D.Yu. Sychugov

Concept of the divertor of a fusion neutron source based on a spherical tokamak

A concept of the divertor and the technology for organizing the edge plasma in a fusion neutron source based on a spherical tokamak (FNS-ST) are described. The experimental data on the characteristics of the peripheral plasma in modern tokamaks are extrapolated to the FNS-ST conditions with the help of semi-analytical models. The effects depending on the magnetic configuration and on the geometry and materials of the divertor and the first-wall elements are considered. Possible designs of the FNS-ST divertor and the first wall are described. Using an original model, it is shown that the maximum density of the heat flux at the divertor plates in a double-null magnetic configuration does not exceed 5–6 MW/m2, which complies with modern engineering capabilities. Methods for further improvement of the FNS-ST divertor concept are analyzed.

Vertical MHD Stability of the T-15M Tokamak Plasma

The objective of the present paper is to investigate the vertical stability of the plasma column in the project of the T-15M tokamak. It has been shown that a passive stabilizing structure consisting of a vacuum vessel and two (upper and lower) passive coils is capable of providing vertical stability of the plasma column.

Is it possible to extract information on the plasma pressure profile from magnetic measurements in stellarators

The free boundary stellarator equilibria are studied in the framework of averaged equations. The structure of external magnetic fields created by currents flowing inside the plasma column is analysed. In all cases considered the dipole component of the magnetic field (in agreement with previous investigations) can only be used to determine the volume averaged value of the parameter beta if the net toroidal current is known. For zero net current operation it is shown that the quadrupole component of the external magnetic field can be used for an estimate of the plasma pressure distribution, for example, in the framework of a one parametric set of profiles. The dependence of the external magnetic field on the current distribution for the given pressure profile is also investigated, and it is shown that there is a possibility of distinguishing at least strongly different current distributions (such as the peaked and off-centred current profiles considered here) either for a large shear system at high beta or for a device with averaged vertical elongation of the vacuum magnetic surfaces

Integrated modelling of DEMO-FNS current ramp-up scenario and steady-state regime

An approach to the integrated modelling of plasma regimes in the projected neutron source DEMO-FNS based on different codes is developed. The consistency check of the steady-state regime is carried out, namely, the possibility of the plasma current ramp-up, acceptance of growth rates of MHD modes in the steady-state regime, heat loads to the wall and divertor plates and neutron yield value. The following codes are employed for the integrated modelling. ASTRA transport code for calculation of plasma parameters in the steady-state regime, NUBEAM Monte Carlo code for NBI incorporated into the ASTRA code, DINA free boundary equilibrium and evolution code, SPIDER free boundary equilibrium and equilibrium reconstruction code, KINX ideal MHD stability code, TOKSTAB rigid shift vertical stability code, edge and divertor plasma B2SOLPS5.2 code and Semi-analytic Hybrid Model (SHM) code for self-consistent description of the core, edge and divertor plasmas based on the experimental scaling laws. The consistent steady-state regime for the DEMO-FNS plasma and the plasma current ramp-up scenario are developed using the integrated modelling approach. Passive copper coils are suggested to reduce the plasma vertical instability growth rate to below ∼30 s −1 .The outer divertor operation in the ‘high-recycling’ regime is numerically demonstrated with a maximal heat flux density of 7–9 MW m −2 that is technically acceptable.

Finite pressure induced destruction of the plasma boundary in stellarators

Free-boundary MHD equilibria in stellarators are investigated. Particular attention is placed on an analysis of the influence of the plasma induced magnetic fields on the shape and sizes of the plasma boundary. Only a weak sensitivity of the plasma boundary destruction on the pressure and current density distributions is found. The existing moderate pressure profile dependence increases on passing from a shearless system to a system with large magnetic shear. One way to reduce the damage is to use external axisymmetrical magnetic fields. The efficiency of such techniques is briefly discussed. The problem of plasma boundary determination from the results of external magnetic measurements is analysed, and the accuracy of the magnetic measurements available for a sufficiently accurate identification of the plasma boundary by the method of field line tracing is estimated. The limiting plasma pressure determined by the conditions for plasma equilibrium is also discussed

Mathematical properties of plasma vertical instability in Tokamak

The model of plasma vertical instability in tokamak has been investigated. Fast, slow and hybrid magnetohydrodynamical instabilities have been studied. The models of passive, active, linear and nonlinear feedback have been studied. Several theorems, which are interesting from the physical point of view, have been proven.

Methods for MHD plasma equilibria mathematical modeling

An overview of the state of the art and of the MHD plasma equilibrium modeling is given in the paper. The statements of the problems, their mathematical properties, and numerical algorithms are considered.

A New Algorithm for Generating Tetrahedral Grids

A new algorithm is presented for generating tetrahedral grids for bound domains of complicated structure and form that consist of the union of several subdomains. The grids obtained using the algorithm have regularity, in that there is no tetrahedron whose vertices lie at different sides of the subdomain boundaries. The algorithm has high fast-action: the time needed to generate a grid consisting of 105−106 tetrahedrons is approximately 20–40 seconds of the work of a standard personal computer. All steps of the algorithm are described in detail, and examples of generated grids are given.

NUMERICAL SIMULATION OF ELECTROMAGNETIC DIAGNOSTICS SYSTEM OF THE TOKAMAK T-15

В настоящее время проводится модернизация токамака Т-15. Одной из задач исследований на Т-15 является получение разрядов с вытянутым поперечным сечением плазмы, окружённым сепаратрисой. Присутствие на сепаратрисе Х-точки, где магнитное поле обращается в ноль, позволяет использовать данную конфигурацию для создания дивертора — устройства для вывода из плазмы примесей и продуктов реакции. Для оптимизации дивертора и контроля плазмы предполагается в каждый момент разряда проводить реконструкцию границы плазменного шнура. Достоверность такой реконструкции зависит от влияния различных факторов на сигналы электромагнитной диагностики. В работе на основе одного из возможных сценариев разряда в Т-15 проведено исследование точности реконструкции границы плазмы и профиля тока в зависимости от погрешности измерений и прочих факторов. Рассматривается задача определения границы плазмы на лимитерной и диверторной стадиях разряда, исследуется влияние распределения тока на сигналы в магнитных датчиках. Так как подобная задача является некорректной, изучается вопрос о выборе параметра регуляризации, в частности, когда погрешность измерений неизвестна. Входными данными задачи являются число магнитных датчиков и их расположение, а также погрешность измерения ими магнитных полей. Исследуется точность восстановления границы плазмы в целом и X-точки сепаратрисы отдельно в зависимости от погрешности измерений и количества датчиков. Моделирование сценария разряда проводилось с помощью кодов TOKAMEQ и DINA. Обратные задачи МГД-равновесия решались на основе кода RPB (Reconstruction of Plasma Boundary).

The numerical code TOKSCEN for modelling plasma evolution in tokamaks

The code TOKSCEN (TOKamak SCENario) for the modelling of plasma evolution is described in this paper. The modelling is based on numerical solution of the Grad–Shafranov equation of plasma equilibrium and circuit equations for eddy currents at each time step. The circuit equations for eddy currents are solved in matrix form using the technique of matrix inversion. The plasma current distribution should be given. The code enables the calculation of an increment of vertical instability at each time step of the plasma evolution. The algorithms of the code were used for the modelling of processes in KTM and other tokamaks.