Fusion Science and Technology / Volume 61 / Number 1 / January 2012 / Pages 19-50
The high-frequency (HF) magnetic sensors for ITER are currently based on a conventional, Mirnov-type pickup coil, with an effective area in the range 0.03 < (NA)EFF (m2) < 0.1; the sensor is required to provide measurements of magnetic instabilities with magnitude around [vertical bar]B/B[vertical bar] [approximately] 10-4 in the 10-kHz to 2-MHz frequency range. The physical, mechanical, and electrical properties of one representative ITER HF pickup coil design have been analyzed with particular attention to the manufacturing and assembly process for the winding pack, as its integrity was found to be of concern when performing a coupled electromagnetic, structural, and thermal analysis of the sensor. Three different options for the guiding grooves in that design have been tested, using copper and tungsten for the winding pack, but none of them has been convincing enough due to the likelihood of breakages of the thin grooving and of the tungsten wire itself. Hence, alternative designs still based on a conventional Mirnov-type pickup coil have been explored, and a nonconventional Mirnov-type pickup coil was produced using direct laser cutting of a Type 316 stainless steel hollow tube, avoiding the difficulties encountered during the winding operations for conventional Mirnov-type sensors. This process of manufacturing appears to be acceptable for HF magnetic sensors of Mirnov-type design in ITER, and it is recommended for future prototyping studies, as the effective area of our first prototype, (NA)EFF [approximately] 0.01 m2 , was well below the ITER requirement. The electrical characteristics and the frequency response of all these prototypes were evaluated up to 8 MHz, with the results in good agreement with model calculations. The conventional Mirnov-type prototypes behave as expected in terms of their main electrical properties and should satisfy the present measurement performance requirements. Finally, a direct measurement of the effective area of these sensors has shown that the geometrical value is a sufficiently correct estimate of its actual value at low frequencies (<10 kHz) when the winding pack closely follows the nominal shape of the coil itself.