This paper considers a fast track to non-energy applications of nuclear fusion that is associated with the ‘fusion for neutrons’ (F4N) paradigm. Being a useful product accompanying energy, fusion neutrons are more valuable than the energy released in DT reactions and they are urgently needed for research purposes and to develop and validate modern technologies. In the near future neutron yield in fusion devices will become significantly larger than that of fission and accelerator sources. This paper describes a compact tokamak fusion neutron source based on a small spherical tokamak (FNS-ST) with a MW range of DT fusion power and considers the key physics issues of this device. The major and minor radii are ∼0.5 and ∼0.3 m with magnetic field ∼1.5 T, heating power less than 15 MW and plasma current 1–2 MA. The production rate of DT neutrons of (3–10) × 1017 n s−1 and their flux at the first wall of 0.2 MW m−2 ensure that the device is capable of fusion–fission demonstration experiments. The problems of major concern are discharge initiation, current drive, plasma—fast ion beam stability and high first wall and divertor loads. The conceptual design provides solutions to these problems and suggests the feasibility of the FNS-ST.

A full tokamak discharge simulator has been developed by combining a freeboundary equilibrium evolution code, DINA-CH, and an advanced transport modelling code, CRONOS. The combined tokamak discharge simulator provides a full simulation of a whole tokamak discharge, including nonlinear coupling effects between the evolution of the free-boundary plasma equilibrium and transport. The free-boundary plasma equilibrium evolution is self-consistently calculated with the plasma current diffusion, in response to currents flowing in the PF coils and the surrounding conducting system. The heat and current source profiles calculated taking the free-boundary plasma equilibrium are used for the plasma transport. The constraints in operating a tokamak, such as the PF coil current and voltage limits, are taken into account. The potential of the combined tokamak discharge simulator is demonstrated by simulating whole operation phases of the inductive 15 MA ELMy H-mode ITER scenario 2. Issues related to ITER operation, such as respecting the coil current limit, vertical instability and poloidal flux consumption, are investigated. ITER hybrid mode operation is studied focusing on the capability of operating the plasma with a stationary flat safety factor profile.

Lower hybrid (LH) assisted plasma current ramp-up in ITER is demonstrated using a free-boundary full tokamak discharge simulator which combines the DINA-CH and CRONOS codes. LH applied from the initial phase of the plasma current ramp-up increases the safety margins in operating the superconducting poloidal field coils both by reducing resistive ohmic flux consumption and by providing non-inductively driven plasma current. Loss of vertical control associated with high plasma internal inductance is avoided by tailoring the plasma current density profiles. Effects of early LH application on the plasma shape evolution are identified by the free-boundary plasma simulation.

By concept development of the compact volumetric neutron source on the spherical tokamak JUST basis for minor actinides transmutation with aspect ratio A ¼ 2, some key plasma physics problems are arising: start of discharge; plasma current maintenance in stationary stage; appropriate neutron fluence for transmutation. On the basis of accepted physical and technical preconditions of the concept the combined scenario of current start and ramp-up, its stationary maintenance due to bootstrap effect and drive by neutral particle injection are considered. The plasma current is proposed to be initiated inductively and then bootstrap current will generate with using additional heating by injection of neutral beams. Necessary neutron flux for effective transmutation (G n % 0.4 MW=m 2 ) will be reached by both plasma-beam reactions and thermal plasma fusion reactions. By the way of preliminary consideration engineering studies of vacuum chamber, toroidal magnetic system, divertor, and blanket with energy multiplication ME % 20-100 are presented.

Frequency locking of edge localized modes (ELMs) to the vertical plasma movements induced by magnetic perturbations first demonstrated in TCV was successfully repeated in ASDEX Upgrade. However, the ELMs were triggered in ASDEX Upgrade when the plasma is moving down towards the X-point with a consequent decrease of the plasma current density in the edge region, contrary to the previous observation on TCV in which ELMs were triggered when the edge current is increased by an upward plasma movement. This opposite behaviour observed in the magnetic triggering of ELMs has been investigated by using a free-boundary tokamak simulator, DINA-CH. The passive stabilization loops (PSLs) located inside the vacuum vessel of ASDEX Upgrade produce similar external linking flux changes to those generated by the G-coil sets in TCV for opposite vertical plasma movements. Therefore, both plasmas experience similar local flux surface expansions near the upper G-coil set and PSL when the ELMs are triggered. In ASDEX Upgrade, however, the localized expansion of the plasma flux surfaces near the upper PSL is observed with the global shrinkage of the plasma column accompanied by the downward plasma movement.

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В статье анализируются различные направления исследований возможности использования компактных токамаков в качестве источника нейтронов. Расчёты основных параметров токамака с «теплыми» обмотками R = 2 и А = 2 показали, что на такой установке можно утилизировать отработавшее ядерное топливо с 10-15 реакторов типа ВВЭР-1000. Анализируются проекты демонстрационного варианта токамака-реактора уменьшенных размеров и пониженной стоимости с «тёплыми» или сверхпроводящими обмотками. Показано, что в этих случаях возможно получение плотностей потоков нейтронов на бланкет, позволяющих решить научно-технические проблемы разработки реактора-токамака для промышленного использования в атомной энергетике. Ключевые слова: токамак-реактор, источник нейтронов, трансмутация, минорные актиниды, плазма.

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