Dissociation of H2+ from a short, intense, infrared laser pulse: proton emission spectra and pulse calibration

The proton energy spectrum from photodissociation of the hydrogen molecular ion by short intense pulses of infrared light is calculated. The time-dependent Schrödinger equation is discretized and integrated. For few-cycle pulses one can resolve vibrational structure, arising from the experimental preparation of the molecular ion. We calculate the corresponding energy spectrum and analyse the dependence on the pulse time delay, pulse length and intensity of the laser for ? ~ 790 nm. We conclude that the proton spectrum is a sensitive probe of both the vibrational populations and phases, and allows us to distinguish between adiabatic and nonadiabatic dissociation. Furthermore, the sensitivity of the proton spectrum from H2+ is a practical means of calibrating the pulse. Our results are compared with recent measurements of the proton spectrum for 65 fs pulses using a Ti:Sapphire laser (? ~ 790 nm) including molecular orientation and focal-volume averaging. Integrating over the laser focal volume, for the intensity I ~ 3 × 1015 W cm-2, we find our results are in excellent agreement with these experiments.

Dynamic control of laser-produced proton beams

The emission characteristics of intense laser driven protons are controlled using ultrastrong (of the order of 10(9) V/m) electrostatic fields varying on a few ps time scale. The field structures are achieved by exploiting the high potential of the target (reaching multi-MV during the laser interaction). Suitably shaped targets result in a reduction in the proton beam divergence, and hence an increase in proton flux while preserving the high beam quality. The peak focusing power and its temporal variation are shown to depend on the target characteristics, allowing for the collimation of the inherently highly divergent beam and the design of achromatic electrostatic lenses.

Proton Probing Measurement of Electric and Magnetic Fields Generated by ns and ps Laser-Matter Interactions

The use of laser-accelerated protons as a particle probe for the detection of electric fields in plasmas has led in recent years to a wealth of novel information regarding the ultrafast plasma dynamics following high intensity laser-matter interactions. The high spatial quality and short duration of these beams have been essential to this purpose. We will discuss some of the most recent results obtained with this diagnostic at the Rutherford Appleton Laboratory (UK) and at LULI - Ecole Polytechnique (France), also applied to conditions of interest to conventional Inertial Confinement Fusion. In particular, the technique has been used to measure electric fields responsible for proton acceleration from solid targets irradiated with ps pulses, magnetic fields formed by ns pulse irradiation of solid targets, and electric fields associated with the ponderomotive channelling of ps laser pulses in under-dense plasmas.

Laser-Driven Proton Beams: Acceleration Mechanism, Beam Optimization, and Radiographic Applications

This paper reviews recent experimental activity in the area of optimization, control, and application of laser accelerated proton beams, carried out at the Rutherford Appleton Laboratory and the Laboratoire pour l’Utilisation des Lasers Intenses 100 TW facility in France. In particular, experiments have investigated the role of the scale length at the rear of the plasma in reducing target-normal-sheath-acceleration acceleration efficiency. Results match with recent theoretical predictions and provide information in view of the feasibility of proton fast-ignition applications. Experiments aiming to control the divergence of the proton beams have investigated the use of a laser-triggered microlens, which employs laser-driven transient electric fields in cylindrical geometry, enabling to focus the emitted
protons and select monochromatic beam lets out of the broad spectrum beam. This approach could be advantageous in view
of a variety of applications. The use of laser-driven protons as a particle probe for transient field detection has been developed and
applied to a number of experimental conditions. Recent work in this area has focused on the detection of large-scale self-generated magnetic fields in laser-produced plasmas and the investigation of fields associated to the propagation of relativistic electron both on the surface and in the bulk of targets irradiated by high-power laser pulses.

Magnetic field measurements in laser-produced plasmas via proton deflectometry

Large magnetic fields generated during laser-matter interaction at irradiances of ~ 5×1014 W?cm-2 have been measured using a deflectometry technique employing MeV laser-accelerated protons. Azimuthal magnetic fields were identified unambiguously via a characteristic proton deflection pattern and found to have an amplitude of ~ 45 T in the outer coronal region. Comparison with magnetohydrodynamic simulations confirms that in this regime the mathTe×mathne source is the main field generation mechanism, while additional terms are negligible.

Solid-bound, proton-driven, fluorescent ‘off–on–off’ switches based on PET (photoinduced electron transfer)

Anthracene-based, H+-driven, ‘off–on–off’ fluorescent PET (photoinduced electron transfer) switches are immobilized on organic and inorganic polymeric solids in the form of Tentagel® and silica, respectively. The environment of the organic bead displaces apparent switching thresholds towards lower pH values whereas the Si–O- groups of silica electrostatically cause the opposite effect. These switches are ternary logic gate tags, one of which can be particularly useful in strengthening molecular computational identification (MCID) of small solid objects.

Application of proton radiography in experiments of relevance to inertial confinement fusion

Multi-Mev proton beams generated by target normal sheath acceleration (TNSA) during the interaction of an ultra intense laser beam (Ia parts per thousand yen10(19) W/cm(2)) with a thin metallic foil (thickness of the order of a few tens of microns) are particularly suited as a particle probe for laser plasma experiments. The proton imaging technique employs a laser-driven proton beam in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in such experiments. The proton probing technique has been applied in experiments of relevance to inertial confinement fusion (ICF) such as laser heated gasbags and laser-hohlraum experiments. The data provides direct information on the onset of laser beam filamentation and on the plasma expansion in the hohlraum's interior, and confirms the suitability and usefulness of this technique as an ICF diagnostic.