Improved neoclassical plasma confinement and turbulence suppression in a magnetic well in tokamak reactors

It has been known that the width of the trapped particles, i.e. bananas, is reduced or squeezed in a magnetic well in tokamaks. The magnetic squeezing factor S (B) depends on the energy and pitch angle of the particles. When rho(p)/a similar to 1, S-B becomes appreciably larger than unity; it can be of the order of 2 for a parabolic well with B (-1) d(2) B/dr(2) similar to 2a(-2). Here, rho(p) is the poloidal gyro-radius, B is the magnetic field strength, r is the local minor radius, and a is the minor radius. However, the real orbit width measured in terms of the poloidal magnetic flux is still less than a because of the magnetic squeezing; for S-B = 2, it is a 30% reduction. The transport consequences of the squeezed bananas in a magnetic well are calculated by solving the drift kinetic equation utilizing the constants of motion in the large aspect ratio limit. Consequently, neoclassical ion heat conductivity chi(h) in the reactor relevant collisionality regime (i.e. banana regime) is reduced by a factor of S-B(3/2); for S-B = 2, it is almost a factor of three reduction for the same temperature. The ion temperature scaling of chi(h) improves from the conventional, and weakly favorable Ti-1/2 T-i(-2)

Automated summary

It has been found that the width of trapped particles like bananas in tokamaks is reduced due to magnetic squeezing, leading to a decrease in ion heat conductivity.