Spectra of magnetic perturbations triggered by pellets in JET plasmas

Aiming at investigating edge localised mode (ELM) pacing for future application on ITER, experiments have been conducted on JET injecting pellets in different plasma configurations, including high confinement regimes with type-I and type-III ELMs, low confinement regimes and Ohmically heated plasmas. The magnetic perturbations spectra and the toroidal mode number, n, of triggered events are compared with those of spontaneous ELMs using a wavelet analysis to provide good time resolution of short-lived coherent modes. It is found that—in all these configurations—triggered events have a coherent mode structure, indicating that pellets can trigger an MHD event basically in every background plasma. Two components have been found in the magnetic perturbations induced by pellets, with distinct frequencies and toroidal mode numbers. In high confinement regimes triggered events have similarities with spontaneous ELMs: both are seen to start from low toroidal mode numbers, then the maximum measured n increases up to about 10 within 0.3 ms before the ELM burst

A new fast ion source and detector for investigating the interaction of turbulence with supra-thermal ions in a simple magnetized toroidal plasma.

A specific experimental apparatus consisting of an ion source and a detector for the investigation of the interaction between supra-thermal ions and drift-wave turbulence is developed on the TORoidal Plasma Experiment (TORPEX). Due to the small plasma temperature (~5eV), a spatially localized, small-size ion source (~4cm) mounted inside the vacuum vessel, with relatively low ion energy (~100eV-1keV) can be used. The source consists of an aluminosilicate Li-6 ion emitter (6 mm diameter, 10-30μA current) installed on a 2D poloidally moving system. The location, energy and current density profile of the ion beam will be measured using a 2D movable gridded energy analyzer

Statistical properties of electrostatic turbulence in toroidal magnetized plasmas (Invited paper)

A unique parabolic relation is observed to link skewness and kurtosis of density fluctuation signals, measured over the whole cross-section of the simple toroidal device TORPEX for a broad range of experimental conditions. This relationship is also valid for density fluctuation signals measured in the scrape-off layer of the TCV tokamak. All the probability density functions (PDFs) of the measured signals, including those characterized by a negative skewness, are universally described by a special case of the beta distribution. In TORPEX, fluctuations in the drift-interchange frequency range are necessary and sufficient to assure that PDFs can be described by this specific beta distribution. For a more detailed plasma scenario, it is shown that electron temperature and plasma potential fluctuations have different statistical properties compared with the density