Büchl, K.

Dux, R.

Fuchs, J.

Gruber, O.

Haas, G.

Kallenbach, A.

Kaufmann, M.

Lang, P.

Lang, R.

Mast, K.

Mertens, V.

Müller, H.

Neu, R.

Neuhauser, J.

Ryter, F.

Salzmann, H.

Schweinzer, J.

Suttrop, W.

Weinlich, M.

Zohm, H.

English

MPG.PuRe

a008(0P6)High Density OperationIn Auxiliary Heated ASDEX Upgrade DischargesV Mertcns, P T Lang, K Buchl, R Dux, Ch Fuchs, O GruherI G Haas, A Kallenbach,M Kaufmann, R Lang. F Mast, H W Muller, R Neu, J Neuhauser, F Ryter, H Salzmann,J Schweinzer, W Suttrop, M Weinlich, H Zohm, ASDEX Upgrade & NBl TeamsMax-Planck-Institut fiir Plasmaphysik, EURATOM Association,85748 Garching, Fed. Rep. of GermanyIntroduction :The highest achievable density in a tokamak is restricted by the normally disruptivedensity limit DL. This limit is quite successfully described, even on machines of differentsizes, by the empirical ’Greenwald‘ limit fifw = r: < j > (Ii is the plasma elongationand <1 > the area averaged plasma current density) [1]. Since the ITER (EDA) conceptaspires an operation density significantly beyond this limit (2 1.5 fifw [2]), one needsreliable scenarios to overcome it without deterioration of the energy confinement time T5.At large heating powers, however, the particle fuelling efficiency of gas injection degradessuch that high densities are difficult to achieve. Furthermore, the corresponding highneutral particle densities cause normally the backtransition of the favourable H—mode toan li-mode, yielding divertor detachment and probably a ‘classic' DL. Presently, the onlymethode to achieve long lasting phases with fit > fifw seems to be the repetitive injectionof pellets.This paper deals mainly with the characteristics of the L—mode phase preceeding everydensity limit disruption including impurity gas injection forced L-modes. The latter isfound to prevent effectively detrimental divertor heat load.Discharge Parameters :Our investigations concentrate on lower single null discharges (R = L65 m. a = 0.5 m,N ~ 1.6) with plasma currents between 0.6 MA and 1.? MA and N81 heating powers upto [0 MW. The corresponding safety factors (195 vary between 2.5 and 5. The plasmafacing vessel components are boronized. The ion VB drift is directed towards the targetplates. A centrifuge enables to inject strings of pellets with velocities up to 1.2 km/s andrepetition rates up to 80 Hz (representing a maximum fuelling rate. of CD ~ 3 - 1022 l)atoms/s) The working gas is mostly deuterium but in a smaller amount of dischargesalso hydrogen. Highly radiative discharges are performed by injecting Ne, Ar or Nitrogengas into the main chaimher.Experimental Observations :We first describe the typical sequence from reaching very high line averaged densitiesfinally ending in a UL disruption. At the applied high heating power levels PM,“ thedischarges stay normally in the ELM‘y H-modc. During this mode the Greenwald limitcould so far not be surpassed only by applying excessive gas pulling ((P S 3 - 1022D atoms/s) as can be seen on Fig. 1 a) summarizing the highest achieved 7‘1! in variousregimes.lf the separatrix electron temperature in the ll-mode decreases below z 130 eV ac-cording to the strong particle influx and radiation cooling the discharge falls back into the15a008(0P6)16L—modc [3]. Owing to the reduced particle confinement of the L-mode the line averageddensity decreases transiently until the density feedback control system rises the externalgas flux and fig grows again. Thereupon the divertor detaches, visible e.g. by the strongdrop of the ion saturation current measured by langmuir probes mounted in the targetplates. After the divertor electron temperature falls to a few eV a small Marfe formsclose to the active X-point observable in a slight enhancement of the bolometer signalsmeasuring the X—point region. If 7'1. raises further, the divertor throats become so coldthat the radiating zone shifts up to the X-point and the Marfe starts to expand smoothlyinto the region of closed flux