Poloidal Localization Research Articles

Poloidal localization of the explosive onset of edge localized modes

In ASDEX Upgrade lower single null H-mode plasmas, the onset of the explosive phase of Edge Localized Modes (ELMs) is poloidally localized in the X-point/outer strike point (X/OSP) region. From this location, ELMs develop on average towards the low-field and high-field sides in typically 100μs . An associated magnetic activity localized in the X/OSP region is also observed in-between ELMs, at a lower level than at the ELM onset, and interpreted as being due to perturbed currents connected to the divertor target-s. Its broadband spectra typically extend up to 100kHz –due to short-lived events of variable frequencies–and are dominated by n=1 toroidal mode numbers rotating in the counter-current direction.

Poloidally localized edge density fluctuation with applied and spontaneous 3-D fields in experimental advanced superconducting tokamak (EAST)

Poloidally localized density fluctuation with applied and spontaneous 3-D fields in high-confinement mode (H-mode) plasma is observed by using poloidally distributed multichannel interferometer diagnostics in the experimental advanced superconducting tokamak. With the application of an n = 1 rotating nonaxisymmetric magnetic perturbation (NAMP) field, a density fluctuation appears on different interferometer channels sequentially. Evidence shows that this fluctuation is located at the plasma edge. When an n = 1 static NAMP field is applied, the density fluctuation amplitude increases in parts of channels but decreases in other channels, implying a poloidal localization. This localization is enhanced with the increase in the NAMP field. In addition, when a spontaneous n = 1 external kink mode with a frequency of ∼3.5 kHz appears in the plasma edge, the amplitude of density fluctuation is modulated by the n = 1 kink. This modulation could be attributed to the localization effect on fluctuation by the 3-D fields of the spontaneous kink mode.

Turbulence and perpendicular plasma flow asymmetries measured at TJ-II plasmas

Dedicated experiments have been carried out for a systematic comparison of turbulence wavenumber spectra and perpendicular rotation velocity measured at poloidally separated positions on the same flux-surface in the stellarator TJ-II. The rationale behind this study is twofold, namely, validation of the spatial localization of instabilities predicted by gyrokinetic simulations in stellarators and validation of the electrostatic potential variation on the flux surface as calculated by neoclassical codes and its possible impact on the radial electric field. Perpendicular wavenumber spectra and perpendicular rotation velocity profiles have been measured using Doppler reflectometry in two plasma regions poloidally separated as both positive and negative probing beam angles with respect to normal incidence can be selected. A systematic comparison has been carried out showing differences in the perpendicular wavenumber spectrum measured at poloidally separated positions on the same flux-surface, that depend on plasma density, heating conditions and magnetic configuration. The asymmetry found in the standard magnetic configuration under some plasmas conditions, reverses in the high iota configuration. The different intensity in the density fluctuation spectra can be related to the poloidal localization of instabilities found in gyrokinetic simulations. Differences in the radial electric field profile are also found that could be explained to be due to on-surface plasma potential variations.

Reduced-magnetohydrodynamic simulations of toroidally and poloidally localized edge localized modes

We use the non-linear reduced-magnetohydrodynamic code JOREK to study edge localized modes (ELMs) in the geometry of the ASDEX Upgrade tokamak. Toroidal mode numbers, poloidal filament sizes, and radial propagation speeds of filaments into the scrape-off layer are in good agreement with observations for type-I ELMs in ASDEX Upgrade. The observed instabilities exhibit a toroidal and poloidal localization of perturbations which is compatible with the “solitary magnetic perturbations” recently discovered in ASDEX Upgrade [R. Wenninger et al., “Solitary magnetic perturbations at the ELM onset,” Nucl. Fusion (accepted)]. This localization can only be described in numerical simulations with high toroidal resolution.

