Plasma production in a small high field force-balanced coil tokamak based on virial theorem

The plasma production and confinement experiments in a novel tokamak device with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is reduced to a theoretical limit determined by the virial theorem are presented. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest stress requires equal averaged principal stresses in all directions. Therefore, the pitch number of a helical coil is determined to satisfy the uniform stress condition. Moreover, the helical winding is modulated in such a way that poloidal field exists only outside of a torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. In the case of the aspect ratio A = 2, our optimal coil theoretically increases the magnetic field to 1.4 times larger than the conventional TF coil. In order to prove the advantage of the VLC concept, we manufactured a small VLC pulsed tokamak "Todoroki-II" with a major radius of 0.3 m, a minor radius of 0.08 m, toroidal magnetic field strengths of B_T < 1.5 T and plasma currents of I_p < 40 kA. External vertical field increased both the plasma pulse length and current to 1 ms and 11 kA, respectively, while they were restricted because of no vertical field control. Using a Cauchy-condition surface (CCS) method, the shape and displacement of plasma boundary was reconstructed, and the validity of CCS under large eddy currents was verified.