Amplifiers for digital neutron detectors

2016, 2020

ITER Center
Measuring amplifiers for the diagnostic fission module of the neutron flux divertor monitor for the ITER project (International Thermonuclear Experimental Reactor).
The International Thermonuclear Experimental Reactor (ITER) is a megaproject aimed at developing commercial applications of fusion reactions. This project involves more than 30 countries. The reactor is planned to start operating in 2025. Russia is manufacturing and supplying high-tech equipment and basic reactor systems. We managed to get involved in developing one of the subsystems of this ambitious project.

The Research and Development Department of the ITER Center coordinating the ITER activities in Russia created a prototype of a pulse signal amplifier with a fiber optic line of analog signal transmission with a bandwidth of up to 100 MHz, with the experimental setup assembled and tested. In order for the sample to be tested in the main reactor in France, it was necessary to elaborate the conceptual prototype into a specialized electronic solution and to implement digital remote control. We were assigned this engineering task.
We were developing the electronics for the diagnostics module in several iterations while the customer’s scientific team was carrying out the research.

With part of the diagnostic device located near the fission ionization chamber in the neutron radiation zone, there should be no people near the device: recording analog signals and controlling the device should be done remotely, hundreds of meters or even several kilometers away. At the same time, the customer demanded that the galvanic isolation be ensured. Given that it was impossible to transmit signals simply by wire and by radio, the only acceptable option was optical fiber. The key difficulty was to ensure that the analog signal could be transmitted over hundreds of meters of fiber without distortion or noise.

We developed an analog fiber optic transmission line at 100 MH. The peculiarity of this optical isolation is that the voltage in the measuring part can differ by hundreds or thousands of volts from the voltage in the laboratory.
The main difficulty was to transmit an analog high-frequency signal over fiber without distortion and noise. This problem was not solved at once, and we had to deal with it a lot.
We also worked through the amplifier and power control boards.

The power board isolates the input power supply from the internal circuits of the amplifier to produce ±12V and ±200V DC voltages for all amplifier boards. We selected a system of converters and added a voltage regulation and measurement unit to monitor the parameters in the control board.

We modified the amplifier board and found a more suitable element base for this task. The board is connected differentially to the fission chamber, and the output is a current signal converted to a voltage with a boost factor of about 100kV/A or 100V/mA.

A controller board was added for remote control of the amplifier and additional equipment.
Finally, we created a system of four microelectronic boards: power board, amplifier board, fiber optic transmitter board, and amplifier controller board.
Further, in order for up to four amplifiers to be connected simultaneously, we expanded the complex with decoupling: we multiplied the opto-receiver module for four channels in a separate case and wrote a new software package for independent control of each amplifier and data consolidation.
The customers were given a complete set of measuring equipment ready for use.

All the design work was formalized in the standard design documentation and provided to the customers.
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