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In the Large Helical Device (LHD), which is able to generate the magnetic field that confines plasma only through superconducting coils, it is possible to maintain high-temperature plasma in a steady state manner. In the future fusion power plant, because steady state operation is demanded, in the LHD we are utilizing its special characteristics and advancing research on steady state plasmas. Here, we introduce research on maintaining plasmas for an extended duration that recently has been developing significantly through the steady state and high power characteristics of heating equipment utilizing electromagnetic waves.
The LHD is a helical-type machine that generates a magnetic field through only coils placed outside the plasma in the doughnut-shaped configuration. On the other hand, there also is the tokamak configuration which by adding an external magnetic field makes a complete confinement magnetic field by delivering an electrical current to the doughnut-style plasma. The tokamak configuration is achieving higher performance than the LHD in short-term plasma generation. But because continuously sending a large electrical current into the plasma in the tokamak is a subject to be solved, to maintain a high-performance steady state plasma for a long period of time is one of the missions of JT-60SA, which is under construction at the Japan Atomic Energy Agency. Thus, research related to the maintenance of steady state plasma and related issues, in addition to being one of the important experimental topics for demonstrating the LHD’s special characteristics, is an extremely important research theme for advancing the design of the future fusion power plant where steady state operation is inevitable.
Currently, the LHD, in order to maintain a steady state plasma, is using primarily a heating system called ion cyclotron heating. The electrons and ions that compose a plasma move in a revolving motion that winds them in magnetic field lines. This method heats ions resonantly through raising the revolving speed by using electromagnetic waves whose frequency is equal to the revolution frequency of ions. In the LHD, we are using the frequency of approximately 40 megahertz, which is close to the frequency of an FM radio.
In experiments conducted last year, we added two high-power steady state heating antennas that radiate electromagnetic waves in plasma. We now have six such antennas. Further, through enhancing the control methods for heating power and the injection method for fuel gas, it became possible to inject a large heating power in a steady state manner and we could maintain higher density plasma for a longer period of time than in the past. As a result, when using approximately one megawatt of heating power, for a density of 12 trillion parts per 1 cc we maintained a plasma with the ion temperature of 23,000,000 degrees for approximately 48 minutes. And when using more than two megawatts of heating power, for a density of 20 trillion parts per 1cc, we succeeded in maintaining a plasma of 17,000,000 degrees for approximately six minutes. Moreover, heating power energy expressed as the product of the heating power and injection time for a 48-minute discharge amounts to approximately 3.4 gigajoules, and this is a world record that greatly surpassed the value of approximately 1.1 gigajoules, which was highest value that has been achieved in a tokamak configuration.
By achieving high-performance steady state plasma, research on the effects of steady state plasma on the vacuum vessel walls also is developing. And through the interaction between plasma and the vacuum vessel walls in some places the walls are scraped-off and in other places sediment accumulates. When a steady state plasma discharge is conducted, that phenomenon is conspicuous. The accumulated layers of sediment sometimes absorb fuel gas and sometimes release fuel gas, and they affect the operation of the plasma. Further, when an accumulated layer of sediment peels away, a portion of it enters the plasma and sometimes causes the plasma to collapse. The termination of a plasma due to this phenomenon is at present the most important issue in maintaining extended duration plasma. Through this research on termination, each year the discharge duration of high-performance steady-state plasma grows longer.
Such research on high-performance steady state plasma lasting several tens of minutes has not yet been achieved on tokamaks or other devices. Thus, knowledge currently being obtained from the LHD is, through research looking at how the future fusion power plant will extend, leading the world. We will develop that research further.
