P. Titus

Design concept of K-DEMO for near-term implementation

A Korean fusion energy development promotion law (FEDPL) was enacted in 2007. As a following step, a conceptual design study for a steady-state Korean fusion demonstration reactor (K-DEMO) was initiated in 2012. After the thorough 0D system analysis, the parameters of the main machine characterized by the major and minor radii of 6.8 and 2.1 m, respectively, were chosen for further study. The analyses of heating and current drives were performed for the development of the plasma operation scenarios. Preliminary results on lower hybrid and neutral beam current drive are included herein. A high performance Nb3Sn-based superconducting conductor is adopted, providing a peak magnetic field approaching 16 T with the magnetic field at the plasma centre above 7 T. Pressurized water is the prominent choice for the main coolant of K-DEMO when the balance of plant development details is considered. The blanket system adopts a ceramic pebble type breeder. Considering plasma performance, a double-null divertor is the reference configuration choice of K-DEMO. For a high availability operation, K-DEMO incorporates a design with vertical maintenance. A design concept for K-DEMO is presented together with the preliminary design parameters.

The Fusion Nuclear Science Facility, the Critical Step in the Pathway to Fusion Energy

The proposed Fusion Nuclear Science Facility (FNSF) represents the first facility to enter the complex fusion nuclear regime, and its technical mission and attributes are being developed. The FNSF represents one part of the fusion energy development pathway to the first commercial power plant with other major components being the pre-FNSF research and development, research in parallel with the FNSF, pre-DEMO research and development, and the demonstration power plant (DEMO). The Fusion Energy Systems Studies group is developing the technical basis for the FNSF in order to provide a better understanding of the demands on the fusion plasma and fusion nuclear science programs.

Iter In-Vessel Coil design and R&D

ITER will incorporate In Vessel Coils (IVCs) as a method of stabilizing “Edge Localized Modes” (ELM) and providing “Vertical Stabilization” (VS). To meet the ELM and VS Coil requirements strong coupling with the plasma is required so that it is necessary for the coils to be installed in the vessel just behind the blanket shield modules. Due to this close proximity to the plasma the radiation and temperature environment is severe and conventional electrical insulation materials and processes cannot be used. The development of mineral insulated conductor technology has been required in the IVC design to deal with this high radiation and high temperature environment. While mineral insulated conductor technology is not new, building a large magnet with high current carrying capability and a conductor diameter larger than the mineral insulated conductor currently manufactured requires R&D and the extension of existing technologies. A 59mm Stainless Steel Jacketed Mineral Insulated Conductor (SSMIC) using MgO is being developed for this application. The IVC ELM and VS coils design includes both the development of the fabrication techniques for the SSMIC and the design and analysis of the ELM and VS Coil assemblies.

DESIGN OF THE ITER IN-VESSEL COILS

Abstract The ITER project is considering the inclusion of two sets of in-vessel coils, one to mitigate the effect of Edge Localized Modes (ELMs) and another to provide vertical stabilization (VS). The in-vessel location (behind the blanket shield modules, mounted to the vacuum vessel inner wall) presents special challenges in terms of nuclear radiation (˜3000 MGy) and temperature (100 °C vessel during operations, 200 °C during bakeout). Mineral insulated conductors are well suited to this environment but are not commercially available in the large cross section required. An R&D program is underway to demonstrate the production of mineral insulated (MgO or Spinel) hollow copper conductor with stainless steel jacketing needed for these coils. A preliminary design based on this conductor technology has been developed and is presented herein.

Progress toward commissioning and plasma operation in NSTX-U

The National Spherical Torus Experiment-Upgrade (NSTX-U) is the most powerful spherical torus facility at PPPL, Princeton USA. The major mission of NSTX-U is to develop the physics basis for an ST-based Fusion Nuclear Science Facility (FNSF). The ST-based FNSF has the promise of achieving the high neutron fluence needed for reactor component testing with relatively modest tritium consumption. At the same time, the unique operating regimes of NSTX-U can contribute to several important issues in the physics of burning plasmas to optimize the performance of ITER. NSTX-U further aims to determine the attractiveness of the compact ST for addressing key research needs on the path toward a fusion demonstration power plant (DEMO). The upgrade will nearly double the toroidal magnetic field BT to 1 T at a major radius of R0 = 0.93 m, plasma current Ip to 2 MA and neutral beam injection (NBI) heating power to 14 MW. The anticipated plasma performance enhancement is a quadrupling of the plasma stored energy and near doubling of the plasma confinement time, which would result in a 5–10 fold increase in the fusion performance parameter nτ T. A much more tangential 2nd NBI system, with 2–3 times higher current drive efficiency compared to the 1st NBI system, is installed to attain the 100% non-inductive operation needed for a compact FNSF design. With higher fields and heating powers, the NSTX-U plasma collisionality will be reduced by a factor of 3–6 to help explore the favourable trend in transport towards the low collisionality FNSF regime. The NSTX-U first plasma is planned for the Summer of 2015, at which time the transition to plasma operations will occur.

