2
Volume 88A, number 8 PHYSICS LETTERS 5 April 1982 REGULAR DENSITY STRUCTURES IN THE PLASMA FOCUS R. HAAS, H. KROMPHOLZ, L. MICHEL, F. Rt)TIL, K. SCHONBACH and G. HERZIGER Institut fur Angewandte Physik, Technische Hochschule Darmstadt, Fed. Rep. Germany Received 17 July 1980 Revised manuscript received 18 December 1981 Density modulations in the compression and pinch phase of the plasma focus observed by means of the Schlieren technique point to strong turbulence initiated by microinstabiities. Optical studies of the plasma focus [1] in the com- pression and pinch phase have been performed using Schlieren techniques [2]. A dye laser beam with a pulse width of 350 ns at a power level of 25 kW is expanded by a telescope. After passing through the focus chamber the beam is refocused on a 1 mm diam- eter disk which is determining the minimum angle of deviation to 0min = 3 mrad. The laser radiation scattered by the focus plasma is photographed through an image converter diode with an exposure time of 5 ns. The plasma radiation is suppressed by means of an interference filter. Fig. 1 shows a Schlieren image of the collapsing plasma sheet 5 ns before pinch time which is charac- terized by the emission of hard X-rays with pulse Fig. 1. Schlieren image recorded in the collapse phase. The widths of less than S ns [3]. The hollow center elec- horizontal bar indicates 1 mm. trode is visible on the left hand side. In the axial direc- tion the spherical collapsing plasma layer isjoined by a plane shock wave, which was launched by the axially Fig. 2 shows Schileren images recorded at pinch moving plasma layer in the rundown phase. The dimen- time. Distinct axial symmetric patterns are observed sions in direction of motion radial for the collapsing which are extending into the hollow center electrode layer, axial for the plane shock wave are determined (left hand side of fig. 2). The pattern with spot dimen- mainly by the exposure time. For a radial velocity of sion of the order of 100 pm is slightly deformed for 2 X i07 cm/s and an exposure time of 5 ns the mo- different shots. Its regularity is correlated to the quality tional displacement is about 0.1 cm. Whereas the of the discharge which is measured in terms of hard axial shock wave is homogeneous, the radially moving X-ray intensity. The sharpness of the spot contours plasma shows a distinct axial density pattern. Periodical is pointing on highly localized events at times small density structures with periods down to 25 pm are compared to the exposure time. observed close to the axis. Superimposed there is a The longitudinal periodical structure in the col- rather irreproducible gross structure with periods of lapsing plasma layer (fig. 1) is due to microinstabiities. about 0.25 cm. Theoretical investigations of the collapsing plasma are 0 031-9163/82/0000—0000/$02.75 © 1982 North-Holland 403

Regular density structures in the plasma focus

  • Upload
    r-haas

  • View
    214

  • Download
    1

Embed Size (px)

Citation preview

Page 1: 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

Page 2: Regular density structures in the plasma focus

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