Search Results (7 total)

A new approach to laser-wakefield acceleration (LWFA) has been analyzed. A seed electron beam bunch precedes the laser pulse into the plasma. This seed bunch initiates formation of plasma waves via a plasma wakefield acceleration mechanism. The amplitude of the plasma waves is subsequently amplified by the laser pulse via a self-modulated LWFA (SM-LWFA) process. This method enables the generation of strong wakefields even when the laser pulse by itself has characteristics that are insufficient for driving resonant LWFA or SM-LWFA. Another advantage is the wakefield formation begins at the seed bunch and does not start from noise as typically occurs in SM-LWFA. This feature may be helpful when the phase of the wakefield must be accurately controlled, for example, when staging multiple LWFA devices in series.

The upgraded Accelerator Test Facility (ATF) CO₂ laser located at Brookhaven National Laboratory offers a unique opportunity to investigate laser wakefield acceleration (LWFA) with a 10.6-μm laser, a wavelength where little experimental work exists. While long laser wavelengths have certain advantages over short wavelengths, our modeling analysis has uncovered another important effect. The upgraded ATF CO₂ laser will have a pulse length as short as 2 ps. At a nominal plasma density of ∼10¹⁶  cm^-3, this pulse length would normally be considered too long for resonant LWFA, but too short for self-modulated LWFA. However, our model simulations indicate that a well-formed wakefield is nevertheless generated with electric field gradients of E_z≳2  GV/m assuming 2.5 TW laser peak power. The model indicates pulse steepening is occurring due to various nonlinear effects. It is possible that this intermediate laser pulse length mode of operation may permit the creation of well-formed, regular-shaped wakefields, which would be needed for staging the LWFA process. Discussed in this paper are the model, its predictions for an LWFA experiment at the ATF, and the pulse steepening effect.

The guided laser pulse propagation and wake-field generation are studied in a wide (in comparison with the laser spot size) gas-filled capillary with an on-axis gas density depletion, which can be produced by a rapid spin of the capillary around its axis or by radially propagating shock waves generated in a piezoceramic tube. A single equation for the wake-field potential, which describes the fully relativistic plasma response in the presence of optical field ionization (OFI) of a gas, is derived and used to demonstrate a guided propagation of a short intense laser pulse over many Rayleigh lengths in a leaky plasma channel produced by the pulse due to OFI in the capillary filled with a radially inhomogeneous gas. The efficient generation of a regular wake field over long distances suitable for the laser wake-field accelerators is shown.

The propagation of a short intense laser pulse in the femtosecond range in a hollow metallic waveguide gives rise to heating of the metallic wall. The temperature of the degenerate electron gas in the wall is increased during the pulse duration and this heating affects the propagation and dissipation of the laser pulse. Analytical and numerical analysis shows that, as the dissipation is increased, the leading edge of the pulse decreases more slowly than the rear, resulting in a pulse shortening.

Three dimensional test particle simulations are applied to optimization of the plasma-channeled laser wakefield accelerator (LWFA) operating in a weakly nonlinear regime. Electron beam energy spread, emittance, and luminosity depend upon the proportion of the electron bunch size to the plasma wavelength. This proportion tends to improve with the laser wavelength increase. We simulate a prospective two-stage ∼1GeV LWFA with controlled energy spread and emittance. The input parameters correspond to realistic capabilities of the BNL Accelerator Test Facility that features a picosecond-terawatt CO₂ laser and a high-brightness electron gun.

The energy penetration depth of a short (100 fs) Ti-sapphire laser pulse (0.8 μm) of intensity 3×10¹⁶ W/cm², in solid density materials has been measured. High-Z (BaF₂) and low-Z (MgF₂) solid layers targets were used. The penetration depth was determined from the measurement of the x-ray emission spectra, as a function of the target thickness. The investigation of these spectra showed that in the low-Z case, solid density material to a depth of 50 nm was heated to a peak electron temperature of ∼150 eV. For the high-Z material, the penetration depth corresponding to this temperature exceeded 100 nm. This is evidence of a larger heat penetration depth in a high-Z material in comparison to a low-Z material. A model based on electron heat conduction is used to estimate the energy penetration depth. It is suggested that the larger heat penetration in high-Z material is due to heating of the material, caused by the radiation flux, generated by the electron heat conduction.

The self-modulational instability of a relatively long laser pulse with a power close to or less than the critical power for relativistic self-focusing in plasma is considered. Strong wake-field excitation occurs as the result of a correlated transverse and longitudinal evolution of the pulse. The dependence of the magnitude of the plasma wave on the duration of a flat-top pulse is investigated. The power necessary to reach 20% electron density modulation behind the laser pulse is shown to decrease as the pulse duration increases, while the phase velocity of the plasma wave remains close to the group velocity of the laser pulse. This provides an opportunity to operate a laser wake-field acceralator in the self-modulated regime at a subcritical laser power, at least twice less than the critical one, and to obtain a sufficiently large accelerating gradient (>20 GV/m) in a region which is longer than that required for the acceleration of an ultrarelativistic particle.