ACTIVE SHIELDING SYSTEM FOR REMOVAL OF STRAY TOKAMAK MAGNETIC-FIELDS IN JT-60 NEUTRAL BEAM INJECTORS

In the JT-60 neutral beam injectors (NBIs), an active method using a set of coils is adopted to shield unneutralized beams from stray magnetic fields, while the usual passive method using high-mu materials is used to shield the ion sources and neutralizers. This active shielding method is a unique solution for the unneutralized beams in the JT-60 NBI under the constraints of the limited space available and minimizing the error field induced by the shielding. A passive shielding method is permissible for the ion sources and the neutralizers because the space to be shielded is limited. The active shielding system is designed by making a one-fourth model of the magnetic system and calculating ion orbits using magnetic fields measured in the model. The shielding characteristics are checked by arrays of thermocouples buried in the beam dump where the unneutralized beams are thermalized. The thermocouple outputs are consistent with those predicted from the ion orbit calculations.