Granetz, R.S.

Greenwald, M.

Hubbard, A.E.

Irby, J.

Kesner, J.

Migliuolo, S.

Pedersen, T.S.

Ramos, J.

Rice, J.

Snipes, J.A.

Stek, P.C.

Takase, Y.

Wolfe, S.M.

DSpace@MIT

PSFC/JA-97-23ELMs and Fast EdgeFluctuations in Alcator C-ModJ.A. Snipes, R.S. Granetz, M. Greenwald, A.E. Hubbard,J. Irby, J. Kesner, S. Migliuolo, T.S. Pedersen,J. Ramos, J. Rice, P.C. Stek', Y. Takase2, S.M. WolfeSeptember 1997Plasma Science and Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139'Present address JET Propulsion Laboratory, Pasadena, CA, USA.2 Present address Dept. of Physics, University of Tokyo, Tokyo, Japan.Submitted to Plasma Physics and Controlled Fusion.This work was supported in part by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole orin part by or for the United States government is permitted.ELMs and fast edge fluctuations in Alcator C-Mod*J A Snipes t , R S Granetzt, M Greenwaldt, A E Hubbardt,I H Hutchinsont, J Irbyt, J Kesnert, S Migliuolot,T S Pedersent, J Ramost, J Ricet, P C Stekt, Y Takase§ andS M Wolfet*Supported by U.S. D.O.E. Contract No. DE-AC02-78ET51013.MIT Plasma Science and Fusion Center, Cambridge, MA USAtpresent address JET Propulsion Laboratory, Pasadena, CA USA§present address Department of Physics, University of Tokyo, Tokyo,JapanAbstract. Just above the H-mode threshold power at low density, smallamplitude high repetition rate (1 - 5 kHz) Type III Edge LocalizedModes (ELMs) are observed. At higher input power, the H-modesbecome ELM-free. At high density and high input power (Ie >2.2 x 1020 m- 3, P," > 2 MW), the plasma often enters a new H-moderegime now called the Enhanced D, H-mode, which is a steady-state H-mode often without distinct ELMs but with high levels of D, emission.Enhanced Da H-modes are accompanied by persistent high frequencybroadband (50 - 400 kHz) density and magnetic fluctuations. Aboveabout i > 3 x 1020 m- 3, the Enhanced D, H-modes may also exhibitbursting or continuous low m, n coherent magnetic fluctuations withfrequencies from 60 - 90 kHz. In H-mode, steep edge gradients areobserved with a nine channel ECE polychromator and a 38 channel highresolution edge soft x ray array. The soft x ray pedestal width remainsaround 3 to 4 mm in ELM-free H-mode and between Type III ELMs.The pedestal collapses within 100 psec during an ELM. Calculationsindicate that the plasma in H-mode is often near ideal ballooning firststability limits, but no Type I ELMs have yet been identified. Edgemicro-tearing modes destabilized by a steep temperature gradient arealso considered as a possible explanation of the observed fluctuationsduring Enhanced D, H-mode.Submitted to Plasma Phys. Control. Fusion12 J A Snipes etal1. IntroductionEdge localized fluctuations have been identified during H-mode since the discovery ofH-modes on ASDEX [1]. Previous work has classified Edge Localized Modes (ELMs)into three types [2] depending on their amplitude and frequency as well as on edgestability properties. On Alcator C-Mod [3], of these types, only Type III ELMs havebeen identified with no characteristic progression to Type I ELMs as the edge gradientsincrease, at least up to the maximum available input power (4.5 MW). The progressionfrom Type III ELMs to ELM-free H-mode is observed as the input power increasesabove the threshold power to achieve H-mode [4]. With increasing input power, theplasma can sustain increased density and remain in H-mode. At high input powerand high density (Pi, > 2 MW, fi, > 2.2 x 1020 m-3 ), instead of entering a Type IELMy regime, the plasma enters a new regime now called an Enhanced Da (EDA)H-mode [5-7], which is a steady-state H-mode that usually has no clear ELMs buthas a high level of D, emission that can reach or exceed L-mode levels. Broadbandhigh frequency (50 - 400 kHz) density and magnetic fluctuations are observed duringthe enhanced D, phase. After several confinement times into a well developed EDAH-mode, when