An Energy-Dependent Finite-Orbit Treatment for Plasma Buildup in Mirror Fusion Devices

Abstract

A computer simulation of hot plasma buildup in mirror fusion devices and results from this model are pesented here. In a small, hot magnetically confined plasma, the ion orbit radius ((rho)(,i)) can be comparable to the plasma radius (R(,p)). In a mirror-confined plasma where (rho)(,i)/R(,p) > 1/25 (such as 2XII-B), a "point kinetic" treatment of ion interactions becomes inaccurate and a finite gyro-radius (FGR) treatment must be used to adequately describe plasma buildup processes. This is particularly true for describing losses due to cold-gas charge exchange (c-x) near the plasma surface, since a particle lost near the vacuum interface may have contributed to the density as far as 2(rho)(,i) radially inward from the c-x point. A similar FGR effect applies to beam-deposited ions whose large orbits influence the density up to 2(rho)(,i) from the trapping point.

These and other important FGR effects are treated by a dynamicmodel called FOREMD (for Finite Orbit, Radial, Energy and Magnetic-field Dependent) which was developed for the present studies. Unlikeearlier finite orbit models, FOREMD is fully energy dependent(important because (rho)(,i) (TURN) SQRT.(E) and includes a quasi-linear treatment for(' )the Drift Cyclotron Loss Cone (DCLC) instability. Also included areeffects of the local magnetic-field on ion orbits.

The FOREMD model has been used to study plasma buildup (particularly early stages) in presently operating and proposed mirror devices (2XII-B, TMX, FERF) and in the Twin Beam Mirror (TBM), a concept proposed by G. H. Miley at UIUC. Results of these simulations compared to experiments or design expectations are presented here.

General factors in plasma buildup and their relative importance are also examined with recommendations on how to achieve plasma buildup under c-x erosion conditions with minimal injected beam power. A detailed description of the model is also included.