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Analysis of lithium driven electron density peaking in FTU liquid lithium limiter experiments
Get PDFThe impact of lithium impurities on the microstability and turbulent transport characteristics in the core of a typical FTU liquid lithium limiter (LLL) (Mazzitelli et al 2011 Nucl. Fusion 51 073006) discharge during the density ramp-up phase is studied. A non-linear gyrokinetic analysis performed with GKW (Peeters et al 2009 Comput. Phys. Commun. 180 2650) accompanied by a quasi-linear fluid analysis is presented. We show that a centrally peaked, high concentration lithium profile contributes to the electron peaking by reducing the outward electron flux, and that it leads to inward turbulent deuterium transport through ion flux separation
Helium injection plasmas in FTU
No full textIn order to extend observations on the increase of electron density peaking in neon doped plasmas, already reported in FTU Mazzotta et al (2015 Nucl. Fusion 55 073027), some sessions have been performed by injecting helium gas on the L-mode plasmas during the last two experimental campaigns. This favorable scenario, which can fall within so called 'highly radiative' or 'plasma detachment' themes, is investigated in this paper. The description of the impact of the helium injection on plasma behavior, by varying plasma parameters and shape, is exposed, especially with respect to the density peaking and edge conditioning. It has been recorded that, not only the total amount of puffed helium, but also the injection rate intervenes in triggering a particle inflow. Finally, by using VUV spectroscopy measurements, a model to estimate concentrations of impurities is outlined here for the first time, in this way the helium amount is deduced
Advances in the FTU collective Thomson scattering system
No full textThe new collective Thomson scattering diagnostic installed on the Frascati Tokamak Upgrade device started its first operations in 2014. The ongoing experiments investigate the presence of signals synchronous with rotating tearing mode islands, possibly due to parametric decay processes, and phenomena affecting electron cyclotron beam absorption or scattering measurements. The radiometric system, diagnostic layout, and data acquisition system were improved accordingly. The present status and near-term developments of the diagnostic are presented.SP
Observation of short time-scale spectral emissions at millimeter wavelengths with the new CTS diagnostic on the FTU tokamak
No full textOn the FTU tokamak, the collective Thomson scattering (CTS) diagnostic was renewed for investigating the possible excitation of parametric decay instabilities (PDI) by electron cyclotron (EC) or CTS probe beams in presence of magnetic islands and measure their effects on the EC power absorption. The experiments were performed launching a gyroton probe beam (140 GHz, 400 kW) and observing the scattered radiation in symmetric and asymmetric directions (with respect to the equatorial plane) in different conditions of plasma density and magnetic field (with or without the EC resonance in the plasma), and with magnetic islands generated by Neon injection. The acquisition with a fast digitizer allowed observing spectral features with very high time and frequency resolution. Shots were performed at 7.2 T, with the fundamental EC resonance out of the plasma region, at 4.7 T, with the resonance on the high field side of the plasma column, and at 3.6 T, in this last case with the plasma between the first and the second EC harmonics both lying outside the plasma volume. Several types of spectral features characterized by their frequency and their fast time evolution were identified in the observed signal after a proper treatment. The paper reports the observations in the different experimental cases and the correlation of the features with the existence of MHD modes as witnessed by magnetic probes signals and with macroscopic plasma parameters.SP
(N)TM Onset by Central EC Power Deposition in FTU and TCV Tokamaks
No full textThe presence in plasmas of conventional and neoclassical tearing modes is a key problem for the
confinement. The investigation of their onset is still an open issue to avoid plasma degradation and possible disruptions. The destabilization of the modes by on-axis or nearly central EC power deposition is a field still not well understood and responses from devices with different shape and comparable size and operation parameters can give more information. D-shaped TCV and circular FTU plasmas show that the NTM onset/amplification are associated with direct effects of ECCD on the stability parameter Δ'0 with concomitant effects on rotation. An EC torque can affect the destabilization role of the ion polarization current. Calculations using transport codes yield small modifications of the plasma current density profile, however the effect on Δ'0 is hard to quantify given the dependence on d2q/dρ2. This is why experimental comparison is important.CRPPSPCConference ID: 46091 (CN-221
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