In the Large Helical Device (LHD), in generating high-temperature plasma, in particular, in heating electrons in a plasma, a powerful electromagnetic wave of 77 gigahertz (77,000 megahertz) frequency is used. In this heating method, called the electron cyclotron resonance heating method, the frequency of the electromagnetic wave is determined in response to the strength of the magnetic field that confines a plasma. This is called the fundamental frequency. On the other hand, when trying to heat high-density plasma, it is necessary to raise the frequency. Thus, we are making the frequency of the electromagnetic waves into “higher harmonic waves” that are two- or three-times the fundamental frequency, heating the plasma, and investigating its characteristics. Here, we introduce research that aims to increase the plasma heating region through higher harmonic wave heating of the electromagnetic waves.
The electrons in a plasma confined by the magnetic field are gyrating at a frequency proportional to the strength of the magnetic field. When we inject an electromagnetic wave of the same frequency (fundamental frequency) as the frequency of the gyration, the energy of the electromagnetic wave moves to the electron and we can generate plasma or raise the electron temperature. In the LHD, through this heating method called the electron cyclotron resonance heating method, though low at a density of two trillion parts per 1 cc, the electron temperature exceeds 230,000,000 degrees. On the other hand, upon deciding the frequency, when the electromagnetic wave enters the plasma, because it cannot proceed further beyond the plasma density (called cut-off density) and is then reflected, the plasma’s core cannot be heated. In the LHD, the fundamental electromagnetic wave (a frequency of 77,000,000,000 oscillations per second) of 77 gigahertz, which corresponds to the strength of the magnetic field, is being used. However, the cut-off density is approximately 74,000,000,000,000 per 1 cc.
In order to try to investigate the density of plasma that exceeds 100,000,000,000,000 parts per 1 cc, which will be necessary in the future fusion reactor, we must increase the strength of the LHD’s magnetic field and raise the frequency of the electromagnetic wave. But this will require a remake of the superconducting conductor, and is not realistic. Thus, we are researching whether it will be possible to raise the frequency of the injected electromagnetic wave by two or three times that of the frequency at which the electrons gyrate and heat them. We call this the electromagnetic wave heating method through higher harmonic electron cyclotron wave.
Actually, the high-frequency high power electromagnetic wave that is used for heating plasma is generated from a massive vacuum tube, called a gyrotron, which is approximately three meters in height and weighs approximately 800 kilograms. Because the frequency of the electromagnetic wave is fixed, the frequency cannot be easily changed. Rather, if we reduce the strength of the magnetic field to one-half or one-third and lower the gyration frequency of the electrons, then we have raised the frequency of the electromagnetic wave equivalently. Using this, when we conduct heating experiments by reducing the strength of the magnetic field and using the frequency of 77 gigahertz, with twice or three times the fundamental waves, we confirmed the heating of plasma although the efficiency falls. Further, this time we learned that the cut-off density rises depending upon the injection method. Moreover, this heating method using higher harmonic waves also has the special characteristic of improving further the absorption of electromagnetic waves when the temperature and density of a plasma rises. And in experiments that used three times the fundamental waves, by raising the plasma temperature step-by-step, 40% of the power of the injected electromagnetic wave was absorbed and we raised the electron density by 3.5 times. This matches well the electromagnetic wave absorption results achieved through computer simulations.
In the LHD, based upon these results, we have developed a gyrotron in which the frequency has been raised to 154 gigahertz, and we are using it in experiments. In addition, in response to the high-temperature, high-density, and large-sized plasmas of the future fusion reactor, because a heating method that uses higher harmonic waves will have even higher absorption is anticipated, this method is being anticipated as one of the powerful methods for heating and controlling plasma.