Non-linear temperature oscillations in the plasma centre on Tore Supra and their interplay with MHD

Regular oscillations of the central electron temperature have been observed by means of ECE and SXR diagnostics during non-inductively driven discharges on Tore Supra. These oscillations are sustained by LHCD, do not have a helical structure and, therefore, cannot be ascribed as MHD phenomena. The most probable explanation of this oscillating regime (O-regime) is the assumption that the plasma current density (and, thus, the q-profile) and the electron temperature evolve as a non-linearly coupled predator-pray system. The integrated modelling code CRONOS has been used to demonstrate that the coupled heat transport and resistive diffusion equations admit solutions for the electron temperature and the current density which have a cyclic behaviour. Recent experimental results in which the O-regime co-exists with MHD modes will be presented. Because both phenomena are linked to details of the q-profile, some interplay between MHD and oscillations may occur. The localisation of magnetic islands allows to obtain an accurate picture of the q-profile in the plasma core. In some case, MHD-driven reconnection helps in maintaining a weakly inverted q-profile that is found to be, in the CRONOS simulations, a necessary condition to trigger the oscillations.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Magnetic Fusion Basics

At the core of the sun and stars, light nuclei combine - or fuse - to create heavier nuclei. This process releases a significant quantity of energy and is the source of the heat and light that we receive. Harnessing this type of reaction on earth for the purpose of generating energy would open the way to almost unlimited resources. This is the aim of fusion research undertaken by the leading industrial nations. The main principles for mastering fusion energy on earth will be described in this paper. After a reminder of the main fusion reactions and the conditions to obtain fusion, this document will focus on the magnetic fusion based concepts. The magnetic confinement principles (particle trajectories, confinement time, Q factor, particles and heat transport, confinement methods, stability limits …) will be exposed with a special emphasis on the Tokamak configuration. The role of the main components of such device will be described: magnetic system, plasma facing components including divertor, fuel injection, heating systems, diagnostics … Methods for creating, heating and mastering plasma inside a tokamak will be explained and specifi cities due to long plasma duration capabilities will be pointed out (plasma wall interactions…)

Discharges with high bootstrap current fraction on Tore Supra

Bootstrap current is regarded as a serious candidate for non-inductively driving a significant fraction of the total current. High bootstrap fraction discharges have already been achieved and analysed in several tokamaks, including JT-60, DIII-D and TFTR. Tore Supra (R=2. 36 m, a=0.80 m) is particularly suited for the study of non-inductive discharges and long pulse operation. It is equipped with several of non-inductive current drive/heating systems including Lower Hybrid Current Drive (LHCD), Fast Wave Electron Heating (FWEH), and in the future Electron Cyclotron Heating. Fully non-inductive discharges with enhanced confinement (LHEP mode) have already been obtained in Tore Supra with LHCD. High {Beta}p ({le}1.6) regimes current nave also been achieved in the presence of FWEH. In particular, a discharge with 70% of the total current generated by the bootstrap current was observed. In this context, non-inductive current density profile determination is essential for understanding current drive experiments and ultimately for implementing current profile control. This paper briefly describes two methods developed on Tore Supra to determine the non-inductive current density profiles. The agreement between the two methods has been tested by applying them to ohmic discharges. These methods are then applied to the high bootstrap fraction discharges heated by FWEH. The non-inductive current density profile of these discharges are carried out. and the results are finally compared to several models of bootstrap current including Hirsman`s with low collisionality, matrix formulation and both Kessel and Houlberg matrix formulation

XUV SPECTROSCOPY IN JET

The 2m extreme grazing incidence XW Schwob-Fraenkel spectrometer has been described in detail [1]. Its use on the TFR tokamak is presented in a parallel paper [2]. The instrument installed on JET differs in that it has two microchannel plates scanning independently two portions of the spectral range from 10 to 335Å. A full scan takes 164 ms, due to the low number of photons. 127 spectra may be taken during a 20s tokamak discharge. The calculated and measured spectral resolution (FWHM) with a 600g/mm Bausch and Lomb grating and 20 µm entrance slit is shown in Fig. 1 for detector positions, y, between 200 and 390 mm (corresponding wavelengths of the central pixels are 85 and 310 Å, respectively)

SOFT X-RAY SPECTROSCOPY ON THE TFR TOKAMAK

The 2m-grating radius, extreme grazing incidence (1.5°) Schwob-Fraenkel spectrograph was developed at the Racah Institute of Physics (under CEA contract) more than 10 years ago. The first results (using photographie plates) on the TFR tokamak permitted the indentification of the spectrum of highly ionised Mo ions in the 5-50 Å spectral region /1/. Subsequently, the system was modified by J.L. Schwob into a duochromator, using two channeltron electron multipliers independently movable along the Rowland circle. It was thus possible to obtain radial profiles of the emissivities of the strongest lines of the H-and He-like isoelectronic sequences of light impurities in the 18-42 Å spectral range /2/. Recently, the duochromator has been converted into a multichannel spectrometer by equipping it with a microchannelplate (MCP) detector again movable along the Rowland circle. The detector consists of a MgF2 coated, funneled MCP, associated with a phosphor screen image intensifier and coupled by a flexible fiber optic conduit to a 1024 element photodiode array (controlled and read-out by a commercially available PAR-1461 EGG Princeton Applied Research optical multichannel analyser system). The first of this type of detector was developed at Princeton for the PLT and TFTR tokamaks and was described by Schwob et al /3/. An identical system has been installed on TFR, using a 20 µm entrance slit and a 600 groove mm-1 Jobin-Yvon holographic grating. This instrument has been routinely used during the last year of TFR operation to monitor spectra of both intrinsic impurities (C, O, Cr, Fe, and Ni, with traces of Mn, Cl, and S) and purposely injected impurity elements in the 10-330 Å spectral range. The spectrometer has been used in both the spectrographic and the polychromator modes. In the former mode, spectra of highly-ionized, unstudied, heavy elements (injected either by the laser blow-off technique or as gaseous elements) have been obtained /4,5/. In the latter utilization (in which selected individual pixels are read-out as function of time) line radiance evolutions of several different Fe ions have been simultaneously obtained on a single discharge. This has allowed the impurity transport to be modelled /6/ even though the system was not absolutely calibrated, since different ionization degrees have different time evolutions