A physical model of the beam impedance method within ideal nested magnetic surfaces

The beam impedance method is used to measure the vacuum magnetic structure of heliotron/torsatron systems. It measures the current that flows from an electron gun inserted into the vacuum chamber to the chamber wall. To interpret experimental results, we have been developing a physical model of the method and a numerical code based on the model. For simplicity, we have focused on a limited model in which only ideal nested magnetic surfaces exist. As a new approach to reproduce the real physical condition, we treat the current that flows across magnetic surfaces and that which is emitted from the electron gun separately. Radial electron distribution forms for the two currents to match each other. The characteristic of electron emission from the gun is obtained experimentally and included in the code. The diffusion process is assumed to obey the neoclassical diffusion theory. The numerically obtained current profile agrees well with the measurement if the suppression of diffusion due to a radial electric field is taken into account. (C) 2000 American Institute of Physics. [S0034-6748(00)00805-4].