New types of toroidally rotating fluctuations (toroidal modenumbers n = 1 and n = 2) of line-integrated electron density and Hαemission, with frequencies ranging from 10 to 100 kHz, are observed inthe sustained spheromak physics experiment (SSPX). The rotatingdirections of these fluctuations are the same as the directiondetermined by E×B, while the E and Bdirections are determined by the gun voltage and gun magnetic fluxpolarities, respectively. These results take advantage of one distinctivesignature of spheromaks, i.e. it is possible to observe toroidal MHDactivity during decay and sustainment at any toroidal angle. Atheoretical constraint on line-integrated measurement is proposed and isfound to be consistent with experimental observations. Fluctuationanalysis in the time and frequency domains indicates that the observeddensity and Hα fluctuations correlate with magnetic modes.Observation of Hα fluctuations correlating with magneticfluctuations indicates that, at least in some cases, MHD n = 1 modes aredue to the so-called `dough-hook' current paths that connect the coaxialgun to the flux conserver, rather than internal kink instabilities.These results also show that electron density and Hα emissiondiagnostics complement other tools for spheromak mode study.
Magnetic fluctuation studies in Extrap T1