Neoclassical tearing modes in the presence of sheared flows

Neoclassical tearing modes (NTMs) pose a potential threat to long pulse tokamak experimentsand future tokamak based reactors, such as ITER [1], where their onset can cause soft disrup-tions and prevent the achievement of high β. There is considerable current research interest inelucidating the evolutionary characteristics of these modes and the conditions for their destabi-lization [2-5]. Among some of the outstanding issues related to them, the interaction of equlib-rium shear flows with NTMs continues to be of paramount importance since sheared velocityflows are known to be widely prevalent in tokamak devices and can be generated by neutralbeams,ioncyclotronheatingandself-consistentdriftturbulence.Anumberofpaststudieshaveexamined the effect of flows on tearing modes, particularly in the linear regime and for simpli-fied geometries [6]. There have also been a few nonlinear studies [7, 8] but the problemis quitecomplex,particularly in realistictoroidal geometries.We have recently begun [5] a detailednu-merical study on the nonlinear evolutionof NTMs in the presence of sheared equilibriumflowswiththehelpofa modifiedversionofafinitedifferencecodeNEAR[10].Thenumericalmodelandthe code havebeen testedin thepast for theeffect of flowson resistivelineartearing modes[9]andsubsequentlyextendedandbenchmarkedfornonlinearresistiveandneoclassicaltearingmodes [5]. In this paper we report on further findings of our numerical work and also present amodel analytic calculation that provides some physical insight into the numerical results.Our numerical simulations are based on the solutions of a set of reduced MHD equations thatwere proposed by Kruger