The　quasineutral　limit　of　Euler-Poisson　system　with　and　without　viscosity　in　plasma　physics　in　the　torus　T-d,　d　greater　than　or　equal　to　l　is　studied.　That　quasineutral　regimes　are　the　incompressible　Euler　or　Navier-Stokes　equations　is　proven.　In　the　mean　time,　long-time　existence　for　large　amplitude　smooth　solutions　of　Euler-Poisson　system　in　torus　T-d,　d　greater　than　or　equal　to　1,　with　or　without　viscosity　as　the　Debye　length　lambda--＞0　is　also　obtained　provided　that　the　smooth　solution　of　incompressible　Euler　or　Navier-Stokes　equations　exists　globally　for　nearby　initial　data.　In　particular,　the　existence　of　large　amplitude　smooth　solutions　of　Euler-Poisson　system　in　torus　T-2　with　or　without　viscosity　and　with　sufficiently　small　Debye　length　is　obtained　on　any　arbitrary　time　interval.　The　proof　of　these　results　is　based　on　a　straightforward　extension　of　the　classical　energy　method,　the　modulated　energy　method,　the　iteration　techniques　and　the　standard　compactness　argument.