E. J. Divall

A 1 TW KrF laser using chirped pulse amplification

Abstract Chirped pulse amplification (CPA) and recompression have been used in a large aperture KrF laser system. The power focused onto target in a 300 fs pulse reached 1 TW with an irradiance of ≈ 10 19 W/cm 2 .

Relativistic plasma surfaces as an efficient second harmonic generator

We report on the characterization of the specular reflection of 50fs laser pulses in the intensity range 10 17 -10 21 Wcm 2 obliquely incident with p-polarization onto solid density plasmas. These measurements show that the absorbed energy fraction remains approximately constant and that second harmonic generation (SHG) achieves efficiencies of 22±8% for intensities approaching 10 21 Wcm 2 . A simple model based on the relativistic oscillating mirror concept reproduces the observed intensity scaling, indicating that this is 8 Author to whom any correspondence should be addressed.

Titania—a 1020 W cm−2 ultraviolet laser

The Titania laser system, based around a 42 cm e-beam pumped KrF amplifier, is currently being installed at the Rutherford Appleton Laboratory and will come on line as a user facility in 1996. Like Sprite, its predecessor, it will operate in both CPA (249 nm) and Raman (268 nm) short-pulse modes, delivering up to 10 TW to target in high-quality beams. With brightness expected to reach 10 21 W cm -2 sterad -1 , it will be the worlds brightest ultraviolet laser. The design of the Titania system includes a number of novel features. The multi-pass Ti :sapphire front-end amplifier uses an unusual beam-folding scheme. The Raman system will involve the first application of Raman multiplexing, combining high KrF efficiency with low transport cost. Reflective coatings with very high damage thresholds are being developed for the CPA compressor gratings and the UV transport optics. A windowless configuration for the final Raman amplifier is presently under analysis, to allow the performance of this maximally stressed component to be extended substantially. Finally the design of the Titania e-beam machine, featuring novel split-cathode diodes, has resulted in unusually high efficiency of electron transport into the laser gas. The lasers infrastructure has involved sophisticated mechanical and electrical design, and a computerized diagnostic, control and safety package is being developed to allow one-man operation of the whole 1000 m 2 installation.

Investigation of the role of plasma channels as waveguides for laser-wakefield accelerators

The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.

Slow protons as a signature of zero-photon dissociation of H2+ in intense laser fields

We have observed protons with near-zero kinetic energy that originate from the interaction of a frequency tripled (λ = 266 nm), 250 fs intense Ti:sapphire laser with H2 molecules. These protons are tentatively ascribed to zero-photon dissociation (ZPD) of H2+. Earlier quantum mechanical calculations show that such a process should occur due to the partial escape of the vibrational wavepacket that is trapped in the laser-induced, adiabatic well above the one-photon crossing of dressed Floquet states. The fact that slow protons are also observed at the second harmonic (λ = 400 nm), but to a lesser extent, is consistent with the ZPD explanation.

Ultrahigh-brightness KrF laser system for fast ignition studies

The main requirements for a fast igniter laser beam are reviewed and shown to favour short wavelength and ultrahigh brightness. These requirements are met by the new KrF laser system at Rutherford Appleton Laboratory called TITANIA. TITANIA uses two schemes to enhance the laser beam brightness. The first is chirped pulse amplification which is used to enhance brightness by compressing the pulse into the femtosecond region. In this mode TITANIA produces in the region of 250 mJ on target in 700 fs. The second mode of operation uses a Raman technique for beam combining and beam clean-up which is designed to give a single beam of 80 joules on target in a pulselength of 60 ps. In this scheme the KrF wavelength is Raman shifted to 268 nm. The Raman amplifiers will use gaseous rather than solid windows and experiments which demonstrate their feasibility will be described. A concept for a reactor scale fast igniter beam using the Raman technique will be discussed.

Observation of a long-wavelength hosing modulation of a high-intensity laser pulse in underdense plasma.

We report the first experimental observation of a long-wavelength hosing modulation of a high-intensity laser pulse. Side-view images of the scattered optical radiation at the fundamental wavelength of the laser reveal a transverse oscillation of the laser pulse during its propagation through underdense plasma. The wavelength of the oscillation λ(hosing) depends on the background plasma density n(e) and scales as λ(hosing)∼n(e)(-3/2). Comparisons with an analytical model and two-dimensional particle-in-cell simulations reveal that this laser hosing can be induced by a spatiotemporal asymmetry of the intensity distribution in the laser focus which can be caused by a misalignment of the parabolic focusing mirror or of the diffraction gratings in the pulse compressor.

Geometry- and diffraction-independent ionization probabilities in intense laser fields : Probing atomic ionization mechanisms with effective intensity matching

We report an experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW/cm{sup 2} to 10 PW/cm{sup 2} using effective intensity matching (EIM), which is coupled with intensity selective scanning (ISS) to recover the geometry-independent probability of ionization. Such measurements, made possible by quantifying diffraction effects in the laser focus, are compared directly to theoretical predictions of multiphoton, tunnel and field ionization, and a remarkable agreement demonstrated. EIM-ISS allows the straightforward quantification of the probability of recollision ionization in a linearly polarized laser pulse. Furthermore, the probability of ionization is discussed in terms of the Keldysh adiabaticity parameter {gamma}, and the influence of the precursor ionic states present in recollision ionization is observed.

Study of near-GeV acceleration of electrons in a non-linear plasma wave driven by a self-guided laser pulse

Electrons are accelerated up to 0.8 GeV in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5 × 1018 cm−3 by a 10 J, 55 fs, 800 nm laser pulse in the blow-out regime. The electron beam stability is correlated with the laser focal spot pointing stability and depends on the target alignment. Variations of the laser pulse energy, focal spot size and energy within the full width at half maximum have little effect on the electron beam profile (stability) but impact the electron energy (stability). The peak electron energy is higher than expected for the initial vacuum intensity. This is evidence for intensity amplification which also explains the observation of polyenergetic beamlets.

Observing time-dependent vibrational quantum dynamics in deuterium hydride molecular ions

Few-cycle laser pulses are used to “pump and probe” image the vibrational wavepacket dynamics of a HD+ molecular ion. The quantum dephasing and revival structure of the wavepacket are mapped experimentally with time-resolved photodissociation imaging. The motion of the molecule is simulated using a quantum-mechanical model predicting the observed structure. The coherence of the wavepacket is controlled by varying the duration of the intense laser pulses. By means of a Fourier transform analysis both the periodicity and relative population of the vibrational states of the excited molecular ion have been characterized.