Bombard, B.

Goetz, J.A.

Hubbard, A.

Hutchinson, I.H.

In, Y.

Irby, J.

Shea, P.J.

Snipes, J.

Stek, P.C.

Takase, Y.

Wolfe, S.M.

DSpace@MIT

PFC/JA-96-42Local Plasma Parameters andH-Mode Threshold in Alcator C-ModA.E. Hubbard, J.A. Goetz, I.H. Hutchinson, Y. In,J. Irby, B. LaBombard, P.J. O'Shea, J.A. Snipes,P.C. Stek, Y. Takase, S.M. Wolfe, and the Alcator GroupOctober 1996To be published in the Proceedings of the 1 6 1h IAEA Fusion Energy Conference.This work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.F1-CN-64/AP2-11LOCAL PLASMA PARAMETERS AND H-MODE THRESHOLD INALCATOR C-MOD'A. E. HUBBARD, J. A. GOETZ, M. GREENWALD, I. H. HUTCHINSON, Y. IN,J. IRBY, B. LABOMBARD, P. J. O'SHEA, J. A. SNIPES, P. C. STEK,Y. TAKASE, S. M. WOLFE and the ALCATOR GROUPMIT Plasma Fusion Center, Cambridge, MA 02139, USAAbstractLocal plasma conditions in the outer region of the Alcator C-Mod tokamakare studied at the transition between L and H mode. Both controlled parameterscans and a database of H-mode discharges are examined. It is found that a mini-mum edge temperature, as measured by electron cyclotron emission diagnostics,must be exceeded in order to enter and remain in H-mode. This condition is not al-ways sufficient, however. Density, and therefore pressure, appear to play a less im-portant role. Comparison of 5.3 T and 8 T discharges shows that the threshold tem-perature increases with BT. The ion larmor radius thus remains in a narrow range atthe transition, while beta and collisionality show a very wide variation. Dischargeswith the ion grad-B drift away from the x-point have L-mode temperatures similarto those with the usual direction toward the divertor. Both the global power thresh-old and local edge temperature at the L-H transition are approximately doubled.1. INTRODUCTIONThe role of the local temperature and density in the transport barrier regionin determining the transitions between the L-mode and H-mode confinement re-gimes has been examined on the Alcator C-Mod tokamak. C-Mod is a high field,compact device, with R=0.67 in and a=0.21 m. H-mode is routinely obtained withICRF heating, at toroidal fields up to 8 T. Ohmic and RF H-modes have also beenseen at Br as low as 2.6 T. These parameters are quite different from those of otherdivertor tokamaks, making C-Mod an interesting device for H-mode threshold stud-ies. Most operation has been with molybdenum walls, but improved H-modes havebeen obtained in 1996 experiments following boronization [1].0.60 [o7 Fig. 1. Global H-modepower threshold for C-0.50 g~rnMod, showing scaling with- -the product of density and4-- p/s - 0-0 magnetic field.0.30-0.20 P/s - 0.022nB0.10 ' +Other Tokomnks0.00.0 5 10 15 20 25n.BT (1020 m- T)1Scalings for the H-mode power threshold on C-Mod in terms of globalquantities show that the power required to obtain an L-H transition is Pthresh(MW)0.02-0.04 ne BTS, where ne is the line averaged density (1020 m-3) and S the surfacearea (m2) [2]. Wall conditions appear to play a role and after boronization thresh-olds tend to lie at the lower end of this range (Fig. 1). Below a density of -0.9 x102Om-3, the threshold power increases sharply, giving a low density limit which isrelatively high. Such global parameter scalings, which include C-Mod data, areused to predict power requirements for ITER [3]. However, with any present scal-ing, there is typically variation of a factor of two or more in the threshold power ona given machine, and nearly an order of magnitude when comparing different ma-chines. A lower threshold is generally found for the H-L transition, and the scalingof this hysteresis is even less well established. A further limitation of global powerscalings is that they provide little insight into the physical mechanisms responsiblefor the transitions in confinement.The limitations in our understanding and predictive capability motivate thesearch for more local threshold conditions. It has been observed since the earliestH-modes on ASDEX [4,5] that edge parameters appear to play a role. In particular,the transition was often correlated with a sawtooth heat pulse, and H-mode could bedelayed by cooling the edge. Recent experiments on DIII-D [6] also point to a rolefor edge electron or ion temperature.2. DIAGNOSTICSThe study of local parameters on C-Mod concentrates on the region