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Local Alfven wave (LAW) resonances are excited in tokamaks by an externally driven electro-magnetic field, below the ion cyclotron frequency. Based on TCABR (Tokamak Chauffage Alfven Bresilien) experiments and numerical calculations, it is shown that a combination of small power deposition in LAW resonances, swept by plasma density variation or scanned by varying generator frequencies, in combination with detection of the density fluctuations in the LAW resonances by reflectometry, can serve as a diagnostic tool for identification of the effective ion mass number Aef and q-profile in tokamaks. The idea is based on the simultaneous detection of the position of m = ±1 LAW resonances, which are excited by M/N = ±1/±2 antenna modes and m = 0 generated by the poloidal mode coupling effect in tokamaks. The m = 0 resonance depends only on the effective ion mass number and not on the q-profile, so that the mass number can be determined unambiguously. Then, we can determine q-factor at the position of m = ±1 LAW resonances. Using the multifluid ALTOK code, we identify mass number in TCABR experiments and demonstrate the possibility of applying this method in the Joint European Torus.

The effect of plasma rotation on low frequency (LF) field penetration, absorption and ponderomotive forces in TEXTOR and in Tokamak Chauffage Alfven Bresilien (TCABR) is investigated in the frequency band of 1–10 kHz. The LF fields are driven by the dynamic ergodic divertor in TEXTOR and the ergodic magnetic limiter in TCABR. Alfven wave mode conversion is responsible for the LF field absorption at the rational magnetic surface where q = −M/N is the integer. Analytical and numerical calculations show the maxima of the LF field absorption at the local Alfven wave resonance |ω − k · U| = |k∥| cA, where ω and k are the frequency and the wave vector, respectively, and cA is the Alfven velocity at the rational magnetic surface q = 2, 3 in TEXTOR and TCABR. The rotation velocity U along the magnetic surfaces, taken into account in the dielectric tensor, can strongly modify the LF field and dissipated power profiles. The absorption in the local AW resonances begins to be non-symmetric in relation to the resonance surface. Calculations show that coil impedance has a maximum related to excitation of some stable (possibly Suydam) modes for waves travelling in the direction of plasma rotation.

Experimental data obtained on the TCABR tokamak (R = 0.61?m, a = 0.18?m) with an electrically polarized electrode, placed at r = 0.16?m, is reported in this paper. The experiment was performed with plasma current of 90?kA (q = 3.1) and hydrogen gas injection adjusted for keeping the electron density at 1.0 ? 1019?m?3 without bias. Time evolution and radial profiles of plasma parameters with and without bias were measured. The comparison of the profiles shows an increase of the central line-averaged density, up to a maximum factor of 2.6, while H? hydrogen spectral line intensity decreases and the C?III impurity stays on the same level. The analysis of temporal behaviour and radial profiles of plasma parameters indicates that the confined plasma enters the H-mode regime. The data analysis shows a?maximum enhanced energy confinement factor of 1.95, decaying to 1.5 at the maximum of the density, in comparison with predicted Neo?Alcator scaling law values. Indications of transient increase of the density gradient near the plasma edge were obtained with measurements of density profiles. Calculations of turbulence and transport at the Scrape-Off-Layer, using measured floating potentials and ion saturation currents, show a strong decrease in the power spectra and transport. Bifurcation was not observed and the decrease in the saturation current occurs in 50??s.

The profile and dissipation of the field excited by dynamic ergodic divertor (DED) coils in tokamak plasmas are calculated, and an estimate is made of the poloidal/toroidal velocities driven by this field. The coils are idealized as an inboard sheet current composed of a toroidal sequence of helical line current segments expanded in Fourier series with poloidal/toroidal mode numbers M/N, and mode amplitudes depending on feeding. Numerical calculations with cylindrical and toroidal codes show maxima of field dissipation due to Alfven wave mode conversion effect taking place at the rational magnetic surfaces where q = M/N. The effects of toroidicity and ion collisions in the dielectric tensor in the upper DED frequency range described (f = 5-10 kHz) are found to be very important in absorption calculations. At the q = 3 resonant magnetic surface typical for DED coil design, it is estimated that ponderomotive forces produced by 20 kW of dissipation can drive local toroidal and poloidal flows of respective orders 8 km s(-1) and 10 km s(-1) in the TEXTOR tokamak.

Direct observations of tokamak plasma responses to an externally applied rotating helical magnetic perturbation (RHMP) have been performed on the small tokamak device HYBTOK-II in order to clarify the penetration process of the RHMP into tokamak plasmas. Not only electromagnetic but also electrostatic fluctuations have been measured with small magnetic probes and a four-pin Langmuir probe, which were inserted inside the plasma. As regards the observed radial profiles of the RHMP in the plasma, the low Doppler-shifted frequency of RHMP produces the amplification of RHMP in the plasma due to magnetic island formation, while the expected attenuation of RHMP in the plasma has been found in the case of the high Doppler-shifted frequency. It is also found that the observed radial electric fields with the four-pin Langmuir probe have strong dependences on the driving frequency of the RHMP and the toroidal magnetic field. A theoretical calculation of the Alfven wave propagation driven by local helical coils in HYBTOK-II has also been carried out. The observed electric fields are discussed in terms of the Alfven resonance and the growth of magnetic islands.

