M. Azumi

Bootstrap current during perpendicular neutral injection in JT-60

The neoclassical bootstrap current effect is investigated in the JT-60 tokamak. The experimental resistive loop voltages are compared with the calculations, using the neoclassical resistivity, with and without the bootstrap current, and the Spitzer resistivity for a wide range of plasma current (Ip = 0.5-2 MA) and poloidal beta (βp = 0.1-3.2). The neoclassical bootstrap current is calculated directly with the force balance equations for viscous and friction forces according to the Hirshman–Sigmar theory. The bootstrap current driven by the fast ion component is also included. The calculated resistive loop voltage is consistent with the neoclassical prediction including the bootstrap current. It is shown that up to 80% of total plasma current is driven by the bootstrap current in the regime with an extremely high poloidal beta value (βp = 3.2) while the beam driven current is negligibly small.

Nonlinear simulation of tearing mode based on 4-field RMHD model

Simulations of dynamics of tearing modes based on a 4-field reduced magneto hydro dynamics model are performed, laying an emphasis on interaction with microscopic and transport processes. The simulation results show the importance of turbulent fluctuations for the onset of tearing mode. The helical current perturbation associated with a magnetic island is studied. The perturbed bootstrap current has a structure in the magnetic island (including a phase difference), indicating the need to improve assumptions in the theory of neoclassical tearing mode.

Self‐sustained plasma turbulence due to current diffusion

Plasma turbulence and anomalous transport by the electrostatic current diffusive interchange mode are studied by the nonlinear simulation based on the magnetohydrodynamic model. The turbulence is found to have a typical characteristic of subcritical turbulence. The saturation level, as a function of the pressure gradient ∇p, is confirmed to scale like ‖∇p‖3/2. This nature holds independent of the ratio ‖∇p‖/‖∇pc‖ where ‖∇pc‖, is a critical pressure gradient against linear instability. The turbulence‐driven transport is also evaluated. The simulation result confirms the theoretical prediction, which is based on the self‐sustained turbulence, with respect to the nonlinear growth and damping. Both the normal cascade and inverse cascade are essential in establishing the stationary turbulent state.Plasma turbulence and anomalous transport by the electrostatic current diffusive interchange mode are studied by the nonlinear simulation based on the magnetohydrodynamic model. The turbulence is found to have a typical characteristic of subcritical turbulence. The saturation level, as a function of the pressure gradient ∇p, is confirmed to scale like ‖∇p‖3/2. This nature holds independent of the ratio ‖∇p‖/‖∇pc‖ where ‖∇pc‖, is a critical pressure gradient against linear instability. The turbulence‐driven transport is also evaluated. The simulation result confirms the theoretical prediction, which is based on the self‐sustained turbulence, with respect to the nonlinear growth and damping. Both the normal cascade and inverse cascade are essential in establishing the stationary turbulent state.

H-mode experiments with outer and lower divertors in JT-60

In JT-60, H-mode experiments with outer and lower divertors have been performed. In the outer divertor discharge, an H-mode similar to the modes observed in the lower/upper divertor discharges is obtained. Its threshold absorbed power and electron density are 16 MW and 1.8 × 1019 m−3. In the two schemes of combined heating with NB + ICRF and NB + LHRF, H-mode discharges are also obtained. Moreover, in the new configuration with the JT-60 lower divertor, H-mode phases with and without edge localized modes are obtained. The improvement in the energy confinement time in both divertor configurations is limited to values within 10%. The paper mainly presents the H-mode results of the outer divertor discharges. Also, typical results of the lower divertor discharges are shown for a comparison of the H-mode characteristics of the two configurations.

Onset condition for ELMs in JT-60U

For the standard shape of JT-60U at low triangularity , the onset condition for giant ELMs is clearly correlated with the high-n ballooning limit in the first stability regime over wide ranges of plasma parameters (, ...). The limit of the normalized edge pressure gradient (-parameter) increases with elongation (1.5 - 1.8) and internal inductance . Recently, a new connection of the poloidal field coils enabled us to scan triangularity from the original value of up to (currently limited at MA). The -scan showed that the limit of edge density, edge pressure and the -parameter increase with . When both and are high (, ), minute-grassy ELMs appear. The edge -parameter during the minute-grassy ELMs can be higher than that for the onset of giant ELMs.

Analysis of the current‐diffusive ballooning mode

The current‐diffusive ballooning mode is analyzed in the tokamak plasma. This mode is destabilized by the current diffusivity (i.e., the electron viscosity) and stabilized by the thermal conductivity and ion viscosity. By use of the ballooning transformation, the eigenmode equation is solved. An analytic solution is obtained by the strong ballooning limit. Numerical calculation is also performed to confirm the analytic theory. The growth rate of the mode and the mode structure are analyzed. The stability boundary is derived in terms of the current diffusivity, thermal conductivity, ion viscosity, and the pressure gradient for the given shear parameter. This result is applied to express the thermal conductivity in terms of the pressure gradient, magnetic configurational parameters (such as the safety factor, shear, and aspect ratio), and the Prandtl numbers.

Effects of metal impurity re-cycling in the scrape-off plasma of a large tokamak

Re-cycling of self-sputtered impurities from a limiter or a divertor neutralizer plate is investigated numerically. An upper limit for the boundary plasma temperature is determined from the condition that wall material impurity ions do not increase. This temperature is shown to be very low in a conventional tokamak but not limited in a divertor tokamak with a molybdenum neutralizer plate.

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

Physics issues of high bootstrap current tokamaks

Physics issues of a tokamak plasma with a hollow current profile produced by a large bootstrap current are discussed based on experiments in JT-60U. An internal transport barrier for both ions and electrons was obtained just inside the radius of zero magnetic shear in JT-60U. Analysis of the toroidal ITG microinstability by toroidal particle simulation shows that weak and negative shear reduces the toroidal coupling and suppresses the ITG mode. A hard beta limit was observed in JT-60U negative shear experiments. Ideal MHD mode analysis shows that the n = 1 pressure-driven kink mode is a plausible candidate. One of the methods to improve the beta limit against the kink mode is to widen the negative shear region, which can induce a broader pressure profile resulting in a higher beta limit. The TAE mode for the hollow current profile is less unstable than that for the monotonic current profile. The reason is that the continuum gaps near the zero shear region are not aligned when the radius of qmin is close to the region of high . Finally, a method for stable start-up for a plasma with a hollow current profile is described, and stable sustainment of a steady-state plasma with high bootstrap current is discussed.

Second stability region against the internal kink mode in a tokamak

Using the ERATO code, the stability of the internal kink mode is studied for a wide range of βp values for tokamak equilibria with a circular cross-section. The internal kink mode becomes unstable when βp exceeds a critical value and becomes stable again when βp is sufficiently high.