Search Results (11 total)

We report in this paper on full scale 2D particle-in-cell simulations investigating laser wakefield acceleration. First we describe our findings of electron beam generation by a laser propagating through a single gas jet. Using realistic parameters which are relevant for the experimental setup in our laboratory we find that the electron beam resulting after the propagation of a 0.8 μm, 50 fs laser through a 1.5 mm gas jet has properties that would make it useful for further acceleration. Our simulations show that the electron beam is generated when the laser exits the gas jet, and the properties of the generated beam, especially its energy, depend only weakly on most properties of the gas jet. We therefore propose to use the first gas jet as a plasma cathode and then use a second gas jet placed immediately behind the first to provide additional acceleration. Our simulations of this proposed setup indicate the feasibility of this idea and also suggest ways to optimize the quality of the resulting beam.

This Letter examines the electron-hosing instability in relation to the drivers of current and future plasma-wakefield experiments using fully three-dimensional particle-in-cell simulation models. The simulation results are compared to numerical solutions and to asymptotic solutions of the idealized analytic equations. The measured growth rates do not agree with the existing theory and the behavior is shown to depend sensitively on beam length, shape, and charge. We find that even when severe hosing occurs the wake can remain relatively stable.

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.

Plasma-wakefield excitation by positron beams is examined in a regime for which the plasma dynamics are highly nonlinear. Three dimensional particle-in-cell simulations and physical models are presented. In the nonlinear wake regime known as the blowout regime for electrons, positron wakes exhibit an analogous “suck-in” behavior. Although analogous, the two wakefield cases are quite different in terms of their amplitudes, wavelengths, waveforms, transverse profiles, and plasma density dependence. In a homogenous plasma, nonlinear positron wakes are smaller than those of the corresponding electron case. However, hollow channels are shown to enhance the amplitude of the positron wakes.

We consider the possibility of using a thin plasma slab as an optical element to both focus and compress an intense laser pulse. By thin we mean that the focal length is larger than the lens thickness. We derive analytic formulas for the spot size and pulse length evolution of a short laser pulse propagating through a thin uniform plasma lens. The formulas are compared to simulation results from two types of particle-in-cell code. The simulations give a greater final spot size and a shorter focal length than the analytic formulas. The difference arises from spherical aberrations in the lens which lead to the generation of higher-order vacuum Gaussian modes. The simulations also show that Raman side scattering can develop. A thin lens experiment could provide unequivocal evidence of relativistic self-focusing.

Using a variational method, we show that an effective attractive force exists between two Gaussian laser beams in a plasma because of a mutual coupling from relativistic mass corrections. The effective force can be generalized to other nonlinearities. This force can cause two laser beams to spiral around each other with a rotation period that is proportional to the Rayleigh length. These orbits are stable if the ratio of the orbit diameter to the laser spot size d₀/W₀≤sqrt[2]. Three-dimensional particle-in-cell simulations are presented which confirm the mutual attraction.

We have used 2D cylindrically symmetric particle-in-cell simulations to investigate the dynamics of a high energy electron beam propagating through an underdense plasma. The simulation parameters are relevant to a recent plasma wakefield experiment conducted at the Stanford Linear Accelerator Center [R. Assmann et al., Stanford Linear Accelerator Center Proposal, 1997]. We model the dynamic development of the beam and wakefield excitation over meters of propagation length. To most clearly illustrate the dynamics of both the beam and the wakefield, a video of the simulation data is presented. The main observation is that the beam dynamics, i.e., its betatron motion in the resulting ion channel, agree well with the theoretical predictions while the plasma wake remains almost invariant over the entire propagation distance. The video illustrates subtle details regarding the interplay between the beam dynamics and wakefield generation. The results presented here complement results published separately [S. Lee et al., Phys. Rev. E 61, 7014 (2000)].

Full-scale particle-in-cell simulations of a meter-long plasma wakefield accelerator (PWFA) are presented in two dimensions. The results support the design of a current PWFA experiment in the nonlinear blowout regime where analytic solutions are intractable. A relativistic electron bunch excites a plasma wake that accelerates trailing particles at rates of several hundred MeV/m. A comparison is made of various simulation codes, and a parallel object-oriented code OSIRIS is used to model a full meter of acceleration. Excellent agreement is obtained between the simulations and analytic expressions for the transverse betatron oscillations of the beam. The simulations are used to develop scaling laws for designing future multi-GeV accelerator experiments.

The nonlinear final state of short-pulse lasers is examined using fully explicit particle-in-cell simulations. A new long-wavelength hosing instability is found to be dominant after a few Rayleigh lengths of propagation. This instability causes self-trapped electrons to be displaced off axis; we find that ion motion is important at the highest densities studied. A possible explanation for this instability is given based on a new variational principle analysis for short-pulse lasers propagating in underdense plasma.

The use of two crossed laser pulses in a plasma for the cathodeless production of high-current low-emittance electron beams [D. Umstadter, J. K. Kim, and E. Dodd, Phys. Rev. Lett. 76, 2073 (1996)] is examined with fully relativistic, two-and-a-half-dimensional particle-in-cell simulations. Estimates for the number of injected particles, their energy spread, and their emittance are given as functions of the amplitude and timing of the injection pulse relative to the drive pulse of the laser wake field accelerator. The physical mechanism of the trapping of particles is examined based on single particle phase space trajectories in the simulations and numerical calculations.