Design and operation of a fast valve for disruption mitigation studies on NSTX-U

NSTX-U research will offer new insight by studying gas assimilation efficiencies for MGI injection from different poloidal locations using identical gas injection systems. In support of this activity, an electromagnetic MGI valve has been built and tested. The valve operates by repelling a conductive disk due to eddy currents induced on it by a rapidly changing magnetic field created by a pancake disk solenoid positioned beneath the circular disk attached to a piston. Results from the operation of an electromagnetic valve, that does not incorporate ferromagnetic materials, are presented. Image currents induced on a conducting piston placed near a pancake solenoid cause it to move away from the solenoid and break the vacuum seal. A new and important design feature is the use of lip seals for the sliding piston. The pressure rise in the test chamber is measured directly using a fast time response baratron gauge. At a plenum pressure of just 7500 Torr (~150 psig), the valve injects 200 Torr.L of Nitrogen with a pressure rise time of 3 ms.