Becker, W.

Braun, F.

Bäumler, J.

Fritsch, R.

Hofmeister, F.

Murphy, A.

Noterdaeme, J.

Ogawa, Y.

Puri, S.

Ryter, F.

Wedler, H.

Wesner, F.

English

MPG.PuRe

lOSS ANALYSIS OF TIlE WADING RESISTANCE FOR ICRF HEATING EXPElUMENfS IN ASDEX Y. Ogava·, F. Hofmei ster. J . -M. Noterdaeme. F. Ryter. F. Wesner. J. Baumler. W. Beeker. F. Braun. R. Fritsch. A.B . Murphy . S. Puri. H. Wedler , ASDEX- , NI- , Pellet-Teams Hax-Planck-Inslilul fur Plasmaphysik . EURATOM Association . 0-8046 Carching, FRG. t Institute of Plasma Physics, Nagoya Univ .• Nagoya, 464-0 1. Japan Introduction Long pulse high- power ICRF heating experiments have heen conduc t ed in the H-mi nor ity (10% H. f=33.5NHz) and second harmonic (2!JcH,j=f57NHz ) r egimes with two l ow- fie ld-side a ntennas, in combination with NI and repetitive pellet refueling {I). In three parti cul ar cases the l oading resistance has been compared vi th theory, usi ng a global wave code with a s lab geometry (2). 1) In H-minority heating experiments . H-phases l asting for up ·to 0.5 sec have been achieved in combination with NI. At the L-H transition the l oading resist ance decreases s harply. But . during the H-phase . the increasing density induces a variation of the l oading resistance. Due to the presence of eigenmodes the pre- transition value can be recovered . 2) In l ong pulse ~ heating eXperiments . a s lowly changing isotope concentration (H concentration: from 70% to 85%) is accompanied by a gradual decrease of the loading resistance. 3) In ~ heating exper i ments with repetitive pellet refueling. in which t wo confinement phases have been identified. it has been observed that the temporal behaviour of the l oading resistance is also quite differen t for the two phases. 11 . H- Minority Heating Experiments In H minority ( to% H) heating exper iments. H- phases lasting for up t o have been achieved in combination with NI . The L- H ·transition induces a sudden decrease of the l oading resistance (Fig . 1) . which corresponds to a decrease of t he density in the SoL. However . the loading resistance does not stay at the reduced l evel during the H- phase. but changes on a l ong time scale (0.2 sec) with many spikes due to ELMs . The s l ow variation i s not correl ated with the plasma position. Despi te the f act tha t the density rises monotonically during the H-phase. the loading resistance has revealed maximum/ minimum values. This i s interpre ted in t erms of e igenmode effects . Indeed. t o understand this s l ow var iation during the H-phase . we have calculated the loading resistance summed up for various toroidal mode numbers n (=k.Ro) with the gl obal wave code. as a function of density (Fig . 2). Peaks of the l oading resi s t ance correspond to eigenmodes standing between two cut-off layers of the fast wave (i.e . • at the two- i on hybrid layer and at the plasma edge) . The experimental loading res i stance (3) during the H- phase is also plotted as a function of the density. The spacing of the peaks and the amplitude of the variation are in agreement with the l oading 1086 resistance calculated for n c= l6. Therefore. we conclude that the slow variation of the loading resistance during the H- phase indicates the existence of the eigenmode . This is the first clear evidence of eigenmodes in ASDEX. Note that the theory predicts the disappearance of the eigenmarle, if the percentage of hydrogen is reduced below 6% . P" IMWI A · H tcm) 10' Real. o 10/ml I.l 1.1 TIME lse(1 ~ H- mode' discharge in H-minority heating experiments . ,Hotdid IQ.u.l 10% 111. Second Harmonic Heating Experiments la 0 4. 6 8 la 12 ne IO)( x1019 m-J , 3 R~xp.. 2 101 Fig . 2 The loading resistance cal culated with a global wave code is plotted as a function of the central density 1le (O) for various sums of the toroidal mode . numbers n (=k.Ro) . The experimental dala shown in Fig. t are also plotted. 111.1 Long pulse discharge _ Figure 3 shows a typical l ong pulse discha rge for 2Oc1f heating experiments, where NI was used only at the beginning of t he ICRF pulse. Let us first discuss the beneficial role of the NI ( either ~ or DO). The loading res istance has been calculated as a function of the hydrogen temperature THCO) . The loading resistance increases monotonically with TH CO) below 1 keY, and saturates at TH CO» I keY. Since the ion temperature in the ohmic phase is 0.6-0.7 keY in ASDEX plasmas and increases with NI pre- heating above 1 keV, the achieved better loading could be one reason why NI at the beginning of the rf pulse is beneficial for rf operation . 