Long timescale density peaking in JET

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The research programme performed on the Mega Amp Spherical Tokamak (MAST) Upgrade experiment has made significant advances in developing the physics understanding of low aspect ratio tokamaks in support of the operation of ITER and design of fusion powerplants. High performance plasma scenarios have been developed to facilitate a broad programme of experiments, in which confinement is constrained by the presence of m/n = 2/1 modes that cause substantial losses of fast ions. The onset of these modes coincide

Nuclear fusion offers a promising route toward a clean and abundant energy source, though harnessing its potential remains an important scientific and engineering challenge. This thesis delves into one of the main challenges of fusion, namely managing the high heat fluxes exhausted from the plasma core. The understanding of scrape-off layer and divertor physics, which define how this heat load will be transported towards the reactor walls, is of crucial importance for the achievement of future reliable ther

The scrape-off layer (SOL) sets the bounday conditions of a tokamak, determining the plasma confinement, the heat exhaust, the impurity levels, and controlling the fuelling of the device. Therefore, a first principles understanding of the physical mechanisms governing SOL turbulence is crucial on the way towards fusion energy. We describe SOL simulations carried out by using GBS, a three-dimensional numerical code that solves the drift-reduced Braginskii equations for the two-fluid model of the plasma and c

The unique flexibility of TCV in plasma shaping has been exploited to address different aspects of tokamak physics in which the plasma shape may play a role. This paper summarizes the experiments undertaken in the TCV tokamak ("Tokamak à Configuration Variable") over the last decade in stability and transport and offers a comprehensive and integrated view of the various effects of plasma shape observed. For each of these effects, the relation between experimental results and theoretical predictions is stres

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors clos

ELMy H-mode experiments at JET in 2000/mid-2002 have focused on discharges with normalized parameters for plasma density, energy confinement and beta similar to those of the ITER Q(DT) = 10 reference regime (n/n(GW) similar to 0.85, H-98(y,H-2) similar to 1, beta(N) similar to 1.8). ELMy H-mode plasmas have been realized reaching or even exceeding those parameters in steady-state conditions (up to similar to5 s or 12tau(epsilon)) in a reproducible way and only limited by the duration of the additional heati

Gyrotrons at high frequency with high-output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion. For the stellarator Wendelstein 7-X, now under construction at IPP Greifswald, Germany, a 10-MW electron-cyclotron-resonance-heating (ECRH) system is foreseen. A 1-MW 140-GHz gyrotron with synthetic diamond window for continuous wave operation and with a single stage depressed collector for energy recovery and improvement of efficiency has been designed, constr

Electromagnetic microinstabilities in tokamak plasmas are studied by means of a linear global eigenvalue numerical code. Ion dynamics is described by the gyrokinetic equation, so that finite ion Larmor radius effects are taken into account to all orders. Nonadiabatic electrons are included in the model, with passing particles described by the drift-kinetic equation and trapped particles through the bounce averaged drift-kinetic equation. A large aspect ratio plasma with circular shifted surfaces is consider

An experimental study of the extraordinary mode (X mode) absorption at the third electron cyclotron harmonic frequency has been performed on the TCV tokamak in plasmas preheated by X mode at the second harmonic. Full single pass absorption of injected X3 power was measured with X2 preheating in co-current drive (CO-ECCD). The measured absorption exceeds that predicted by the linear ray tracing code TORAY-GA by more than a factor of 2 for the CO-ECCD case. Experimental evidence indicates that a large fractio

Alfven eigenmodes (AEs) with intermediate toroidal mode numbers are modelled using the global gyrokinetic PENN code to determine the stability of high performance tokamak discharges in the presence of energetic particles. A large plasma pressure and a weak magnetic shear in the core give rise to radially extended kinetic AEs, which are stabilized by the high shear at the edge of a divertor (X point) configuration. Large values for the safety factor and the ion Larmor radius in reversed shear operation may h

In Se.C.R.E.T.S. (Segregated Copper Ratio Experiments on Transient Stability), the stability performance under transverse field transient is compared for two Nb3Sn cable-in-conduit conductors which differ only because of the different distribution of the stabilizing copper, either included in the Nb3Sn strand cross section or segregated as bundled copper wires. If the segregated copper is found to be effective for stability purposes, the copper fraction in the Nb3Sn strands can be Substantially reduced, wit

Recent stability results of external kink modes and vertical modes in compact stellarators are presented. The vertical mode is found to be stabilized by externally generated poloidal flux. A simple stability criterion is derived in the limit of large aspect ratio and constant current density. For a wall at infinite distance from the plasma, the amount of external flux needed for stabilization is given by F-i = (kappa(2) - kappa)/(kappa(2) + 1), where kappa is the axisymmetric elongation and F-i is the fract

Predictions from a gyrokinetic toroidal plasma model reproduce for the first time the evolution of Alfven eigenmode (AE) dampings over a range of discharges. The coupling between shear-and kinetic-Alfven waves is responsible for the main source of damping through Landau interactions and can be an order of magnitude larger than fluid predictions neglecting global kinetic effects. Strong stabilization occurs when the wave field gets localized radially by a rise in the edge magnetic shear, explaining why globa

The energy confinement time of TCV Ohmic, L-mode discharges is observed to depend strongly on the plasma shape, improving slightly with elongation and degrading strongly as triangularity is increased from zero to positive values. The thermal conductivity of these plasmas is found to be independent of the shape. This observation, combined with geometrical effects on the temperature gradient and a degradation with increasing input energy flux, can explain the observed variation in the energy confinement time.

The paper describes measurements of the sawtooth instability in JET, in which the instability wave function is shown to extend to the edge where it is measured using magnetic coils. The numerous magnetic probes in JET allow the time evolution of the (n = 0, 1, 2, 3) toroidal Fourier components to be analysed. The n = 1 magnetic component is similar to the m = 1 soft X-ray centroid motion. This fact indicates the potential of edge signals in retrieving the poloidal mode spectrum of the q = m/n = 1 surface. T

Long timescale density peaking in JET

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Overview of the MAST Upgrade physics programme: testing novel concepts at low aspect ratio to inform future devices

Performance assessment of a reactor-scale tightly baffled divertor using the SOLPS-ITER software

Plasma refuelling at the SOL simulated with the GBS code

Physics Insight and Performance Benefit from Plasma Shaping Experiments in MHD and Energy Transport in the TCV Tokamak

Plasma wall interaction and its implication in an all tungsten divertor tokamak

Towards the realization on JET of an integrated H-mode scenario for ITER

Advanced high-power gyrotrons

Global-gyrokinetic study of finite beta effects on linear microinstabilities

Full absorption of third harmonic ECH in TCV tokamak plasmas in the presence of second harmonic ECCD

Stability of Alfven eigenmodes in optimized tokamaks

Se.CRETTS : A stability experiment on the role of segregated copper in Nb3Sn cable-in-conduit conductors

Magnetohydrodynamics stability of compact stellarators

Prediction of Alfven eigenmode dampings in the Joint European Torus

Influence of plasma shape on transport in the TCV Tokamak

Global Sawtooth Instability Measured by Magnetic Coils in the Jet Tokamak