Regular density structures in the plasma focus

Volume 88A, number8 PHYSICSLETTERS 5 April 1982

REGULAR DENSITY STRUCTURES IN THE PLASMA FOCUS

R. HAAS, H. KROMPHOLZ, L. MICHEL, F. Rt)TIL, K. SCHONBACH and G. HERZIGERInstitut fur AngewandtePhysik,TechnischeHochschuleDarmstadt,Fed. Rep.Germany

Received17 July 1980Revisedmanuscriptreceived18 December1981

Densitymodulationsin thecompressionand pinch phaseof theplasmafocusobservedby meansof theSchlierentechniquepoint to strongturbulenceinitiated by microinstabiities.

Optical studiesof the plasmafocus [1] in thecom-pressionandpinchphasehavebeenperformedusingSchlierentechniques[2]. A dyelaserbeamwithapulsewidth of 350nsat a powerlevel of 25 kW isexpandedby a telescope.After passingthroughthefocuschamberthebeamis refocusedon a 1 mm diam-eterdiskwhichis determiningtheminimumangleof deviationto 0min = 3 mrad. Thelaserradiationscatteredby thefocusplasmais photographedthroughan imageconverterdiodewith an exposuretime of 5ns. Theplasmaradiationis suppressedby meansof an

interferencefilter.Fig. 1 showsa Schlierenimageof thecollapsing

plasmasheet5 nsbeforepinchtime whichis charac-terizedby the emissionof hardX-rays with pulse Fig. 1. Schlierenimagerecordedin the collapsephase.The

widthsof less than S ns [3]. The hollow centerelec- horizontalbar indicates1 mm.

trodeisvisible on theleft handside. In theaxial direc-tion the sphericalcollapsingplasmalayerisjoined byaplaneshockwave,whichwaslaunchedby the axially Fig. 2 showsSchilerenimagesrecordedat pinchmovingplasmalayerin therundownphase.The dimen- time. Distinctaxial symmetricpatternsare observedsionsin directionof motion — radialfor thecollapsing which areextendinginto thehollow centerelectrodelayer, axial for the planeshockwave — are determined (left handside of fig. 2).The patternwith spotdimen-mainlyby the exposuretime. Fora radialvelocity of sionof the order of 100pm is slightly deformedfor2 X i07 cm/sandan exposuretime of 5 nsthe mo- different shots.Its regularityis correlatedto the qualitytionaldisplacementis about0.1 cm.Whereasthe of the dischargewhich ismeasuredin termsof hardaxial shockwave is homogeneous,theradially moving X-ray intensity.Thesharpnessof the spotcontoursplasmashowsa distinct axial densitypattern.Periodical is pointing on highly localizedeventsat timessmalldensitystructureswith periodsdownto 25 pmare comparedto theexposuretime.observedcloseto the axis.Superimposedthereis a Thelongitudinal periodicalstructurein thecol-ratherirreproduciblegrossstructurewith periodsof lapsingplasmalayer(fig. 1) is dueto microinstabiities.about0.25 cm. Theoreticalinvestigationsof the collapsingplasmaare

0 031-9163/82/0000—0000/$02.75© 1982 North-Holland 403

Volume88A, number8 PHYSICSLETTERS 5 April 1982

to a wavelengthof 6 pm alsoconfirmthe existenceof microinstabilitiesin the collapsingfocusplasma.The grossmodulationmay be relatedto thedevelop-ment of m = 0 instabilities,which havebeenobserved

Theregulardensitypatternin the pinchplasmais~ at comparabledistances.

pointingon strong turbulenceeventsin the final stageof the focusdevelopment.Similar structuresare ex-pectedin plasmawith strongLangmuirturbulence[7].Obviously a self-organizationof plasmastructuresisdevelopingdue to thehigh powerdensityin the focusplasmawhich is far from thermalequilibrium.The in-teractionof axially movingrelativistic electrons[81with regularly arrangeddensitymodulationsmay ex-plain the occurrenceof highly collimatedsoft X-raysfrom theplasmafocusL91.

______ -~ This researchwassupportedby theDeutsche~ .~ Forschungsgemeinschaft.

~ References

[1] L. Michel,K.H. SchdnbachandH. Fischer,Appl. Phys.Lett. 24 (1974)57.

[2] J.W.Mather,Proc. Conf. on PlasmaPhysicsandContr.NuclearFusionRes.(Cuiham, 1965)CN-21/80,Vol. 2,p. 389.

Fig. 2. Schlierenimagesrecordedin the pinchphase.(a) [3] H. Krompholz,W. Neff, K. SchdnbachandG. Herziger,Focusdischargewith low hardX-ray intensity;(b) focusdis- Phys.Lett. 76A (1980)388.chargewith hardX-ray intensityoneorderof magnitudehigh- [4] R. DeutschandH.J. Kaeppeler,Microinstabilities in aer than(a).Thehorizontalbar indicates1 mm. Radially ContractingInhomogeneousCylindrical Plasma

Slab,Institut für Plasmaforschung,UniversitàtStuttgart,Bericht IPF-80-9(1980).

[5] G. Herziger,H. Krompholz,L. Micheland K. Schonbach,pointingon thelowerhybrid drift instability aspoten- Phys.Lett. 64A (1977) 51.

tial origin for theperiodicalmodulation[41. [6] A. Bernard,J.P.Carconnet,A. Jolas,J.P.Le BretonandWavelengthscomputedfor the lowerhybrid drift J. deMascureau,Proc.SeventhConf. on PlasmaPhysics

instability with plasmaparametersT~ 100 eV, mag- andContr. NuclearFusionRes. (Innsbruck,1978) IAEA-neticinduction 20 T andwavelengthsinferred [7] Yu. S. Sigor andV.E. Zakharov,J. dePhys.40

from observedmicrowaveemissionfrequencies[5] (1979) C7-63.

agreewell with valuesobservedby Schlierenmethods. [8] G. Herziger,H. Krompholz, L. Michel, D. SchleiferöckEnhancedlight scatteringlevels [6] at a k-vectorof andK. Schdnbach,Phys.Lett. 69A (1978) 37.

1.1 X i04 cm1 parallelto thefocusaxisaccording [9] G. Herziger,H. Krompholz,L. Michel andK. Schonbach,

Phys.Lett. 64A (1978)390.

404