A double plasma device with multipole surface magnetic confinement is used to investigate the beam relaxation length of beam plasma interaction as a function of the beam plasma density ratio, 10-4 <FONT FACE="Symbol">£ </font>nb/no <FONT FACE="Symbol">£</font> 2 x 10-2, and beam energy Wb <FONT FACE="Symbol">£</font> 200 eV in an unmagnetized plasma, using argon and helium gases. The measured relaxation lengths show a transition from quasi-linear to nonlinear regime, as predicted by Galeev et al., 1977. There is evidence that for some range of the beam plasma parameters the threshold for modulational instability is reached. In that case it is possible to observe that the beam propagates relatively unperturbed for larger distances than the ones predicted by quasi-linear theory.