Accélérateur MHD

Résumé

Un accélérateur MHD (magnétohydrodynamique) est un convertisseur MHD qui met en mouvement un fluide conducteur, grâce à un champ électrique et un champ magnétique combinés. Le principe de base est le même que celui d'un moteur électrique. Tous deux possèdent un inducteur (électroaimant) générant un champ magnétique dans un induit.

Dans le cas d'un moteur conventionnel, cet induit est solide : c'est une bobine constituée d'un enroulement de fil métallique.
Dans le cas d'un accélérateur MHD, cet induit est fluide : liquide conducteur (eau salée, métal liquide) ou gaz ionisé (appelé plasma).

Les accélérateurs MHD n'utilisent donc pas de pièce mécanique mobile, contrairement aux moteurs électriques traditionnels, ils convertissent directement l'énergie électromagnétique en énergie cinétique. Un fluide est mis en mouvement dans un champ magnétique, par un champ électrique débitant un courant électrique aux bornes d'électrodes immergées dans le fluide. Principe Mouvem

Source officielle

https://fr.wikipedia.org/wiki/Acc%C3%A9l%C3%A9rateur_MHD

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Controlling the rotation of drift tearing modes by biased electrode in ADITYA-U tokamak

Sameer Kumar, Rohit Kumar, Ankit Kumar, Harshita Raj, Abhijit Sen

The influence of background plasma poloidal rotation on the rotation frequency of the m/n = 2/1 drift tearing mode (DTM) has been studied in ADITYA-U tokamak. The poloidal rotation velocity of the background plasma in the ion diamagnetic direction is increased or decreased by inducing an outward or inward radial electric field, respectively, through a biased-electrode placed in the edge region of the plasma. The rotation frequency of the preexisting drift tearing mode, rotating in the electron diamagnetic direction, concomitantly decreased or increased with the application of bias depending on its polarity. The positive-bias increases the background plasma rotation in the ion-diamagnetic direction from its pre-bias value, hence decreasing the DTM rotation frequency, whereas the negative bias reduces the plasma rotation velocity in the ion-diamagnetic direction, hence increasing the mode rotation. In addition to that, a short gas puff introduced during the positive and negative bias pulse further reduces the mode frequency, however, with different amplitudes in different bias-polarities. These observations suggest that the background plasma rotation contributes significantly toward the rotation of DTMs, and the rotation frequency of the magnetohydrodynamic modes can be modified by varying the poloidal rotation of the background plasma and/or the diamagnetic drift frequency.

2021

Chargement

The need of durable, economically acceptable and safe energy sources continues to stimulate studies in a field of thermonuclear fusion. The most successful solution for controlled magnetic fusion is the tokamak. The improvement of tokamak performance depends on the optimization of pressure and current density spatial distributions which can be modified by means of an auxiliary heating and a current drive. Electron cyclotron heating, in particular is a very important tool for the study and control of basic physical processes governing plasma confinement and stability, particularly because it allows the injection of highly localized intense power. ECH power deposition location plays a crucial role in sawtooth control and suppression, it is also important for tearing mode stabilization, and for implementation of closed loop systems for automatic control/suppression of magnetohydrodynamic activity. A part of the ECH power can be modulated (MECH), and used to identify where the ECH power has been deposited, and can also be useful in the experimental analysis of the electron transport in general. Nevertheless, despite the goal of MECH being a diagnostic and analysis tool, MECH can couple to plasma oscillations, such as sawteeth. MECH-sawtooth phase coupling adds significant complications in ECH deposition location and transport analysis, in some cases making the interpretations of results misleading. This is why it is important to get an insight into the phenomenon of MECH-sawtooth interaction, and to establish the boundaries where conventional types of modulation analysis can be successfully implemented. This thesis presents the analysis and interpretation of perturbative MECH experiments performed in the TCV tokamak with particular attention paid to the non-linear phase coupling of heat waves. TCV is equipped with a very flexible and high power ECH system. Two independent ECH systems permit MECH to be deposited at two different spatial locations, with two different types of modulating signals. This set-up was used to study simultaneous propagation of heat waves induced by MECH in non-sawtoothing plasmas, and in discharges with sawtooth activity. A new analysis method for the characterization of the plasma non-linear dynamic response to modulated heating was developed on the basis of Higher Order Spectral Analysis (HOSA) technique. This method applied to signals from different diagnostics, such as electron cyclotron emission and soft X-ray measurements, was extensively used to quantitatively characterize the effect of nonlinear phase coupling. In sawtooth free discharges a detailed analysis of the propagation of heat waves demonstrated that their phase coupling is solely related to properties of heat sources. It was demonstrated that if two heat waves are induced by non-coupled power sources (multi-beam MECH) then no phase coupling occurs. In the opposite case, when a source of perturbation (MECH) contains coupled harmonics, the corresponding heat waves demonstrate phase coupling. It was shown that these coupled heat waves loose their phase coherence while propagating in plasma. The dissipation of phase coupling is due to different phase velocities of heat waves and their diffusive damping. The new type of ECH power modulation accompanied with bicoherence analysis was proposed as a candidate for a reliable identification of EC power deposition location in a case of high frequency and low modulation depth MECH, including multi-beam heating. This type of MECH can be particularly important for real time control applications. In cases when MECH is applied to sawtoothing plasmas a direct experimental evidence of MECH-sawtooth non-linear phase coupling has been demonstrated using HOSA techniques, in particular bispectrum and bicoherence profiles. The detailed analysis presented here demonstrates a direct proof of periodic modification of sawtooth behavior by modulated ECH. It was shown that a simple diffusive model for the perturbed electron temperature with MECH source term can not be used for transport analysis in the presence of MECH-sawtooth coupling. Self-consistent time dependant transport simulations including a sawtooth relaxation model is capable of properly reproducing the effect of MECH-sawtooth phase coupling and can be used for transport analysis. On the basis of these simulations, an hypothesis for MECH-sawtooth coupling based on periodic modifications of magnetic shear was proposed. Additional information provided by HOS analysis enabled a correct interpretation of the plasma dynamic response to modulated heating in the presence of MECH-sawtooth coupling.

EPFL2008

Chargement

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

Chargement

EPFL2008