We consider a Vlasov-Poisson problem, with ionization terms, that models an
ion beam. We show that in some physical situations it does not admit solut
ions. To overcome this difficulty, we take into account collisions and we m
odel them with a velocity diffusion term. An existence result is then prove
d. (C) 1999 Academie des sciences/Editions scientifiques et medicales Elsev
ier SAS.