A concept of cost competitive fusion reactor, based on a beam generated reversed shear eqilibrium, is considered . Our previous study on tokamak reactor cost shows that, in order to achieve the acceptable COE (cost of electricity) by tokamaks, the Troyon coefficient beta-N must be increased up to about 5. Although no experiment achieves such a high beta-N in tokamaks, this is an inevitable requirement from the economical point of view for commercial use. A reversed shear configuration might have a possibility to realize such cost competitive tokamak with a high beta-N. A conceptual design of water cooled commercial reactor CREST-1 (Compact REversed Shear Tokamak) based on a reversed shear high beta equilibrium was proposed by us previously, as a possible scinario to economical and feasible reactor concepts succeeding to the ITER project. Although the design parameters are highly advanced, the aspect ratio of CREST is 3.4. Therefore the CREST concept might be tested by ITER with plasma as large as possible.

In the previous study, the equilibrium with beta-N=5.5 has been sustained by the beam driven current (23 MW) with assistance by RFCD in the plasma periphery (24 MW). The self-consistent MHD and current drive calculation shows that the equilibrium of CREST-1 is stable against low-n kink and high-n ballooning modes. Such an NB+RF combination driver provides a good control flexibility of current profile, which is quite important in the reversed shear configurations. However, it also brings about undesirable complication in the reactor design, and the extremely high beam energy (2.5 MeV) has been necessary to attain the desirable beam deposition. In this study, we removed the rf driver and reduced the beam energy down to 1.5 MeV successfully by off axis beam injection with assuming a small amount of current (<200kA) to be driven by MHD turbulence near the plasma axis.

The optimized parameters of CREST-1 are; major radius R=5.4m, R/a=3.4, plasma elongation *=2, q $B&W (B=4.3, Bmax =12.5 T (5.6 T at plasma center), =15.0 keV, =16.2 keV, He accumulation=15%, Zeff=1.5, fusion power Pf = 3.04 GW with mean neutron wall load of 4.6 MW/m2. The stable current profile can be sustained by 77 MW of beam power with high flexibility in the current profile control. Although the injection power is larger than the previous NB+RF case(47MW), the powerful beam injection induces a large toroidal rotation, which may be desirable for the reversed shear equilibria. The rotation velocity attains up to about 10^5 m/sec. This is an important advantage of NBCD system.