Demand for electricity is increasing at a rate of 5 percent a year worldwide, which creates a pressing need for fusion energy generation. Annual global expenditure on fusion energy R&D is about $3 billion, and any serious scientific effort requires a tokamak for fusion research with the latest magnet technology. Our company, Tokamak Energy, develops small spherical tokamaks intended for use as neutron sources and plasma research instruments in the 300 plasma research centers around the world.
We have built an early prototype of a small, fully operational tokamak ( the ST25) with the potential to speed up the fusion R&D process to make fusion power widely available. While huge experiments such as JET (Joint European Torus) and ITER (International Thermonuclear Experimental Reactor) tackle major problems in fusion R&D, small tokamaks can address many fusion challenges and are amenable to rapid development with a small device.
The projected cost of ITER is well over $15 billion USD, whereas a small ST25 Tokamak costs a few million dollars to build and run. In other words, Tokamak Energy aims to provide research tools for rapid incremental innovation in fusion that complements and speeds up mainstream fusion R&D.
A tokamak is a device which uses magnetic fields to confine plasma in the shape of a torus. A stable plasma discharge requires a magnetic field that moves around the torus in a helical path. This is accomplished by combining a toroidal field, which orbits the vertical axis of the torus, and a poloidal field, which encircles the central axis of the torus.
Confining plasma with magnetic fields is necessary as plasma can reach temperatures of millions of degrees, which would melt through any solid confinement barriers. In a tokamak, electromagnets that surround the torus produce the toroidal and poloidal fields. A third electromagnet, the solenoid, induces an electric current that flows toroidally within the plasma, ionizing and heating it in the process.
The ST25 uses eight 1-farad capacitors switched by insulated gate bipolar transistors (IGBTs) to provide the necessary kiloampere-level current discharges to copper windings around the tokamak, which create a toroidal field of 0.2 tesla. Working with the poloidal field and solenoid coils supplied by separate high-voltage capacitor banks, the system induces a plasma current of 10 kA to 20 kA. We can increase both field and current by adding extra capacitors. The plasma is also ionised and driven using 2.45 GHz microwaves generated by a 3 kW magnetron for long pulse operation. We sustain it by injecting hydrogen gas from a piezoelectric valve synchronised with the electrical discharge, which we can dope with other gases if required.
Design and Implementation
In our system, NI LabVIEW software and NI CompactRIO hardware handle the plasma control and data acquisition. An NI PCI Express frame grabber captures video of the plasma discharge. We chose NI because LabVIEW is well known and widely used in big physics, giving us a well established community for support.
LabVIEW integrates seamlessly with the CompactRIO platform and third-party hardware and software, which cemented the choice. In addition to CompactRIO and the NI PCIe-1433 frame grabber, the ST25 systems incorporate a Siemens programmable logic controller (PLC) for the interlock and safety systems, The MathWorks, Inc. MATLAB® software for data analysis, an Ocean Optics spectrometer to analyse the plasma for impurities and contaminants such as water and other gases, and various vacuum pump controllers and pressure gauges. The wide variety of instrument drivers available from the NI website helped us integrate the third-party devices.
Two CompactRIO devices run two separate parts of the system. The first CompactRIO system collects data from voltage loops and coils mounted around the tokamak vessel. These tell us about the plasma conditions and position within the vessel. We also digitise signals from photodiodes to measure plasma duration and intensity. We networked this system to a second CompactRIO controller that manages the synchronisation of the capacitor bank discharges into the coils via IGBT devices and keeps the plasma discharge stable. This CompactRIO controller also manages hydrogen injection into the plasma chamber. The speed necessary to acquire waveform data, process signals, and react to control the plasma discharge means that CompactRIO and the LabVIEW FPGA Module are perfect for this application. We can control triggering the IGBTs and other devices to initiate the plasma discharge with microsecond resolution using the FPGA. The LabVIEW and CompactRIO approach allows for the processing speed of a custom chip with the reconfigurability of software.
The NI PCIe-1433 frame grabber uses NI-IMAQ drivers to collect high-speed video of the plasma at more than 1,000 frames per second from a Basler ace acA2000-340kc camera running Camera Link for monitoring and discharge analysis. With the NI-IMAQ drivers, we can easily configure the acquisition, and the system can easily convert the video files to AVI.
The whole system runs on a master PC controlled by LabVIEW which networks the CompactRIO systems, the video capture, and, using NI-VISA and an RS232 link, the spectrometer, the PLC, and the surveillance cameras. We enclose the ST25 within a Faraday cage and control it from a safe room.
The ST25 paved the way for the design of a new tokamak, the ST25 HTS. Tokamak Energy is working with Oxford Instruments to develop and demonstrate the world’s first tokamak with magnets made from high-temperature superconductors (HTS)[VS1] . All of the ST25 code is new and written specifically for this application; however, we can reuse the code in the ST25(HTS) and future iterations and take full advantage of the NI graphical system design benefits. The small tokamak is easy for students to use and researchers can perform groundbreaking plasma research and fusion science. Specifically, it will sustain intense hot plasmas for long pulses, capitalizing on the low energy dissipation in superconducting magnets.
Simplified Design, Reduced Time
Using a networked CompactRIO approach simplified the control system and DAQ design. Without these devices, the cabling and layout would be more complicated and have taken longer to implement. We can also use this scalable design to control much larger and more complex devices, which is another advantage.
Overall, NI products simplified the whole setup. We quickly brought together hardware and software from many manufacturers into an incredibly compact, powerful, and cost-effective tokamak. The ST25 took less than six months to design and build, of which the control system took about three months to plan and implement. This working prototype has yielded promising results in a short time.
We used LabVIEW and CompactRIO to construct a scalable, distributed embedded control and DAQ system without the need for specialist embedded electronic engineers. We will continue to use this approach as the project scales up in size and complexity so engineering physicists can work closely on the system implementation for HTS magnet control, plasma control, and advanced diagnostics.
MATLAB® is a registered trademark of The MathWorks, Inc.
The ST25 HTS, High Temperature Superconducting version of the ST25 Tokamak is currently being commissioned. This will be the world’s first demonstration of a fully HTS Tokamak device and will enable continuous operation with low temperature plasma discharges lasting for many minutes and hours. This will pave the way for a new high magnetic field device capable of fusion temperatures.
This promises exciting results and is currently in the design and planning phase. It will make extensive use of NI software and hardware building upon the work already done on the ST25 and indeed other much larger Tokamaks currently under construction around the world.
Tokamak Energy, Culham Innovation Centre, D5 Culham Science Centre
Abingdon OX14 3DB
Tel: 01865 536 6060