Artaserse, G.

Belohony, E.

Boom, J.

Buratti, P.

Coelho, R.

Delabie, E.

Figueiredo, A.

Hillesheim, J.

Lerche, E.

Meneses, L.

Refy, D.

Rimini, F.

Sips, A.

Solano, E.

Vianello, N.

Zoletnik, S.

English

MPG.PuRe

Experimental characterization of M-Mode in JET tokamakN. Vianello1, E. R. Solano2,3, E. Delabie4, J. Hillesheim5, D. Refy6 , S. Zoletnik6P. Buratti7, J. E. Boom8, R. Coelho9, A. Figueiredo9, E. Lerche10, L. Meneses9, F. Rimini5ACC Sips11, G. Artaserse7, E. Belohony3 and the JET Contributors ∗EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK, 1 EPFLCRPP CH-1015 Lausanne, Switzerland 2EUROfusion PMU, Culham Science Centre,Abingdon, 3 Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain 4ORNL, Oak Ridge,TN, USA 5 CCFE, Culham Science Centre, Abingdon UK, 6Wigner Research Centre forPhysics, Budapest, Hungary, 7ENEA for EUROfusion, Frascati, Italy, 8 IPP Garching,Germany 9Instituto de Plasmas e Fusao Nuclear IST, Portugal, 10LPP-ERM/KMS, Brussels,Belgium 11 European Commission, Brussels, Belgium, ∗ See the Appendix of F Romanelli etal., Proc. 25th IAEA Fusion Energy Conference 2014, St Petersburg, Russia02468ne [1019m−2]LID4 LID3Pulse No: 858970.51.52.5 ICRH [MW]Wdia [MJ]50100150f [kHz]Ḃθ10 15 20t[s]0.51.01.52.02.5f [kHz]DαEF14.346-1cV1Figure 1: From top to bottom: Core(red) andedge(black) line integrated density, ICRHpower (black) and Wdia (red), spectrogramof Ḃθ and spectrogram of DαIn recent years detailed investigations on the L-H tran-sition in tokamaks have revealed that at power closeto the L-H power threshold an intermediate oscillatoryphase between the L and H phases exists [1]. This phase,generally appearing prior to a final transition to a properH-Mode, is characterized by dithering cycles with pe-riodic oscillations between high and low confinementregime and it has been confirmed in a variety of devices[2–5]. In all these devices the temporal dynamics hasbeen often association to limit cycle oscillations (LCO)describing the interaction between turbulence (generallyelectrostatic turbulence is considered) and E×B flow,although some of the machines have been observing alsoa magnetic oscillation during these phases [2, 6, 7]. It isin any case accompanied by periodic oscillations of thepedestal profiles. A weak ELM-free H-Mode regimes,dubbed M-Mode has been observed at JET [8], with a clear increase of pedestal density anda weak temperature pedestal. A peculiarity of the M-Mode is the presence of a clear low-frequency coherent magnetic oscillation, with periodicity (n,m) = (0,1) around 1-2 kHz. Ithas been observed in the pedestal region, as confirmed by ECE, Reflectometer and Li-Beam di-agnostic, but it also modulates particle and heat flux at the target as confirmed by Dα , Langmuir42nd EPS Conference on Plasma Physics P2.133probes and fast infrared cameras.EF14.346-2cFigure 2: Top:Toroidal mode spectrum.Bottom: zoom on two ḃ probe, located atdifferent poloidal position, pass-band fil-tered between 60 and 80 kHz. The insetmagnify the two signals to highlight thepoloidal periodicityThe present contribution will continue the descriptionof the experimental observation associated to this state,highlighting the existence of an associated high frequencymagnetic oscillations and the observed modulation ofedge and SOL density gradient. In Figure 1 a typicalICRH heated pulse with the appearance of the M-Modeis shown. The M-mode oscillation is clearly visible inthe spectrogram of the Dα in the bottom panel where thedominant frequency around 1 kHz and the second har-monic are shown. The mode appears whenever the ICRHpower is increased and it is accompanied by a modestincrease in the energy content and in the edge and coredensity. In the same plot the third panel shows the spec-trogram of a magnetic pick-up coil located in the HFS.The inset amplify the evolution of the spectrogram in thelower frequency branch where the M-Mode appears butalso a second branch at higher frequency (50-80 kHz) isclearly evident. This high frequency branch (HFB) exhibits similar spectral properties with re-spect to the dominant low frequency oscillation as shown in Figure 2. The HFB has a clearn = 0 toroidal periodicity and it is worth noting that these broad-band oscillation is present evenbefore the appearance of the M-mode, marked with a vertical white line. At the appearance ofthe M-Mode the HFB branch starts pulsating at the characteristic frequency of the M-mode asshown in the bottom panel, where the band-pass filtered ḃ signal obtained from two poloidallyseparated probes are shown. We have marked the typical periodicity of the M-Mode with an ar-row and in the smaller inset we provide a zoom on the two signal to highlight that also the HFBmaintain the m = 1 periodicity. As stated at the beginning the M-Mode is generally observablein different diagnostic apart from the magnetic, both in the pedestal and in the Divertor region.In the top panel of figure 3 the Power Spectral Density (PSD) of the magnetic field (in the HFSof the tokamak, above the equator), is shown together with the same quantity calculated for afast ECE channel located at the top of the pedestal the reflected phase of the correlation reflec-tometry Φr f l . The dominant frequency of the M-Mode, located near 1.5 kHz for this pulse, isclearly visible in all signals together with higher harmonics. The proper phase between mag-netic and kinetic fluctuations (i.e density and temperature) has been analyzed in the frequency42nd EPS Conference on Plasma Physics P2.133domain and shown in the same figure 