Sensor Algorithms of the Plasma Vertical Position to Avoid a Vertical Displacement Event During Plasma-Current Quench on JT-60U

A high toroidal eddy current induced in a vacuum vessel during plasma-current quench, I_p quench, results in errors in determining the vertical position of the plasma-current center, Z_J, calculated from standard linear regression sensor algorithms. These deviations result in a vertical displacement event (VDE) that must be avoided because of the expected severe damage on the first wall in tokamak fusion reactors like the International Thermonuclear Experimental Reactor (ITER). On the other hand, high Z_J calculation accuracy must be maintained at steady state to obtain reasonable plasma performance. Thus, real-time sensor algorithms for the calculation of Z_J applicable to the two cases of steady state and slow I_p quench are investigated. When a statistical method is applied to the Z_J calculation, its deviation from the actual Z_J cannot be completely reduced at the same time for both cases. On the contrary, a neural network demonstrates high accuracy in the calculation of Z_J for both cases, which enables real-time feedback control of Z_J during slow I_p quench, avoids VDE, and keeps reasonable plasma performance during steady state.