Ion cyclotron range of frequency heating with hydrogen and helium minority species in Heliotron E

Fusion Engineeringand Design 26 (1995) 173-178

ELSEVIER

Fusion Engineen[ng and Design

Ion cyclotron range of frequency heating with hydrogen and helium minority species in Heliotron E H . O k a d a a, T. M u t o h b, M . O k a m o t o b, M . O h n i s h i c, A. F u k a y a m a d, H . Z u s h i a, K . K o n d o a, T. M i z u u c h i a, S. B e s h h o u a, F. S a n o a, K . N a g a s a k i a, M . W a k a t a n i a, T. O b i k i a Plasma Physics Laboratory, Kyoto University, Gokasho, Uji 611, Japan b National Institute for Fusion Science, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan c Institute of Atomic Energy, Kyoto University, Uji 611, Japan d Department of Electrical and Electronic Engineering, Okayama University, Okayama 700, Japan

Abstract

In ion cyclotron range of frequency heating of Heliotron E, hydrogen and helium 3 were used as minority species. From the charge exchange neutral particle energy measurement, the bulk deuteron temperature was higher in the helium minority case by 20%-40% in the high power region. Helium minority has an advantage in this experiment because its loss region in the velocity space is small, the ion-ion collision cross-section is large and its charge exchange loss ratio is less than that of hydrogen. In this paper, we discuss the comparison of the heating efficiencies and the power loss by escaping particles between the experimental data and the calculation by the Monte Carlo code. The results of the calculation showed that approximately 40% higher efficiency was obtained in the helium minority case. This result agreed with the experimental data in the higher power region.

1. Introduction

The ion cyclotron range of frequency (ICRF) heating method has the advantage of less expensive device cost and the variety of heating schemes. For example, there are minority heating, second harmonic heating, ion-Bernstein heating and so on. In the Heliotron E device, up to 1.6 MW I C R F power was injected in a helical plasma of major radius R = 2.2 m and averaged minor radius ap = 0.2 m by using the eight antenna loops from the four ports [1,2]. All antennas are installed in the high field side of the heliotron vacuum chamber wall.

With ICRF heating, high energy particles can be produced easily. The energy of such fast particles is determined by balancing the acceleration by I C R F wave with the damping by Coulomb collisions with bulk particles and the orbit loss. High energy particles must be confined for enough time to establish the high energy tail and to make the collisional damping effective. For helical devices, the high energy particle confinement is important because they have a relatively large loss cone in the velocity space for such particles. Since the helium 3 ion has a heavier mass and double electric charge number, the orbit loss and the charge exchange loss are smaller and, there-

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H. Okada et al. / Fusion Engineering and Design 26 (1995) 173 178

fore, more effective heating with helium is expected. In this paper, we mention the results of the minority heating experiment with He and H minorities in the next section. The orbit loss and heating efficiency will be discussed in the following sections.

2. Experimental results The experimental setup in I C R F experiment in Heliotron E is shown in Fig. 1. Two I C R F transmitters are used. The frequency can be changed from 17.8 MHz to 54 MHz according to the experimental objects. Four antenna pairs (eight antenna loops) are used in the heating experiment. Each antenna loop is fed with the same phase r.f. current. Cross-sections of the helical windings, vacuum chamber, I C R F antenna loops, plasma

surface, cyclotron resonance and cut-off layers are shown in Fig. 2. These experiments were performed with a toroidal magnetic field strength B,p o at the centre of the minor radius of 1.9 T and a line-averaged electron density of (1.5-2.5) x 1019 m -3. A plasma was produced by electron cyclotron resonance heating (ECRH). The E C R H power input was about 450 kW. The electron temperature of the target plasma was about 500 eV. An ICRF pulse was imposed during the E C R H pulse. During the I C R F pulse, the electron density was almost constant. The experimental results of the minority heating are shown in Fig. 3. The ion temperature of the bulk deutrons was measured by a charge exchange particle energy analyser, which is located in the opposite side of the torus against the ICRF antenna section. The ion temperature was higher in helium minority case by 20%-40% especially in high power region over

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H. Okada et al./ Fusion Engineering and Design 26 (1995) 173 178 Helical Field Coil

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These differences are expected to be due to the differences in the direct orbit loss and the charge exchange loss. By using a Monte Carlo calculation code [3], the energy input and the heating efficiency in the hydrogen and helium cases are estimated.