1087 After the turn-off of NI . the gradual decrease of the loading resistance has been observed on a long time scale (1 sec), accompanied by B s lowly changing i sotope concentration ( the H concentration increases from 70% t o 85%) . Other parameters are constant . From the computer code we found indeed that. as the H concentration is increased from 50% to t (X)%. the loading resistance decreases slightly, in agreement with the experimental observat i ons . This can be under stood in terms of the difference in the cut-off density f or the fas t wave between deuterium D and hydrogen H; i. e" nH (cut -off)/nD(cut -off)=(~+ntH)/(w+OtD )= 1. 2 f or all various toroidal mode numbers n{=k.Ro) . Figure 4 s hows the positions of the cut-off for Hand D ions. When 0 i s replaced by H. the position of the cut- off for the fast wave moves about 0.33 cm away from the antenna. giving a s l ight deteriora tion of the antenna- plasma coupling. ", • 1 )C1019 m-)1~ o. BD nH 60 nH + nO " 1%1 20 ' .0 Rn 3.1 IQ! 3.0 2.1 " Prel. IkWI 20 'IF 2) 961 _n. ~c . -PMI ~ PlC. NI o IMWI H,-Rn 0.1 1.0 TIME I sec I Mass 1 AnalysiS 10.u.1 m.J Long pulse discharge for 3lcH heating, with the help of the NI at the beginning of the rf pulse . The gradual decrease of ; the l oading resistance experimentally observed could thus be explained by the increase of the cut- off density for H enriched plasmas. although the role of eigenmodes or changes in the SOL can not be excluded. I ! , I •• O.U .n Tlm) Ui ." Fig . 4 Positions of the cut-off densities for the fast wave at the SOL for H and 0 ions. 111 .2 Repetitive Pellet Refueling In 2ncH heating experiment refue led by r epetitive pelle t injection. two confinement phases (phase I and 11) have been observed (1]. The improved confinement phase (phase 11) is characterized by a peaked density profile. accompanied by a deeper penetration of the pellet. Figure 5 s hows the Max- P l aiis !~ - \", ," , :j ,-! t i [ i Pl:'l"'"':l:phys!K r .r "'j ;.~l.',,:::·en 1088 -loading resistance of the two antennas (called SO and NW) for the two phases. " Synchronized to the pellet injection. a large positive spike has been observed clearly only on the signal of the SO antenna in phase I. Since the pellet injector is toroidally only 45" away from the SO antenna, but 135° from the NW antenna, this s pike could be an indication of a strong perturbation transiently produced rather l ocally by the pellet in the boundary. The temporal behaviour of the loadin-g resistance after the pellet injection is quite different for the two phases. In phase I. the signal is characterized by a quick recovery( ..... ISmS) followed by a gradual decrease, and in phase 11 the loading resistance is staying at the r educed level for a relatively long time. The plasma position seems to bear ·no clear correlation with it, as shown in Fig. 5. The time scales of these gradual changes seem to be corresponding not to those of SoL parameters, but to those of the bulk plasma (e .g .• the changes of the density profile. W, I k JJ Ion I.U 1.11 1.U Phose I ~ 70 . I I I 60 1·1 1 1 1 1 1 i w, ~ 50 , 2.]1 2:30 D2 Phose 11 I I I I .,. 23 996 " OL-~~~-L~~~7-~~ 1.6 1.& 2.0 2.2 H TIME I sec J Acknowledgements H temperature and its high energy tail. and so on). Further work is in progress. Fig . 5 2f1cH heating experiment in combination with repetitive pellet refueling, where two confinement phases (I and 11) have been identified. The loading resistance for two· antennas (SO and NW) and the plas ma position 61, are shown for the two phases. Arrows show the times at which pellets are injected . The authors wish to thank the ~embers of the technical and operation teams of ASDEX. the ion cyclotron heating, the neutral beam injection and the pellet groups. One of the authors(Y.O.) acknowledges the members of Technology, ASDEX and W-V1I groups for their hospitality. Computations are performed using the mAY- computer in the Computer Center of the Max- Planck- 1nstitut fur Plas maphysik. References (I) Noterdaeme. J.-H .• et al .• 12th Int . Conf. on Contr. Fusion and Plasma PhYsics. Nice 1988. lAEA- CN-50/E-2- 2. (2) Fukuyama, A., et al . • Comp. Phys. Rep. 4 (1986) 13'7. (3) Hofmeister. F .• e t al., Proc . 14th Symp. of Fusion Technology (1986. Avignon) , CEC. vol. 1, 801. 

Analysis of the loading resistance for ICRF heating experiments in ASDEX

http://hdl.handle.net/21.11116/0000-000A-2C60-9

Analysis of the loading resistance for ICRF heating experiments in ASDEX

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