Channeling of terawatt laser pulses by use of hollow waveguides

Guiding of laser pulses in plasma waveguides created by linearly-polarized femtosecond laser pulses

We experimentally demonstrate that plasma waveguides produced with ultra-short laser pulses (sub-picosecond) in gas jets are capable of guiding high intensity laser pulses. This scheme has the unique ability of guiding a high-intensity laser pulse in a plasma waveguide created by the same laser system in the very simple and stable experimental setup. A hot plasma column was created by a femtosecond class laser that expands into an on-axis parabolic low density profile suitable to act as a waveguide for high intensity laser beams. We have successfully guided ~10¹⁵ W cm⁻² laser pulses in a 8 mm long hydrogen plasma waveguide with a 35% guiding efficiency.

Clustered gases as a medium for efficient plasma waveguide generation

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.

Guiding of intense laser pulses in plasma waveguides produced from efficient, femtosecond end-pumped heating of clustered gases

We demonstrate intense pulse guiding in efficient femtosecond end-pumped waveguides generated in clustered gases. This novel scheme provides a route to significantly lower on-axis plasma density (< 10(18) cm(-3)) more than is feasible in conventional hydrodynamic plasma waveguides. Self-focused propagation and strong absorption of intense femtosecond laser pulses are used to produce long centimeter scale channels in an argon cluster jet, and a subsequent pulse is guided with 3 x 10(17) W cm(-2) intensity and approximately 50% coupling efficiency. Preliminary results with hydrogen clusters also show guiding.

Electron acceleration in an ultraintense-laser-illuminated capillary

An ultraintense laser injected a 10 J of power at 1.053 microm in 0.5 ps into a glass capillary of 1 cm long and 60 microm in diameter and accelerated plasma electrons to 100 MeV. One- and two-dimensional particle codes describe wakefields with 10 GV/m gradient excited behind the laser pulse, which are guided by a plasma density channel far beyond the Rayleigh range. The blueshift of the laser spectrum supports that a plasma of 10(16) cm(-3) is inside the capillary. A bump at the high energy tail suggests the electron trapping in the wakefield.

Demonstration of a collisionally excited optical-field-ionization XUV laser driven in a plasma waveguide

We describe the first demonstration of a collisionally excited optical-field-ionization laser driven within a waveguide. Lasing on the 4d(9)5d-4d(9)5p transition at 41.8 nm in Xe8+ was observed to be closely correlated to conditions under which the pump laser pulses were guided well by a gas-filled capillary discharge waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.

Guiding of high-intensity laser pulses with a hydrogen-filled capillary discharge waveguide

We report guiding of laser pulses with peak input intensities greater than 10(17) W cm(-2) in 30 mm and 50 mm long H2-filled capillary discharge waveguides. Under conditions producing good guiding the coupling and propagation losses of the waveguide were <4% and (7+/-1) m(-1), respectively. The spectra of the transmitted pulses were not broadened significantly, but were shifted to shorter wavelength. It is concluded that this shift is not associated with significant temporal distortion of the laser pulse.

Laser propagation in cylindrical waveguides

Laser propagation in cylindrical waveguides is studied theoretically, assuming that the guide medium and the internal medium have permittivities and identical permeabilities that are uniform in space and time and independent of the fields. Approximate solutions to the cylindrical dispersion relation are found and compared with numerical solutions. For high refractive indices and small radii the modes are transverse electric and transverse magnetic, as in the loss-less case. As the refractive index is lowered or the radius increased the lower-order modes become hybrid electric and hybrid magnetic, and the lower-order transverse magnetic modes are modified. The higher-order modes, in any waveguide, are always transverse. The transition to hybrid modes is marked by the disappearance of the fundamental electric mode and the appearance of an additional magnetic mode. This mode and the losses of the magnetic modes adjacent to it are only adequately described by numerical solutions. The losses of the transverse modes are accurately reproduced by a simple model based on a mean reflectivity.

Propagation of short intense laser pulses in gas-filled capillaries

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.

