Experimental study of the structure of the plasma current sheath on the PF-1000 facility

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by  Krauz V., Mitrofanov K., Scholz M., Paduch M., Karpinski L., Zielinska E., Kubes P.

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Experimental study of the structure of the plasma-current sheath on the PF-1000 facility
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IOPPUBLISHING PLASMAPHYSICSANDCONTROLLEDFUSION
PlasmaPhys.Control.Fusion54(2012)025010(14pp) doi:10.1088/0741-3335/54/2/025010
Experimental study of the structure of the
plasma-current sheath on the PF-1000
facility
VKrauz1,KMitrofanov2,MScholz3,MPaduch3,LKarpinski3,
EZielinska3 andPKubes4
1NRC‘KurchatovInstitute’,Moscow,Russia
2SRCRFTRINITI,Troitsk,MoscowOblast,Russia
3IPPLM,Warsaw,Poland
4CVUT,Prague,CzechRepublic
E-mail: krauz@nfi.kiae.ru
Received28September2011,infinalform3December2011
Published19January2012
Onlineatstacks.iop.org/PPCF/54/025010
Abstract
Theresultsofstudiesoftheplasma-currentsheathstructureonthePF-1000facilityinthestage
closetotheinstantofpinchformationarepresented. Themeasurementswereperformedusing
variousmodificationsofthecalibratedmagneticprobes. Studiesoftheinfluenceoftheprobe
shapeanddimensionsonthemeasurementsaccuracyweredone. Thecurrentflowinginthe
convergingsheathatadistanceof40mmfromtheaxisofthefacilityelectrodeswasmeasured.
Intheoptimaloperatingmodes,thiscurrentisequaltothetotaldischargecurrent,which
indicatesthehighefficiencyofcurrenttransportationtowardtheaxis. Insuchshotsacompact
high-qualitysheathformswithshockwaveinfrontofthemagneticpiston. Itisshownthatthe
neutronyielddependsonthecurrentcompressedontotheaxis. Thisdependenceagreeswell
withtheknownscaling,Y ∼I4. Theuseofthetotaldischargecurrentinconstructingthe
n
currentscaling,especiallyforfacilitieswithalargestoredenergy,isunjustified.
(Somefiguresmayappearincolouronlyintheonlinejournal)
1. Introduction capacitor bank exceeded a certain critical value, the scaling
in terms of energy was violated and the neutron yield was
It is well known that, in plasma focus (PF) systems, the saturatedatalevelof∼1011–1012 neutrons/shot. Thenature
intensity of ionizing radiation emitted from the pinch region of this effect is one of the key subjects of study in PF
depends strongly on the pinch current I. In particular, systems; in particular, it is of great importance to find out
the experimental neutron scaling based on measurements whethersaturationisrelatedtonaturalphysicalconstraintsor
performed at facilities with a stored energy, W, from several determinedbydisadvantagesofthefacilitydesign[13].
units of kilojoules to a few hundred kilojoules has the form We believe that saturation is not related to any physical
Y ∼ I3.2–5 [1–3]. For this energy range the scaling agrees constraints. At least, on the Z-facility (Sandia National
n
wellwithanotherwell-knownexperimentalscalingY ∼W2. Laboratories),aneutronyieldof∼5×1013D–Dneutronsper
n
Suchdependence,slightlydivergentinpowerfrom2.1to2.5, shotwasachieved, however, ataveryhighdischargecurrent
was found on many devices in the early stages of PF-studies of17MA[14]. Althoughtheabovescalingwasnotsatisfied
[4–6]. In the beginning of the 1970s H Rapp analyzed the inthiscase,theseexperimentsdemonstratedthefundamental
results obtained on more than 10 facilities in the world, and possibilityofobtaininghighneutronyieldsatrealisticcurrents
showed that this scaling has a general character [7]. Later inZ-pinchsystems.
