Lengyel, L.

Rozhansky, V.

Veselova, I.

English

MPG.PuRe

1 549Drift motion in the scrape—off layer during hard disruptionsL.L. Lengyel“, V.A. Rozhansky“, I.Yu, Veselova”*Max-Planck-Institut fur Plasmaphysik, Euroatorn Association, D-85748 Garching, Germany**St,Petersburg State Technical University, StPetersburg, Russia1. IntroductionDuring hard disruptions, intense vaporization of the divertor plates takes place. The process isaccompanied by the formation of a dense and partially ionized vapor layer over the plates,which significantly reduces the particle and heat fluxes reaching the divertor tiles and thusdetermines the evaporation rate of the surface The energetic plasma particles moving along theskewed magnetic field lines are stopped inside the layer at different depths, Hence, in order toproduce a return current that compensates the current carried by the plasma particles, a self»consistent electric field is generated {ambipolarity constraint). Since the temperature of thevapor plasma is of the order of some eV—s, the electrical conductivity is rather low and theresulting electric field is of considerable strength, In a tokarnak with conducting divertor plates,the local electric field has to be normal to the plates, thus creating plasma drifi across the fluxsurfaces. This electric field and the resulting drift motion are modelled in a self—consistentmanner. In the code applied, the deceleration of hot particles of diiferent energy groups iscalculated, while the cold layer is described by means of resistive MID) equations by takinginto account all important elementary processes taking place in the plasma [1]. In the present1-D model, the electric field is calculated from the condition of zero net current through theplates. The analysis shows that drift velocities of the order of 103 to 10‘ m/s may be expected,which may notably impair the shielding characteristics of vapor layers and may thus increasethe erosion rate of the divertor plates. Under analogous conditions, this type of drift maybecome relevant also for detached divertor plasmas in normal tokamak regimes.2.ModelThe 1D geometry is considered, Fig. 1, 6/ awe/ex :0, BX is assumed to be zero. Thecurrent of hot particles eFi(y)-el'e(y) is parallel to fl, and its projection on y direction is e(l";-F.)sinct. It is calculated by tracing the depletion of particle groups of difi‘erent energies [l]. Thepotential drop between the cold and the hot plasmas [2], [3] is also taken into account.The current has to be balanced by the return current created by the self-consistent electricfield E. The return current jy is the sum ofthe projections of the B-parallel current jttsinct andthe B—perpendicular current jLoosen. The parallel current is primarily the electron current driven1550by the parallel projection of electric field EH=Eysinotr The perpendicular current j L is associatedwith El. The condition of vanishing not current in the y-direction throughout the vapor layercan be written assintt+jlcoscx+e(l"i—I}) sinot=0. (1)It differs from the corresponding condition discussed in [3] by the second term. Thecomponent j“ is determined from the parallel projection of the electron momentum balanceequationjn = 0'H sin ot(Ey +eTaaln ne lay +gTeal"a /3y), (2)where on is the electrical conductivity which depends both on Coulomb and electron-neutralcollisions [3], the coefiicient ET is larger than unity due to the thermal force, From theperpendicular component of the electron momentum balance equation (neglecting theperpendicular thermal force), we have1 will x§1=slrfil +[Vx§]+:;p‘), (3)where the Hall parameter flccve; a)“ is the electron cyclotron frequency, V5 is the electroncollision frequency, and ‘7 is the ion velocity, ol=constoHi Due to high collisionality, the ionand neutral gas velocities are assumed to be equalAnother relation between the perpendicular current and plasma velocity is given by the x-component of the momentum equation Vm+ 6(pVn) :at 6yThe velocity component Vy is calculated fi'cm the y component of the momentum equation by[it xfaii (4)solving simultaneously the complete set of resistive MHD conservation equations, thusdetermining the evolution of the vapor layer. The electric field component B" is assumed to bezero,3. Results of calculationsA set of typical results is presented for a scenario in which a carbon (or carbonized) divertorplate is exposed to a thermal disrupting plasma at t=0. The follovving input parameters wereassumed: Teo=5keV, nw=5.5x10”m‘3, B=6T, 0L=5°, A cold and dense shielding layer isevolving at the surface, The temperature (electron temperature being equal to the heavyparticle temperature) and the density distributions, monitored at t = 50ps , are shown in Figs 21551and 3. At this time instant, ionization degrees significantly less than unity only exist in a narrowvapor layer adjacent to the surface (y less than 2 cm} in which the neutral atom density is oftheorder of loum‘a. The corresponding potential distribution is shown in Fig. 4. The divertor plateas well as the vapor layer are biased negatively with respect to the rest of the SOL. Thepotential difl‘erence is of the order of the energy of the incident hot electrons, In Fig, 5, thecalculated velocity component V1. is displayed for three difi'erent time instants. As can be seen,large lateral velocities may exist. Outside of the weakly ionized region the value of Vxpractically coincides with the drift velocity Vm=ElfBeEy/B. In another scenario calculationswith Tso=lkeV, nw=6x1019m‘3, which correspond to larger Hall parameter values [3,, lateralvelocities of the same order of magnitude were obtained. Within the ionized part of the vaporlayer, the values of Vx are found to be close to the values of Van-n,4. ConclusionsDrift velocities of the order of 103 to 104 rn/s may he expected during hard disruptions. Thisefl‘ect may notably impair the shielding characteristics of the vapor layer and increase theerosion rate of the divertor plates.AcknowledgmentThe study was supported in the framework of the Geman-Russlan Scientific TechnicalCooperation in the Field of Peaceful Use of Atomic Energy, Project No, RUS-554-96; by theRFFI, Grant No. 96-02-16919.References. .1. Lulousis, P. et al. in Controlled Fusion and Plasma Physics (Proc. 22nd Eur. Conf.Bournemouth, 1995), Vol. 19C, Part 11, European Physical Society, Geneva (1995) 285,2. Lengyel, L.L., Rozhansky, VA, Veselova, I. YU., Nucl. Fusion 36, No, 12 (1996).3. Rozhansky, V.A., Lengyel, LL, Calculation of Electric Fields in Vapor Shields Evolving atAblating Surface, Tech. Rep. IPP 5/53, Max-Planck Institut rm Plasmaphysik, Garching(1993). .Fig. 1 The geometry cans! dered1552nono‘amo‘ .gumu‘ .Lono‘ -‘100.00 0.05 0103’0")015 0.20VI(1115)0.000 0001 win: winyou)0.004 0.005Fig.5 5 ktfimpflngplama: distributions ofV,(solid lines) and and EJ/B (dotted lines) in thevapour layerFig.2 5 keV disrupting plasma: temperature distributionin the vapour layer 50 m afierplasmanwll canmm‘.a)t=3,us,@0110” -mm)” -I 2.0m“ - -mum” -. l . 1 I v n«m 0.05 (no 1115 um ‘fioou was 0.1110 mm «on an:Hm) Hm)Fig.3 SkeVdimpting plasma: density dlsm‘buflon m b) I = 10 ,m.the vapour layer 50 ,w afler [alumna-wall contact.ow v ‘amo’8 40nd '9 4mm?40m?mo (insyou)5.0110) am [105Fig.4 5 kei/dlsrupflng plasma: potential distributionacross the vapour layer. (=50 p.5-

Drift Motion in the Scrape-off Layer during Hard Disruption

http://hdl.handle.net/21.11116/0000-000A-7E20-5

Drift Motion in the Scrape-off Layer during Hard Disruption

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