Enhanced pinch effect due to the electrostatic potential

Abstract

An inward particle pinch appears necessary to explain experimental results in tokamaks. Neither the neoclassical pinch effect, which is too small, nor the off-diagonal quasilinear term, which is usually outward in the trapped particle regime, can account for the observations. A mechanism for an enhanced inward pinch is proposed, based on results for an asymmetric magnetic field bump [Phys. Fluids 15, 2211 (1972)]. Because turbulent fluctuations also break toroidal symmetry, an enhanced inward pinch driven by the fluctuations and the Ohmic inductive field E is expected. To demonstrate this effect, an inward particle flux is calculated for a model tokamak configuration that has an electrostatic potential bump Φ0 at toroidal angle ζ=2π. For the parameter regime r/R<eΦ0/Te<1, the flux is found to be Γ=−4.47K(q)(r/R)L(Φ0)(vte/Rνe) 1/2cNE/B, where r(R) is minor (major) radius, B is the magnetic field strength, vte is the electron thermal speed, νe is the electron–ion collision frequency, q is the safety factor, and K(q) and L(Φ0) are functions of q and Φ0, respectively. The results are also applicable to an asymmetric potential bump created externally to enhance the inward pinch flux of high energy, collisionless particles.