this scaling has been expanded to sub-kJ machines [8,9]. The problem of achieving high discharge currents in PF
However, experiments carried out on megajoule facilities systemsisbeingactivelydiscussed. Inparticular,in[15]itwas
[10–12] demonstrated that, when the energy stored in the assertedthatthefurtherincreaseinthecurrentamplitudecan
0741-3335/12/025010+14$33.00 1 ©2012IOPPublishingLtd PrintedintheUK&theUSA
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
be provided only by increasing the initial discharge voltage, classical ‘shock wave-magnetic piston’ model. In particular,
whichinevitablyleadstoagradualconversionofPFsystems magnetic probe measurements often demonstrate a two-peak
tofastZ-pinchconfigurationsandthelossofoneofthemain PCSstructure(see,e.g.,[25]). Three-fluidMHDsimulations
advantagesofPFsystems,namely,thesimplicityofdesign. In of the Z-pinch PCS [26] have shown that, since the plasma
contrast,calculations,madeintheframeworkoftheLeemodel temperatureand,accordingly,plasmaconductivityinthePCS
[16–18], show that there was no saturation in pinch current intheinitialstageofthedischargearelow,themagneticfield
and Y : they continue to rise with condenser bank capacity diffusesintotheshockwave,whichleadstotheformationof
n
C althoughataslowerrate. Butthesecalculations,madefor a current density peak ahead of the shock front. In [27], it
0
PF-1000 parameters, show also, that, in spite of high power wassupposedthat,inthestageofdischargechambertraining
inneutronscaling(Y ∼ I4.7 andY ∼ I3.9 )[17],theD–D (degassing),asignificantfractionofthecurrentcanflowinthe
n pinch n peak
neutron yield Y ∼ 1013 can be achieved only at W close to shockwaveandthecurrentisredistributedintothemagnetic
n
19MJ[16]. piston only when the facility reaches its standard operating
In this study, we do not consider this problem in detail, mode. In[28],itwasdemonstratedbymeansofsynchronized
because it obviously requires more thorough examination. high-speed photography and magnetic probe measurements
There is another (no less important) problem that manifests that, in the standard operating mode, more than 85% of the
itselfalreadyatcurrentsachievableintheexistingfacilities— currentflowsinthemagneticpiston. Thus,thePCSstructure
the problem of the transportation efficiency of the total depends substantially on the discharge mode. Systematic
discharge current toward the system axis, where the pinch measurements of the PCS structure, however, have not been
forms. The saturation effect can manifest itself if a fraction performedasyet.
ofthetotalPFcurrentflowsbeyondthepinch. Measurements Inthispaper,wepresenttheresultofmeasurementsofthe
of the current distribution performed on the megajoule PF PCScurrentinthestageclosetotheinstantofpinchformation
facility at Frascati showed that a fraction of the total current on the PF-1000 facility [29] operating with deuterium at
flowed in the residual plasma near the PF insulator and did an energy stored in the capacitor bank of 250–500kJ. The
notcontributetotheprocessofpinchcompression[19]. Thus, measurements were performed using absolutely calibrated
despiteinfringementoftheneutronscalingintermsofenergy, magneticprobes. ThePCSstructurewasstudiedusingoriginal
scalingintermsofthecurrentflowinginthesheathcontinued magneto-optical probes simultaneously recording the dB/dt
tobevalid. Itbecameclearthatthecurrentinthesheath(or signal and the plasma optical glow. Along with magneto-
thepinchcurrent)sofardoesnotalwayscoincidewithatotal opticalprobes, aRogowskicoilrecordingthetotaldischarge
dischargecurrent. In[20]thedistinctionofthesecurrentsfor currentandthreemagneticprobeslocatedinsidethePF-1000
variousinstallationsisestimated. Itwasconcludedthat(i)the collector were also used. The main objective of this paper
relevantparameterofneutronproductionisthesheathcurrent wastostudythedependenceoftheneutronyieldonthetotal
I and not total current I and (ii) all effectively operated discharge current and the current flowing in the PCS in the
sh t
facilities at a comparative low level of input energy have the finalstageofradialplasmacompression.
relationI /I closeto1.
sh t
Atthesametimetheresultsreportedin[19]andobtained 2. Designofmagneticprobesandschemeofthe
on very fast high-voltage facility SPEED2 [21] have shown experiment
that this problem becomes more challenging with increasing
discharge energy. The same situation was observed on the Magnetic probe measurements are widely used to study the
PF-3facility(KurchatovInstitute,Moscow),whereabsolutely dynamicsandstructureofthePCSinPFdischarges[19,22–
calibratedmagneticprobeswereusedtomeasurethecurrent 25,27,28,30–35]. Inmoststudies,however,thismethodwas
distribution in PF experiments with noble gases. The results usedtoinvestigatethePCSbehaviorintheearlystageofthe
ofprobemeasurementsshowedthat,inthePF-3experiments discharge in the regions located far from the pinching zone,
carried out with neon, only a fraction of the total discharge because the magnetic probe diagnostics is a contact method
current contributed to the process of pinch compression and questions always arise concerning the influence of the
[22–24]. detectorontheplasmacharacteristicsandaccuracyofmagnetic
Theefficiencyofcurrenttransportationtowardthesystem field measurements. Magnetic probes introduced directly in
axisdependssubstantiallyontheefficiencyofthe‘snowplow’ thepinchformationregionmaydisturbthepinchingprocess.
