Control of a global motion on field-reversed configuration

An n=1 mode global motion on a field-reversed configuration (FRC) plasma is observed by means of a newly developed optical system. The deviation of the FRC from the coil axis reaches 20%–40% of the plasma radius. In order to push back the FRC to the equilibrium position, a multipole field (quadrupole or hexapole field) is applied. The n=1 motion can be easily controlled by the quadrupole field, the critical field strength of which is required to be about 15% of the confinement field. It is found that the n=2 rotational instability can also be stabilized by strength of the same order. The critical strength for the n=1 motion is theoretically obtained from a model such that the driving energy of the motion given at the formation phase balances with the work done by the multipole field. The theoretical estimation agrees within a factor of 2 with the experimental results.