M. Rosing

Demonstration of electron beam self‐focusing in plasma wake fields

In this paper, the direct observation of wake‐field self‐focusing of an electron beam in plasma is reported. The dynamics of beam self‐pinching and the fast collisionless evolution of a Bennett‐like, near‐equilibrium profile are examined theoretically and computationally. The experimental results are compared to predictions of the analysis and discussed in the context of application to the plasma lens and the plasma wake‐field accelerator.

Measurement of deflection-mode damping in an accelerating structure

We have directly measured the damping of wake‐field deflection modes in a slow‐wave accelerating structure consisting of a dielectric‐lined waveguide with segmented conducting boundaries wrapped with rf absorbing material. Such damping of deflection modes is desired to prevent beam breakup instabilities. Attenuation e‐folding times of 246 ps were recorded for deflection modes at the Advanced Accelerator Test Facility while the quality of the desired accelerating mode remained unaffected.

The Argonne Wakefield Accelerator-overview and status

The Argonne Wakefield Accelerator (AWA) is a new facility for advanced accelerator research, with a particular emphasis on studies of high gradient (/spl sim/100 MeV/m) Wakefield acceleration. A novel high current short pulse L-Band photocathode gun and preaccelerator will provide 100 nC electron bunches at 20 MeV to be used as a drive beam, while a second high brightness gun will be used to generate a 5 MeV witness beam for Wakefield measurements. We present an overview of the various AWA systems, the status of construction, and initial commissioning results.<<ETX>>

The Argonne Wakefield Accelerator high current photocathode gun and drive linac

The Argonne Wakefield Accelerator (AWA) is a new facility for advanced accelerator research. A major component of the AWA is its drive linac, consisting of a unique high current short pulse L-band photocathode based gun and special standing wave preaccelerator designed to produce 100 nC, 30 ps electron bunches at 20 MeV. Commissioning on the drive linac is now underway. We report on our initial operating experience with this novel machine, including bunch length and emittance measurements.

The measurement of Twiss parameters using segmented Faraday cups

The theory behind the use of segmented Faraday cups (SFCs) to measure beam emittance using a scanning slit is presented. A description of the electronics is given, and a sample of a beam profile and the results of an emittance measurement are presented. It is shown that the ability to combine the data using computers allows much more flexibility than only being able to center the beam in the pipe. Emittance measurements in two dimensions are straightforward and automatic. Beam center and width can be matched to expected values from beam optics codes, and magnets can be adjusted for corrections to very fine resolution.<<ETX>>

NPBTS-overview and capabilities

The Neutral Particle Beam Test Stand (NPBTS) provides a versatile facility for scientific and engineering studies on large-diameter, low-divergence neutral and charged particle beams. It consists of a linac that accelerates H/sup -/ atoms to 50 MeV at 10-12 mA and two experimental areas. Typical pulse widths are 30-150 mu s at repetition rates of 0.5-30 Hz. A small RMS-emittance is achieved by using a series of collimators to shave the 1.6- pi -mm-mr emittance measured at the output of the linac. Typical current in the experimental areas is 500-600 mu A. Experimental area A has been used to study the physics of beam diagnostics and foil neutralization and to measure (p,n) reaction cross sections. Experimental area B has a series of quadrupole objectives built by Los Alamos National Laboratory to reduce beam divergence. Typical beam characteristics are RMS diameters of 10-20 cm and a full-angle divergence (RMS) of 12-24 mu r. The facility contains a wide variety of diagnostics including segmented Faraday cups, beam toroids, stripline beam-position monitors, and wire scanners. In addition, several new diagnostic systems for large-diameter beams have been developed by Argonne and Los Alamos National Laboratories.<<ETX>>

A beam characterization of H/sup -/ particles

A pinhole-scintillator diagnostics system has been developed to determine the characteristics of a 50-MeV H/sup -/ beam. The device consists of an aluminium plate (1.27 mm thick) with a series of pinholes (125- mu m diameter) covered by a thin neutralizer foil, and a downstream scintillator plate and associated camera. The foil-covered pinholes produce a series of H/sup 0/ beamlets that are detected downstream by the scintillator plate. The use of H/sup 0/ particles allows measurement of the beam parameters without changing the magnetic fields of downstream magnetic elements. Measuring the intensities of the beamlets and the beamlet width allows determination of the beam parameters. The H/sup +/, produced by the plate, are sufficiently scattered by the plate, such that a sweep magnet is not required,. The device has been used at the Argonne National Laboratory (ANL) Neutral Particle Beam Test Stand (NPBTS) to measure the parameters at the input of the beam expanding telescope.<<ETX>>

Experimental studies of plasma wake-field acceleration and focusing

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.

An update on Argonne's AWA

The Argonne Wakefield Accelerator (AWA) is a research facility which should possess unprecedented research capabilities for the study of wake fields and related areas requiring short, intense electron bunches. The AWA is designed to produce 100-nC, 14-ps (full width) electron bunches at rep rates up to 30 Hz. Phase-I of the AWA, now under construction, will provide these pulses at 20 MeV for various experiments. Current designs, related research and development, and construction status are presented in this general overview and project update.<<ETX>>

Drive linac for the Argonne Wakefield Accelerator

The drive linac in Phase I of the Argonne Wakefield Accelerator (AWA) will be used to accelerate short duration (10 ps), high charge (100 nC) electron bunches from 2 MV to 20 MV for use in a variety of wakefield acceleration and measurement studies. The high charge is required since this drive bunch will generate the wakefields of interest in various test sections and their amplitudes are proportional to bunch charge. The short bunch duration is required to drive high-frequency wakefields without intra-bunch cancellation effects. The drive linac design was a balance between having a small wake function to maintain a drive bunch energy spread of /spl les/10% and obtaining an adequate accelerating gradient of /spl ges/10 MV/m. This yielded a large aperture, low shunt impedance, high group velocity, L-band, standing-wave linac. Details of the design, fabrication, and testing are presented in the following.<<ETX>>