mechanism. Poor snowplowing of the neutral gas by the Moreover,intenseplasmaandradiationfluxescandestroythe
magnetic piston in the outward expansion and the ‘runaway’ probe during one discharge, which substantially complicates
stages can result in the high density of the residual plasma experiments.
behind the plasma-current sheath (PCS) and, accordingly, Taking into account the aforesaid, modified absolutely
in substantial current leakages. The spatial distribution of calibratedmagneticprobesforinvestigatingthePCSdynamics
the plasma density and current in the final stage of plasma underthePFdischargeconditionsonthePF-1000facilitywere
compression should substantially affect plasma stability and designed and manufactured. The new probes were designed
theprocessesresponsibleformagneticenergydissipationand on the basis of magnetic probes developed earlier for the
radiationgeneration. Therefore, studiesofthePCSstructure experiments carried out on the Angara-5-1 facility [36,37].
are of great importance. The actual current distribution in Indesigningtheseprobes,wetookintoaccountmanyfactors
the PCS can differ substantially from that predicted by the limiting the application of magnetic probes in high-power
2
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
Figure1. Sensitiveelementsof(a)theplaneprobeand(b)themagneto-opticalprobe.
discharges, such as the evaporation of the probe case under The probes were calibrated using a homogeneous ac
the action of radiation and particle fluxes, screening of the magneticfieldwithaknownamplitudeandknownfrequency
probebytheplasma,andplasmaperturbationsarisingdueto by the method described in [37]. The probe sensitivity was
plasmaflowaroundtheprobe[38]. Toimproveconditionsfor ∼(1.0–5.0)×10−11VG−1s−1. Thecalibrationaccuracywas
plasmaflowaroundtheprobeandminimizetheinfluenceofthe betterthan5%,andtheaccuracyindeterminingthemagnetic
probeontheprocessesunderstudy,thesensitiveelementofthe induction in plasma with account of calibration was about
probewasmadeplanarwithatransversesizeof0.7–0.8mm. 15–20%.
Application of such probes on the Angara-5-1 facility for ThePCSstructurewasstudiedusingspeciallydeveloped
measurements of magnetic fields during the implosion of magneto-optical probes. Along with the time derivative of
20mm diameter cylindrical wire arrays demonstrated their the azimuthal magnetic field, these probes also recorded the
efficiency [39,40]. Later on, these probes were successfully plasma optical glow at their location. Similar probes were
adapted to the experimental conditions of the PF-3 plasma earlier used to study the process of shock wave formation
focusfacility[22–24]. during the compression of a transverse magnetic field by a
The probes designed for measurements of the azimuthal supersonic(M > 3)plasmaflow[41,42]. Lateron, sucha
A
magneticfielddistributioninthestageofPCScompressionin probe was used to investigate the PCS structure in the early
the PF-1000 facility consist of two ∼300µm-diameter coils stageofradialplasmacompressioninthePF-3facility[43].
wound in clockwise and counterclockwise directions. The The design of the magneto-optical probe is shown in
design of the sensitive element of the probe is shown in figure 1(b). The shape of the sensitive element of the probe
figure 1(a). The coils are packed in a common plane screen
wasmodified:theplaneheadwasreplacedbya2mmdiameter
madeofaniobium-titaniumfoilwithathicknessof10–15µm,
ceramictube. Twochannelsforrecordingthemagneticfield
which is less than the skin depth for the characteristic times
weresupplementedwitha0.38mmdiameterpolymeroptical
of the processes under study. The time of electromagnetic
fiber placed in the common case with the magnetic coils. A
fielddiffusionthroughsuchascreenwascalculatedinMHD shortsegmentoftheopticalfiber(∼1mm)protrudedfromthe
approximation for supersonic plasma flow with a frozen-in
protectingceramictubeintothedischarge.
