The effect of electron transport on electron cyclotron current drive in tokamak plasmas

The operation of the most promising concept for the future fusion rector, tokamak, relies on the generation of plasma current. Traditionally, this is created by magnetic induction which makes the tokamak operation pulsed. With electro-magnetic waves, in the range of the electron cyclotron (EC) frequency of the plasma electrons, current can be driven non-inductively. This provides the possibility of a continuous tokamak operation. In the TCV tokamak, a fully non-inductive operation is a routine experiment. This allows not only the possibility to study the non-inductive tokamak operation, but also the EC wave driven current is easily measurable. In addition, the plasma confinement has been observed to improve when the EC waves drive current in certain locations inside the plasma. This mode of operation is called the internal transport barrier (ITB) and it has been attainable in TCV since 2002. It is assumed that a specific shape of the current density profile causes the ITB to form. However, in TCV no measurement of the current density profile is available. Theoretically, the linear modeling of EC wave driven current fails in some cases to reproduce the experimentally observed current. In addition, TCV is expected to operate on the quasilinear regime. Yet the predicted quasilinear enhancement in the efficiency of the EC wave driven current is not observed, the simulated current being too large by a factor up to ten. These facts suggest that in traditional modeling some important mechanism is missing. For these reasons, modeling experiments where the plasma current is driven by EC waves is an interesting challenge. In this thesis we assume that radial transport of electrons is the missing part in the models. Transport acts as a partially linearizing mechanism in bringing the theoretical current to the observed level. Radial transport also causes current diffusion, affecting strongly the current density profile. Various experiments including non-inductive operation, plasmas with a varying fraction of inductive current, and the ITB mode have been modeled with this technique. The simulated results are consistent with respect to many measured plasma parameters: the driven current, the total plasma energy, the temperature profile, and the observed hard X-ray spectra. In addition, the transport level is in agreement with the experimental limit. Also, in ITB plasmas, the resulting current density profile is in agreement with the theories of ITB formation. These results show that radial transport has a major role in EC wave driven current experiments in TCV. Therefore, only quasilinear simulations taking into account radial transport effects provide the correct experimental results in all cases in TCV.

Electron Cyclotron Heating and Suprathermal Electron Dynamics in the TCV Tokamak

Silvano Gnesin

This thesis is concerned with the physics of suprathermal electrons in thermonuclear, magnetically confined plasmas. Under a variety of conditions, in laboratory as well as space plasmas, the electron velocity distribution function is not in thermodynamic equilibrium owing to internal or external drives. Accordingly, the distribution function departs from the equilibrium Maxwellian, and in particular generally develops a high-energy tail. In tokamak plasmas, this occurs especially as a result of injection of high-power electromagnetic waves, used for heating and current drive, as well as a result of internal magnetohydrodynamic (MHD) instabilities. The physics of these phenomena is intimately tied to the properties and dynamics of this suprathermal electron population. This motivates the development of instrumental apparatus to measure its properties as well as of numerical codes to simulate their dynamics. Both aspects are reflected in this thesis work, which features advanced instrumental development and experimental measurements as well as numerical modeling. The instrumental development consisted of the complete design of a spectroscopic and tomographic system of four multi-detector hard X-ray (HXR) cameras for the TCV tokamak. The goal is to measure bremsstrahlung emission from suprathermal electrons with energies in the 10-300 keV range, with the ultimate aim of providing the first full tomographic reconstruction at these energies in a noncircular plasma. In particular, suprathermal electrons are generated in TCV by a high-power electron cyclotron heating (ECH) system and are also observed in the presence of MHD events, such as sawtooth oscillations and disruptive instabilities. This diagnostic employs state-of-the-art solid-state detectors and is optimized for the tight space requirements of the TCV ports. It features a novel collimator concept that combines compactness and flexibility as well as full digital acquisition of the photon pulses, greatly enhancing its potential for full spectral analysis in high-fluency scenarios. Additional flexibility is afforded by the possibility to rotate the orientation of two of the cameras, permitting the crucial comparison of radiation emitted perpendicular and parallel to the primary magnetic field. The design of the HXR system was optimized through an extensive iterative simulation process with the aid of tomographic reconstruction codes as well as quasilinear Fokker-Planck modeling of ECH-driven TCV plasmas. In parallel, the selection of the detectors for this system was performed through comprehensive laboratory testing of several candidate detectors available on the market. While the design was completed in the course of the thesis work, commissioning of the system has only commenced recently with one of the four cameras installed on TCV. The first preliminary results, discussed in the last part of this thesis, include basic parameter scans of ECH wave-plasma interaction and the investigation of the dynamic response of suprathermal electrons to modulated ECH. In addition, the cameras possess the novel ability to discriminate against very high-energy γ-ray radiation that cannot be collimated and must thus be excluded from spatial distribution analysis. A basic study of the conditions for γ-ray suppression was conducted in preparation for future experiments. The Fokker-Planck modeling tool used in this diagnostic development was acquired through a collaboration with CEA-Cadarache, initially with the primary motivation of studying the simultaneous plasma heating by 2nd and 3rd harmonic electron cyclotron waves that is uniquely possible on TCV. This motivated a dedicated study, both theoretical and experimental, of one particular instance of this combined heating, which became a second primary subject of this thesis work. The particular scenario studied here is one in which a single ECH frequency is resonant at both harmonics in the same plasma. The primary objective of this study was to determine whether a synergy effect existed, permitting an enhancement of the intrinsically weak 3rd harmonic absorption by the suprathermal electrons generated at the 2nd harmonic resonance. An associated question was whether this effect, if it existed, was experimentally measurable or was in fact observed in TCV. The simulations performed in the course of this study indeed predict the existence of such a synergy, although the answer to the second question was ultimately negative, at least within the current technical limitations. This study has proven nevertheless highly valuable in providing new insight into the complex velocity-space dynamics that govern ECH wave-particle interaction and suprathermal electron dynamics.

