BOOTSTRAP CURRENT AND ION THERMAL-CONDUCTIVITY IN AN ULTRALOW-ASPECT RATIO TOKAMAK

The bootstrap current Jb = -IcP′B/〈B 2〉 and χi = √2NνiM iTi(Ic/e)2〈1/B2〉/ |∇ψ|2 are valid for both the banana and the Pfirsch-Schlüter regimes for any finite value of the collision frequency at a radius where the local aspect ratio A approaches unity. Here, I = RB t with R the major radius, Bt the toroidal magnetic field strength, and the prime denoting the derivative with respect to the poloidal flux ψ. Thus, the bootstrap current does not vanish, even in the collisional regime, when A approaches unity. The physical reason for this dramatic result is that the magnitudes of the electron and ion parallel viscosity approach infinity as A approaches unity. This also indicates that the conventional theory underestimates the magnitude of bootstrap current in an ultralow-aspect-ratio tokamak. © 1995 American Institute of Physics.