the density exceeds about ?i, > 3 x 1020 m-3, bursting or continuoushigh frequency coherent magnetic (60 - 90 kHz) fluctuations have also been observed[8]. These high frequency density and magnetic fluctuations appear to be localizednear the edge of the plasma and they occur only at high density and high input power.The edge gradients under these conditions, and often also during ELM-free and TypeIII ELMy H-modes, are near ballooning or micro-tearing stability limits [9,10].2. Edge Fluctuation DiagnosticsMagnetic fluctuations in C-Mod are measured with several arrays of poloidal fieldpick-up coils at the inner wall and on the outboard limiters. The inner wall coils arebeneath molybdenum tiles, which limit the frequency response to about 150 kHz. Theoutboard limiter coils are covered only by a 0.5 mm stainless steel cover that limitsthe frequency response to about 300 kHz. Presently, 48 coils channels can be sampledat rates up to 1 MHz each with 128 K of samples.Density fluctuations are measured with an AM reflectometer[11] at 88 GHzoperating in O-mode with a density cutoff at 9.6 x 1019 m- 3 , which is in the steepgradient region just inside the separatrix during H-mode. This diagnostic was designedto measure the density profile rather than fluctuations. Fluctuations in the phase aremeasured while fluctuations in the amplitude (k/ne) cannot be determined with thepresent system. The 88 GHz channel was sampled at 1 MHz with a bandpass filterthat allows up to 400 kHz fluctuations to be measured.Edge electron temperature pedestals are measured with a 9 channel ECE gratingpolychromator, which normally has about 2 cm channel separation and 50 psec timeresolution. The effective radial resolution can also be somewhat improved to ~ 1 cm byELMs and fast edge fluctuations in Alcator C-Mod* 3ramping the magnetic field and sweeping the channels across the edge steep gradientregion. A high resolution edge soft x ray imaging array of 38 photodiodes with aspatial resolution of about 1.7 mm and a time resolution of 12 psec also measures acombination of temperature and density profiles in the edge. In the absence of moredetailed edge density and temperature measurements, the edge soft x ray pedestalwidths are used as a qualitative indicator of how the edge density and temperaturepedestals change with time in the H-mode.-. 40 40e Inner Wall3020 - EDA H-mode <0.947,0.957>a' 100 100 200 300 400Frequency (kHz)40 60 < f < 80kHz203.00.920 0.930 0.940 0.950Time (sec)Figure 1. Fast Fourier transform of an inner wall poloidal field pick-up coil signalduring an EDA H-mode averaged from 0.947 to 0.957 s. The time history of thesignal has been filtered from 60 to 80 kHz to enhance the coherent fast edge modes.Comparison with a central electron temperature signal shows that the magneticfluctuations occur at all phases of the sawtooth oscillations.3. Fluctuations during Enhanced D, H-modeThe fluctuations present in Enhanced D, H-mode do not exhibit the characteristicsof previously defined ELM types. There are bursting or continuous coherent fastedge magnetic oscillations with frequencies from 60 to 90 kHz that have low poloidaland toroidal mode numbers, typically m=4, n=1. There are also continuous densityfluctuations with peak frequencies from 50 to 200 kHz as well as broadband magneticand density fluctuations out to 300-400 kHz. The coherent magnetic fluctuationsexhibit no ballooning character in that, once the signals are corrected for the distanceto the mode rational surface, the outboard limiter signals are about the same4 J A Snipes etalamplitude as the inner wall signals (Be/Bo ~ 5 x 10-). The mode rational surfaceis calculated to be only 4 mm inside the last closed flux surface. The density andmagnetic fluctuations may appear together or separately with nearly the same orwith quite different frequencies, but no clear coherence has been found between themagnetic and density fluctuations