Neutron Anisotropy and X-Ray Prodution
of the FN-II Dense Plasma Fous Devie
F. Castillo, J.J.E. Herrera, J. Rangel,A. Alfaro, M.A. Maza, V. Sakaguhi,
Instituto deCieniasNuleares
UniversidadNaionalAutonomade Mexio
A.P.70-543,CiudadUniversitaria,04511 Mexio, D.F.Mexio
G. Espinosa,and J.I.Golzarri
Instituto deFsia
UniversidadNaionalAutonomade Mexio
A.P.20-364, 01000Mexio,D.F.Mexio
Reeivedon26June,2001
TheFN-IIDense PlasmaFous isasmall(<5kJ) Mathertypedevie,where thedependeneof
neutronyieldanditsanisotropy,intermsofdeuteriumllingpressure, andtheneutronemission
angular distributionhavebeen studied. Two dierenteletrodeongurations have beentested,
showingthattheir geometryplaysanimportantrole bothonneutronyieldandanisotropy. Time
integratedanisotropy hasbeenmeasuredwithsilver ativationounters,ona shotto shot basis.
CR-39nuleartrakdetetorsareusedtodeterminetheangulardistributionofneutrons,averaged
overtensofshots,showing thatanisotropipedestalaountsfor 70%ofthe emission,whilethe
anisotropy omponent aounts for the remaining 30%. Theneutron yield shows a tendeny to
inreasewithanisotropy,aswellaswiththeemissionofhardX-raysobservedon-axis.
Sintillator-photomultiplierdetetors show a slight Doppler shift inthe neutron energyat bakwardangles,
supportingthebeam-targetmehanism. Additionalinformationhasbeenobtainedfromtime
inte-gratedX-raydiagnostis,whihinludelteredmulti-pin-holeameras.
I Introdution
TheDensePlasmaFous(DPF)hasbeenstudiedsine
the late 1950's, and its researh has been stimulated
from its earlydaysbeauseofthe simpliity ofits
en-gineering, anditspropertiesas anintense,pulsed, low
ost, soure of neutrons, eletron and ion beams, as
well as of soft and hard x-rays [1-3℄. Although some
projetionsmaintaintheDPFmightstillbeonsidered
afeasibleandidateforaontrolledfusionreator[2,4℄,
most of thereent work has beenmotivated by
ongo-inginterestasaosteetivepulsedneutronandx-ray
soureforawiderangeofappliations[5-9℄.
TheDPF isaoaxialgun, in whih theinner
ele-trode,theanode,iseletriallyinsulatedfromtheouter
eletrode, the athode, by eramis or Pyrex. After
ahievingahighvauum,gasisadmittedatapressure
of afewtorr. Theplasmaoriginateswhenaapaitor
bankisdishargedthroughalowindutane
transmis-sionline,assparkorrailgapswithesarelosedwithin
the breeh of the gun, through the insulator surfae,
aradialurrentsheath develops, givingrise to an
az-imuthalmagnetield,whihdrivesthesheathtowards
themuzzle. As aresult,thesheath sweepsandionises
theneutral gas it ndson its way. Finally, it fouses
intoaplasmaolumn atthetipoftheinnereletrode,
with densities in the range of 10 18
10 20
m 1
, and
eletrontemperaturesin therangeof0:1 2:0keV,
de-pendingonthedimensionsofthedevieandtheenergy
storedin theapaitorbank. Connementis ahieved
by the pinh eet, due to the axial urrent, whih
rangesfrom200kAin smalldevies upto2MAin the
largestones. During this phase, soft X rays are
pro-duedbybremsstrahlung. Thellinggasanbehosen
inordertoproduestronglineradiation,andthushard
X-rays[10℄. Thetip of the eletrode an be designed
in suh a way that the impinging eletron beam an
enhanethem.
