De Pena Hempel, S.

Fuchs, J.

Gruber, O.

Meister, H.

Murmann, H.

Ryter, F.

Salzmann, H.

Schweinzer, J.

Stober, J.

Suttrop, W.

English

MPG.PuRe

IAEA-F1-CN-69/EXP2/03 (R)Dimensionally Similar Studies of Connementand H-mode Transition in ASDEX Upgrade and JETF. Ryter, J. Stober, W. Suttrop, J. C. F uchs, O. Gruber, H. Meister, H. D. Murmann,S. de Pe~na Hempel, H. Salzmann, J. Sc hweinzer and the ASDEX Upgrade T eamMax-Planc k-Institut fur Plasmaph ysik, EURATOM-IPP Association,Garching and Berlin, German yJ.P. Christiansen, G. Huysmans, P . Lomas, C. Lo wry, B. Schunke, B. Tubbingand JET TeamJET Joint undertaking, Abingdon, OX 14 3EA EnglandAbstractJoint experiments on connement and L-H transition were performed in ASDEX Upgrade and JET.The connement experiments suggest that the invariance principle is not always fullled at high density.For the L-H transition studies, the dimensionless variables taken at the plasma edge can be in generalonly made identical per pair, due to the condition imposed by the L-H transition. This new approach toinvestigate the L-H physics suggests a weak dependence of the L-H transition mechanism on collisionality.1. INTRODUCTIONThe H-Mode transition and connemen t are key issues for future fusion devices. Compar-isons between tokamaks are expected to bring insight into the physics mechanisms and to nallyallow a reliable extrapolation to fusion reactors. A possible physical way to address this problemis to compare experimen ts performed in devices in which dimensionless parameters are madeequal [1,2]. The experiments presented here are part of a collaboration between JET and AlcatorC-Mod, ASDEX Upgrade and DIII-D, reported in a companion paper [3]. This dimensionallysimilar approach has already been applied to connement studies, for instance in JET and DIII-D [4,5]. The experiments carried out between ASDEX Upgrade and JET, address not onlyconnement but also the L-H transition. A new approach had to be developed for the L-Htransition case because it introduces an additional condition, as discussed in section 3.2. IDENTITY CONFINEMENT EXPERIMENTSIn the "connement identity experiments" the usual dimensionless variables (; ; ; q) aswell as plasma geometry m ust be the same in the two devices. Phenomena occurring on theDebye length or related to atomic physics are assumed to be negligible. In addition the heatingpower deposition proles must be similar. The identity of the dimensionless variables determinesthe size scaling (a = minor radius) of the con trollable parameters, [1,2,5]: BT  a 5=4, Ip a 1=4, ne  a 2, Ptot  a 3=4, T  a 1=2, leading to BT  th = const. If energy transport isdetermined by plasma physics only, heat conductivity  must scale as a+3=4 between the twodevices under such conditions. A important consequence of the above scaling is that JET mustbe operated at the lower boundary of its parameter range (BT = 1.1T, IP = 1 MA) whereasASDEX Upgrade (AUG) was run close to its higher values(BT = 2.3 T, IP = 1.2 MA).A set of pulses has been performed initially in JET in which the plasma shape of ASDEX Upgrade1has been matched. It includes low density (ne;AUG  7  1019m 3, ne;JET  2:2  1019m 3)and high density cases (ne;AUG  10 1019m 3, ne;JET  3:3 1019m 3), at q95  3 .For the low density case, a good match of the global parameters and of the scaled proles(nea2, Tea1=2 and Tia1=2), as well as their gradient lengths, could be achieved by adjustingdensity and heating power in AUG (shot pair: AUG 11229, JET 43868). This results in a goodprole agreement of the dimensionless parameters in the core (tor  0:75), shown in Fig. 1.0.000.020.040.060.080.100.12β0.0 0.2 0.4 0.6 0.8 1.0ρtor0.010.101.0010.000.0 0.2 0.4 0.6 0.8 1.0ρtorυ*0.00.51.01.52.02.50.0 0.2 0.4 0.6 0.8 1.0ρtorρ*Figure 1: Radial prole of ; , and  for electrons in AUG (solid) and JET (dashed).For the pair 11229/43868, the BT  th values agree within the uncertainties: 0.27 for AUG,0.28 for JET. This is in agreement with the expectation from the invariance principle for theglobal analysis. However, the transport analysis of this matched pair reveals that the powerdeposition proles in the 2 devices dier signicantly: it is more peaked in JET than in AUGbecause the NBI acceleration voltage was not scaled. This leads to a dierence in heat uxesof about a factor of 2 between the two devices at mid-radius. The scaled proles being wellmatched, transport analysis indicates that the scaled values of  dier by about a factor of 2at mid radius. The fact that the heat deposition proles are not matched well enough does notallow to apply