A.A. Chmyga

Plasma potential and turbulence dynamics in toroidal devices (survey of T-10 and TJ-II experiments)

A direct comparison of the electric potential and its fluctuations in the T-10 tokamak and the TJ-II stellarator is presented for similar plasma conditions in the two machines, using the heavy ion beam probe diagnostic. We observed the following similarities: (i) plasma potentials of several hundred volts, resulting in a radial electric field Er of several tens of V?cm?1; (ii) a negative sign for the plasma potential at central line-averaged electron densities larger than 1\times 10^{19}\,{\rm m}^{-3} SRC=http://ej.iop.org/images/0029-5515/51/8/083043/nf381326in001.gif/>, with comparable values in both machines, even when using different heating methods; (iii) with increasing electron density ne or energy confinement time ?E, the potential evolves in the negative direction; (iv) with electron cyclotron resonance heating and associated increase in the electron temperature Te, ?E degrades and the plasma potential evolves in the positive direction. We generally find that the more negative potential and Er values correspond to higher values of ?E. Modelling indicates that basic neoclassical mechanisms contribute significantly to the formation of the electric potential in the core. Broadband turbulence is suppressed at spontaneous and biased transitions to improved confinement regimes and is always accompanied by characteristic changes in plasma potential profiles. Various types of quasi-coherent potential oscillations are observed, among them geodesic acoustic modes in T-10 and Alfv?n eigenmodes in TJ-II.

Internal measurements of Alfvén eigenmodes with heavy ion beam probing in toroidal plasmas

Energetic ion driven Alfven eigenmodes (AEs) are believed to be an important element disturbing the transport in a future fusion reactor. The studies of the AE properties in modern toroidal devices have made crucial contributions to the reactor relevant physics. AEs are conventionally studied by magnetic probes (MPs), which provide the poloidal m and toroidal n mode numbers and their spectral characteristics. Heavy ion beam probing (HIBP) has become a new tool to study AEs with high spatial and frequency resolution. HIBP in the TJ-II heliac observes locally (~1 cm) resolved AEs over the whole radial interval. The set of low-m (m < 8) modes, detected with the high-frequency resolution (<5 kHz), present different types of AEs. AEs are pronounced in the local density, electric potential and poloidal magnetic field oscillations, detected simultaneously by HIBP in the frequency range 50 kHz < fAE < 300 kHz. Various AE modes are visible in the neutral beam injector (NBI)-heated plasma for co-NBI (<450 kW), counter- (<450 kW) and balanced NBI (<900 kW) from the plasma centre to the edge. A high coherence between MP and HIBP data was found for specific AEs. When the density rises, AE frequency decreases, , and the cross-phase between the plasma density, poloidal magnetic field and potential remains constant. The amplitude of the AE potential oscillations δAE ~ 10 V was estimated. Poloidally resolved density and potential measurements may provide information about the AE poloidal wavelength and the AE contribution to the poloidal electric field Epol and the turbulent particle flux ΓE×B. The typical range of Epol oscillations for AEs is . Depending on the δne and δEpol amplitudes and cross-phase, AEs may make a small or a significant contribution to the turbulent particle flux ΓE×B for the observed wavenumbers kθ < 3 cm−1.

Electron internal transport barrier formation and dynamics in the plasma core of the TJ-II stellarator

The influence of magnetic topology on the formation of electron internal transport barriers (e-ITBs) has been studied experimentally in electron cyclotron heated plasmas in the stellarator TJ-II. e-ITB formation is characterized by an increase in core electron temperature and plasma potential. The positive radial electric field increases by a factor of 3 in the central plasma region when an e-ITB forms. The experiments reported demonstrate that the formation of an e-ITB depends on the magnetic configuration. Calculations of the modification of the rotational transform due to plasma current lead to the interpretation that the formation of an e-ITB can be triggered by positioning a low order rational surface close to the plasma core region. In configurations without any central low order rational, no barrier is formed for any accessible value of heating power. Different mechanisms associated with neoclassical/turbulent bifurcations and kinetic effects are put forward to explain the impact of magnetic topology on radial electric fields and confinement.

Plasma Potential Evolution Study by HIBP Diagnostic During NBI Experiments in the TJ-II Stellarator

Abstract The heavy ion beam probe diagnostic is used in the TJ-II stellarator to study directly the plasma electric potential with good spatial (up to 1 cm) and temporal (up to 2 μs) resolution. Singly charged heavy ions, Cs+, with energies of up to 125 keV are used to probe the plasma column from the edge to the core. Both electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI)-heated plasmas (PECRH = 200 to 400 kW, PNBI = 200 to 400 kW, ENBI = 28 keV) have been studied. Low-density ECRH [[over bar]n = (0.5 to 1.1) × 1019 m-3] plasmas in TJ-II are characterized by positive plasma potential on the order of 1000 to 400 V. A negative electric potential appears at the edge when the line-averaged density exceeds 0.5 × 1019 m-3. Further density rises are accompanied by a decrease in the core plasma potential, which becomes fully negative for plasma densities [over bar]n ≥ 1.5 × 1019 m-3. The NBI plasmas are characterized by a negative electric potential across the whole plasma cross section from the core to the edge. In this case, the absolute value of the central potential is on the order of -500 V. These results show a clear link between plasma potential and density in the TJ-II stellarator.