Ion-temperature gradient modes affected by helical magnetic field of magnetic islands

Ion temperature gradient mode (ITG) affected by static magnetic field of magnetic islands is investigated numerically by means of Landau fluid model. The ITG is localized around O-points of magnetic islands, and the localization in poloidal direction is similar to the poloidal localization of toroidal ITG. This is because the helical magnetic field of magnetic islands causes geometrical coupling, and thus Fourier modes that have the same helicity as the islands are coupled together. The strength of coupling is characterized by the square of island width, and it corresponds to the fact that the strength of mode coupling of toroidal ITG is characterized by the inverse aspect ratio of torus in reduced fluid models.

Reduction of RF-sheaths potentials by compensation or suppression of parallel RF currents on ICRF antennas

Radio frequency (RF) sheaths are suspected of limiting the performance of present-day ion cyclotron range of frequencies (ICRFs) antennas over long pulses and should be minimized in future fusion devices. Within the simplest models, RF-sheath effects are quantified by the integral VRF = ∫ E∥ · dl where the parallel RF field E∥ is linked with the slow wave. On ‘long open field lines’ with large toroidal extension on both sides of the antenna it was shown that VRF is excited by parallel RF currents j∥ flowing on the antenna structure.In this paper, the validity of this simple sheath theory is tested experimentally on the Tore Supra (TS) ITER-like antenna prototype (ILP), together with antenna simulation and post-processing codes developed to compute VRF. The predicted poloidal localization of high-|VRF| zones is confronted to that inferred from experimental data analysis. Surface temperature distribution on ILP front face, as well as ILP-induced modifications of RF coupling and hot spots on a magnetically connected lower hybrid current drive antenna, indicates local maxima of dc plasma potential in both the upper and lower parts of the ILP. This result, qualitatively conforming to VRF simulations, is interpreted in terms of j∥ flowing on ILP frame.Once the validation is done, such reliable theoretical models and numerical codes are then employed to provide predictive results. Indeed, we propose two ways to reduce |VRF| by acting on j∥ on the antenna front face. The first method, more adapted for protruding antennas, consists of avoiding the j∥ circulation on the antenna structure, by slotting the antenna frame on its horizontal edges and by partially cutting the Faraday screen rods.The second method, well suited for recessed antennas, consists of compensating j∥ of opposite signs along long flux tubes, with parallelepiped antennas aligned with (tilted) flux tubes.The different concepts are assessed numerically on a two-strap TS antenna phased [0, π] using near RF fields from the antenna code TOPICA. Simulations stress the need to suppress all current paths for j∥ to substantially reduce |VRF| over the whole antenna height.

Filament structures at the plasma edge on MAST

The boundary of the tokamak core plasma, or scrape-off layer, is normally characterized in terms of average parameters such as density, temperature and e-folding lengths suggesting diffusive losses. However, as is shown in this paper, localized filamentary structures play an important role in determining the radial efflux in both L mode and during edge localized modes (ELMs) on MAST. Understanding the size, poloidal and toroidal localization and the outward radial extent of these filaments is crucial in order to calculate their effect on power loading both on the first wall and the divertor target plates in future devices. The spatial and temporal evolution of filaments observed on MAST in L-mode and ELMs have been compared and contrasted in order to confront the predictions of various models that have been proposed to predict filament propagation and in particular ELM energy losses.

Discrete compressional Alfvén eigenmode spectrum in tokamaks

The spectrum of compressional Alfvén eigenmodes (CAE) is analysed and shown to be discrete in tokamaks with low aspect ratio, such as the National Spherical Torus Experiment (NSTX), as well as in conventional tokamaks, such as DIII-D. The study is focused on recent similarity experiments on NSTX and DIII-D in which sub-cyclotron frequency instabilities of CAEs were observed at similar plasma conditions (W.W. Heidbrink et al 2006 Nucl. Fusion 46 324). The global ideal MHD code NOVA recovers the main properties of these modes predicted by theory and observed in both devices. The discrete spectrum of CAEs is characterized by three quantum mode numbers for each eigenmode, (M, S and n), where M, S and n are poloidal, radial and toroidal mode numbers, respectively. The expected mode frequency splitting corresponding to each of these mode numbers seems to be observed in experiments and is consistent with our numerical analysis. The polarization of the observed magnetic field oscillations in NSTX was measured and is also consistent with the numerical analysis, which helps to identify them as CAE activity. CAE mode structure was obtained and shown to be localized in both radial and poloidal directions with typical radial localization toward the plasma edge and poloidal localization at the low field side of the plasma cross section.

