Experimental studies of plasma wake-field acceleration and focusing (1990)

More than four years after the initial proposal of the Plasma Wake-field Accelerator [ 11 (PWFA), it continues to be the object of much investigation, due to the promise of the ultrahigh accelerating gradients that can exist in relativistic plasma waves driven in the wake of charged particle beams. These large amplitude plasma wake-fields are of interest in the laboratory, both for the wealth of basic nonlinear plasma wave phenomena which can be studied, as well as for the applications of acceleration and focussing of electrons and positrons in future linear colliders. Plasma wake-field waves are also of importance in nature, due to their possible role in direct cosmic ray acceleration [2]. The purpose of the present work is to review the recent experimental advances made in PWFA research at Argonne National Laboratory, in which many interesting beam and plasma phenomena have been observed. Emphasis is given to discussion of the nonlinear aspects of the PWFA beam-plasma interaction. Experimental investigations of the PWFA at the Argonne Advanced Accelerator Test Facility [3] (AATF) have produced a multitude of accomplishments in the short time since their inception in 1986. The results of these experiments have directly verified the existence of electron acceleration in plasma wake-fields by the injection of a test bunch of electrons into a beam-driven plasma wave, thus measuring the longitudinal dependence of the accelerating and deflecting wake-fields. These proof-of-principle experiments [4], which will be described below, validated major relevant predictions of linear wake-field theory [5-71, including the fundamental excitation of the electron plasma waves with their associated longitudinal and transverse electrostatic fields, the electromagnetic self-pinching forces of a high intensity driving beam, and the dependence of these phenomena on beam and plasma characteristics. After the proof-of-principle experiments were completed, improvements in facility operation allowed a second round of experiments which explored an entirely different, nonlinear regime of the PWFA. High resolution measurements of the wake-fields excited by more intense driving beams revealed large nonlinearities in the wake-fields, indicating that the driving beams became significantly self-pinched within the plasma by their transverse wake-fields [8]. Both the nonlinearity and the self-pinching are exciting observations, as the nonlinear wake-fields can theoretically yield large transformer ratios [9], and the self-pinching wake-fields have been proposed for use in a powerful final focussing lens for a linear e+ e- collider [5, lo]. The results and analysis of the nonlinear PWFA experiments will also be presented below. A third set of experiments have also been recently completed, in which the focussing effect on the driving electron beam, which was inferred from the nonlinear PWFA measurements, was directly observed. A streak camera-based diagnostic allowed time-resolved measurement of the beam density transverse profiles at the end of the plasma column used in the PWFA experiments. These self-pinched profiles showed the development of Bennett-like [l 11 equilibria, with radii smaller than a plasma skin-depth. The experimental results, as well as theoretical analysis of the self-focussing process, are discussed in detail. Particular attention is paid to analysis of the Bennett-like equilibria which can be approached through nonlinear phase space filamentation. After the discussion of the experimental investigations which have been completed to date, we will turn to the subject of future efforts in this field. Prospects for future experiments, to be performed with a much more intense beam derived from a new laser photo-cathode source at the AATF, will be examined. Theoretical expectations for nonlinear wake-field amplitudes in these experiments will be estimated; they are predicted to be greater than I GeV/m.