Experimental studies toward long-pulse steady-state operations in LHD

Stable discharges longer than one minute have been obtained in LHD with all the heating schemes including electron cyclotron heating (ECH). Plasma is sustained with neutral beam injection (NBI) or with ion cyclotron resonance frequency (ICRF) with 0.5-1 MW. Central plasma temperature is higher than 1.5 keV with a density of 1-2x1019m-3 until the end of the pulse. Full installation of the carbon divertor has contributed to this achievement. This gives a sufficient base for physics and technology studies from the next campaign. The long pulse operation indicates new possibilities in diagnostics and in physics studies. Higher accuracy and reliability is obtained with diagnostics parameter scan, longer integration of signals or two-dimensional measurement. The mechanism of a slow oscillation called breathing is discussed. Hydrogen recycling analysis has been carried out and preliminary results are obtained. Based on these results, the future program is divided into two categories, that is, i) physics and technology experiments utilizing long-pulse discharges up to 5 minutes, and ii) extension of the pulse-length toward one hour. (author)