3.0.20.40.60.81.0PSD(a)Pulse No: 85915Ḃθ HFS@ Z = 0.642ECE Φrefl0.20.40.60.8γ(b)ECE-Bθ Φrefl−Bθ 10-1 100 101f [kHz]-π-π/20π/2πα [radians](c)EF14.346-4cV11 0 1Z [m]-π-π/20π/2πα [radians](d)# 80913# 85915# 87879# 80911Figure 3: (a) PSD of Ḃθ (black), ECE Channel (blue),Φre f l(red). Coherence (b) and Phase (c) between in-tegrated magnetic and ECE (blue) and magnetic Re-flectometer (red). (d) Phase between ECE and Mag-netic field at different Z computed at the M-Mode fre-quencyThe magnetic signal has been integrated witha proper high-pass filtering above 50 Hz andcoherence and phase with the ECE and Reflec-tometer have been computed. The observed highcoherence at the mode frequency between mag-netic and kinetic signals, ensures that the phaseestimate is reliable in this frequency range. Ac-tually, consistently with the idea that the M-Mode is associated to a small up-down motion ofpedestal plasma surface [8], the phase is foundto depend on the Z value of the chosen probe.In the same Fig. 3 panel (c) we show the phase,between ECE and magnetic field, computed atthe M-Mode frequency as a function of the ver-tical position for a series of pulses. Consideringthe ECE LOS is located at Z = 0.248 we can conclude that, within the uncertainty due to thepoloidal resolution, the phase between Te and bp is close to zero. We have checked that the sameconsideration holds for the density oscillations suggesting we have a pressure perturbation inphase with Bθ .EF14.346-5cFigure 4: Coherence (a) and Phase (b) between den-sity gradient and magnetic field computed at differentpoloidal position. (c) Phase computed at the M-Mode fre-quency as a function of vertical positionInvestigation with a Li-Beam reveals thatthe entire pedestal gradient is oscillating atthe frequency of the M-Mode. The time evo-lution of the entire pedestal and SOL den-sity profile has been computed using Li-Beam data and a combination of hyperbolictangent and linear fit (corresponding to thepedestal and SOL region respectively). It hasbeen observed that there is no modulation ofthe pedestal position neither of the pedestalheight, whereas the gradient is found to oscil-late at the M-Mode frequency, as confirmedby the figure 4 where phase and coherence between ∇ne and the integrated magnetic field com-puted at different vertical position are shown. As done for the temperature and density we have42nd EPS Conference on Plasma Physics P2.133also computed the phase at the M-Mode frequency, and 4 (c) shows this phase as a functionof the vertical position of the probes. We can conclude that the density gradient is almost inquadrature with the magnetic field.1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5〈Ne〉 [1019  m−3 ]0123456789PLH [MW]DH1.5 2.0 2.5 3.0 3.5 4.0〈Bp〉Ψ =0.97/√µ0ρi [105  m/s]0.00.51.01.52.02.53.03.54.0f max [kHz]DHEF14.346-6cV1Figure 5: Top: L-H Power threshold vsdensity for Hydrogen and Deuterium dis-charges. Bottom: scaling of the M-Modefrequency vs Poloidal alfven speed. Thecolor code is consistent with the top panelFinally, hints of the scaling of the Mode frequency withthe poloidal Alfvén speed have already been provided [8].A further consistent observation has been obtained by com-paring similar discharges obtained during L-H thresholdinvestigation in Hydrogen and Deuterium plasmas [9]. Thetwo sets have been obtained comparing similar dischargeswith Ip = 1.7MA and BT =1.8T. The mode frequency hasbeen tracked in time and the scaling with the time evolu-tion of the poloidal alfven speed as computed at ρ = 0.97 isshown in the bottom panel of figure 5. Apart from a slightdeviation at lower frequency, where we should take into ac-count the feedback system reacting to vertical oscillation atJET, a linear trend is visible, consistent with the poloidalAlfvén speed scaling with an appropriate isotope depen-dence.Concluding further investigation of the M-Mode on JET tokamak is reported. The oscillationof the poloidal magnetic field is in phase with density and temperature. An higher frequencybroadband fluctuation is observed with the same symmetry of the M-Mode. At the appearanceof the M-Mode this HFB starts pulsating. The pedestal gradient is found to oscillate at the modefrequency, resembling similar observation in the dithering or I-phase. The dynamical interplaywith the density gradient is presently under investigation. Finally by considering an isotopescaling the dependence of the M-Mode on the poloidal alfven speed has been confirmed.This work has been carried out within the framework of the EUROfusion Consortium and has received fundingfrom the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views andopinions expressed herein do not necessarily reflect those of the European Commission1L Schmitz et al., Physical Review Letters 108, 155002 (2012).2G. S. Xu et al., Nuclear Fusion 54, 103002 (2014).3J Cheng et al., Nuclear Fusion 54, 114004 (2014).4S. H. Müller et al., Physics of Plasmas 21, 042301 (2014).5T Estrada et al., Nuclear Fusion 55, 063005 (2015).6K. H. Burrell et al., Physics of Fluids B: Plasma Physics 2, 1405–7 (1990).7Y Xu et al., Plasma Physics and Controlled Fusion 57, 014028 (2015).8E. R. Solano et al., in 40th EPS Conference on Plasma Physics (2013), P4.111.9E Delabie et al., in Proceedings of the 24th iaea fec conference (2014), EX/P5–24.42nd EPS Conference on Plasma Physics P2.133

Experimental characterization of M-Mode in JET tokamak

http://hdl.handle.net/21.11116/0000-0008-85DD-9

Experimental characterization of M-Mode in JET tokamak

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