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September 2, 2013
Seeking Long Sustainment of High-performance Plasma: Ion Cyclotron Resonance Heating

Having completed maintenance on the Large Helical Device (LHD), we began the vacuum pumping of the vacuum vessel on August 12, 2013, and this year’s operation of the LHD has now started. Plasma experiments, continuing from the cooling of the superconducting coils, are planned for about three months, from early October to the end of this calendar year. In this year’s experiments, one of the goals is long duration sustainment of high-performance plasma that will exceed several tens of minutes. In order to generate and sustain high-performance plasma over a long period of time a heating method called ion cyclotron resonance heating, in which plasma is irradiated by an antenna, is being utilized. Looking forward to this year’s experiments, here we will introduce ion cyclotron resonance heating, which utilizes a new type of antenna.

Plasma is in a condition in which the positive-charge ions and the negative-charge electrons are moving at random. The charge-bearing ions and electrons circulate at high speeds around the magnetic field line. In the LHD, using that characteristics we confine plasma in the magnetic field’s container, which is in the shape of a doughnut made by the superconducting coils. The number of times that these ions circle the magnetic field line can be approximately 40,000,000 times per second in the case of hydrogen ions. Here, what might happen if an oscillating electric field is added so that the plus and the minus alternate to the hydrogen ions that bear positive charge? If we do this, it will push the forward-moving swing from the rear (plus), and when it returns it will pull (minus) from behind, similar to the affixing of heat, and we can increase the speed of the rotating ions. We call this method for heating the positive-charge by adding oscillating electric field in synchronization to the ions rotating around the magnetic field line “ion cyclotron resonance heating.”

Through ion cyclotron resonance heating we irradiate the electro-magnetic waves with plasma using the antenna installed near the plasma and force the generation of an oscillating electro-magnetic field. In order to synchronize the 40,000,000 rotations that occur each second, an electro-magnetic wave of 40 megahertz, which is close to the frequency of an FM radio wave, is used. Ion cyclotron resonance heating utilizes the oscillator for the frequency band that has already been developed for radio, and it is used in a long time operation, one of the principal features of the LHD. In the previous year, using 1,000 kilowatts of heating power, we succeeded in maintaining high-performance plasma for nineteen minutes. At that time, of the heating power used, ion cyclotron resonance heating accounted for approximately 70% of the input power.

In ion cyclotron resonance heating, the role of the antennas, which are used to radiate the electromagnetic waves into the plasma’s ions, is important. In this year’s experiments, we have added two new antennas, and with these we plan to increase the heating power. The characteristic of these new-type antennas is that we set them at an incline so that oscillating electric fields are vertical with respect to the direction of the magnetic field line, and were able to more effectively heat the ions. Further, we designed a feeding system for sending the electro-magnetic wave to the antennas and we improved the tenability of the electro-magnetic wave sent to the plasma. This corresponds to improving the tuning performance when receiving the radio wave. Thus, in addition to being able to increase the electric power for heating, we anticipate improvement of the operation performance when generating long-duration plasma.

With the introduction of this new type of antenna, it has become possible to utilize ion cyclotron resonance heating for longer periods of time using even greater amount of electrical power than in the past. One of the goals of the LHD, steady-state operation using 3,000 kilowatts of heating input, has come into view. Looking toward the realization of a future fusion power plant in which steady-state operation is required, we anticipate an important contribution.