Search Results (29 total)

Positron trapping and acceleration in a plasma wake using a four-bunch scheme [X. Wang , Phys. Rev. Lett. 101, 124801 (2008)] is numerically investigated through 2D particle-in-cell simulations. This scheme that integrates positron generation, trapping, and acceleration into a single stage is a promising approach for investigating positron acceleration in an electron-beam-driven wake. It consists of a plasma with an embedded thin foil target into which two closely spaced electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Some of the outstanding issues related to the quality of the accelerated positron beam load are discussed as a function of the beam and plasma parameters. Simulations show that a large number of positrons (10⁷–10⁸) can be trapped when the plasma wake is modestly nonlinear, and the positron-generating foil target must be immersed into the plasma. Beam loading can reduce the energy spread of the positron beam load. The quality of the positron beam load is not very sensitive to the exact bunch spacing between the drive electron bunch and the positron beam load.

Multi-GeV trapped electron bunches in a plasma wakefield accelerator (PWFA) are observed with normalized transverse emittance divided by peak current, ϵ_N,x/I_t, below the level of 0.2  μm/kA. A theoretical model of the trapped electron emittance, developed here, indicates that emittance scales inversely with the square root of the plasma density in the nonlinear “bubble” regime of the PWFA. This model and simulations indicate that the observed values of ϵ_N,x/I_t result from multi-GeV trapped electron bunches with emittances of a few  μm and multi-kA peak currents.

A novel approach for generating and accelerating positron bunches in a plasma wake is proposed and modeled. The system consists of a plasma with an embedded thin foil into which two electron beams are shot. The first beam creates a region for accelerating and focusing positrons and the second beam provides positrons to be accelerated. Monte Carlo and 3D PIC simulations show a large number of positrons (10⁷∼10⁸) are trapped and accelerated to ∼5  GeV over 1 m with relatively narrow energy spread and low emittance.

An ultrarelativistic 28.5 GeV, 700-μm-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n_e between ≈10¹³ and ≈5×10¹⁴  cm^-3. Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of ≈3 in the high emittance plane of the beam ≈1  m downstream from the plasma exit. As n_e increases, the formation of a beam halo containing ≈40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of ≈3 and emittance ratio of ≈5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances.

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.

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al.

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36  GV/m, in good agreement with an analytical model and PIC simulations.

Positrons in the energy range of 3–30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1–50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

Plasma production via field ionization occurs when an incoming particle beam is sufficiently dense that the electric field associated with the beam ionizes a neutral vapor or gas. Experiments conducted at the Stanford Linear Accelerator Center explore the threshold conditions necessary to induce field ionization by an electron beam in a neutral lithium vapor. By independently varying the transverse beam size, number of electrons per bunch, or bunch length, the radial component of the electric field is controlled to be above or below the threshold for field ionization. Additional experiments ionized neutral xenon and neutral nitric oxide by varying the incoming beam’s bunch length. A self-ionized plasma is an essential step for the viability of plasma-based accelerators for future high-energy experiments.

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam’s space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8×10¹⁰ electrons is compressed to 20  μm longitudinally and focused to a transverse spot size of 10  μm at the entrance of a 10 cm long column of lithium vapor with density 2.8×10¹⁷  atoms/cm³. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

A high-gradient, meter-scale plasma-wakefield accelerator module operating in the electron blowout regime is demonstrated experimentally. The beam and plasma parameters are chosen such that the matched beam channels through the plasma over more than 12 beam beta functions without spreading or oscillating over a range of densities optimum for observing both deceleration and acceleration. The wakefield decelerates the bulk of the initially 28.5 GeV beam by up to 155 MeV; however, particles in the back of the same beam are accelerated by up to 280 MeV at a density of 1.9×10¹⁴   cm^-3 as the wakefield changes sign.

Tunnel ionizing neutral gas with the self-field of a charged particle beam is explored as a possible way of creating plasma sources for a plasma wakefield accelerator [Bruhwiler et al., Phys. Plasmas (to be published)]. The optimal gas density for maximizing the plasma wakefield without preionized plasma is studied using the PIC simulation code OSIRIS [R. Hemker et al., in Proceeding of the Fifth IEEE Particle Accelerator Conference (IEEE, 1999), pp. 3672–3674]. To obtain wakefields comparable to the optimal preionized case, the gas density needs to be seven times higher than the plasma density in a typical preionized case. A physical explanation is given.

Plasma wakefields are both excited and probed by propagating an intense 28.5 GeV positron beam through a 1.4 m long lithium plasma. The main body of the beam loses energy in exciting this wakefield while positrons in the back of the same beam can be accelerated by the same wakefield as it changes sign. The scaling of energy loss with plasma density as well as the energy gain seen at the highest plasma density is in excellent agreement with simulations.

We report on the first study of the dynamic transverse forces imparted to an ultrarelativistic positron beam by a long plasma in the underdense regime. Focusing of the 28.5 GeV beam is observed from time-resolved beam profiles after the 1.4 m plasma. The strength of the imparted force varies along the ∼12  ps full length of the bunch as well as with plasma density. Computer simulations substantiate the longitudinal aberration seen in the data and reveal mechanisms for emittance degradation.

