Short Recovery Time, Multianode, Microchannel Plate Photomultiplier For Plasma Diagnostics.

A multianode MCP photomultiplier with a recovery time two orders of magnitude shorter than in conventional MCP devices has been developed for plasma diagnostics by laser scattering. The detector has an S20 photocathode, a low resistivity Z--plate electron multiplier and a 10 x 10 square array anode configuration. The Z-plate has a diameter of 25 mm, a resistance of 8.8 MO (at 20 °C) and is made of MCPs rated for operation at a DC strip current density in excess of 100 pA/cm 2 . Once inside the vacuum tube the Z-plate must be operated with pulsed high voltage, to avoid overheating. For this reason this PMT is suitable for the detection of fast bursts of light pulses, occurring at a repetition rate sufficiently low to permit the heat dissipation between successive bursts. The thermal behaviour of the detector has been investigated. First the thermal coefficient of the resistance was measured giving a value of -6.00 ± 0.04 x 104 0/°C (around 20 °C); then the temperature increase of the Z-plate due to a single high voltage square pulse was measured as a function of the pulse energy. The heat capacity and the thermal time constant were found to be 0.93 J/°C and 175 s respectively. The PMT electrical behaviour has also been studied in pulse current mode. At 900 V/plate and with 25 ns FWHM input light pulses, the maximum output charge density was found to be 8.2 x 10 -10 C/cm 2 in linear regime and 4.9 x 10-9 C/cm2 in saturated gain mode. The Z-plate recovers from the maximum linear pulse in about 50 las.