R. Seki

Integrated discharge scenario for high-temperature helical plasma in LHD

The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti).

Development of steady-state operation using ion cyclotron heating in the Large Helical Device

Using a handshake shape (HAS) antenna phasing dipole for ion cyclotron heating (ICH), the heating efficiency was higher than that using a previous poloidal array antenna in the Large Helical Device. In order to sustain the dipole operation, real-time feedback for impedance matching and maintaining the same phase and power was adopted during long-pulse discharge. The HAS antenna was designed to reduce parasitic losses associated with energetic particle and radio-frequency (RF) sheath effects by field-aligned current concentration on the midplane. Local hot spots and the inhomogeneity of the diverter heat profile in the toroidal direction were reduced. The long-pulse discharge with an electron density (ne0) of 1u2009×u20091019 m−3, center electron temperature (Te0) of 2.5u2009keV, a plasma duration time (td) of 19u2009min, and RF heating power (PRF) of 1u2009MW was achieved by ICH and electron cyclotron heating.

Studies of dust transport in long pulse plasma discharges in the large helical device

Three-dimensional trajectories of incandescent dust particles in plasmas were observed with stereoscopic fast framing cameras in a large helical device. It proved that the dust is located in the peripheral plasma and most of the dust moves along the magnetic field lines with acceleration in the direction that corresponds to the plasma flow. ICRF heated long pulse plasma discharges were terminated with the release of large amounts of dust from a closed divertor region. After the experimental campaign, the traces of exfoliation of carbon rich mixed-material deposition layers were found in the divertor region. Transport of carbon dust is investigated using a modified dust transport simulation code, which can explain the observed dust trajectories. It also shows that controlling the radius of the dust particles to less than 1 mm is necessary to prevent the plasma termination by penetration of dust for the long pulse discharges. Dust transport simulation including heavy metal dust particles demonstrates that high heating power operation is effective for shielding the main plasma from dust penetration by an enhanced plasma flow effect and a high heat load onto the dust particles in the peripheral plasma. It shows a more powerful penetration characteristic of tungsten dust particles compared to that of carbon and iron dust particles.

Development and application of a ray-tracing code integrating with 3D equilibrium mapping in LHD ECH experiments

The central electron temperature has successfully reached up to 7.5 keV in large helical device (LHD) plasmas with a central high-ion temperature of 5 keV and a central electron density of m−3. This result was obtained by heating with a newly-installed 154 GHz gyrotron and also the optimisation of injection geometry in electron cyclotron heating (ECH). The optimisation was carried out by using the ray-tracing code LHDGauss, which was upgraded to include the rapid post-processing three-dimensional (3D) equilibrium mapping obtained from experiments. For ray-tracing calculations, LHDGauss can automatically read the relevant data registered in the LHD database after a discharge, such as ECH injection settings (e.g. Gaussian beam parameters, target positions, polarisation and ECH power) and Thomson scattering diagnostic data along with the 3D equilibrium mapping data. The equilibrium map of the electron density and temperature profiles are then extrapolated into the region outside the last closed flux surface. Mode purity, or the ratio between the ordinary mode and the extraordinary mode, is obtained by calculating the 1D full-wave equation along the direction of the rays from the antenna to the absorption target point. Using the virtual magnetic flux surfaces, the effects of the modelled density profiles and the magnetic shear at the peripheral region with a given polarisation are taken into account. Power deposition profiles calculated for each Thomson scattering measurement timing are registered in the LHD database. The adjustment of the injection settings for the desired deposition profile from the feedback provided on a shot-by-shot basis resulted in an effective experimental procedure.

Dynamic transport study of heat and momentum transport in a plasma with improved ion confinement in the Large Helical Device

Dynamic transport study taking account of the slowing-down effect on the neutral beam injection heating is applied to a high ion temperature plasma with an ion internal transport barrier (ITB) obtained by carbon pellet injection, which records the highest ion temperature of around ~7 keV in the Large Helical Device. The transient increase in ion heating is clarified during the density decay phase just after the carbon pellet injection by considering the slowing-down effect. The dynamic transport analysis also includes the change in heat flux due to the change in kinetic energy inside the plasma with the time scale of the global energy confinement time, which is required to investigate the heat and momentum transport during the transient phase more exactly. The characteristics of the ion heat and momentum transport improvement during the ion ITB formation phase are clarified by the dynamic transport study.

Impact of carbon impurities on the confinement of high-ion-temperature discharges in the Large Helical Device

Effects of carbon impurities on the thermal confinement properties were discussed for carbon-pellet-injected high-Ti discharges in the Large Helical Device (LHD). To clarify their role, the amounts of carbon impurities introduced into plasmas were scanned by varying the number of injections in a discharge or the size of the pellet, and the changes of thermal confinement properties with these variations were examined. In all cases, strong correlations between the densities of carbon impurities and the thermal diffusivities of plasmas were found. Thermal diffusivities were small when the carbon densities were greater than a certain value, e.g. ~1.5 × 1017 m3 at r/a ≈ 0.72, and the thermal diffusivity degraded drastically below this density. This indicates the existence of a threshold density of carbon impurity for an improved confinement in plasmas. Combined with the formation of the impurity hole, the variation of confinement property with carbon density can explain the degradation of ion temperature in high-Ti discharges with carbon pellet injection in the LHD.

Investigation of ion and electron heat transport of high-Te ECH heated discharges in the large helical device

An analysis of the radial electric field and heat transport, both for ions and electrons, is presented for a high-

3-D effects on viscosity and generation of toroidal and poloidal flows in LHD

{ {T}_{text{e}}}