Laser-based diagnostic for tracing magnetic-field lines in spheromaks and other self-organized magnetically confined plasmas

Abstract

We are in the process of testing a technique for measuring the magnetic-field line topology in magnetically confined plasmas. The basic idea is to use a high-powerful short-pulse laser to launch a burst of energetic (∼100 keV) electrons from a target passing through the plasma of interest; these electrons then generally follow field lines until they strike a solid surface, where a burst of x rays is produced and then detected. The field line connection length can be determined from the time delay between the laser pulse and the burst of x rays. The topology of the field lines can be inferred by measuring the connection length as a function of initial target location inside the plasma. Measuring the spatial distribution of the x-ray production will provide further information on the field topology, including the effects of magnetic-field fluctuations and stochasticity. The work will eventually include testing the appropriate x-ray detectors, measuring the background x-ray emission in a spheromak plasma, measuring the energetic electron production by a short-pulse high-power laser, and making preliminary measurements of the edge field line topology in the Sustained Spheromak Physics Experiment using a pulsed electron-beam source as a prototype for a laser-based source. This technique may have broad application to a variety of plasma configurations and provide physics data applicable to a wide range of plasma physics problems.