L. Meneses

Type-I ELM precursor modes in JET

Clear low frequency (typically 5?25?kHz) coherent type-I ELM precursor modes have been identified in JET. They are detected through various diagnostics, especially in the ECE and Mirnov signals, but also on the SXR cameras and the multichannel O-mode edge reflectometer. The modes propagate in the direction of the ion-diamagnetic drift and are usually accompanied by a slight rise in the divertor D?-emission. The precursors show no radial phase inversions and are localized to a few centimetres inside the separatrix, in the pedestal region. The time by which the precursor onset precedes the ELM event varies greatly, and ranges from 0.2?ms up to several tens of milliseconds, though the timescale has no effect on the characteristics of the precursors. During their lifetime, the precursor amplitude is often seen to fluctuate, and the frequency and dominant n-number of the precursors can change rapidly, typically within 1?ms. The range of toroidal precursor mode numbers n which have been observed is 1?13. Parametric studies show that n seems to be mainly prescribed by a combination of the electron pedestal pressure and the (normalized) electron pedestal collisionality , with increasing leading to higher n-numbers. Above a certain edge collisionality (roughly 1hbox{--}3 SRC=http://ej.iop.org/images/0029-5515/44/5/005/nf177921in002.gif/>) the precursors seem to be absent, indicating that a sufficiently high edge current is important to destabilize the precursors. The low-n precursors are known to be external kinks (also known on JET as Outer Modes), while experimental findings and their comparison with stability calculations suggest that the precursor modes with higher n are not pure external kinks but coupled ballooning?kink modes. Several hundred discharges have been analysed, and, in discharges with low to moderate edge collisionality, the precursors could be detected prior to most type-I ELMs, while prior to type-III ELMs the modes were not encountered. In spite of their regular occurrence, there is no evidence of the precursor mode growth rate rapidly accelerating before the ELM, indicating that type-I ELMs on JET are not triggered by these modes.

Optimization of ICRH for core impurity control in JET-ILW

Ion cyclotron resonance frequency (ICRF) heating has been an essential component in the development of high power H-mode scenarios in the Jet European Torus ITER-like wall (JET-ILW). The ICRF performance was improved by enhancing the antenna-plasma coupling with dedicated main chamber gas injection, including the preliminary minimization of RF-induced plasma-wall interactions, while the RF heating scenarios where optimized for core impurity screening in terms of the ion cyclotron resonance position and the minority hydrogen concentration. The impact of ICRF heating on core impurity content in a variety of 2.5 MA JET-ILW H-mode plasmas will be presented, and the steps that were taken for optimizing ICRF heating in these experiments will be reviewed.

Discriminating the trapped electron modes contribution in density fluctuation spectra

Quasi-coherent (QC) modes have been reported for more than 10 years in reflectometry fluctuations spectra in the core region of fusion plasmas. They have characteristics in-between coherent and broadband fluctuations as they oscillate at a marked frequency but have a wide spectrum. This work presents further evidences of the link recently established between QC modes and the trapped electron modes (TEM) instabilities (Arnichand et al 2014 Nucl. Fusion 54 123017). In electron cyclotron resonance heated discharges of Tore Supra, an enhancement of QC modes amplitude is observed in a region where TEM cause impurity transport and turbulence. In JET Ohmic plasmas, QC modes disappear during density ramp-up and current ramp-down. This is reminiscent of Tore Supra and TEXTOR observations during transitions from the linear Ohmic confinement (LOC) to the saturated Ohmic confinement (SOC) regimes. Evidencing TEM activity then becomes experimentally possible via analysis of fluctuation spectra.

Characterization of scrape-off layer transport in JET limiter plasmas

The JET scrape-off layer (SOL) has been characterized with a reciprocating probe in inner wall (IW), and outer wall (OW), limited plasmas. Experiments revealed that SOL profiles are substantially broader (by a factor of ~5–7.5 in the power e-folding length) for IW limited than in OW limited plasmas. Results are consistent with the larger radial turbulent transport found for IW limited plasmas. Major differences are observed between IW and OW limited plasmas on the density and electron temperature e-folding lengths, parallel flow, radial turbulent transport as well as on the temporal and spatial characteristics of the fluctuations. Experimental findings on JET suggest that the differences in the SOL characteristics for both configurations are due to a combination of a poloidal asymmetry in radial transport with a reduced cross-field transport across the last closed flux surface associated with the confinement improvement observed for OW limited plasmas.The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found. Finally, it is shown that the SOL radial transport and the amplitude of the fluctuations increase with plasma current and decrease with line-averaged density for IW limited plasmas.

Global and pedestal confinement and pedestal structure in dimensionless collisionality scans of low-triangularity H-mode plasmas in JET-ILW

A dimensionless collisionality scan in low-triangularity plasmas in the Joint European Torus with the ITER-like wall (JET-ILW) has been performed. The increase of the normalized energy confinement ...

A multichannel reflectometer for edge density profile measurements at the ICRF antenna in ASDEX Upgrade

A multichannel reflectometer will be built for the new three-straps ICRF antenna of ASDEX Upgrade (AUG), to study the density behavior in front of it. Ten different accesses to the plasma are available for the three reflectometer channels that can be interchanged without breaking the machine vacuum. Frequency is scanned from 40 GHz to 68 GHz, in 10μs, which corresponds to a cut-off density ranging from 1018÷1019m−3 in the Right cut-off of the X-mode propagation, for standard toroidal magnetic field values of AUG.

Implementation of the new multichannel X-mode edge density profile reflectometer for the ICRF antenna on ASDEX Upgrade

A new multichannel frequency modulated continuous-wave reflectometry diagnostic has been successfully installed and commissioned on ASDEX Upgrade to measure the plasma edge electron density profile evolution in front of the Ion Cyclotron Range of Frequencies (ICRF) antenna. The design of the new three-strap ICRF antenna integrates ten pairs (sending and receiving) of microwave reflectometry antennas. The multichannel reflectometer can use three of these to measure the edge electron density profiles up to 2 × 1019 m-3, at different poloidal locations, allowing the direct study of the local plasma layers in front of the ICRF antenna. ICRF power coupling, operational effects, and poloidal variations of the plasma density profile can be consistently studied for the first time. In this work the diagnostic hardware architecture is described and the obtained density profile measurements were used to track outer radial plasma position and plasma shape.

Experimental investigation of geodesic acoustic modes on JET using Doppler backscattering

EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Universidade Lisboa, PT 2 CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK 3 Laboratorio Nacional de Fusion,CIEMAT, 28040 Madrid, Spain 4 Max-Planck-Institut fur Plasmaphysik, Boltzmannstr. 2, D-85748 Garching, Germany 5 FOM Inst. DIFFER, Nieuwegein, Netherlands

Plasma turbulence measured with fast frequency swept reflectometry in JET H-mode plasmas

In this work we present recent achievements to provide precise measurements of turbulence on JET H-mode plasmas using frequency sweeping reflectometry diagnostic. The plasma density fluctuations re ...