FAST IONIZATION GAUGE

[Objectives]
Measurement of the neutral gas fluxes into the divertor region, pumping duct, and the outside midplane in the presence of strong magnetic fields

[Detectors, Diagnostic Method]
Linear tetrode with hot cathode
(similar to the usual ionization gauges)
Chopping emission current to pick up the minute ion current
(related to the concept of lock-in amplifier)
Installed eleven gauges in the vacuum vessel

[Specifications]

Measurable range	: from ultra high vacuum to several Pascal
Vessel temperature	: up to 400C
Magnetic fields	: up to 5.4 Tesla
Time response	: 3 - 4 ms (determined by conductance)
Chevron in front of gauge head
	: to avoid plasma exposure : to provide thermalization of particles

[Acknowledgement]
We would like to thank Dr. G. Haas at ASDEX, and Dr. J. Ehrenberg at JET for their discussion at the occasion of introducing the fast ionization gauges.