It is well known from the early days of fusion
destroyed, usuallywithin 10 50ns. Inthe proessof
suh instabilities, ions are aelerated away from the
anode, while eletrons are aelerated into it,
assoi-ated with theappearene ofhot-spots[11℄. A
reason-ablehypothesisis that strongeletrields are
gener-ated,whihaelerateionsuptoenergiesgreaterthan
10MeV, evenin small devies [12-15℄. Unfortuantely,
disrepaniesbetweenonitingtheorieshavenotbeen
solvedyetbyexperimentalevidene. Suhissueof
out-mostimportane fallsbeyond thesopeofthepresent
paper. Ifthellinggasisdeuteriumin aDPF,the
a-elerated ions reatagainstthe bakgroundhotdense
plasmaions,originatinganotoriousneutronpulsethat
ranges from 10 6
per shotin small devies, upto 10 12
in someof thelargestones. Thisneutronyieldfollows
a power law with urrent I that goesas I 4
[16℄, and
with the stored energy W
0
that goesas W 2
0
[2℄. F
ur-therstudies onthesalinglawsofDPFmahineshave
beenarriedoutbyLeeandSerban[17℄.
Throughout the past forty years, the operation of
plasma foi has been doumented for stored energies
thatrangefrom2kJ[6,10,18,19℄upto1MJ[2,20℄. The
behaviourof smalldevies around2kJ hasbeen
thor-oughlystudied. Theinueneofeletrodeandinsulator
materialsontheneutronemission,hasbeenstudiedby
Routetal. [18℄,andomparativestudies ofion,x-ray
andneutronemissionhavebeenarriedout[19℄. Small
portablemahineshavebeenproposedforintrospetive
imagingofmetalliobjetsandforneutronidetetion
of water[8,9℄. Furthermore, smallerdevieswith high
repetition rate have been developed for appliations
suh as miroeletronis lithography [6℄ and
biomed-ial studies [7℄. It an be stated that plasma fous
researh has spread beause of its interesting physis
phentomena, its potential appliations, the simpliity
ofitsengineering,anditsaordability.
The present work stems from the Fuego Nuevo II
DensePlasmaFousDeviewithtwodierent
oxygen-free oppereletrodes. Theouter eletrode is arrayed
asasquirrelage. Oneoftheobjetivesofthisworkis
toomparetheperformaneoftwodierentinner
ele-trodes with equal length but dierent diameter. The
rst eletrode, hereafter alled eletrode I, is a solid
ylinderfully ontainedwithin theinsulator,soits
di-ameteris smallerthan theinner diameter ofthe
insu-lator. Thediameter oftheseond one,hereafteralled
eletrode II, by ontrast, mathes the outer diameter
of the insulator. It may be surmisedthat the seond
one may allow a better breakdown, not only beause
itspathisdereased,but alsobeausethere isanedge
eet attheeletrode-insulatorjoint. Itwillbeshown
thisisthease.
The struture of the paper is as follows: A brief
desriptionoftheFN-IIdevieandthediagnostis
rel-evant to this work is made in setion II. The latter
inlude time integrated silver ativation and nulear
trakdetetors, whih were used to obtainanisotropy
and angular distribution measurements,
sintillator-photomultiplier systems,used for timeresolved
dete-tion of hard X-rays and neutrons, and the
multi-pin-hole amera. The results for time integrated neutron
yield and anisotropy for eah eletrode will be
pre-sentedanddisussedinsetionIII.Timeresolvedhard
X-raysandneutronsignalswillbepresentedinsetion
IV, showing that a slight Doppler eet for the
neu-tronenergyanbedetetedatbakwardangles. Time
integrated pin-hole x-raysignals, and radiographs
ob-tainedonaxiswillbeshowninsetionV.Finally,some
onlusionswill bedrawnin setionVI.
