Engineering innovation is required to reduce cost and risk in fabrication for the Quasi-Poloidal Stellarator being developed to test key physics issues at very low plasma aspect ratio. Complex, highly accurate, stainless steel modular coil winding forms are cast and machined; conductor is wound directly onto the winding forms; a vacuum-tight cover is welded over each coil pack; the coils are vacuum pressure impregnated; the completed coils are installed in an external vacuum vessel. An internally cooled, compacted cable conductor that can be wound into complex 3-D shapes was developed. The largest and most complex of the winding forms has been cast using a patternless process (machined sand molds) and a high-temperature pour. The resulting casting required <1/10 the major weld repairs of similar sand castings using conventional patterns. As a result, QPS differs significantly in design and construction from other toroidal devices. Published by Elsevier B.V.

The Quasi-poloidal stellarator (QPS), currently in the early design phase, is a low-aspect-ratio (R/a=2.7), concept exploration experiment with a non-axisymmetric, near-poloidally-symmetric magnetic configuration. The QPS design parameters are 〈R〉=0.9 m, 〈a〉=0.33 m, B=1 T, and a 1 s pulse length. The QPS device will be located at the Oak Ridge National Laboratory. Lyon et al. [http://qps.fed.ornl.gov/pvr/pdf/qpsentire.pdf, 2001] describes the physics and engineering features in detail. The QPS device is estimated to require 4 years from start of design to first plasma in 2007.

The engineering design status of the Quasi-Poloidal Stellarator Experiment (QPS) is presented. The purpose, configuration, and possible manufacturing and assembly techniques of the various components of the core are described.

The Advanced Toroidal Facility is a large torsatron device with a major radius R0 = 2.1 m, an average plasma minor radius a ≈ 0.3 m, and a magnetic field B0 ≤ 2T. The sheared magnetic configuration [τ(0) ≈ 0.3, τ(a) ≈ 1] is produced by an l = 2, M = 12 field period helical winding set and associated circular vertical field coils. The segmented helical windings were constructed with a tolerance of ±1-mm deviation from the ideal winding law using computer-aided manufacturing and assembly techniques. Nevertheless, in the initial operating period, it was found that field errors produced significant magnetic islands (island width ≈6 cm at τ = ½), which reduced the effective plasma radius by ∼30%. The main cause of these islands was the toroidally asymmetric field perturbation produced by the geometry of the electrical coil feeds. After “symmetrization” of the buswork, the dominant magnetic islands were reduced in size to ≤1 cm at the operating field of 1 T.

An overview of recent AFT (Advanced Toroidal Facility) experimental results and program plans is presented, with emphasis on the role of magnetic configuration controls in transport studies. The ATF operating space is bounded by a density limit that effectively sets a limit on the energy confinement time τE. Although this limit is not solely due to impurities, it has recently been raised by improved cleanliness following titanium gettering. This has led to collapse-free neutral beam injection discharges with global τE ≈16 ms. Preliminary experiments show that stored energy and bootstrap current are sensitive to details of the magnetic configuration

The Advanced Toroidal Facility (ATF) torsatron was designed on a physics basis for access to the second stability regime and on an engineering basis for independent fabrication of high-accuracy components. The actual construction, assembly, and initial operation of ATF are compared with the characteristics expected during the design of ATF. 31 refs., 19 figs., 2 tabs.

A prototype ohmic heating coil for the proposed Spherical Torus Experiment (STX) has been designed, fabricated, and successfully tested to 16 T, well above its design operating field and stress levels. The conductor material was Glidcop Al-15 with a copper tube for water cooling soldered into a groove. The magnet winding inner and outer radii were 7.5 cm and 14.2 cm, respectively, and the axial length was 48 cm. The magnet heated to approximately 90°C during a pulse and was cooled within 2 min. All design and performance goals were met or exceeded.