Braun, F.

Bäumler, J.

Hofmeister, F.

Noterdaeme, J.

Wedler, H.

Wesner, F.

English

MPG.PuRe

ICRF heating for the ASDEX-Upgrade tokamak

AB TRACT 173 ICRF HEATING FOR THE ASDEX-UPGRADE TOKAMAK F. Wesner, J. - M. Noterdaeme J . Baumler, F. Br aun, F. Hofmeister, H. Wedler, Max-Planck-Institut fur Plasmaphysik , EURATOM Association, D-8046 Garching, FRG The 12 MW of additional power for ASDEX Upgrade will be provided by 6 ~~ of neutral beam in jection and 6 MW of ICRH. Such a combi nation has proved to be very efficien t in ASDEX . In order t o enabl e minority and second harmonic heating schemes , 4 equal RF systems with a f requency r ange of 30 - 120 ~ffiz and a generator output power of 2 MW each are being built. The water-cool ed , wide antennae allow the rad i ation of waves with dif f erent K-spectra by exchangeable central conductor geomet ries at a moderate voltage l evel. INTRODUCTION The major aim of ASDEX- Upgrade is to investigate a tokamak plasma and its interaction with the walls in a reactor relevant open poloidal diverto r configuration and with an energy flux density in the plasma boundary comparable to r eactor conditions / 1, 2/ . To reach this aim a heating powe r in t he plasma of about 12 MW fo r hydrogen and 8 . 5 MW fo r deuterium seems necessary /3/, this estimation being based on a confinement similar to t he H-mode found in ASDEX. The heating power of 12 MW was dec i ded to be supplied partly by Neutral Injection and par tly by I CRH. I f exper imental results call for e ven more power, the concept of ASDEX- Upg rade a l lows to add further ICRH or NI devices as well as t he application of other RF heating methods . ICRH GENERAL CONCEPT To provide 6 MW heating power in the plasma 4 equal systems will be built with a genera t or power of 2 HW each . This power should be available f or standard magne tic field values of ASDEX- Upgrade i n the single- and double-nul l diverto r - and the limiter configuration . Heating should be possible, too , up to the maximum magnetic field , a somewhat reduced power being unavoidabl e. High fie ld side antennae bei ng nearly impossible due to the poloidal divertors , heating by mode c onvers ion is not practicable. The ICRH a pplication is thus restricted t o second ha rmonic and minority heating. Fi gure 1 gives t he frequency r anges necessary for these heating methods for the main operational regimes of ASDEX- Upgrade. In view of the still experimental status of ICRH it was decided t o buil d a system providing max imum experimental flexibility: A frequency range of 30 - 120 MHz allowing second harmonic heat i ng in D and H as well as minority heating with H or He3 in D. An ant enna concept allowing variations of the k .. - spectrum as well as fitting the antennae to different plasma c ross- sections (singl e- and double-null divertors ) by relatively simpl e modifications. Every provis i on has been made fo r a possible later increase of the hea ting powe r by 2 additional sys tems . I f needed , the total numbe r of ante nnae could even be increased to 8. 174 EXPERIMENTAL EXPERIENCE Besides the large experimental experience of PLT and TFR /e.g. 4 , 5/ , more recent results of ASDEX , JET /6/ and rEXTOR /7/ can now be taken into account. Especially ASDEX results /8, 9/ a re highly relevant due to the divertors and the similar heating scenarios . A summary of these results in view of the choice of heating methods for ASDEX-Upgrade show: Second harmonic and minority heating are very efficient heating methods, the measured heating efficiencies of 20-25 kJ/MW being larger than for NI in the L-mode. A combination with NI improves the available parameter range for ICRH (e.g. the impurity problems being reduced). ICRH is accompanied by high-Z impurities leading to about 30 % of power radiation from the plasma centre. This can largely be r educed by low-Z wall material (e.g. carbonization). The physics of the impurity release i s still under investigation. It is possible to get the H-mode with ICRH /9/. This seems to be somewhat easier with minority heating . Further intense investigtions are necessary in this field. Second harmonic heating is largely independent of the gas composition (e.g . for znH the content of H ions should only be ;l!:. 40 %) allowing a larger experimental flexibility than minority heating. So the choice of a combination of NI and ICRH as well as the large fre-quency range all owing second harmonic and minority heating are fully justi-fied by recent resul ts. A restriction to a smaller frequency range and only one ICR-heating scheme, despi te of its attractiveness from t he economic point of view, seems not yet reasonable for the next generation of experi-ments. ANTENNAE The development of antennae for ASDEX-Upgrade has the following aims: Optimization for single-null operation and second harmonic heating (80 MHz). The antenna will be used for minority heating, too, with somewhat worse coupling characteristics. The antenna should allow different s pectra for K .. by using different centre conductor geometries. Fitting to different plasma cross- sections (single-null, double-null divertor and l imiter operations) is possible by changing simple side wall elements, the Faraday screen and side limiters being reused. The antenna has to be cooled. The antenna geometry was optimized with respect to the electric field and voltage levels in the antenna and transmission lines. For this purpose the coupling calculations with 2- and 3- dimensional codes were combined with ray-tracing calcula,ions in order to take into account the effect of the geometry on the power absorbed in the plasma, no.t only on the power radiated by the antenna /10/ (Fig . 