E. Ascasíbar

Plasma performance and confinement in the TJ-II stellarator with lithium-coated walls

In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 1019u2009m−3 under Li-coated walls.

Mitigation of NBI-driven Alfvén eigenmodes by electron cyclotron heating in the TJ-II stellarator

Alfven eigenmode (AE) activity driven by NBI-produced fast ions is observed in TJ-II plasmas. A two-step response of the measured AEs to electron cyclotron heating (ECH) power is seen. In a first step, the continuous character of the unstable AEs changes to a chirping character of the marginally unstable AEs when moderate values of ECH power are applied to the NBI-only-heated plasma. In a second step, a significant reduction of the AE amplitude is observed when the ECH power is doubled. This stabilizing effect has been experimentally confirmed both on a shot-by-shot basis and along a single discharge by means of ECH modulation. The observed stabilizing effect is stronger with on-axis ECH than with off-axis ECH power injection.

Overview of TJ-II flexible heliac results

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R = 1.5 m, a 0.22 m, B0 = 1T , PECRH600 kW, PNBI3 MW under installation). This paper reviews the main technical aspects of the TJ-II heliac as well as the principal physics results obtained in the most recent TJ-II experimental campaign carried out in 2000.

MHD mode activity and the velocity shear layer at TJ-II

Low-frequency MHD mode activity was studied at the TJ-II stellarator. A spatiotemporal Fourier technique was used to resolve frequency-degenerate modes. By means of this technique, several MHD modes could be identified in discharges with a spontaneous confinement transition in different but similar magnetic configurations. The configurations differed mainly with respect to the radial position of the rational surfaces, thus allowing the reconstruction of a poloidal mode rotation profile based on the mode activity, which was found to be consistent with earlier work. The detected mode spectrum also provided an explanation for the bicoherence observed in one of the configurations after the confinement transition. Both the mode spectrum and the velocity profile were closely reproduced by nonlinear resistive MHD calculations in simplified geometry. As a consequence, the magnetic Reynolds stress is hypothesized to play an important role in the establishment of the velocity shear layer in TJ-II and the concomitant confinement transition.

Limit cycle oscillations at the L–I–H transition in TJ-II plasmas: triggering, temporal ordering and radial propagation

The spatiotemporal evolution of the interaction between turbulence and flows has been studied close to the L–H transition threshold conditions in the edge of TJ-II plasmas. As in other devices the temporal dynamics of the interaction displays limit cycle oscillations (LCO) with a characteristic predator–prey relationship between flows and turbulence. At TJ-II, the turbulence-flow front is found to propagate radially outwards at the onset of the LCO and in some particular cases, after a short time interval without oscillations, a reversal in the front propagation velocity is observed. Associated to this velocity reversal, a change in the temporal ordering of the LCO is measured. However, the change in the temporal ordering is not related to an intrinsic change in the nature of the LCO. In all cases the turbulence increase leads the process and produces an increase in the E × B flow shear. Dedicated experiments have been carried out to investigate the physical mechanisms triggering the onset of the LCO. At TJ-II the LCO are preferentially observed close to the transition threshold conditions at specific magnetic configurations having a low order rational surface located at the inner side of the E × B flow shear location. The behaviour of different frequency modes has been analysed and interpreted in terms of a geodesic acoustic mode generated by the non-linear mode coupling of Alfven eigenmodes that evolves towards a low frequency flow, plus a MHD mode linked to the low order rational surface, as precursors of the LCO.

Relationship between MHD events, magnetic resonances and transport barriers in TJ-II plasmas

In the TJ-II stellarator, a medium-sized heliac device that operates at high rotational transform and low magnetic shear, a close relationship is identified between abrupt changes in transport (in the form of radial fluxes, edge or internal transport barriers), broadband magnetohydrodynamic (MHD) events and the location of magnetic resonances. The observations are based on the local behaviour of plasma radiation. Phenomena that appear like sawteeth and crashes in tokamaks are found in practically all kinds of plasmas and show intensities and repetition rates that depend on plasma parameters. The magnetic resonances are suggested as important transport regulators in normal operation.

