Plasma accumulation in a device fed by energetic-ion trapping

Abstract

Plasma accumulation (without energy losses) in a device in which energetic ions are injected and trapped has been discussed by Simon [J. Nuclear Energy C 1 (1960) 215]. Simon has presented a general steady-state theory for devices, such as the OGRA device, where trapping is initiated by interactions with background gas. For such devices there is usually a critical input current or critical plasma density (a function of input current) above which plasma density builds up to a value limited by Coulomb scattering losses. For a specific regime of operation (600-keV hydrogen molecular ion injection and dissociation, highly efficient ion-pumping action of the trapped plasma), simple approximate formulas are derived which describe the critical current or density for plasma build-up. Second-approximation formulas are derived which describe with about 10 % error the overlapping segments of the density versus current curve for a broad range of parameters.The analysis is then applied to a system in which the initial trapping is accomplished by a Luce carbon arc, e.g. DCX-2. Two new aspects emerge: (1) if the arc is turned off slowly or if an arc of lower density is used, then the critical current for plasma build-up with arc is nearly an order of magnitude less than previously estimated; and (2) the critical arc density, below which plasma build-up occurs after the arc is removed, is sufficiently low that energy losses to fixed-temperature arc electrons are not significant.The time-dependence of plasma build-up, certain aspects of the impurity problem, and electron heating effects are also considered. Numerical examples are given for OGRA and DCX-2.