The multiplier‐assisted discharge: A new type of cold cathode

摘要

A multiplier‐assisted discharge is a new type of electron source obtained by operating a high‐gain electron multiplier in the presence of a feedback mechanism. At sufficient multiplier gain such a closed feedback loop results in self‐sustaining currents. In one such feedback configuration—called ion feedback—the multiplier is operated in a background gas pressure of 10-6–10-4 Torr. At this pressure, and with multiplier gains of ?105, positive ions that are created by electron‐gas collisions are accelerated to the multiplier input and produce sufficient secondary electron emission to sustain exponential current buildup. The operation of a nonlinear mechanism, most notably electron space charge, serves to ’’saturate’’ the current at a stable dc level. A second feedback mechanism, optical feedback, is obtained by operating a multiplier with a photocathode at the input that is spectrally matched to a phosphor at the output. The feedback mechanism in this case consists of that part of the phosphor emission that reaches the photocathode and causes photoelectrons that enter the multiplier. Again, space charge acts to saturate the currents to a stable dc level. Both theoretical expressions and experimental measurements were obtained for the operating parameters and the dynamic characteristics of feedback cells. For ion feedback, the relation that was developed between the multiplier gain and the pressure required for sustained emission was found to agree with the experimental values over the range of rare gases from helium to xenon. Current rise times were measured for this range of gases and agreed with predictions; exponential rise times as short as 26 nsec are available in cells using helium as background gas. Space‐charge effects, in the form of both cell saturation due to electrons and neutralization due to the positive currents, were observe-nd; the latter effect showed the dependence on pressure and gas species that is expected on the basis of simple theory. There was also measured the production and decay rates of metastable atoms, which are created along with positive ions as a result of electron‐gas collisions. A similar correlation of theory with experiment was achieved for optical feedback. Particularly noteworthy in the optical feedback case is the very fast—1.5 nsec—rise time available with a short‐time‐constant phosphor such as cerium‐doped LaPO4.