J.A. Jiménez

First plasmas in the TJ-II flexible Heliac

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (, ) with a pulse length of . The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

Improved confinement regimes induced by limiter biasing in the TJ-II stellarator

The influence of limiter biasing on plasma confinement, turbulence and plasma flows is investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.

Ballistic transport phenomena in TJ-II

Perturbative transport experiments have been performed at the stellarator TJ-II. Both the inward propagation of edge cooling pulses induced by the injection of nitrogen, and the outward propagation of heat pulses due to spontaneous spikes of the central temperature have been analysed. It has been found that the observed propagation is incompatible with diffusive transport models. Simultaneous inward and outward propagation eliminates an explanation in terms of a pinch. A numerical simulation with a resistive interchange turbulence model suggests that the observed propagation is related to the successive destabilizations of pressure gradient driven modes associated with rational surfaces.

Statistical properties and radial structure of plasma turbulence in the boundary region of the L2-M stellarator

The structure of fluctuations and turbulent transport have been investigated in the plasma boundary region of the L2-M stellarator. Normalized fluctuation levels are in the range (3-20)% and fluctuations are dominated by frequencies below 300 kHz. In the edge plasma region located inside the last closed magnetic flux surface the radial coherence of fluctuations is due to high-frequency fluctuations (>100 kHz). The poloidal coherence is dominated by low frequencies. Linear coupling of resistive interchange modes is considered a candidate to explain the existence of highly radially correlated fluctuations in the high-frequency range.

Magnetic configuration effects on the TJ-IU torsatron plasma edge turbulence

A study of plasma edge turbulence carried out in the ECRH heated TJ-IU torsatron is presented. Radial profiles of ion saturation current and floating potential, together with the fluctuation levels of these have been evaluated in the plasma edge by means of Langmuir probe arrays. The existence of two different propagation modes in the proximity of the velocity shear layer has been observed. In the plasma bulk side of the limiter radius, high-frequency fluctuations are negligible and only one propagation mode stands. A detailed examination of the data shows the existence of a quasi-coherent mode probably related to the local magnetic configuration. A radial probe scan reveals an increase in the turbulent particle flux for the location of a rational surface as calculated by the VMEC free-boundary 3D equilibrium code.

Experimental evidence of fluctuation-induced inward transport linked to rational surfaces in the TJ-II stellarator

Radial profiles of electrostatic fluctuations and fluctuations-induced transport have been measured in the proximity of rational surfaces in the plasma boundary region of the TJ-II stellarator. Structures in plasma profiles linked to the location of low-order rational surfaces (n = 8/m = 5, n = 4/m = 2) have been observed. Local turbulent E×B driven fluxes are significantly modified in the proximity of rational surfaces. In the case of low-order rationals (n = 4/m = 2) turbulent transport reverses from radially outwards to radially inwards. These changes are due to a modification in the phase relation between density and electric field fluctuations.

Review of confinement and transport studies in the TJ-II flexible heliac

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

Software and hardware developments for remote participation in TJ-II operation. Proof of concept using the NPA diagnostic

The operation protocols for the TJ-II require that all diagnostics be controlled remotely during machine operation. For this reason, most diagnostics have control systems that permit remote actions to be performed on their various subsystems. An Internet browser, which connects to a server where the human machine interfaces (HMIs) for all diagnostics are situated, is used for this. Real time control has been resolved using programmable automates for each diagnostic. Dedicated application software is in operation to provide a user interface for programming digitisers, for signal visualisation and for data processing during TJ-II discharges. This software is an event-based application that can be remotely launched from any X terminal. Other resources provided to users are graphical and computational tools for data analysis, data compression, off-line access to databases, reception of data and its integration in the databases at any moment. Two neutral particle analysers (NPAs) are operating on TJ-II to measure the energy spectra of charge exchange (CX) neutral particles. Protocols require remote control over several actions and subsystems of this diagnostic. With all these tools a proof of the concept of remote participation has being performed during the present experimental campaign. The NPA diagnostic has been operated from UPC-ETSEIB in Barcelona, 600 km of distance from the machine.

On the radial scale of fluctuations in the TJ-II stellarator

The TJ-II stellarator was designed to have a high degree of magnetic configuration flexibility. The rotational transform can be varied between 0.9 and 2.5, and the magnetic well may be changed from -1% to 6%. This flexibility is a powerful tool for physics studies in fusion plasmas. The magnetic-well scan experiments in TJ-II support the view that plasma turbulence displays universality. These results emphasize the importance of the statistical description of transport processes in fusion plasmas as an alternative approach to the study of transport based on the computation of effective transport coefficients. Comparative studies in TJ-II stellarator and other devices show that fluctuations and E ? B sheared flows organize themselves to be close to marginal stability. This property should be considered as a critical test for L-H transition models. The magnetic-configuration scan experiments in TJ-II show the complex interplay between transport and radial electric fields in the proximity of rational surfaces.

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.