Baldzuhn, J.

Brakel, R.

Fiedler, S.

Geist, T.

Grigull, P.

Hartfuss, H.

Hirsch, M.

Hofmann, J.

Holzhauer, E.

Jaenicke, R.

Koponen, J.

Wagner, F.

Weller, A.

Wobig, H.

English

MPG.PuRe

1601Dynamic behaviour of the H—mode edge transport barrierin the W7-AS stellaratorM. Hirsch, E. I-Iolzhauer *, J. Baldzuhn, R. Brakel, S. Fiedler, T. Geist, P. Grigull.H. J. Hartfufi, J. Hofmann, R. Jaenicke, J. Koponen, F. Wagner, A. Weller, H. Wobigand the W7-AS teamMax—Planck Institutfilr Plasmaphysik, EURATOM Ass. D-85748 Garching. FRG* Institutfir Plasmaforschung, Univ. Stuttgart, 70569 Stuttgart, FRGIntroductionIn the stellarator W7~AS the Hnmode is characterized by an edge transport barrier whichis associated with a strong reduction of magnetic and density turbulence and a sheared poloidairotation in the electron diamagnetic direction [1,2,3]. A specific feature of W7-AS is therestriction of the H-mode operational range to narrow windows of the edge rotational transformwhere the H-mode is obtained even at the lowest available heating power. The quiescent ELM—free H—mode is always obtained through a phase with quasi periodic ELMs [3]. Once thequiescent state is reached, edge profile gradients and the spectroscopically measured poloidaiplasma rotation develop on a timescale of about 20 ms while the energy confinement improvesby AW/W=30%.Influence of edge parameters on the H-mode operational rangeIn the low-shear stellarator W7~AS Hnmode operation is obtained within narrowwindows of the edge rmationai transform la ( e.g. at 13 = O.525i0.005 and La z 0.48 ) at thelowest available heating power, 200 kW of ECRH (one gyrotron) or 340 kW of NBI (onesource), respectively. The actual poWer threshold might even be Iowa. At the onset of an ELM-free quiescent H-mcde the energy flux density across the separatrix is comparable or less to thatfound for the L~H transition in tokarnaks [4].The magnetic field topology within the operational windows is characterized by acomparatively large plasma minor radius and a plasma boundary determined by the innerseparatrix of a natural island chain ( e.g. 5/10 and 5/9 ). The limiter does not disturb thisLCFS. Under these conditions two mechanisms are believed to contribute to the easy accessinto the H-mode: (1) outside the LCFS the connection lengths decrease to a value of somemeters within a radial distance Ar z 1 cm. This allows for the development of a strong radialvariation of the radial electric field and a corresponding velocity shear layer already before afully developed H—mode is achieved [5]. (2) the poloidai viscosity is lower than for other valuesof la» as the island structures which create strongly corrugated flux surfaces increasing themagnetic pumping are shifted out of the confinement region [2,6].1602The quiescent H—mode is always reached through a phase with quasi periodic ELMs [2]with a typical repetition frequency fELM > lld-Iz. Between the ELMs turbulence is stronglyreduced and indistinguishable from the quiescent H-mode phase. Global confinement in thisELMy H—mode is close to the L—state which only exists outside the H—mode operational range.Within the H-mode operational windows no stationary turbulent L—mode is found to proceed thetransition. Instead short (<lms) quiescent phases and sequences of periodic ELMs are observedintermittently even in the early phase of the discharge. For the onset of an ELM-free quiescentH—mode a threshold density nth is required which depends on the edge rotational transform asan important parameter. In Fig.1a the operational window around La = 0.525 is marked by thehatched area and the observed values of nth are given by the squares. The numbers inserted inthe Figure indicate the repetition frequencies fELM observed in ELMy H—rnodes. As the densityapproaches the threshold fELM decreases to a minimum of 1 kHz. Stationary ELMy H—modeshave been obtained below the threshold for up to 700 ms if the line averaged density was keptconstant. In Fig.1b the same range of the total edge rotational transform ta is scannedcontinuously within 130 ms in a single discharge by adding a small ohmic current( 0 kA < Itor < 5 RA ). The average density is kept constant at <n>=4 1019 m3. ELMs‘appear as 1a crosses the operational range and fELM approaches a minimum as the chosendensity is close to the corresponding threshoid density for a quiescent H—mode.a b‘3’ >E 6 os.3 oO l—"‘ s‘A CGV a>~i': .g =o 2 w '1::o a a t ._ r. a».0.52 0.525 0.53 0-52 0.5 5 0.53edge rotational transform edge rotational transformFig. 