Naohiro Yamaguchi

In situ calibration of a flat-field grazing incidence spectrograph with an x-ray charge-coupled device camera using a transmission grating and a laser-produced plasma for x-ray laser research

An in situ calibration experiment of a soft x-ray spectrograph for x-ray laser research has been performed. The calibrated spectrograph was a flat-field grazing incidence spectrometer with an x-ray charge-coupled device camera. The absolute sensitivity of the spectrograph was obtained for the wavelength range from 2.6 to 4.0 nm using a transmission grating spectrograph as a reference spectrograph set along the axial direction of a line-focused laser-produced plasma. The absolute sensitivity determined in this work shows nearly the same value as that deduced from the calibration experiment using synchrotron radiation.

Behavior of neutral-hydrogen and particle confinement on GAMMA 10 tandem mirror plasmas

Abstract Behavior of neutral hydrogen in the central-cell of the GAMMA 10 tandem mirror is investigated by measuring spatial-profiles of Hα line-emission and neutral transport simulation. Hα-emission detectors are newly installed horizontally in the vacuum chamber and detailed profiles of the Hα emission are measured. It is found that hydrogen atoms introduced from the gas puffer at the mirror throat are localized around the gas puffer in the steady state phase of ICRF-heated plasmas. The DEGAS neutral transport code is applied to calculate axial density profiles of atomic and molecular hydrogen. In the DEGAS code, the mesh model is modified to take into account variations along the magnetic-field line. The simulation result fairly agrees with the above experimental result. In a standard ICRF-heated plasma, an experiment with modulated gas-puffing is performed. Characteristics of particle confinement in the main plasma are discussed by using the experimental and the simulation results.

Neutral particle transport under strong hydrogen recycling condition in the GAMMA 10 central cell

Abstract In order to understand the behavior of neutral hydrogen in ICRH plasmas of the GAMMA 10 tandem mirror, we tried new simulations of neutral transport by using the DEGAS Monte Carlo code in which the effects of hydrogen recycling were taken into account. This simulation has been performed by introducing a multiplication coefficient in DEGAS to control the amount of desorbed test particles, which enables us to simulate the neutral transport for the first time under the conditions in which the recycling coefficient exceeds unity. By using the multiplication coefficient, it was found that the axial profile of the atomic hydrogen density obtained from the simulation changed significantly. On the other hand, there is little influence on the radial profile of the simulated hydrogen density. The simulation result, taking the axial variation of the multiplication coefficient into consideration, well predicted the result of the H α -emission measurements. This indicates that strong hydrogen recycling is localized near the midplane. These calculation results are discussed from the view point of the wall-reflux coefficient deduced from the DEGAS simulation.

Absolute calibration of space-resolving soft X-ray spectrograph for plasma diagnostics

Abstract A grazing incidence flat-field soft X-ray (20–350xa0A) spectrograph was constructed and applied for impurity diagnostics in the GAMMA 10 fusion plasma. The spectrograph consisted of a limited height entrance slit, an aberration-corrected concave grating, a microchannel-plate intensified detector and an instant camera/a high speed solid state camera. An absolute calibration experiment for the SX spectrograph was performed at the Photon Factory in the High Energy Accelerator Research Organization with monitoring the incident synchrotron beam intensity by using an absolutely calibrated XUV silicon photodiode. From the results of absolute calibration of the spectrograph, the radiation loss from the plasma was obtained.

Calibration of space-resolving VUV and soft X-ray spectrographs for plasma diagnostics

Vacuum ultraviolet (VUV) and soft X-ray measurements are important means of diagnosing impurities in magnetically confined plasmas used in fusion research. Recently, space- and time-resolving flat-field VUV (150-1050 A) and soft X-ray (20-350 A) spectrographs have been constructed by using aberration-corrected concave gratings with varied-spacing grooves which give a wide simultaneous spectral coverage on a microchannel-plate intensified detector. Calibration experiments have been performed at beamlines 11A and 11C at the Photon Factory of the High Energy Accelerator Research Organization. The relative efficiency of the VUV spectrograph has been measured for P-polarization geometry in the spectrograph. In the soft X-ray spectrograph, efficiencies have been obtained for several different points of irradiation on the grating along the groove direction and for two (S and P) polarization geometries.