On the poloidal localization and stability of multi-faceted asymmetric radiation from the edge (MARFE)

A possible explanation for differences in the behavior of multi-faceted asymmetric radiation from the edge (MARFE) in normal and reversed toroidal field configurations, based on the influence of poloidal E×B drift, is offered. The poloidal E×B drift creates poloidal rotation that tends to shift the MARFE away from its stable position near the inner midplane. For a neoclassical radial electric field (Er≡−dΦ/dr<0), and in normal toroidal field configurations (ion ∇B drift towards the X-point), the E×B poloidal rotation shifts the MARFE towards another stable position, near the X-point. In the reversed toroidal field configurations (ion ∇B drift away from the X-point), the E×B poloidal rotation is shifting the MARFE away from the X-point. These trends are in agreement with experimental observations.

Improvement of the highly energetic passing particle confinement in torsatrons and heliotrons

The configurational effects on the confinement of highly energetic passing particles, especially the helical-field ripple effects, are considered in torsatrons and heliotrons from the viewpoint of increasing the efficiency of tangential neutral beam injection (NBI) heating of plasma. The drift resonances of passing particles (ιΩφ=ΩD, where ι is the rotational transform, Ωφ is the particle toroidal transit frequency, and ΩD is the particle drift frequency) are investigated. It is discovered that the resonant passing particles oscillate poloidally through the variation of the field-line curvature. The passing particles with the energies located in the vicinity of the resonance separatrix are revealed to be as dangerous for confinement, as superbananas are. The existence of the highly poloidally localized resonant particles with lower radial deviations than the corresponding values for the nonresonant particles was found. The particles with the energies close to the state of the high poloidal localization are proposed as the most preferable beam particles for the tangential NBI heating of plasma.

Correlation spectroscopy diagnostics in the H-1 heliac

A simple diagnostic arrangement is suggested for measuring density and temperature fluctuations. The diagnostic uses a cross-correlation technique to extract local frequency spectra, spatial distribution or (in the case of low mode numbers) the mode structure from the chord-average fluctuating intensities of the visible light in the H-1 heliac. For low temperature H-1 plasmas ( T e ≈ 20 eV, n e ≈ 2 × 1012 cm −3), the spectral line emission from neutral atoms is used For strong turbulence, where the coherence length of the fluctuations is much shorter than the plasma radius, the amplitude of the cross-correlation between two crossed-sightline fluctuating intensities is proportional to the fluctuation amplitude in the intersection volume. The optical arrangement on the H-1 heliac uses two orthogonal views with mirrors that allow the intersection volume to be scanned over the full plasma poloidal cross-section. Under certain discharge conditions, the plasma is dominated by strong, low frequency oscillations with frequencies in the range 15–20 kHz. Although such discharges are not very suitable for implementing the main idea of the diagnostic (because, in this case, the fluctuation correlation length is of the order of the plasma diameter), the measurements allow the poloidal mode number and spatial localization of the fluctuations to be determined after comparison with numerical modelling results; these show good agreement with the Langmuir probe results.

