C. A. Cecchetti

Ultrafast laser-driven microlens to focus and energy-select mega-electron volt protons

We present a technique for simultaneous focusing and energy selection of high-current, mega–electron volt proton beams with the use of radial, transient electric fields (107 to 1010 volts per meter) triggered on the inner walls of a hollow microcylinder by an intense subpicosecond laser pulse. Because of the transient nature of the focusing fields, the proposed method allows selection of a desired range out of the spectrum of the polyenergetic proton beam. This technique addresses current drawbacks of laser-accelerated proton beams, such as their broad spectrum and divergence at the source.

Energetic protons generated by ultrahigh contrast laser pulses interacting with ultrathin targets

A regime of laser acceleration of protons, which relies on the interaction of ultrahigh contrast laser pulses with ultrathin targets, has been validated using experiments and simulations. Proton beams were accelerated to a maximum energy of ∼7.3MeV from targets as thin as 30nm irradiated at 1018Wcm−2μm2 (1J, 320fs) with an estimated peak laser pulse to pedestal intensity contrast ratio of 1011. This represents nearly a tenfold increase in proton energy compared to the highest energies obtainable using non contrast enhanced pulses and thicker targets (>5μm) at the same intensity. To obtain similar proton energy with thicker targets and the same laser pulse duration, a much higher laser intensity (i.e., above 1019Wcm−2μm2) is required. The simulations are in close agreement with the experimental results, showing efficient electron heating compared to the case of thicker targets. Rapid target expansion, allowing laser absorption in density gradients, is key to enhanced electron heating and ion acceleration i...

Impulsive electric fields driven by high-intensity laser matter interactions

Theinteractionofhigh-intensitylaserpulseswithmatterreleasesinstantaneouslyultra-largecurrentsofhighlyenergetic electrons, leading to the generation of highly-transient, large-amplitude electric and magnetic fields. We report results of recent experiments in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channeling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation andMeVionfrontexpansionattherearoflaser-irradiatedthinmetallicfoils.Laser-drivenimpulsivefieldsatthesurface of solid targets can be applied for energy-selective ion beam focusing.

Dynamics of Self-Generated, Large Amplitude Magnetic Fields Following High-Intensity Laser Matter Interaction

The dynamics of magnetic fields with an amplitude of several tens of megagauss, generated at both sides of a solid target irradiated with a high-intensity (~10(19) W/cm(2)) picosecond laser pulse, has been spatially and temporally resolved using a proton imaging technique. The amplitude of the magnetic fields is sufficiently large to have a constraining effect on the radial expansion of the plasma sheath at the target surfaces. These results, supported by numerical simulations and simple analytical modeling, may have implications for ion acceleration driven by the plasma sheath at the rear side of the target as well as for the laboratory study of self-collimated high-energy plasma jets.

Observation and characterization of laser-driven Phase Space Electron Holes

The direct observation and full characterization of a phase space electron hole (EH) generated during laser-matter interaction is presented. This structure, propagating in a tenuous, nonmagnetized plasma, has been detected via proton radiography during the irradiation with a ns laser pulse (Iλ2≈1014 W/cm2) of a gold hohlraum. This technique has allowed the simultaneous detection of propagation velocity, potential, and electron density spatial profile across the EH with fine spatial and temporal resolution allowing a detailed comparison with theoretical and numerical models.

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.

Recent results from experimental studies on laser?plasma coupling in a shock ignition relevant regime

Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity Iλ 2 >10 15 Wc m −2 µm 2 at the end of the compression phase. In this intensity regime, laser–plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS). We study the laser–plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 10 13 Wc m −2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 10 15 –10 16 Wc m −2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250–850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target. Preliminary results show that the backscattered energy is in the range 3–15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers.

The application of laser-driven proton beams to the radiography of intense laser–hohlraum interactions

Plasma expansion following the interaction of an intense laser beam with the inner surface of gold hohlraums, emulating conditions relevant to indirect drive inertial confinement fusion (ICF), has been investigated by a radiographic technique which employs a beam of laser-accelerated protons. This probing technique has made it possible to measure the electric field distribution associated with the plasma front and its propagation throughout the interior of the hohlraum with a temporal and spatial resolution of the order of a few ps and μm, respectively. The data indicate that the expanding plasma slows down approaching the opposite walls, possibly due to the interaction with x-ray heated plasma from the non-irradiated walls. The electric field at the plasma front shows a bipolar structure, suggesting the presence of ion-acoustic soliton-like structures cotraveling with the front. Data obtained using enclosed hohlraums suggest the feasibility of this type of diagnosis in gas-filled hohlraums, as currently employed in ICF experiments.

Laser Triggered Micro-Lens for Focusing and Energy Selection of MeV Protons

We present a novel technique for focusing and energy selection of high-current, MeV proton/ion beams. This method employs a hollow micro-cylinder that is irradiated at the outer wall by a high intensity, ultra-short laser pulse. The relativistic electrons produced are injected through the cylinders wall, spread evenly on the inner wall surface of the cylinder, and initiate a hot plasma expansion. A transient radial electric field (10 7 –10 10 V/m) is associated with the expansion. The transient electrostatic field induces the focusing and the selection of a narrow band component out of the broadband poly-energetic energy spectrum of the protons generated from a separate laser irradiated thin foil target that are directed axially through the cylinder. The energy selection is tunable by changing the timing of the two laser pulses. Computer simulations carried out for similar parameters as used in the experiments explain the working of the micro-lens.

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 ∇Te×∇ne source is the main field generation mechanism, while additional terms are negligible.