Abstract
The central bulk electron temperature of more than 20 keV is achieved in LHD as a result of increasing the injection power and the lowering the electron density near 2 10 18 m 3 . Such collision-less regime is important from the aspect of the neoclas- sical transport and also the potential structure formation. The presences of appreciable amount of high energy electrons are indicated from hard X-ray PHA, and the discrep- ancy between the stored energy and kinetic energy estimated from Thomson scattering. ECE spectrum are also sensitive to the presence of high energy electrons and discussed by solving the radiation transfer equation. The ECRH power absorption to the bulk and the high energy electrons are dramatically affected by the acceleration and the confinement of high energy electrons. The heating mechanisms and the acceleration process of high energy electrons are discussed by comparing the experimental results and the ray tracing calculation under assumed various density and mean energy of high energy electrons.
Highlights
Recent upgrade of the ECRH system in LHD[1] enabled to study the plasma confinement properties at the far low collisional or collision-less regime in the helical system where specific confinement features are predicted from the neo-classical transport theory
The ECRH power absorption to the bulk and the high energy electrons are dramatically affected by the acceleration and the confinement of high energy electrons
In order to discuss the confinement properties in such collision-less regime, the accurate estimation of the behavior of the high energy electrons as well as that of the bulk electron temperature is required, since the high energy electrons can absorb injected ECRH power at the relativistically down shifted frequency and can deposit their energy to the bulk electrons out of the region where they absorb the energy from injected EC wave
Summary
Recent upgrade of the ECRH system in LHD[1] enabled to study the plasma confinement properties at the far low collisional or collision-less regime in the helical system where specific confinement features are predicted from the neo-classical transport theory. In case of low density, high energy electrons are created under high power ECRH and the ECE can be non-thermal. The ECE intensity at high frequency side are ascribed to the non-thermal feature of the high energy electrons of relativistically down shifted emission from the core region. While those at low frequency side are ascribed to the down shifted second or third harmonic emission at optically thin region in the plasma periphery
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