surfaces. This leads to a concentration of the boundarydensity into the X-point region observed by Li-beam and Thomson scattering diagnosticsmeasuring close to the midplane and near the X—point, respectively. Correspondingly, theelectron temperature close to the X-point reduces strongly from values of up to 100 eV toa few cV. An analysis of the. electron pressure at the outside midplane boundary and infront of the divertor plates shows a clear pressure drop as signature of detachment. Thesmooth Marfe expansion is an unambiguous precursor of the local instability of the Marfeand hence of the DL. There is a large density operational window found with detachedplasmas between qgs values of z 3 and 4.. n . , , . . . . ..— 15 a) _ 1.4 - b) _E - .IE 1 1 ,. .. 7 'ji .................< i a A‘E u - u <'- QT‘L 9 5'5‘1‘ A"i""~ 0.8- o Jc: 0-5 ' {a mud. ‘. ' _ .'\7‘ ,r : IA Palln:ll.L : A mJ‘ x \: Sf,“ .' 0.2 - n 0’'. .‘ 0 I“.0 J I I 77777 I. I l | 4 lo 0.5 l 1.5 2 o 2 4 5 a 10fi,R/BT[10’“m'7T'] PM [MW]Figure l: a) shows the Grecnwnld plol indicating the typical ASDEX Upgrade densityoperating space. The dark area represents the region offcedbnck controlled pellet fuelledplasmas in H— as well as in L-mode. The while dashed surrounded area in Fig. [1) corres-ponds lo (he fie achieved up to now. Technical reasons prevent. the furlher increase affil.HRL denotes highly [adiuling L-mades.To obtain quickly and stationary high densities, we combined moderate gas puffingand repetitive pellet injection (with pellet injection alone F1, could not be successfullyincreased). Each pellet causes a quick (< 1 ms) raise in density, followed up by a slowdecay of the density increment. In parallel, each pellet triggers an ELM expelling a partof the injected fuel [4]. The magnitude of the pellet induced increment is governed bythe pellet fuelling efficiency 6,. As during the injection sequence plasma cooling occurs(see Fig. 2 a), the pellets penetrate deeper into the plasma starting with roughly halfminor radius ending up to approximately central deposition. Concomittantly, the fuellingefficiencies rise markedly. It has been shown that c, is a. clear function of the penetrationdepth ranging between 30 % and 40 % in lrmode plasmas and dropping down to z 0 %a008(0P6)in ELM’y H—mode phases for shallow penetration.As a special tool to maintain long lasting high density phases we performed a controlcircuit using a bremsstrahlung signal as a measure of the line averaged density and inhibitthe injection of pellets when the preprogrammed fin is reached. Using this setup stationaryphases (>> 75) of up to 1.5 fif’w have been achieved as demonstrated in Fig. 2 a). Inthis example a smooth H —> L transition occurs at about 1.9 s.1 l5 ‘3'm r a) -------n"' ‘WW 1 TE ___________________ _m — , a; o"'0 i n: TIETIDIIID //. capo/3° «um / oO 06 . Net” 0 3/00s /o,/o// 000.2 rb) a [tonal-’1i . i i . .0.4 0.6 0.3 11 'TE ._____.._..._0......./ 113(0) 2 _: TIEDDIll-D o 1/ a)l " £3” 00.6 —.7— J‘O— — “a / ' 43)Ill 0 I . ’01 - /' o o r0‘2 {6 TI;L\\ ‘C) PM [MW] I, a‘ l'n . . l i r -.Timc m i 4 8 12Figure ‘2: a) shows a feedback controlled pellet fuelled discharge with preprogrammed1.5179”.dm/erinm L-mode plasmas.The shaded area in 1)) corresponds to the full squares of c) and represent cleanThe fact that clean hydrogen/deuterium discharges normally do not detach in the H-mode at high densities, since at least the ELM’S always burn through the divertor plasma,forced us to reduce the divertor power load by means of impurity gas injection. DuringELM’s peak power densities of up to 25 MWm'2 have been measured on the plates. Thisso achieved highly radiating and gomplctely detached fl—mode is known as CDH-mode[5]. The impurity injection can lead to radiative fractions Fwd/Pk“, > 90 % before theplasma falls back into the L-mode. The highly radiating L-mode HRL shows qualitativelythe same detachment and Marlo sequence as the clean hydrogen/deuterium discharges.No disruption has been