TOMOGRAPHIC WIRE-GRID IMAGING OF MAGNETIC-SURFACES

We describe a new, fast, and accurate method for obtaining magnetic surface puncture plots by the use of a rotating grid of fine wires. Electrons launched from a small gun follow the magnetic field lines, and a fraction are intercepted by the wire grid. The current signals, chordal integrals of electron beam intensity, are inverted by an iterative pseudotomographic method to obtain an image of the electron beam density in the plane of the grid, that is, a puncture plot of the magnetic surface. The method is capable of high resolution while maintaining a high transparency to electrons, and is demonstrated on a small toroidal stellarator, the SHEILA heliac.