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May 13, 2013
Accelerating the Negative Ion Beam: Aiming toward Heightened Performance of the Beam Injection Heating Device

One method for heating plasma in the Large Helical Device (LHD) is by injecting a high-energy neutral beam. Last year, using this heating method, we achieved an ion temperature of 85,000,000 degrees. In the LHD, in order to generate an excellent neutral beam of high efficiency and high energy, we were the first to develop and are utilizing a neutral beam injection heating device that uses negative ions. Here we will introduce research that aims at heightening the efficiency of the negative ion source that generates negative ions in order to further heighten the efficiency of the beam injection heating device.

When injecting a high-energy hydrogen atom beam into plasma, we can heat the plasma by passing the energy held by the beam into the ions and electrons in the plasma. However, because hydrogen atoms are neutral particles, they cannot be given high energy electrically. Thus positive ions bearing positive electricity with the electrons removed, or, negative ions bearing negative electricity to which electrons are attached, gain energy through acceleration due to the high voltage and become high-energy ion beams. Then, by making the ion beam pass through the hydrogen gas that beam changes into a neutral particle beam. If one uses negative ions, compared to the positive ions, one can change the beam into a neutral particle beam with higher efficiency. In the LHD, the neutral beam injection heating device using negative ions is operating only in the world, and is contributing greatly to the high performance of plasma.

In the source of the negative ions which generate the negative ion beam is an accelerator for accelerating the ion beams into high energy. Through the LHD’s ion-source accelerator, using three electrodes we are accelerating the ion beam to 180,000 volts. However, because though but a small amount of gas particles exist in the accelerator, too, when negative ions that are accelerating collide with gas particles floating nearby sometimes their electrons are stripped away and become neutral particles. Or, conversely, gas particles are forced to separate into electrons and positive ions. Neutral particles and the electrons and the positive ions that were generated in this way are called secondary particles. A part of these collide with electrodes and become heat load, and sometimes they cause damage to the accelerator.

Decreasing this heat load, in order to generate a high-energy negative ion beam with stability over a long period of time it is necessary to clarify the place of generation of secondary particles and the generation mechanism, and also their behavior. We are investigating these points through computer simulations and experiments. When undertaking numerical simulations through modeling of the accelerator’s negative ion source operating in the LHD, upon comparing those results to experiments, in addition to the importance of lowering the density of the gas particles in the accelerator, we have learned that it is important to simulate the behavior of the negative ions that enter the accelerator, that is, the process by which negative ions are generated. At present, we are moving forward together with researchers in Italy and France in developing simulation code for measuring the process by which negative ions are generated and the process by which they accelerate. In the future, we will clarify the behavior of secondary particles, and we plan to develop an accelerator for the negative ion source that will control the heat load at its most extreme, and to further increase the efficiency of beam injection heating devices.