Heavy ion beam probes for stellarators

A heavy ion beam probe is being designed for the ATF torsatron at Oak Ridge National Laboratory. Beam probes are also either being designed or under consideration for stellarator type devices in Japan and the Soviet Union. The primary purpose for this diagnostic on a stellarator is direct determination of the plasma potential structure which is largely responsible for the particle drift surfaces in a nonaxisymmetric toroidal confinement device. Beam probes have been successfully implemented on tokamaks and bumpy tori but the complex magnetic geometry of a stellarator presents some significant difficulties not encountered before. The helical field coils make it impossible to locate the input and output ports used by the beam probe in the same toroidal plane. They are separated by 15 degrees on ATF. There are also substantial return fields in the region of the beam line optics and the electrostatic energy analyzer electric fields. The separation of the ports and the open field structure produces a large dispersion in ion trajectories. The design for the ATF system has successfully addressed these problems