PPPL-2354 DE86 013642Nonlinear evolution of drift instabilities in the presence of electron-ion collisions in a shear-free slab has been studied by using gyrokinetic particle simulation tech niques as well as by solving, both numerically and analytically, model mode-coupling equations. The purpose of the investigation is to determine the mechanisms respon sible for the nonlinear saturation of the instability and for the ensuing steady-state transport. Such an insight is very valuable for understanding drift wave problems in more complicated geometries. The results indicate that the electron E x B convection is the dominant mechanism for saturation. It is also found that the saturation amplitude and the associated quasilinear diffusion are greatly enhanced over their collisionless values as a result of weak collisions. In the highly collisional (fluid) limit, there is an upper bound for saturation with e
°1 712 ? Au analytical explanation is provided for phenomena observed in gyrokinetic patckle simulations, which were used to study the nonlinear evolution of the universal insta bility. The nonlinear £ x B advection of nearly resonant electrons is related to the deactivation of the instability mechanism, leading to oscillations of the perturbation amplitude around a nonzero saturated level. An equivalent description is given in terms of nonresonant mode coupling. A self-consistent evolution equation for the am plitude is obtained for the case where this mechanism predominates, and is successfully compared to the gyrokinetic simulations. DISCLAIMER This report was prepared aS au .««..« 0. work .ponsorcd by an agency <* «« ""£»' f t0 "= Government. Neither the Uniteo-a.es Government nor any-^ «^-* »^"££ employees makes any warranty. express or implied, or assumes any legal liability or ftspMsi Mi o hTVccuracy. compos. ", »Sefu,ness " any information ^~f < ^ proL disclose, or represents W its « wou.d no, infringe p,".» y .^^'^ ence herein w any specific commercial product. process, or semce by trade name, '"«"»"' ^Surer, o/otn^ doc, no, necessarily consulate or ^'jM^™^ mendalion. o: favoring by the Uniled State, Government or any ^"P^^JZ and opinions of authors expressed herein do no. necessarily stale or reflect those ot Uniled Slates Governmenl or any agency thereof.
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