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An analytical formula systematically predicts the observed frequency of pressure relaxation limit cycle oscillations in the vicinity of the transition to high confinement in four tokamaks (JET, ASDEX Upgrade, COMPASS, Globus-M). The experimental dataset spans the widest available range of frequencies, machine sizes and plasma ion species. The machine size dependence is explained by the connection length scale of plasma flows parallel to the magnetic field. The model also explains the observed up-down poloidal current asymmetry and the impact of the plasma ion species mass and charge. [ABSTRACT FROM AUTHOR]

The non-linear dynamics of a chain of stationary, saturated magnetic islands is studied by solving a four-field system of equations that include non-ideal effects, lowest order finite Larmor radius corrections and neoclassical terms. The magnetic island rotation velocity is calculated self-consistently with the fields profiles. The solutions for the island rotation velocity and for the ion polarization current are determined as a function of the characteristic parameters of the system and the results are discussed. The results of the calculations show that island rotation velocity and the ion polarization current depend in a non-trivial way on the parameters characterizing the system, and some clear patterns emerge only in particular cases. An analysis of magnetic island rotation velocity is performed on experiments in COMPASS tokamak. Measured island rotation velocity is compared with the calculated ion and electron flow velocities, for different hypotheses on the toroidal rotation of the plasma. The comparison shows that the island rotation velocity is consistent with the ion flow velocity, under the hypothesis of slow toroidal rotation and low collisionality. Theoretical calculation of the island rotation velocity according to the model here developed suggests that the islands rotate weakly in the ion direction, in the hypothesis of slow toroidal rotation and high collisionality. The impossibility of directly measuring the plasma rotation velocity makes it difficult to distinguish between these different regimes. [ABSTRACT FROM AUTHOR]

The understanding of the halo current properties during disruptions is key to design and operate large scale tokamaks in view of the large thermal and electromagnetic loads that they entail. For the first time, we present a fully self-consistent model for halo current simulations including neutral particles and sheath boundary conditions. The model is used to simulate vertical displacement events (VDEs) occurring in the COMPASS tokamak. Recent COMPASS experiments have shown that the parallel halo current density at the plasma-wall interface is limited by the ion saturation current during VDE-induced disruptions. We show that usual magneto-hydrodynamic boundary conditions can lead to the violation of this physical limit and we implement this current density limitation through a boundary condition for the electrostatic potential. Sheath boundary conditions for the density, the heat flux, the parallel velocity and a realistic parameter choice (e.g. Spitzer's resistivity and Spitzer–Härm parallel thermal conductivity) extend present VDE simulations beyond the state of the art. Experimental measurements of the current density, temperature and heat flux profiles at the COMPASS divertor are compared with the results obtained from axisymmetric simulations. Since the ion saturation current density () is shown to be essential to determine the halo current profile, parametric scans are performed to study its dependence on different quantities such as the plasma resistivity and the particle and heat diffusion coefficients. In this respect, the plasma resistivity in the halo region broadens significantly the profile, increasing the halo width at a similar total halo current. [ABSTRACT FROM AUTHOR]

Microsecond probe measurements of the electron temperature during the tokamak edge localised mode (ELM) instability show that the peak values significantly exceed those obtained by conventional techniques. The temperatures measured at the plasma facing component (divertor) are around 80% of the initial value (at the pedestal). This challenges the current understanding, where only several percent of the pedestal value are measured at the divertor. Our results imply a negligible energy transfer from the electrons to the ions during the ELM instability, and therefore no associated increase of the ion power loads on the divertor. This observation is supported by the simple analytic free-streaming model, as well as by full kinetic simulations. The energetic ELM ion loads are expected to be one of the main divertor damaging factors; therefore, the obtained results give an optimistic prediction for next generation fusion devices. [ABSTRACT FROM AUTHOR]

Recently the Lithium-Beam Emission Spectroscopy (Li-BES) system on COMPASS has reached its full diagnostic power in terms of routine automatic operation in any kind of plasma scenarios and it is normally used as a standard tool for reconstruction of ultra fast density profiles in the edge region of COMPASS plasmas. The purpose of this study is to investigate the advantages and limitations of the COMPASS Li-BES system in characterizing plasma electron density fluctuations. We show how the atomic physics of plasma-beam interactions can affect the interpretation of the measurement at different radial positions and for different electron density profiles. We also demonstrate the usability of generalized sequential probability ratio test for automatic event detection. Using non-perturbative diagnostic, we verify the validity of the stochastic Garcia-model for scrape-off layer filaments and accompanying holes (density deficits). [ABSTRACT FROM AUTHOR]