Eigenmodes for capillary tubes with dielectric walls and ultraintense laser pulse guiding

The properties of the eigenmodes of a capillary tube are examined in the context of ultrashort intense laser pulse guiding. The dispersion relation for the eigenmodes of a cylindrical hollow waveguide is derived and the family of eigenmodes EH(nus) is shown to be a solution of the wave equation up to the first order under the condition k(0)a >>1, where k(0) is the light wave number and a the capillary tube radius. The expressions of the fields for the eigenmodes are given at zero and first order of a small parameter equal to the ratio of the perpendicular to longitudinal wave number and the absorbed intensity at the wall is estimated.

Simulations of a hydrogen-filled capillary discharge waveguide

A one-dimensional dissipative magnetohydrodynamics code is used to investigate the discharge dynamics of a waveguide for high-intensity laser pulses: the gas-filled capillary discharge waveguide. Simulations are performed for the conditions of a recent experimental measurement of the electron density profile in hydrogen-filled capillaries [D. J. Spence et al., Phys. Rev. E 63, 015401 (R) (2001)], and are found to be in good agreement with those results. The evolution of the discharge in this device is found to be substantially different to that found in Z-pinch capillary discharges, owing to the fact that the plasma pressure is always much higher than the magnetic pressure. Three stages of the capillary discharge are identified. During the last of these the distribution of plasma inside the capillary is determined by the balance between ohmic heating, and cooling due to electron heat conduction. A simple analytical model of the discharge during the final stage is presented, and shown to be in good agreement with the magnetohydrodynamic simulations.

Nonlinear propagation of short intense laser pulses in a hollow metallic waveguide

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.

Simulation and design of stable channel-guided laser wakefield accelerators

Most laser wakefield accelerator (LWFA) experiments to date have operated in the self-modulated (SM) regime and have been self-guided. A channel-guided LWFA operating in the standard or resonant regime is expected to offer the possibility of high electron energy gain and high accelerating gradients without the instabilities and poor electron beam quality associated with the SM regime. Plasma channels such as those produced by a capillary discharge have demonstrated guiding of intense laser pulses over distances of several centimeters. Optimizing the performance in a resonant LWFA constrains the on-axis plasma density in the channel to a relatively narrow range. A scaling model is presented that quantifies resonant LFWA performance in terms of the maximum accelerating gradient, dephasing length, and dephasing-limited energy gain. These performance quantities are expressed in terms of laser and channel experimental parameters, clearly illustrating some of the tradeoffs in the choice of parameters. The predicted energy gain in this model is generally lower than that indicated by simpler scaling models. Simulations agree well with the scaling model in both low and high plasma density regimes. Simulations of a channel-guided, self-modulated LWFA are also presented. Compared with the resonant LWFA regime, the requirements on laser and channel parameters in the SM regime are easier to achieve, and a channel-guided SM-LWFA is likely to be less unstable than a self-guided SM-LWFA.

Basic physics of laser propagation in hollow waveguides

The basic theory of laser propagation in hollow waveguides is considered in the context of laser-plasma physics. The physical model of waves reflecting between the guide walls is used to show that there is a discrete series of modes, and to give the mode dispersion relation and losses in terms of a given reflectivity. The mathematical connection between this model and the solution of Maxwell's equations for lossless propagation in a cylinder is given. Thus the solutions for low loss propagation for any given reflectivity can be obtained, provided it is close to 1. Results are given using Fresnel reflectivity for perfect dielectric and finite conductivity waveguides. The relationship of the breakdown intensity in dielectric waveguides to known breakdown intensities is also derived. The practical implications for the guiding of intense laser pulses and the limitations of the model are discussed. The theory is shown to explain, at least qualitatively, a number of previous experimental results.

Formation of plasma channels in the interaction of a nanosecond laser pulse at moderate intensities with helium gas jets

We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)].

High efficiency guiding of terawatt subpicosecond laser pulses in a capillary discharge plasma channel

Transmission efficiencies in excess of 75% were obtained in the optical guiding of subpicosecond, terawatt laser pulses in a 2-cm-long capillary discharge plasma channel at the Naval Research Laboratory. The guided laser beam size at the exit of the channel was measured using far field imaging and Thomson scattering techniques. The guided laser intensity was >1 x 10(17) W/cm(2) at a guided beam diameter of 35 microm for a propagation length of 22 Rayleigh ranges. There is evidence that the plasma channel extends beyond the ends of the capillary and affects the far field beam structure of the transmitted laser pulse.

Backscattered supercontinuum emission from high-intensity laser-plasma interactions

We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0) > 1) extending from ~500 to >1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.