II Desription of the
experimen-tal devie
Ashemeoftheexperimentalarrangementisshownin
Fig. 1. Twodierentinnereletrodes(anodes)ofequal
length(40mm long), but dieringdiameter havebeen
tested. Theinsulatorusedinbothasesisa10mmlong
Pyrexsleeve,withanouterdiameterof50mm. The
di-ameter ofeletrodeI is40mm,while thatof eletrode
IImathestheouterdiameterofthePyrexsleeve. The
squirrel age outer eletrode (athode) onsists of 12
opper bars, of 8mm diameter and 36mm long eah,
arrayed on a 100mm diameter irle. The apaitor
bank onsists of four 1.863 F apaitors in parallel,
withaninternalindutaneof24nH eah. Theenergy
isonduted throughaparallelopperplate
transmis-sion line insulated by six Mylar sheets, with a total
thiknessof2:2mm. Theindutaneofthesystemhas
beenmeasuredby short-iruitingthe gun, andfound
tobe54nH. Theiruitislosedbyasparkgapswith,
pressurisedwithair,andtriggeredbyamiroMarx
gen-erator,whihproduesa100kV pulsewith a20nsrise
time. Theresultsobtainedinthiswork wereprodued
byhargingtheapaitorbankat 36kV, whih means
Figure1. Shemeoftheexperimentalarrangement,showingeletrodeIandthepin-holeamera.
The behaviour of the iruit is monitored with a
Rogowskioilwhihyieldsthederivativeoftheurrent
signaldI=dt. Fig. 2showsatypialosillogram,where
(a) is the dI=dt signal, and (b) is the signal from a
sintillator-photomultiplier detetor, desribed below.
ThedipneardI=dt=0,orrespondstoasuddendrop
of urrent,whih oursasimpedanerisesduring the
ompression of the plasmaolumn. This is the
signa-tureofthefousingoftheplasmaolumn.
Timeintegratedmeasurementsoftheneutronyield
anditsanisotropyaremadewithtwodierentmethods.
Silverativationdetetorsareusedonashottoshot
ba-sis. TheseonsistintwoVitoreen1B85Geiger-Muller
ounters,withlateralwindow,10mlong,wrappedup
ina500msilverfoil,andsurroundedby6mof
paraf-n. The totalsize of eah detetoris 22m high with
a15:5mdiameter. Theirpurposeis tomeasurethe
deayof 108
Agand 110
Agnuleithatresultfromthe
a-tivation of 107
Ag and 109
Ag,respetively. TheparaÆn
moderatesthe neutrons, therefore inreasing the
ati-vation ross setion. Both ounters were plaed54m
away from the fous, one of them on the axis of the
devie, andtheotheroneat90 o
. Theywerealibrated
with an Am-Be neutron soure, and the bakground
ounts were monitored on a daily basis, in order to
Figure2. AtypialosillogramshowingthedI=dtsignal(a)
alongwiththeoneoftheNE211sintillator-photomultiplier
detetorplaed5.4mawayfromtheolumnat90 o
(b). The
dipinsignal (a)isthesignatureof thepinhatmaximum
urrent. Therstpulsein(b),whihalmostoinideswith
thedip,orrespondstohardX-rays,andtheseondoneto
neutronemission.
tothe alibration,thebehaviourofbothounterswas
very similar, and ould be onsidered to be the same
ountsolletedin60semultipliedby(9:40:3)10 3
.
Theminimumneutronyieldthresholdpersteradianper
minutethatanbemeasured,determinedbythe
bak-ground,is(4:7:9)10 4
. Furtherinformationonthis
methodanbefoundinRef.[21℄.
The seond method used to measure the neutron
anisotropy, and speially the averageneutron
angu-lar distribution, onsisted in using CR-39 plasti
nu-lear trak detetors. Fast neutrons an be deteted
with the CR-39 polyarbonate (C
12 H
18 O
7
), either by
elasti sattering with protons from the detetor
ma-terial or radiatorfoils, suh aspolyethylene plaed in
front[22℄,orbypartilesproduedby 10
B(n;)
rea-tions[23℄. Athigher energies,evensatteredC andO
atoms generateethable traks [22℄. These traks an
behemiallyethedbyagentssuhasNaOHorKOH.