the invariance principle. It must be stressed, however, that the excellent proleagreement achieved under such conditions suggests that the reaction of proles to changes inheat ux is much weaker than one would expect from a diusive transport model.The high density case was proposed to investigate the connement degradation at high density.In this case the NBI heating deposition is strongly o-axis in AUG, whereas that of JET remainedpeaked. This eect was partly counterbalanced by providing a fraction of the required powerwith ICRH, yielding heat uxes similar to that of the low density case. Three JET pulses atconstant density with dierent heating power are available for the comparison and time slicesfrom 15 AUG shots with dierent heating power at similar densities.0.150.200.250.300.350.400.450.501BTτ E2 3 4 5Pheata3/41.01.21.31.51.62.2 10-4 2.5 10-4 2.9 10-4 3.2 10-4β Nρ*0.030.040.050.060.070.090.100.110.12ν*2.2 10-4 2.5 10-4ρ*2.9 10-4 3.2 10-4Figure 2: High density dataset: normalised connement versus scaled power(a); dimensionlessparameters (b, c). + for JET, others for AUG (see text)The results are summarised in gures 2 a-c in which 3 classes of AUG points are presented:triangles BT  th close to JET values, crosses far from JET values and squares spread overa large range of BT  th. Figure 2.a reects the power degradation for both devices and alsoindicates that BT  th is systematically higher in JET. This can be attributed to the eectof heating power mentioned above and/or to the somewhat higher triangularity in JET. The2triangles have similar ; ;  as in JET, whereas the crosses do not, as expected. Surprisinglythe squares exhibit quite dierent BT  th values but similar ; ; . They demonstrate that; ;  and plasma energy can be identical at the same density for quite dierent heatingpowers, in contrast to the usual monotone increase of plasma energy with heating power (trian-gles and crosses). We veried that the squares also have very similar density and temperatureproles. They are taken from phases in the discharges during which the density can be keptat the required high value, but with less gas pung. As gas pung is experimentally observedto cause connement degradation, less gas pung allows for higher connement. This eectcan occur when heating power is reduced after a high density/high power phase. Followingthe reduction of heating power the ELM frequency decreases and less gas pung is requiredto maintain the density. Thus, after an initial decrease caused by the reduction of the heatingpower, plasma energy increases in relation with the reduced gas pung. This evolution canreaches a quasi-stationary state. Finally the same density and plasma energy are reached thanpreviously but with weaker particle fuelling and at lower power. This behaviour only appearsin the high density shots, little gas pung being required at low density. It is so far not clearwhether such a phenomenon can be observed in JET.This observation sheds new light on transport and identity experiments at high density: therelation between transport degradation and power can be broken and transport can be dierentfor the same local values of ; ; . This suggests several interpretations: i) a strong vari-ation of transport for very small variation of at least one on the parameters ; ; , in thisregion of operational space; ii) the evolution described above reects a change of plasma statefor identical ,  and ; iii) edge particle balance has a strong inuence on transport. Anadditional parameter is perhaps required to characterise the plasma and determine the corre-sponding pairs in the two devices. These high density experiments also provide examples of verysimilar temperature proles obtained with quite dierent heating proles, observation known asprole resilience and widely observed in the tokamaks. This also implies that for very similarlocal values of ;  and  heat diusivity can be dierent.3. DIMENSIONNALY SIMILAR L-H TRANSITION EXPERIMENTSExperiments in which dimensionless variables at the plasma edge are equal may contributeto a better understanding of L-H physics. The situation is dierent from connement studiesbecause data are taken at the time of the L-H transition, introducing an additional condition: theheating power is not free. More details on this work are to be found in [6]. It was experimentallyobserved that the edge temperature has a critical value at the L-H transition [7,8]. Thereforeit is in general not possible to make ;  and  respectively equal in two devices at the L-Htransition but only one of the 3 possible pairs (; ), (; ) or (; ). The