Proceedings of ITC/ISHW2007

Compact Stellarator Path to DEMO
J. F. Lyon for the U.S. stellarator community
Oak Ridge National Laboratory, Fusion Energy Division, Oak Ridge TN, USA
lyonjf@ornl.gov

The US fusion program is considering what new facilities are needed in addition to ITER andexisting facilities to prepare the way to a DEMO reactor. The broad issues are familiar:
sustainment of an ignited/high-Q plasma in steady state, avoidance of disruptions and large
variations in power flux to the wall, adequate confinement of thermal plasma andalpha-particles, control of a burning plasma, particle and power handling (divertors), etc.
Stellarators have key reactor advantages: steady-state high-plasma-density operation without
external current drive or disruptions, stability without a close conducting wall or active
feedback systems, and low recirculating power. While stellarators are at present lessdeveloped than tokamaks because of the higher parameters obtained in large tokamaks, D-T
operation in TFTR and JET, and the decision to proceed with ITER, stellarators do present an
attractive alternate path to a DEMO reactor.
Compact stellarators, with moderate plasma aspect ratios, good confinement, and high-betapotential lead to the smaller-size reactors that are favored by the US utility industry. TheARIES group, which had previously analyzed different tokamaks, an ST, an RFP and a
modular stellarator (SPPS) as reactors, has recently examined compact stellarators. TheARIES-CS study established that compact stellarators can be economically competitive withtokamaks as reactors, but face serious issues with divertors, as do other toroidal concepts.
The favorable physics properties of compact stellarators follow from their quasi-symmetricmagnetic geometry, analogous to the axisymmetry of |B| in tokamaks and RFPs. Basicphysics properties depend only on |B| in the Boozer coordinate system in which magnetic flux
surfaces are concentric, axisymmetric tori. This fundamental property allows mapping ofphysics properties from tokamaks to analogous stellarators. The different types ofquasi-symmetry (helical, toroidal and poloidal) manifest themselves in the magnitude,
direction, and variation of flow and flow shear, and instability thresholds for trapped particle
modes. Initial results from HSX, a helically symmetric stellarator, are in general agreementwith expectations for quasi-symmetry, and demonstrate its effectiveness.
Many of the issues that need to be resolved before committing to a compact stellarator DEMO
can be answered using the results of the large tokamaks and stellarator experiments already inoperation (LHD, TJ-II, Heliotron J, HSX, CTH), under construction (W 7-X, NCSX) or indevelopment (QPS). The compact stellarator route to a DEMO can also incorporate resultsfrom ITER D-T experiments and fusion materials, technology and component development
programs. However, a large next-generation stellarator experiment will be needed to addresssome physics issues: size scaling and confinement scaling at DEMO-relevant density,
temperature, and beta; the burning plasma issues of adequate fast-ion (alpha-particle)
confinement, robustness to Alfven instabilities, and helium ash removal; and operation with astrongly radiative divertor. Technology issues include simplification of the 3-D coil,structure, and divertor fabrication, the use of rapid prototyping as in the aerospace industry,and reliable costing techniques.