1a: Operational range (hatched area) and threshold Edge electron temperaturefrom ECE (Fig. 1b) anddensity (squares) for the quiescent H—mode in the la Horrignal (Fig. lc) as the total edge rotationalwindow around 1320.525. The numbers inserted indicate transform is tuned over the operational windowELM repetition frequencies afELMy H-nwdes in kHz. shown in Fig la by adding an increasing ohmiccurrent. <n> is kept constant at4 1019 m'3. Thetime window shown lasts [30 ms.1603The influence of magnetic shearThe H—mode can also be achieved if positive or negative magnetic shear is induced inaddition to the vacuum configuration by positive and negative ohmic currents. An improvementof the edge confinement can be obtained by magnetic shear even outside the H-rnode operationalwindows [7]. With increased shear the fluctuation spectra observed with reflectometry arefound to broaden and shift further into the electron diamagnetic drift direction.Dynamic behaviour of edge parametersThe ELMS within the ELMy H-mode display characteristics similar to type III ELMsfound in tokamaks [8]. They show up as bursts of magnetic and density turbulence with arepetition frequency >ikHz and a typical length of 200m followed by an interval with stronglyreduced turbulence identical to that in the quiescent H—mode phase. In most cases at the onset ofthe ELM magnetic coils measure a quasi coherent precursor activity with a frequency around400 kHz. Edge profiles of TC and me are obtained with high time resolution from 130 and SK-emission, reflectometry and a 10 channel mm—wave interferometer, respectively As thebroadband magnetic and density turbulence level starts to grow the edge profiles of Te and n3flatten over the first 3 cm inside the separatrix, emphasising the edge localized character of thephenomenon and the associated loss of confinement As soon as the level of turbulence beginsto decrease edge gradients again begin to steepen.The radial propagation of the change of density perturbations is followed withreflectometry on a shot to shot basis using the onset of the Hot-burst as reference. Over adistance from 3‘ cm inside to 1 cm outside the separatrix the increase of the density fluctuationlevel occurs within a time interval as short as 20 us.The poloidal prepagation of density perturbations can also be measured with thereflectometer system since the antenna beams are tilted with respect to the normal of themagnetic surfaces. The measured Doppler-shift of the reflected rum-wave results from theselected poloidal wavevector component and the poloidal propagation velocity of the densityperturbation [9]. As an example Fig.2 shows a frequency powar spectrum of the reflected waveduring an ELMy H-mode. The spectrum is Scanned over 20 ms, therefore about 30 ELMs andintermitted quiescent phases are covered Selecting only time intervals during ELMs Fig.3shows the radially resolved Doppler-shift on a shot to shot basis. The observed Doppler-shifttowards negative frequencies is due to the poloidal propagation of the density perturbations inthe electron diamagnetic drift direction. The poloidal propagation velocity of fluctuationsderived from the Dopplernshift depends critically on the resulting tilt angle Le. on the details ofthe complex edge topology. For positions inside the separatrix region a maximum poloidalvelocity of about 20 kin/s is estimated. Note that the observed velocity of the turbulencestructures Vpdl is the sum of poloidal plasma rotation and the intrinsic phase velocity of theturbulence itself.1604-4D I N-a.2— r 1 | _E Ig-sr -5ELMy Hmode '0-4” —\ \-602 “'M' i i-70 Eo1: iii-oer--3-80 m54“) 3-1DH HQ i o-100 1 1 1 94.2 1 1 I | 1-400 -200 o 200 400 0.13 0.14 0.15 0.16 0.17frequency shift I kHz effective radius / mFig 2 .' Power spectrum ofa rq’iected min-wave measured Fig 3 : Doppler—shift of the reflected rignalover 20m: ofan ELM)» H—mode consisting of about 30 during ELMs. Each point is an average over 20ELMS and imennittentquiescent intervals. The imrhifted ELMS. Different radial positions have beenline reflectedfrom the rear tom: wall is given as a reference. probed on a shot to shot basis. The repararrbcposition estimated from an interpolation ofequilibrium calculations is 16 cm. For the steepedge gradients observed the acmai separarrix maybe shifted more outward.Termination of the quiescent H-modeAfter a quiescent H~m0de of more than about 50 to 100 ms confinement tends todegrade which is accompanied by an increase of bolometiy and SX radiation. During this phasestrong isolated ELMs can appear as a distinct (duration less than 300115) burst of magnetic anddensity turbulence associated with a huge spike in Her-emission and a reduction of 11.3 and Tegradients at the edge (Le. less than 3 cm inside the separatrix). In contrast to the quasi periodicELMS found in the ELMy H-mode the edge gradients do not recover immediately after the eventbut remain flat for a typical period of several ms. During that time the decrease of ne and Tepropagates to the inner part of the plasma. In comparison to the quiescent H—phase thefluctuation level at the plasma edge in most cases remains significantly higher during thisperiod.References[1] Wagner et 21]., Plasma Phys. Control. Fusion 36 A61 (1994).[2] Holzhauer et al., Plasma Phys. Control. Fusion 38 1267 (1996).[31 Hirsch et al., Proc. of the 16th IAEA Conference, Montreal, 7—11 Oct 1996.[41 Ryter et a1. Plasma Phys. Control. Fusion 36 A99 (1994).[51 Wagner et al, Proc. of the IAEA Conference, Sevilla. V1.1 559 (1994).[6] Wobig and KiBlinger, this conference.[7] Brake] et al. this conference.[8] Zohm. Plasma Phys. Control. Fusion 38 105 (1996).[9] Hirsch, Holzhaucr. Proc. of IE Workshop on Reflectometry. Madrid, May 5—7 1997.

Dynamic Behaviour of the H-Mode Edge Transport Barrier in the W7-AS Stellarator

http://hdl.handle.net/21.11116/0000-000A-96A2-5

Dynamic Behaviour of the H-Mode Edge Transport Barrier in the W7-AS Stellarator

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