Progress in cathode research was reported at the second Tri-Service Cathode Workshop held last year at the Rome Air Development Center, New York. Selected papers were later published, and in one of these the authors M. C. Green, H. B. Skinner and R. A. Tuck of EMI-Varian Limited at Hayes, Middlesex, presented studies of tungsten-osmium alloys and their relevance to improved M-type cathodes (Appl. Surf. Sci., 1981, 8, (1 and 2), 13–35).

When half the emitting area of a B-type cathode was sputtered with osmium the coated area performed as M-type, and the expected difference in the work function of the two parts was observed. However an interfacial band gave enhanced emission, and was of obvious practical interest. Mixed matrix cathodes were prepared from 80 tungsten-20 osmium sintered compacts impregnated with barium calcium aluminate. Initially these gave disappointing results but after 500 hours the performance became most promising, the zero field emission being some 50 per cent better than M-type.

A definite correlation between the formation of the tungsten-osmium σ intermetallic phase and the improved emission has been demonstrated. As a result an improved dispenser cathode has been constructed.

Experimental work is continuing to determine the mechanism of emission enhancement.