even though they appear to originate from the sameedge region within the plasma.Figure 1 shows an example of fast edge modes measured with a poloidal field pick-up coil at the inner wall during an EDA H-mode together with a Fourier spectrumof the raw signal. The time trace of the signal has been filtered from 60 to 80 kHzto enhance the features of the mode. A central electron temperature signal is alsoshown to compare the timing of the fast edge mode bursts with the sawtooth crashes.The bursts occur independent of the phase of the sawtooth. The Fourier spectrumshows several harmonics of the -70 kHz fast edge mode, which could be higher orderharmonics of the dominant m=4, n=1 mode. Note that the EDA H-mode often occurswith no evidence of coherent fast edge modes, though there is generally an increase inthe high frequency broadband density and magnetic fluctuations.960214030SReflecto eter EDA H-mode <0.88,0.93>2 ~- ELM-free <0.80,0.85>U_U_ 20 100 200 300 400Frequency (kHz)1.0--0.0 -15 Midplane Dro5- ELM-free Enhanced D.0.82 0.84 0.86 0.88 0.90 0.92Time (sec)Figure 2. Comparison of fast Fourier transforms of an edge reflectometer signalaveraged from 0.88 to 0.93 s during an EDA H-mode (dashed) and averaged from 0.8to 0.85 s during a preceding ELM-free H-mode (solid) showing an increase in broadbandfluctuations out to at least 400 kHz as well as a clear peak around 120 kHz duringEDA H-mode. The time histories of the root mean square density fluctuations andthe D, emission show that the fluctuations increase when the Da emission begins toincrease and remain higher throughout the EDA H-mode.In addition to the magnetic fluctuations, the high frequency broadband densityELMs and fast edge fluctuations in Alcator C-Mod 5fluctuations, observed on the edge reflectometer signal, increase markedly during anEDA H-mode. Figure 2 shows a clear example of an ELM-free H-mode becoming anEnhanced D, H-mode at about 0.86 sec as the D, emission begins to increase. Atthe same time, the broadband density phase fluctuations on the reflectometer signalincrease out to at least 400 kHz and a broad peak in the spectrum appears around120 kHz. The RMS of the reflectometer phase fluctuations increases coincident withthe increase in the D, emission and remains higher throughout the EDA phase. Themagnetic fluctuations during the EDA phase of this discharge do not show any clearpeaks in the spectrum.4. Comparison with TheorySince fast edge modes appear to be quite localized in the steep gradient region atthe edge in H-mode, it is plausible that temperature, density, or pressure gradientscould be involved in destabilizing these modes. The coherent magnetic fluctuationshave frequencies that are many times the estimated diamagnetic drift frequency, w.,suggesting that a strong edge rotation may be present, which is in the oppositedirection to the measured core rotation [12].100S80G()4()Figure 3. Profile of the reconstructed edge soft x ray emissivity versus time showing aseries of three Type III ELMs. A steep edge pedestal exists between ELMs that extendsover only two channels each separated by 1.7 mm radially. The radii are mapped tothe outboard midplane assuming constant emission on a flux surface.Initial estimates from edge electron temperature measurements gave pedestalwidths of about 1 cm in H-mode. Recent edge soft x ray measurements indicate5"t 9706250206 J A Snipes etalthat a combination of the edge temperature and density gradients is much steeperwith pedestal widths of about 2 to 4 mm (Figure 3). The steep gradient region in thereconstructed emission between Type III ELMs occurs over only two spatial channelsseparated by about 1.7 mm when mapped back to the midplane assuming constantemission on a flux surface. The pedestal collapses within about 100 psec duringeach ELM. Assuming similar temperature and density pedestals, initial calculationsindicate that the soft x ray pedestal width is about 