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July 14, 2014

Confining High-energy Particles: Detecting Lost Ions

In the Large Helical Device (LHD) we are heating high-temperature, high-density plasma confined in the magnetic field by high-energy particles whose energy is much higher than that of ions and electrons in the plasma. Such high-energy particles are generated by sometimes accelerating a hydrogen ion by high voltage outside the plasma and then injecting that beam into the plasma, and sometimes by injecting an electro-magnetic wave, selectively accelerating ions in the plasma, and producing high energy particles. In order to increase the temperature of the plasma, it is necessary to effectively confine the high-energy particles, which are the source of plasma heating. But because these particles are of high energy, depending upon the configuration of the magnetic field container and other conditions, they escape from the plasma. Here, we introduce research on designing optimal confinement conditions for high-energy particles by measuring the high-energy particles that escape from the plasma and investigating their behavior inside plasma.

Because the high-energy particles that heat plasma are ions, they can be confined in the magnetic field container. These ions sometimes have from several tens to several hundreds the amount of energy of plasma ions. Depending upon the configuration of the magnetic field container, some of the high-energy particles interact with the magnetic field and cause waves, and those waves force the loss of high-energy particles from inside the plasma. Detecting the high-energy particles lost from this plasma, by investigating in detail the behavior of those high-energy particles we can clarify the conditions in which high-energy particles have fled from the plasma. Then, based upon those results we seek to make more appropriate the configuration of the magnetic field container that confines the high-energy particles and to further raise the temperature of plasma. In plasma in the future fusion power plant, because the high-energy particles that are generated when plasma is burned will heat plasma and then continuously burn it, it is important to investigate in detail the special characteristics of confinement of high-energy particles and to clarify their behavior.

At the LHD, we have developed a diagnostic tool called the “lost high-energy ion probe.” We place this probe in a location slightly distant from the plasma, and we are undertaking detection of high-energy ions from the plasma. We use the fluorescent material emitted when a high-energy ion hits, and then identify the location of the detection. From the strength of the emission we measure the energy of the detected ions and their volume. Considering the characteristic that the course of the moving ion enveloped in the magnetic field lines changes according to the strength of the energy, we can clarify the movement of a high-energy ion from its escape from the plasma until its arrival at a detection instrument.

Using this lost high-energy ion probe we investigated in detail the phenomenon of high-energy ions escaping from plasma caused by a wave caused by a high-energy ion. Then, when we conducted parallel numerical simulations using a computer to analyze the experiment results, by constructing the configuration of the magnetic field container that confined the plasma we were able to confirm conditions in which confined high-energy ions did not escape from the plasma even when high-energy ions caused waves. In the future, we will advance further in composing an appropriate configuration for the magnetic field container that confines high-energy ions, and utilize this in design research for the future fusion power plant.