This method hasbeen usedearlier at otherdevies to
detetneutronsofenergyaround2.45MeV[24℄.
How-ever itwas not used to evaluatethe role of the
angu-lardistribution in the determination of thetotal
neu-tron yield, whih is oneof the purposes of this work.
Retangular hips, with a 1:9m0:9m surfaeand
500m thikness were plaedon a irular ar of 100
m radius, entred at thefous, outsidethe disharge
hamber. Fivedierentanglesrelativetothegun axis
werehosen: 0 o ;45 o ;90 o ;135 o and170 o
. Itwasfound
that,inordertodetetneutrons,theCR-39neededto
beoveredwithpolyethylenesheets,2mmthik. The
detetors were exposed to 65 shots, for a lling
pres-sureof2.75torr. Thispartiularhoieofpressurewill
beexplainedbelow. Thehemialething ofthe
plas-ti hips wasperformed by immersing them in KOH,
6.25Msolution, at (601) o
C for 6 hours. The trak
ounting wasdonewith anoptial mirosope,with a
200magniation,xingaeldof0:36mm 0:3mmas
measurementarea. Fortyelds perhipwereounted,
in eah hip, in orderto obtainbetterstatistis. The
optialmirosopeis oupledto aCCD ameraand a
DigitalImageAnalysisSystem(DIAS)[25℄,makingall
themeasurementsproessautomatially.
Two sintillator-photomultiplier detetors have
been used for time resolved neutron detetion. The
sintillators, NE211 and NE218, are both ontained
within ylinders of equal size; 7m long and 5m
di-ameter. They are oupled to AMPEREX
photomul-tiplier tubes model 56 AVP. In order to minimise the
hardX-raysignal,thesintillatorsaresurroundedbya
1mleadshield,andtheirfrontisoveredbyleadand
opperlters. Signal (b)in Fig. 2wasobtainedwith
the NE211 sintillator and a Pb lter 5:53mm thik,
followed bya 2:65mm Cu foil. The peak that almost
oinideswiththedipoftheRogowskioilsignalis
pro-duedbyhardX-rays,anditislearlyresolvedfromthe
neutronpulse bytime ofight. Thus,if theenergyof
the neutronsvaries with theangle, this an be
deter-mined as aDoppler shift, measuring the dierene in
peaksat dierentangles. Furtherdetails will begiven
in setionV.
Althoughtheinnereletrodeisasolidylinder,the
entreofthetipishollow,sothathardX-raysfromthe
ollisionoftheeletronbeamwithitarehiddenfrom
di-retobservationat90 o
fromtheaxis. Amulti-pin-hole
amerawasplaedinthatdiretion,fortimeintegrated
X-rayimagingoftheplasmaolumn. Fiveirular
pin-holes, 400m in diameter, and separated by 4:5mm
weremadeona2mmthikCusreen,19mawayfrom
the axis. Eah hole was overed by 25;10;5;15 and
20mthikAlltersrespetively. Althoughthe400m
diameter of thepinholes maybe too large for getting
aproperresolution, it was hosenin orderto
guaran-tee reproduibility of the veholes. The images were
reordedonX-raydental lm,plaedwithin an
evau-ated amera7:6mawayfrom theoppersreen, ona
revolver-likelmbakwithapaityforfourlms. This
yields a.40magniationof theobservedobjet. The
time (3min.) andtemperature(20 o
C)ofthe
develop-ingproess wereontrolled,sotheintensities obtained
fordierentshotsouldbeompared.
Besides the pin-hole amera, a 300m Al window
wasplaedonaxis,35mawayfromtheinnereletrode,
whih allowsobservation of thehard X-raysprodued
onthefaeoftheinnereletrode.