Alfven cyclotron instability and ion cyclotron emission

Two-dimensional solutions of compressional Alfven eigenmodes (CAEs) are studied in the cold plasma approximation. For finite inverse aspect ratio tokamak plasmas the two-dimensional eigenmode envelope is localized at the low magnetic field side with the radial and poloidal localization on the order of a/ square root m and a/4 square root m, respectively, where m is the dominant poloidal mode number. Charged fusion product driven Alfven cyclotron instability (ACI) of the compressional Alfven eigenmodes provides the explanation for the ion cyclotron emission (ICE) spectrum observed in tokamak experiments. The ACI is excited by fast charged fusion products via Doppler shifted cyclotron wave-particle resonances. The ion cyclotron and electron Landau dampings and fast particle instability drive are calculated perturbatively for deuterium-deuterium (DD) and deuterium-tritium (DT) plasmas. Near the plasma edge at the low field side the velocity distribution function of charged fusion products is localized in both pitch angle and velocity. The poloidal localization of the eigenmode enhances the ACI growth rates by a factor of square root m in comparison with the previous results without poloidal envelope. The thermal ion cyclotron damping determines that only modes with eigenfrequencies at multiples of the edge cyclotron frequency of the background ions can be easily excited and form an ICE spectrum similar to the experimental observations. Theoretical understanding is given for the results of TFTR DD and DT experiments with valpha 0/vA approximately=1 and JET experiments with valpha 0/vA>1

Quenching of external kink modes by localized radio-frequency fields

The quenching of external kink modes by a spatially localized, externally applied surface force is studied in a low-beta cylindrical tokamak model. It is shown that m=2 modes may be stabilized by the application of a radio frequency (rf) field in the ion cyclotron range of frequencies (ICRF) excited by either a toroidal or a poloidal antenna structure surrounding the plasma. Whereas linear mode coupling induced by toroidal localization is negligible, that caused by poloidal localization may degrade the stabilization and, thus, require increased power levels for stability. The model is directly applicable to the ponderomotive force exerted by an evanescent slow wave in the ion cyclotron range of frequencies. Quantitative estimates are made for ponderomotive stabilization of kink modes by means of ion-Bernstein-wave (IBW) antennas. This method of stabilization has the advantage that the antenna system does not have to match the spatial or temporal structure of the unstable modes. For typical IBW antenna systems, it is found that the required rf fields for kink stabilization are reasonable if the current profile is not too flat.

Two- and three-dimensional tokamak equilibrium modifications driven by the localized ponderomotive force of applied radio frequency waves

Large ponderomotive forces can occur when waves in the ion cyclotron range of frequencies (ICRF) are employed in stabilization or high power heating schemes. It is shown that the magnetohydrodynamic (MHD) equilibrium of the plasma is modified when the rf antenna is localized poloidally and/or toroidally. In the two-dimensional (2-D) problem (toroidally uniform rf) a modified Grad–Shafranov equation describes ponderomotively driven magnetic surface distortions. At electric field strengths of interest for ICRF stabilization, the equilibrium distortions are small provided that Lrf≪a and the poloidal localization of the rf is not severe. The character of the 3-D problem (toroidally and poloidally varying rf) depends on whether or not the ponderomotive force couples to rational surfaces within the plasma. For the case without rational surfaces, the 3-D component of the distortions is found to be small by 1/q2a relative to the 2-D component. In the case where rational surfaces are excited, magnetic islands result. A threshold condition for island overlap determines the rf electric field at which ergodicity may be expected. It is shown that ergodicity of the edge plasma may occur for some cases of interest for heating and stabilization, but should be controllable.

Low-frequency microinstabilities in the PLT tokamak

A turbulence with wavelengths in the range 2–0.2 cm and with a spectrum in the range of frequencies of drift waves has been observed in the PLT discharge with scattering of microwaves. The total density fluctuation was ñe/n̄e≃5×10−3–10−2. It is estimated that this turbulence can cause an anomalous transport much larger than that predicted by neoclassical theory. As the PLT discharge developed a central minimum of the electron temperature, an increase in the turbulence level was observed with a very strong poloidal localization on the outside of the plasma torus.