This contribution presents the experimentally observed edge plasma evolution during limit cycle oscillations (LCO) measured with a new Langmuir and ball-pen multi-pin probe head at the COMPASS tokamak. The observed LCO regime modulates the intensity of density fluctuations , radial electric field Er and intensity of emission with a frequency 3–5 kHz. The density fluctuations grow after Er decreases in strength which appears to be strongly correlated with the evolution of the pressure gradient . The magnetic signature of the LCO shows a left–right asymmetry with propagation from the low to high field side. High-frequency (above 100 kHz) precursor-like oscillations are observed as well. [ABSTRACT FROM AUTHOR]

The radial distributions of the main plasma parameters in the scrape-off-layer of the COMPASS tokamak are measured during L-mode and H-mode regimes by using both Langmuir and ball-pen probes mounted on a horizontal reciprocating manipulator. The radial profile of the plasma potential derived previously from Langmuir probes data by using the first derivative probe technique is compared with data derived using ball-pen probes. A good agreement can be seen between the data acquired by the two techniques during the L-mode discharge and during the H-mode regime within the inter-ELM periods. In contrast with the first derivative probe technique, the ball-pen probe technique does not require a swept voltage and, therefore, the temporal resolution is only limited by the data acquisition system. In the electron temperature evaluation, in the far scrape-off layer and in the limiter shadow, where the electron energy distribution is Maxwellian, the results from both techniques match well. In the vicinity of the last closed flux surface, where the electron energy distribution function is bi-Maxwellian, the ball-pen probe technique results are in agreement with the high-temperature components of the electron distribution only. We also discuss the application of relatively large Langmuir probes placed in parallel and perpendicularly to the magnetic field lines to studying the main plasma parameters. The results obtained by the two types of the large probes agree well. They are compared with Thomson scattering data for electron temperatures and densities. The results for the electron densities are compared also with the results from ASTRA code calculation of the electron source due to the ionization of the neutrals by fast electrons and the origin of the bi-Maxwellian electron energy distribution function is briefly discussed. [ABSTRACT FROM AUTHOR]

We report the latest results on fast measurements of the electron temperature and parallel heat flux in the COMPASS tokamak scrape-off layer (SOL) and divertor region during ELMy H-mode plasmas. The system of ball-pen and Langmuir probes installed on the divertor target, the horizontal reciprocating manipulator and the fast data-acquisition system with sampling frequency rate f  =  5 MSa s−1 allow us to measure the electron temperature and parallel heat flux during inter-ELM and ELM periods with high temporal resolution. The filamentary structure of the electron temperature and parallel heat flux was observed during ELMs in the SOL as well as in the divertor region. The position of the filaments within ELMs is not regular and therefore the resulting conditionally averaged ELM neglects the peak values of the electron temperature and parallel heat flux. We have found a substantial difference between the value of the radial power decay length in the inter-ELM period λq,inter  =  2.5 mm and the decay length of the peak ELM heat flux λq,ELM  =  13.1 mm. The decay length of the ELM energy density was found to be λE,ELM  =  5.4 mm. [ABSTRACT FROM AUTHOR]

It has been shown on several tokamaks that application of a resonant magnetic perturbation (RMP) field to the plasma can lead to suppression or mitigation of edge-localized mode (ELM) instabilities. Due to the rotation of the plasma in the RMP field reference system, currents are induced on resonant surfaces within the plasma, consequently screening the original perturbation. In this work, the extensive set of 104 saddle loops installed on the COMPASS tokamak is utilized to measure the plasma response field for two n  =  2 RMP configurations of different poloidal mode m spectra. It is shown that spatially the response field is in opposite phase to the original perturbation, and that the poloidal profile of the measured response field does not depend on the poloidal profile of the applied RMP. Simulations of the plasma response by the linear MHD code MARS-F (Liu et al 2000 Phys. Plasmas7 3681) reveal that both of the studied RMP configurations are well screened by the plasma. Comparison of measured plasma response field with the simulated one shows a good agreement across the majority of poloidal angles, with the exception of the midplane low-field side area, where discrepancy is seen. [ABSTRACT FROM AUTHOR]

Multiple quasicoherent electromagnetic modes with steady-state frequency and different nature and location were observed in the COMPASS tokamak (R = 0.56 m, = 0.2 m) at Bt = 1.14 T with Co-NBI (PNBI = 0.2–0.5 MW, Eb = 32 keV) at frequencies 5 kHz < f < 250 kHz. Modes were observed in both low and high confinement (L- and H-modes) plasmas. Lower frequency modes with f < 50 kHz were identified as low m tearing and kink MHD modes, while higher frequency modes with 50 kHz < f < 250 kHz were considered as having Alfvénic nature. Unexpectedly, such modes were only observed in the H-mode, both in neutral beam injector-assisted and Ohmic, so the mode driving force is not yet clear. Using the linear MHD code KINX, we initially identified the observed mode with a ballooning structure is as beta induced Alfvén eigenmode (BAE) with m, n < 5, while an antiballooning mode is initially identified as toroidal Alfvén eigenmode (TAE) with m, n < 9. [ABSTRACT FROM AUTHOR]