Analyses of visible images of the plasma periphery observed with tangentially viewing CCD cameras in the Large Helical Device

Magnetic field produced by helical and poloidal coils in LHD forms a complicated structure of the magnetic field lines in the plasma periphery (ergodic layer and divertor legs), which can change the radial position of the magnetic axis, the shape and size of magnetic surfaces and the location of the strike points, etc. CCD cameras have observed complicated structure of the visible emission depending on the magnetic configurations. The dependence of the images of visible emission on three magnetic parameters which specify magnetic configurations (the position of the magnetic axis, coil pitch parameter, quadruple magnetic components) is investigated by tracing magnetic field lines. The images of the three-dimensional plots of the magnetic field lines quite agree with the observations in various magnetic configurations. Safe operational range of the three magnetic parameters from the viewpoint of minimizing the direct heat load onto the vacuum vessel is found by calculating the distributions of strike points

MHD stability of low-n ideal external mode in Large-Helical-Device plasma

0000-0002-3541-6298The characteristics of low-n ideal external modes in the Large-Helical-Device are studied. The low-n analysis is carried out with the TERPSICHORE code [W.A. Cooper, Plasma Phys. Control. Fusion 34, 1011 (1992)]. Under fixed boundary conditions, the low-n instability region is limited within the Mercier unstable region and the external mode does not appear. On the other hand, with free boundary conditions, the low-n unstable region spreads and the external modes become unstable. The effects of a conducting wall similar in shape to that of the plasma show that stability to external modes of higher growth rate can be obtained even with the wall further away from the plasma.journal articl

Low-n ideal MHD analysis in limiter plasma on LHD

0000-0002-3541-6298Serious MHD instabilities leading to the degradation of the energy confinement time and the termination of the discharge have not been observed in experiments on the Large Helical Device (LHD) [S. Sakakibara et al., NucL Fusion 41, ll77 (2001)]. The stability of low-n modes is calculated with the three-dimensional MHD stability analysis code of TERPSICHORE [W.A. Cooper, Plasma Phys. and Control. Fusion 34, 1011 (1992)1. The results show that the high beta plasma becomes MHD unstable with an external mode (nlm = l/1). In case the rotational transform at the edge decreases below 1"6r"/2rt=1.0, the external mode is also destabilized.By using the limiter, L"4".|2TE can be controlled and it is possible to imitate the high beta plasma from the viewpoint of the iota profile. The characteristics of the MHD instabilities in the high beta plasma need to be examined both theoretically and experimentally. We propose, therefore, a reasonable limiter experiment to check the characteristics of the MHD instabilities with external modes.journal articl

Energetic ion losses caused by magnetohydrodynamic activity resonant and non-resonant with energetic ions in Large Helical Device

Experiments to reveal energetic ion dynamics associated with magnetohydrodynamic activity are ongoing in the Large Helical Device (LHD). Interactions between beam-driven toroidal Alfvén eigenmodes (TAEs) and energetic ions have been investigated. Energetic ion losses induced by beam-driven burst TAEs have been observed using a scintillator-based lost fast-ion probe (SLIP) in neutral beam-heated high β plasmas. The loss flux of co-going beam ions increases as the TAE amplitude increases. In addition to this, the expulsion of beam ions associated with edge-localized modes (ELMs) has been also recognized in LHD. The SLIP has indicated that beam ions having co-going and barely co-going orbits are affected by ELMs. The relation between ELM amplitude and ELM-induced loss has a dispersed structure. To understand the energetic ion loss process, a numerical simulation based on an orbit-following model, DELTA5D, that incorporates magnetic fluctuations is performed. The calculation result shows that energetic ions confined in the interior region are lost due to TAE instability, with a diffusive process characterizing their loss. For the ELM, energetic ions existing near the confinement/loss boundary are lost through a convective process. We found that the ELM-induced loss flux measured by SLIP changes with the ELM phase. This relation between the ELM amplitude and measured ELM-induced loss results in a more dispersed loss structure.journal articl

Impact of Energetic Ion Driven Global Modes on Toroidal Plasma Confinements

Excitation of energetic-ion-driven Alfv6n eigenmodes (AEs) and their impact on energetic ion confinement are widely and intensively studied in helical devices such as CHS and LHD as well as major tokamaks. The excitation of AEs sensitively depends on the parameter space defined by the averaged beam beta and the velocity ratio V6nlV6 (V611 : injected beam ion velocity, Va: Alfv6n velocity). In LHD, these two relevant parameters are widely scanned without suffering from current disruptions. So far, toroidicity induced AE (TAE), global AE (GAE) and energetic particle mode (EPM) or resonant TAE (R-TAE) were identified during tangential neutral beam injection (NBI) in CHS and LHD. Moreover, a new coherent mode with the frequency by about 8 times higher than the TAE frequency was observed in NBI heated plasmas of LHD at low magnetic field (<0.6T). This mode may be induced by helical field components of the confinement field. Nonlinear phenomena of bursting amplitude modulation and fast frequency chirping are clearly seen for TAEs and EPMs in CHS and LHD. EPMs in CHS and bursting TAEs in LHD enhance radial transport of energetic ions in certain plasma conditions

Design of a Closed Helical Divertor in LHD and the Prospect for Helical Fusion Reactors

A new closed helical divertor configuration for efficient particle control and reduction of the heat load on the divertor plates is proposed. The closed divertor configuration practically utilizes an ergodic layer and magnetic field line configuration on divertor legs in helical systems. For optimization of the design of the closed divertor, the distribution of the strike points is calculated in various magnetic configurations in the Large Helical Device (LHD). It suggests that the installation of the closed divertor components in the inboard side of the torus under an inward shift configuration (Rax=3.60m) is the best choice for achieving the above two purposes. This divertor configuration does not interfere with plasma heating and diagnostic systems installed in outer ports. The prospect of the closed divertor configuration to a helical fusion reactor is investigated using a three-dimensional neutral particle transport simulation code with a one-dimensional plasma fluid calculation on the divertor legs. The investigation shows efficient particle pumping from the in board side and reduction of the heat load due to the combined effect of the optimized closed divertor geometry, ergodized divertor legs, and low electron temperature in the ergodic layer. It indicates a promising closed divertor configuration for helical fusion reactors

Study of the effect of a closed divertor configuration on neutral particle control in the LHD plasma periphery

An optimized closed divertor configuration for effective particle control in LHD is proposed from the viewpoints of the distribution of the strike points and neutral particle transport. Calculations of the distribution of the strike points indicate that 50% of the strike points locate in the inboard side of the torus in a standard magnetic configuration (Rax = 3.60 m). The ratio increases to 80% by installing target plates near lower/upper ports. A three-dimensional neutral particle transport simulation shows that installation of closed divertor components with the target plates raise the neutral pressure in the inboard side by more than one order of magnitude compared to that in the present open divertor case. The analysis of the neutral particle transport predicts that enhancement of the neutral pressure becomes moderate in outward shift configurations (Rax > 3.75 m)