Becoulet, M.

Eich, T.

Fundamenski, W.

Jachmich, S.

Laux, M.

Loarte, A.

Matthews, G.

Philipps, V.

Stamp, M.

English

MPG.PuRe

Novel method to study SOL-response to ELMs by divertor targetprobes in JETS. Jachmich1, M. Laux2, M. Becoulet3, T. Eich4, A. Loarte3, G. Matthews3, V. Phillips4, W.Fundamenski3, M. Stamp3 and contributors to the EFDA-JET workprogramme1 Laboratory for Plasma Physics, Ecole Royale Militaire/Koninklijke Militaire   School, Association “Euratom-Belgian State”, Brussels, Belgium*2 Max-Planck-Institut für Plasmaphysik, Association EURATOM, Bereich  Plasmadiagnostik, Berlin, Germany3 UKAEA Fusion, Culham Science Center, Abingdon,UK4 Institut für Plasmaphysik, Forschungszentrum Jülich, Jülich, Germany** Partners in the Trilateral Euregio Cluster (TEC)1. IntroductionThe particle fluxes and heat energy loads, which accompany edge localised modephenomena (ELM) during high confinement discharges cause losses in density and plasmaenergy and may lead to enhanced erosion of the divertor target plates. In the past the globalproperties and scalings of the ELMs have been explored [1,2]. However, the underlyingphysics is still concealed due to the stochastic nature, of which ELMs are. In order to revealdetailed time information of a typical ELM, a new ELM identification algorithm has beenapplied to ELMy H-mode periods to find ELM parameters in the Dα signal and in Isat, Vfloatand Te from triple probes in the divertor targets. Once an ELM-event has been identified, theinverse time interval of successive ELM events, the width, height and integral of the ELMburst, and the average signal between ELMs is calculated for every individual ELM in theseries. In addition, the data of all ELMs, which belong to the same phase during an ELM, arecombined and averaged. The so reconstructed time trace, which represents a typical ELM, isdiscussed in more detail elsewhere [3]. In order to discover and to exclude irregular ELMs,their total deviation to the averaged ELM is determined. For the calculation of more complexplasma quantities, which consist of different diagnostics, one signal is used as a marker and asecond one is averaged accordingly to the first one. In most cases the Dα recycling has beenused as a marker signal, since it shows usually a very sharp peak.2. Changes of individual ELM parameters during H-mode dischargeFrom an H-mode discharge (Ip= 3 MA, Ptot = 15 MW, <ne>0=2.8 1019 m-3) the series ofELMs, which is indicated in Figure 1, has been extracted and analysed. It is obvious that theELM-frequency is slightly increasing in time. A simple relation between the time since thelast ELM and the peak area of the subsequent ELM seems to hold at least in a statisticalsense. The postulate that more particles accumulate until the next ELM-event is triggeredleads to an inverse proportionality between the peak area and the ELM-frequency. This isconfirmed in Figure 2, where the Dα peaks are decreasing with the inverse time betweenELMs. The ratio of the peak to the background flux decreases until an almost constant levelis achieved.The individual standard deviation of all ELMs with respect to the averaged one isvery small (2-6%!, see Fig. 3). This proves that the averaged ELM is in fact the typical ELMof this series (shown in Fig. 4, where also the individual ELMs are plotted). However itbecomes also clear that there are four exceptional ELMs present. Those short and smalladditional peaks in Dα are strewn in between the regular ELMs on a surprisingly regular timepattern with a period of 77 ms, which is much lower than the present ELM-frequency. Thedeviation of the ion saturation current from the average ELM is reasonably small (5-10%),but for the floating potential and electron temperature it is higher (respectively 10-40%, 30-200%).In some cases the post-ELM signals do not return to their pre-ELM values until thenext ELM approaches and, therefore, higher particle flux and electron temperature can buildup. This non-stationary behaviour usually leads to a transition in ELM type. Both probes onthe outer target exhibit such features (Fig. 5). The gradient of the SOL-profile is at the ELM-burst higher than in between ELMs.3. Detailed temporal evolution of an ELMThe electron temperature and floating potential are peaking during the rise of the particleflux (Fig. 6). This early rise leads