just in-side the separatrix. Since this is where temperature and density gradients are ob-served to steepen and fluctuation levels to drop in H-mode, it is likely that this is animportant region for the threshold. The primary edge diagnostics are electron cyclo-tron emission (ECE) for Te(R) and reflectometry for ne(R). ECE diagnostics consistof a Michelson interferometer, which is absolutely calibrated, and a nine channelgrating polychromator, which gives higher time and radial resolution and is cross-calibrated against the Michelson. Estimated systematic errors are + 10% on T,with radial resolutions approximately 1 cm. The random noise level is typically -20 eV. Density profiles are measured by an interferometer with 10 vertical chords.For some discharges, these are supplemented by edge profiles from a 5-channel o-mode amplitude-modulated reflectometer. A scanning Langmuir probe is used tomeasure profiles outside the separatrix, where the ECE becomes optically thin.Probably the biggest source of uncertainty for all of these measurements, given thesteep gradients characteristic of the edge region, is the mapping of data to a givensurface of normalized poloidal magnetic flux, V. The mapping is done using theEFIT magnetic reconstruction package [7] and its estimated accuracy is 3 mm at thehorizontal midplane. The effects of these uncertainties can be minimized by keep-ing magnetic equilibria similar while doing controlled threshold scans.s3.0 _,____)Fig. 2. Time evolutionof plasma parameters2- during an ELM-free H-8 mode with I = 1.0 MA3~~ 3 v r-(9_______________________ and B,.=5.3 '. The ver-tical lines show the L-Hand H-L transitions..1.5-- P..(Mw)2-0.60 0.70 0.80 0.90 1.00 1.10t (s)2The time behaviour of plasma parameters during a typical ELM-free H-mode with boronized wall conditions is shown in Fig. 2. In the RF-heated L-mode,electron temperature increases throughout the plasma cross-section while the densityis unaffected. Once the H-mode transition occurs, as evidenced by a drop in Da,there is a rapid formation of an edge pedestal in both density and temperature. Thecentral Te and ne also increase, as does the radiated power due to an improvement inimpurity confinement. This gradually erodes the edge Te pedestal, and an H-L tran-sition occurs at 1.02 seconds. ELMy and enhanced Da H-modes are also observedon C-Mod. For threshold studies, it is the conditions just before the L-H and H-Ltransition, rather than the details of H-mode behaviour, which are of interest.3. RESULTSThe most striking result to date is that for a given magnetic configuration,ie. constant I,, BT and grad-B drift direction, the L-H transition occurs at a nearlyconstant edge Te over a wide range of plasma densities and input powers. This ismost clearly evident in a controlled parameter scan such as that summarized in Fig.3, which shows results at 0.8 MA and 5.3 T. For each discharge, the power wasstepped up and the temperature at the V=0.95 surface increased correspondingly. Ineach case the L-H transition (solid symbols) occurred when Te(lyl=0. 9 5 ) reached0.12 keV. At the lowest densities in this scan, the optical depth of the ECE at thisradius is becoming marginal. However, the same trend is observed further in theplasma (V-=0.85), where Te and ne are higher. The power required to reach thethreshold temperature generally increases with density, consistent with the globalpower scaling. The exception is near the low density limit, when the edge is muchcooler and where more power is required to reach H-mode. In other low ne dis-charges of this scan, the edge remained below 0.1 keV even at maximum availablepower and H-mode was not achieved.0.25 1(10m~ Fig. 3. Dependence of0.20 hmic H- ode) Te at Nf=0.95 on total1.8 input power in L-mode,0 - ( -m0 for a series of dis-0 charges with differentC - ------- - -- densities. The solid0.10 symbols indicate condi-tions just prior to the L-0.05 H transition.0.00 10 1 2 3Pw (MW)The trend of a minimum edge temperature for achieving H-mode is alsoseen in the wider database of C-Mod shots. Figure 4 shows a number of dischargesfrom the post-boronization run period, with I = 1 .0-1. 