Effects Of Magnetic Islands On Parallel Current Of Fast Ions

Tearing mode is a class of MHD instability,it changes the topology of magnetic field and generates a special structure,which is called magnetic islands.Magnetic islands enhance the radial transport of particle,so reduce the confinement of plasma in tokamak.There are lots of fast particles in the burning plasma of tokamak,the interaction between fast particle and plasma can not only drive a series of new plasma phenomena,but also suppress some instabilities in tokamak.Toroidal plasma current is necessary to the operation of tokamak,neutral beam injection is one of inductive current drive.The magnetic islands and fast particles are all so important in tokamak,so the parallel current of fast ions in magnetic islands deserves study.Understanding effects of magnetic islands on parallel current of fast ions can also help to analyze the influence of fast ions on the neoclassical tearing mode conversely,and explore the mechanism of suppressing the tearing mode.We apply numerical computation to study effects of magnetic islands on parallel current of passing fast ions by using GTC,and observe the variation tendency of parallel current by changing the ratio of orbit width of passing fast ions and island width.According to the result,we find that the parallel current of fast ions is flattened by magnetic islands.And the ratio of orbit width of passing fast ions and island width becomes larger,the flattened width will becomes smaller,until finally disappeared.We also see another phenomena.The position of flattened current is not in the magnetic islands due to the one-way movement of fast ions.The ratio of orbit width of passing fast ions and island width gets bigger,the offset distance gets larger.Next we calculate the value of ?2,which represents the percentage of overlap area of island and orbit of passing fast ions to island.The depending on the ratio of orbit width of passing fast ions and island width is ?2=1/1+?b/w by fitting.It canbe seen that the ratio of orbit width and island width becomes larger,?2 becomes smaller,namely the fast ions can not be constrained well by magnetic islands.So r2 can explain the changing trend of flatten width of passing fast ions,both results decrease with the increase of Ab/w.We also study linear tearing modes in pair plasmas.It is found that electron inertia can also cause magnetic reconnection in electron-positron plasmas,and the conditions under which either resistively or electron inertia dominates depend strongly on the limit of ?',where ?' is the instability criterion of tearing mode.