to broadening or even to a multiple peak structure of thepower load Qp (Fig. 7), which has been derived by assuming a sheath transmission factor γof 8 (with Ti=Te). Although the electron temperature reaches in maximum 30-40 eV, theapplied voltage is sufficient to measure the Isat-current. The so measured power fluxrepresents the heat flow due to the electrons. Despite a possible change of γ due to higher Tior Te, the rate of change is small enough to assure a good measurement of Qp. Since noprobe was located at the separatrix, it is not possible to derive the spatially maximum powerload. Comparisons with IR-data are underway. More information can be found in [4].If strong gas puffing is applied the peaks are broadened. Although the gas has beenreleased into the inner divertor base, also the outer divertor recycling seems to be influencedby the neutrals (Fig. 8). At low power crossing the separatrix or at low densities the doublepeak seen in the power flux can also be present in Dα  as it is illustrated in Figure 9.Comparisons with the ion saturation current confirmed this observation.4. ConclusionsELMs can be studied in a more systematic manner by averaging probe signals using acommon marker signal for an event. The relatively small standard deviations of the singleELMs from the averaged one demonstrated clearly the fact that the ELMs appear in a veryregular manner, which makes such analysis feasible. The technique can be applied to anymeasured quantity and thus allows the calculation of even more complex plasma quantities.A first analysis has shown an early rise in Te and Vfl, which is in contradiction to the delayedIsat-rise and therefore indicates a double peak structure of the power load. The heat fluxconsists of two components: an early short small pulse of hot particles, followed by a longerlarge one at much lower temperature.The study of inner/outer divertor recycling asymmetries has elucidated particlefluxes, which appear after the ELM burst at lower time scales. Depending on the density andgas puffing, they are seen either on the outer or on the inner divertor box.The results presented here are only a small collection of the detailed ELM-behaviour.In order to retrieve more information it is planned to extend the procedure to slow probevoltage ramps and slow X-point sweeps. In the first case I(V)-characteristics can bereconstructed for each ELM time, in the second changes of the SOL-profiles during an ELMcan be analysed.References[1] JET Team, Nucl. Fusion 39 (1999), 1687.[2] G. Saibene et al., Nucl. Fusion 39 (1999), 1133.[3] M. Laux et al., this conference.[4] Th. Eich et al., this conference.FiguresFig. 1: Time traces of neutral beam power, plasmadiamagnetic energy, radiated power, line integratedcentral density, gas flow Γ  and recycling at the outerdivertor box (Dα). The grey shaded box indicates theseries of analysed ELMFig. 2: (a) changes of the ELM-frequency. (b) area ofELM peak decreases with frequency.(a)(b)Fig. 3: Peak maximum, peak width and standarddeviation of each ELMs. The four spikes in σreloccur on a regular time pattern.solid: with gasdashed: w/o gasouter div.inner div.Fig. 8: Average ELM course of Dα with and w/ogas-puff. The humps in the solid curves indicate asecond particle flow during strong gas puffing.Fig. 9: Average ELM course of Dα for variousdensity and separatrix power. Note the pre-peakfor low ne or/and low Psep.fGw=0.42, Ptot/Pthr=3.1fGw=0.51, Ptot/Pthr=2.2fGw=0.48, Ptot/Pthr=2.9fGw=0.65, Ptot/Pthr=1.6Fig. 7: Electron power flow of averaged type IIIELMs (t=47-47.5) and during type I ELMs (t=48.8-49.2). The type I are plotted with an arbitrary timeoffset.Fig. 6: Ion saturation current, floating potential,return voltage and electron temperature of averageELM. The peak of Isat is delayed.probe at r-a= 7 mm probe at r-a= 10 mm probe at r-a= 12 mm51915 - Inner divertorIsatVretTeVflFig. 5: Ion saturation current in between ELMs ofthree probes at the outer target is increasing in time.Fig. 4: All individual ELMs and the resultingaverage ELM. Note the four small exceptionalELM-events.

Novel Method to Study SOL-Response to ELMs by Divertor Target Probes in JET

http://hdl.handle.net/21.11116/0000-0008-471E-8

Novel Method to Study SOL-Response to ELMs by Divertor Target Probes in JET

Abstract

Similar works

Full text

Available Versions

MPG.PuRe