In a DC neon discharge with moderately strong magnetic field, nonlinear drift waves are studied under the condition of strong dispersion. Forced harmonics and multimode spectra are observed and compared to theoretical expectations.

Nonlinear periodic drift waves are investigatcd on the basis of a simple perturbation scheme for both the amplitude and inverse frequency. The coefficients for the generation of the forced harmonics are derived, a nonlinear dispersion relation is suggested and a criterion for the onset of the modulational instability is obtained. The results are compared with those obtained with the help ofa standard KBM ireaimeni. Moreover, cnoidai driii waves are suggested and compared !O an experimentai observation.

Density gradient-driven coherent drift waves are studied in straight-cylindrical geometry at low magnetic fields, where the finite Larmor corrections of the ions gain increasing influence. For the prominent peaks in multimode spectra the dispersion properties and radial distribution of the density fluctuations are found in reasonable agreement with the nonlocal model of Ellis et al. [Phys. Fluids 22, 2137 (1979)], whereas the local slab model turns out to be inadequate. The azimuthal wavenumbers are determined by digital cross-correlation techniques with fixed probe pairs. The phase velocity of minority peaks in the spectra is found to be close to that of the prominent peaks.

The e l e ~t r o ~t a t i ~ character of the self-excited drift wave in a magnetized plasma column is demonstrated. The level of the self-excited mdgnetic fluctuation is small. With the help of external coils fixed onto the glass tube a rotating field component perpendicular to the steady state axial magnetic field is produced and the inhence nn i'ne self-excited electrostatic drift wave spectrum is siudied. Positive frequency shifts, modulation of the drift-wave amplitude and phase, suppression of the drift wave and mode competition and locking effects are studied. Furthermore, enhanced plasma losses arc observed which increase approximately with the square of the amplitude of the oscillating magnetic field. 1. I N T R O D U C T I O N ELECTROSTATIC PLASMA EDGE INSTABILITTES and turbulence Of illhOmOgeneOUS magneticaiiy confined piasma devices have been subject t o several studies during the iast decades [see for instance the review articles of LIEWER (1985) and HORTON (1984, 1990)]. The density gradient driven electrostatic drift wave is discussed to be one possible origin of these fluctuations (Vn-model), while another is the resistivity gradient driven mode (Vq-model), see for instance W ~T T O N er al. (1990).The understanding of the edge fluctuations is important with respect to the understanding of the induced particle and heat transport. Moreover, the low frequency fluctuations scatter and damp high frequency waves, for example lower hybrid waves, that are transmitted through the plasma device for the purpose of plasma heating. These low frequency waves are a good example for basic wave studies in dispersive media, such as coherent nonlinear structures as well as turbulence (KAUSCHKE and SCHLUTER, 1990, 1991).In addition to the electrostatic turbulence magnetic fluctuations are also a central topic in the description and understanding of the edge turbulence and particle and heat transport in several studies (HORTON, 1984; THYAGARAJA and HAAS, 1989; WOOTTON et al., 1988). A comparison of theory and experiment concerning heat transport induced by magnetic fluctuations is given by MCCOOL et al. (1989). HOW-LING and ROBINSON (1986) demonstrate on the TOSCA Tokamak that the electrostatic convective thermal transport due to fluctuations is the dominant heat loss mechanism. 'See footnote opposite. * I n this paper we use the following notations. The index " E indicates oscillating quantities caused by the externally coupled perlurbation(l,, B E : effective r.m.s. value^ of current and magnelic induction and B, the amplitude, respectively). The index "0" indicates non-time dependent steady slate quantities. The index "1" indicates time-dependent (fluctuating) quantities (amplitudes), n , / n , is therefore the level of rclativc density fluctuation.

In this study it is demonstrated that low-hequency turbulent spectra of a magnetized DC discharge consist of flute-type and drift waves. The existence of waves showing several characteristics of solitons or vortices are made plausible through the properties of propagation and amplitude conditions for self-binding. Self-organized phase-locked waves appear as well as spontanous transitions to coherent structures at velocities complementary to those of periodic linear modes. Spectra obtained from fluctuations of density and potential only agree for components belonging to periodic drift waves and slow drift soliton-like perturbations.In the regime of turbulence wave coupling occurs, whereas in the windows of order the coherent waves show no coupling with the remaining spectral contributions.The scenario of transition to turbulence is such that, at small magnetic inductions B, waves of short wavelengths establish a regime of order. With increasing B , broad spectra occur with modes of short and long wavelengths. Within this regime of turbulence windows of ordered soliton-like behaviour reappear. The present study extends a previous publication on forced harmonics and multimode spectra of drift waves by Kauschke and Schliiter.