observed without proceeding violent Marie expansion.The Greenwald scaling supposes the density limitation to be independent of heatingpower. The experiments however, exhibit In the case of purely hydrogen/deuteriumpuffing a clear increase of the DL with heating power. At low plasma currents, flaw issurpassed by up to 40 %. In contrast the impurity dominated DL seems to be independenton PM“, (see Fig. 1 b). In the HRL~mode the disruption occurs normally at densities 0.6 —I7a008(0P6)0.8 fiGW. The line averaged 2c]! there is about 4, whereas it is z 1.8 in the clean plasmas.Gene‘rally, the DL of clean hydrogen discharges is systematically about 10 % higher thanthose in deuterium.if one compares the energy confinement times of the two extremes (clean deuteriumH URL) one gets the interesting situation that the highly radiating plasmas have signifi-cantly larger 7-5 than the clean ones. Taking the JET/Dlll-D H-mode scaling as reference[6] since it does not include a for these high densities weakly founded fie dependenceréET/DIH'D o< 1;.03 . [137“, one can see in Figures ‘2 b) and c) that at high densitiesand heating powers TE approaches ll-mode confinement characteritics. The densities r‘zEare taken just before the Marfe grows into the bulk plasma. The decrease of T5 of theclean deuterium plasmas is connected with the increasing recycling fluxes necessary forstrong density rise. Moreover, the PM", dependence of these plasmas is well describedby the scaling, whereas the confinement of URL discharges deteriorates not noticeably.The improved confinement in the HRL mode is connected with markedly peaked electrondensity profiles. On the other hand, the clean plasmas exhibit very flat ones. The pelletrcfuelled discharges have significantly larger central densities (up to 6 - 10707714) butthe boundary behaves as in the purely gas refuelled cases. it shorrld be mentioned thatdespite strong density peaking the pellet discharges do not show improved 75.Summary :The aim to achieve densities far above the Greenwald limit is successfully obtained withstrong gas puffing and alternatively with repetitive pellet injection, however mostly inlr—mode. H-mode plasmas in ASDEX Upgrade so far do not surpass this limit (withthe exception of the pellet fuelled ones). in JF'l, ELM free ll— mode plasmas have beenobserved exceeding fifw [7]. At low I, we attained densities up to 1.4 fifw with prrre gasrefnelling. lu accordance with earlier experiments e.g. of ASDEX [8] and in contrast tothe Greenwald scaling, 11?" has been shown to increase clearly with PM“. In the extremecase of strong impurity puflirig the BL is markedly reduced. the heating power dependenceon the DL evanesccs but TE can reach ll mode quality. With pellet injection we achievedcontrolled long lasting phases up to 1."J eW and transientlv maximum densities of about2 f1?“ in type I ELM y ll— modes and x 2.5116“, in L-modes. The related confinement Inthe ll-mode is, however, degraded.With respect to lTER one can conclude that operation at the projected density seemsto be possible, brrt there are serious difficulties to maintain H—mode confinement. Theinjection of impurities unburdens the (livcrtor and improves the L-mode confinement butreduces noticeably the achievable densities.Refeer[llrM Greenswald et al Nuclear Fusion 28 1988, 2190[2] G Janeschitz et al Plasma Physics and Controlled Fusion 37 11A, A19[3] W Suttrop et al this conference[4] P T Lang et al Nuclear Fusion in press[5] O Grrrber et al Phys Rev Lett 74 1995 4217{6] D Schissel ct al, Nuclear Fusion 31‘ 1991, 73[T] D Campbell et al, Controlled Fusion and Plasma Physics (Proc. 21th Europ. Conf.Morrtpellier 1994) 1813 l ‘2[8] A Stabler et al, Nuclear Fusion 32. 1992. 155718

High Density Operation in Ausxiliary Heated ASDEX Upgrade Discharges

http://hdl.handle.net/21.11116/0000-000A-96FE-F

High Density Operation in Ausxiliary Heated ASDEX Upgrade Discharges

Abstract

Similar works

Full text

Available Versions

MPG.PuRe