R. Kumazawa

T. Watari

T. Yamamoto

Kumazawa,  R.

Yamamoto,  T.

Watari,  T.

NIFS-Repository (National Institute for Fusion Science)

§26. Dependence of ICRF Heating Performance on Magnetic Axis Kumazawa, R., Yamamoto, T., Watari, T. The position of the magnetic axis, Rax is a key parameter to retermine the behavior of the reeply trapped ions, e.g., accelerated by RF electric field and the MHD stability aspect. The inward-shifted configuration reduces the reviation of the particle orbit from the magnetic surface in spite of reteriorating the MHD stability. The standard configuration has been selected to be Rax=3. 75m by compromising these two aspects. When the orbits of ions with perpendicular pitch angle are compared between the different magnetic axis configurations, i.e. Rax=3.6m and =3. 75m, the improvement of the drift surface of a trapped particle is remarkable in the inward-shifted configuration.[!] The bulk plasma confinement was different in two different magnetic axis. The energy confinement time was found to be 1.5-1.6 times longer than that of International Stellarator Scaling 95 at Rax=3.6m [2]; however no enhancement was not observed at the outward shifted configuration, i.e., Rax=3.75m. The most of ICRF heating was carried out at the inward-shifted magnetic axis, i.e., Rax=3.6m. In adfition to the selection of the position of the magnetic axis, the location of the ion cyclotron resonance layer affected the effective ICRF heating. So far the optimum ICRF heating has been found at the experimental condition of Rax=3.6m and B=2. 75T, where the proton cyclotron resonance layer is just located at the sad.Ile point of the contour of the magnetic field strength at the applied frequency of f=38.47MHz. In the 4th experimental campaign, the performance of the ICRF heated plasma was compared at the different magnetic axis configuration. Time evolutions of the plasma stored energy and the ICRF and the NBI heating powers are shown in Fig.l(a) and (b); The ICRF heating was applied to the NBI heated plasma during 1. 7 to 2.3 seconds, and (a) and (b) is the case at Rax=3.6m and 3.75m, respectively. The injected NBI power is 0. 9MW in both plasma discharges and the average electron rensity is the same of ne=l.Oxl019m-3• The absorbed ICRF heating power P abs is 0.84MW at (a) and 0.45MW at (b), respectively. The achieved plasma stored energy WP is 210kJ and lOOkJ at the ad:litional ICRF heating period; this was caused by the difference in the energy confinement time of the bulk plasma The energy distribution of high energy ions was measured using a Natural Diamond Detector (NOD) in the energy range of 30keV to 250keV as shown in Fig.2(a) and (b). The high energy ions extendxl to 200keV and the tail temperature is calculated at the same value in both cases. Here the tail temperature Ttaii can be compared using Stix formula as follows, T == T (1 + pabs rs ) eff- e 3nH VH~ Ttail was calculated to be almost the same, i.e., 28 (au.) and 25(a.u.), corresponding at Rax=3.6m and 3.75m. Here the electron temperature and the minority ratio to the electron rensity, nJne were used at Te0=2.0keV and 1.6keV, and nJne=8% and 4%, respectively. The confinement of the high energy ions was assumed to be ootermined by the electron slowing process only. When there exists an another energy loss of the high energy ions, the total energy confinement tEtail is expressed as, 1 1 I --=--+--rtail r I 2 rtoss E S E Here 't/2 is a slowing down time due to electrons and 'tEioss is a power loss time of high energy ions, which incluoos the orbit loss and the charge exchange loss and is thought longer at Rax=3.6m than that at Rax=3.75m. As 't/2 was much shorter than 'tEtoss at the present experiment, the difference in the tail temperature was not observed; therefore the difference of the measured ion temperature will be observed at a lower density plasma or a higher temperature plasma. 10000 ..-..----.--..,.---,---, 1000 "" § 100 0 0 10 50 100 150 200 250 50 100 150 200 250 Energy(keV) Energy(keV) Fig.2 Energy spectrum of high energy ion ; (a) at Rax:=3.6m and Rax=3.75m. References [1] S.Murakami et al., Nuclear Fusion, 3 9 1165(1999). [2] R.Kumazawa et al., Physics of Plasmas, 8 2139(2001). 31 

Dependence of ICRF Heating

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