P.J. Knight

Integrated plasma physics modelling for the Culham steady state spherical tokamak fusion power plant

Integrated modelling of important plasma physics issues related to the design of a steady-state spherical tokamak (ST) fusion power plant is described. The key is a steady-state current drive, and 92% of this is provided by a combination of bootstrap and diamagnetic currents, both of which have a substantial toroidal component in a ST. The remaining current is to be provided by either neutral beam injection or radio-frequency waves, and various schemes for providing this are discussed and quantified. The desire to achieve a high bootstrap current drives the design to high plasma pressure, ? (normalized to the magnetic field pressure), and high elongation. Both these requirements have implications for ideal magneto-hydrodynamic instability which are discussed. Confinement is addressed both through comparison with the recent scaling laws developed from the conventional tokamak database and self-consistent one-dimensional modelling of the transport processes. This modelling shows that the power required for the current drive (~50?MW) is sufficient to heat the plasma to a regime where more than 3?GW of fusion power is produced, taking into account the dilution due to He ash and prompt ?-particle losses, which are small. A preliminary study of the micro-instabilities, which may be responsible for the turbulent transport is provided. Given assumptions about the particle confinement, we make estimates of the fuelling requirements to maintain the steady state. Finally, the power loading due to the exhaust is derived using theory-based scalings for the scrape-off layer width.

Global two-fluid turbulence simulations of L-H transitions and edge localized mode dynamics in the COMPASS-D tokamak

It is shown that the transition from L-mode to H-mode regimes in tokamaks can be reproduced using a two-fluid, fully electromagnetic, plasma model when a suitable particle sink is added at the edge. Such a model is implemented in the CUTIE code [A. Thyagaraja et al., Eur. J. Mech. B/Fluids 23, 475 (2004)] and is illustrated on plasma parameters that mimic those in the COMPASS-D tokamak with electron cyclotron resonance heating [Fielding et al., Plasma Phys. Contr. Fusion 42, A191 (2000)]. In particular, it is shown that holding the heating power, current, and magnetic field constant and increasing the fuelling rate to raise the plasma density leads spontaneously to the formation of an edge transport barrier (ETB) which occurs going from low to higher density experimentally. In the following quiescent period in which the stored energy of the plasma rises linearly with time, a dynamical transition occurs in the simulation with the appearance of features resembling strong edge localized modes. The simulation...

A model of the availability of a fusion power plant

The availability of a fusion power plant is dependent on the frequency and duration of both planned and unplanned outages. Planned outages result from a maintenance plan for major component replacement, and are entirely predictable once divertor and blanket lifetimes are established. Unplanned outages result from failures of one or more of many diverse components, and can be estimated only by a probabilistic approach taking into account distributions of failure frequencies and repair times. A Monte Carlo model, PAMPAS, has been developed which will enable studies of new conceptual power plant designs once sufficient design detail and component reliability data are available. It can already be used for more generic studies and results are shown which give insight into the magnitude of the problem of achieving adequate plant availability. A simplified model of plant availability has been incorporated into the PROCESS systems code, in order to assess the economic impact of plant unavailability. Initial results show a strong influence on optimum plant parameters dependent on the speed of maintenance operations. This work is continuing as part of preparations for a European fusion power plant conceptual study.

A conceptual design of a spherical tokamak power plant

Abstract The spherical tokamak (ST) has a very low aspect ratio, typically below ∼1.8, which allows operation at high beta. The ST also offers stability at high elongation which permits operation at high bootstrap current fraction leaving only a modest external current drive requirement. Results from START indicate the ST has resilience to disruptions and low halo currents when disruptions are triggered. In order to explore these potential advantages, a conceptual design of a steady-state ST power plant is being developed for which the thermodynamic, neutronic and mechanical design of the plant have been iterated, together with the plasma parameters, to give a consistent design. The baseline design uses water cooled copper for the centre rod and return limbs of the toroidal field (TF) coils with minimal steel shielding around the rod giving a simple coil design. A helium cooled ceramic pebble bed blanket with beryllium multiplier is used to generate the required tritium and achieve a high thermal efficiency.

CENTORI: A global toroidal electromagnetic two-fluid plasma turbulence code

A new global two-fluid electromagnetic turbulence code, CENTORI, has been developed for the purpose of studying magnetically-confined fusion plasmas on energy confinement timescales. This code is used to evolve the combined system of electron and ion fluid equations and Maxwell equations in toroidal configurations with axisymmetric equilibria. Uniquely, the equilibrium is co-evolved with the turbulence, and is thus modified by it. CENTORI is applicable to tokamaks of arbitrary aspect ratio and high plasma beta. A predictor–corrector, semi-implicit finite difference scheme is used to compute the time evolution of fluid quantities and fields. Vector operations and the evaluation of flux surface averages are speeded up by choosing the Jacobian of the transformation from laboratory to plasma coordinates to be a function of the equilibrium poloidal magnetic flux. A subroutine, GRASS, is used to co-evolve the plasma equilibrium by computing the steady-state solutions of a diffusion equation with a pseudo-time derivative. The code is written in Fortran 95 and is efficiently parallelised using Message Passing Interface (MPI). Illustrative examples of output from simulations of a tearing mode in a large aspect ratio tokamak plasma and of turbulence in an elongated conventional aspect ratio tokamak plasma are provided.

A filament code for tokamak axisymmetric stability

Abstract A program for simulating the axisymmetric time evolution of plasma position within a tokamak is described. The conductors in the system are represented by sets of toroidal filaments which are given the relevant electrical and geometrical properties. The plasma position is determined by force balance, and a matrix equation is solved for the resulting filament currents. The program theory and structure together with some comparisons with analytical theory are presented. Also, to illustrate the capability of the code, some results that have contributed to the design of a possible new system of fast active plasma control are described.

Full orbit simulation of collisional transport of impurity ions in the MAST spherical tokamak

Transport analysis of MAST discharges indicates that collisions are an important loss mechanism in the core of a tight aspect ratio tokamak. In the strongly varying equilibrium fields of MAST many of the assumptions of drift kinetic and neoclassical theory (e.g. small plasma inverse aspect ratio and low ratio of toroidal Larmor radius to poloidal Larmor radius) are not met by all particle species and it becomes appropriate to use full orbit analysis to evaluate heat and particle fluxes. Collisional transport of impurity ions (C6+ and W20+) has been studied using a full orbit solver, CUEBIT, to integrate the test-particle dynamics. Electromagnetic fields in MAST plasma have been modelled using the cylindrical and toroidal two-fluid codes CUTIE and CENTORI. A detailed study of the scaling of the test-particle diffusivity with collisionality in the equilibrium field reveals deviations from the standard neoclassical theory, in both the Pfirsch–Schluter and banana regimes, and difficulties in defining a local diffusivity at low collisionalities. The effect of electric and magnetic fluctuations is also briefly addressed. It is found that field fluctuations enhance the non-diffusive nature of transport. The full orbit analysis presented here predicts levels of transport and confinement times for the examined species broadly consistent with the experimental observations.

The effect of optimal wall loads and blanket technologies on the cost of fusion electricity

Abstract This paper presents a discussion of trends in fusion economics based on technology, as well as, physics arguments. Based on relatively simple physics considerations, supported by detailed systems code calculations, it is shown that optimal wall loads are not high. The results of systems code calculations, focussing on the economic impact of different blanket technologies, are described. These suggest that the economically favourable thermodynamic efficiencies of some blankets capable of operating at higher temperatures may be counterbalanced by the economic penalties of shorter lifetimes.