A novel direct core heating fusion process is introduced, in which a preimploded core is predominantly heated by energetic ions driven by LFEX, an extremely energetic ultrashort pulse laser. Consequently, we have observed the D(d,n)^{3}He-reacted neutrons (DD beam-fusion neutrons) with the yield of 5×10^{8} n/4π sr. Examination of the beam-fusion neutrons verified that the ions directly collide with the core plasma. While the hot electrons heat the whole core volume, the energetic ions deposit their energies locally in the core, forming hot spots for fuel ignition. As evidenced in the spectrum, the process simultaneously excited thermal neutrons with the yield of 6×10^{7} n/4π sr, raising the local core temperature from 0.8 to 1.8 keV. A one-dimensional hydrocode STAR 1D explains the shell implosion dynamics including the beam fusion and thermal fusion initiated by fast deuterons and carbon ions. A two-dimensional collisional particle-in-cell code predicts the core heating due to resistive processes driven by hot electrons, and also the generation of fast ions, which could be an additional heating source when they reach the core. Since the core density is limited to 2 g/cm^{3} in the current experiment, neither hot electrons nor fast ions can efficiently deposit their energy and the neutron yield remains low. In future work, we will achieve the higher core density (>10 g/cm^{3}); then hot electrons could contribute more to the core heating via drag heating. Together with hot electrons, the ion contribution to fast ignition is indispensable for realizing high-gain fusion. By virtue of its core heating and ignition, the proposed scheme can potentially achieve high gain fusion.

The surface shape and optical thickness variation of a lithium niobate (LNB) wafer were measured simultaneously using a wavelength-tuning interferometer with a new phase-shifting algorithm. It is necessary to suppress the harmonic signals for testing a highly reflective sample such as a crystal wafer. The LNB wafer subjected to polishing, which is in optical contact with a fused-silica (FS) supporting plate, generates six different overlapping interference fringes. The reflectivity of the wafer is typically 15%, yielding significant harmonic signals. The new algorithm can flexibly select the phase-shift interval and effectively suppress the harmonic signals and crosstalk. Experimental results indicated that the optical thickness variation of the LNB wafer was measured with an accuracy of 2 nm.

The first use of a high-repetition-rate laser-diode (LD)-pumped laser in a fusion target experiment is demonstrated. An LD-pumped Nd-solid state laser's output is coupled to a Ti:sapphire laser, enabling the resulting HAMA laser to generate 2-J, 815-nm-wavelength output with a pulse width of 150 fs and a repetition rate of 10 Hz. A photon-to-photon efficiency of 1.25% (electric-to-photonic 0.7%) is achieved, which is an order of magnitude higher than that of current flash-lamp lasers. Irradiation of a 500-µm-thick deuterated polystyrene film by a 0.6 -J pulse yielded 10 5 DD fusion neutrons. The efficiency from the electric input to the neutron yield is 10 times higher than the flash-lamp-pumped table-top lasers. The National Ignition Facility (NIF) [1] is expected to begin fuel burning soon. However, the flash-lamp-pumped lasers typically used are unsuitable for power plants, because the electric-to-photonic conversion efficiency of the flash lamp is less than 0.5%. A power plant requires a laser with an efficiency of more than 10%. The solution is the use of high-repetition-rate laser-diode (LD)-pumped lasers [2][3][4][5][6]. Here we demonstrate the first use of an LD-pumped laser, HAMA, in fusion target experiments. HAMA generated 2 J with a pulse width of 150 fs at 10 Hz and its efficiency was 1.25%. Irradiation of a 500-µm-thick deuterated polystyrene film by a 0.6-J HAMA pulse yielded 10 5 DD fusion neutrons [7]. The efficiency from the electric input to the yield is 8×10 5 neutrons/kW, 10 times higher than those using flash-lamp-pumped tabletop lasers [8][9][10]. The results will also assist in developing industrial and commercial neutron sources. The present result is still far from a true working accelerator sources (a neutron generator). Typically a deuteron beam of 1 mA generates 10 9 n/s, whereas, our results show 10 5 n/s from 15 nC, corresponding to 7×10 9 n/mA. However, since current neutron sources using accelerators are too big to meet industrial demands, a compact neutron source using a laser must have an advantage over the rf sources.author's e-mail: kitagawa@gpi.ac.jp Figure 1 shows the key issues together with the roadmap for achieving an inertial confinement fusion (ICF) power plant [11]. Soon the National Ignition Facility (NIF) Other regions indicate the main path using repetitive-shot mode toward a fusion power plant. This main path is divided into three phases: engineering test and neutron production(zeroth or pre-phase), break-even machine (first phase), and demonstration (second phase). Yellow boxes indicate problems that need to be resolved. We focuse here on the first step toward achieving the main path, by a star.

Inverse Compton X-rays from laser-accelerated multiple electron bunches are observed. A Ti:sapphire laser (pulse energy: 500 mJ; pulse width: 150 fs) beam is divided into two beams. The main beam is focused onto an edge of a helium gas jet to accelerate electrons to energies of 14 and 23 MeV, which inversely scattered the head-on colliding secondary laser beam into 6 and 12 keV X-rays; this agrees well with that calculated from the electron spectra obtained. This demonstrates a first on-axis inverse Compton scattering X-ray energy detection beyond 10 keV induced by laser-accelerated electrons.

A tailored-pulse-imploded core with a diameter of 70  μm is flashed by counterirradiating 110 fs, 7 TW laser pulses. Photon emission (>40  eV) from the core exceeds the emission from the imploded core by 6 times, even though the heating pulse energies are only one seventh of the implosion energy. The coupling efficiency from the heating laser to the core using counterirradiation is 14% from the enhancement of photon emission. Neutrons are also produced by counterpropagating fast deuterons accelerated by the photon pressure of the heating pulses. A collisional two-dimensional particle-in-cell simulation reveals that the collisionless two counterpropagating fast-electron currents induce mega-Gauss magnetic filaments in the center of the core due to the Weibel instability. The counterpropagating fast-electron currents are absolutely unstable and independent of the core density and resistivity. Fast electrons with energy below a few MeV are trapped by these filaments in the core region, inducing an additional coupling. This might lead to the observed bright photon emissions.