Turbulence studies in TCV using the Correlation ECE diagnostic

In this thesis work, the flexibility of the Tokamak à Configuration Variable (TCV) is exploited to study the influence of plasma parameters on turbulent fluctuations. The correlation electron cyclotron emission (CECE) diagnostic is used to measure low amplitude, large bandwidth radiative temperature fluctuations, associated with the anomalous transport terms that constitute the largest contribution to heat and particle loss in tokamak plasmas. In a series of L-mode limited plasmas, fluctuations are characterized over a large range of plasma parameters, obtained varying collisionality, Te/Ti and plasma triangularity. The influence of negative triangularity on confinement and fluctuations is also investigated.

Temperature fluctuations profiles have been measured in low density, ohmic, positive and negative triangularity discharges, with matched current and density profiles. Strong suppression of fluctuations is observed in negative triangularity discharges from the plasma edge to the plasma mid radius. This is despite triangularity quickly decreasing when moving away from the edge. The existence of a region of low stiffness at the plasma edge is postulated to be responsible for this. Negative triangularity is confirmed having beneficial effects on confinement and reducing fluctuations amplitude in regimes with high Te/Ti.

The neutral beam injector (NBI) has been used on positive and negative triangularity discharges in order to study the effect of negative triangularity on plasmas where Te/Ti~1. This condition is of particular interest since future reactor-like tokamaks are expected to work with thermalized electrons and ions. A pair of discharges with positive and negative triangularity has been realized, where matched temperature profiles are attained with ~385 kW and ~145 kW of NBI power respectively for the two shapes. This is evidence of negative triangularity improving plasma confinement also in conditions of low Te/Ti. In these discharges, CECE measurements find suppressed fluctuations in the negative triangularity discharges.

Linear, flux tube, gyrokinetic simulations show that, in the same plasmas, the dominant turbulent regime is a mix of ion temperature gradient (ITG) instabilities and trapped electron modes (TEM). This is different with respect to all previous works in TCV, in which no ion heating had been available and only pure TEM regimes were observed. The results of the linear simulations indicate that, also in this mixed turbolence regime, negative triangularity has a stabilizing effect on instabilities, more visible for radial positions closer to the plasma edge, where the magnitude of triangularity is higher.

A database of fluctuations measurements has been constructed from a series of discharges with positive and negative triangularity covering a large range of plasma conditions, particularly focusing on the effects of different combinations of collisionality and the Te/Ti ratio. Collisionality is found to be the main parameter to control the relative suppression of fluctutations in negative triangularity plasmas. No direct effect of Te/Ti on the fluctuation amplitudes is observed in the explored parameter range. Measurements taken in positive and negative triangularity plasmas, with comparable conditions and matched normalized temperature scale lengths still show reduced fluctuation levels for negative triangularity, suggesting influence on the threshold gradients for the onset of fluctuations.

Effects of collisionality and T e /T i on fluctuations in positive and negative δ tokamak plasmas

Stephan Brunner, Ajay Chandrarajan Jayalekshmi, Stefano Coda, Ambrogio Fasoli, Matteo Fontana, Gabriele Merlo, Laurie Porte, Olivier Sauter

The effects of negative triangularity () on confinement and fluctuations in plasmas covering a large range of parameters were investigated on the tokamak à configuration variable (TCV). The conditions explored in this paper include discharges where neutral beam (NB) heating was employed to obtain an electron-ion temperature ratio across a large fraction of the plasma profile. This significantly extended the range of negative plasmas studied on TCV towards conditions more relevant to future reactor-like tokamaks. Negative triangularity was found to improve confinement over the full range of collisionality studied () and (). The amplitude of radiative temperature fluctuations, measured using a correlation electron cyclotron emission diagnostic over the range , was found to be reduced, in negative with respect to positive plasmas, for all combinations of parameters explored. This was, in particular, verified for a pair of positive and negative plasmas with comparable density and under different conditions of NB heating. Linear gyrokinetic simulations found the dominant turbulence regime, in the strongly NB heated discharges, to be a mixture of trapped electron modes (TEMs) and ion temperature gradient driven modes. This is in contrast to ohmic or electron cyclotron heated discharges, for which the dominant turbulence regime was found to be pure TEM. Negative triangularity was found to lead to partial stabilization of the most unstable modes for low wavenumbers in both turbulence regimes. These findings demonstrate that negative triangularity could provide significant confinement improvement over a large range of parameters, that include conditions closer to future reactor-like machines (, low collisionality).

