The response of a fast scintillator screen (YAP:Ce) to low energy ions (0-40 keV) and its use to detect fast-ion-loss in stellarator TJ-II.

A systematic study of scintillation materials was undertaken to improve the time resolution of the fast ion diagnostic currently installed at TJ-II stellarator. It was found that YAP:Ce (formula YAlO:Ce, Yttrium Aluminum Perovskite doped with Cerium) ionoluminescence offers better sensitivity and time response compared to the standard detector material, SrGaS:Eu (TG-Green), currently used in TJ-II. A comparison between both materials was carried out by irradiating them with H ions of up to 40 keV using a dedicated laboratory setup.

Doppler reflectometer system in the stellarator TJ-II.

A Doppler reflectometer system has recently been installed in the stellarator TJ-II. The system is optimized for the Q-band (33-50 GHz) and the high-curvature plasmas produced in TJ-II. The launch angle of the microwave beam can be controlled by a steerable mirror to obtain angles between +/-20 degrees enabling the measurement of perpendicular wave numbers in the range of 3-15 cm(-1).

Comments on "An accelerated learning algorithm for multilayer perceptrons: optimization layer by layer".

This letter analyzes the performance of the neural network training method known as optimization layer by layer. We show, from theoretical considerations, that the amount of work required with OLL-Learning scales as the third power of the network size, compared with the square of the network size for commonly used conjugate gradient training algorithms. This theoretical estimate is confirmed through a practical example.

Profile structures of TJ-II stellarator plasmas.

Fine structures are found in the TJ-II stellarator electron temperature and density profiles, when they are measured using a high spatial resolution Thomson scattering system. These structures consist of peaks and valleys superimposed to a smooth average. Some irregularities remain in an ensemble average of discharges with similar macroscopic parameters such as line density, central temperature, and plasma current.