2 MA and BT = 5.3 T. While,not unexpectedly, there is more scatter in the data, all the L-H transitions (solid tri-angles) occur at Te > 100 eV and most are at Te < 150 eV, independent of density.A few discharges are found to have higher temperature at the transition, in particularthose with ne approaching the low density limit. This implies that the edge tempera-ture threshold may be a necessary but not always sufficient condition for enteringH-mode. Similar trends were found in the 1995 data [2]. The lower threshold Tequoted here is a result of more accurate calculation of the location of ECE channelswith respect to flux surfaces; there is not a systematic change due to boronization.3m0 L Fig. 4. Edge Te vs lineS-mode averaged density forA L-H transition 1 discharges with I = 1.0-H-L transition *P>04 mode . * 1.2 MA and B,5.3 T.1 0.- H-Note the large increaseo in T during H-Mode,0 0and the clustering ofII * V values at the L-H transi-0.2 - tion.U -0.0 .0 1 2 3 4A, (m- 3)Scalings of local discharge parameters with global parameters show that thestrongest dependence is on toroidal field. The L-H transition temperature increaseswith BT (Fig. 5), with a power between 1.2 and 1.5, depending on the dataset usedand the other parameters included in regression fits. This is consistent with a globalpower threshold which increases with BT, as seen on C-Mod and in multi-machinedatabases. No significant dependence on I is seen. There is a weak dependence ofthe average threshold temperature on ne, due mainly to the increased scatter at lowdensity. The minimum Te at which H-modes have been seen is, however, nearly in-dependent of density.0.25. Fig. 5. Dependence of Tethreshold on toroidal field,0.20 for a group of 1996 dis-charges with I,=1.0-1.1A MA and density well above0.15 the low density limit.0 Dashed line is best fit, pro-10.10 / portional to BT1.460.050.000 2 4 6 8 10BT (T)The behaviour at the H-L transition is more complicated, and depends onthe type of H-mode and radiation conditions. After the H-mode edge temperaturepedestal is formed, it tends to decrease as density and Prad increase due to improvedparticle confinement, especially in ELM-free H-modes. If the edge temperature isreduced below the L-H threshold temperature, the plasma reverts to L-mode. Thiswas the typical behaviour in pre-boronization experiments, and leads to a relativelyslow loss of confinement. On many boronized discharges, particularly ELMy H-modes, the edge T. remains well above the L-H threshold throughout a long H-mode. When the H-L transition occurs, there is a rapid collapse of the edge pedes-tal. At least some of these discharges, which typically have very steep edge pressure4gradients, are computed to be unstable to ballooning and low n ideal modes. MHDactivity is sometimes seen prior to such H-L transitions. It again appears that theedge temperature threshold is a necessary but insufficient condition for remaining inH-mode, and that this local condition, unlike global power thresholds, does notshow hysteresis. In this context, the lower H-L power threshold may be seen as aresult of the improved confinement in H-mode, ie. for a given P/ne, an H-modeplasma will have a hotter edge. Given the wide variation in H factors which is seenunder various conditions and on different experiments, it is not surprising that theH-L global power threshold shows a wide variation [3]. The added requirement forMHD stability further complicates this power threshold.An interesting and often-observed feature of the H-mode is that approxi-mately twice as much power is required when the ion grad-B drift direction is awayfrom instead of towards the active divertor. In the context of a local threshold, it isthus of interest to compare the edge plasma conditions in the two configurations.While most C-Mod operation, including all of the results quoted above, has beenwith the favourable drift direction, a few days were devoted to operation with 're-verse' BT and I . It was found that in L mode, edge temperatures follow the samescaling with total power per particle as in discharges with 'normal' field direction(Fig. 6). The main difference is that they remained in L-mode with much morepower, and correspondingly higher edge temperatures. In the few discharges whichdid briefly enter H-mode, the temperatures at the L-H transitions were in the range260-320 eV, approximately twice the threshold values with the more favourabledrift direction. A similar observation has been mentioned on ASDEX Upgrade [8].This shows that the reason for higher global thresholds with reverse field is not thatthe edge is cooler. Whatever the mechanism for the H-mode transport improve-ment, it clearly is affected by field direction, either via drift or other flows.0.40 ' Norrnal B; L-mode Fig. 6. Comparison ofA Normal BT L-H Te (N=O.95 ) vs0 Reverse BT L-mode * for similar groups of dis-0.30 *Reverse BT L-H charges with ion grad-Bdrift towards and awayfrom the divertor. Pa-0.20 - rameters are BT =5.3-1A AA *5.6 T, I =0.8-1.0 MA,0 0 ne=1.8-3.0 x 10 32 0.10 A and RF power 0-3.1MW.0.00.0.0 0.5 1.0 1.5 2.0P t./h.