The Ti3+-Ti4+ ion pair is proposed experimentally to be responsible for the infrared residual absorption of Ti:Al2O3. However, no theoretical verification has been presented to date. In this paper, the DV-Xalpha calculation is performed upon clusters of (Ti3+O62-)9-, (Ti3+Al3+O92-)12- and (Ti3+Ti4+O92-)11-. Our work not only has interpreted the infrared residual absorption but also has clarified the confusion about the origin of the ultraviolet absorptions. Therefore the Ti3+-Ti4+ pair postulation is strongly supported by our present work.

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In nuclear fusion power supply systems, the thyristors often need to be connected in parallel for sustaining large current. However, research on the reverse recovery transient of parallel thyristors has not been reported yet. When several thyristors are connected in parallel, they cannot turn-off at the same moment, and thus the turn-off model based on a single thyristor is no longer suitable. In this paper, an analysis is presented for the reverse recovery transient of parallel thyristors. Parallel thyristors can be assumed as one virtual thyristor so that the reverse recovery current can be modeled by an exponential function. Through equivalent transformation of the rectifier circuit, the commutating over-voltage can be calculated based on Kirchhoff's equation. The reverse recovery current and commutation over-voltage waveforms are measured on an experiment platform for a high power rectifier supply. From the measurement results, it is concluded that the modeling method is acceptable.

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