The focusing effects of a 1.4 m long, (0–2)×10¹⁴   cm^-3 plasma on a single 28.5 GeV electron bunch are studied experimentally in the underdense or blowout regime, where the beam density is much greater than the plasma density. As the beam propagates through the plasma, the density of plasma electrons along the incoming bunch drops from the ambient density to zero leaving a pure ion channel for the bulk of the beam. Thus, from the head of the beam up to the point where all plasma electrons are blown out, each successive longitudinal slice of the bunch experiences a different focusing force due to the plasma ions. The time-changing focusing force results in a different number of betatron oscillations for each slice depending upon its location within the bunch. By using an electron beam that has a correlated energy spread, this time-dependent focusing of the electron bunch has been observed by measuring the beam spot size in the image plane of a magnetic energy spectrometer placed at the plasma exit.

The transverse dynamics of a 28.5-GeV electron beam propagating in a 1.4 m long, (0–2)×10¹⁴  cm^-3 plasma are studied experimentally in the underdense or blowout regime. The transverse component of the wake field excited by the short electron bunch focuses the bunch, which experiences multiple betatron oscillations as the plasma density is increased. The spot-size variations are observed using optical transition radiation and Cherenkov radiation. In this regime, the behavior of the spot size as a function of the plasma density is well described by a simple beam-envelope model. Dynamic changes of the beam envelope are observed by time resolving the Cherenkov light.

The successful utilization of an ion channel in a plasma to wiggle a 28.5-GeV electron beam to obtain broadband x-ray radiation is reported. The ion channel is induced by the electron bunch as it propagates through an underdense 1.4-meter-long lithium plasma. The quadratic density dependence of the spontaneously emitted betatron x-ray radiation and the divergence angle of ∼(1–3)×10^-4 radian of the forward-emitted x-rays as a consequence of betatron motion in the ion channel are in good agreement with theory. The absolute photon yield and the peak spectral brightness at 14.2-keV photon energy are estimated.

In a recent Brief Comment, the results of an experiment to measure the refraction of a particle beam were reported [P. Muggli et al., Nature 411, 43 (2001)]. The refraction takes place at a passive interface between a plasma and a gas. Here the full paper on which that Comment is based is presented. A theoretical model extends the results presented previously [T. Katsouleas et al., Nucl. Instrum. Methods Phys. Res., Sect. A 455, 161 (2000)]. The effective Snell's law is shown to be nonlinear, and the transients at the head of the beam are described. 3D particle-in-cell simulations are performed for parameters corresponding to the experiment. Additionally, the experiment to measure the refraction and internal reflection at the Stanford Linear Accelerator Center is described.

Emissions produced or initiated by a 30-GeV electron beam propagating through a ∼1-m long heat pipe oven containing neutral and partially ionized vapor have been measured near atomic spectral lines in a beam-plasma wakefield experiment. The Cerenkov spatial profile has been studied as a function of oven temperature and pressure, observation wavelength, and ionizing laser intensity and delay. The Cerenkov peak angle is affected by the creation of plasma, and estimates of neutral and plasma density have been extracted. Increases in visible background radiation, consistent with increased plasma recombination emissions due to dissipation of wakefields, were simultaneously measured.

We report measurements of very large output intensities corresponding to a gain larger than 10⁵ for a single pass free-electron laser operating in the self-amplified spontaneous emission (SASE) mode at 12 μm. We also report the observation and analysis of intensity fluctuations of the SASE radiation intensity in the high-gain regime. The results are compared with theoretical predictions and simulations.

The transverse-diffusion coefficients for Na⁺ ions drifting in Kr gas have been experimentally measured with estimated accuracies of better than 3% at E/N values ranging from 5 to 500 Td (1 Td=10^-21 V m²) at about 303 K. In addition, elaborate calculations of the transport properties for the Na⁺-Kr system have been made by using Monte Carlo simulations (MCS’s) and the generalized Einstein-relation techniques. The experimental results are compared with values obtained from MCS’s using the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] as input. Agreement within 3% for the entire range of E/N is observed in the comparisons. MCS-calculated values of longitudinal-diffusion coefficients are also compared with those determined experimentally by Thackston et al. [J. Chem. Phys. 73, 2012 (1980)].

Using various recently proposed interaction potentials for the Li⁺-He system, elaborate calculations of the mobility, longitudinal, and transverse diffusion coefficients of Li⁺ swarms in helium have been made by high-speed Monte Carlo simulations (MCS’s). In addition, the transverse diffusion coefficients for this ion–neutral-atom pair have been experimentally measured with total errors of ±3%. The close agreement of the present experimental results with those of Skullerud, Eide, and Stefansson [J. Phys. D 19, 197 (1986)] as well as the faithful MCS reproduction of all three transport coefficients using the interaction potential proposed by Larsen et al. [J. Phys. B 21, 2519 (1988)] over the entire range of E/N (electric field to neutral-particle number density ratio) studied not only provides confirmation of their transport coefficient values, but also lends strong support for their proposed interaction potential. In comparison, the more recent potentials of Ahlrichs et al. [J. Chem. Phys. 88, 6290 (1988)] and of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] did not reproduce the experimental data quite as well. As a benchmark the MCS calculations have also provided evidence of the accuracy of the Kramers-Moyal expansion method in calculating the transport coefficients.