In the Large Helical Device (LHD), one of the methods used for heating plasma that has been confined by the magnetic field lines container is the NBI (a neutral particle injection heating device; Neutral Beam Injection), which is a method that injects into plasma a hydrogen atom beam that has been accelerated by high energy. The NBI is the principal heating device for the LHD, and, in experiments conducted in 2013 we achieved an ion temperature above 94,000,000 degrees by using this device. In the LHD’s neutral beam injector in order to generate a hydrogen beam we are using the world’s most efficient negative ion source. Here we will introduce research on the hydrogen negative ion source that is being advanced with an aim toward further improving the NBI device.
The NBI is a device that injects into a plasma a beam of hydrogen atoms that held a high energy that had been accelerated at an extremely high speed and raises a plasma’s temperature by supplying the beam’s energy to the ions and the electrons in the plasma. In western television dramas, we often see a scene in which when the gunman turns toward a tree and fires his gun, smoke appears from around the tree into which the bullets were shot. This occurs because the energy held by the high-speed bullets has been supplied to the tree and the temperature near the tree that was shot has risen. The heating of plasma by the neutral beam injector occurs similarly.
In previous neutral beam injectors, using a positive ion we injected a hydrogen beam accelerated by several tens of thousands of volts. But with LHD plasma, because the diameter of the donut is large at 8 meters, in order to effectively inject the hydrogen beam into the plasma core it is necessary to inject a hydrogen beam that has been accelerated by more than 100,000 volts. For that reason, we had to change to the positive ions of hydrogen and accelerate the negative ion by high voltage, and we were able to advance in the production of a hydrogen negative ion source.
When we can take an electron holding negative (minus) electricity from an atom, it becomes a positive ion holding positive (plus) electricity. On the other hand, when an electron attaches to an atom, it becomes a negative ion holding negative electricity. However, it is difficult to generate negative ions in the vacuum, and when the LHD was constructed in the 1990s a sufficient negative ion source for producing hydrogen negative ions had not yet been achieved. At NIFS, we are actively moving forward in developing, and realizing, a negative ion source for use in the neutral beam injector. We are in the first in the world to have succeeded in plasma heating by a neutral beam injector that employed a negative ion beam accelerated by 190,000 volts. Based upon this, every year we are raising the LHD plasma performance.
When having advanced research in attempting to further raise the performance of the hydrogen negative ion source, inside the hydrogen negative ion source there was a deeply interesting discovery of the generation of “plasma with only a hydrogen positive ion and a hydrogen negative ion.” A typical plasma is composed of electrons and positive ions. However, as a result of succeeding in greatly increasing the density of negative ions, we created the plasma in which there are almost no electrons with same negative electricity. The quality of the plasma was much more greatly dependent upon light electrons than upon positive ions. However, because the electrons are replaced with negative ions of the same weight as positive ions, a phenomenon that differed from those already known was seen. At present we are working on explicating this.
In the future, we will investigate the special characteristics of plasma only through positive and negative ions that do not have an electron. And we plan to raise further the performance of hydrogen ion source used in the neutral beam injector.