Plasma current start-up by the outer ohmic heating coils in the Saskatchewan TORus Modified (STOR-M) iron core tokamak

Abstract

A plasma current up to 15 kA has been driven with outer ohmic heating (OH) coils in the STOR-M iron core tokamak. Even when the inner OH coil is disconnected, the outer OH coils alone can induce the plasma current as primary windings and initial breakdown are even easier in this coil layout. This result suggests a possibility to use an iron core in a spherical tokamak to start up the plasma current without a central solenoid. The effect of the iron core saturation on the extension of the discharge pulse length has been estimated for further experiments in the STOR-M tokamak.

Plasma current start-up experiments without a central solenoid in the iron core STOR-M tokamak

“…The plasma current (figure 3(a)) is reproducibly generated because of a wide null field region due to the absence of the reversed bias OH coil current. We noted that breakdown was not affected when the bias current was applied to the outer OH coil because the stray field itself by the bias current is small [8]. The loop voltage is shown in figure 3(b) and its numerically integrated flux ( ) is shown in figure 3(e), indicating that the iron core is near saturation at the end of the discharge.…”

“…Since the image fields from the outer OH coil, the feedback BV coil and the plasma current are functions of the relative permeability, they are separately calculated by the infinitely long iron core approximation of the image field formula with the finite permeability [12]. Thus the calculated image fields are fitted by a function of the relative permeability µ r [8].…”