70% of the temperature or densitywidth.Preliminary calculations of ideal and resistive modes have been performed withthe BALOO code [13], the CAXE and KINX codes [14], and the MARS code [15].C-Mod equilibria in which fast edge modes are observed, with edge pressure pedestalwidths of 7 mm, are found to be stable to ideal ballooning modes, though the edgepressure gradients are within 15 - 30% of being unstable. If the pressure gradients areas steep as the soft x ray gradients, then ideal or resistive ballooning modes may beunstable.m=4, n=1 Micro-tearing Eigenmodes Be 40.25-0.00-0.25- Re(Be)0 -0.50Ea 0.022oW 0C............ .. ...............E Re(4)0z -0.02-1.0 -0.5 0.0 0.5 1.0R - Ro (cm)Figure 4. Normalized m=4, n=1 micro-tearing eigenmodes of the poloidal field,Be, and the electrostatic potential, 1D,showing their localization to the mode rationalsurface R0 .Other possible modes whose stability has been examined are the edge micro-tearingmodes [9,10]. Their eigenmodes are localized to a few ion gyroradii (pi ~ 1 mm) ofthe rational surface (Figure 4) and are driven by steep temperature gradients. Micro-tearing modes are predominantly electromagnetic but have a smaller electrostaticcomponent and they are not ballooning. The eigenmode solver from Ref. [10], whichELMs and fast edge fluctuations in Alcator C-Mod*assumes concentric, circular flux surfaces in a large aspect ratio limit, has been usedto examine the stability of m=4, n=1 radial eigenmodes in a plasma characterized byLn ~ LT ~ 0.5 cm and 6 ~ 0.002 and ne = 2.7 x102 0 m 3 Te =Ti = 300 eV at thetop of the pedestal. The fundamental m/n=4/1 radial eigenmode displays the largestgrowth rate. Unstable higher radial harmonics and higher m modes also appear to bepresent. The higher radial harmonics of the m=4, n=1 eigenmodes and the higher mmodes may only be detectable by internal density fluctuation measurements becauseof the rapid decay with distance from the rational surface.Since the coherent fast edge modes are nearly in-out symmetric and have lown=1, it seems unlikely that they could be related to ballooning modes. The highfrequency broadband turbulence may be unresolved multiple high m,n modes and morecalculations still need to be done to make detailed comparisons of resistive ballooning,micro-tearing, and other modes with the experiment.References[1] Wagner F, etal 1982 Phys. Rev. Lett 49 1408[2] Doyle E J, Burrell K H, Lehecka T, Luhmann N C Jr, Matsumoto H, Osborne T Peebles W A,Philipona R and Rettig C 1991 Proc. 18th EPS Conf. on Controlled Fusion and Plasma Physics(Berlin) 15C Part I p 285[3] Hutchinson I H, etal 1994 Phys. Plasmas 1 1511[4] Ryter F, H-mode Database Working Group, 1996 Nucl. Fusion 36 1217[5] Takase Y, etal 1997 Phys. Plasmas 4 1647[6] Greenwald M, etal 1997 Nucl. Fusion 37 793[7] Snipes J A, etal 1997 Proc. 24th EPS Conf. on Controlled Fusion and Plasma Physics(Berchtesgaden) to be published[8] Hutchinson I H, Granetz R S, Hubbard A, Snipes J A and Wolfe S M, 1997 Proc. 24th EPS Conf.on Controlled Fusion and Plasma Physics (Berchtesgaden) to be published[9] Gladd N, Drake J, Chang C, Liu C S, 1980 Nucl. Fusion 23 1182[10] Lau Y T, 1990 Nucl. Fusion 30 965[11] Stek P C, Reflectometry Measurements on Alcator C-Mod PhD thesis, MIT 1997[12] Rice J E, etal , "Observations of Central Toroidal Rotation in ICRF Heated Alcator C-ModPlasmas", this conference[13] Miller R L, Lin-Liu Y R, Osborne T H, and Taylor T S, "Ballooning mode stability for self-consistent pressure and current profiles at the H-mode edge", this conference[14] Medvedev S, etal 1993 Proc. 20th EPS Conf. on Controlled Fusion and Plasma Physics (Lisbon)17C Part IV 1279[15] Bondeson A, Vlad G, Liitjens H, 1992 Phys. Fluids 4 18897

ELMs and fast edge fluctuations in Alcator C-Mod

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ELMs and fast edge fluctuations in Alcator C-Mod

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