LHD Diagnostics towards Steady State Operation

The Large Helical Device, LHD, is the world largest helical system having all superconducting coils. After completion of LHD in 1998, 6 experimental campaigns have been carried out successfully. The maximum stored energy, electron temperature, and beta value are 1.2 MJ, 10 keV, 3.2 %, respectively. The confinement time of the LHD plasma appears to be equivalent to that of tokamaks. One of the most important missions for LHD is to prove steady state operation, which is also significant to ITER and to future fusion reactors. LHD is quite appropriate for this purpose based upon the beneficial feature of a helical system, that is, no necessity of the plasma current. So far, the plasma discharge duration was achieved up to 127 sec. The plasma density was kept constant by feedback control of gas puffing with real time information of the line density. The issue for demonstrating steady state operation is whether divertor function to control particle and heat flux is effective enough. Relevant to this, LHD diagnostics should be consistent with: (a) Continuous operation of main diagnostics during long pulse operation for feedback control and physics understanding, (b) Measurement of fraction of H, He, and impurities in the plasma, (c) Heat removal and measure against possible damage or surface erosion of diagnostic components inside of the vacuum chamber, (d) Data acquisition system for handling real time data display and huge data amount. (e) Although there are already some achievements on the above subjects, there remain still several issues to be resolved. On the other hand, the long pulse operation of the plasma gives benefits to the diagnostics. For example, the polarizing angle of ECE emission can be changed during the discharge, and the intensity dependence on the polarizing angle has been obtained. The spatial scanning of the neutral particle analyzer and the spectroscopy can supply the spatial profiles of the fast neutral particle flux and the specific impurity lines. The present status and future plan of the above issues will be described.