Observation of Alfvén Eigenmodes driven by off-axis neutral beam injection in the TCV tokamak

Fast-particle driven Alfv´en Eigenmodes have been observed in low-collisionality discharges with off-axis neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and a reduced toroidal magnetic field. During NBI and ECRH, toroidicity induced Alfv´en Eigenmodes (TAEs) appear in frequency bands close to 200 kHz and energetic-particle-induced geodesic acoustic modes (EGAMs) are observed at about 40 and 80 kHz. When turning off ECRH in the experiment, those beam-driven modes disappear which can be explained by a modification of the fast-ion slowing down distribution. In contrast, coherent fluctuations close to the frequency of the beam-driven TAEs are present throughout the experiment. The modes are even observed during ohmic plasma conditions, which clearly demonstrates that they are not caused by fast particles and suggests an alternative drive, such as turbulence. The mode-induced fast-ion transport has been found to be weak and marginal in terms of the fast-ion diagnostic sensitivities. Measurements of the plasma stored energy, neutron rates, neutral particle fluxes and fast-ion D-alpha spectroscopy show good agreement with neoclassical modelling results from TRANSP. This is further supported by a similarly good agreement between measurement and modelling in cases with and without ECRH and therefore with and without the modes. Instead, a significant level of charge exchange losses are predicted and observed which generate a bump-on-tail fast-ion distribution function that can provide the necessary free energy to EGAMs.

Extension of the operating space of high-beta(N) fully non-inductive scenarios on TCV using neutral beam injection

Javier García Hernández, Timothy Goodman, Alexander Karpushov, Mengdi Kong, Antoine Pierre Emmanuel Alexis Merle, Olivier Sauter, Duccio Testa, Matteo Vallar

The fully non-inductive sustainment of high normalized beta plasmas (beta(N)) is a crucial challenge for the steady-state operation of a tokamak reactor. In order to assess the difficulties facing such scenarios, steady-state regimes have been explored on the tokamak a configuration variable (TCV) using the newly available 1 MW neutral beam injection (NBI) system. The operating space is extended towards plasmas that are closer to those expected in JT-60SA and ITER, i.e. with significant NBI and electron cyclotron resonance heating and current drive (ECRH/CD), bootstrap current and fast ion (FI) fraction. beta(N) values up to 1.4 and 1.7 are obtained in lower single null L-mode (H-98 (y, 2) similar to 0.8) and H-mode (H-98 (y, 2) similar to 1) plasmas, respectively, at zero time averaged loop voltage and q(95 )similar to 6. Fully non-inductive operation is not achieved with NBI alone, whose injection can even increase the loop voltage in the presence of EC waves. A strong contribution to the total plasma pressure of thermal and FIs from NBI is experimentally evidenced and confirmed by interpretative ASTRA and NUBEAM modeling, which further predicts that FIcharge-exchange reactions are the main loss channel for NBH/CD efficiency. Internal transport barriers, which are expected to maximize the bootstrap current fraction, are not formed in either the electron or the ion channel in the plasmas explored to date, despite a significant increase in the toroidal rotation and FI fraction with NBI, which are known to reduce turbulence. First results on scenario development of high-beta(N) fully non-inductive H-mode plasmas are also presented.

2019

Self-consistent interaction of fast particles and ICRF waves in 3D applications of fusion plasma devices

Jonathan Marc Philippe Faustin

Tokamaks and stellarators are the most promising reactor concepts using the magnetic confinement to contain the plasma fuel. Reactors capable of sustaining deuterium-tritium (D-T) fusion reactions requires the confinement of a very high temperature plasma (above 100 millions kelvin). In addition to external heating methods, the slowing down of alpha particles (helium-4 nuclei) born from D-T fusion reactions on the background plasma represents a significant source of plasma heating. The good confinement of fast particles is therefore one of the most important aspect of magnetic fusion devices. Furthermore, long plasma operation in future fusion reactors requires the control of inherent plasma instabilities. These instabilities are particularly dangerous in tokamaks because of the large plasma current necessary to establish the confining magnetic field. In this thesis we use the numerical code package SCENIC to study the application of Ion Cyclotron Range of Frequency (ICRF) waves to tokamak and stellarator devices. This numerical tool was built to self-consistently solve, in three dimensional configurations, the plasma magnetohydrodynamic (MHD) equilibrium, the ICRF wave propagation and the resonant ion distribution function. SCENIC is used to interpret how the sawtooth instability can be controlled in tokamaks by appropriate application of ICRF waves. This control method was successfully tested in the JET tokamak and it is foreseen to be applied in the future ITER tokamak. Such plasma degrading instabilities are not, however, expected in stellarators because they operate with no plasma current. The recently started stellarator Wendelstein 7-X (W7-X) must however prove experimentally that fast particles particles can be confined in an optimised quasi-isodynamic magnetic configuration. An efficient auxiliary source of fast ions is required in W7-X since it is not designed to procude alpha particles via D-T fusion reactions. In this thesis, we address the possibility of generating a significant fast ion population with ICRF waves in W7-X. SCENIC simulations are employed in order to identify relevant fast ion loss channels that may still exist in the W7-X quasi-omnigenous equilibrium. These simulations show that ICRF minority heating may not be suitable for producing fast ions in W7-X plasmas. It is found that a high energy tail is more likely to be developed if a three-ion species scheme is applied.

EPFL2017

Development of a reduced model for energetic particle transport by sawteeth in tokamaks

Mario Podesta, Francesca Maria Poli, Anna Teplukhina, Matteo Vallar, Jiangyan Yang

The sawtooth instability is known for inducing transport and loss of energetic particles (EPs), and for generating seed magnetic islands that can trigger tearing modes. Both effects degrade the overall plasma performance. Several theories and numerical models have been previously developed to quantify the expected EP transport caused by sawteeth, with various degrees of sophistication to differentiate the response of EPs at different energies and on different orbits (e.g. passing vs. trapped), although the analysis is frequently limited to a single time slice during a tokamak discharge. This work describes the development and initial benchmark of a framework that enables a reduced model for EP transport by sawteeth retaining the full EP phase-space information. The model, implemented in the ORBIT hamiltonian particle-following code, can be used either as a standalone post-processor taking input data from codes such as TRANSP, or as a pre-processor to compute transport coefficients that can be fed back to TRANSP for time-dependent simulations including the effects of sawteeth on EPs. The advantage of the latter approach is that the evolution of the EP distribution can be simulated quantitatively for sawtoothing discharges, thus enabling a more accurate modeling of sources, sinks and overall transport properties of EP and thermal plasma species for comprehensive physics studies that require detailed information of the fast-ion distribution function and its evolution over time.

IOP Publishing Ltd2022

Self-consistent interaction of fast particles and ICRF waves in 3D applications of fusion plasma devices

Jonathan Marc Philippe Faustin

EPFL2017