MHD beta limits for advanced scenarios on JET

Abstract

Ideal MHD limits to beta and bootstrap fraction are computed for pressure profiles similar to those of JET discharges with both internal and H mode transport barriers. Several non-monotonic current profiles are tested, and the pressure profile is constrained so that no negative current drive is required in steady state. Calculations are made both with and without an ideally conducting wall. The main result is that, for such peaked pressure profiles, the limits to beta and bootstrap fraction improve at low internal inductance, in particular when wall stabilization is taken into account. The highest limits to beta, and often also to normalized beta, occur for the maximum plasma current. There is also a weak dependence on qmin , for which three favourable regions have been identified. A highly advantageous region is found at qmin 1.6, where the limits to β * are 7.0% with, and 4.8% without, wall stabilization. The corresponding limits are 68 and 50%, respectively, for the bootstrap fraction and 4.0 and 2.9 for the normalized beta. These equilibria have low internal inductance, li = 0.62. For higher inductance, an optimum occurs when qmin 1.2, where the limit to β * is 5.3% with a wall and 4.7% without. The corresponding bootstrap fractions are about 46 and 38%, respectively. A third type of equilibrium that is interesting for steady state operation has qmin 2.1 and low inductance. Here the β * limits are lower, 4.9 and 3.4%, but the bootstrap fractions are higher, 77 and 60%.

Internal transport barriers in tokamak plasmas*

“…The effect of the wall is to decrease the growth rate of the radial displacements, caused by the ideal instability, to a level where they can be counteracted by external saddle coils (Fitzpatrick and Jensen, 1996). While toroidal plasma rotation is predicted to support the effect of a conducting wall (Bondeson andWard 1994, Ward andBondeson 1995), this does not improve the situation in a burning fusion plasma with little or no rotation.…”

“…In JET optimized shear plasmas (s 0) the β-limit is determined by an ideal n = 1 pressure driven kink mode (Bondeson et al 1999, Huysmans et al 1999. In figure 49 the calculated stability limit is compared to the evolution of the experimental β N and pressure peaking factor, β(0)/ β .…”

Toroidal modeling of interaction between resistive wall mode and plasma flow

“…2(b), results in a large fraction of the bootstrap current in the middle region of the plasma column, which in turn creates a rather flat current profile and a slightly reversed q profile in the plasma core. The equilibrium with a broad current profile and a peaked pressure profile tends to be more susceptible to the external kink instability [23], which becomes an RWM in the presence of a close fitting resistive wall. This motivates our choice of the equilibrium for studying the interaction between the RWM and the plasma flow.…”

Magnetic drift kinetic damping of the resistive wall mode in large aspect ratio tokamaks

“…It is well known that advanced tokamaks 1 have good transport properties in the plasma core region, but suffer from a pressure limit set by pressure-driven external kink modes, which are especially vulnerable to instability for a plasma with a broad current profile and a rather peaked pressure profile. 2 A surrounding, highly conducting wall reduces the growth rate of the external kink to a time scale of the vertical field penetration time through the wall, resulting in the so-called resistive wall mode ͑RWM͒, which needs to be stabilized for a steady-state operation of advanced scenarios, for example, the Scenario-4 ͑Ref. 3͒ designed for the ITER.…”