Abstract
Magnetic field sensors are fundamental for control and physics exploitation of fusion devices. Their inductive nature implies high dynamic ranges and a broad bandwidth, thus a precise characterization and calibration of these probes is paramount.
COMPASS-U will have a completely new set of magnetic diagnostics, from sensors to data acquisition. Sensors installed in-vessel will operate at 300-500 degrees C and should survive transients of even higher temperatures and thus materials used should be high-temperature compatible.
These design limitations will have an impact on the dynamic range and bandwidth of the sensors, which needs to be reliably quantified, optimally with the same test bench for all sensor types. The first part of this work presents a test bench and the process of how to calibrate the effective area using a large solenoidal coil and frequency response of the magnetic sensor prototypes with a Helmholtz coil.
In the second part, test results of the sensor prototypes are presented and discussed in detail. The low-bandwidth sensors made of mineral insulated cable (MIC), intended for plasma control and machine protection, show negligible attenuation up to 10 kHz, sufficient for their role.
For fast coils consisting of bare wire wound on ceramic mandrel and Thick Printed Copper (TPC) sensors the negligible attenuation measured below 1 MHz is again sufficient for their intended purpose of detecting fast coherent plasma fluctuations. Resonances introduced by the capacitance of long cables from the vacuum vessel feedthroughs to the data acquisition systems are measured, to model their influence on the signal.