NEGATIVE-ENERGY PERTURBATIONS IN CIRCULARLY CYLINDRICAL EQUILIBRIA WITHIN THE FRAMEWORK OF MAXWELL-DRIFT KINETIC-THEORY

The conditions for the existence of negative-energy perturbations (whi ch could be nonlinearly unstable and cause anomalous transport) are in vestigated in the framework of linearized collisionless Maxwell-drift kinetic theory for the case of equilibria of magnetically confined, ci rcularly cylindrical plasmas and vanishing initial field perturbations . For wave vectors with a nonvanishing component parallel to the magne tic field, the plane equilibrium conditions (derived by Throumoulopoul os and Pfirsch [Phys Rev. E 49, 3290 (1994)]) are shown to remain vali d, while the condition for perpendicular perturbations (which are foun d to be the most important modes) is modified. Consequently, besides t he tokamak equilibrium regime in which the existence of negative-energ y perturbations is related to the threshold value of 2/3 of the quanti ty eta(nu) = partial derivative InTnu/partial derivative InNnu, a new regime appears, not present in plane equilibria, in which negative-ene rgy perturbations exist for any value of eta(nu). For various analytic cold-ion tokamak equilibria a substantial fraction of thermal electro ns are associated with negative-energy perturbations (active particles ). In particular, for linearly stable equilibria of a paramagnetic pla sma with hat electron temperature profile (eta(e) = 0), the entire vel ocity space is occupied by active electrons. The part of the velocity space occupied by active particles increases from the center to the pl asma edge and is larger in a paramagnetic plasma than in a diamagnetic plasma with the same pressure profile. It is also shown that, unlike in plane equilibria, negative-energy perturbations exist in force-free reversed-field pinch equilibria with a substantial fraction of active particles. The present results, in particular the fact that a thresho ld value of eta(nu), is not necessary for the existence of negative-en ergy perturbations, enhance even more the relevance of these modes.