Generation of femtosecond electron bunches and hard-X-rays by ultra-intense laser wake field acceleration in a gas jet

Summary form only given. Femtosecond electron beams and hard X-rays with /spl lambda//spl sim/0.1 nm may find various applications in biology, chemistry, and molecular electronics giving a new time-scale probe analysis. Such short electron beams can be produced in the wake field acceleration by short relativistically intense laser pulses and then Thomson scattering of a second laser pulse can serve for efficient generation of very short X-rays with use of such electron beams. We study experimentally with 12 TW, 50 fs Ti-sapphire laser set-up and numerically through a multidimensional particle-in-cell simulation two mechanisms of generation of femtosecond electron bunches in gas jet suitable for efficient Thomson scattering. The first is the LWFA of electrons injected due to wave-breaking on a shock-wave produced by a laser prepulse in a He gas-jet. This mechanism allows us to produce a narrow-coned electron bunch with duration around 40 fs. Results of measurements agree well with two-dimensional hydrodynamics and particle-in-cell simulations. Such a beam can scatter up to 10/sup 9/ photons per pulse in 1/spl deg/ cone. Spectrum of scattered light is discussed. Because of large energy spread of electrons in a bunch, the X-ray spectrum is broad. To overcome this problem another mechanism, self-injection of plasma electrons, is proposed and studied numerically. The self-injection of plasma electrons which have been accelerated to relativistic energies by a laser pulse moving with a group velocity less than the speed of light appears when a/sub 0//spl ges//spl radic/2(/spl omega///spl omega//sub pl/)/sup 2/3/ where a/sub 0/ is normalized laser field. In contrast to the injection due to wave-breaking processes, self-injection allows extraction of a beam-quality bunch of energetic electrons. This injection is also expected to be useful in generation of very short pulse, /spl sim/10 fs, electron beams with the charge /spl sim/100 pC. The diameter of such a beam in a gas jet after acceleration is only 5-10 /spl mu/m that makes possible the production of high-brightness hard-X-rays with few percent energy spread by using contrary propagating laser pulses. The efficiency and spectrum of such X-rays are calculated and discussed. We also study density effects on the dynamics of a cavity produced in the wake of an ultra-intense (a/sub 0//spl Gt/1) femtosecond laser pulse via 2D particle-in-cell simulation. Formation of a non-breaking cavity is a crucial part of relativistic self-injection of plasma electrons and their further acceleration leading to a beam-quality femtosecond bunch. This self-injection appears in a uniform plasma when the group velocity of the pulse becomes smaller than the maximal electron energy mc/sup 2/a/sub 0//sup 2//2 so that a/sub 0/>(2/sup 1/4//spl omega///spl omega//sub pl/)/sup 2/3/. With increasing density, the total charge of the bunch increases, however, the energy distribution becomes Maxwellian-like; the bunch lengthens due to the effect of the wave-breaking injection and mutual effects on the cavity.