magnetic field and for parameters of the Angara 5-1-facility
The second end of the optical fiber was connected to
which is closed to parameters of PF-1000: n = 5×1017–
e a photomultiplier tube (PMT). In processing the recorded
1019cm−3,plasmavelocity106–5×107cms−1,magneticfield
signals, all the cable time delays, the propagation time of
0–100kG,T =1–30eV[37]. Forthetotalsizeofthesensitive
photonsalongtheopticalfiber,andthePMTelectrontransition
element of the probe in the direction perpendicular to the
time(ETT)weretakenintoaccount. Themagneticchannels
incidentplasmaflow0.7–0.8mm,thecalculateddiffusiontime
of these probes were also calibrated by the abovementioned
is∼1.5ns. Twosymmetricsignalsofdifferentpolarityfrom
method[37].
two coils allow one to be sure of their magnetic origin. At
TheexperimentswereperformedonthePF-1000Mather-
the instant of probe destruction, the symmetry of the signals
type facility. The large-scale PF-1000 facility (figure 2)
violates.
consistsofthefollowingmainunits:
Eachprobemeasuredthetimederivativeoftheazimuthal
magnetic field dB (r,ϕ)/dt at its location. To determine • The condenser bank and pulsed electrical power circuit
ϕ
the current flowing within the radius at which the probe was withacollectorandlow-inductancecables.
installed, the signal was integrated numerically under the • The mechanical vacuum and gas system consist of the
assumptionthattheplasmacurrentwassymmetricwithrespect vacuum chamber, coaxial electrodes and gas handling
tothesystemaxis. system.
3
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
the recording system. To prevent high-voltage breakdowns
ontotheprobe,ameasurementschemewasusedinwhichthe
entire ‘probe-oscilloscope’ measurement channel was under
theanodepotential.
In order to monitor the discharge quality, along with
magnetic probe measurements, the electrotechnical and
emissioncharacteristicsofthesystemweremeasuredineach
shotusingstandarddiagnosticsappliedonthePF-1000facility.
Thesediagnosticsareasfollows.
Neutron emission was recorded using four integral
activation detectors. The integral neutron yield in these
experiments was the main parameter characterizing the
discharge quality. In combination with magnetic probe
measurements,itallowsonetoanalyzetheneutronscalingin
termsofthedischargecurrent. FourdetectorsbasedonSTS-5
and STS-6 counters covered with a silver foil and placed in
a neutron moderator were used to increase the measurement
Figure2. GeneralviewofthePF-1000facility.
accuracy. The detectors were calibrated in their standard
positionsinthepreviousPF-1000experiments.
Theelectricalenergyistransferredtoacollectorandelectrodes
For time-resolved measurements of radiation we used
by means of low-inductance cables. The vacuum chamber,
a scintillator detector (SD) based on PMT with a plastic
whichsurroundstheelectrodes,hasalargevolume(1400mm
scintillatorinstalledatadistanceof7mfromthepinch(neutron
indiameterand2500mminlength). Twocoaxialelectrodes
andhardx-rayradiation)andp–i–nsilicondiodecoveredby
are shown on figure 3. The outer electrode (OE) (cathode)
filter10µmBeandinstalledatadistance0.8mfromthepinch
consists of 12 stainless steel rods with 80mm in diameter.
(softx-rayswithenergyabove600eV).
The OE and copper center electrode (CE) radii are 200mm
Theinterelectrodevoltagewasmeasuredbyacapacitive
and115.5mm, respectively, withCElengthof460mm. The
dividerwithasensitivityof4kV/V.
cylindricalaluminainsulatorsitsontheCEandthemainpart
The total discharge current was recorded using a
oftheinsulatorextends85mmalongtheCEintothevacuum
calibratedRogowskicoilwithasensitivityof470kAV−1.
chamber. The insulator prescribes the shape of the initial
The time derivative of the total discharge current was
current sheet between the CE and the back plate of the OE.
measured using three single-loop magnetic detectors located
Thecondenserbankofcapacitance1332µFcanbechargedto
near the facility collector. The detectors were arranged
voltagerangingbetween20and40kV,whichcorrespondedto
with a step of 120◦ along the collector perimeter, which
dischargeenergiesrangingfrom266to1064kJ.
