Recent observations indicate that low‐altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V⊥ is generally small but spatial gradients in V⊥ can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large‐scale Kelvin‐Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Furthermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novel two‐step mechanism for ion energization is applied to typical observations of low‐altitude thermal ion upwelling events.

A theoretical model for the investigation of the ICRF fundamental minority plasma heating scheme in tokamak configurations is developed. The wave differential operator is obtained by including in a selfconsistent manner the effects of strong wave damping, linear mode conversion and a one-dimensional non-uniform equilibrium configuration. It is found that the use of a self-consistent equilibrium distribution function yields important modifications of the ICRF wave differential operator applicable to this heating regime. In particular, the paper presents a set of new terms which are resonant at the fundamental cyclotron frequency and which ensure the self-adjointness of the resulting wave operator in the limit k,. ->• 0. A numerical scheme is developed with which solutions for the ICRF electromagnetic field and the corresponding power deposition and energy flux profiles can be obtained. An extensive parametric study is carried out for a range of wave and plasma parameters illustrative of current and proposed JET operating regimes. The results are considerably different from those obtained using a WKB fast wave model. In particular, the 'full wave' model presented in this paper yields a percentage for the wave power absorbed by the ionic species which is much larger than the one predicted by the WKB theory. The model presented also shows that the majority species can absorb a much higher proportion of the incident wave power than previously reported. Finally, the results obtained for JET indicate that in the case of low magnetic field incidence a sizeable percentage of the launched wave energy can be reflected on the fast wave branch for values of k,. < 6 m" 1 and that at higher plasma temperatures electron heating becomes appreciable.

A study is presented of the nonlinear evolution of a magnetized plasma in which a localized electron cross-field flow is present. The peak velocity of the flow is denoted by Ve; L, represents the flow's shear scale length; and the regime pe < L, < pi is considered, where pi and pe denote the ion and electron Larmor radii, respectively. It is shown that if the shear frequency os= VdLE is larger than the lower-hybrid frequency, OLn, then the system dynamics is dominated by the onset of the electron-ion-hybrid (EIH) mode which leads to the formation of coherent (vortexlike) structures in the electrostatic potential of the ensuing lower-hybrid waves. The wavelength of these structures is on the order of LE, and correlates well with that predicted by the linear theory of the EIH mode. Since the characteristic wavelength is longer than pe, the corresponding phase velocity is low enough that there results significant direct resonant ion acceleration perpendicular to the confining magnetic field. When w, > 3tiLH, the system exhibits significant anomalous viscosity (typically an order of magnitude larger than that due to Coulomb collisions), which increases as the shear frequency is increased. As w, is reduced below tiLn, shear effects are no longer dominant and a smooth transition takes place in which the system dynamics is governed by the short wavelength (on the order of pe) lower-hybrid drift instability.

The stability of a magnetized plasma is investigated in which a sheared electron flow channel is present. The flow’s peak velocity and shear scale length are denoted by V and L, respectively. If the velocity channel is perpendicular to the confining magnetic field and L≤ ρi (ρi is the ion Larmor radius) an electrostatic instability develops whose frequency is on the order of the lower hybrid frequency. For V/(ΩeL) ≳ 0.02 (Ωe denotes the electron cyclotron frequency), the peak growth rate is on the order of the lower hybrid frequency when k∥ = 0 (in here, k∥ is the wave number along the magnetic field). For V/(ΩeL) ≳ 0.1 and k∥ = 0, the spectrum peaks when kyL ∼ 1, where ky is the wave number in the direction of the flow velocity. For this mode it is shown that (i) a net cross-field current is not required for the onset of instability and (ii) the growth rate is not reduced by a velocity profile with no net flow (spatially averaged). Hence we conclude that velocity shear is the only source of free energy. Further, it is shown that density gradients do not stabilize this mode. It follows that the mode presented in this work cannot be identified with the well-known modified two-stream instability. If the velocity channel is parallel to the confining magnetic field and the plasma is weakly magnetized, an instability driven by velocity shear is shown to exist, provided that V/(ωpeL) ≳ 0.32, where ωpe is the electron plasma frequency. It is shown that a net plasma current is not required in order for this instability to be excited.

Observations are presented which show that plasma parameters vary on a scale length smaller than the ion gyroradius at the interface between the plasma sheet boundary layer and the lobe. The Vlasov equation is used to investigate the properties of such a boundary layer. The existence, at the interface, of a density gradient whose scale length is smaller than the ion gyroradius implies that an electrostatic potential (e ф ∼ 2 κTe, where e and Te are the charge and electron temperature, respectively) is established in order to maintain quasineutrality. Strongly sheared (scale lengths smaller than the ion gyroradius) perpendicular and parallel (to the ambient magnetic field) electron flows develop whose peak velocities are on the order of the electron thermal speed and which carry a net current. The free energy of the sheared flows can give rise to a broadband spectrum of electrostatic instabilities starting near the electron plasma frequency and extending below the lower hybrid frequency.