Optimization of helical hybrid coil for high field tokamak

A new toroidal field (TF) coil whose stress is much reduced is proposed for tokamaks by use of the virial theorem. It is a hybrid coil of a conventional TF coil and an ohmic heating (OH) coil helically wound on a torus. The winding is modulated in such a way that the poloidal field exists only outside of the torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest averaged stress requires equal averaged principal stresses of all directions. Therefore, the pitch number of the helical coil is determined to satisfy the uniform stress condition. The enhancement factor of the magnetic field in our TF coil increases as the aspect ratio A decreases where the distribution of the stress by the toroidal effect is important. In the case of A = 2, our optimal coil increases the magnetic field to 1.4 times larger than the conventional TF coil. As concerning stress, it is reduced by a factor of 2 in comparison with the conventional TF coil.