Sawtooth-like oscillations and steady states caused by the m/n=2/1 double tearing mode

The sawtooth-like oscillations resulting from the m/n=2/1 double tearing mode(DTM)are numerically investigated through the three-dimensional,toroidal,nonlinear resistive-MHD code(CLT).We find that the nonlinear evolution of the m/n=2/1 DTM can lead to sawtooth-like oscillations,which are similar to those driven by the kink mode.The perpendicular thermal conductivity and the external heating rate can significantly alter the behaviors of the DTM driven sawtooth-like oscillations.With a high perpendicular thermal conductivity,the system quickly evolves into a steady state with m/n=2/1 magnetic islands and helical flow.However,with a low perpendicular thermal conductivity,the system tends to exhibit sawtooth-like oscillations.With a sufficiently high or low heating rate,the system exhibits sawtooth-like oscillations,while with an intermediate heating rate,the system quickly evolves into a steady state.At the steady state,there exist the non-axisymmetric magnetic field and strong radial flow,and both are with helicity of m/n=2/1.Like the steady state with m/n=1/1 radial flow,which is beneficial for preventing the helium ash accumulation in the core,the steady state with m/n=2/1 radial flow might also be a good candidate for the advanced steady state operations in future fusion reactors.We also find that the behaviors of the sawtooth-like oscillations are almost independent of tokamak geometry,which implies that the steady state with saturated m/n=2/1 islands might exist in different tokamaks.

Stability of Tokamak Equilibrium with Internal Transport Barrier against High-n MHD Ballooning Mode

A new eigen-mode equation for the tokamak high-n （the toroidal mode number） ideal magnetohydrodynamic （MHD） ballooning mode in tokamak plasmas is derived to include the toroidal effects that are significant for stability of configurations with internal transport barriers （ITBs）, Fot tokamak equilibria of shift circular flux surfaces, these toroidal effects basically are the finite inverse aspect ratio and the Shafranov shift. The former yields the averaged favorable curvature stabilization while the latter further strengthens this effect, leading to a low shear stable channel connecting the first and second stability regions, and to the shrinkage of unstable region in the （8,α） diagram. The dependence of the critical shear, below which the plasma is stable, on these effects is given. These results are important for understanding the ITB physics to some regards.

A Simulation of the Effect of Collisional Dissipation of Fast Electrons on the Radial Profiles of the Current Driven by Lower-Hybrid-Waves on the HT-7 Tokamak

For the effect of the collisional dissipation of fast electrons driven by the lower-hybrid waves, a predictive simulation is made for the HT-7 plasma. The simulation results show that the dissipation of fast electrons counteracts the effect of radial diffusion to some extent, thereby making the lower-hybrid driven current profile closer to the power deposition profile. So, in the case of an off-axis lower-hybrid wave power launching, the dissipation is helpful in maintaining a center-hollowed current profile in lower hybrid current drive （LHCD） plasmas, and thus possibly maintains the desired reversed magnetic shear.