Kinetic or slow Alfven waves, which appear in tokamak plasmas as a result of RF wave conversion, have small radial localization. For this reason, these waves are used here to create strongly sheared plasma flows at any point of the plasma minor radius and to obtain the locally reversed magnetic shear configuration. It is also demonstrated that Alfven waves may be a convenient trigger for formation of transport barriers in tokamaks. They can be used for suppressing plasma turbulence and squeezing ion banana and potato orbits. Estimates of the absorbed Alfven wave power, which is necessary to achieve these effects in some tokamaks, are about the level of the absorbed power in early completed experiments. The use of Alfven waves can also be helpful for self-organization processes in tokamaks.

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.

Long-distance correlations (LDCs) of plasma potential fluctuations in the plasma edge have been investigated in the TCABR tokamak in the regime of edge biasing H-mode using an array of multi-pin Langmuir probes. This activity was carried out as part of the scientific programme of the 4th IAEA Joint Experiment (2009). The experimental data confirm the effect of amplification of LDCs in potential fluctuations during biasing recently observed in stellarators and tokamaks. For long toroidal distances between probes, the cross-spectrum is concentrated at low frequencies f f f = 13–15 kHz and f ∼ 40 kHz and low wave numbers with a maximum at k = 0. The effects of MHD activity on the LDCs in potential fluctuation are investigated.

Absorption of radiofrequency (RF) waves within the Alfven range of frequencies (Ω < ωHi) and current drive generation by Alfven waves are studied in a cylindrical plasma column, with particle, heat and magnetic field diffusion processes taken into account. To this end, a 1-D numerical code describing RF field propagation, absorption and current drive processes has been developed, with a self-consistent evolution of the plasma parameters. The code structure includes electrodynamic and transport equations that are solved alternatingly with a 1 ms time order step. – Two different regimes have been found in the model, depending on the direction of the RF field rotation. Below the Alfven threshold frequency, drift Alfven mode pumping and RF field conversion into an ion acoustic wave have been found for a hollow current density distribution. It is shown that the helical (kink) resonance regime with wavenumbers m < 0 and n < 0 is the most promising for plasma heating. The efficiency of steady state current sustainment for the INTOR device parameters is of the order of 0.5 AW−1.

The problem of RF field excitation and absorption in an inhomogeneous plasma cylinder with axial uniform direct current is investigated. The RF power absorption is connected with the conversion of the incident wave generated by a helical antenna in the slow Alfven (kinetic) mode in the vicinity of the point of 'local Alfven resonance' r = rA. Effects of the plasma gyrotropic properties are included. Analytical expressions for RF field amplitudes and absorbed RF power, W, are obtained in terms of series expansions. If the axial current is large enough, sharp resonance curves, W(rA), are possible, with the maxima near the plasma centre. These maxima virtually coincide with the kink mode frequencies of the plasma column.

This study deals with the stability of a plasma in the field of a self-consistent Alfven wave. Decay instabilities for a plasma with β 1 are considered, and the growth rate and widths of the instability zones are found. The problem of determining the critical wave amplitude is discussed.The solution as a whole was reached for two types of Alfven waves – standing and travelling.An instability associated with the scattering of the Alfven wave was found in the second approximation with respect to the parameter h0/H0 (h0 being the amplitude of the Alfven wave and H0 the intensity of the constant magnetic field). The excitation conditions for this instability are less stringent than those for decay instabilities.Problems connected with the build-up of instabilities of the type under consideration in systems bounded along the magnetic field are discussed.

In the paper, the decay instabilities of the hydromagnetic oscillations in a plasma cylinder placed in the field of a direct magneto-sonic wave Hz = 0I0(k0r)cosΩ0t (I0 is the Bessel function, k0 is the radial wave number, ) and 0 is the amplitude of the high-frequency field) are studied. Instability growth rates were derived for decay in cases where the result is the excitation of short-wave Alfven and slow magneto-sonic oscillations. Calculations were carried out for various distributions of the field of the background wave over the cylinder cross-section: k0a 1; k0a = 2.405; a is the radius of the cylinder. The growth-rates are determined by formulae of the type γ = Ω0A0/H0, where H0 is the strength of the static field, A is a coefficient less than or of the order of unity, depending on the type of decay, the plasma parameters and the wave numbers for the excited waves. Formulae are presented for the determination of the threshold amplitudes for excitation of the instabilities. The possibility of using these instabilities for plasma heating is briefly discussed.