Longitudinally excited N2 laser with large-diameter discharge tube.

To realize an ultraviolet laser with high beam quality and high output power, we are developing a master oscillator power amplifier system for a longitudinally excited gas laser. In this system, high beam quality is produced by an oscillator stage, and high output power is produced by an amplifier stage. Here, the beam profile of the longitudinally excited gas N2 laser had a quasi-Gaussian shape when using a large-diameter discharge tube, which enabled the use of the amplifier stage. The discharge tube consisted of a dielectric pipe with an inner diameter of 14 mm and a length of 30 cm. The discharge tube was covered with a metallic tube for pre-ionization. The strength and spatial distribution of pre-ionization were optimized by adjusting the gap between the metallic tube and the discharge tube. The uniform discharge achieved by controlling the pre-ionization produced a quasi-Gaussian laser beam profile with a correlation factor of 0.99513, a laser pulse energy of 343 µJ, and an input energy of 2.95 J.

Heating efficiency evaluation with mimicking plasma conditions of integrated fast-ignition experiment.

A series of experiments were carried out to evaluate the energy-coupling efficiency from heating laser to a fuel core in the fast-ignition scheme of laser-driven inertial confinement fusion. Although the efficiency is determined by a wide variety of complex physics, from intense laser plasma interactions to the properties of high-energy density plasmas and the transport of relativistic electron beams (REB), here we simplify the physics by breaking down the efficiency into three measurable parameters: (i) energy conversion ratio from laser to REB, (ii) probability of collision between the REB and the fusion fuel core, and (iii) fraction of energy deposited in the fuel core from the REB. These three parameters were measured with the newly developed experimental platform designed for mimicking the plasma conditions of a realistic integrated fast-ignition experiment. The experimental results indicate that the high-energy tail of REB must be suppressed to heat the fuel core efficiently.

Comparison of modified driver circuit and capacitor-transfer circuit in longitudinally excited N2 laser.

We developed a modified driver circuit composed of a capacitance and a spark gap, called a direct-drive circuit, for a longitudinally excited gas laser. The direct-drive circuit uses a large discharge impedance caused by a long discharge length of the longitudinal excitation scheme and eliminates the buffer capacitance used in the traditional capacitor-transfer circuit. We compared the direct-drive circuit and the capacitor-transfer circuit in a longitudinally excited N2 laser (wavelength: 337 nm). Producing high output energy with the capacitor-transfer circuit requires a large storage capacitance and a discharge tube with optimum dimensions (an inner diameter of 4 mm and a length of 10 cm in this work); in contrast, the direct-drive circuit requires a high breakdown voltage, achieved with a small storage capacitance and a large discharge tube. Additionally, for the same input energy of 792 mJ, the maximum output energy of the capacitor-transfer circuit was 174.2 µJ, and that of the direct-drive circuit was 344.7 µJ.

Temperature dependence of laser-induced damage threshold of optical coatings at different pulse widths.

The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser pulses from 123 K to 473 K at different temperature. For pulses longer than a few picoseconds, the laser-induced damage threshold of coated substrates increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to explain the differences in the temperature dependence.

Pulse compression and beam focusing with segmented diffraction gratings in a high-power chirped-pulse amplification glass laser system.

Segmented (tiled) grating arrays are being intensively investigated for petawatt-scale pulse compression due to the expense and technical challenges of fabricating monolithic diffraction gratings with apertures of over 1m. However, the considerable freedom of motion among grating segments complicates compression and laser focusing. We constructed a real compressor system using a segmented grating for an 18cm aperture laser beam of the Gekko MII 100TW laser system at Osaka University. To produce clean pulse shapes and single focal spots tolerant of misalignment and groove density difference of grating tiles, we applied a new compressor scheme with image rotation in which each beam segment samples each grating segment but from opposite sides. In high-energy shots of up to 50J, we demonstrated nearly Fourier-transform-limited pulse compression (0.5ps) with an almost diffraction-limited spot size (20microm).

Split-aperture laser pulse compressor design tolerant to alignment and line-density differences.

We introduce a four-pass laser pulse compressor design based on two grating apertures with two gratings per aperture that is tolerant to some alignment errors and, importantly, to grating-to-grating period variations. Each half-beam samples each grating in a diamond-shaped compressor that is symmetric about a central bisecting plane. For any given grating, the two half-beams impinge on opposite sides of its surface normal. It is shown that the two split beams have no pointing difference from paired gratings with different periods. Furthermore, no phase shift between half-beams is incurred as long as the planes containing a grating line and the surface normal for each grating of the pair are parallel. For grating pairs satisfying this condition, gratings surfaces need not be on the same plane, as changes in the gap between the two can compensate to bring the beams back in phase.

Longitudinally excited N(2) lasers without high-voltage switches.

We have developed novel excitation circuits without high-voltage switches for two longitudinally excited N(2) lasers (wavelength: 337 nm). One uses a single tube without a trigger and the other uses a tandem tube with a trigger. In both systems, the discharge tube acts as a switch. In the single-tube system, the laser output energy was 125.8 microJ and the efficiency was 0.16% at 18 Torr (2.4 kPa) when a slow-rising voltage pulse of -28 kV was applied (rise time: 21.3 micros). In the tandem-tube system, the laser output energy was 259.4 microJ and the efficiency was 0.11% at 18 Torr when a slow-rising voltage pulse of -48 kV was applied (rise time: 27 micros).