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August 11, 2014
Measuring the Energy of Light Emitted from Plasma: Bolometer Diagnostics

In the Large Helical Device (LHD), high-temperature plasma is confined by a container made from magnetic field lines. Hydrogen ions and electrons, which are high-temperature plasma particles, are confined so that they do not escape, and they control the loss of energy held by the plasma. On the other hand, because light that impurities in a plasma emit cannot be confined, this causes a loss of the plasma’s energy. For that reason, in order to understand the plasma’s energy balance and the plasma condition, it becomes important to measure the total amount of energy released as light from the plasma. Here we will introduce bolometer diagnostics that are utilized in LHD research in order to measure the light energy released from a plasma.

In the LHD and other magnetic confinement devices, plasma particles such as the hydrogen ions that carry positive charge and electrons carrying negative charge are confined in a magnetic field lines container. Those particles are heated by various means which raise the plasma to a high temperature. In order to efficiently achieve such a condition in which the plasma holds high thermal energy, together with efficiently inputting heating power into plasma particles, it is necessary to control the loss of energy from the plasma. In the loss of energy, plasma particles escape from the magnetic field lines container. Through the small amount of impurities that are inside the plasma and the hydrogen atoms and the hydrogen molecules which have not become part of the plasma and are outside the magnetic field lines container, energy is emitted also as light (electromagnetic waves). The magnetic field lines container can tightly confine plasma particles, but it cannot confine light.

In the LHD, a diagnostic tool called a “bolometer” is used in order to measure the energy emitted as light. The bolometer is a device that was originally developed for measuring infrared light in the scientific field of astronomy. This English-language word derives from the Greek words bolē (light ray) and metron (to measure), and means “to measure light rays.” Similar to the warming of the body when in sunlight, the sensors made from gold of a bolometer are warmed by the heat from the plasma. Because the electric resistance of metal generally increases when the temperature rises, we can measure light energy from changes in that electric resistance.

At present there are more than 50 bolometers installed to the LHD. These devices are measuring from various directions the light that a plasma emits. Various elements only receive light from one direction. But by using somewhat difficult math called “singular value decomposition” we can know of a light emission from nested sections of the doughnut-like plasma. Using such data, we can know of the energy emitted as light from everywhere in the plasma within the doughnut configuration.

This bolometer, which is only used for measuring the heat of light energy, is an important diagnostic device for research on the energy conditions of high-temperature plasma. In particular, it contributes to investigation of the energy balance of long-duration discharge plasmas that are unique to the LHD.