JET (He-3)-D scenarios relying on RF heating: survey of selected recent experiments

Recent JET experiments have been devoted to the study of (He-3)-D plasmas involving radio frequency (RF) heating. This paper starts by discussing the RF heating efficiency theoretically expected in such plasmas, covering both relevant aspects of wave and of particle dynamics. Then it gives a concise summary of the main conclusions drawn from recent experiments that were either focusing on studying RF heating physics aspects or that were adopting RF heating as a tool to study plasma behavior. Depending on the minority concentration chosen, different physical phenomena are observed. At very low concentration (X[He-3] < 1%), energetic tails are formed which trigger MHD activity and result in loss of fast particles. Alfven cascades were observed and gamma ray tomography indirectly shows the impact of sawtooth crashes on the fast particle orbits. Low concentration (X[He-3] < 10%) favors minority heating while for X[He-3] >> 10% electron mode conversion damping becomes dominant. Evidence for the Fuchs et al standing wave effect (Fuchs et al 1995 Phys. Plasmas 2 1637-47) on the absorption is presented. RF induced deuterium tails were observed in mode conversion experiments with large X[He-3] (approximate to 18%). As tentative modeling shows, the formation of these tails can be explained as a consequence of wave power absorption by neutral beam particles that efficiently interact with the waves well away from the cold D cyclotron resonance position as a result of their substantial Doppler shift. As both ion and electron RF power deposition profiles in (He-3)-D plasmas are fairly narrow-giving rise to localized heat sources-the RF heating method is an ideal tool for performing transport studies. Various of the experiments discussed here were done in plasmas with internal transport barriers (ITBs). ITBs are identified as regions with locally reduced diffusivity, where poloidal spinning up of the plasma is observed. The present know-how on the role of RF heating for impurity transport is also briefly summarized.

Plasma production and transport in a simple magnetised toroidal plasma

Mario Podesta

This Thesis addresses questions related to transport phenomena and the plasma production mechanisms by injection of microwaves in the electron-cyclotron frequency range in the simple magnetised toroidal plasma TORPEX. The second subject is investigated in detail in Part II. The mechanisms of the interaction between the injected microwaves and the plasma are identified. The experimental results highlight the different roles played by the electron-cyclotron and upper-hybrid plasma resonances in the absoprtion of the microwave power by the plasma. The effects of the plasma-wave interaction on the electron distribution function are investigated, confirming that the high-energy electrons that are able to ionise the neutral gas mainly come from interactions at the upper-hybrid resonance. Based on the experimental results, an expression is derived for the particle source term, which can be used in numerical codes simulating the plasma dynamics on TORPEX. The plasma production mechanisms are then related to the properties of the time-averaged plasma profiles. A set of control parameters, including the injected microwave power and the vertical magnetic field, are identified. These allow one to vary in a systematic way the plasma profiles, as needed for a detailed study of plasma instabilities and related transport. The study of particle and heat transport is undertaken in Part III. A number of experimental and analysis techniques, including a method based on the combination of "conditional-average sampling" and "boxcar-averaging", are applied to identify and quantify specific contributions to the total fluxes. The two-dimensional temporal behaviour of density, electron temperature and plasma potential is simultaneously reconstructed, thus contributing significantly to the characterisation of transport mechanisms at play in TORPEX plasmas. Two clearly distinct mechanisms are mainly responsible for the transport across the magnetic field. They are respectively associated to unstable low-frequency electrostatic modes, identified as drift waves and interchange modes, and to intermittent high-density plasma structures (or blobs). It is shown that the blobs originate from the intermittent radial expansion of the unstable modes in a region of strongly sheared E × B flow.

EPFL2007

Modelling of sawtooth-induced fast ion transport in positive and negative triangularity in TCV

Marcelo Baquero Ruiz, Basil Duval, Alexander Karpushov, Antoine Pierre Emmanuel Alexis Merle, Dmytry Mykytchuk, Mario Podesta, Olivier Sauter, Lorenzo Stipani, Duccio Testa, Matteo Vallar

Internal kinks are a common magneto hydro-dynamic (MHD) instability observed in tokamak operation when the q profile in the plasma core is close to unity. This MHD instability impacts both the transport of the bulk plasma (current, particle and energy transport) and minority species, such as fast ions. In tokamak a configuration variable (TCV) (R (0)/a = 0.88 m/0.25 m) the fast ion population is generated in the plasma by neutral beam tangential injection of energies up to 28 keV. TCV features 16 active shaping coils permitting a great flexibility in plasma shape, including negative triangularity (delta) configurations that show surprisingly high confinement. This study focuses on the transport of fast ions induced by sawteeth, by comparing two triangularity cases and simulation results with experimental data. Comparison of two equilibria with opposite delta shows that the fast ion drifts are larger for delta < 0. Furthermore, the sawtooth-induced transport in this case is larger than delta > 0 in similar conditions. Comparison with experimental data confirms the dominance of the modification of thermal kinetic profiles following the sawtooth crash in explaining drops in the neutron rates and fast ion D-alpha signals. Additional fast ion diffusion, however, improves the interpretation of the experimental data. For delta < 0, the amplitude of the perturbation better representing the experimental data is larger. Finally, an exploratory study for 50 keV particles (soon available in TCV) shows that the situation does not worsen for such particles.

IOP Publishing Ltd2022

Overview of alfven eigenmode experiments in jet

Ambrogio Fasoli, Pierre Lavanchy, Jonathan Bryan Lister, Jean-Marc Moret, Laurie Porte, Laurent Villard

Results from the first experiments to drive Alfven Eigenmodes (AEs) with antennas external to a tokamak plasma are presented. In ohmically heated plasma discharges, direct experimental measurements of the damping of toroidicity induced AEs (TAEs) have allowed an identification of different regimes corresponding to different dominant TAE absorption mechanisms with a wide range of damping rates, 10(-3) less than or equal to gamma/omega less than or equal to 10(-1). In plasmas heated by ion cyclotron resonance heating, neutral beam injection heating, lower hybrid heating and high plasma current ohmic heating, a new class of weakly damped Alfven eigenmodes, the kinetic Alfven eigenmodes, predicted in theoretical models that include finite Larmor radius and finite parallel electric field effects, has been identified experimentally.

1995

Plasma production and transport in a simple magnetised toroidal plasma

Mario Podesta

EPFL2007