Electromagnetic wall torques from magnetically confined plasmas

A direct method to measure the localized magnetic shear stress and hence electromagnetic torque or force exerted on a wall by a fluctuating magnetic field is presented. A model of the wall is needed when the magnetic measurements do not include the normal component. Explicit formulae for thin and thick walls are given. Estimates of the force in illustrative Alcator C-Mod tokamak plasmas provide evidence against the theory that resistive magnetic drag determines mode locking. (author)