Evaluation of noise limits of a precision ADC for direct digital signal integration of magnetic measurements

Abstract

The characterization of laboratory plasma instabilities, magnetic reconnection and turbulence associated phenomena, require the simultaneous signal sampling from arrays of magnetic sensors (hundreds or even thousands) to obtain spatial resolution, with several hundred kHz for time resolution. Magnetic measurements based on pick-up (Mirnov) coil are quite common in experimental pulsed devices for plasma research, thanks to their simplicity and reliability. Being the signal from this type of sensor proportional to the time variation of the magnetic field ($dB/dt$), it has to be time-integrated to recover the instant value of magnetic field. Depending on the required integration time usually either analog integrators or chopped integrators are used. However these solutions tend to limit the frequency bandwidth in the kHz range, they are not easy to design and build, and require additional fast channels to directly acquire the $dB/dt$ signal to recover the plasma dynamic features at higher frequencies. In this paper we evaluate the feasibility of using a direct single channel precision ADC to allow the simultaneous acquisition of $dB/dt$ measurements and to provide the integrated B measurement by means of digital integration on the new RFX-mod2 device. On this purpose we interfaced an existing ADC-module to a Xilinx Zynq FPGA, in order to evaluate the intrinsic noise and to investigate the feasible integration window of this configuration. The result opens the door to a compact, cost-effective and reliable acquisition system, usable for simultaneous real-time control and transient signal recording, scalable from tenths to thousands channels, applicable to a broad class of pulsed plasma experimental devices.