III Neutronyield andanisotropy
results
Two silver ativation detetors, as desribed above,
wereusedtomeasuretheneutronyieldandanisotropy
foreletrodesI andII.Thenumberofountsobtained
during sixtyseondsafter eahshot,were reordedfor
onedetetorplaedonaxis(0 o
)andtheotherat90 o
. A
widerangeofllingpressureswasexplored,goingfrom
1.0 to 4.5 torr. Theaveragenumber ofountsN
0 for
thedetetoronaxis,andN
90
forthedetetorat90 o
,as
afuntionofllingpressureareshownin Fig. 3(a)for
eletrodeI andFig. 3(b) foreletrodeII.The aseof
eletrodeIinludes400shots,whilethatofeletrodeII
inludes 539shots. Itmustbenotedthatforeletrode
Ithenumberofountsisbarelyabovethebakground
forpressuresbelow1.5torr,whileeletrodeIIshoweda
largerneutronyieldatlowpressures. Itisalsoobserved
that theerrorbars,whihshowthemeansquare
devi-ation,aresmallerforeletrodeII.Thisreetsabetter
reproduibility for this geometry. Generally speaking,
theneutronyieldis learlylargerforeletrodeIIthan
foreletrodeI,themaximumbeingroughly3timesas
largefortherstone. Thismaximumappears around
3:50torrforeletrodeI,andisshifteddownto2:75torr
foreletrodeII.TheorrespondingsalefordY
Figure3.Numberofountsobtainedduringoneminute
af-tereahshotwiththesilverativationdetetorsat0 o
,(N
0 )
and 90 o
, (N
90
), as funtions of the deuteriumlling
pres-sure. Results for eletrodeI are shownin(a), while those
foreletrodeIIareshownin(b). Therighthandsaleshows
thediferentialyieldpersolidangle.
Thedataforbotheletrodesshowalearlydened
anisotropyA =N
0 =N
90
, whihis shownas afuntion
ofthedeuteriumllingpressurein Fig. 4. EletrodeII
showsastrongerA,whihtends tobelargerfor lower
pressures, up to 2:75torr, and falls thereafter. The
anisotropyfor eletrodeI generallyfalls below thatof
eletrodeII,andremainsroughlyonstant,within the
errorbars,throughouttherangeofpressuresexplored.
AbetterreproduibilityisfoundforeletrodeIIagain.
Itmayalsobeinterestingtolookatthesattergramsof
A forseveralshots vs. thenumberofountsofthe
de-tetors. Asabest asesample,Fig. 5showsA vs. N
0
for eletrodeII.A linear regressionshowsalear
ten-denyofanisotropytogrowwiththenumberofounts.
This resultshowstherelevaneof thebeam-target
ef-fet as a neutron generation mehanism. It may be
surmisedthat thebettertheionbeamgeneration,the
larger theneutron yield will be fora given stored
en-Figure 4. Time integrated anisotropy for eletrodes I (a)
andII(b)asafuntionofthedeuteriumllingpressure.
Figure 5. Sattergram of anisotrpy A vs. the number of
ountsonaxisN0 foreletrodeII.
Onetheoptimumpressure2:75torr was foundfor
eletrodeII,theaverageangulardistributionofthe
neu-tronyield wasdetermined using the CR-39detetors.
Theangulardistributionofthetrakdensity,obtained
with65shots isshowninFig. 6,normalisedtothe
av-eragevalueat0 o
. Thedataobtainedforpositiveangles
anbeadjusted byaGaussianfuntion superposedon
apedestal;
f()=B+ C
p
2 exp(
2
=2 2
); (1)
where is the angle in degrees, B = :507, C = 82:7
and = 70:6. The values obtained for the samples
at 90 o
onrm the symmetry of the distribution, as
anbeseenfrom thegure. Eq.(1) anbeused in
or-derto estimatebyintegration thetotalneutronyield,
introduing a proper saling fator for B and C,
theanisotropi Gaussian distributionaountsforthe
remainig30%. Fortheaseof the65 shots,forwhih
Fig. 6wasobtained,the averagenumberof ountson
axisandat90 o
are<N
0
>=2034and<N
90
>=1265,
whihyielddY
0
=d=1:910 7
anddY
90
=d=1:210 7
neutrons/str,respetively. From these values,andthe
onesobtainedfromtheadjustedurve,eq.(1),forboth
positions, an optimisti saling fator S
1
= 2:010 7
isobtainedat 0 o
, andapesimistioneS
2
=1:710 7
at90 o
. Theyagreewithin18%. Integratingeq.(1)over
the sphere, and taking the average between the
val-uesobtainedforS
1 andS
2
,thetotalneutronyieldper
shot, for this partiularset of 65 shots is found to be
Y
N
= (1:660:14)10 8
. This value is 11% higher
thanthe oneexpeted from themeasurementsat 90 o
,
whihwould be1:510 8
,assumingtheemission were
isotropi.
