Recent proposals for using plasma wakefield accelerators (PWFA) as a component of a linear collider have included intense electron beams with densities many times in excess of the plasma density. The beam's electric fields expel the plasma electrons from the beam path to many beam radii in this regime. We analyze here the motion of plasma ions under the beam fields, and find for a proposed PWFA collider scenario that the ions completely collapse inside of the beam. Simulations of ion collapse are presented. Implications of ion motion on the feasibility of the PWFA-based colliders are discussed.

In this paper we discuss a variety of physical aspects of the nonlinear regime of the plasma wakefield accelerator: general nonlinear properties, thermal effects, particle trapping, and maximum acceleration. The unifying motivation for this investigation is to discover the limit on the size of the plasma waves, as the interesting characteristics of the nonlinear scheme depend strongly on amplitude. Implications for laboratory and cosmic-ray acceleration are discussed.

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In this paper we present various treatments of plasma wake-field phenomena which employ multiple-fiuid models. These models generalize the one-dimensional, nonlinear, relativistic singlefluid model which has been used extensively in previous plasma wake-field calculations. Using a two-fluid model, we discuss the interaction of a low-energy continuous electron beam with wakefield-generated plasma waves. The phenomena of continuous-beam modulation and wave period shortening are discussed. The relationship between these effects and the two-stream instability is also examined. Also, using a three-fluid model, effects due to plasma electron temperature in nonlinear plasma wake-fields are examined and compared to previous work. Finally, the consequences of ion motion induced by large-amplitude electron plasma waves are calculated by including the fluid behavior of the ions.