Table of contents

Electron temperature fluctuations are correlated with the dominant (m, n) = (0, 1) edge-resonant magnetic tearing mode during sawtooth events in the Madison Symmetric Torus reversed-field pinch. Electron temperature fluctuations in phase with the (0, 1) tearing mode are measured using high-repetition-rate Thomson scattering. Immediately prior to the sawtooth the (0, 1) island structure appears to be heat-confining, while during the sawtooth it assumes an isothermal character. Core electron temperature variation is also phase correlated with the (0, 1) mode, suggesting that the edge-resonant tearing mode has an effect on core electron thermal confinement during sawtooth events.

All the main features of the scrape-off layer turbulence, magnitude, frequency spectrum and perpendicular wave vector, ξ_t, are strongly affected by the injection of lower hybrid (LH) power into the FTU tokamak. The governing parameters are the local last closed magnetic surface values of density, n_e,LCMS, and temperature, T_e,LCMS. n_e,LCMS determines the perpendicular wave vector of the LH waves, which is a key parameter for the multiple scattering processes, and together with T_e,LCMS the collisionality that exerts a stabilizing effect on the fluctuations. This effect, still to be examined in the light of theoretical models, leads to an asymptotic value for the fluctuation relative amplitude in the ohmic phase close to 25%, and ∼10% in the LH phase, or even less, since the saturation level is not yet attained. The LH waves also can strongly raise ξ_t, about 3 times, and double the root mean square frequency. The transfer of momentum and energy in the mutual scattering of LH and turbulence 'waves' drives these changes. An increase also of the cross-correlation between temperature and electric potential fluctuations should occur in order to explain the magnitude of the fluctuation amplitude drop and the large increment of the temperature e-folding decay, by more than a factor of 2.5. Particle transport, however, does not appear to be affected to a large extent—the density e-folding decay length is almost unchanged but the power flow typical length rises by about a factor of 1.5, which is a relevant figure in view of the problem of mitigating the power loads on divertor targets in future reactors. These changes are confined mainly within the flux tube connected with the LH waves launching antenna, but start to spread significantly out of it at high plasma densities.

The code COREDIV, self-consistent with respect to both the interaction of plasma core-edge and main plasma impurities, is used to simulate nitrogen-seeded discharges in JET. The model is fully described and numerical results are compared with experimental data pertaining to two series of discharges differing in input power, confinement, the level of power radiated and the puffing rate of the main gas. Impurity sources, their transport and densities are considered as functions of the edge and core parameters and consistency of their radiated power is discussed. Special emphasis is given to analysis of the fluxes of carbon and recycled deuterium.

This paper describes the use of nonlinear gyrokinetic simulations to assess the feasibility of a new correlation electron cyclotron emission (CECE) diagnostic that has been proposed for the Alcator C-Mod tokamak (Marmar et al2009 Nucl. Fusion49 104014). This work is based on a series of simulations performed with the GYRO code (Candy and Waltz 2003 J. Comput. Phys.186 545). The simulations are used to predict ranges of fluctuation level, peak poloidal wavenumber and radial correlation length of electron temperature fluctuations in the core of the plasma. The impact of antenna pattern and poloidal viewing location on measurable turbulence characteristics is addressed using synthetic diagnostics. An upper limit on the CECE sample volume size is determined. The modeling results show that a CECE diagnostic capable of measuring transport-relevant, long-wavelength (k_θρ_s < 0.5) electron temperature fluctuations is feasible at Alcator C-Mod.

This paper presents particle-in-cell simulations of the plasma behaviour in the vicinity of gaps in castellated plasma-facing components (PFCs). The point of interest was the test limiter of the TEXTOR tokamak, a PFC designed for studies of plasma–wall interactions, in particular, related to impurity transport and fuel retention. Simulations were performed for various plasma conditions in the vicinity of the castellated surface, where the gaps can be either shaped or unshaped. It was observed that depending on plasma parameters the transport of plasma particles inside the gap can be either in potential- or geometry-dominated regimes. The mechanisms responsible for the formation of a potential peak inside the poloidal gap and its consequences on plasma deposition profiles are discussed. A study of gap shaping was performed in order to validate its effectiveness.

Modulation of the amplitude of externally injected electron cyclotron (EC) power is a frequent method used to determine the radial power deposition profile in fusion plasmas. There are many tools to analyze the plasma response to the power modulations under quasi-stationary conditions. This paper focuses on the unique ability of the break-in-slope (BIS) method to retrieve a quasi-instantaneous estimate of the power deposition profile at each power step in the modulation, an outcome particularly relevant to track the power deposition location under non-stationary conditions. Here, the BIS analysis method is applied to the signals of a fast and high radial resolution wire-chamber soft x-ray camera in the Tokamak à Configuration Variable (TCV) where the plasma magnetic configuration and thus the EC resonance location are varied during the plasma discharge. As a step to validate this technique before real-time control experiments, the time-varying EC power deposition location of a single beam is successfully monitored by off-line BIS analysis. Simultaneous tracking of deposition locations of two EC beams gives promising results.

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m². An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.