made it possible to estimate the current homogeneity at the
During the described experiment, several shots with
collector.
the PF-1000 upgraded current generator and a new alumina
The PCS dynamics and structure were also investigated
insulatorweremade.
using a 16-channel laser interferometer with a 1ns time
As was mentioned above, a probe located near the
resolutionandatimeintervalof10–20betweenframes[44].
system axis is subject to intense radiation and particle fluxes
and, therefore, is destroyed during one shot. Before each
subsequentdischarge,itisnecessarytoreplacethedestroyed 3. Studyoftheperturbationarisingduetoplasma
probe with a new one. For this purpose, a vacuum lock was flowaroundthemagneticprobe
createdtointroducetheprobeintheaxialregionofthesystem
withoutviolatingvacuumconditionsinthedischargechamber Themagneticprobediagnosticsofplasmaisacontactmethod,
of the facility. This is necessary for the initial discharge which assumes the plasma impact on the probe and vice
conditions(thegaspressure,theconcentrationsofadmixtures versa. In other words, we are dealing with the plasma
intheworkinggas,etc)toremainunchanged,becausethePF perturbationintroducedbytheprobe. Inthiscase,theproblem
discharge is very sensitive to these conditions. The probes offlowaroundaprobebyamagnetizedplasmaisofprimary
were introduced from the collector side through the vacuum importance. The accuracy and time resolution of magnetic
lock along the axis of the hollow anode. The system design probe measurements depend on whether the magnetic field
allowedonetoplacethesensitiveelementoftheprobenearthe has enough time to diffuse from the plasma into the probe
systemaxis,namely,neartheanodesurfaceatradiiof1.2and case. The situation is especially complicated in the case of
4cm and at a distance of 0.5–1.5cm from the surface of the supersonicflowaroundtheprobebyplasmawithafrozen-in
endflangeofthecentralelectrode(anode). Thearrangement magneticfield,accompaniedbytheformationofashockwave.
oftheprobesinthedischargechamberisshowninfigure3. The question naturally arises concerning the characteristic
It should be noted that, in these experiments, the anode scale length and relaxation time of the induced plasma
was at a high voltage. In this case, high-voltage breakdown perturbation.
can occur between the anode and the probe, which can lead Thepapersdevotedtostudyingtheproblemofsupersonic
not only to the probe destruction, but also to a damage of flowaroundbodiesofdifferentshapesbyamagnetizedplasma
4
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
Figure3. GeometryoftheelectrodesofthePF-1000facilityandarrangementofthemagneticprobes: (1)anode,(2)cathode(12rods),
(3)insulator,(4)vacuumlockthoughwhichtheprobeisintroduced,and(5,6)magneticprobesmeasuringtheazimuthalmagneticfieldat
theradiiof12mmand40mm,respectively. Probe(5)isnotshownonthephoto.
in high-current discharges are few in number [38,45,46]. theprobeandmakeanappreciableimpactontheaccuracyof
In our experiments, we used two probe modifications with a themeasurementofthemagneticfieldsandthedegreeofthis
plane(0.5×3mm)caseandacylindricalcase(withadiameter influencestronglydependsontheprobeshape.
of 2.0–2.5mm). The perturbations introduced by the probes The influence of the plasma flow with a frozen-in
werestudiedusinglaserinterferometry. magnetic field on the probe signal for probes of different
Figures 4 and 5 show interferograms obtained in shapes was numerically calculated in [37]. It follows from
dischargeswithaplaneprobe(shotno9120)andacylindrical thesecalculationsthatmagneticfieldperturbationsintroduced
probe (shot no. 9127) for the same initial conditions (D , by a plane probe are about 7%. It was found that the
2
P = 1.8Torr, U = 24kV and W = 384kJ). Figure 4(a) measurementerrorofprobeswithaplanescreenislessthan
0 0 0
corresponds to the time at which the PCS passes the probe. that of probes with a conventional cylindrical screen. For a
Afterpassingtheplaneprobe,thePCSperturbationoccupiesa cylindricalprobe,1mmindiameter,thiserrorisalready25%.
narrowregion∼1mmwidthalongtheradius(figure4(b)),and This result was confirmed experimentally on the Angara-5-1
40nslater(figure4(c)),theperturbationcompletelyvanishes. facility[37].
