Confinement improvement in high-ion temperature plasmas heated with high-energy negative-ion-based neutral beam injection in the Large Helical Device

Abstract

An increase in the ion temperature due to transport improvement has been observed in plasmas heated with high-energy negative-ion-based neutral beam injection (NBI), in which the electrons are dominantly heated, in the Large Helical Device. When centrally focused electron-cyclotron-resonance heating is superposed on the NBI plasma, the ion temperature is observed to rise, accompanied by the formation of the electron internal transport barrier (electron-ITB). In the electron-ITB plasmas, an increase in a positive radial electric field is observed, and the transport analysis indicates that the ion transport in the half-radius region is improved with a reduction of the anomalous transport. Thus, this ion temperature rise is ascribed to the ion transport improvement by the transition to the neoclassical electron root. In high-Z plasmas, the ion temperature is increased with an increase in the ion heating power and reaches 13.5 keV. The central ion temperature increases with an increase in a gradient of the electron temperature in an outer plasma region of ρ = 0.8, suggesting the ion transport improvement in the outer plasma region induced by the neoclassical electron root. These results indicate the effectiveness of the electron-root scenario for obtaining high-ion temperature plasmas in helical systems.

Ion Heating Experiments Using Perpendicular Neutral Beam Injection in the Large Helical Device

“…It is noted that the heating power dependence of T i shown in Fig. 3 continuously connects to the higher heating power region obtained in high-Z experiments, which supports the extension of these results obtained by the high-Z discharges to the low-Z plasmas [7]. Plasmas with T i higher than T e were obtained after superimposition of tangential NBs over the P-NB heated plasma, although T e tends to be higher than T i in LHD because of the high electron heating power of tangential NBs.…”

“…Before the 9 th experimental campaign of the Large Helical Device (LHD), ion heating experiments were performed by tangential neutral beam (NB) injection [7]. There are three beam injectors of negative-ion-based NB with a very high beam energy of 180 keV, which are the main heating devices in LHD, and the total port-through beam power was over 14 MW [8].…”

“…In LHD a high T i of 13.5 keV was realized with Argon gas puffing. Although the experiments were performed in a high-Z plasma because of the low ion heating power, the achievement shows the high potential of high-T i plasma confinement in a helical configuration [7,10]. The next step is the production of high-T i plasmas in low-Z discharges.…”

Development of net-current free heliotron plasmas in the Large Helical Device

“…The ion temperature is raised to 6.8 keV at the moderate density of 2×10 19 m -3 [7]. In an earlier study, a high ion temperature as high as 13.5 keV was realized by applying a heavier gas, argon, instead of hydrogen [26]. This scenario was attributed to the enhancement of ion heating under the condition of a tangential NBI energy as high as 180 keV which primarily heats electrons in hydrogen operation.…”

Observation of Hydrogen and Cesium Spectra in a Negative Ion Source for a Neutral Beam Injector using a Multi-Channel Spectrometer

“…We have adopted six large H − sources used arc discharge for the Large Helical Device (LHD) experiment, and these are successfully operated. Injection beam power has been reached 13 MW totally three beam lines [4]. The current density of H − at the exit of ion source is achieved 350 A/m 2 [5].…”