Upgrade of the magnetic diagnostic system for restart of HT-6M operation

The HT-6M tokamak at the Thailand Institute of Nuclear Technology has been restarted.In order to ensure the smooth breakdown of plasma and obtain plasma discharge parameters,optimization of the poloidal field coils and upgrade of the magnetic diagnostics are described in this article.A perfect null field(stray field in the main chamber<10 G)is obtained using an ohmic heating field.To obtain important information about the plasma,an external magnetic diagnostics system is designed and calibrated,including a Rogowski coil(measuring plasma current),a magnetic probe(measuring external field),diamagnetic loops(measuringβ_(p))and so on.In order to realize high-frequency signal measurement and transmission,a series of frequency responses with the magnetic probe and transmission line are tested.Later,to verify the null field,a fitting code is developed to reconstruct the stray field in the vacuum chamber based on magnetic probe measurements and flux loops.The results show that the error is within 1.5%.This indicates the accuracy of the magnetic measurement system and ensures the preparation for the breakdown of plasma.