Magnetic field measurements are indispensable for control and diagnostics of a tokamak plasma. The existing methods for the measurement are (i) integration of time-derivative of magnetic field, and (ii) direct measurements of absolute magnetic field using rotating coils, Hall-element sensors, etc. The latter seems to be incompatible with 14-MeV neutron field, while the former has a problem of signal drift inevitable in an integrator, and thus it could not work for a long period of discharge (e.g. 1000 s for ITER).

Three integration methods have been applied or investigated: (a) analog integration, (b) numerical integration by a DSP with a fast ADC, and (c) a VF (voltage-to-frequency) converter with a UDC (up-down counter). We chose the VF-UDC method for the development from the following view points: Its high accuracy is expected equivalent to analog integration. Wider dynamic range is allowed in a large digital accumulator, and drift can be compensated more precisely in a digital system.

The current results of the development are as follows:
(1) Technical causes of drift in the old VF-UDC system for JT-60 were analyzed. A new VF converters, then, have been developed with taking all measures identified in the analysis.
(2) The first version of the new VF converter (VF#1) showed small amount of drift for 24 hours in a stand-alone integration test with input short.
(3) Conversion linearity of the VF#1 is much improved from 0.027%FS (the old VF) to 0.002%FS (VF#1) in the ranges of 110V and 110.5MHz. A deadband width at 0-V input is reduced from a few mV to several tens of micro-volts.
(4) A new method for drift compensation is being installed in the VF#2, which can suppress the amount of drift with constant speed to several counts for 1000-s integration.
(5) For further improvement of signal/drift ratio for ITER, a new method is proposed that two VF-UDC sets with different amplifier gains are used for one signal input, and a supervisory processor calculates the precise integrated signal using a low-gain signal for plasma breakdown/termination and a high-gain one for the flattop.

The measurement with VF (#1/#2)-UDC in JT-60 experiments will be conducted in June, 1997. The detail results will be discussed in the conference.