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This article presents the realistic initial design studies of a 300-GHz, 1-MW, conventional-cavity gyrotron for its probable application in the next-generation thermonuclear fusion reactors. Keeping the design goals, parameters, and constraints in view, the very high-order TE49,18 mode is chosen as the operating mode after a careful mode-selection calculation considering realistic ohmic cavity losses. After mode selection and mode competition studies, the cold-cavity design and initial design of a triode-type magnetron injection gun (T-MIG) and a gyrotron magnet are carried out and an electron beam radius of 8.11 mm is obtained with 2.4% velocity spread. Furthermore, investigation on RF behavior of the cavity is performed with the T-MIG beam parameters. By varying the nominal beam parameters, single-mode self-consistent calculations are conducted and achieved the desired output power. Then, multimode time-dependent self-consistent calculations are carried out before and after space-charge neutralization (SCN) with realistic velocity spread (up to 6%) and different beam radii for the assessment of the start-up scenario. Before SCN without velocity spread, the beam voltage is depressed to 70.08 kV and 0.72-MW output power is obtained, whereas with velocity spread (6%), 0.69-MW output power is obtained with 8.11 mm of beam radius. After 60% of SCN in the start-up scenario with velocity spread (6%), the beam voltage increases to 74.83 kV, and thereby, an output power of 0.91 MW is obtained. [ABSTRACT FROM AUTHOR]

The European roadmap to the realisation of fusion energy has identified a number of technological and scientific challenges towards the development of a DEMO reactor. Mission 2 'Heat-exhaust systems' includes the investigation of alternative divertor configurations such as the snowflake, X and super-X divertors as a reliable solution for the power exhaust problem. This paper, starting from the geometrical description of a conventional European DEMO scenario with an aspect ratio of 3.1 and a reference Single Null configuration, firstly illustrates the objectives, the figure of merits and the constraints considered in the design of alternative configurations. DEMO descriptions for Double Null, Snowflake, X-divertor and Super-X configurations optimizing the plasma shape, the machine geometry and the PF coil system will be proposed. A comparison of costs and benefits of the various configurations is given, with particular reference to the power exhaust issues. [ABSTRACT FROM AUTHOR]

This paper addresses the issue of magnetic control of the DEMO device through the equatorial in-vessel coils. While vertical stabilization and radial control are typical applications of equatorial coils in plasma control, the cumbersome compatibility of in-vessel divertor coils with the DEMO divertor structures and remote handling draws attention to the possibility of using the in-vessel equatorial coils also for active plasma wobbling, as an alternative to traditional strike-point sweeping. In this work, the technical specifications of the DEMO equatorial in-vessel coils and the design requirements of the vertical stabilization and radial control systems are assessed. Finally, an optimized strategy for efficient power-exhaust control through active wobbling in DEMO is proposed. • Active wobbling and fast plasma control on DEMO with the in-vessel equatorial coils. • Technical specifications and design requirements of the equatorial in-vessel coils. • Numerical validation of the plasma wobbling control techniques on DEMO. [ABSTRACT FROM AUTHOR]

Since the year 2013, the Swiss Plasma Center (SPC) has proposed a Toroidal Field (TF) layout for the DEMO- EUROfusion tokamak, based on a graded winding pack made of layers of Nb3Sn (react-and-wind) conductors. In summer 2015, a new reference baseline is issued for the DEMO- EUROfusion tokamak, leading to an update of the TF coil requirements, e.g. the operating current has been reduced from 80 kA to 63 kA. Consequently, the conductor layouts for every graded layer of the TF coil winding pack is re-designed in order to match the new requirements. Each layer of TF coil winding pack has to be connected electrically in series to form the coil. The inter-layer Nb3Sn splice joint design which does not exceed the conductor dimensions is proposed in this paper for the updated 63 kA Nb3Sn TF conductor. This proposed joint design allows a continuous winding of TF coil winding pack from layer to layer, housing the joint at the zone of inter-layer transition. Ultimately, the all inter-layer joints should be arranged within the winding pack at the low-field and low mechanically stressed region of D-shaped TF coil. [ABSTRACT FROM AUTHOR]

Thermal radiation analysis of the DEMO tokamak based on the 2015 baseline CAD design model was performed. For the purpose of analysis, Vacuum Vessel Thermal Shield (VVTS) and Cryostat Thermal Shield (CTS) were designed on a conceptual level to complement the baseline DEMO CAD model. The Finite Element (FE) code ABAQUS was used to perform numerical analyses. A special care was taken to adapt the geometry and mesh of the FE model to reduce the heat transfer error to an acceptable level. Thermal loading on DEMO components and refrigeration power for the base case scenario with actively cooled thermal shields was determined. Besides the base case, the sensitivity analysis with passive CTS was performed. [ABSTRACT FROM AUTHOR]

• A new strategy for the sweeping control on DEMO device is developed. • The possibility to improve the sweeping control performance with in-vessel coils is investigated. • The compatibility of the sweeping control with vertical stability and shape control is investigated. • The sweeping performance has been validated by numerical simulations. In this paper, the sweeping control strategy is applied to DEMO single null configuration as possible solution for the power exhaust problem. Firstly, a possible sweeping control strategy is proposed to impose a periodic movement of the plasma divertor legs at a desired frequency with an almost fixed plasma shape. Then, the performance of the sweeping control using ex-vessel coils is analyzed in terms of current, voltage and maximum sweeping frequency. Finally, the possibility to implement the sweeping with the in-vessel coils is considered. Two possible solutions for the in-vessel coils have been adopted: a preliminary solution compatible with the actual DEMO port locations and an optimized solution that is more performing for the sweeping but needs to be validated for the compatibility with the remote handling. Numerical simulations demonstrate the effectiveness of the proposed approach coupling the sweeping control with the feedback control strategies for the plasma shape and position. [ABSTRACT FROM AUTHOR]