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JET Vertical Stabilization system

The JET Vertical Stabilization project provides a good example where demanding requirements from a fusion experiment (JET) have driven the adoption of ATCA based solutions.

Elongated plasmas are vertically unstable leading to loss of control if plasma reaches the vessel protecting tiles leading to considerable heat loads on JET plasma facing components. Therefore, dedicated MIMO systems are designed to make the plasma vertically stable so that other controllers can successfully control the plasma position and shape. While at JET, a Vertical Displacement Event (VDE) can generate disruptions with a reduced impact in the machine, in ITER the loss of vertical plasma position control will cause thermal loads on Plasma Facing Components of 30-60 MJ/m2 for ~0.1s. With the present knowledge the Plasma Facing Components cannot be designed to sustain such (repetitive) thermal loads. Furthermore,  VDEs also generates the highest electromagnetic loads: (i) A phenomenological extrapolation of horizontal forces from worst JET cases implieshorizontal loads ~45MN  on ITER vacuum vessel;  (i) MHD wetted kink model developed to simulate the horizontal loads predicts ~20MN; and (iii) Vertical loads ~90MN. This leads to the conclusion that the plasma vertical position control in ITER must be robust and reliable to ensure a vertical plasma position control loss is a very unlikely event. Therefore, JET project already had these stringent demands into consideration. In its specification was required to aim at a reduction of: (i) the loop delay on the signal acquisition/generation endpoint (down to 10 ms); (ii) the data interconnect links form and to the processing unit; (iii) the analogue filter path. It was also required high processing power on the acquisition/generator endpoints, on the system controller and for the improvement of the MIMO algorithm performance. The synchronization of all digitizer/generator endpoint was also required. There was a strong emphasis on choosing an architecture designed for maintainability, upgradability and scalability at a low cost per channel.

A Multi-Input-Multi-Output controller for the plasma Vertical Stabilization (VS) was implemented and installed on the JET tokamak. The system currently attains a control loop-cycle time of 50 µs using x86 multi-core processors but targets 10 µs via FPGA based processing. The hardware, complying to the Advanced Telecommunications Computing Architecture (ATCA) standard, was specially designed to achieve such a performance mindful of its suitability for ITER needs.  It consists of: (i) a total of 6 synchronized ATCA control boards, each one with 32 analog input channels, provide up to 192 galvanically isolated channels used mainly for magnetic measurements (Fig. 1). (ii) Each board contains 512 MBytes of DDR memory and a FPGA, which performs digital signal processing and includes a PCI Express communications interface; (iii) An ATCA Rear Transition Module, which comprises up to 8 galvanically isolated analog output channels for controlling the Fast Radial Field Amplifier (±10 kV, ±2.5 kA); (iv) An optical link to allow the digital control of the Enhanced Radial Field Amplifier (±12 kV, ±5 kA); (v) Up to 8 EIA-485 digital IO channels for timing and monitoring information; (vi) An in-house developed ATCA processor blade, with a quad-core processor where the control algorithm is running presently, connected to the 6 ATCA control boards through the PCI Express interface. All FPGAs are interconnected by low-latency links via the ATCA full-mesh backplane, allowing all channel data to be available, in the control cycle, on each FPGA running an upcoming distributed control algorithm.


Publications

F. Sartori et al., “The JET PCU project: An international plasma control project”, Fusion Engineering and Design, Volume 83, Issues 2-3, April 2008, Pages 202-206

A. J. N. Batista et al., “ATCA digital controller hardware for vertical stabilization of plasmas in tokamaks”, Rev. Sci. Instrum. 77, 10F527 (2006)


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