Multiple wave radio frequency current driven tokamak reactors in the first stability regime

Abstract

Using a ray tracing calculation, a number of different waves are simultaneously launched and followed as they generate a steady state tokamak MHD equilibrium via RF current drive. By adjusting the power, it is possible to create equilibria which in these calculations are in the first stability regime. The measure of global current drive efficiency, 7 ~ n e RoI/PcD> is computed for a wide range of tokamak plasmas in steady state power balance to obtain a general scaling law which includes a dependence on temperature, Z e ff and beta. On the basis of the general expression for 7, a series of reactor designs are created which illustrate the options available for optimizing steady state, first stability regime tokamaks; high temperature (Ti J> T e « 20 keV) and large size (R o = 7 m) appear desirable for maximum Q.

Bootstrap currents in radiofrequency driven tokamak equilibria

“…The conventional RF current drive efficiency, y, can be improved up to a factor of -10. Whereas a previous study [5] of tokamakreactors with self-consistent RF-driven equilibria reported rather dismal power multiplication, 0, » (fusion power)/Pco ~ 4-6, our present calculations show that Q is likely to be enhanced by a factor of two to ten, under favorable conditions, by the bootstrap effect. Thus, an efficient tokamak reactor with the advantages of steady-state operation may be more readily achieved, although the experimental demonstration of bootstrap current in tokamaks has not yet been fully achieved.…”

“…In particular, it has been shown that, using multiple RF waves, it is possible to generate the current required for steady-state tokamak reactors in the first stability regime. [5] However, the required RF power appears to be a significant fraction of the fusion power. For example, the normalized current drive efficiency for a wide range of cases surveyed in Reference 5 is close to the minimum required value [1] of Y-0-4.…”

Inductive currents in an rf driven plasma

“…where < > is a flux surface average [2], and G(_), H(_), and I(_) describe the RF, bootstrap, and inductive effects as a function of poloidal flux. The flux is the solution to the Grad-Shafranov equation,…”

“…where K is the elongation, Jo and J2 are the spherical Bessel functions, and d is the triangularity [2]. We now wish to replace Wesson's function, g(Z) = N(Z)Z, with some convenient expression depending upon Zef f. A reasonable fit for g(Z) versus Z yields g(Z) = .591 (I + Z).…”

Divertor erosion study for TPX and implications for steady-state fusion reactors

“…It is known from previous studies e.g. [2] that non-inductive current drive may result in higher edge temperatures and hence more difficult plasma-surface interaction problems at the divertor. The higher edge temperature results from the need to minimize core plasma density to achieve acceptable current drive efficiency.…”