M. Nakatsutsumi

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

Space and time resolved measurements of the heating of solids to ten million kelvin by a petawatt laser

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32–0.73 PW and intensities of up to 4×1020 W cm−2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ~107 K (kT/e~ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15–30%, and tends to increase with laser energy.

Measurements of fast electron scaling generated by petawatt laser systems

Fast electron energy spectra have been measured for a range of intensities between 1018 and 1021Wcm−2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5to5ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p-polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions.

Reentrant cone angle dependence of the energetic electron slope temperature in high-intensity laser-plasma interactions

Energy spectra of fast electrons, generated when high-intensity laser pulses irradiated hollow conical targets, have been measured experimentally. It is shown here that the slope temperature of the fast electrons is strongly dependent on the opening angle of the cone, and has a maximum value at 25°. The data confirms optical guiding of the laser pulse, by comparison of the measured electron temperature with ray-tracing calculations that include absorption in plasmas. The enhanced energy flow and intensity induced by optical guiding of the laser pulse inside the cone as a function of the opening angle as well as the f-number of the focusing optics is discussed.

Correlation between laser accelerated MeV proton and electron beams using simple fluid model for target normal sheath acceleration

High density energetic electrons that are created by intense laser plasma interactions drive MeV proton acceleration. The correlation between accelerated MeV protons and escaped electrons is experimentally investigated at laser intensities in the range of 1018–1019 W/cm2 with S-polarization. Observed proton maximum energies are linearly proportional to escaped electron slope temperatures with a scaling coefficient of about 10. In the context of the simple analytical fluid model for transverse normal sheath acceleration, hot electron sheath density near the target rear surface can be estimated if an empirical acceleration time is assumed.

Recent experiments on electron transport in high-intensity laser matter interaction

We present the results of some recent experiments performed at the LULI laboratory using the 100 TW laser facility concerning the study of the propagation of fast electrons in gas and solid targets. Novel diagnostics have been implemented including chirped shadowgraphy and proton radiography. Proton radiography images did show the presence of very strong fields in the gas probably produced by charge separation. In turn these imply a slowing down of the fast electron cloud as it penetrates in the gas and a strong inhibition of propagation. Indeed chirped shadowgraphy images show a strong reduction in time of the velocity of the electron cloud from the initial value, which is of the order of a fraction of c. We also performed some preliminary experiments with cone targets in order to verify the guiding effect and fast electron propagation in presence of the cone. Finally we compared results obtained by changing the target size.Here we only give a first presentation and preliminary analysis of data, which will be addressed in detail in a following paper.

Collimated fast electron beam generation in critical density plasma

Significantly collimated fast electron beam with a divergence angle 10° (FWHM) is observed when an ultra-intense laser pulse (I = 1014 W/cm2, 300 fs) irradiates a uniform critical density plasma. The uniform plasma is created through the ionization of an ultra-low density (5 mg/c.c.) plastic foam by X-ray burst from the interaction of intense laser (I = 1014 W/cm2, 600 ps) with a thin Cu foil. 2D Particle-In-Cell (PIC) simulation well reproduces the collimated electron beam with a strong magnetic field in the region of the laser pulse propagation. To understand the physical mechanism of the collimation, we calculate energetic electron motion in the magnetic field obtained from the 2D PIC simulation. As the results, the strong magnetic field (300 MG) collimates electrons with energy over a few MeV. This collimation mechanism may attract attention in many applications such as electron acceleration, electron microscope and fast ignition of laser fusion.

Heating of solid target in electron refluxing dominated regime with ultra-intense laser

Propagation of electron beams generated in laser-plasma interactions is strongly influenced by self-induced electrostatic fields at target-vacuum interfaces, resulting the refluxing of electrons. We confirmed the refluxing and propagation of electrons with three different kinds of target configurations; thin-wide foil, thin-narrow foil, and long-wire geometry. Enhancement of target heating, effective guiding and collimation of high density MeV electrons were observed.

Self-generated surface magnetic fields inhibit laser-driven sheath acceleration of high-energy protons

High-intensity lasers interacting with solid foils produce copious numbers of relativistic electrons, which in turn create strong sheath electric fields around the target. The proton beams accelerated in such fields have remarkable properties, enabling ultrafast radiography of plasma phenomena or isochoric heating of dense materials. In view of longer-term multidisciplinary purposes (e.g., spallation neutron sources or cancer therapy), the current challenge is to achieve proton energies well in excess of 100 MeV, which is commonly thought to be possible by raising the on-target laser intensity. Here we present experimental and numerical results demonstrating that magnetostatic fields self-generated on the target surface may pose a fundamental limit to sheath-driven ion acceleration for high enough laser intensities. Those fields can be strong enough (~105 T at laser intensities ~1021 W cm–2) to magnetize the sheath electrons and deflect protons off the accelerating region, hence degrading the maximum energy the latter can acquire.Laser-generated ion acceleration has received increasing attention due to recent progress in super-intense lasers. Here the authors demonstrate the role of the self-generated magnetic field on the ion acceleration and limitations on the energy scaling with laser intensity.

Collimated propagation of fast electron beams accelerated by high-contrast laser pulses in highly-resistive shocked-carbon

Collimated transport of ultrahigh intensity electron current was observed in cold and in laser-shocked vitreous carbon, in agreement with simulation predictions. The fast electron beams were created by coupling high-intensity and high-contrast laser pulses onto copper-coated cones drilled into the carbon samples. The guiding mechanism-observed only for times before the shock breakout at the inner cone tip-is due to self-generated resistive magnetic fields of ∼0.5-1  kT arising from the intense currents of fast electrons in vitreous carbon, by virtue of its specific high resistivity over the range of explored background temperatures. The spatial distribution of the electron beams, injected through the samples at different stages of compression, was characterized by side-on imaging of hard x-ray fluorescence.