Two data points in the shaded zones of Figs. 3(a) and 3(b) (one point for each figure), were omitted in the published version of these figures. The correct Fig. 3 is shown below.FIG. 3. Radial profiles of (a) electron density, ( b) electron temperature, and (c) ion temperature at 6 s and (d) safety factor at 5.9 s of the discharge shown in Fig. 2. The volume-averaged plasma minor radius is 1.01 m.

A concept of a steady state tokamak fusion reactor based on the bootstrap current is presented. Operation at high poloidal beta (βp ≥ 2.0) and high q (4–5) with a relatively small limit on ∈βp (< 0.5) makes it possible to drive a bootstrap current constituting up to 70% of the total plasma current without exceeding the Troyon beta limit. The rest of the plasma current can be driven by the high energy neutral beam with an energy multiplication factor Q of 30. Energy confinement scaling laws predict that the reactor condition is attainable by increasing the major radius up to 9 m in such a high βp and high q plasma at a relatively low plasma current (12 MA) with a confinement enhancement factor of 2 compared with the L-mode scaling. This reactor has a reasonable size (Vp = 1500 m3) and fusion output power (2.5 GW) and is consistent with present knowledge regarding tokamak plasma physics, namely the Troyon limit, the energy confinement scalings, the bootstrap current and the current drive efficiency (neutral beam current drive with a total power of 70 MW and a beam energy of 1 MeV) with good prospects for the formation of a cold and dense divertor plasma.

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The paper discusses edge stability, beta limits and power handling issues for negative triangularity tokamaks. The edge MHD stability is the most crucial item for the power handling. For the case of negative triangularity the edge stability picture is quite different from that for conventional positive triangularity tokamaks: the 2nd stability access is closed for localized Mercier/ballooning modes due to the absence of magnetic well, and nearly internal kink modes set the pedestal height limit weakly sensitive to diamagnetic stabilization just above the margin of localized mode Mercier criterion violation. While negative triangularity tokamak is thought to have low beta limit with its magnetic hill property, it is found that plasmas with reactor relevant values of normalized beta β N > 3 can be stable to global kink modes without wall stabilization with appropriate core pressure profile optimization against localized mode stability and also with increased magnetic shear in the outer half radius. The beta limit is set by n=1 mode for the resulting flat pressure profile. The wall stabilization is very inefficient due to strong coupling between external and internal modes. The n>1 modes are increasingly internal when approaching the localized mode limit and set a lower beta in case of peaked pressure profile leading to Mercier unstable core. With the theoretical predictions supported by experiments, a negative triangularity tokamak would become a perspective fusion energy system with other advantages including larger separatrix wetted area, more flexible divertor configuration design, wider trapped particle free SOL, lower background magnetic field for internal poloidal field coils and larger pumping conductance from the divertor room.

The bootstrap current is a plasma current associated with trapped parliclicles in a toroidal plasma. Magnetic measurements, such as the surface voltage. the internal inductance, and the Faraday rocation, are consistent with the existence of the neoclassical bootstrap current in tokamaks. The neoclassical trapped-particle correction to the electrical conductivity is also systematically validated against experiments. These results support the assertion that the generalized Ohm's law along the magnetic field is valid, as predicted by the neoclassical hanspott theory, while the perpendicular transport deviates from the neoclassical transport theory.