( 1 0-W/m-5 )4. DISCUSSIONThresholds based on local parameters offer a basis for comparison with H-mode transition theories. Most prevalent theories involve stabilization of turbulencevia ExB shear, though there is debate as to the mechanism for the shear [9]. Itshould be noted that the measured ECE temperatures inside the separatrix are in-dicative of the temperature gradient in the transport barrier region. Measured gradi-ent scale lengths are typically 1 cm or less, limited by diagnostic resolution. At thehigh densities on C-Mod we expect Tj - Te, but are presently lacking spatially re-solved measurements of Ti in the edge region. It will thus be difficult to distinguisha threshold in T. from one in Tj or dT/dR. Similar thresholds and scalings to those5shown are found plotting dTe/dR, or Te at the 85% or 90% flux surface, so 4f=0.95is not necessarily the critical location for H-mode transitions. However, since up-down asymmetries mainly affect the region outside or just inside the separatrix, theresults of reverse field experiments imply that the extreme edge region plays an im-portant role in H-mode physics.The threshold scalings can be used to help identify which dimensionlessvariables may be important for H-mode transitions. The observed temperaturethreshold is not strongly dependent on ne. This implies either that Ti* and 0 are notimportant variables, or that their functional dependences are such as to cancel outthe density dependence. Given the positive dependence of threshold Te with field,the ion gyroradius p emerges as a possibly important parameter. Figure 7 shows pvs 0 at the 95% flux surface, calculated using BT at the outer midplane and assum-ing that Ti=Te, for all L-H transitions in the database for the spring 1996 campaign.While there is scatter in both parameters, it is clear that the ion gyroradius showsless variation at the transtion than does P and is nearly independent of BT. The cal-culated p - 0.4 mm is to be compared with gradient scale lengths of 1-2 mm typicalof the C-Mod SOL near the separatrix. The normalized ion collisionality Ti* atN'=0 .95 varies from 2.1 to 32 at the L-H transition, making it an unlikely candidatefor a dimensionless threshold.0.8.A 5.3 T L-H Fig. 7. Variation of P andA 7.9 T L-H ion gyroradius at xV=0.950.6 -A for L-H transitions with aE A s wide range of plasma pa-A A sA A s A~ rameters.S0.4-0A11Q- 0.20.00.00 0.02 0.04 0.06 0.08P,p =0.95 (Z)Comparing the observed local and dimensionless parameters on machines ofdifferent sizes and global parameters, as is proposed in a new version of the ITERthreshold database, may provide further opportunities for scalings and tests of the-ory. Planned experiments on C-Mod include extending the density range of H-modes at 8 T and more systematic study of the H-L transition and low density limit.REFERENCES[1] TAKASE, Y. et al, Paper FI-CN-64/A5-4, these proceedings.[2] SNIPES, J. A., HUBBARD, A. E., GARNIER, D. et al, Plasma Physics andControlled Fusion 38 (1996) 1127.[3] RYTER, F. AND THE H-MODE DATABASE WORKING GROUP, JETReport JET-P(95)55, submitted to Nuclear Fusion (1995).[4] WAGNER, F. et al, Phys. Rev. Letters 53(15) (1984) 1453.[5] ADSEX TEAM, Nuclear Fusion 29(3) (1989) 1959.[6] T. N. CARLSTROM, T. N. and GROEBNER, R.J., Physics of Plasmas 3(5)(1996) 1867-1874.[7] LAO, L.L., ST. JOHN, H., et al, , Nuclear Fusion 25 (1985) 1611.[8] ZOHM, H., Plasma Physics and Controlled Fusion 38(1996) 105-128.[9] GROEBNER, R.J.,Phys. Fluids B 5 (7) (1993) 2343.6

Local plasma parameters and H-mode threshold in Alcator C-Mod

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Local plasma parameters and H-mode threshold in Alcator C-Mod

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