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.

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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

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