Figure 6. Angular distribution of the trak population
obtained from the neutron emission, normalised to the
maximum ount obtained for 0 o
. CR-39hips overing a
1:9m0:9mareawereplaed1mawayfromthefousat
0 o ;45 o ;90 o ;135 o and170 o
fromtheaxis.
The value of A obtained from the CR-39 data
is 1:37 :05. For this partiular set of shots, the
anisotropy measured with the silver ativation
dete-tors, obtainedon a shot to shot basis, is 1:550:22,
whihis onsistentwithinthe experimentalerror.
Be-sides ounting the trak density by means of DIAS,
theirdiameterswerealsomeasured. Allvehipswere
onsideredforthismeasurement. Morethan90%ofthe
traksfall withinastronglypeakedGaussian
distribu-tionentredaround19:8mm, withafullwidth at half
maximumof 4:3mm. These are presumably produed
byprotons kikedo thepolyethyleneplate byelasti
(n,p)sattering. Thisisanindiationthattheneutrons
that are being deteted are strongly mono-energeti.
Theremaining10%of thetraksare attributableto a
neutronbakground, whih resultsfrom reetionson
IV Time integrated X-ray
re-sults
Time integrated soft X-ray images of the plasma
ol-umnhavebeenobtainedwithamulti-pin-holeamera,
asthatdesribedinsetionII.Typialimagesobtained
with eletrode I are shown in Figs. 7a and b. In the
rstasem=0andbright-spotsanbeobserved. The
length of the entral image is 8mm, whih means the
maximumsize oftheplasmaolumn isaround20mm.
This ase orresponds to shot 2578, where N
0 = 283
andN
90
=164. An interestingexampleisthat ofshot
2582, shown in Fig. 7b, where the olumn is kinked,
andahot-spot appearsin thekinkedolumn. Itlooks
likean m =0instability ours well after the plasma
olumnhasbeenbentasaresultofanm=1instability.
Obviously,thisstatementannotbeonlusivewithout
the support of time resolved diagnostis. In this ase
N
0
=523,while N
90
=372. However,these twoases
diersigniantlyfrom theimages obtainedwith
ele-trodeII,whiharegenerallydimmer. Agoodexample
is that of Fig. 8a, orresponding to shot 2745, where
the neutron yield is onsiderably larger: N
0
= 8876
andN
90
=3347. This hintsat thepossibilitythat the
radiationfromeletrodeIhasastrongerlineradiation
omponentoriginatedbyimpurities,than thatof
ele-trode II. Sine the neutron yield is learly greaterfor
the latter, it is apparent that the impurity radiation
oolingatsindetrimentofneutronprodution.
