L G Askinazi

Radial current in a tokamak caused by a biased electrode

It is demonstrated that measurements of radial electric currents excited from biased electrodes in TUMAN 3 and other tokamaks provide a crucial test in validating different models for L-H transitions. The results are assessed from the viewpoint of a previously developed theory, which is briefly described. There is evidence from the voltage-current characteristics in TUMAN 3 which corroborates features of this theory. A spontaneously occurring Ohmic H-mode is switched off when a substantial positive biasing voltage is applied to the electrode

Radial electric field evolution in the vicinity of a rotating magnetic island in the TUMAN-3M tokamak

Radial electric field is known to be an important factor affecting transport and confinement in toroidal fusion plasmas. Langmuire probe measurements of peripheral radial electric field evolution in the presence of a rotating MHD island were performed on the TUMAN-3M tokamak in order to clear up the possible connection between the radial electric field and the island rotation, both in L and H-modes. The measurements showed that Er became positive, if the island was large enough, in spite of the constant direction of the islands rotation. Comparing similar ohmic H-mode discharges with or without a rotating MHD island, it was found that in the presence of the large island Er was always more positive. Possible explanations of this observation are discussed.

Plasma rotation evolution near the peripheral transport barrier in the presence of low-frequency MHD bursts in TUMAN-3M tokamak

The experiments described in the paper are aimed at investigating the possible influence of the low frequency magnetohydrodynamic (MHD) activity burst on the Ohmic H-mode in the TUMAN-3M tokamak. During the MHD burst a transient deterioration of improved confinement was observed. The study has been focused on the measurements of plasma fluctuation poloidal velocity performed by microwave Doppler reflectometry. The plasma fluctuation rotation observed before the MHD burst in the vicinity of the edge transport barrier was in the direction of plasma drift in the negative radial electric field. During the MHD activity the measured poloidal velocity was drastically decreased and even changed its sign. Radial profiles of the poloidal velocity measured in a set of reproducible tokamak shots exhibited the plasma fluctuation rotation in the ion diamagnetic drift direction at the location of the peripheral transport barrier. The possible reasons for this phenomenon are discussed.

Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

An extensive (INTAS) research programme started in 2002 to investigate the correlations between, on the one hand, the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand, electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution, and various active means to externally control plasma transport. It also requires one to characterize fluctuations of various important plasma parameters inside and outside transport barriers (TBs) and pedestal regions with high spatial and temporal resolution using advanced diagnostics, and to elucidate the role of turbulence driving and damping mechanisms, including the role of the plasma edge properties. Furthermore, one needs to determine the cross-field transport from the measurements and compare this with available theoretical models. This has been done in a strongly coordinated way, exploiting the complementarity of TEXTOR and T-10 and the backup potential of the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise. Physical mechanisms of several TBs have been studied: electron internal transport barriers in T-10 and TEXTOR, ergodization-induced TB in TEXTOR, TB in ohmic discharges in TUMAN-3M, periodic bias-induced TBs in CASTOR. Geodesic acoustic modes (GAM) have been investigated in T-10, TEXTOR and TUMAN-3M. Core turbulence has been characterized in T-10, and small-scale turbulence has been revealed in FT-2.

Heavy ion beam probe development for the plasma potential measurement on the TUMAN-3M tokamak

The peculiarities of the heavy ion beam probe implementation on the small aspect ratio tokamak TUMAN-3M are analyzed. The toroidal displacement of beam trajectory due to the high Ipl/Btor ratio is taken into account when designing the layout of the diagnostic. Numerical calculation of beam trajectories using realistic configuration of TUMAN-3M magnetic fields and parabolic plasma current profile resulted in proper adjustment of probing and detection parameters (probing ion material, energy, entrance angles, detector location, and orientation). Secondary ion energy analyzer gain functions G and F were measured in situ using neutral hydrogen puffed in the tokamak vessel as a target for secondary ions production. The detector unit featured split-plate design and had additional electrodes for secondary electron emission suppression. As a result, the diagnostic is now capable of plasma potential evolution measurement and is sensitive enough to trace the potential profile evolution at the L-H mode transition.

