The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser–plasma interaction and electron transport numerical simulations.

Neutron emission as high as 10 7 is observed when a high intensity (a few 10 19 W͞cm 2 ) subpicosecond laser pulse at 529 nm wavelength is focused on a deuterated polyethylene target. Neutron emission is also measured in different directions. The emission of neutrons along the laser axis is higher than in the transverse direction. Nonisotropic emission is consistent with neutrons generated by D͑d, n͒-3 He reaction for 0.3 -1 MeV deuterons accelerated in the direction of the laser beam. The energy transferred to the ions is roughly estimated and compared with the energy carried out by the electrons. [S0031-9007(98)08299-4] PACS numbers: 52.40.Nk, 52.50.Jm, 52.70.Nc The development of chirped pulse amplification has made possible the generation of energetic subpicosecond laser pulses [1]. The interaction of the laser pulse with a target generates energetic particles, like MeV electrons and ions [2]. The fast ignitor concept [3], relevant to the inertial confinement fusion (ICF), enhances the interest in this process: Hot particles could heat to thermonuclear temperature an already compressed deuterium-tritium fuel.High-intensity subpicosecond laser pulses produce fast neutrons when they interact with a deuterated target [4]. Hot deuterium ions create neutrons from D͑d, n͒-3 He reaction. Measurements of this neutron emission is a useful method to diagnose fast ions (in the keV to MeV range) generated by the interaction of the laser with the target. Particle-in-cell (PIC) calculations show that highenergy ions are accelerated by a shock wave propagating inside the target [5][6][7]. It knocks the ions along the direction of the laser propagation [8], but collisions stop the ions in the thickness of the target. Neutron emission can identify these ions which cannot be directly measured.Here, we report neutron emission from a deuterated polyethylene target irradiated with a subpicosecond 529 nm laser. The focused intensity is a few 10 19 W͞cm 2 . The experimental conditions are similar to other experiments [1,9]. The laser system provides a chirped pulse with energies up to 30 J at the fundamental wavelength of 1.058 mm. After compression by a pair of diffraction gratings, the pulse duration, measured by an autocorrelation method, is routinely 400 fs. A KH 2 PO 4 (KDP) crystal is used to convert the laser beam to 529 nm with an efficiency of 70%. The pulse duration is less than 300 fs. An f͞3 off-axis parabolic mirror focuses the laser pulse to a 5-mm-diameter spot containing about 30% of the total energy. The highest intensity is 3.5 3 10 19 W͞cm 2 for a laser energy of 7 J at 529 nm.At 1.058 mm, the contrast ratio measured at a few tens of picosecond before the main pulse by a third order cross correlator is 10 8 . Three dichroic mirrors located after the KDP crystal increase the contrast ratio to 10 12 at 529 nm. The optical intensity before the pulse is insufficient to ionize the target, so the pulse interacts directly with the solid target and not with a plasma.The target is made from deuterated polyethylene powd...

The National Ignition Facility (NIF) successfully completed its first inertial confinement fusion (ICF) campaign in 2009. A neutron time-of-flight (nTOF) system was part of the nuclear diagnostics used in this campaign. The nTOF technique has been used for decades on ICF facilities to infer the ion temperature of hot deuterium (D(2)) and deuterium-tritium (DT) plasmas based on the temporal Doppler broadening of the primary neutron peak. Once calibrated for absolute neutron sensitivity, the nTOF detectors can be used to measure the yield with high accuracy. The NIF nTOF system is designed to measure neutron yield and ion temperature over 11 orders of magnitude (from 10(8) to 10(19)), neutron bang time in DT implosions between 10(12) and 10(16), and to infer areal density for DT yields above 10(12). During the 2009 campaign, the three most sensitive neutron time-of-flight detectors were installed and used to measure the primary neutron yield and ion temperature from 25 high-convergence implosions using D(2) fuel. The OMEGA yield calibration of these detectors was successfully transferred to the NIF.

The design of plasma diagnostics for the future MJ class lasers (LMJ–Laser MégaJoule—in France or NIF—National Ignition Faciliy— in the USA) must take into account the large increased radiation field generated at the target and the effect on the diagnostics components. These facilities will focus up to 1.8 MJ ultraviolet laser light energy into a volume of less than 1 cm3 in a few nanoseconds. This very high power focused onto a small target will generate a large amount of x rays, debris, shrapnel, and nuclear particles (neutrons and gamma rays) if the DT fuel capsules ignite. Ignition targets will produce a million more of 14 MeV neutrons (1019 neutrons) by comparison with the present worldwide most powerful laser neutron source facility at OMEGA. Under these harsh environmental conditions the survivability goal of present diagnostic is not clear and many new studies must be carried out to verify which diagnostic measurement techniques, can be maintained, adapted or must be completely changed. Synergies with similar environment studies conducted for magnetic fusion diagnostic design for ITER facility are considered and must be enhanced.