Development and use of the calutron during and since World War II is a fascinating chapter in the history of physics. William Parkins points out that the success of the calutron was made possible by an unexpected natural process—that of ion-beam space-charge neutralization by trapped electrons. He calls such a process “a gift of Nature.”
Although the unexpected beneficial process in the calutron was crucial for the practicality of electromagnetic separation of isotopes, one wonders if such processes are as rare as Parkins declares. A clarification of his criteria for distinguishing the “small number of beneficial natural physical processes” from all others would have been helpful.
For example, I wonder whether a particular physical process with which I have much experience qualifies as such a rare gift. The cesium vapor arc, essential for practical thermionic energy conversion, depends crucially on electron trapping in the plasma as the mechanism for space-charge neutralization just as electron trapping was required for space-charge neutralization in the calutron. It also requires the trapping of excitation radiation in the plasma so that the cesium vapor ionizes efficiently, which allows the dense plasma to be maintained with only a 0.5-volt arc potential drop even though the ionization potential of cesium is 3.9 volts. 1 Furthermore, the cesium vapor also adsorbs on the electrodes to increase electron emission 10 000-fold and to give a very low work function that provides more than enough potential difference to maintain the arc. 2
These multiple and remarkable phenomena combined to give much higher performance than expected for energy conversion, as space-charge neutralization alone gave for the calutron. Does this qualify the cesium vapor arc as a gift of Nature? If not, what specifically does qualify those few physical processes that do?
