[en] Ultrashort pulses of uv and soft x-ray radiation with durations ranging from femtoseconds to attoseconds can be produced as high-order harmonics of the fundamental frequency of a laser beam focused into gas. Applications to fields such as spectroscopy and attosecond metrology require the control and characterization of spectral and spatial properties of the emitted radiation. These are determined by both single atom and macroscopic response of the interaction medium to the laser field. Here we present evidence that microscopic effects have a larger influence than previously thought, and can induce a splitting and a frequency shift of the harmonic lines. These results not only offer a direct diagnostic for high-order harmonic generation, but also enable us to better tune the parameters of the produced radiation, while giving a deeper insight into the fundamental physics underlying this nonlinear optical process

[en] Time of flight (TOF) distribution of YO in laser ablated plasma from superconducting sample (YBa₂Cu₃O₇) is studied using emission spectroscopy. Up to a short distance (<1.5 cm) from the target, TOF distributions show twin peaks. At greater distances (>1.5 cm) all of them exhibit single peak distribution. (author). 3 refs., 3 figs

[en] Gas-filled capillary discharge waveguides are commonly used as media for plasma wakefield accelerators. We show that effective waveguides can be manufactured using a femtosecond laser micromachining technique to produce a linearly tapered plasma density, which enables the energy of the accelerator to be enhanced significantly. A laser guiding efficiency in excess of 82% at sub-relativistic intensities has been demonstrated in a 40 mm long capillary with a diameter tapering from 320 μm to 270 μm, which gives rise to an on-axis, time-averaged plasma density that varies from 1.0 x 10¹⁸ cm^-3 to 1.6 x 10¹⁸ cm^-3.

[en] The development, understanding and application of laser-driven particle accelerators require accurate measurements of the beam properties, in particular emittance, energy spread and bunch length. Here we report measurements and simulations showing that laser wakefield accelerators can produce beams of quality comparable to conventional linear accelerators.

[en] Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5x10¹⁵ A m^-1 rad^-1 makes it suitable for compact XUV FELs.

[en] Raman amplification in plasma has been proposed to be a promising method of amplifying short radiation pulses. Here, we investigate chirped pulse Raman amplification (CPRA) where the pump pulse is chirped and leads to spatiotemporal distributed gain, which exhibits superradiant scaling in the linear regime, usually associated with the nonlinear pump depletion and Compton amplification regimes. CPRA has the potential to serve as a high-efficiency high-fidelity amplifier/compressor stage.

[en] We present an all solid-state, high voltage pulsed power supply for inducing stable plasma formation (density ∼10¹⁸ cm^-3) in gas-filled capillary discharge waveguides. The pulser (pulse duration of 1 μs) is based on transistor switching and wound transmission line transformer technology. For a capillary of length 40 mm and diameter 265 μm and gas backing pressure of 100 mbar, a fast voltage pulse risetime of 95 ns initiates breakdown at 13 kV along the capillary. A peak current of ∼280 A indicates near complete ionization, and the r.m.s. temporal jitter in the current pulse is only 4 ns. Temporally stable plasma formation is crucial for deploying capillary waveguides as plasma channels in laser-plasma interaction experiments, such as the laser wakefield accelerator.

[en] Electron beams from laser-plasma wakefield accelerators have low transverse emittance, comparable to those from conventional radio frequency accelerators, which highlights their potential for applications, many of which will require the use of quadrupole magnets for optimal electron beam transport. We report on characterizing electron bunches where double bunches are observed under certain conditions. In particular, we present pepper-pot measurements of the transverse emittance of 120–200 MeV laser wakefield electron bunches after propagation through a triplet of permanent quadrupole magnets. It is shown that the normalized emittance at source can be as low as 1 π mm mrad (resolution limited), growing by about five times after propagation through the quadrupoles due to beam energy spread. The inherent energy-dependence of the magnets also enables detection of double electron bunches that could otherwise remain unresolved, providing insight into the self-injection of multiple bunches. The combination of quadrupoles and pepper-pot, in addition, acts as a diagnostic for the alignment of the magnetic triplet. (paper)

[en] We demonstrate the efficient generation of 4 mm and 8 mm long plasma waveguides in hydrogen and helium. These waveguides have matching spots sizes for 13 to 34 μm laser beams. The plasma waveguides are created by ultra-short laser pulses (sub-picosecond) of moderate intensities, ∼10¹⁵–10¹⁶ W cm⁻², that heat the plasma to initial temperatures of tens of eV in order to create a hot plasma column that will expand into a plasma waveguide. We have determined that the main heating mechanism when using fs laser pulses and plasma densities ∼10^18–19 cm⁻³ is Above Threshold Ionization. Detailed time and space electron density measurements are presented for the laser produced plasma waveguides

[en] The laser–plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator. (paper)