Compatibility between the plasma and low activation ferritic steel, which is a candidate material for fusion demonstration reactors, has been investigated step by step in the JFT-2M tokamak. We have entered the third stage of the Advanced Material Tokamak EXperiment (AMTEX), where the inside of the vacuum vessel wall is completely covered with ferritic steel plates ferritic inside wall (FIW). The effects of a FIW on the plasma production, impurity release, the operation region, and H-mode characteristics have been investigated. No negative effect has been observed up to now. A high normalized beta plasma of β N ∼ 3, having both an internal transport barrier and a steady H-mode edge was obtained. A remarkable reduction in ripple trapped loss from 0.26 MW m −2 (without ferritic steel) to less than 0.01 MW m −2 was demonstrated by the optimization of the thickness profile of FIW. A code to calculate fast ion losses, taking into account the full three-dimensional magnetic structure was developed, and values obtained using the code showed good agreement with experimental results. Thus, encouraging results are obtained for the use of this material in the demo-reactor.
Non-axisymmetric Alfvén waves are excited with a helical coupler in a finite beta and a cylindrical inhomogeneous plasma surrounded by a conducting wall. The helical coupler, which consists of two symmetrical helical windings, exhibits a strong propagation directionality; slow waves of the m=-1 mode and fast waves of the m=+1 mode can be launched simultaneously from the coupler, but propagate in opposite directions along the static magnetic field. Dispersion relations including the attenuation length for both modes are compared with a magnetohydrodynamic theory given by Woods on the assumption of appropriate boundary conditions. As a nonlinear phenomenon, subharmonic slow Alfvén waves of the m=-1 mode have been observed when the pump frequency is close to or above the ion cyclotron frequency.
Axisymmetric compressional Alfven (fast) waves, which propagate into a region of increasing magnetic field in a cylindrical plasma, are observed to be converted into ioncyclotron (slow) waves via ion-cyclotron resonances.PACS numbers: 52.50.Gj The mode conversion of Alfv §n waves or their resonant coupling has received much attention because of the relevance to the origin of micropulsations in the magnetosphere, 1 * 2 and to the additional plasma heating to thermonuclear temperature. The main idea of Alfven wave heating is to use the Alfven wave "resonance" and the resulting mode conversions in an inhomogeneous density plasma, as proposed by Hasegawa and Chen 3 and by Tataronis and Grossmann. 4 In recent years, the importance of the two-ion hybrid resonance on fast-wave damping has been observed in several tokamaks, 5 " 7 and has received considerable theoretical analysis. 8 " 10 When the minorityion concentration is sufficiently high, the tunneling and mode conversion of the fast wave occur in the vicinity of the ion-ion hybrid resonance and lead to wave damping by both the electrons and the resonant ion species. However, instead of these extensive studies, few observations of mode conversions related to AlfvSn waves have been made so far.In this Letter, we report the observation of a conversion of Alfv6n waves which occurs in an inhomogeneous magnetic field; i.e., the compressional (fast) Alfv£n waves propagating along a static mirror magnetic field are found to be converted into ion-cyclotron waves via the ion-cyclotron "resonance layer," at which substantial plasma heating can be expected.The experiments were carried out in the TPH device of Nagoya University which has been described in detail previously 11 (chamber diameter, 15 cm; chamber length, 2 m). In this device, with linear or mirror magnetic field configuration, a quasisteady (~ 1 ms), current-free, highdensity streaming plasma has been established by developing a pulsed magneto-plasma-dynamic (MPD) arcjet with an anode of 3-8 cm diameter. In this experiment, a 5-cm-diam anode was used. Plasma parameters were measured with an HCN laser interferometer (x = 337 jum), a spectrometer, double probes, magnetic probes, and diamagnetic loops. Typical plasma parameters are as follows: electron and helium-ion temperatures T e «T,~4 eV; plasma density n e~4 x 10 14 cm" 3 ; ionization degree > 70%; /3 value ~ 1%. Inhomogeneous magnetic fields were produced by modifying the configuration of the magnetic coils. Axisymmetric compressional waves were launched with use of a Helmholtz-like coil (6.8 cm diam x 6.8 cm) placed outside the plasma column. This coil is very inefficient for the excitation of shear Alfven waves. Wave magnetic fields (b z component) were detected by a small magnetic probe (5 mm diameter) which can be moved along the field B and were analyzed by means of fast Fourier transform in order to obtain an auto-or cross-power spectrum and correlation functions.As is well known, axisymmetric compressional waves propagating along a uniform field B i...
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