National Institute for Fusion Science

High-temperature plasma, on which research seeking fusion energy in the future advances, does not make direct contact with the vacuum vessel wall because it is maintained while confined in a magnetic field container inside the vacuum vessel. However, low-temperature plasma that has leaked out of the magnetic field container terminates at a certain specified place, interacts with the wall surface, and on those occasions sometimes scrapes away wall material. That quantity is a minute amount, but collected together it becomes dust, and sometimes it is inside plasma. Here we will introduce research that observes dust in real-time in the Large Helical Device (LHD)

Dust is generated when wall of the vacuum vessel into which particles are impinging from inside the plasma and when the plasma-facing material where plasma is terminated, are little by little carved and worn away. In the future fusion reactor, in order to maintain continuous operation for more than one year, it is important to evaluate the amount of wear of these materials over a long period of time. Thus, in the LHD, by measuring in real time the generation frequency of dust, we are investigating the relationship between the condition of a plasma and the amount of dust generation and advancing with research that speculates on the amount of wear on materials inside the vacuum vessel.

Through international joint research, we attached inside the LHD’s vacuum vessel a real-time dust detector developed at Princeton University, in the United States. This dust detector has copper wire in parallel lines set in minute intervals. The voltage is applied between two copper wires, and the dust is detected by the electrical current measured by the detector when dust lands in that interval. The principle is simple, but the electrical current detected is extremely minute, and because it is easily influenced by noise from large electrical apparatuses placed nearby, in order to receive even but an accurate signal this detector’s special characteristic is that a special circuit design has been provided. Using this circuit design as an example, we may say that this is an operation in which when we plant seeds for plants in the summer (signal) the plants are quickly surrounded by various grasses (noise), the grasses are removed so that only the plants remain.

The size of the dust that can be detected is determined by the breadth between two copper lines. Even if dust does not touch a copper line, if dust approaches the electrical current flows, but when the dust is too small vis-à-vis the interval between the copper lines the electrical current will not flow. According to research to date, the size of dust measured by the LHD is from 10 nanometers (1/100,000,000 meters) to 20 microns (2/1,000,000 meters). The interval between copper lines on the dust detector is 25 microns, and according to an earlier experiment, it has been confirmed that dust larger than several microns can be detected. Thus dust generated by the LHD can be detected by this dust detector.

We affixed this dust detector to the LHD and together with researchers from Princeton University we took measurements. The quantity of dust collected equaled one square centimeter, and results were achieved in which several nanograms were collected each second. This amount collected is less than compared to results to date from other devices, and when considered with regard to the fusion energy plant of the future, this indicates that the amount of damage is low, which may be considered a desirable result. Next, we plan to investigate the different conditions of plasma and the quantity of dust in long-duration plasmas, such as those longer than 10 minutes.

As this dust detector has already been attached to several large-size tokamak devices in addition to the LHD, we may anticipate advances in comparative research regarding the frequency of dust generation, the materials from which the dust is composed, and the differing sizes of the helical and tokamak devices.

Photograph caption 1: A photograph of the dust detector. Only one side of the board may be seen, but the copper lines are set at intervals of 25 microns.

Photograph caption 2: Dr. Charles H. Skinner, a member of Princeton University’s research team working jointly with the National Institute for Fusion Science, confirms the signal before attaching the dust detector to the LHD. The dust detector may be seen in the lower part of the photograph.