required scaling ofthe global parameters has been calculated using the experimental knowledge about the criticaltemperature [7,8], yielding the following scaling for the edge data: (; ) matching: BT / a 1=2T 3=2, ne / a T 2; (; ) matching: BT / a 1 T 1=2, ne / a2; (; ) matching: BT / a 1 T 1=2,ne / a 1 T 2.Thirteen pulses have been rst done in JET followed by 22 in ASDEX Upgrade in an iterativeway to match the edge parameters by varying density. In these discharges the heating powerwas slowly increased across the L-H transition. The measurements allow only the comparisonof electron parameters, but in ASDEX Upgrade analyses of available ion temperatures indicatethat the results are qualitatively valid for ions too. The dataset shown in Fig. 3 provides avariation of , ,  and scaled temperature Tea1=2, at 95% of the poloidal ux, for q95  3and q95  4. The range in  is moderate within each q95 value but  and  vary by  3.In Fig. 3 dierent symbols indicate matched pairs of shots. For the circles the 3 dimen-sionless parameters are quite similar which is a particular situation for the L-H transition asexplained above. In this case, the scaled electron temperatures are similar as well. The heatingpowers at the L-H transition in this case scale as a 1=3, somewhat weaker than a 3=4 (previoussection) but in clear disagreement with a+1 required for divertor similarity in which the identity30.0 1002.0 10-44.0 10-46.0 10-48.0 10-46.0 10-4 8.0 10-4 1.0 10-3 1.2 10-3βρ*0.01.02.03.04.05.06.0 10-4 8.0 10-4 1.0 10-3ν∗ρ*1.2 10-30501001502002503003506.0 10-4Te a0.58.0 10-4 1.0 10-3 1.2 10-3ρ*Figure 3: Edge data at L-H transition: closed symbols for AUG, open for JET; circles goodmatch and squares no match of  (see text), triangles q95  3 , diamonds q95  4in  is replaced by that in temperature [9]. Squares highlight matched (; ) which dier in by a factor of  3 and in scaled temperature by  2. The complete analysis of the datasetshows that  and  can be matched simultaneously within the experimental errors, dominatedby the uncertainty on edge temperature ( 10%), while  varies in one machine by 50% ormore. We conclude that  and  are critical parameters for the L-H transition physics whichhowever depends weakly on , as observed previously in ASDEX Upgrade [7]. The variationof q95 yields similar  and  values, indicating a weak q95 dependence of the L-H threshold.Finally, in a plasma shape variation in JET the L-H transition occurred at the same values ofthe dimensionless parameters.4. CONCLUSIONThe common work between JET and ASDEX Upgrade shows that "dimensionless similarityexperiments" are quite fruitful in investigating both connement and L-H transition physics.The imperfections on plasma shape and heat proles of this rst set of discharges should beminimised in future campaigns. In-vessel modications in ASDEX Upgrade will allow highertriangularity, which is expected to change the situation at high density in particular. The in-crease of the beam acceleration in ASDEX Upgrade from 60 kV to 100 kV and operation of theNBI in JET at lower voltage should allow a better match of the heat proles. The reaction ofconnement to power changes at high density in both devices should be compared in "behavioursimilarity experiments". The results of the new approach to study the L-H transition with di-mensionless parameters at the plasma edge, which suggest that the Kadomtsev constraints arevalid in this case, should be pursued and the extension to other devices is highly recommended.We are glad to acknowledge the excellent support of the ASDEX Upgrade and JET Teams.References[1] CONNOR, J. W. et al., Nucl. Fusion 17 (1977) 1047.[2] LACKNER, K., Comments Plasma Phys. Controlled Fusion 13 (1990) 163.[3] CHRISTIANSEN, J. et al., this conf. paper EXP2/02 (R) .[4] PETTY, C. C. et al., Phys. Plasmas 2 (1995) 2342.[5] CORDEY, J. et al., Plasma Phys. Controlled Fusion 38 (1996) A67 .[6] SUTTROP, W. et al., H-Mode Threshold Edge Parameter Similarity Discharges in JET and ASDEXUpgrade, Proc. of 25th EPS Conf. Contr. Fus. Plasma Phys., Prague, 1998, to be published .[7] SUTTROP, W. et al., Plasma Phys. Controlled Fusion 39 (1997) 2051 .[8] RIGHI, E. et al., Plasma Phys. Controlled Fusion 40 (1998) 721 .[9] LACKNER, K., Comments Plasma Phys. Controlled Fusion 15 (1994) 359.4

Dimensionally Similar Studies of Confinement and H-mode Transition in ASDEX Upgrade and JET

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Dimensionally Similar Studies of Confinement and H-mode Transition in ASDEX Upgrade and JET

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