K. Freudenberg

ITER Central Solenoid design

The Central Solenoid (CS) is a critical component in the ITER tokamak providing plasma current drive and shaping. The CS final design is being completed at the US ITER Project Office (USIPO) in Oak Ridge, TN under a Procurement Arrangement with the ITER Organization (IO). Key design decisions have been made and CAD models and drawings developed. Interfaces have been established. An extensive R&D program has been completed. Analyses have been conducted to verify the design meets requirements. Design documentation is being completed in anticipation of a Final Design Review in the fall of 2013. The paper describes the key features of the CS final design.

Moving Toward Manufacture of the ITER Central Solenoid

After several years of design optimization, the Central Solenoid (CS) of the ITER Magnet system is now moving towards manufacture. The design has evolved to take into account on one hand the results of the R&D carried out by the US ITER team in charge of the development of the design and on the other hand the feedback provided by the involvement of industry in preparation of the manufacture. To address specific issues, dedicated mock-ups have been manufactured and tested. Electromagnetic, structural and thermo-hydraulic analyses have been carried out to verify the compliance of the design with the ITER design criteria. A review of the Final Design is planned in 2013, preparing then to move into the manufacturing phase.

Addressing the Technical Challenges for the Construction of the ITER Central Solenoid

The Central Solenoid (CS) of the ITER Magnet system is split into six independently powered coils enclosed inside an external structure which provides vertical precompression thus preventing separation of the coils and acting as a support to net resulting loads. The analyses include an assessment of the mechanical behavior of the different components of the CS, under the normal and fault conditions, aiming at demonstrating the ability of the CS to achieve 30 000 cycles of plasma operation at nominal current (15 MA). A comprehensive material testing program is developed for the conductor jacket, the impregnated glass-epoxy insulation and the structure. The paper describes the architecture of the analysis and qualification programs and provides an overview of the results obtained so far.

Starting Manufacture of the ITER Central Solenoid

The central solenoid (CS) is a key component of the ITER magnet system to provide the magnetic flux swing required to drive induced plasma current up to 15 MA. The manufacture of its different subcomponents has now started, following completion of the design analyses and achievement of the qualification of the manufacturing procedures. A comprehensive set of analyses has been produced to demonstrate that the CS final design meets all requirements. This includes in particular structural analyses carried out with different finite-element models and addressing normal and fault conditions. Following the Final Design Review, held in November 2013, and the subsequent design modifications, the analyses were updated for consistency with the final design details and provide evidence that the Magnet Structural Design Criteria are fully met. Before starting any manufacturing activity of a CS component, a corresponding dedicated qualification program has been carried out. This includes manufacture of mockups using the real manufacturing tools to be tested in relevant conditions. Acceptance criteria have been established for materials and components, winding including joints, cooling inlets and outlets, insulation, precompression, and support structure elements.

Engineering Accomplishments in the Construction of NCSX

The National Compact Stellarator Experiment (NCSX) was designed to test a compact, quasi-axisymmetric stellarator configuration. Flexibility and accurate realization of its complex 3D geometry were key requirements affecting the design and construction. While the project was terminated before completing construction, there were significant engineering accomplishments in design, fabrication, and assembly. The design of the stellarator core device was completed. All of the modular coils, toroidal field coils, and vacuum vessel sectors were fabricated. Critical assembly steps were demonstrated. Engineering advances were made in the application of CAD modeling, structural analysis, and accurate fabrication of complex-shaped components and sub-assemblies. The engineering accomplishments of the project are summarized.

