Search Results (13 total)

First measurements of the breakdown threshold in a dielectric subjected to GV/m wakefields produced by short (30–330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of 100  μm inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to 27  GV/m to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches 13.8±0.7  GV/m. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.

A slab-symmetric dielectric-loaded accelerator structure, consisting of a vacuum gap between dielectric-lined conducting walls, is described. The device is resonantly excited by an external drive laser which is side coupled into the acceleration region; a novel coupling scheme, which consists of an array of narrow, equally spaced slots in the upper structure boundary, is presented and analyzed in detail. This structure partakes of the advantages of earlier slab-symmetric optical acceleration proposals, but will use a terahertz-frequency external radiation source (λ=340   μm), allowing realistic electron beams to be used in a proof-of-principle experiment. Two- and three-dimensional electromagnetic simulations are used to verify the mode patterns and study the effects of the couplers, including time-dependent calculations of the filling of the structure and particle-in-cell computations of the beam wakefields. Details of the resonance are found to be highly sensitive to the coupling slot geometry: the presence of the couplers can lead to frequency detuning, changes in the field breakdown limits and overall Q factor, and distortions of the field pattern. Beam wakefields are enhanced by the presence of the slots, but found to have no significant effect on the beam transport. The resonant accelerating fields, which are nearly constant along the short transverse direction, are found to have between 10 and 15 times the amplitude of the driving radiation, with only a small (<10%) admixture of other nonaccelerating modes. Field gradients are computed to be near 100  MV/m when the structure is driven with 100 MW of terahertz laser power. Possible manufacturing methods for a prototype device are discussed.

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.

Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an inverse-free-electron-laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a 400 GW CO₂ laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, ∼1.8  cm, is much shorter than the undulator length yielding a diffraction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the first half of the undulator at a gradient of 70  MeV/m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator.

We examine the use of sextupole magnets to correct nonlinearities in the longitudinal phase space transformation of a relativistic beam of charged particles in a dispersionless translating section, or dogleg. Through heuristic analytical arguments and examples derived from recent experimental efforts, augmented by simulations using the particle tracking codes PARMELA and ELEGANT, sextupole corrections are found to be effective in optimizing the use of such structures for beam compression or for shaping the current profile of the beam, by manipulation of the second-order longitudinal dispersion. Recent experimental evidence of the use of sextupoles to manipulate second-order horizontal and longitudinal dispersion of the beam is presented. The theoretical and experimental results indicate that these manipulations can be used to create an electron bunch with a current profile having a long ramp followed by a sharp cutoff, which is optimal for driving large-amplitude wake fields in a plasma wake field accelerator.

Velocity bunching has been recently proposed as a tool for compressing electron beam pulses in modern high brightness photoinjector sources. This tool is familiar from earlier schemes implemented for bunching dc electron sources, but presents peculiar challenges when applied to high current, low emittance beams from photoinjectors. The main difficulty foreseen is control of emittance oscillations in the beam in this scheme, which can be naturally considered as an extension of the emittance compensation process at moderate energies. This paper presents two scenarios in which velocity bunching, combined with emittance control, is to play a role in nascent projects. The first is termed ballistic bunching, where the changing of relative particle velocities and positions occur in distinct regions, a short high gradient linac, and a drift length. This scenario is discussed in the context of the proposed ORION photoinjector. Simulations are used to explore the relationship between the degree of bunching, and the emittance compensation process. Experimental measurements performed at the UCLA Neptune Laboratory of the surprisingly robust bunching process, as well as accompanying deleterious transverse effects, are presented. An unanticipated mechanism for emittance growth in bends for highly momentum chirped beam was identified and studied in these experiments. The second scenario may be designated as phase space rotation, and corresponds closely to the recent proposal of Ferrario and Serafini. Its implementation for the compression of the electron beam pulse length in the PLEIADES inverse Compton scattering (ICS) experiment at LLNL is discussed. It is shown in simulations that optimum compression may be obtained by manipulation of the phases in low gradient traveling wave accelerator sections. Measurements of the bunching and emittance control achieved in such an implementation at PLEIADES, as well as aspects of the use of velocity-bunched beam directly in ICS experiments, are presented.