Plasma flow, turbulence and magnetic islands in TJ-II

The effect of magnetic islands on plasma flow and turbulence has been experimentally investigated in ohmically induced magnetic configuration scans at the stellarator TJ-II. This operational mode allows sweeping the radial position of a low order rational surface from the plasma core towards the edge in a controlled way, what reveals effects that are difficult to notice in scans performed on a shot to shot basis. The main diagnostic used in the present work is a two-channel Doppler reflectometer that allows the measurement of the perpendicular rotation velocity of the turbulence and density fluctuations with good spatial and temporal resolution. A characteristic signature of the n/m = 3/2 magnetic island as it crosses the measurement position is clearly detected: the perpendicular flow reverses at the center of the magnetic island and a flow shear develops at the island boundaries. Fluctuations of the perpendicular flow and density have been also measured along the 3/2 magnetic island. An increase in the low frequency flow oscillations is measured at the magnetic island boundaries together with a reduction in the density fluctuation level; the later being more pronounced at the inner island boundary. These observations could explain the link between magnetic islands and transport barriers observed in a number of fusion devices.

Transition from chirping to steady NBI-driven Alfvén modes caused by magnetic configuration variations in the TJ-II stellarator

Beam-driven Alfven eigenmodes (AEs) have been studied in the TJ-II low-magnetic-shear flexible heliac (B 0 = 0.95 T, 〈R〉 = 1.5 m, 〈a〉 = 0.22 m), in L-mode hydrogen plasmas with hydrogen NBI and ECR heating (P NBI ≤ 1.0 MW, E NBI = 32 keV, P ECRH ≤ 0.6 MW). In low-density plasmas in the range = (0.3–1.5) × 1019 m−3, a large variety of AEs have been observed in the plasma core with the heavy ion beam probe diagnostic and Mirnov coils, in the frequency range 50 kHz < f AE < 380 kHz. In experiments in which the vacuum rotational transform is varied during the shot (dynamic configuration scan), some AEs exhibit changes in their nonlinear evolution from bursting-amplitude AEs with chirping frequency to steady-frequency AEs, and back. The range of intervals within which the AEs studied are chirping or steady-state is determined.

Analysis of magnetohydrodynamic instabilities in TJ-II plasmas

Abstract Magnetohydrodynamic instabilities in the TJ-II stellarator are being experimentally characterized in various plasma parameter regimes and heating scenarios. Magnetic field fluctuations data are collected, using various Mirnov coil sets distributed at different toroidal sectors of the vacuum vessel, with frequency resolution up to 1 MHz. Specific analysis is carried out with the signals from a poloidal array of 15 coils measuring poloidal magnetic field fluctuations. The appearance of low-frequency modes (some tens of kilohertz) in electron cyclotron heated plasmas depends on the rotational transform profile and plasma density. In neutral beam injection plasmas, high-frequency (150- to 300-kHz) modes have been found in plasmas with line densities in the range 0.6 × 1019 m-3 to 3 × 1019 m-3 and heated with on/off-axis electron cyclotron heating. They are good candidates for global Alfvén eigenmodes related to the low-order resonance n/m = 3/2.

Spatiotemporal and wavenumber resolved bicoherence at the low to high confinement transition in the TJ-II stellarator

Plasma turbulence is studied using Doppler reflectometry at the TJ-II stellarator. By scanning the tilt angle of the probing beam, different values of the perpendicular wavenumbers are probed at the reflection layer. In this way, the interaction between zonal flows and turbulence is reported with (a) spatial, (b) temporal and (c) wavenumber resolution for the first time in any magnetic confinement fusion device.We report measurements of the bicoherence across the low to high (L?H) confinement transition at TJ-II. We examine both fast transitions and slow transitions characterized by an intermediate (I) phase. The bicoherence, understood to reflect the non-linear coupling between the perpendicular velocity (zonal flow) and turbulence amplitude, is significantly enhanced in a time window of several tens of milliseconds around the time of the L?H transition. It is found to peak at a specific radial position (slightly inward from the radial electric field shear layer in H-mode), and is associated with a specific perpendicular wavenumber (k????6?12?cm?1, k??s???0.8?2). In all cases, the bicoherence is due to the interaction between high frequencies (?1?MHz) and a rather low frequency (?50?kHz), as expected for a zonal flow.