. . . "This has the important advantage of protecting the remote cathode 82 from contamination, and enables a very extended cathode life, a feature which is especially important to enable use of the preferred LaB6 cathodes, which are particularly sensitive to degradation from chemically active species. [0207] Water Cooled Block and Demountable Ionization Chamber [0208] In the embodiment of FIG. 9B (see a" .