Formation of microbeam using tabletop soft X-ray laser

Abstract An X-ray microprobe with a sub-micron size beam and high intensity can provide X-ray analyses with a remarkable spatial resolution. We have performed focusing of an X-ray laser output into a sub-micron beam for the first time. In our experiment, an X-ray laser of Li-like Al 3d–4f transition at 15.47 nm was delivered from an unstable cavity consisting of a concave mirror and a flat mirror with a square orifice of 100×100 μm in size. The beam from the orifice was then focused by using a Schwarzschild mirror coated with a Mo/Si multilayer. An X-ray beam size with a diameter of about 0.45 μm and an estimated photon number of about 2×106 photons per shot was achieved. Such sources could be well suited for the realization of X-ray microprobes.

Space-resolving VUV and soft X-ray spectroscopy in the tandem mirror gamma 10 plasma

Vacuum ultraviolet (VUV) and soft X-ray (SX) spectroscopic measurements are important means to diagnose radiation power loss, impurity ion densities and effective charge of confined plasma, Zeff, in magnetically confined plasmas such as fusion plasmas. We have constructed space- and time-resolving flat-field VUV (150–1050A) and SX (20–350A) spectrographs by using aberration-corrected concave gratings with varied line spacing. Absolute calibration experiments have been conducted at the Photon Factory in the High Energy Accelerator Research Organization. Absolute sensitivities of the VUV and SX spectrographs have been obtained for the two (S and P) polarization geometries. Thus, absolute intensities of emission spectra from impurity ions can be measured together with their radial distributions in plasmas. The total radiation power was determined to be less than 6 kW within ±20% of error in our normal plasma operation. Density profiles of impurity ions were reduced by using absolute emissivities of impurity lines and a collisional-radiative model. Moreover, the value of Zeff is estimated to be 1.00 in the tandem mirror GAMMA 10 plasma.

Absolute Calibration of Space- and Time-Resolving Flat-Field Vacuum Ultraviolet Spectrograph for Plasma Diagnostics

Measurement of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft X-ray is an important means to diagnose impurities in magnetically confined plasmas used in fusion plasmas such as a GAMMA10 plasma. Recently, a space- and time-resolving flat-field grazing-incidence VUV spectrograph was constructed for the simultaneous observation of spatial, temporal and spectral distributions of plasma radiation in the wavelength range of 150–1050 A. Absolute calibration experiments were performed at beamline 11C in the Photon Factory at the High Energy Accelerator Research Organization. The absolute efficiency of the VUV spectrograph was measured for P polarization geometry in the spectrograph.

Debris from the Target of an X-Ray Laser System and the Effect on Cavity Mirrors

The spatial distributions of debris generated from an X-ray laser system are measured in the directions of the cavity mirrors set orthogonal to the target surface both in the horizontal and vertical directions. The thickness of the vaporized particles deposited increases in the horizontal direction and decreases in the vertical direction with distance from the origin, the intersection of the mirror plane and a line drawn from the center of the irradiated area along the longer side. The number of droplets deposited also shows similar trends. Upon analyzing the spatial distributions of the debris, as an appropriate measure for preventing debris, a combination of sidewalls and pinhole cells were installed between the edge of the tape target and the cavity mirrors. The sidewalls stopped all the droplets from reaching the mirror surfaces but had only a small effect on the vaporized particles. The vaporized particles were however reduced substantially when installing pinhole cells with the diameter of the pinhole corresponding to the lasing region. Using the combination of sidewalls and pinhole cells, the lifetime of the cavity mirrors was prolonged by about 40-fold. This demonstrates that a reasonable cycle operation of the X-ray laser system can be realized by using the sidewalls and pinhole cells.

Contactless excitation and measurement method for inspection of microstructures and thin films

A specially designed resonator to avoid the change of the resonance characteristics by thermal influences of the laser beam was fabricated on a silicon on insulator wafer. This resonator design enables one to apply a lateral driving force to generate lateral vibration. The resonator was excited using a laser diode which was driven with sinusoidal current, and the vibration was detected by measuring intensity fluctuation of the He–Ne laser beam reflected on the resonator mass. Four evident resonance frequencies were successfully observed in the range from 100 Hz to 100 kHz.