J. Sousa, A. J. N. Batista, A. Combo, R. Pereira, N. Cruz, P. Carvalho, C.A.F. Varandas, S. Conroy, G. Ericsson e J. Källne

A PCI time digitizer module with eight independent Time-to-Digital Converter (TDC) channels is being developed for the Time-of-Flight spectrometer designed for Optimized Rate (TOFOR). TOFOR is a new instrument for 2.5-MeV neutrons to diagnose deuterium plasmas of the JET tokamak with the high accuracy that comes with the ability to operate at signal rates approaching the theoretical limit of the coincidence measurements (up to 0.5 MHz for the application at hand). The module shall measure with high accuracy and resolution flight-times of neutrons in the 100-ns range as given by two groups of scintillation detectors operating at event rates up to several MHz.
To attain the requirements of the TOFOR, the time digitizer module was designed to handle a peak event rate of 1.25 Gevents/s/channel and a sustained event rate of 5 Mevents/s/channel. The digitizer time resolution and the dead-time between events are as low as 400 ps. A maximum of 8 Mevents/channel can be stored in the 512 Mbytes onboard memory and the time marks cover a period of up to 300 days. The digital signal processing (DSP) features of the modules include real-time control/monitoring and data reduction capabilities; the latter can encompass algorithms appropriate for the phenomena being measured, permitting operation for long time periods and, even, real-time execution. The use of a System-On-Chip device complements the DSP capabilities, which allow programmable logic changes to be implemented without the need for costly hardware modifications. The high channel count, the implementation of the TDCs around a 2.5-Gbit/s digital communications device and the use of fast price decay components, contribute to reduce the overall cost per channel.

*) The TOFOR project is a JET diagnostic enhancement within the EFDA-JET work program performed as a EURATOM collaboration between VR, ENEA/CNR-Milano, IST and UKAEA/JET.