Search Results (12 total)

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.

The first hydrodynamic experiments were performed on the National Ignition Facility. A supersonic jet was formed via the interaction of a laser driven shock (∼40  Mbar) with 2D and 3D density perturbations. The temporal evolution of the jet’s spatial scales and ejected mass were measured with point-projection x-ray radiography. Measurements of the large-scale features and mass are in good agreement with 2D and 3D numerical simulations. These experiments provide quantitative data on the evolution of 3D supersonic jets and provide insight into their 3D behavior.

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.

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.

The concept of using short plasma sections several meters in length to double the energy of a linear collider just before the collision point is proposed and modeled. In this scenario the beams from each side of a linear collider are split into pairs of microbunches with the first driving a plasma wake that accelerates the second. The luminosity of the doubled collider is maintained by employing plasma lenses to reduce the spot size before collision.

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.