HarderX-rays ross the300m Al window on the
axis whih an be used for radiographial purposes,
suh as the those shown in Fig. 8, where the inner
details of a penil an be observed. There is an
in-verse relationship between the intensities of suh
on-axis X-rays and the soft X-rays at the pin-hole
am-era. This anbe seen as follows: An array of
super-posed Pb foils, eah 63m thik was prepared, suh
that the maximum thikness orresponds to ve foils
315m. Forshot 2745, shown in Fig. 8a, the
radio-graph of the Pb lter obtained on axis is shown in
Fig. 8b. In ontrast, for shot 2744, where the
neu-tron yield is lower (N
0
= 1329;N
90
= 706), the soft
X-rays at the pinhole amera are stronger, as shown
in Fig. 10a, while the radiograph on axis, Fig. 10b,
is onsiderablydimmerthanfor theformerase. This
pattern is systematially observed. It may surmised
that, sine the hard X-rays observed on axis are
pro-dued bythe eletronbeamthat ollideson theinner
eletrode,thestrongerthebeam,thegreateristhe
neu-tronyield. Thus, the eletronaelerationmehanism
may be orrelated with the neutron generation
meh-anism. Further time resolved studies may larify this
possibility. Ontheotherhand,thepinholeameras
in-diatethatpurer,impurity-freeplasmasallowabetter
Figure7.X-rayimagesforshots2578(a)and2582(b),
ob-tainedwiththemulti-pin-holeamerashowninFig. 2. In
bothasesm=0instabilitiesanbeobserved.Inshot2582
the plasmaolumnis kinked,showinganm=1instability,
andahotspotanbeobservedatthetip.
Figure 8. (a) X-ray pin-hole images for shot 2745. The
neutron yield inreasessigniantlywith eletrode II, but
the imagesare dimmer. (b)Radiographof anarrayof ve
Pbfoils,obtainedwiththeX-raysemittedonaxis forshot
2745. Themaximumthikness,315m,appearsonthe
up-perleft orner, whiletheminimum,63m,appearsonthe
lowerrightorner.
V Time resolved hard X-ray and
neutron measurements
Two sintillator-photomultiplier systems have been
usedforhardX-rayandneutronmeasurements,as
de-sribedinsetionII.Thegeometryissimilarforboth,
but in one ase the sintillator used is NE-211, while
intheotheroneitisNE-218. Signal(b)in Fig. 2was
obtainedwiththeNE-211at5:4mfrom theaxisofthe
devie. Therst peak that almost oinides with the
dip of the dI=dtsignal is a hard X-ray pulse, related
to the eletron aeleration in the fous phase, while
the neutron pulse is observedin the seond peak,
re-tardedbytheirtimeofight. Forthispartiularshot,
2909,N
0
=8545andN
90
=3284. Thetwosintillators
usedinthis workhavesimilarresponse totheneutron
signalwithin 20ns. Intheaseof thex-raysignalthe
diereneisindistinguishable.
The NE-211 wasfound to bemore eetive. This
an be seen in Fig. 11, where both detetors were
plaedat thesame distane of 5:4m, with 6:4mmPb
and1:3mmCulters. Forthis shot,3461, N
0
=1039,
and N
90
=735. In the osilogram shown in Fig. 11,
bothdetetorswereplaedat5:4mfromthefous,but
atdierentangles. TheNE218detetor(signal(b))was
plaedat90 o
from theaxis,whiletheNE211 detetor
(signal(a))wasplaedata170 o
bakwardsangle. The
former was ltered with 6:4mm Pb and 1:3mm Cu,
and the latter with a set of 5:35mm Pb and 2:65mm
Cu. The hard X-ray peak an be used as atime
ref-ereneforthe ourreneofthe fous, and theenergy
of the neutrons an be measured in terms of the
dif-ferene of the time elapsed between both peaks, the
main problem being that neutrons emitted at a later
timeanoverlapneutronsemittedearlierwithsmaller
energy. Thus, for5.4 m, theequivalene between the
timedierene andthe energyof neutronsisgivenby
(t)(se) = :39710 6
E(MeV) 1=2
. From the
dif-fereneofourreneofthemaximaofbothdetetors,
a Doppler shift in the energy is learly observed,
de-spiteoftheexperimental error. Theresultsareshown
inTable1.
Table 1. Energy Doppler shiftsof neutronsmeasured
atbakwardangles. Theseondolumngivesthetime
ofightdierenethatexistsbetweentheneutronpeak
measuredbythe detetor plaedperpendiular to the
axis, and that measured by the detetor at an angle,
givenbytherstolumn.
