Tomographic density profile measurements of spheromak plasma targets for MTF applications*

A method of plasma sustainment, imposed-dynamo current drive (IDCD), could improve the performance of compact, magnetized plasma targets of interest to ARPA-E’s ALPHA fusion portfolio. A new tomography system has been built and installed on the HIT-SI3 spheromak experiment at the University of Washington. Several hundred chords of light from three poloidal planes and the toroidal mid plane are simultaneously collected by 13 wide-angle lenses and imaged onto custom fiber optic bundles. The plasma light travels through a beam splitter after which each leg is filtered to a different He I emission line (668 nm and 728 nm) and collected by a single Phantom v1212 high-speed camera. The plasma density is calculated from the ratio of the lines. This diagnostic is used to assess the effect of imposed electron velocity shear symmetrizing and stabilizing compact, spheromak plasma targets. An improvement in the symmetry of compact plasma targets while preserving gross plasma stability may lead to high energy confinement quality, thereby enabling higher temperature magnetized plasma targets.