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The typical MHD disruption at safety factor {ital q}{sub {ital a}}=3 has been suppressed by off-central electron cyclotron heating with power of 70--80 kW in the JFT-2M tokamak. For the suppression, the electron cyclotron resonance layer has to be placed radially in a very narrow region of width about 1 cm near the {ital q}=2 surface. The observed narrow suppression window suggests that the basic mechanism responsible for the suppression is direct island heating. The actual time scale of the suppression is much faster than the time scale of the change of the overall current profile.

A new operational regime has been observed in neutral-beam--heated JFT-2M limiter discharges. This regime is characterized by confinement times close to those of Ohmic and divertor H-mode (good confinement mode) discharges. The properties of these discharges are similar to the divertor H mode. The incremental energy-confinement time due to the heating is found to be independent of the plasma density up to the maximum density.

The chaotic or turbulent behavior seen in a physical system which is governed by deterministic equations has attracted intense interest recently. Ikeda, Daido, and Akimoto' have pointed out that chaotic behavior can occur in an optically bistable system which is described by a differential-difference equation. Experimentally, Gibbs et al. ' have observed such optical turbulence or Ikeda instability in an optical hybrid device with a delay in the feedback. In a recent article' they showed that their system takes the period-doubling route to chaos. It is surprising that the period-doubling scenario seems to be realized in many different

It is observed that fast waves launched into an electron cyclotron heated plasma from a phased four-strip-line antenna array couple effectively with core electrons. This coupling produces enhanced electron cyclotron emission and a drop in the loop voltage. These effects depend on the phase difference between adjacent strip lines and on the electron density. An absorption efficiency of 20% is deduced under the conditions {ital v}{sub {ital ph}}/v{sub e0}{similar to}3 and {ital f}{sub 0}/f{sub LH}(0){similar to}0.38. The experimental results agree well with predictions based on electron Landau damping of fast waves.

Decay time of stored energy was measured when additional heating power decreased abruptly. The energy decay time is almost constant at 15 ms, irrespective of the gross energy confinement time tau/sub E//sup G/, which is changed by ion cyclotron radio frequency heating up to 1.5 MW. The decay time coincides with a confinement time of the additionally heated power defined by tau/sub ad/ = ..delta..W/sub T//..delta..P/sub Tot/, which indicates that the incremental energy from the additional heating behaves independently of the gross plasma energy. It is also shown that tau/sub ad/ is almost independent of density and plasma current as well as heating power.

It is observed that the extraordinary mode near the second harmonic effectively couples to electrons at largely down-shifted frequencies. From measurements of soft x-ray radiation and third-harmonic electron-cyclotron emission, selective heating of electrons with an energy satisfying the relativistic cyclotron resonance condition is deduced. It is demonstrated that the selective electron-cyclotron heating of 70 kW produces a 65-kA/sec ramp-up of the lower-hybrid--driven plasma current.

The {ital H} mode is observed by the edge heating of a tokamak plasma solely by the electron cyclotron heating with the wave cutoff region in the plasma core. Threshold power for the {ital H}-mode transition can be as low as 120 kW, which is the lowest threshold power observed in spite of the peripheral power deposition. These experimental results show clearly that the {ital H} mode can be produced without additional heating of the plasma core, and provide direct confirmation that the {ital H}-mode mechanism is closely related to an increase in the plasma edge electron temperature.

A Comment on the Letter by Akira Tomita and Raymond Y. Chiao, Phys. Rev. Lett. 57, 937 (1986).