Theplasmaperturbationintroducedbyacylindricalprobe It is natural to assume that the error increases with the
is wider, ∼4mm, which is about 1.6 of the probe size (see increase in the probe size. We have undertaken comparative
figure5). AlthoughthePCSperturbationinthiscaseislarger studiesofprobesofadifferentconfigurationsonthePF-1000
than that introduced by the plane probe, it rapidly decays as facility. In figure 7 the result of current measurements with
the PCS propagates toward the system axis. An even larger thedifferentprobesobtainedinthedischargeswithpractically
PCSperturbationisobservedinexperimentsperformedwith thesamevalueofthetotaldischargecurrentandtheneutron
a cylindrical probe at a higher gas pressure (P = 2.4Torr, output is shown. One can see that the current measured
0
figure 6). The size of the perturbed region behind the PCS with the cylindrical probe is essentially less. The typical
just after it has passed the probe is ∼6mm (figure 6(a)), valueofcurrentamplitudeunderstatingunderourconditions
which is about 2.5 of the initial probe size. After 30ns, makes ∼1.75. The measurement error decreases during the
the size of the perturbed region behind the PCS decreases to pulseandatthecertaininstant,theindicationsoftwoprobes
∼2.5mm(figure6(b))and,afteranother40ns,theperturbation start to coincide. It is caused by the gradual disintegration
practicallyvanishes(figure6(c)). Inthiscase,anunperturbed of the plasma surrounding a probe, and shielding reduction.
pinchisobservedontheaxis(seefigure6(d)). It should be noted that, in similar experiments on the FF-3
Itshouldbenotedthattheradialprobesizewithaccount facility[43],suchastronginfluenceoftheprobedimensionon
oftheprojectionontotheobservationdirectionis∼3.5–4mm, theaccuracyofmeasurementswasnotrevealed. Apparently,
which is somewhat larger than the initial probe size. This it is caused by the strong influence of the parameters of the
can be related to the residual PCS plasma and the plasma plasmasurroundingtheprobe:themeasurementsonthePF-3
formed on the probe surface under the action of radiation in weredoneonthelargedistancesfromtheaxisunderconditions
thefinalstageofplasmacompression. Thisplasmacanshield oflessdenseandlesshotplasma.
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PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
Figure4. PCSinterferogramsrecordednearthesystemaxisinshotno9120(D ,P =1.8Torr,U =24kVandW =384kJ)withplane
2 0 0 0
probeatdifferentinstantsfromthebeginningofthedischarge:(a)5824,(b)5844and(c)5884ns. TheneutronyieldisY ∼1.3×1011
n
neutrons/shot.
Thefollowingimportantconclusionscanbedrawnfrom sessionswithdifferentinitialconditions. Takingintoaccount
theseresults: that the number of shots was limited by the number of
available probes, the first experiments were performed in a
(i) In the case of a plane probe, the PCS perturbation is
well-reproduciblemodewithdeuteriumataninitialpressure
much smaller than in the case of a cylindrical probe.
of P = 3Torr and a discharge voltage of U = 27kV.
It is necessary to take into account at performing the 0 0
In these experiments, the stored energy was about 485kJ.
quantitativemeasurements.
Beforeeachshotwiththemagneticprobe,aseriesoftraining
(ii) In spite of a relatively large plasma perturbation
discharges under standard conditions were performed until
introducedbyacylindricalprobe,itrapidlyrelaxesasthe
the neutron yield became higher than 1010 neutrons/shot.
PCS propagates toward the axis, so that no perturbation
Since the probes were destroyed in each shot, the working
introducedbytheprobeisobservedneartheaxis.
gas in the chamber was usually refreshed before the next
(iii) Theprobeparametersdonotrenderanessentialinfluence
experiment.
onitstimecharacteristics. Itallowsusingtheprobesofthe
Figure 8 shows the results of measurements with a
cylindricalform,forexample,themagneto-opticalprobes
magneticprobelocatedatadistanceof4cmfromthesystem
describedabove,forstudiesofthePCSstructure.