Counter-NBI assisted LH transition in low density plasmas in the TUMAN-3M

This paper reports observations of the LH transition at very low density in the experiments on counter-current neutral beam injection (NBI) in the TUMAN-3M tokamak. The transition has been found at a target average density of as low as 0.5 ? 1019?m?3, which is lower by a factor of 2.5 than the LH transition density boundary in ohmic and co-current NBI heated plasmas. Relatively low input power in counter-NBI experiments is noticed: Pinput = 130?270?kW. In contrast, in the case of co-NBI the LH transition is difficult at low density. No transition is possible at the above density with co-NBI Pinput up to 500?kW. A model that predicts the generation of a negative radial electric field Er, which is thought to help LH transition during counter-NBI, is suggested. The model conjectures the development of the Er and toroidal rotation V in the presence of large ion orbit losses in the counter-NBI scheme. Measurements of the plasma potential by the heavy ion beam probe technique and edge radial electric field by electrostatic probes indicating negative Er emergence in the counter-NBI scenario are presented. Doppler spectroscopy of B3+ impurity ions has shown an increase in the V of 16 ? 6?km?s?1 after counter-NBI switch on. The measured V agrees with the above model estimations.

Physics of GAM-initiated L–H transition in a tokamak

Based on experimental observations using the TUMAN-3M and FT-2 tokamaks, and the results of gyrokinetic modeling of the interplay between turbulence and the geodesic acoustic mode (GAM) in these installations, a simple model is proposed for the analysis of the conditions required for L–H transition triggering by a burst of radial electric field oscillations in a tokamak. In the framework of this model, one-dimensional density evolution is considered to be governed by an anomalous diffusion coefficient dependent on radial electric field shear. The radial electric field is taken as the sum of the oscillating term and the quasi-stationary one determined by density and ion temperature gradients through a neoclassical formula. If the oscillating field parameters (amplitude, frequency, etc) are properly adjusted, a transport barrier forms at the plasma periphery and sustains after the oscillations are switched off, manifesting a transition into the high confinement mode with a strong inhomogeneous radial electric field and suppressed transport at the plasma edge. The electric field oscillation parameters required for L–H transition triggering are compared with the GAM parameters observed at the TUMAN-3M (in the discharges with ohmic L–H transition) and FT-2 tokamaks (where no clear L–H transition was observed). It is concluded based on this comparison that the GAM may act as a trigger for the L–H transition, provided that certain conditions for GAM oscillation and tokamak discharge are met.

GAM observation in the TUMAN-3M tokamak

Results of an experimental study of geodesic acoustic modes (GAM) in the TUMAN-3M tokamak are reported. With Doppler backscattering (DBS) the basic properties of the GAM such as frequency, conditions for the GAM existence and the GAM radial location have been identified. The two-frequency Doppler reflectometer system was employed to reveal an interplay between low frequency sheared poloidal rotation, ambient turbulence level and the GAM intensity. Bicoherence analysis of the DBS data evidences the presence of a nonlinear interaction between the GAM and plasma turbulence.

Study of internal transport barriers in the initial phase of Ohmic discharges in TUMAN-3M

A regime with electron heat confinement improvement was recently found in the initial phase of discharges in the TUMAN-3M tokamak. An internal transport barrier (ITB) formation in this regime was confirmed by Thomson scattering measurements and by transport modelling. Two possible reasons for the ITB formation are discussed in the paper: by reduction of turbulent transport in the presence of low magnetic shear or by plasma sheared rotation. It is demonstrated that low magnetic shear formation is possible in the current ramp-up phase of the Ohmic discharge. The low magnetic shear does not seem to be the only reason for the transport reduction. Results of Doppler reflectometry measurements of poloidal rotation of density fluctuations are presented. It is found that core confinement improvement correlates with the appearance of sheared rotation of the density fluctuations and with a burst of the MHD activity. The ITB formation in the regime seems to be a result of a combined action of reduced magnetic shear and plasma sheared rotation.

Bicoherence analysis of geodesic acoustic modes in the Tuman-3M and Globus-M Tokamaks

We present the results of a comparative investigation of geodesic acoustic modes (GAMs) by the method of bicoherence analysis in tokamaks (Tuman-3M and Globus-M) with different aspect ratios. The obtained data provide indirect evidence of the mutual influence of GAMs and background turbulence of tokamak plasma. The cross-bicoherence spectra were calculated using the signals of Doppler reflectometers and temporal dependences of the velocity of plasma rotation in the electric field of GAMs. The level of bicoherence in both tokamaks increases before transition to the H-mode and then decays down to the noise level in the H-mode.