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January 5, 2015
Investigating Periphery Plasma by Inserting a Needle: The Electrostatic Probe Diagnostic

In the Large Helical Device (LHD) a magnetic field container is composed by a helical-shaped electromagnet, and plasma is confined therein. For that reason, high-temperature plasma is maintained in a condition of floating in space, and does not touch the vessel wall. However, not all of the plasma stays in the magnetic field container, for a small amount leaks out from the magnetic field container. The escaped plasma is called edge plasma. It moves along the magnetic field lines that shape the magnetic field container, and disappears at the heat board called the “divertor board.” Due to its influence upon high-temperature plasma confined in the magnetic field container, edge plasma is being energetically investigated regarding its movement, including its interaction with the divertor board. Here we will introduce edge plasma research using a diagnostic instrument called the “electrostatic probe.”

The magnetic field lines which guide edge plasma are heading toward a specific place on the vessel wall called the divertor while revolving as if circling the magnetic field container. The plasma that escaped from the magnetic field container lowers its temperature sufficiently and strikes the divertor board while following the magnetic field line, and becomes neutral gas. When this gas returns to the plasma in the core region, because it cools the plasma, the neutral gas should be effectively evacuated. In the LHD we are devising a divertor structure that includes the divertor board.

The temperature of the edge plasma is sufficiently low, but the divertor board will be contacted directly by the plasma and receive a heat load of a size that cannot be ignored. In the future fusion reactor, based upon the prediction that this heat load will become larger, research is advancing on material development for the divertor board, and also on skillfully cooling plasma near the divertor board and reducing the heat load. In research on plasma near the divertor board, a diagnostic instrument called the “electrostatic probe” is demonstrating effectiveness.

Because plasma is composed of positively charged ions and negatively charged electrons, similar to the metals iron and copper, if we insert two electrodes into the plasma and apply voltage an electrical current will flow between the two electrodes. When we investigate the relationship between the voltage and the electrical current, we can evaluate the qualities of the plasma’s density of electrons and of ions, the temperatures, and other aspects. We call such a diagnostic method the electrostatic probe method. And we call an electrode inserted into a plasma an electrostatic probe. In the electrostatic probe method, because a small electrode shaped like a needle and called a “probe” is inserted into the plasma, it surpasses other diagnostics in a localized diagnostic. However, in a high-temperature plasma the electrodes suffer damage. Regarding this point, for an edge plasma that is not an issue because its temperature is low.

In the LHD at present we have inserted approximately 460 electrostatic probes into various places on the divertor board, and we are understanding more about the three-dimensional and complicated phenomena generated in edge plasma near the divertor board. Recently, from outside we added locally a small magnetic field. By changing the shape of the magnetic field lines that head toward the divertor board, we discovered a condition called “the detached divertor” which decreases remarkably the heat load carried toward the divertor board by edge plasma. And we have successfully maintained the condition in a stable manner. At that time, we clarified from electrostatic probes set in various places that the distribution of places where there are reductions in heat load changes depending upon the condition of the magnetic field added from outside. In the future, by comparing experimental data that we have obtained and theoretical calculations from computer simulations, we will clarify a more detailed spatial distribution and contribute to the design of the future fusion reactor.