Abstract
A simultaneous high ion temperature (Ti) and high electron temperature (Te) regime was successfully extended due to an optimized heating scenario in the LHD. Such high-temperature plasmas were realized by the simultaneous formation of an electron internal transport barrier (ITB) and an ion ITB by the combination of high power NBI and ECRH. Although the ion thermal confinement was degraded in the plasma core with an increase of Te/Ti by the on-axis ECRH, it was found that the ion thermal confinement was improved at the plasma edge. The normalized ion thermal diffusivity at the plasma edge was reduced by 70%. The improvement of the ion thermal confinement at the edge led to an increase in Ti in the entire plasma region, even though the core transport was degraded.
Highlights
Confinement improvement is necessary for realization of high-temperature plasmas and is one of the most important issues in toroidal devices
The present paper shows the high-temperature plasma regime extended recently and the effect of an electron cyclotron resonance heating (ECRH) on the ion thermal transport in the LHD and is organized as follows
Simultaneous high Ti and high Te have been obtained by the on-axis ECRH superposition on the high-Ti plasmas sustained by the neutral beam injection (NBI)
Summary
Confinement improvement is necessary for realization of high-temperature plasmas and is one of the most important issues in toroidal devices. Since the first observation of H mode [1], various kinds of improved confinement modes have been observed in tokamaks and helical/stellarator devices Of these improved confinement modes, an internal-transport barrier (ITB) is characterized by a steep gradient formation in temperature profiles and a decrease in thermal diffusivity in a plasma core region [2,3,4]. In the Large Helical Device (LHD), electron-ITB (e-ITB) relating to the production of high Te plasmas has been realized by strongly centre-focused ECRH beams [10,11,12,13]. The present paper shows the high-temperature plasma regime extended recently and the effect of an ECRH on the ion thermal transport in the LHD and is organized as follows. The total injection power of the NBI and the ECRH reached 28 MW and 5.4 MW, respectively
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