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3. Calculation results

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To explain the results of the heating experiments, the Monte Carlo code was modified for the Heliotron E configuration. In this calculation code, minority particles are treated as test particles. Particle guiding centre orbits and changes in momentum and energy are calculated. Orbit calculation is performed based on Boozer's model [4]. When a guiding centre of a particle goes out from the last close surface of a plasma, the particle is lost in this model. Re-entering from the outside of the last closed surface and the charge exchange process are not included. The energy loss by direct orbit loss is evaluated from the sum of the energies of particles escaping from the plasma. The bulk ions and electrons receive energy by the collision with test particles. For this calculation, typical plasma parameters were chosen as follows: the ion and electron temperature, 500eV, line-averaged electron density, 2 x 1019 m-3; effective charge of the plasma ions, 3. Radial profiles were assumed to be parabolic in all calcu-

PIc~-(MW) Fig. 3. Ion temperature measured by neutral particle energy analyser for the various I C R F powers. The powers are presented as transmitter power. Radiated power from the antennas is 60%-70%.

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500 kW. The increment of the ion temperature in H minority case saturated beyond 500 kW. In such a power region, a high energy tail is formed up to 50 keV or more. We observed that the tail energy increased linearly with the input power even in the high power region for the hydrogen experiments. On the contrary, the bulk ion temperature in the helium case increased even in the high power region. However, there is no measurement of energy spectrum of helium 3 particles in Heliotron E experiments.

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H. Okada et al. / Fusion Engineering and Design 26 (1995) 173-178

lations. The acceleration term caused by I C R F was assumed to be as follows; Av± - qE~ 2~, Jn-~(k±P) e-i'~°

ergy distributions of helium 3 and proton after they reached the steady state are shown in this figure. Power input is about 1.3 M W in both cases. The tail temperature of the helium minority is higher than that of protons• The energy loss ratio of the hydrogen minority in this case is 0.5. On the contrary, the loss ratio of helium case is 0.3. When the input energy is the same, the transferred power to the bulk particles is higher in the helium case by 40% with these parameters.

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formed at that direction• Coulomb collisions drag these particles to the low energy region and scatter the distribution in the velocity space. If the high energy particles enter into the loss cone during slowing down, the heating efficiency becomes reduced. The velocity space loss cone of the particles departing at a normalized radius r/ap of 0.5 in the outer midplane is shown in Fig. 5(a) for helium and Fig. 5(b) for protons. The loss cone of protons appears at the lower energy region. This causes the difference in the high energy tails of heliums and protons in Fig. 4. In the minority heating experiment in Heliotron E, the higher ion temperature is obtained in the helium 3 minority case. The results of Monte

Carlo calculations agree with the experimental results. This calculation shows that the orbit loss can be effective especially in the hydrogen case and the high power experiment for both minority species in Heliotron E. The helium 3 minority has better performance for the high power minority heating experiment. The density and input power are sensitive parameters for the orbit loss ratio because they decide the high energy tail formation. For more effective heating, higher density plasma operation or a different heating scheme in the experiment in which there is less of a tail component is required. In this paper, charge exchange loss was not discussed. For more detailed discussion, it must be

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H. Okada et al. / Fusion Engineering and Design 26 (1995) 173-178

considered. F r o m the estimation using the neutral density measurement o f similar plasmas, the charge exchange loss ratio for the high energy tail particles seems to be negligible in this parameter range.

Acknowledgment A u t h o r s would like to t h a n k the Heliotron E experiment g r o u p for carrying out the I C R F experiment. One o f the authors (H.O.) acknowledges

Dr. S. M u r a k a m i for discussion o f the M o n t e Carlo code.

References [1] T. Mutoh et al., Nucl. Fusion, 24 (1984) 1003. [2] T. Mutoh et al., Proc of llth Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Kyoto, 1986, IAEA, Vienna, Vol. 2, 1987, p. 473. [3] M. Ohnishi et al., Res. Rep. IPPJ-884, 1988. [4] A.H. Boozer and G. Kuo-Petravic, Phys. Fluids, 24 (1981) 851.