S. Okamura

Spectrograph of electric field fluctuation in toroidal helical plasma

This is a report on fluctuation measurements using twin heavy ion beam probes in CHS. The observation shows that dozens of coherent modes with intermittent nature co-exist in an electron cyclotron heated plasma. The modes are found to have a long-distance correlation in the toroidal direction. The radial structure of the modes is evaluated in potential fluctuation. The characteristics are consistent with those of the geodesic acoustic modes.

Edge transport barrier formation in compact helical system

The edge transport barrier (ETB) for particle transport is formed in the neutral beam (NB) heated hydrogen discharges in compact helical system (CHS). The transition to the ETB formation and the back transition are controlled by the heating power. The existence of the heating power threshold is confirmed and it is roughly proportional to the density. The Ha emission signal shows a clear drop at the transition (the timescale of signal decrease is ∼1 ms for the high heating power case). The ETB formation continues for the full duration of NB injection (100 ms) with a moderate level of radiation power loss. Local density profile measurement shows increase of the edge density and the movement of the density gradient region towards the edge.

Edge and internal transport barrier formations in CHS

Edge transport barrier (ETB) formation was observed in a compact helical system. A sharp decrease in Hα emission indicates the quick transition of edge particle transport. The increase in the density gradient at the edge was measured by using various profile diagnostics. The transition process has a slowly developing pre-phase, and a quick transition for which the fastest case is less than 0.2 ms. There exists a heating power threshold which is roughly proportional to the density and the magnetic field. The transition and back transition is controlled by the heating power. The magnetic configuration effect on ETB formation was also studied. The local density measurement by beam emission spectroscopy shows intermittent bursts of low frequency fluctuations during the ETB formation phase. When the initial density profile is very hollow, ETB formation together with the electron temperature increase (electron internal transport barrier) in the core region, was observed for the NBI discharges without ECH.

Scaling of power threshold for edge transport barrier formation in CHS with density, magnetic field and magnetic configuration

An improved confinement state by the formation of an edge transport barrier can be obtained in the compact helical system. A power threshold exists, as is the case for the H-mode in tokamaks. The threshold scales as . These dependences are similar to those in tokamaks. The threshold also depends on magnetic configuration parameters such as the position of the magnetic axis. The absolute value of the threshold is 1–3 times larger than the tokamak H-mode power scaling.

Confinement characteristics of the quasi-axisymmetric stellarator CHS-qa

A low aspect ratio stellarator CHS-qa with a quasi-axisymmetric configuration has been designed at NIFS, whose main objective is the experimental study of confinement improvement. The high beta equilibrium was obtained with the HINT code which does not require the assumption of nested magnetic surfaces. The equilibria with bootstrap current were calculated using the SPBSC code and their global MHD stability was evaluated with the TERPSICHORE code. The boundary structure of the magnetic field was examined to scope divertor options. A new method of calculating the neoclassical transport properties was developed for toroidal configurations with very small non-axisymmetry. The control knobs of the configuration properties are proposed for the study of improved confinement.

Improved confinement and related physics study in the compact helical system

Abstract Various types of transport barriers have been studied in the Compact Helical System. In addition to the neoclassical transport barrier, the edge transport barrier (H-mode) was studied using the high-power heating of two coinjection neutral beam injections. A density pedestal is formed after the transition that is indicated by the drop of Hα emission signal. The heating power threshold for the transition was investigated by varying the heating power. Its dependence on the density and the magnetic field is close to the H-mode scaling obtained in tokamaks. The dependence of the power threshold on the magnetic field configuration was also found. Local density fluctuation was measured with beam emission spectroscopy, which observed harmonic oscillations appearing after the density pedestal was formed. For L-mode plasma, long-distance coherence of the potential fluctuations were measured with two sets of heavy ion beam probes (HIBPs). Those coherent modes are supposed to be the geodesic acoustic mode part of zonal flow. Turbulent particle flux was also measured with HIBP, and its change with internal transport barrier formation is demonstrated.