Electron internal transport barriers, rationals and quasi-coherent oscillations in the stellarator TJ-II

The evolution of core quasi-coherent modes has been investigated during the formation of electron internal transport barriers (e-ITB) in the TJ-II stellarator. These modes have been characterized using heavy ion beam probe and electron cyclotron emission diagnostics. The quasi-coherent mode evolves during formation/annihilation of the e-ITB and vanishes as the transport barrier is fully developed. These observations can be interpreted in terms of the influence of sheared flows in the stability of quasi-coherent modes.

Physics of sheared flow development in the boundary of fusion plasmas

The link between edge sheared flows and turbulence is investigated in the plasma edge region of the TJ-II stellarator and the results are compared with results in other devices like JET tokamak. In the TJ-II stellarator there is a threshold density to trigger the development of edge shear flows. During sheared flow development the degree of turbulence anisotropy ((v ∥ v r )) is modified. The fact that different quadratic terms in fluctuating velocities ((v ∥ v r ) and (v ⊥ v r )) change during edge sheared flow generation means that shear flow physics involves 3D physics phenomena in which both perpendicular and parallel dynamics are involved. A new strategy has been recently applied to plasma physics to quantify the local energy transfer between flows and turbulence by computing the production term. Experimental results show that turbulence can act as an energy sink and energy source for the mean flow near the shear layer. Measurements of the turbulence production show the importance of 3D effects on the energy transfer between flows and turbulence.

Changes in plasma potential and turbulent particle flux in the core plasma measured by heavy ion beam probe during L–H transitions in the TJ-II stellarator

Plasma potential and turbulent particle flux ΓE×B in the plasma core were measured for the first time during L–H/H–L transitions in the TJ-II heliac using a heavy ion beam probe. The L–H transition is characterized by a simultaneous potential drop ~−100 V over the whole plasma radius, formation of density pedestal and a strong radial electric field Er between −100 to −150 V cm−1 in the edge (0.8 < ρ < 0.9), strong suppression of the broadband density and potential oscillations and of ΓE×B in the edge and core plasma.

ELECTRON INTERNAL TRANSPORT BARRIERS AND MAGNETIC TOPOLOGY IN THE STELLARATOR TJ-II

Abstract In most helical systems, electron–internal transport barriers (e-ITBs) are observed in electron cyclotron heated (ECH) plasmas with high heating power density. In the stellarator TJ-II, e-ITBs are easily achievable by positioning a low-order rational surface close to the plasma core because this increases the density range in which the e-ITB can form. Experiments with different low-order rationals show a dependence of the threshold density and barrier quality on the order of the rational (3/2, 4/2, 5/3 …). In addition, quasi-coherent modes are frequently observed before and/or after the e-ITB phenomenon at the radial location of the transport barrier foot. Such modes vanish as the barrier is fully developed.

Dynamics of flows and confinement in the TJ-II stellarator

This work deals with the results on flow dynamics in TJ-II plasmas under Li-coated wall conditions, which produces low recycling and facilitates the density control and access to improved confinement transitions. The low-density transition, characterized by the emergence of the shear flow layer, is described from first principles and within the framework of neoclassical theory. The vanishing of the neoclassical viscosity when approaching the transition from below explains the observation of a number of turbulent phenomena reported in TJ-II in recent years; a unifying picture is provided in which zonal, i.e. large scale, radially structured, perturbations are observable when the neoclassical damping is sufficiently small. Preliminary linear, collisionless gyrokinetic simulations are carried out to assess that the measured time scale of relaxation of such perturbations is reasonably understood theoretically. In higher density regimes, the physical mechanisms behind the L–H transition have been experimentally studied. The spatial, temporal and spectral structure of the interaction between turbulence and flows has been studied close to the L–H transition threshold conditions. The temporal dynamics of the turbulence-flow interaction displays a predator–prey relationship and both radial outward and inward propagation velocities of the turbulence-flow front have been measured. Finally, a non-linear relation between turbulent fluxes and gradients is observed.

First HIBP results on the WEGA Stellarator

The heavy ion beam probe (HIBP) is an established non perturbing diagnostic for measuring the spatial distributions of plasma potential, density, temperature and poloidal magnetic field (axial current) of magnetically confined fusion plasma. These are determined from the change in the primary ion beam parameters (charge, intensity and trajectory) passing through a plasma volume due to collisions with electrons and interactions with the confining magnetic field. A heavy ion beam probe plasma diagnostic system has been installed and tested on the WEGA stellarator in Greifswald, Germany in 2006–2007. The WEGA HIBP operates with a beam of singly charged sodium ions with an energy of up to 60 keV, ion current up to 100 μA, and beam diameter of 5–6 mm in the confined plasma region. Plasma experiments with the HIBP diagnostic system were carried out at a magnetic field strength of B0 = 0.489 T. In the experiments, an argon plasma was heated with ECRH at 28 GHz. In this work the first plasma potential and total c...