ADVANCED CONCEPTS IN FAST IGNITION AND THE RELEVANT DIAGNOSTICS

摘要

Recent developments in high power lasers have allowed the study of new regimes of laser-matter interactions relevant to astrophysics, nuclear physics, and fusion energy research. In the context of inertial fusion energy research, the 'fast ignitor; (FI) concept was proposed in order to relax the strict symmetry requirements for the laser irradiation of the spherical target and to reduce the drive energy needed to achieve high densities and the formation of the spark. However, other problems have gradually become clear with this scheme. These are relevant to the temporal and spatial stability of the propagation of the additional heating source, for example the high-energy electron beam, due to losses and deflection of the ultra-intense laser pulse in the surrounding plasma and the transport through a considerable length of a plasma. Here we describe a new compression geometry that eliminates these problems utilizing the cone-guided compression scheme, or the so-called 'advanced fast ignitor'. The attraction of this scheme is that it makes easy to access to fuel core without the temporal and spatial uncertainty. Using this scheme, colleagues from Japan and UK reported the first experimental results of fast heating of the compressed super-solid density matter using an ultra-intense short pulse laser and showed that efficient compression and heating are both possible simultaneously. This new approach provides a route to efficient fusion energy production. It is also important for fast ignitor scheme to understand how to accelerate the electrons and ions using short-pulse intense laser pulses in the overdense plasma and also how the particles propagate into the super-critical plasma from the viewpoint of an additional heating source. Recently there have been many reports in both simulation and experiment that demonstrate a number of different acceleration mechanisms. Some of these suggest the acceleration strongly depend upon the laser and the plasma conditions such as the laser polarization, intensity and the density scale-length. In particular, recent simulations predict that the energy of the accelerated ions suddenly increases with higher laser intensity and that the ion acceleration direction is changed from the longitudinal (or laser) direction to the transverse direction as plasma density scale-length is increased. Therefore, measurement of the accelerated ion momentum distribution helps us to understand the laser-plasma interactions under various controlled conditions. Neutron spectroscopy is one of the most fascinating methods to investigate the accelerated ion distribution processes generated by ultra-intense laser-plasma interactions (using beam-fusion neutron spectroscopy). For this purpose, we are re-constructing the multi-channel neutron spectrometer, LaNSA. The system records the timing when the neutrons are detected at each scintillator in the array. These timings are converted into neutron spectrum via the time-of-flight (TOF) method. Neutron energy spectra taken at several different directions can then be used to determine the momentum distribution of the accelerated ions by taking into account the Doppler shifts of neutron spectra from 2.45MeV for each viewing angle. In this paper, we discuss the re-commissioning of the LaNSA system and the future experiments that are planned for the RAL PW laser system.