ITER Central Solenoid support structure analysis

The ITER Central Solenoid (CS) is comprised of six independent coils held together by a pre-compression support structure. This structure must provide enough preload to maintain sufficient coil-to-coil contact and interface load throughout the current pulse. End of burn (EOB) represents one of the most extreme time-points during the reference scenario when the currents in the CS3 coils oppose those of CS1 & CS2. The CS structure is performance limited by the room temperature static yield requirements needed to support the roughly 180 MN preload to resist coil separation during operation. This preload is applied by inner and external tie plates along the length of the coil stack by mechanical fastening methods utilizing Superbolt® technology. The preloading structure satisfies the magnet structural design criteria of ITER and will be verified during mockup studies. The solenoid is supported from the bottom of the toroidal field (TF) coil casing in both the vertical radial directions. The upper support of the CS coil structure maintains radial registration with the TF coil in the event of vertical disruptions (VDE) loads and earthquakes. All of these structure systems are analyzed via a global finite element analysis (FEA). The model includes a complete sector of the TF coil and the CS coil/structure in one self-consistent analysis. The corresponding results and design descriptions are described in this report.

ITER CS Conductor Helium Inlet Design Optimization and Evaluation

Abstract The ITER central solenoid (CS) is wound from cable-in-conduit-conductor (CICC) and cooled by supercritical Helium (He) delivered to ~120 inner diameter (ID) turns through integrally welded “inlets.” The flow to each inlet splits and passes through two pancakes, exiting at outlets. While both the He supply and return points (outlets) require penetrating the conduit wall, the inlets reside in the highest stress field, and thus become the more critical structural element. The CS Conceptual Design Review (CRD) reference He inlet design has a long, narrow slot in the inside diameter (ID) turn wall with pencil-tip shaped ends. This shape is optimized in order to minimize the hoop stress concentration. The slot length is chosen to expose each of the six superconducting (SC) sub-cables to the He cooling supply. Implementing this design at 120 inlet sites requires substantial machining and welding operations where even virgin conduit has minimal structural margin. A design space exploration produces numerous inlet options. One configuration emerges as the new reference configuration: the oblong, heavy-wall boss. It addresses all of the critical issues: bi-axial stress field, pressure drop and sub-cable flow uniformity, manufacturing costs (complexities and risks) and in-service robustness (least invasive, greatest margin). Finite element (FE) simulations are presented which highlight the results of the optimization and evaluation process.

ITER CENTRAL SOLENOID COIL INSULATION QUALIFICATION

An insulation system for ITER Central Solenoid must have sufficiently high electrical and structural strength. Design efforts to bring stresses in the turn and layer insulation within allowables failed. It turned out to be impossible to eliminate high local tensile stresses in the winding pack. When high local stresses can not be designed out, the qualification procedure requires verification of the acceptable structural and electrical strength by testing. We built two 4×4 arrays of the conductor jacket with two options of the CS insulation and subjected the arrays to 1.2 million compressive cycles at 60 MPa and at 76 K. Such conditions simulated stresses in the CS insulation. We performed voltage withstand tests and after end of cycling we measured the breakdown voltages between in the arrays. After that we dissectioned the arrays and studied micro cracks in the insulation. We report details of the specimens’ preparation, test procedures and test results.

Testing of compact bolted fasteners with insulation and friction-enhanced shims for NCSX

The fastening of the National Compact Stellarator Experiments (NCSX) modular coils presented a number of engineering and manufacturing challenges due to the high magnetic forces, need to control induced currents, tight tolerances and restrictive space envelope. A fastening method using high strength studs, jack nuts, insulating spacers, bushings and alumina coated shims was developed which met the requirements. A test program was conducted to verify the design. The tests included measurements of flatness of the spacers, determination of contact area, torque vs. tension of the studs and jack nuts, friction coefficient tests on the alumina and G-10 insulators, electrical tests, and tension relaxation tests due to temperature excursions from room temperature to liquid nitrogen temperatures. This paper will describe the design and the results of the test program.

Status of design and manufacturing of the ITER Central Solenoid and Correction Coils

The Final Design of the Central Solenoid (CS) of the ITER Magnet system is currently being completed by the US ITER Domestic Agency (USDA) and the manufacturing line of the coil under installation at the suppliers premises in the USA. The Central Solenoid includes 6 identical Nb3Sn coil modules independently powered and enclosed inside a precompression structure preventing their separation. The CS structure includes 9 subsets, made of Nitronic 50 high strength austenitic stainless steel, evenly distributed around the stack of the 6 modules.