaingoodpenetration,ofthe
order of 7m far from the last losed ux surfae, by
Figure6. ShematidrawingofFNLBdevieprobingtheETEtokamak.
supplies to better foalize and aelerate the beam.
Theseimprovementswillopenpossibilitiestothe
mea-surementsofdensityutuations,sinehigherintensity
beamoutputisbeingexpeted,allowingmeasurements
with high signal to noiseratio. Speial arehas been
taken in the onstrution of this new devie to avoid
anykindofinterfereneofthis diagnostistotheETE
dishargeandvie-versa.
The probing of a target plasma with an old 3keV
FNLBPwassuessful,withthedensityvaluesobtained
in goodagreementwithLangmuirprobedata. Allthe
optimization tasks that were performed in this devie
willbeinorporatedin thenewFNLBP.
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