2019

Poloidal CX visible light plasma rotation diagnostics in TCV

Claudio Marini

Controlled thermonuclear fusion is the main goal of plasma physics. At the Swiss Plasma Center, the Tokamak `a Configuration Variable (TCV) constitutes the main experiment on fusion research, where high temperature plasmas are confined by means of magnetic fields. The confinement of plasma energy and particles is limited by transport arising from the gradients between the hot-dense plasma core and the cold-rarefied plasma edge. Due to the tokamak topology, plasma can rotate in the toroidal and poloidal directions. Plasma rotation has a strong influence on confinement and stability, which makes its understanding a priority. There are many discrepancies between the theoretical rotation description and experiments, which stimulated research in the field. In this context this work provided experimental results of unprecedented accuracy, where plasma impurity parameters are measured with the charge exchange recombination spectroscopy (CXRS) diagnostic. CXRS exploits the CX signal induced by a diagnostic neutral beam injector (DNBI), permitting localised measurements of impurity rotation, density and temperature. During this work, the CXRS diagnostic was extended with the development of a new high resolution system, termed CXRS-EDGE, devoted to the study of edge profiles. The accuracy improvements with respect to the legacy systems were obtained through an high throughput lens spectrometer and numerical aperture matching optics, resulting in rotation uncertainties

EPFL2017

Stability and toroidal rotation properties of highly shaped plasmas in the TCV tokamak

Andrea Scarabosio

Magnetohydrodynamic (MHD) instabilities and plasma rotation have various impacts on particle and thermal transport in toroidal plasmas. MHD instabilities degrade the confinement, limit the maximum achievable plasma pressure, and can lead to plasma disruptions. Plasma rotation is observed to have beneficial effects on global instabilities and improve confinement through the reduction of turbulent transport. Plasma rotation and stability are strongly coupled, influencing each other in a complicated fashion. The Tokamak à Configuration Variable (TCV), unique in its plasma shaping capabilities, is equipped with a flexible auxiliary heating system (ECRH) and several diagnostics allowing us to study the effect of the plasma shape on stability in a wide range of scenarios. TCV also provides for measurements of carbon toroidal velocity in the absence of external momentum input, an experimental condition poorly studied in the past but of major interest for an accurate prediction of the toroidal rotation in future large experiments. The work presented in this thesis may be divided into two parts. In the first part, we focus on plasma instabilities appearing in three different TCV scenarios. These instabilities limit the achievable maximum pressure and current density. New features and previously unnoticed dependencies are shown. In the second part of this thesis, we experimentally study the plasma rotation properties and their relation with the plasma parameters and MHD activity. The new insight on spontaneous rotation may help in constructing a complete model of plasma rotation in tokamaks. During the initial plasma current rise, edge MHD instabilities are commonly observed in most tokamaks when the edge safety factor qa approaches low rational values. These instabilities may lead to plasma disruptions and can effectively limit the edge safety factor to qa > 3, which is above the well-known operational limit qa ≥ 2. We report on a detailed analysis of the MHD modes leading to disruptions and show experimental evidence of the key role of mode coupling. The beneficial effect of plasma shaping on current rise experiments was early noted in the TCV operation. In this thesis we characterise the effect of the plasma shape on the instability, showing how plasma elongation and triangularity (positive and negative) act as stabilising factors. The complex shape dependence of such MHD modes is interpreted on the basis of the theory of coupled tearing modes. Three stabilising mechanisms linked with plasma shaping are shown to be important for TCV and likely to other tokamaks. Sawteeth, which are central relaxation oscillations common to most tokamak scenarios, also have a significant effect on central plasma parameters. In highly elongated TCV discharges heated with far off-axis ECRH, the sawtooth oscillations are observed to disappear and to be replaced by a continuous MHD mode resonant on the q = 1 surface. The combination of a flat current profile and small q = 1 radius determines the change in the plasma stability, which we study using ideal and resistive MHD models. Plasmas with internal transport barriers (ITBs) are routinely produced in TCV using high power ECRH and current drive. A variety of MHD activity is observed during these discharges including classical m/n = 3/1 and neoclassical m/n = 2/1 tearing modes, and more exotic q = 2 pseudo-sawteeth central relaxations. Disruptive modes are observed in ITB plasmas with strong reverse magnetic shear. In general, the MHD instabilities limit the maximum achievable plasma pressure. The experimental β-limit in this scenario is observed to depend strongly on the peaking of the electron pressure profile, in agreement with the ideal MHD theory. The properties of the toroidal carbon impurity rotation in Ohmic limited L-mode plasmas are studied in detail in stationary conditions. The dependence and scaling of the toroidal velocity with plasma parameters, such as the plasma current and density, are highlighted, as well as the effect of the sawtooth activity on the rotation profile. We show that the toroidal rotation in TCV is generally directed in the counter-current or electron diamagnetic drift direction. We also compare the experimental results with the neoclassical prediction for stationary toroidal rotation in absence of external momentum input. Angular momentum relaxations are observed in TCV Ohmic plasmas. Large magnetic islands cause strong losses of angular momentum, flattening the rotation profile. Once the MHD mode has disappeared, the stationary angular velocity profile is restored over a typical time scale, 100–200 ms, providing experimental evidence of the spontaneous torque spinning the plasma column. We show that we can reproduce the phenomena of "toroidal spin-up" and rotation inversion observed in TCV Ohmic plasmas with a phenomenological momentum transport model. We infer transport coefficients, such as the momentum diffusivity, and compare them with theoretical predictions.

EPFL2006

Stability and toroidal rotation properties of highly shaped plasmas in the TCV tokamak

Andrea Scarabosio

EPFL2006