Mitsutoshi Aramaki

Development of a high power supercontinuum source in the 1.7 μm wavelength region for highly penetrative ultrahigh-resolution optical coherence tomography

We developed a high power supercontinuum source at a center wavelength of 1.7 μm to demonstrate highly penetrative ultrahigh-resolution optical coherence tomography (UHR-OCT). A single-wall carbon nanotube dispersed in polyimide film was used as a transparent saturable absorber in the cavity configuration and a high-repetition-rate ultrashort-pulse fiber laser was realized. The developed SC source had an output power of 60 mW, a bandwidth of 242 nm full-width at half maximum, and a repetition rate of 110 MHz. The average power and repetition rate were approximately twice as large as those of our previous SC source [20]. Using the developed SC source, UHR-OCT imaging was demonstrated. A sensitivity of 105 dB and an axial resolution of 3.2 μm in biological tissue were achieved. We compared the UHR-OCT images of some biological tissue samples measured with the developed SC source, the previous one, and one operating in the 1.3 μm wavelength region. We confirmed that the developed SC source had improved sensitivity and penetration depth for low-water-absorption samples.

Experimental studies on ion acceleration and stream line detachment in a diverging magnetic field

The flow structure of ions in a diverging magnetic field has been experimentally studied in an electron cyclotron resonance plasma. The flow velocity field of ions has been measured with directional Langmuir probes calibrated with the laser induced fluorescence spectroscopy. For low ion-temperature plasmas, it is concluded that the ion acceleration due to the axial electric field is important compared with that of gas dynamic effect. It has also been found that the detachment of ion stream line from the magnetic field line takes place when the parameter |f(ci)L(B)∕V(i)| becomes order unity, where f(ci), L(B), and V(i) are the ion cyclotron frequency, the characteristic scale length of magnetic field inhomogeneity, and the ion flow velocity, respectively. In the detachment region, a radial electric field is generated in the plasma and the ions move straight with the E×B rotation driven by the radial electric field.

Development of a Compact Divertor Simulator Excited by Helicon-Wave Discharge

We have developed a high-density H2 plasma source excited by helicon-wave discharge. By optimizing the antenna shape, the diameter of the discharge tube, and the magnetic field strength, a high electron density close to 1×1013 cm-3 was achieved at an rf power of 3 kW and a H2 gas pressure of 30 mTorr. The high-density H2 plasma source can be used as a compact divertor simulator in nuclear fusion research.

Determination of electron densities by diode-laser absorption spectroscopy in a pulsed ICP

A novel method to determine electron densities in a low pressure (1 Pa) pulsed ICP discharge via absorption spectroscopy on argon metastables is presented. By use of an external cavity diode laser tuned at a vacuum wavelength of 696.73 nm the time behaviour of the absorption from metastable argon atoms in the Ar1s5 state in the afterglow is recorded. An analytical model motivates the assumption of a homogeneous metastable density distribution under our experimental conditions. Further, a detailed model for the metastable density decay is developed, including the spatial electron density distribution and its temporal decay. This allows determination of the electron density from the measured data. Comparison between densities obtained by this technique and by Langmuir probe measurements shows excellent agreement. Furthermore, the electron density decay time, the ambipolar diffusion constant and the effective electron temperature in the afterglow are determined comparing data and model. The scaling of the electron decay time with density indicates recombination. The obtained low electron temperature reduces diffusion very much and can be expected to cause an increased recombination rate.

Measurements of gas temperature in high-density helicon-wave H2 plasmas by diode laser absorption spectroscopy

The gas temperatures in high-density H2 plasmas excited by helicon-wave discharges were measured by absorption spectroscopy using a diode laser as the light source. The gas temperature was evaluated from the Doppler broadening of the absorption line profile at Hα. The gas temperature increased with rf power from 0.05 to 0.18 eV at a gas pressure of 50 mTorr. The temporal variations of the gas temperature after the initiation of discharge and the termination of the rf power were investigated. The power consumed by heating the gas was evaluated using the temperature and the time constant of the temporal variation.

