Raman spectra of H 2 0 and 0 2 0 ice VIII have been measured up to 50 GPa at lOOK. The changes in line positions and intensities of ice VIII and the appearance of a new band above 40 GPa point to the formation of symmetric hydrogen bonds in ice X. A lattice dynamical analysis of the frequency shifts up to 36 GPa suggest a second order phase transition to symmetric hydrogen bonds near 47.5 GPa. However, the observed changes in the lattice modes indicate that this phase transition is already formed by a first order transition at 42 GPa.

By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3-J pulse energy, 300-ps pulse duration, 0.5-Hz repetition rate) together with a 700-MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial point yields low statistical errors (ΔTe /Te ≊6%, Δne /ne ≊4% at Te =6 keV, ne =3×1019 m−3 ). Goodness-of-fit tests indicate that the systematic errors are within the same limits. The system is described and examples of measurements are given.

No abstract

Flat regions observed in the profiles of the electron temperature measured by LIDAR Thomson scattering provide evidence for the existence of helical magnetohydrodynamic resistive mode structure in JET discharges. Comparison with profiles of the safety factor q, determined from magnetic equilibrium calculations, shows that the most prominent regions are located close to rational values of q. The flat regions are also correlated with perturbations observed with other independent experimental measurements such as soft X-ray, electron cyclotron emission and Mirnov oscillations

No abstract

For the analysis of ion dynamics (i.e. ion temperature and ion flow velocity distributions) in the divertor I of ASDEX Upgrade an optical spectroscopic system with high spectral, temporal and spatial resolution was brought into operation. Integrated along up to 74 lines of sight with different orientations to the magnetic field lines, emission spectra of atoms and ions were recorded with high resolution by an Echelle spectrometer and a 2D CCD camera. With this setup, ion flow velocities and temperatures in a divertor plasma could be determined experimentally for the first time. In the attached high-recycling divertor regimes analysed, neutrals such as helium do not flow preferentially parallel to any of the installed lines of sight. In contrast, all the ions investigated always showed pronounced Doppler shifts. The corresponding ion flow is directed towards the target plates, in the outer as well as in the inner divertor. For typical L-and H-mode discharges (attached high-recycling divertor conditions) the C 2+ ions flow at speeds of about 10 km s −1 in the L-mode and 10-20 km s −1 in the ELMy H-mode phases. The temperatures of the C 2+ ions near the divertor plates range from about 5 eV (L-mode) to up to 20 eV (ELMy H-mode phases). The comparison of these results with computer code calculations showed good agreement.

The Raman spectra of NH4I and ND4I are reported at pressures up to 130 kbar and the II–IV transition in this compound is observed to occur at 27(5) kbar at room temperature. The Raman spectral changes which characterize this transition are very subtle and it is concluded that a considerable amount of disorder can still be found immediately above the II–IV transition in the presumably fully ordered phase IV. The behavior of both the N–H stretching modes as well as the librational mode of the NH+4 ions show that at both 298 and 100 K the strength of hydrogen bonding increases when the pressure is raised in phase IV. Two different potentials are used to calculate the barrier heights for the librational motions in NH4I and it is found that anharmonicity is of much less significance in this compound than in NH4Cl and NH4Br. At higher pressures in phase IV the behavior of ν6, however, shows that a volume effect becomes important and this most probably gives rise to the IV–V transition which occurs at 54(4) kbar in NH4I at 298 K and 40(1) kbar at 100 K. The Raman spectra of NH4I (V) are characterized by the appearance of a large number of lattice modes as well as by the disappearance of the LO–TO splitting of ν3 and changes in the relative intensities of the N–H stretching modes. The librational mode ν6 shifts upwards in frequency rather rapidly in NH4I (V) but the N–H stretching modes show only a very slight downward frequency shift. It is not possible to elucidate the structure of NH4I (V), from a consideration of the Raman spectra of powdered samples only, however, similarities and differences between phase V spectra and those of the fictitious phase ’’V’’ of NH4Br, as well as those of other well-known structures of NH+4 compounds are pointed out.

The LIDAR Thomson Scattering System on the JET tokamak is described. Backscattering geometry and the time of flight method are employed to measure profiles along the major radius in the equatorial plane. First electron temperature profiles with a spatial resolution of better than 15 cm obtained with this new diagnostic technique are presented.