The energy loss and gain of a beam in the nonlinear, “blowout” regime of the plasma wakefield accelerator, which features ultrahigh accelerating fields, linear transverse focusing forces, and nonlinear plasma motion, has been asserted, through previous observations in simulations, to scale linearly with beam charge. Additionally, from a recent analysis by Barov et al., it has been concluded that for an infinitesimally short beam, the energy loss is indeed predicted to scale linearly with beam charge for arbitrarily large beam charge. This scaling is predicted to hold despite the onset of a relativistic, nonlinear response by the plasma, when the number of beam particles occupying a cubic plasma skin depth exceeds that of plasma electrons within the same volume. This paper is intended to explore the deviations from linear energy loss using 2D particle-in-cell simulations that arise in the case of experimentally relevant finite length beams. The peak accelerating field in the plasma wave excited behind the finite-length beam is also examined, with the artifact of wave spiking adding to the apparent persistence of linear scaling of the peak field amplitude into the nonlinear regime. At large enough normalized charge, the linear scaling of both decelerating and accelerating fields collapses, with serious consequences for plasma wave excitation efficiency. Using the results of parametric particle-in-cell studies, the implications of these results for observing severe deviations from linear scaling in present and planned experiments are discussed.

There has been much recent experimental and theoretical interest in the blowout regime of plasma wakefield acceleration, which features ultrahigh accelerating fields, linear transverse focusing forces, and nonlinear plasma motion. A quantitative understanding of the blowout regime including all these effects has, to this point, been available only through detailed simulations. This paper represents an initial step towards an analytical theory of this regime, in which the mechanism of energy loss in the drive beam is investigated. We find, first from examination of electromagnetic particle-in-cell simulations, and then through analytical investigations, that under short pulse, high-charge conditions, the plasma electrons receive a strong initial push along the direction of beam motion. This nonlinear effect is unanticipated by linear theory, where the return current motion is in the opposite direction. In the limit of short pulses (the δ-function limit), the beam energy loss is shown to be linear in charge even with a nonlinear plasma response dominated by relativistic, electromagnetic effects, despite the fact that the initial plasma electron response changes qualitatively from the familiar electrostatic, nonrelativistic limit.

Enhanced energy gain of externally injected electrons by a ∼3  cm long, high-gradient relativistic plasma wave (RPW) is demonstrated. Using a CO₂ laser beat wave of duration longer than the ion motion time across the laser spot size, a laser self-guiding process is initiated in a plasma channel. Guiding compensates for ionization-induced defocusing (IID) creating a longer plasma, which extends the interaction length between electrons and the RPW. In contrast to a maximum energy gain of 10 MeV when IID is dominant, the electrons gain up to 38 MeV energy in a laser-beat-wave-induced plasma channel.

The nonresonant beat-wave excitation of relativistic plasma waves is studied in two-dimensional simulations and experiments. It is shown through simulations that, as opposed to the resonant case, the accelerating electric fields associated with the nonresonant plasmons are always in phase with the beat-pattern of the laser pulse. The excitation of such nonresonant relativistic plasma waves is shown to be possible for plasma densities as high as 14 times the resonant density. The density fluctuations and the fields associated with these waves have significant magnitudes, facts confirmed experimentally using collinear Thomson scattering and electron injection, respectively. The applicability of these results towards eventual phase-locked acceleration of prebunched and externally injected electrons is discussed.

Experiments are reported on inverse free-electron-laser acceleration, including for the first time observations of the energy change as a function of relative injection phase of the electron bunches. The microwave accelerating structure consists of a uniform circular waveguide with a helical wiggler and an axial magnetic field. Acceleration of the entire beam by 6% is seen for 6 MeV electron bunches at optimum relative phase. Experimental results compare favorably, for accelerating phases, with predictions of a three-dimensional simulation that includes large-orbit effects.

First experimental results are reported on the operation of a multimegawatt 2.856 GHz cyclotron autoresonance accelerator (CARA). A 90–100 kV, 2–3 MW linear electron beam has had up to 6.6 MW added to it in CARA, with an rf-to-beam power efficiency of up to 96%. This efficiency level is larger than that reported for any fast-wave interaction between radiation and electrons, and also larger than that in normal conducting rf linear accelerators. The results obtained are in good agreement with theoretical predictions.

Raphael's modified effective-range expansion for nucleon-nucleon scattering is usually truncated at the second term. It is explicitly shown that the resulting series is not a good representation of the predictions of reasonable potential models for moderate-energy nucleon-nucleon scattering. In particular, it can lead to the wrong sign for the shape parameter.