One of the most critical issues for the realization of the fusion reactor is sustaining high-performance plasma at a steady state and for a long duration. To achieve a steady-state fusion reactor, we must demonstrate confinement of high-performance plasma and examine such physics as plasma material interaction. However, high-temperature plasma has not yet been sustained longer than several minutes. Thus, we need to research and analyze the plasma behavior and the fusion reactor when the plasma is confined for a long duration. Figure 1 and Figure 2 show our experimental device, the Large Helical Device (LHD). Figure 1 provides an external view of the LHD and the numerous heating devices on it. Figure 2 shows one part of the inside of the LHD vessel. Inside the vessel, a plasma at a temperature of more than 10 million ˚F (20 million ˚C) is generated and sustained as a long-duration plasma. One of the important missions of the LHD is to sustain the high-temperature plasma at a steady-state. To sustain the plasma for a long period of time, we need to continuously supply plasma heating and gas fueling as required. When we supply less gas fuel, the plasma becomes thinner and vanishes. When we supply too much gas, the plasma vanishes either by cooling or by thickening. Heating plays an important role here. If the heating is not strong enough, the plasma becomes cold and vanishes. Maintaining the health of the devices while sustaining high-power heating (at megawatt levels) requires sophisticated heating technology. We collect and then measure data for the purpose of calculating solutions for gas fueling and heating power requirements. We need this procedure for feedback control and to sustain the ideal condition. For this, we must develop an integrated system we can use to control the plasma parameters. This type of complex control for the steady-state plasma is also important for the future fusion reactor.
The challenges for the success of a steady-state operation are stabilization of the plasma parameters and stabilization of the injection heating power. To stabilize plasma parameters, various observed information such as plasma density, temperature, and optical emission are important for feedback to the constant parameters. Using these parameters, we need the quantities of the gas fueling and the heating power to decide the next quantities for gas fueling and heating power. However, heating power control is difficult because it is greater than thousands of household microwave ovens. The voltage of the transmission lines exceeds 30,000 V, and accidental power reflection causes the transmitter to breakdown or the cooling water to leak onto the antenna head. These types of accidents sometimes cause terrible damage to the heating devices. Thus, in the past, we required two or three operators for complex monitoring and response.
Our experiment on the LHD is a large science project. We cannot stop or delay the experimental schedule even when the system or the device encounters difficulties. We must replace the system immediately after a problem occurs. In that sense, FPGA suits this system because we can easily modify and copy with high reliability and performance.
Stabilizing the plasma parameters and stabilizing the injection heating power are distinct challenges, which are also linked to each other. Therefore, we developed an integrated system using CompactRIO with FPGA. This empowered us to complete the complex operation, which includes the data collection, calculation, and control signal output, at high speed. Also, in our experiment we have two control rooms and many experiment operators in each room. The LabVIEW system we developed enables the operators to control the devices from both rooms. The GUI of the LabVIEW program makes operation intuitive. This system is very useful for inputting the target plasma parameters and for discussing strategies and approaches during the experiment. Thus, we could reduce the number of people involved in the operation, which means that responses to problems become quicker and more accurate. Furthermore, the operator can engage in another task during the experiment. The system also helps prevent unexpected equipment damage. This system’s fast interlocks prevented a critical accident on the heating devices.
By developing this advanced control system, researchers can discuss with other experiment participants the plasma operation and related issues during the experiment, as seen in Figure 3. As a result of the CompactRIO system with FPGA, we have sustained high-performance plasma for more than 48 minutes at more than 10 million ˚F and 10 trillion/cc in an experiment that required extremely complex processing. This temperature is higher than the temperature on the surface of the sun. This total heating power of 3.4 GJ exceeded by more than three times the world record of 1.0 GJ set more than a decade ago.
National Institute for Fusion Science, Department of Helical Plasma Research