Experimental comparison of compact RFPs, spheromaks and STs under controlled current drive

A comparative experiment on low aspect ratio (A ~ 1.5) torus plasmas, including compact reversed field pinches (RFPs), spheromaks and spherical tokamaks (STs), has been performed in the TS-3 and TS-4 compact toroid (CT) devices. In particular, a compact RFP with qo as large as 0.3 was found to have a low n dynamo mode, n = 3, while a spheromak with qo ~ 0.5 had an n = 2 mode. Under OH current sustainment, the magnetic fluctuation and loop voltage of the CT were observed to increase inversely with its q value. A new method of edge current drive was developed using axial CT merging, which together with the OH current drive created a balanced current drive. This current drive was found to reduce the dynamo fluctuations of compact RFPs and spheromaks by a factor of 3. Multiple CT merging is a promising method of edge current drive because of its intermittent current drive and its detailed control of current profiles using varied size of colliding CTs.