Evidence of halo currents in JET

There is evidence in JET (Joint European Torus) and in other tokamak machines that large currents flow to the vessel walls during vertical instabilities and the subsequent current quench. The extent and the location of damage to the first wall components in JET caused by the interaction of the magnetic field with currents through these components are reported. An analysis of the damage allows one to estimate the currents. The difference in the toroidal field measured at the top and the bottom Inside the vessel gives an indication of the total poloidal current flowing between the plasma and the vessel. The voltage difference along 2.25 m long poloidal sections at the top and bottom of the vessel gives an indication of the net repelling (stabilizing) force between the plasma and the vessel. In addition, current shunts on strategically located tiles in the vessel were used to measure local current flowing across the tile supports. The estimates are compared, and the direction of the forces exerted on the in-vessel components and the implications that these currents have for the design of the new JET pumped divertor are discussed