Shinichi Namba

Parametric amplification of attosecond pulse trains at 11 nm

We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11u2005nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2u2005fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.

Reinjection of transmitted laser light into laser-produced plasma for efficient laser ignition

For improving the laser absorption efficiency in laser ignition, the transmitted laser light was returned to the laser-produced plasma by using a corner cube. In the experiments, the transmitted light was reinjected into the plasma at different times. The laser absorption efficiency was found to be substantially improved when the transmitted light was reinjected into the plasma after adequate plasma expansion. Furthermore, through visualization experiments on gas-dynamics phenomena, it was found that the reinjection of the transmitted light affected not only the laser absorption efficiency but also the gas dynamics after breakdown, and thereby the initial flame kernel development.

Avalanche of stimulated forward scattering in high harmonic generation.

Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.

Construction of a magnetic bottle spectrometer and its application to pulse duration measurement of X-ray laser using a pump-probe method

To characterize the temporal evolution of ultrashort X-ray pulses emitted by laser plasmas using a pump-probe method, a magnetic bottle time-of-flight electron spectrometer is constructed. The design is determined by numerical calculations of a mirror magnetic field and of the electron trajectory in a flight tube. The performance of the spectrometer is characterized by measuring the electron spectra of xenon atoms irradiated with a laser-driven plasma X-ray pulse. In addition, two-color above-threshold ionization (ATI) experiment is conducted for measurement of the X-ray laser pulse duration, in which xenon atoms are simultaneously irradiated with an X-ray laser pump and an IR laser probe. The correlation in the intensity of the sideband spectra of the 4d inner-shell photoelectrons and in the time delay of the two laser pulses yields an X-ray pulse width of 5.7 ps, in good agreement with the value obtained using an X-ray streak camera.

Measurement of Ion Mach Number of Arcjet Plasmas by a Directional Langmuir Probe Under High-Gas Pressure

Spatial profile of an ion Mach number for arcjet He and Ar plasmas are measured by using a directional Langmuir probe. The Ar arcjet has bright–dark emission structures, implying that expansion–compression waves are formed, whereas no structure is observed for the He discharge. It is found that for He plasma, the Mach number monotonically decreases along the jet axis, while in the Ar jet the prominent peaks appear. The peak positions are almost the same with the bright emission regions, indicating that there is a strong correlation between the plasma emission structure and the Mach number.

Development of a cascade arc discharge source for an atmosphere-vacuum interface device.

To realize a novel vacuum-atmosphere interface that does not require a large differential pumping system, a robust cascade arc discharge source called a plasma window is constructed and tested for long-term operation. By modifying a test plasma with a direct current discharge, a vacuum interface with a high gas pressure ratio of 1/407 between the discharge and expansion sections is demonstrated for currents as high as 20 A. No significant damage to the electrodes is observed during the operation. Analysis of the visible emission spectra reveals that a stationary, stable argon plasma having a temperature of 1 eV and a density of 1.5 × 10(16) cm(-3) is generated in the plasma channel.

Progress and Prospects of X-Ray Laser Research in QST

This paper reviews recent progress made in the development of coherent x-ray sources and their applications in material science and laser processing in the National Institutes for Quantum and Radiological Science and Technology (QST). The upgrade of Ti:Sapphire laser for a grazing incidence pumping (GRIP) scheme was started, to further develop a coherent x-ray source. Investigating the applications of x-ray lasers, we observed temporal evolution of metal surface ablation using an x-ray laser probe, and the damage to an EUV mirror structure by the x-ray pulse irradiation.

Attosecond Dynamics of Parametric Amplification at 11 nm

X-ray parametric amplification is a high order parametric process with the capability to amplify ultrashort laser pulses with broad spectrum ranging from the extreme ultraviolet to the soft x-ray. In this contribution, the dynamics of this broadband amplification schema is studied with sub-20-as temporal resolution by seeding the amplifier with a train of 200-as-long pulses at 11 nm central wavelength.

Pump-Probe Experiment for Temporal Profile Measurement of Plasma X-Ray Laser

Temporal behavior of a soft x-ray laser pulse is measured by means of a pump-probe spectroscopy (cross correlation). In this scheme, first the innershell electron 4d of Xe atom is photoionized with 13.9 nm x-ray pulse (x-ray pump). Simultaneously, an ir probe laser pulse is irradiated to dress the electrons. By varying the time delay between both pulses, the sideband spectra associated with ir photon absorption/emission processes appear at a separation of the ir photon energy from the photo- and Auger main peaks. By analyzing the sideband intensity, we can evaluate the pulse width of the x-ray laser to be ~6.2 ps, which is in excellent agreement with the value measured by an x-ray streak camera

Effects of Equation of State on Fluid Simulations for Laser-Produced Plasmas

We developed a new fluid simulation code for laser-produced plasmas that applied HLL-HLLC scheme for equations of conservation for mass and momentum and upwind difference method for electron temperature and ion temperature. We applied new equation of state library named BADGER v1.0 to our simulation code and compared with the case of use of SESAME library. Our simulation results show the sensitivity of fluid dynamics for the equation of state .