Anglewith Timeofight Energyshift
respettoaxis dierene(nse) MeV
120 Æ
185 0:30:1
150 Æ
225 0:40:1
170 Æ
Figure9. Radiographofapenilobtainedwiththe X-rays
thatrossa300mAlwindowonaxis. Innerdetailsanbe
learlyobserved.
Figure10. SameasFig.8,butforshot2744,whihprodued
asmallerneutronyield:(a)SoftX-rayimagesobtainedwith
the multi-pin-hole amera. (b) The orresponding
radio-Figure 11. Hard X-ray and neutron signals, obtained for
shot 3830, withthe NE211 detetor(a) at 5:4mfrom the
devieanda170 o
bakwardangle,andtheNE218detetor
(b)atthesamedistane,butperpendiularto theaxis. A
Doppler shift is learly seenas a diereneof theneutron
timeofightforeahdetetor.
VI Conlusions
Two eletrode ongurations have been tested: one
where the eletrode is fully ontained within the
in-sulator (eletrode I), and one where the eletrode
di-ametermathestheoneoftheinsulator(eletrodeII).
Theneutronyieldandanisotropywasfoundtobe
sub-stantially largerfor thelatter ase, while an
improve-ment in reproduibility was also observed. This may
berelatedto abetterurrentsheath formationat the
breakdownphase,asaresultofboththeedgeeetof
theeletrode, loseto theinsulator,andaderease in
thebreakdownlength.
This work provides evidene that neutron yield
tendsto inreasewithanisotropyin thisdevie, whih
points tothe onvinieneofthe beam-targeteet for
neutron generation. From the average angular
distri-bution of theneutron emission, measuredwith CR-39
nulear trak detetors, it is found that there is an
isotropipedestalthataountsfor70%oftheemission,
while the anisotropy omponent aounts for the
re-maining30%. Theestimationofthetotalneutronyield
using this information is 11%larger thanwhat would
be expeted form the silver ativation detetor
mea-surementat 90 o
, assumingtheemssion wereisotropi.
Further evidene of the existene of the beam-target
eet is demonstratedbyaDoppler eet observedat
bakward angles with sintillator-photomultiplier
sys-tems.
Thepinhole ameraimages tend to be dimmerfor
high neutron yield shots, whih may be related to a
dimmer for eletrode II than for eletrode I. On the
otherhand,thefatthat adimmeremissionofsoft
X-raysat90 o
,togetherwithabrighteremissionofX-rays
on axisare observed when theneutronyield is larger,
seemstoindiatethattheneutrongenerationisrelated
totheformationoftheeletronbeamthatimpingeson
the eletrode. It maybeinterestingto notethat after
560shots,theeletronbeamdrilledanalmost
ylindri-alhole,3mmwideand6mmdeep,ineletrodeI,while
theerosionin eletrodeII,after956shotsshowedasa
one, 6mmwideand12mmdeep. Bothintheasesof
soft x-ray emissions at 90 o
, and hard x-ray emissions
ontheaxis,spetrosopywill bemadein future work,
in ordertolarifythenatureofthephenomena.
Regardingthepin-holex-rayimages,itisinteresting
to point outthat,in someases, brightspots that are
related to m = 0 instabilities are observed in kinked
olumns whih presumably arise from m = 1
insta-bilities. This might mean that under ertain
irum-stanesthem=1instabilitymayhaveafastergrowth
ratethanm=0instabilities,againstommonwisdom.
However, study of this phenomenawith time resolved
optialdiagnostisareneededinordertolarifythis.
Aknowledgments
The authors wish to aknowledge the fruitful
in-teration with JorgePouzo. F. Castillois partiularly
grateful forhishospitalityattheUniversidaddel
Cen-trodelaProviniadeBuenosAires. Thisworkwas
par-tiallysupportedbyDGAPA-UNAMprojetIN105100,
and the International Atomi Energy Ageny, that
made possible the attedane of J.J.E. Herrera to this
meeting.
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