The shear stabilization of collisional drift waves

Current-driven resistive drift instabilities in sheared magnetic fields

“…This letter contains an analytic proof of the absolute stability of the universal mode for collisional, non-isothermal electrons (generalizing a similar proof for isothermal electrons given in Ref. [5] and substantiating and extending numerical and approximate analytical results in Refs [5,6]), and presents numerical calculations of the current-driven instability, where, again, the thresholds are unrealistically high.…”

“…The main conclusion of Refs [5,6] is that dissipative effects enhance the convective loss due to shear and so play a stabilizing role. Consider first the collisional universal mode (lower curves in the figures).…”

“…The desire to understand the short-scale-length density and electric-field fluctuations observed in tokamak experiments has led to extensive investigation of electrostatic instabilities in inhomogeneous plasmas immersed in sheared magnetic fields. Within the context of a sheared slab model, the 'universal mode' (electron diamagnetic drift wave) has recently been shown to be at worst marginally stable in a collisionless plasma [1][2][3][4] and to be stabilized by both shear and dissipation in a resistive plasma [5,6]. The introduction of an equilibrium electron current along the magnetic field has been demonstrated to give rise to absolute instabilities [7 -9]; the threshold currents required are, however, very large compared to those present in typical experiments.…”

“…[5]; it has the same structure (different numerical coefficients) as the eigenvalue equation of Ref. [6] in the limit [Tj ->• 0], and it has been investigated extensively in Ref. [5] in the limit *C| | -•<» [isothermal electron response; seeEq.…”

Theoretical interpretation of experimental dibaryon resonance data

“…With the reservation that the near-axis region may deserve refined theoretical treatment (in view of the small gradients), the latest theories then predict both the collisionless [40] and the collisional [41 ] drift waves to be absolutely stable in the discharges considered, except perhaps in the case of the Frascati Tokamak where there is, at small radii (0.175 < p <0.375), a window of mode numbers in the range kfla s = 1 for which shear damping is ineffective. We recall, however, that, for the collisional and the collisionless drift modes -and in contrast to the trapped-electron modepositive values of r} e = dlnT e /d In N have a stabilizing influence on the non-adiabatic electron response.…”