S. Perfilov

Investigation of geodesic acoustic mode oscillations in the T-10 tokamak

Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics. Regimes with Ohmic heating and with on- and off-axis ECRH were studied. It was shown that GAMs are mainly the potential oscillations. Typically, the power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with the contrast range 3–5. They are the manifestation of the torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. The frequency of GAMs changes in the region of observation and decreases towards the plasma edge. After ECRH switch-on, the frequency increases, correlating with growth in the electron temperature Te. The frequency of the GAMs depends on the local Te as , which is consistent with a theoretical scaling for GAM, where cs is the sound speed within a factor of unity. The GAMs on T-10 are found to have density limit, some magnetic components and an intermittent character. They tend to be more excited near low-q magnetic surfaces.

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.

The features of the global GAM in OH and ECRH plasmas in the T-10 tokamak

Zonal flows and their high-frequency counterpart, the geodesic acoustic modes (GAMs) are considered as a possible mechanism of the plasma turbulence self-regulation. In the T-10 tokamak GAMs have been studied by the heavy ion beam probing and multipin Langmuir probes. The wide range of the regimes with Ohmic, on-axis and off-axis electron cyclotron resonance heating (ECRH) were studied (Bt = 1.5–2.4 T, Ip = 140–300 kA, , PEC < 1.2 MW). It was shown that GAM has radially homogeneous structure and poloidal m = 0 for potential perturbations. The local theory predicts that , that means the frequency increases with the decrease of the minor radius. In contrast, the radial distribution of experimental frequency of the plasma potential and density oscillations, associated to GAM, is almost uniform over the whole plasma radius, suggesting the features of the nonlocal (global) eigenmodes. The GAM amplitude in the plasma potential also tends to be uniform along the radius. GAMs are more pronounced during ECRH, when the typical frequencies are seen in the narrow band from 22 to 27 kHz for the main peak and 25–30 kHz for the higher frequency satellite. GAM characteristics and the range of GAM existence are presented as functions of Te, density, magnetic field and PEC.

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.

Investigation of the plasma potential oscillations in the range of geodesic acoustic mode frequencies by heavy ion beam probing in tokamaks

Specific oscillations within a range of 20 kHz (“20 kHz-mode”) were investigated on the T-10 and TEXT tokamaks using Heavy Ion Beam Probe (HIBP) diagnostic. Regimes with ohmic heating on both machines, and with off-axis ECRH in T-10 were studied. It was shown that “20 kHz-modes are mainly the potential oscillations. The power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with a contrast range of (3–5). They are the manifestation of torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. It was shown that in TEXT the radial electric field oscillations exist in a limited radial range of 0.65 > ρ < 0.95. The frequency of “20 kHz-mode” is varied in the region of observation; it diminishes to the plasma edge. In T-10, after ECRH switch-on, the frequency increases, correlating with the growth of the electron temperature Te. In both machines the frequency of the “20 kHz-mode” varies with local Te: f ∼ Te1/2, which is consistent with theoretical scaling for geodesic acoustic modes (GAM): fGAM ∼ cs/R ∼ Te1/2, where cs is the speed of sound. The absolute frequencies are close to GAM values within a factor of unity.

Recent results of the T-10 tokamak

Poloidal asymmetry and radial correlation lengths of turbulence were investigated in T-10 at low field side and high field side by correlation reflectometry. Correlation of plasma confinement with the turbulence type was observed. Improvements in heavy ion beam probe diagnostic enabled us to measure the plasma potential during electron cyclotron resonance heating (ECRH) in a wide range of radial positions and operational regimes. The turbulence appeared to rotate close to E × B velocity. The concept of electron internal transport barrier (e-ITB) formation at low-order rational surfaces under conditions of low density of the rational surfaces was proved by the observation of e-ITB formation near the q = 1.5 surface in discharges with non-central ECRH and current ramp-up. The kinetic phenomena were investigated by means of electron cyclotron emission (ECE) under the strong on-axis ECRH. Lithium gettering of the limiter and the wall allowed us to significantly reduce the impurity level and obtain a recycling coefficient as low as 0.3. The rates of carbon film deposition were measured in the working and cleaning discharges. Second harmonic EC assisted start-up was investigated. ECRH allowed us to control the generation of runaway electrons and the current decay rate after the energy quench at the density limit disruption. (Some figures in this article are in colour only in the electronic version)

A global resonance phenomenon at the TJ-II stellarator

A new phenomenon at the TJ-II stellarator is reported. The line-integrated density measured by a microwave interferometer systematically shows bursts of fluctuations at regularly spaced values of the mean density across a very large number of discharges. Related modes are also observed with other diagnostics, such as the magnetic pickup coils and the heavy ion beam probe. The mode is tentatively identified with an acoustic mode, and its relation to confinement transitions at TJ-II is discussed.

The dynamics of the formation of the edge particle transport barrier at TJ-II

A large set of discharges with both spontaneous and forced confinement transitions at low electron density is studied at the stellarator TJ-II in different magnetic configurations. By means of a Bayesian technique, using reflectometry and interferometry, electron density profiles are reconstructed in the outer half of the plasma. A scanning heavy ion beam probe delivers information on the global evolution of the plasma potential. Langmuir probes are used to measure long-range correlations. Together, a comprehensive picture is obtained of the evolution of various key global plasma profiles and parameters across forward and backward transitions associated with the formation of an edge sheared flow layer. The impact of low-order rationals on the transition is elucidated.

Damping of radial electric field fluctuations in the TJ-II stellarator

The drift-kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows to simulate density ramp-up experiments and to describe from first principles the formation and physics of the radial electric field shear, which is associated to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighbourhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution. We present simulations of collisionless relaxation of zonal flows, in the sense of the Rosenbluth and Hinton test, that show an oscillatory behaviour in qualitative agreement with the experiment close to the transition.