Identification of coherent magneto-hydrodynamic modes and transport in plasmas of the TJ-II heliac

Resumen

Magneto hydrodynamic (MHD) instabilities in magnetically confined plasmas are responsible for different phenomena of relevance in the transport of particles and energy. Plasma oscillations can limit the operational space of most experimental devices, therefore their study and understanding may lead to find the way to control them and achieving better plasma performance. In the flexible heliac device TJ-II, an almost current-free and low magnetic shear stellarator, collective oscillations are commonly observed in plasmas heated with microwave or neutral particle beams. Frequently, low mode number oscillations, systematically linked to low order rational values of the rotational transform (namely, to their associated magnetic islands chain), are found to lead fast-particle and/or thermal-plasma confinement degradation. The present work summarizes some studies performed in the TJ-II stellarator to investigate the role of magnetic islands dynamics in plasma confinement. Tearing mode (TM) activity is often detected as intense and coherent modes in Mirnov coils signal spectra during the low-confinement phase (L-mode) of neutral beam (NBI) heated plasmas. Their rotation speed is generally close to the plasma flow velocity and the resolved mode numbers correspond to the lowest rational values contained in the rotational transform profile of each magnetic configuration, so the measured mode frequencies are rather low (tens of kHz) and scale with their poloidal mode number. Nevertheless, under co-NBI heating (that flattens the magnetic shear profile), the rotating structures can become faster than the plasma flow, apparently driven by an additional velocity in the electron diamagnetic drift direction. At some critical plasma conditions dependent on plasma profiles, the TM may change their stable behaviour into a pulsed magnetic activity. Mirnov coil signals show repeated magnetic bursts and their spectrograms reveal that stable modes change into intermittent (with≈1 ms repetition period) frequency stripes starting at around doubled frequency and rapidly (≈0.3 ms) decreasing to frequencies slightly above the stable rotation value. Coinciding with the magnetic pulses, collapses of small intensity local transport barriers are observed, akin to minor disruptions, and during the magnetic fluctuations-free periods, barriers build up again. This cyclic process, resembling an ELMy-like phase, can be sustained, if heating and/or fuelling are not modified. As well, rotating islands, which in essence provide a wave for the magnetic components, can interact with shear Alfvén waves giving rise to new coherent modes. This new coupling of TM with Alfvén waves has been identified for the first time in TJ-II in L-mode plasmas under full NBI power (≈0.9 MW). On the other hand, magnetic islands can be locked in the laboratory frame, as is the magnetic field itself in stellarators. The interaction of shear-Alfvén waves with static magnetic island chains might open new gaps in the continuum spectrum, which would enable for the existence of weakly damped Alfvénic modes. This kind of wave coupling, MIAEs (magnetic island induced Alfvén eigenmode), has been proposed to explain the origin of some Alfvénic waves in the intermediate frequency range ( fTM < f < fHAE) during NBI heating in TJ-II. Acoustic like modes with frequencies in the range of 15-40 kHz have been first identified in TJ-II during Electron Cyclotron Resonance (ECR) heating, where a considerable population of fast electrons is generated. They exhibit standing wave like behaviour in the poloidal plane, as geodesic acoustic modes (GAM), but propagate at around the ion sound speed in the toroidal direction, as ion sound waves (ISW). Their fundamental mode number is (m=2, n=1), and frequency scales with acoustic speed. Modes can be occasionally bursting in intensity with rapid decreases in frequency (≈20 %), in coincidence with pulses of fast electron lost from the plasma core, which may suggest a nonlinear interaction of acoustic mode with fast electrons in velocity space. The necessary condition for exciting the acoustic modes is having finite size low-order magnetic islands in the plasma bulk, likely because the long confinement time of fast electrons in the vicinity of islands favours wave-fast electron interaction. The results of the studies described in this thesis show that rational surfaces may help to regulate plasma transport due to the multiple means that they interact with thermal plasma, fast particle populations, and plasma waves. We hope that this work can contribute to future stellarator design and operation procedures.