A new type of toroidally localized kink instability, which we named the "slinky mode," was observed in a reverse-field-pinch plasma in the OHTE device. It is found that the slinky mode is the result of the phase locking of several internal kink modes due to nonlinear coupling and is an effective way to approach the Taylor relaxed state.PACS numbers: 52.35.Py, 52.55.EzReverse-field pinches (RFP's) were successfully operated in the OHTE device with the resistive shell, and discharges were sustained over 10 ms, much longer than the resistive-shell time constant of 1.5 ms. Global plasma characteristics with the resistive shell were very similar to those obtained with the conducting shell.' Since the linear MHD theories predict that RFP plasmas are unstable with a resistive shell and that unstable modes grow on the resistive-shell time scale, ^""^ magnetic fluctuations were studied in detail. A new type of toroidally localized kink instability was observed. In this Letter we report characteristics of the instability and its role in RFP discharges.The OHTE device has a major radius of 1.24 m and a plasma radius of 0.183 m. The resistive shell is placed at a minor radius of 0.200 m. The details of the device and the operation mode are described elsewhere. ^ In order to investigate detailed MHD behavior of the plasma, an ar-300

A numerical code has been developed to calculate the loading of a cavity loop antenna in threedimensional geometry. The loading of the cavity is calculated for fast wave and ion Bernstein wave coupling and compared with the loading of a conventional loop antenna. For fast waves, the cavity loading increases with increasing edge density, while the conventional loop loading is less sensitive to the edge density but shows a slight decrease of loading because of a steeper density gradient. For ion Bernstein waves, the two types of antenna behave similarly; however, in contrast to the loading for fast waves, the loading for ion Bernstein waves increases with decreased edge density and a steeper density gradient.

Toroidal effects on the outer magnetic surfaces of a configuration having a large toroidal plasma current that is stabilized by a pitch reversal produced by an external helical coil (OHTE) are investigated by both field-line tracing and an analytic expansion of the magnetic surfaces. Results of the two methods are compared. It is shown that both plasma position control and the helical coil winding law can be used to compensate undesired toroidal effects at both low and high beta.

A successive iteration method has been developed to investigate the MHD equilibrium of high-current plasma in an ℓ > 1 toroidal-helical configuration of large aspect ratio. In this scheme, helical and toroidal effects are treated as perturbations. The order of perturbation determines possible Fourier mode numbers, and the equilibrium configuration can be expressed by a small number of Fourier terms. The differential equations for the Fourier coefficients have been studied and solved numerically up to the toroidal-helical coupling effect for OHTE configurations.

The nonlinear efficiency of the cyclotron autoresonance maser (CARM) amplifier is calculated numerically following the self-consistent evolution of the amplitude and phase of the wave. As the beam density is increased, the nonlinear efficiency and linear growth rate of the instability increase, and the interaction length for maximum efficiency decreases. Increasing the beam density also plays a favorable role in reducing the sensitivity of the instability to adverse velocity-spread effects.