Octave spanning coherent supercontinuum generation using 51 fs high-power ultrashort pulse from Er-doped similariton amplifier

A 51 fs, 4.4 nJ, almost-pedestal-free, high-power ultrashort pulse was generated using an Er-doped similariton amplifier. Using the generated ultrashort pulse, high-power, octave-spanning coherent supercontinua broadened from 1.05 to 2.15 and 0.95 to 2.3 µm were generated in highly nonlinear fibers. The characteristics of the similariton amplifier were investigated experimentally.

High resolution laser induced fluorescence Doppler velocimetry utilizing saturated absorption spectroscopy

A high resolution laser induced fluorescence (LIF) system has been developed to measure the flow velocity field of neutral particles in an electron-cyclotron-resonance argon plasma. The flow velocity has been determined by the Doppler shift of the LIF spectrum, which is proportional to the velocity distribution function. Very high accuracy in velocity determination has been achieved by installing a saturated absorption spectroscopy unit into the LIF system, where the absolute value and scale of laser wavelength are determined by using the Lamb dip and the fringes of a Fabry-Perot interferometer. The minimum detectable flow velocity of a newly developed LIF system is +/-2 m/s, and this performance remains unchanged in a long-time experiment. From the radial measurements of LIF spectra of argon metastable atoms, it is found that there exists an inward flow of neutral particles associated with neutral depletion.

Spectroscopic Studies on Impurity Transport of Core and Edge Plasmas in LHD

Spectroscopic diagnostics have been extensively developed for studies of impurity and neutral particle transports at core and edge plasmas in LHD. Diagnostics of core plasmas are similar to a tokamak case, i.e., Zeff from visible bremsstrahlung, K-x-ray measurements from x-ray spectroscopy using Si(Li) detectors and a compact crystal spectrometer, and high-Z impurity diagnostics from VUV spectroscopy using a flat-field EUV spectrometer. A combination of impurity pellet injection and visible bremsstrahlung is an active tool for determination of the diffusion coefficient D and convective velocity V. Using this tool the spatial structures of D and V are obtained and discussed with a neoclassical effect. On the other hand, the spectroscopic method for edge diagnostics is considerably different from the tokamak case because of the existence of a thick ergodic layer in addition to the x-points necessarily included into the diagnostic chord view. In order to break this negative situation, Zeeman and polarization spectroscopy are adopted to LHD edge plasmas. As a result, 2-dimensional emission contours of HeI and Hα are successfully obtained. Laser absorption spectroscopy is tried to measure hydrogen neutrals directly. Radial profiles of edge impurities are also measured with a mirror-assembled 3 m VUV spectrometer. Recent results of and progress in LHD spectroscopy are briefly reviewed.

Heat Transport Simulation for Atmospheric-Pressure High-Density Microgap Plasma

Atmospheric-pressure cw high-density plasma can be produced in a microgap between two knife-edge electrodes by microwave excitation. A possible application of such a plasma is as an excimer light source and for this purpose the gas temperature in the plasma is a particularly important parameter. In this paper we report a fluid dynamic simulation of heat transport in the microgap plasma and compare the results with previously studied experimental gas temperature characteristics (e.g., dependence on the microwave power and the forced gas flow rate). The simulation explains reasonably well the experimental results when the effect of local gas density change on the gas heating process is taken into consideration. Discussion is given that the existence of thermally driven convection in the microgap plasma indicated in a preliminary report is incorrect.

Spatially and temporally resolved optical spectroscopic investigations inside a self-pulsing micro thin-cathode discharge

A fibre probe is developed to investigate the temporal development and the spatial distribution of the population density of argon 1s5 state and the emission in the afterglow in a self-pulsing micro thin-cathode discharge at atmospheric pressure. The fibre probe with a diameter (125 µm) of about half the discharge diameter (200 µm) is moved along the discharge axis. A tunable diode laser beam at 696.73nm at resonance with the argon 1s5 → 2p 2 transition is coupled into the far end of the fibre probe. The spatially differentiated absorbance yields the axial distribution of the 1s5 state. In addition, the fibre is used to determine similarly the spatially resolved optical emission of various argon lines. Both the emission and the absorption profiles confirm that, in the afterglow, the plasma is strongly localized inside the hole of the discharge. Qualitatively, the measurements agree very well with the results of a zero-dimensional simulation. The long living afterglow is related to the high population density of the metastable atoms. (Some figures in this article are in colour only in the electronic version)