axisinadischargewithahighneutronyield. Inthisdischarge,
theneutronyieldwas1.24×1011neutrons/shot,whichisone
4. Experimentalresults
ofthebestresultsobtainedinthisexperimentalsession. Thus,
itcanbeassertedthat,inthegivenexperimentalscheme,the
4.1. Studyoftheazimuthalmagneticfield
probelocatedinthePCScompressionregionatadistanceof
The dynamics of the azimuthal magnetic field during PCS 4cmfromtheaxisinsignificantlyaffectsthepinchingprocess
radial compression was investigated in four experimental and,accordingly,theneutronyield. Itfollowsfromfigure8(a)
6
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
Figure5. PCSinterferogramsrecordednearthesystemaxisinshotno9127(D ,P =1.8Torr,U =24kVandW =384kJ)with
2 0 0 0
cylindricalprobeatdifferentinstantsfromthebeginningofthedischarge: (a)5793and(b)5913ns. TheneutronyieldisY ∼1.7×1011
n
neutrons/shot.
thatthesignalfromthemagneticprobeappearsabout6.2µs A more abrupt drop in the signal from the Rogowski coil at
after the beginning of the discharge. As a rule, it has one or the instant of dip (a peak in the time derivative of the total
twomaxima,whichindicatesthatthecurrentdistributioninthe current) can be attributed to the partial differentiation of this
PCSisrathercomplicated. Inparticular,inthisshottwopeaks signal.
were recorded, the full width at half-maximum (FWHM) of Undertheconditionsinwhichthetotaldischargecurrent
eachpeakbeing∼35–40ns. Aprecursorintheformofasmall- varies insignificantly while the PCS passes the probe, the
amplitudepulsebeforethemainsignalwasalsoobserved. The signalmaximumcorrespondstotheinstantatwhichtheentire
presenceofsuchaprecursor, whichcancarry∼5–7%ofthe PCS current begins to flow inside the 4cm radius region.
total PCS current, is typical of many our discharges and its After this, until the probe is destroyed, the behavior of the
originisunclear. currentmeasuredbytheprobecoincideswiththatofthetotal
The signals from two different probe coils remain current,i.e.themeasuredcurrentdecreasesinaccordancewith
practically symmetric over ∼200ns, which confirms their the reduction in the total discharge current. An important
magnetic nature. Later on, the symmetry of these signals is consequence of this result is that the discharge current is
abruptlyviolated(seetheverticaldashedlineinfigure8(a)), efficientlycompressedtowardtheaxis(atleasttoaradiusof
which indicates that at least one coil is destroyed. After this 4cm),whichresultsinthehighvalueofneutronyieldinthis
instant, the signals were not processed. However, even this discharge.
timeintervalisquitesufficienttodeterminethecurrentflowing At the instant of dip (at the peak of the total current
inthePCS.Thiscurrentwasrecoveredundertheassumption derivative), an abrupt increase in the voltage up to 50kV is
thatthePCSwasaxisymmetric. observedinthevoltagewaveform.
Itcanbeseenthat,over∼100ns,thePCScurrentreaches Figure 9 shows the results of similar measurements
itsmaximumvalueof∼1.7MA,whichcorresponds,towithin performedwiththeuseoftwomagneticprobes,oneofwhich
measurement errors, to the total discharge current measured wasinstalledatadistanceof12mmfromtheaxis.
by the Rogowski coil or recovered using the signals from Thesignalfromthemagneticprobelocatedattheradius
the loop detectors measuring the time derivative of the total of40mmalsoappearsabout6.2µsafterthebeginningofthe
current. A certain difference in the signals from of the discharge, whereas the signal from the probe located at the
loop detectors can be caused by the inhomogeneous current radius of 12mm appears 130ns later. From the time delay
distribution at the collector, because the loop detectors are between these signals, we find that the average PCS radial
arranged with a step of 120◦ along the collector perimeter. velocitybetweentheseprobesis≈2.1×107cms−1. Forthis
7
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
Figure6. PCSinterferogramsrecordednearthesystemaxisinshotno9131(D ,P =2.4Torr,U =24kVandW =384kJ)with
2 0 0 0
cylindricalprobeatdifferentinstantsfromthebeginningofthedischarge:(a)6479,(b)6509,(c)6549and(d)6569ns. Theneutronyieldis
Y ∼8.7×1010neutrons/shot.
