Observation of the internal structure of energetic particle-driven MHD modes in a Large Helical Device

For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfven eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation. (author)