Austenitic stainless steel SUS 316L was nitrided by active screen plasma nitriding (ASPN) using screens with various open areas to investigate the effect of the screen's open area ratio on the nitriding response. The sample was placed on the sample stage in a floating potential and isolated from the cathodic screen and anode. The screen, which was SUS 316L expanded metal mesh with 38%, 48%, or 63% open area ratio, was mounted on the cathodic stage around the sample stage. Nitriding was performed in a nitrogen-hydrogen atmosphere with 25% N2 + 75% H2 for 18 ks at 693 K under 600 Pa by the ASPN process. After nitriding, the nitrided microstructure was examined using a scanning electron microscope and an electron probe microanalyzer. The phase structures on the nitrided surface were determined by X-ray diffraction. In addition, the surface hardness and cross section of the nitrided samples were measured by the use of a Vickers microhardness tester. The thickness of the nitrided layer of the S-phase decreased with increasing open area ratio of the screen.KEY WORDS: surface engineering; active screen plasma nitriding; cathodic cage; S-phase; stainless steel; open area ratio.

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Emphases in recent JT-60 experiments are placed on 1)lower-hybrid (LH) current drive characteristics with a multi-junction type launcher and 2 ) the confinement study with combination of neutral beam injection, LH current drive and pellet injection. The new multi-junction LH launcher provides a 2 . 7 x 1 0 1 9 m -3 and Ip=1-1.7 MA.Volt-sec saving of -2volt-sec was demonstrated by 2 sec long, 1.6MW LHCD during the plasma current ramp of 0.4MA/s.A broad radial distribution of high energy electron current and -30% reduction in sawtooth inversion radius were obtained by high N I I (-2.5) LH injection. In order to fully suppress the sawtooth activity, low NII (-1.3) injection was found to be more effective, in which up to 1.8 sec sawtooth-free phase was obtained by 2MW LHCD for lOMW NB heating of Ip=I.SMA discharge, Improved energy confinement has been obtained with hydrogen pellet injection. Energy confinement time was enhanced up to 40% relative to usual gas fuelled discharges. The discharge has a strongly peaked electron density profile with ne(O)/-5 and n e ( 0 ) -2 . 0 ~ 1020,-3,The improved discharges are characterized by a strongly peaked pressure profile within the q=l magnetic surface, and degrades when a large sawtooth recovers or the pressure gradient may reach a critical value. When large (3mm,4mm) and fast (2.2km/s) pellets were injected, 30% energy confinement improvement was obtained even during the NB heating of 14MW. Further investigations of IDC characteristics have been made.The oxygen impurity lines from the main plasma and the main radiative loss drop first. Then the plasma stored energy starts to rise. The particle recycling is reduced around the main plasma, and is localized in the neighborhood of the X-point with a time lag of -0.2 sec. Eventually the discharge shows a significant remote radiative cooling power at the divertor region. Most of the IDC characteristics is totally different from that of H-mode, although there are some similarities i.e., gas puff degrades the confinement, both have the same favorable BT direction.

From June to October 1987, JT-60 achieved fusion product(ne(0) .r~*.Ti(Oj ) of 6 ~1 0 ' ~ m 3 . k e V . s with hydrogen plasma at plasma current of 2.8 to 3.1 MA with neutral beam power of -20 MW. The central electron density of 1 . 3 ~1 0 2 0 m-3 was obtained at plasma current of 3 MA with 13-20 MW neutral beam power and the confinement time reached 0.14-0.18 s. It is found that an offset linear scaling law like the Shimomura-Odajima scaling on confinement time will be able to reproduce experimental data better than that of the Goldston type scaling. With low beam energy injection, -40 keV, confinement degradation was found. Many short periods (0.05-0.1 s) of H-mode phase were found in outside X-point divertor discharges with NB or NB+RF(LH or IC) heating power above 16 MW. However, improvement in energy confinement time was limited to 10 %. The ballooning/interchange stability analysis were JT-60 TEAM also made for the outside X-point divertor equilibrium in connection with H-phase capability. Heating power of 9.5 MW and 1.9 MW was obtained by LHRF, ICRF injection, respectively. In combined LHRF and NB heating, the incremental energy confinement time of 0.064 s was obtained, which is the same level of that of NB heating only. In combined NB and on-axis ICRF heating of low ne discharge, an incremental energy confinement time of 0.21 s was obtained, which is three times as long as those of NB or ICRF heating only. It was also observed that high energy beam ions were accelerated by ICRF in the central region of the plasma.

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