From current-driven to neoclassically driven tearing modes

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The EC Upper Launcher (UL) and Ex-vessel waveguide (EW) are integral components of the ITER Electron Cyclotron (EC) system [1], which plays a key role in achieving and maintaining the high temperatures required for nuclear fusion within the tokamak, providing plasma heating and current drive, and magnetohydrodynamics (MHD) stabilization, which are crucial for ITER’s operational success. The EC system is increasingly becoming the reference mechanism for plasma heating in both present and future devices. Nota

Simulating turbulence in the tokamak boundary, which is crucial for understanding and optimizing future fusion power plants performance, is computationally challenging due to the complex, multi-physics, and multi-scale nature of plasma dynamics. In the boundary region, plasma interacts with neutral particles recycled from the walls, leading to nonlinear phenomena that control confinement and heat exhaustion. Accurately capturing these coupled processes requires solving large-scale, stiff, and non-local pro

This thesis presents a study on the statistical characterization and predictability of plasma discharges in the TCV tokamak, based on time-dependent transport simulations performed with the ASTRA code. The work combines physical modeling, numerical implementation, and nonlinear time-series analysis to bridge the gap between transport simulations and the dynamical complexity of experimentally observed plasma behavior.

The effect of the limiter position on the scrape-off layer (SOL) width, radial electric field and intrinsic flows is investigated via global, three-dimensional turbulence simulations in four different limiter configurations. The limiter position affects the SOL dynamics in a number of ways, for example by changing the effective connection length or by modifying the unstable modes present in the system. The simulations show that the SOL width is much smaller and less poloidally asymmetric when the plasma is

All tokamak experiments using ECCD require setting of the beam elliptical polarization for proper coupling to the plasma. This is done either in the matching optics unit (MOU) at the output of the gyrotron, or in a couple of miter bends of the transmission line. Similarly, oblique ECE receivers require selection of the correct elliptical polarization to provide localized measurements. For the TCV tokamak at the CRPP, gyrotron and oblique-ECE polarizers are characterized during either high- or low- power tes

The existence of a critical pressure gradient needed to drive the interchange instability is experimentally demonstrated in the simple magnetized torus TORoidal Plasma EXperiment [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)]. This gradient is reached during a scan in the neutral gas pressure pn. Around a critical value for pn, depending on the magnetic configuration and on the injected rf power, a small increase in the neutral gas pressure triggers a transition in the plasma behavior. The pressure pro

Localized direct electron heating (EH) by mode-converted (MC) ion cyclotron range of frequencies (ICRF) waves in D(H) tokamak plasmas has been clearly observed for the first time in Alcator C-Mod. Both on- and off-axis (high field side) mode conversion EH (MCEH) have been observed. The MCEH profile was obtained from a break-in-slope analysis of electron temperature signals in the presence of radio frequency shut-off. The temperature was measured by a 32-channel high spatial resolution (less than or equal to

Localized electron heating and current drive, like those produced by electron cyclotron heating (ECH) systems, are powerful tools for controlling the sawtooth period. They allow the direct modification of the plasma parameters which determine the sawtooth stability. In this paper we report a set of new experimental results obtained in the TokamakConfiguration Variable (TCV) and a set of related simulations obtained applying a sawtooth period model in a transport code. The TCV device, equipped with a very fl

A characterization of the H-mode in Tokamak de Varennes (TdeV) is presented, combining results obtained from spectroscopic measurements with observations of global plasma parameters. Controlled L- and H-modes are obtained by applying an external bias on the divertor plate and/or by varying the plasma triangularity using similar steady-state discharges with electron cyclotron resonance heating (ECRH). A qualitative study of the impurity emission asymmetries is presented and discussed by proposing a simple mo

Compact optimized stellarators offer novel solutions for confining high-beta plasmas and developing magnetic confinement fusion. The three-dimensional plasma shape can be designed to enhance the magnetohydrodynamic (MHD) stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the

The original TCV control system was designed in the late 1980s to be inherently network based in that all control operations were performed from X-windows terminals and the control computers were networked. Using VMS-VAX technology, the acquisition system (CAMAC) and control system (BITBUS) were connected to controllers situated in the machine bus of these machines that were linked homogeneously by the VMS 'cluster' technology. Although the computing power and network bandwidth have improved considerably ov

The prediction of the thermal energy confinement time, tau(th), for ITER size experiments is based on power law scalings obtained using data sets of present tokamak results in specific regimes, the most relevant being the ELMy H mode regime. A thorough statistical approach has provided a bust fit to these data with an estimation of the error bars which forms the basis for the ITER EDA design parameters. In this article the range of variation of the main parameters in the database are studied individually an

A 'standard', single null lower diverted discharge has been developed to enable continuous monitoring of the first wall conditions and to characterise the effectiveness and influence of wall conditioning in the TCV tokamak. Measurements over a period encompassing nearly 2000 ohmic discharges of varying configuration and input power show the global confinement time and main plasma impurity concentrations to be good general indicators of the first wall condition, whilst divertor target profiles demonstrate st

The linear relativistic theory of electron cyclotron wave propagation predicts partial absorption of the extraordinary mode at the third electron cyclotron harmonic for typical parameters of present day tokamak plasmas. Since the waves are not totally damped, a quantitative confirmation of such predictions is important to establish this particular resonance scheme firmly for plasma heating and current drive. Specific experiments have been performed on Tore Supra, to measure the optical thickness of the thir

From current-driven to neoclassically driven tearing modes

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The design of the EC Upper Launcher and Ex-vessel waveguide systems for ITER

Acceleration of Solvers for Plasma Simulation in the Tokamak Boundary

Statistical Analysis of Plasma Discharge Predictability in Transport Simulations of TCV Experiments

Effect of the limiter position on the scrape-off layer width, radial electric field and intrinsic flows

Polarization Issues with High Power Injection and Low Power Emission in Fusion Experiments

Observation of a critical pressure gradient for the stabilization of interchange modes in simple magnetized toroidal plasmas

Ion cyclotron range of frequencies mode conversion electron heating in deuterium-hydrogen plasmas in the Alcator C-Mod tokamak

Effects of localized electron heating and current drive on the sawtooth period

Characterization of impurities in L- and H-modes created by divertor biasing and plasma triangularity in TdeV

Physics of the compact advanced stellarator NCSX

Evolution not revolution in the TCV tokamak control and acquisition system

Considerations on energy confinement time scalings using present tokamak databases and prediction for ITER size experiments

Divertor target profiles and recycling studies in TCV single null lower standard discharges

Measurement of the optical depth at the third electron cyclotron harmonic in tore supra