EPFL2011

Turbulence studies in TCV using the Correlation ECE diagnostic

Matteo Fontana

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.

EPFL2018

ECH physics and new operational regimes on TCV

Stefano Alberti, Yanis Andrebe, Kurt Appert, Gilles Arnoux, Roland Behn, Patrick Blanchard, Paolo Bosshard, Yann Camenen, René Chavan, Stefano Coda, Basil Duval, Damien Fasel, Ambrogio Fasoli, Jean-Yves Favez, Timothy Goodman, Jean-Philippe Hogge, Jan Horacek, Pierre-François Isoz, Alexander Karpushov, Igor Klimanov, Jonathan Bryan Lister, Xavier Llobet, Jean-Claude Magnin, Adriano Manini, Blaise Marlétaz, Philippe Marmillod, Yves Martin, Andrei Martynov, Jean-Marc Moret, Petri Nikkola, Albert Perez, Richard Pitts, Antoine Pochelon, Laurie Porte, Olivier Sauter, Andrea Scarabosio, Edgar Scavino, Ugo Siravo, Gilbert Tonetti, Minh Quang Tran, Henri Weisen, Marco Wischmeier

The physics of tokamak plasmas, in which electrons are heated by electron cyclotron heating (ECH) and whose current is driven by electron cyclotron current drive (ECCD), is investigated in this paper together with applications on tokamak A configuration variable (TCV) using modifications of the pressure and current profiles to improve the operational regimes. In order to explain the experimentally determined current drive efficiency and hard x-ray and electron cyclotron emission measurements, it is shown that quasi-linear effects and radial transport of the suprathermal electrons are necessary. Plasmas with fully non-inductively driven currents were obtained with 0.9 MW of off-axis ECCD and 0.45 MW of on-axis counter ECCD. The combination of the driven current and the bootstrap current, accounting for 50% of the total current and peaking off-axis, yields a reversed safety factor profile and a wide and stable electron internal transport barrier. This barrier leads to an enhancement in the energy confinement by a factor of 4.5. ECH is also used to broaden the current profile of high elongation, low normalized-current plasmas whose vertical position would otherwise be uncontrollable on TCV, but whose MHD stability properties should allow high beta values. An elongation of 2.47 at a normalized-current of 1.05 MA mT(-) 1 is obtained with off-axis ECH absorbed at an optimized normalized radius between 0.55 and 0.7. Finally, third harmonic ECH is tested in various scenarios, all using vertical beam launching. In particular, high density Ohmic target and preheating with second harmonic ECH are presented. The fraction of third harmonic power absorbed reaches 65% and 85%, respectively.