Development of a precise long-time digital integrator for magnetic measurements in a tokamak

A new digital integrator based on the method using a voltage-to-frequency converter (VFC) and an up-down counter (UDC) has been developed for a precise long-time integration of magnetic signals in a tokamak. Three trial VFC boards have been made corresponding to the development steps: (a) to remove technical causes of drift by analyzing the old VPC-UDC system for JT-60, (b) to improve the precision of integral results, and (c) to suppress the drift speed to an allowably low amount. After various modifications, a precise VFC having a feature of low drift enough for ITER was successfully completed, where it is premised that the noise in the magnetic sensors and common ground at ITER is similar to that at JT-60. For a remaining problem of large integral error that related to plasma MHD activities, we present a design of a new system that can correct the integral result using two VFC´s for one sensor in parallel