Progress of design studies on an LHD-type steady-state reactor

Helical Heliotrons such as the Large Helical Device (LHD) and Stellarators (H and S systems) have a high potential to realize a current-less steady-state and stable magnetic fusion energy reactor as an alternative to the tokamak DEMO-reactor. H and S systems ideally have an intrinsic property of Q=infinite. Here it is very important to remember that the understanding of the physics of 3-D toroidal magnetic confinement system is naturally extended to tokamak systems. The physics is universal among these two types of systems and the technology is common. We present our recent results from LHD experiments and reactor studies of a next generation LHD-type DEMO Reactor called FFHR. (1) Development of 3-D superconducting (SC) coil technology Due to the successful results of the LHD construction from 1990 to 2007, and steady operation over 8 years from 1998 to 2007, more than 2,000 hrs/year at a high field of around 3 Tesla, we have a large enough data base to demonstrate that 3D coil technology has become the standard technology for a fusion energy reactor. LHD is the largest SC fusion device in the world, contributing to the development of the SC technology necessary for fusion research. The poloidal coils of LHD adopted a super critical forced flow cooling system and their dimensions are almost the same as the ITER toroidal coils. (2) Extended physics understanding of high beta, high T, high nτT, and steady state operation Recent LHD experiments have demonstrated the broad and advanced capabilities of LHD as a toroidal magnetic confinement device, which are highlighted by the achievements of 5% volume averaged beta, electron and ion temperatures of 10 keV, super high density of 10E15/cc and 1 hr discharges. We plan to increase the heating power up to 35 MW, and to use deuterium gas for confinement improvement. The nτT will be improved to the design nominal value of Q=0.3 within several years and ultimately would approach unity. The key issue for this is the demonstrated ability to produce super-high density plasma stably by the formation of an Internal Density Barrier (IDB), which far exceeds the tokamak Greenwald limit. IDB is formed by the careful edge control of particle and energy flux by the Local Island Divertor (LID). IDB will make it possible to pursue the new approach of a Super Dense Core Reactor (SDCR). Our results will contribute to the improvement of tokamak confinement physics. (3) Feasability Study of Reactor Design The SDC Reactor Concept represents a new viable scenario to build an LHD type reactor. Since the super high density core of 10E15/cc is possible, the required temperature is around 7 keV. In addition we are developing the slow reactor up scenario based on the disruption free property of LHD. A mass based comparison of FFHR construction costs to the ITER cost database demonstrates economic viability and reasonable electricity cost. The cost of the SC helical coils does not represent a critical path. (orig.)