All-optical thermometry using a single multimode fiber endoscope and diamond nanoparticles containing nitrogen vacancy centers.

Photoluminescence (PL) spectra from diamond nanoparticles containing negative nitrogen vacancy centers were measured by using a single multimode fiber endoscope combined with a high-sensitivity spectroscopy system. A laser light spot was produced at the distal end of the endoscope and the PL spectra from a temperature-controlled ensemble of diamond nanoparticles were measured. After calibrating the sensitivity and wavelength of the spectroscopy system, the temperature dependence of the zero-phonon line peak wavelength similar to those previously reported was obtained.

Spatially resolved measurement of helium atom emission line spectrum in scrape-off layer of Heliotron J by near-infrared Stokes spectropolarimetry.

For plasma spectroscopy, Stokes spectropolarimetry is used as a method to spatially invert the viewing-chord-integrated spectrum on the basis of the correspondence between the given magnetic field profile along the viewing chord and the Zeeman effect appearing on the spectrum. Its application to fusion-related toroidal plasmas is, however, limited owing to the low spatial resolution as a result of the difficulty in distinguishing between the Zeeman and Doppler effects. To resolve this issue, we increased the relative magnitude of the Zeeman effect by observing a near-infrared emission line on the basis of the greater wavelength dependence of the Zeeman effect than of the Doppler effect. By utilizing the increased Zeeman effect, we are able to invert the measured spectrum with a high spatial resolution by Monte Carlo particle transport simulation and by reproducing the measured spectra with the semiempirical adjustment of the recycling condition at the first walls. The inversion result revealed that when the momentum exchange collisions of atoms are negligible, the velocity distribution of core-fueling atoms is mainly determined by the initial distribution at the time of recycling. The inversion result was compared with that obtained using a two-point emission model used in previous studies. The latter approximately reflects the parameters of atoms near the emissivity peak.

Population kinetics of many-electron atoms in ionizing plasmas studied using a continuous collisional radiative model.

Collisional radiative (CR) models based on ab initio atomic structure calculation have been utilized over 20 years to analyze many-electron atomic and ionic spectra. Although the population distribution of the excited states in plasmas and their emission spectra are computed using CR models, systematic and analytical understanding of the population kinetics is still lacking. In this work, we present a reduced model of the population dynamics in many-electron atomic ions, in which we approximate the dense energy structure of complex many-electron atoms by a continuum, a continuous CR model (CCRM). Using this simplification, we show an analytical population distribution of many-electron atoms in plasmas and its electron-density and temperature dependence. In particular, the CCRM shows that the population distribution of highly excited states of many-electron atoms in plasmas resembles a Boltzmann distribution but with an effective excitation temperature. We also show the existence of three typical electron-density regions and two electron-temperature regions where the parameter dependence of the excitation temperature is different. Analytical representations of the effective excitation temperature and the boundaries of these phases are also presented.

Data-driven sensitivity inference for Thomson scattering electron density measurement systems.

We developed a method to infer the calibration parameters of multichannel measurement systems, such as channel variations of sensitivity and noise amplitude, from experimental data. We regard such uncertainties of the calibration parameters as dependent noise. The statistical properties of the dependent noise and that of the latent functions were modeled and implemented in the Gaussian process kernel. Based on their statistical difference, both parameters were inferred from the data. We applied this method to the electron density measurement system by Thomson scattering for the Large Helical Device plasma, which is equipped with 141 spatial channels. Based on the 210 sets of experimental data, we evaluated the correction factor of the sensitivity and noise amplitude for each channel. The correction factor varies by ≈10%, and the random noise amplitude is ≈2%, i.e., the measurement accuracy increases by a factor of 5 after this sensitivity correction. The certainty improvement in the spatial derivative inference was demonstrated.

Development of a polarization-modulation spectroscopy system for the temporally resolved measurement of linear polarization in plasma emission.

A system to measure linear polarization in the HeI 21P-31D emission line (667.8 nm) was developed for application to plasma polarization spectroscopy. To verify the system performance, the normalized Stokes parameters were evaluated. A measurement error of less than 1% with a time resolution of 1 ms was achieved for monochromatic light in the state of complete linear polarization.

Z(eff) measurement using extreme ultraviolet bremsstrahlung emission in LHD.

Radial profile measurement of Z(eff) using visible bremsstrahlung (5300 Å) in the Large Helical Device (LHD) has often encountered difficulties because the intensity profile was largely deformed by the nonuniform visible bremsstrahlung emissions from the edge ergodic layer surrounding the core plasma. A space-resolved flat-field extreme ultraviolet (EUV) spectrometer has been newly adopted to measure the Z(eff) profile using the EUV bremsstrahlung continuum in the wavelength range of 70-75 Å. The EUV bremsstrahlung intensity profiles have been measured and checked for all the magnetic configurations with totally different magnetic field structures in the ergodic layer of LHD. It is found that the nonuniform bremsstrahlung emission from the thick ergodic layer can be entirely eliminated by use of the EUV emission with relatively high photon energy of 170 eV. As a result, the Z(eff) profile can be successfully measured for most of discharges regardless of magnetic field structures of the ergodic layer. The Z(eff) profiles measured in the EUV range are compared with those measured in the visible range at a magnetic configuration with the thinnest ergodic layer thickness. The result verifies that the use of the EUV bremsstrahlung continuum is an alternative way for the Z(eff) measurement in toroidal plasmas with nonuniform bremsstrahlung emissions at the edge. Typical results from the EUV bremsstrahlung measurement are presented showing a fairly flat Z(eff) profile with error bars of ±14%.

Absorption enhancement of an electric quadrupole transition of cesium atoms in an evanescent field.

We have observed for the first time reflection spectra of an electric quadrupole transition for the cesium atom (6 (2)S(1/2)-5 (2)D(5/2)) line at an angle of incidence from theta(c)-11.9 to theta(c)+107.5 mrad, where theta(c) is the critical angle for total reflection. From a comparison with the calculated absorption in the attenuated total reflection, the oscillator strengths for s and p polarizations were found to increase with an increase in the angle of incidence by a factor up to 1.5 at theta(c)+83.8 mrad and 2.4 at theta(c)+107.5 mrad, respectively, in the experiment. The dependences of the observed enhancement on the angle of incidence were in good agreement with the calculated ones for the oscillator strength of the quadrupole transition in the evanescent light.