Abstract
The possibility of a high-performance/low-cost fusion reactor concept which can
simultaneously satisfy (1) high beta, (2) high bootstrap fractio (self-sustaining) and (3) high
confinement is discussed. In CDX-U, a tokamak configuration was created and sustained
solely by internally generated bootstrap currents, in which a ’seed’ current is created through
a nonclassical current diffusion process. Recent theoretical studies of MHD stability limits in
spherical tori [e.g. the National Spherical Torus Experiment (NSTX)] produced a promising
regime with stable beta of 45% and bootstrap current fraction of > 99%. Since the bootstrap
current is generated by the pressure gradient, to satisfy the needed current profile for MHD
stable high beta regimes, it is essential to develop a means to control the pressure profile. It is
suggested that the most efficient approach for pressure profile control is through the creation of
transport barriers (localized regions of low plasma transport) in the plasma. As a tool for creating
the core transport barrier, poloidal-sheared-flow generation by ion Bernstein waves (IBW) near
the wave absorption region appears to be promising. In PBX-M, application of IBW power
produced a high-quality internal transport barrier where the ion energy and particle transport
became neoclassical in the barrier region. The observation is consistent with the IBW-inducedpoloidal-
sheared-flow model. An experiment is planned on TFTR to demonstrate this concept
with D–T reactor-grade plasmas. For edge transport control, a method based on electron ripple
injection (ERI), driven by electron cyclotron heating (ECH), is being developed on CDX-U. It
is estimated that both the IBW and ERI methods can create a transport barrier in reactor-grade
plasmas (e.g. ITER) with a relatively small amount of power ( 10 MW Pfusion).