In order to realize fusion energy the plasma temperature must be raised to more than 100,000,000 degrees. In the Large Helical Device (LHD), we are conducting research on the confinement of high-temperature plasma in a twisted nested container with magnetic fields that are invisible to the eye and produced by electromagnets. In order to raise the plasma temperature, we input energy to electrons or to ions in the plasma and heat the plasma. Among heating methods, that which uses high frequency electromagnetic waves for electron heating is called electron cyclotron resonance heating. In this method, an important issue is how powerful electromagnetic waves are absorbed into a plasma. In this Research Update, we introduce research on the enhancement of efficiency of electron cyclotron resonance heating (ECH).
Plasma confined inside the magnetic field container maintains a high temperature in the core region, and the temperature falls toward the edge region. In order to efficiently heat plasma in the core region using powerful electromagnetic waves, it is necessary to know beforehand how to inject those electromagnetic waves into the plasma. In order to know the direction for the injection, we calculate how the electromagnetic waves will propagate in the plasma and where the electromagnetic waves will be absorbed. Important here is the refractive index. Light that propagates from air to water bends at the boundary based upon differences in the refractive index for air and for water. Plasma, too, has a refractive index, and that index changes depending upon the spatial distribution of electron density. For that reason, when the electron density distribution of the plasma changes, the trajectory of the electromagnetic wave changes in response. To be able to effectively heat the plasma core, spatial distribution data of the electron density becomes necessary in order to know the most appropriate injection direction for the electromagnetic waves. In order to know the spatial distribution of the electron density it is necessary to know the condition of the magnetic field container through calculations. However, because the calculations take time, it is difficult to know in real time the precise condition of the container.
Thus, the “High-speed plasma analysis system” (For details, please see Research Update 261.) is helpful in experiments. The high-speed plasma analysis system investigates the condition of the plasma immediately after the plasma shot using the immense amounts of data collected during the plasma experiments. Using data collected by this system, we have become able to decide beforehand the most appropriate injection direction for the electromagnetic wave for the next plasma experiment by calculating the change in the refractive index of the previous plasma experiment and knowing the absorption location. We have thus been able to greatly improve the efficiency of core heating, and in LHD experiments conducted during 2014 these achievements contributed to simultaneously achieving a high ion temperature of 70,000,000 degrees and a high electron temperature of 88,000,000 degrees.
In the future, we will improve further the accuracy of our calculations. And we will endeavor for improvement not only of the injection direction, but also for optimization of the wave polarization (the direction of the oscillation of the electromagnetic waves), and aim for further improvement of efficiency of electron cyclotron resonance heating.