Stress multipliers for the NSTX upgrade digital coil protection system

Conceptual design of the upgrade to NSTX, explored designs sized to accept the worst loads that power supplies could produce. This produced excessive structures that would have been difficult to install and were much more costly than needed to meet the scenarios required for the upgrade mission. Instead, the project decided to rely on a digital coil protection system (DCPS). Initial sizing was then based on the 96 scenarios in the project design point with some headroom to accommodate operational flexibility and uncertainty. This has allowed coil support concepts that minimize alterations to the existing hardware. The digital coil protection system theory, hardware and software are described in another paper at this conference. The intention of this paper is to describe the generation of stress multipliers, and algorithms that are used to characterize the stresses at key areas in the tokamak, as a function of either loads calculated by the influence coefficients computed in the DCPS software, or directly from the coil currents.

Digital Coil Protection System (DCPS) algorithms for the NSTX centerstack upgrade

A significant upgrade is planned for the National Spherical Torus eXperiment (NSTX) in which plasma current and confining magnetic field intensities will nearly double while plasma duration will more than double. Changes will include replacing the existing centerstack with a new one containing a thicker TF inner Leg, a new OH solenoid coil, and an expansion to six of the complement of PF1 coils controlling plasma divertor shape. Other coils will remain in service with hardware modifications as needed for their approximately three-fold increases in magnetic forces.

Overview of Design and Analysis Activities for the W7-X Scraper Element

The Wendelstein 7-X stellarator is in final stages of commissioning, and will begin operation in late 2015. In the first phase, the machine will operate with a limiter, and will be restricted to low power and short pulse. But in 2019, plans are for an actively cooled divertor to be installed, and the machine will operate in steady state at full power. Recently, plasma simulations have indicated that, in this final operational phase, a bootstrap current will evolve in certain scenarios. This will cause the sensitive ends of the divertor target to be overloaded beyond their qualified limit. A high heat flux scraper element (HHF-SE) has been proposed in order to take up some of the convective flux and reduce the load on the divertor. In order to examine whether the HHF-SE will be able to effectively reduce the plasma flux in the divertor region of concern, and to determine how the pumping effectiveness will be affected by such a component, it is planned to include a test divertor unit scraper element (TDU-SE) in 2017 during an earlier operational phase. Several U.S. fusion energy science laboratories have been involved in the design, analysis (structural and thermal finite element, as well as computational fluid dynamics), plasma simulation, planning, prototyping, and diagnostic development around the scraper element program (both TDU-SE and HHF-SE). This paper presents an overview of all of these activities and their current status.

Analysis of NSTX Upgrade OH Magnet and Center Stack

Abstract The new ohmic heating (OH) coil and center stack for the National Spherical Torus Experiment (NSTX) upgrade are required to meet cooling and structural requirements for operation at the enhanced 1 Tesla toroidal field and 2 MA plasma current. The OH coil is designed to be cooled in the time between discharges by water flowing in the center of the coil conductor. We performed resistive heating and thermal hydraulic analyses to optimize coolant channel size to keep the coil temperature below 100 C and meet the required 20 minute cooling time. Coupled electromagnetic, thermal and structural FEA analyses were performed to determine if the OH coil meets the requirements of the structural design criteria. Structural response of the OH coil to its self-field and the field from other coils was analyzed. A model was developed to analyze the thermal and electromagnetic interaction of centerstack components such as the OH coil, toroidal field (TF) coil inner legs and the Bellville washer preload mechanism. Torsional loads from the TF interaction with the OH and poloidal fields are transferred through the TF flag extensions via a torque transfer coupling to the rest of the tokamak structure. A 3D FEA analysis was performed to qualify this design. The results of these analyses, which will be presented in this paper, have led to the design of OH coil and centerstack components that meet the requirements of the NSTX-upgrade structural design criteria.

Design of the C-Mod Advanced Outer Divertor

Operational requirements and research considerations make a high-temperature, toroidally continuous outer divertor an important aspect for future operation of the Alcator C-Mod tokamak. Leading edge melting of tiles, nonuniform heat loads, large electromagnetic forces, and localized impurity sources limit the performance of bulk plasmas. These issues can be addressed by the installation of a well-aligned, toroidally continuous outer divertor. In addition, future proposed long pulse operation of C-Mod will cause the temperature of the outer divertor to reach bulk temperatures as high as 500-600°C. This future operational requirement combined with the strong temperature dependence of plasma surface interactions (especially fuel retention), makes a controllable, high-temperature outer divertor desirable and necessary. The design and development of the C-Mod Advanced Outer Divertor (AOD) is discussed.