  We study the quasineutral limit of a Vlasov-Poisson system that describes the
dynamics of ions in a plasma. We handle data with Sobolev regularity under the
sharp assumption that the profile of the initial data in the velocity variable
satisfies a Penrose stability condition.
  As a by-product of our analysis, we obtain a well-posedness theory for the
limit equation (which is a Vlasov equation with Dirac distribution as
interaction kernel) for such data.