Electromagnetic ion cyclotron (EMIC) instabilities with an isotropic ion beam and general loss-cone distribution of hot core plasmas are discussed. The growth rate of the wave, perpendicular heating of ions, parallel resonant energy and marginal instability of the EMIC waves in homogeneous plasmas are obtained using the dispersion relation for hot plasmas consisting of H⁺, He⁺,O⁺ ions and electrons. The wave is assumed to propagate parallel to the static magnetic field. The whole plasma is considered to consist of resonant and non-resonant particles permeated by the isotropic ion beam. It is assumed that the resonant particles and the ion beam participate in energy exchange with the wave, whereas the non-resonant particles support the oscillatory motion of the wave. We determined the variation in energies and growth rate in hot plasmas by the energy conservation method with a general loss-cone distribution function. We also discuss the effect of positive and negative ion beam velocity on the growth rate of the wave. The thermal anisotropy of the ions of the core plasma acts as a source of free energy for EMIC waves and enhances the growth rate. Heating of ions perpendicular to the magnetic field is discussed along with EMIC wave emission in the polar cusp region.

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that in the presence of in–out asymmetric impurity populations the zero-flux impurity density gradient (the so-called peaking factor) is significantly reduced. A sign change in the impurity flux may occur if the asymmetry is sufficiently large. This may be a contributing reason for the observed outward convection of impurities in the presence of radio frequency heating. This paper extends a previous work (Fülöp and Moradi 2011 Phys. Plasmas18 030703), by including the effect of ion parallel compressibility on the peaking factor, which is found to have a significant contribution in the presence of poloidal asymmetry. It is shown here that in the ion temperature gradient mode dominated plasmas the presence of an in–out poloidal asymmetry can lead to a negative impurity peaking factor, and it becomes more negative in regions with larger ion temperature gradients. In the trapped electron mode dominated plasmas an in–out poloidal asymmetry results in a strong reduction of the peaking factor; however, it remains positive for typical experimental parameters. Furthermore, it is shown that an up–down asymmetry reduces the peaking factor while an out–in asymmetry increases it.

A solitary drift wave structure and its quasi-periodic change are discovered in a linear magnetized plasma. Detailed investigations show that the periodic changes are accompanied by an emission of splash, that is, a short lifetime event characterized by much higher frequencies than that of the fundamental drift waves. Dynamic interaction between the solitary wave and zonal flows is found to cause the emission of splashes since the splashes occur in a particular phase of zonal flows. The findings should give a deep insight into the dynamics and structural formation of magnetically confined plasmas and rotating planets, which are governed by drift and Rossby waves, respectively.

Pedestal profiles that span the ELM cycle have been obtained and used to test the idea that the pedestal pressure gradient in MAST is limited by the onset of kinetic ballooning modes (KBMs). During the inter-ELM period of a regularly type I ELM-ing discharge on MAST, the pressure pedestal height and width increase together while the pressure gradient increases by only 15% during the ELM cycle. Stability analyses show that the pedestal region over which infinite-n ballooning modes are unstable also broadens during the ELM cycle. To test the relationship between the width of the region that is unstable to n = ∞ ideal magnetohydrodynamic ballooning modes and KBMs the gyrokinetic code, GS2, has been used for microstability analysis of the edge plasma region in MAST. The gyrokinetic simulations find that KBM modes with twisting parity are the dominant microinstabilities in the steep pedestal region, with a transition to tearing parity modes in the shallower pressure gradient core region immediately inside the pedestal top. The region over which KBMs are unstable increases during the ELM cycle, and a good correlation is found between the region where KBMs dominate and the region that is unstable to infinite-n ideal ballooning modes.

A model for the experimental results of the periodic oscillation of the electric field, so-called the electric field pulsation, observed in the Compact Helical Device (Fujisawa et al 1998 Phys. Rev. Lett.81 2256) and the Large Helical Device (Shimizu et al 2010 Plasma Fusion Res.5 S1015) is presented. A self-generated oscillation of the radial electric field is shown as the simulation result in helical plasmas. The reduction of the anomalous transport diffusivity in the core region is observed due to the strong shear of the radial electric field when the positive electric field is shown in the core region in the periodic oscillation of E_r. Two different time scales are found in the self-generated oscillation, which are the transport time scale and the fast time scale at the transition of the radial electric field. This oscillation because of the hysteresis characteristic is attributed to the electric field pulsation observed in helical plasmas. The parameter region of the condition for the self-generated oscillation is derived. It is shown that the multiple solutions of the radial electric field for the ambipolar condition are necessary but not sufficient for obtaining the self-generated oscillation.

In a recent paper (Cairns and Fuchs 2010 Nucl. Fusion80 095001) we have shown how the asymptotic method of stationary phase can be used to find the radiation pattern from an antenna in the far field region. The novel feature of this work is that it describes how to obtain the wave amplitude and phase in complex geometries where an explicit solution in terms of a phase integral is not available. Instead the necessary information is obtained from ray-tracing methods. Here we show how this method can be adapted to give the evolution of a short pulse in a plasma with arbitrary space and time dependence. Again the exact wave form of the pulse, including phase information, can be obtained from ray tracing. This provides a computationally simple way of calculating the behaviour of a short pulse that may be useful in studying some problems in laser–plasma interactions.

The mobilization and acceleration of metallic dust in the gap region between the last closed confinement surface and the vessel wall of the Frascati Tokamak Upgrade (FTU) is studied numerically for the definition of the appropriate location of diagnostics devoted to dust dynamics.

Calculations of the bootstrap current for the TJ-II stellarator are presented. DKES and NEO-MC codes are employed; the latter has allowed, for the first time, the precise computation of the bootstrap transport coefficient in the long-mean-free-path regime of this device. The low error bars allow a precise convolution of the monoenergetic coefficients, which is confirmed by error analysis. The radial profile of the bootstrap current is presented for the first time for the 100_44_64 configuration of TJ-II for three different collisionality regimes. The bootstrap coefficient is then compared with that of other configurations of TJ-II regularly operated. The results show qualitative agreement with toroidal current measurements; precise comparison with real discharges is ongoing.