2). . A simple loop antenna leads to a k .. -spectrum peaked at K .. = 0 , mainly exciting fast waves with small k .. which are weakly damped by the plasma. It possibly excites coaxial waves, too, which may contribute to the impurity release. Since experimental results are still contradic tory concerning these ques tions, the spectra of the ASDEX-Upgrade antenna will be variable by exchanging the cent re conductor array (Fig. 3a ,b) . 2D- calculations show, that for an antenna wide enough the voltage necessary for radiati on 175 and absorption of a certain power, e.g. in an array like Fig. 3b mus t no t be l arger than for a single l oop antenna (Fig. 2). It can be seen further in Fig. 2, that for such an array and with a given antenna width there exists an optimum for the widths of the centre conductors and gaps. Figure 4 shows the principle design of t he antenna . It is based on the technology of the ASDEX antenna providing TiC- coated, optically open Faraday screens / 11/ , carbon protection limiters and water cooling. For single null operation the antennae are asymmetric with respect to the midplane of the expe riment in order to utilize the full available space for a large di s t ance between central and return conductor . LINE AND MATCHING SYSTEM The antennae are fed at their both end s by 6 1/8" vacuum lines with bellow sections to compensate thermal expansions. They a re connected to the pressurized 9" lines by two feedthroughs with intermedia t e vacuum. First stubs near t he upper and lower antenna feeder will reduce the VSWR for the maj or frequency range (~CH corresponding to a bout 80 MHz) and allows cooling of the inner conduc t or of the vacuum l i ne. The antenna impedance is matched to that of the generato r by double stub-tune r systems near the experiment, the distances between the two stubs and between stubs and antennae being adjustable by exchangeable elements. RF GENERATORS The 4 RF generators will be very s i milar to those of ASDEX-W VII /12/ . Besides mino r modifications based on the operation experiences with ICRH experiments on ASDEX and the extension of the upper frequency limit from 115 to 120 MHz the output power of each generator will be extended from 1.5 to 2 MW up t o 80 MHz and up to 1 . 2 MW at 120 MHz, Special tests with an ASDEX-W VII gener ator showed that these values can be achieved / 13/ . At least at lower frequenci es a further i ncrease of the power should be possible. REFERENCES /1,2/ IPP Garching Reports Nr. 1 / 197, 1/211 (1982) and 1/217 (1983) /3/ Additional hea ting fo r ASDEX- Upgrade, IPP Garching Rep.l/237 (1985) /4/ e.g. J. Hosea et al ., Proc. of 4th Int. Symp. on Heating in Toroida l Plasmas, Roma 1984, P.261-275 Proc. of 12th European Conference on Controlled Fusion and Plasma Physics , Budapest, 1985 /5/ e.g. J, Adam et al., 4th Int . Symp. on Heating in Tor oida l Plasmas, Rams 1984, P . 277-290 /6/ J , Jacquinot et al ., 12 th Europ. Conf. on Contr ol led Fusion and Plasma Physics, Budapest, 1985, o r p , p , Lallia, this conference /7/ A.M. Messiaen et al., ibid /8/ K. Ste inmetz et al ., ibid /9/ K. Steinme tz e t al . , t his confe r ence / 10/ J.-M. Noterdaeme, M. Soll, 12th Eur op. Conf . on Controlled Fusion and Plasma Phys ics , Budapest , 1985 / 11 / F. Wesner et al . , 4th Int.Symp. on Heating in Toroidal plasmas, Rams 1984, P.1103 , a nd J.-M. Noterdaeme et al . , this conference /12/ w. Schminke, F. Hofmeister, F. Wesner,10th Symp. on Fusion Engineering , Philadelphia, 1983, p. 1498 /13 / J. Wys s , K.Holm, Applications of RF Waves t o Tokamak Plasmas, Varenna , 1985, p.882 176 Frecp.Jency range ~: ASDEX- Upgrade operational regimes and ICRH frequencies Arb. Un. 3.0 2.5 2.0 3b 3a 2w, = 42cm:<= Ocm 2w1 =14cm:<= 14cm 2w, = 11 cm :<= 20cm 2w, = 9 cm :< = 24 cm Fig. 3a: Spectra of an antenna array like Fig. 3b for different distances and a total width of 70 cm. Spectrum (a) is fo r one loop . 10 Votues lor the 1 • 2 onh!MO array 30 ...!::. 12vl z42 cm, d= 14cm. some total width 70 cm I os 0.4 0.3 20 .,V 'l»iW 02 -/ -v--p-;---· , ~HW • - /'•"" ........ yiiCI ~.---· ' 0.1 10..:!.. OIL-~~~~~~~~~--~ 1 10 203010 -£1cml ~: Power absorption and antenna voltage for an array like Fig. 3b and simple loops ~: Cross sectional views of the ICRH ant ennae in the vacuum vessel. 

http://hdl.handle.net/21.11116/0000-0009-15D6-E

ICRF heating for the ASDEX-Upgrade tokamak

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