n
velocity,theskindepthestimatedfromtheformula which resulted in a reduction in the neutron yield. In this
(cid:1) (cid:2) discharge, the discharge current was also almost completely
V
I˙≈I × r compressed to the radius of 4cm, but the neutron yield was
δ
skin about one-half order of magnitude lower than in the above
is δ ≈ 1.6cm. Under the assumption of the Spitzer discharge. Unfortunately, theprobeinstalledattheradiusof
skin
conductivity, this value corresponds to the plasma electron 12mmwasdestroyedverysoon;therefore,itwasimpossible
temperature of T ≈ 50eV. Discharge current somewhat tomeasurethecurrentamplitudeinthisregion. Itisnaturalto
e
increases (to 1.9MA in our case); however, the fraction of assumethattheprobeinstalledsoclosetotheaxiscouldaffect
the current compressed to the axis decreases substantially, the process of pinch formation. This conclusion, however,
8
PlasmaPhys.Control.Fusion54(2012)025010 VKrauzetal
2
1
1.5
A
2
M 1
I,
0.5
0
6200 6400 6600 6800 7000 7200 7400
t,ns
Figure7. Totaldischargecurrent(1)andcurrentrecoveredfromthe
probemeasurement(2)obtainedunderthesameinitialconditions
(D ,P =1.8Torr,U =24kVandW =384kJ)bytheplane
2 0 0 0
probe(solidline,shot9341,Y =6.9×1010n/pulse)andbythe
n
cylindricalprobe(dashedline,shot9340,Y =9.8×1010n/pulse).
n
Probelocationis4cmfromthedischargesystemaxis.
can occur too hastily. First, earlier experiments carried out
on the PF-3 facility with the use of x-ray pinhole cameras
and frame cameras [22,24,43] demonstrated that a similar
probe located at a distance of 2cm from the system axis
practicallydidnotaffecttheprocessofpinchformation. On
theotherhand,thelowerneutronyieldinthisdischargecanbe
explainedbyotherfactors,inparticular,alowerPCScurrent
(∼1.5MA) and a broader PCS with a complicated structure.
The neutron yield measured in this discharge lies within the
admissiblescatterobservedinaseriesofreferenceexperiments
performed without magnetic probes. Nevertheless, taking
into account fast destruction of the probe located so close
to the axis and, accordingly, the impossibility of obtaining
reliableinformationonthePCSparametersatthisradius,we
abandonedtheapplicationofsuchprobesatthisposition.
Another experimental session was performed after the
power supply of the PF-1000 facility was upgraded. This
made it possible to attain discharge currents of 1.5–1.9MA
atalowerbankvoltageof20–24kV,whichcorrespondstoa
lowerdischargeenergy,W =260–380kJ.However,although
the energy deposited in the discharge was lower than in the
previousseriesofexperiments,theneutronyieldremainedat
thesamelevelof1010–1011 neutrons/shot. Thisindicatesthe
conditionalcharacteroftheenergyscalingandtheimportance
ofthecurrentscaling.
This fact was clearly demonstrated in our experiments.
In particular, we performed measurements in the stage of
dischargesystemtraining,duringwhichthedischargevoltage
wasgraduallyincreased. Inthecourseoftraining,theneutron
yield gradually increased with increasing discharge voltage
from20to23kV(figures10(a)and(b)). However,whenthe
voltageincreasedto24kV,theneutronyielddegradedabruptly. Figure8. Resultsofthemeasurementoftheazimuthalmagnetic
fieldinshotno8220(D ,P =3Torr,U =27kVand
Probemeasurementsshowedthat,inthiscase,theefficiencyof 2 0
W =485kJ):(a)currenttimederivativesmeasuredbythetwocoils
currenttransportationtowardtheaxisdecreasedsubstantially.
oftheprobe(theverticaldashedlineshowstheinstantofprobe
The probe located at the radius of 40mm recorded only destruction)and(b)currentmeasuredbythemagneticprobe(I ),
p
aboutone-halfofthetotalcurrentmeasuredbytheRogowski totaldischargecurrentmeasuredbytheintegratingRogowskicoil
coil (figure 10(c)). It should be noted that, along with the (IR),totaldischargecurrentmeasuredbytheloopdetectorsatthe
collector(I ),timederivativeofthetotaldischargecurrent
upgrade of the power supply, the insulator of the discharge loop
(dI/dt) andvoltage(U). TheprobeisatR=40mmand
systemwasalsoreplaced. Itiswellknownthat,inthiscase, Z=10tmm. Theneutronyieldis1.24×1011neutrons/shot.
itisnecessarytograduallytrainthedischargesysteminorder
9