B. Labit

As the operation of future devices will require target heat fluxes of less than 10-20 MW/m, finding ways of dissipating the power is a critical issue. This can be achieved by operating in a detached regime where a drop of the target ion flux is attributed to the onset of parallel momentum losses. To further understand this process in the TCV open divertor, we investigate detachment in a lower single-null X-Divertor geometry in range of flux expansions of 2 to 20, approaching detachment through nitrogen seeding at fixed integrated core density (25% or 45% of the Greenwald limit) or integrated core density ramps (from 30% to about 70% of the Greenwald limit) in L–Mode plasmas. We particularly focus on the relation between the roll-over of the target ion flux, the development of a parallel pressure gradient along the divertor leg and the evolution of the upstream profiles. The plasma profiles at the outboard midplane reveal a saturation, and eventually a roll-over of the upstream press...

Recent experiments have been performed on the TCV tokamak to investigate confinement in electronheated discharges in an attempt to simulate reactor conditions with dominant electron heating. For the first time, L-mode confinement properties have been analyzed as a function of the electron heating power profile width. Discharges with on-axis peaked electron-cyclotron heating power profiles with HWHM~0.12-0.4 as well as a scenario with off-axis heating peaked at ~0.7 have been studied in the range of heating power 0.5-2 MW at plasma density 210 19 m -3 =0.15 nGw. The confinement appears to be independent of the power deposition profile width, provided a significant fraction is inside the q=1 radius. Confinement degradation scales with the heating power in agreement with ITER scaling prediction for on-axis peaked power deposition, whereas a stronger degradation is observed for off-axis heating, E~ Ptot -0.9 . Plasma behavior has also been investigated in discharges with higher plasm...

B. Labit, G. Canal, H. Reimerdes, B. Tal1, W. Vijvers, S. Coda, B.P. Duval, T. Morgan2, G. De Temmerman2, J. Zielinski2 and the TCV team École Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas (CRPP), Association EURATOM-Confédération Suisse, CH-1015 Lausanne, Switzerland. 1Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, EURATOM Association, Budapest, Hungary. 2Dutch Institute for Fundamental Energy Research (DIFFER), Association EURATOM-FOM, Nieuwegein, The Netherlands.

Heat flux profiles inferred from a reciprocating probe at the outer midplane of the TCV tokamak during inner wall limited discharges feature radial fall-off lengths that shorten near the last closed flux surface (LCFS) consistent with the so-called narrow feature. The narrow feature is significantly wider on the outboard side compared with that measured on the inner wall by infrared thermography, so it is difficult to discern from the main scrape-off layer feature. After small shifts were applied for alignment, the fraction of the power contained in the narrow feature matches between inboard and outboard measurements, and they scale together with plasma current Ip, suggesting that we are observing the same phenomenon. The outboard side fall-off length within the narrow feature is found to scale closely with the radial correlation length of the edge turbulence as expected if the narrow feature arises due to radially sheared E × B flows. This is found to hold true even for cases where the narrow feature is ...

Naked foil bolometers can reflect a significant fraction of incident energy and therefore cannot be used for absolute measurements. This paper outlines a novel coating approach to address this problem by blackening the surface of gold foil bolometers using physical vapour deposition. An experimental bolometer was built containing four standard gold foil bolometers, of which two were coated with 100+ nm of carbon. All bolometers were collimated and observed the same relatively high temperature, ohmically heated plasma. Preliminary results showed 13%-15% more incident power was measured by the coated bolometers and this is expected to be much higher in future TCV detached divertor experiments.

The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted on free-free and three-body recombination. During density ramps, the plasma at the target detaches as inferred from a drop in density at, and ion current to, the target. At the same time the Balmer 6 → 2 and 7 → 2 line emission near the target is dominated by recombination, indicating that the ionization region has also detached from the target to be replaced by a recombining region with densities more than a factor 2 higher than at the target. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in...

ABSTRACT TORPEX is a device for studying turbulence and transport, in which toroidal plasmas (n≃10^17 m-3, T_e≃10 eV) are produced by microwaves at f=2.45GHz in the electron cyclotron (EC) range. The mechanisms for the EC wave absorption and sustainment of plasmas with different profiles are investigated as a function of the B-field configuration, gas pressure and injected power. In addition to the broad ionization from bulk electrons, a localized source is provided by electrons accelerated at the upper hybrid resonant layer, where most of the microwave power is absorbed. Different plasma profiles correspond to different fluctuation characteristics, measured by Langmuir probes. Argon plasmas are usually characterized by coherent peaks with f ˜ 10kHz, identified as drift waves, and by large-scale structures that move in the ExB direction, while Hydrogen plasmas appear more turbulent and show no evidence of coherent structures. Experiments on the link between the plasma production scenarios, determining the plasma profiles, and the characteristics of the fluctuations, are discussed.

ABSTRACT Electrostatic turbulence, related structures and their effect on transport are investigated on TORPEX simple magnetized plasmas using high resolution diagnostics and several control parameters, fluid models and numerical simulations. A critical pressure gradient to drive the interchange instability is experimentally identified, consistently with linear theory. Interchange modes nonlinearly develop blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from probe measurements using pattern recognition and are described by an analytical expression that includes ion polarization currents, parallel sheath currents and ion-neutral collisions. Limiter configurations with varying angles between field lines and the metal surface are explored. To complement probe data, a fast framing camera and a movable gas puff system are installed. Density and light fluctuations show similar signatures of drift-interchange activity. Further developments of optical diagnostics, including an image intensifier and LIF, will be discussed. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized.

ABSTRACT Low frequency phiphici electrostatic fluctuations are ubiquitous in laboratory plasmas and play an important role in anomalous cross-field transport. In TORPEX plasmas, Langmuir probes covering the whole plasma cross-section are used to determine the local dispersion relation. The low frequency fluctuations are identified as drift interchange-modes. Coupling to the stable modes using an antenna is a first step in understanding the underlying excitation mechanism. Moreover, this may lead to methods that alter the instability spectrum. The antenna consists of a movable poloidal arrangement of four electrodes. Each electrode is phased independently and is driven with a sinusoidal potential. The wave field is obtained using a synchronous detection technique applied to the probes in the ion saturation regime. Results of the wave field dependency on the launched frequency, on the relative phase shift between electrodes, and on the driving amplitude are presented. On the passive side (with no excitation), an investigation of the energy transfer between distinct frequency bands on the unstable modes is conducted. The status of the implementation of non-pertubative optical diagnostics on TORPEX will also be discussed.

ABSTRACT TORPEX is a toroidal device where plasmas produced by microwaves are embedded in a helical magnetic field. It is mainly dedicated to basic plasma physics studies on instabilities and transport. In this type of devices, the plasma is primarily lost along the open field lines. Nevertheless, particle fluxes across the magnetic field are clearly measurable using different, complementary techniques. The local particle flux can be estimated over most of the cross-section from the plasma response to a modulation of the injected microwave power, and quantified on the basis of a diffusive-convective model. The fraction of the total particle flux caused by plasma instabilities, identified as drift-interchange modes, is also measured and related to the observed spectral features. The results are compared with the transport associated with macroscopic fluctuation structures, reconstructed from a 2D imaging of the spatio-temporal behavior of the density fluctuations.

presented. Experimental setup The experiments are performed in TORPEX, a toroidal device with major and minor radius of 1 and 0:2 m. The helical magnetic field results from a toroidal component < 100 mT and a vertical component » 1 mT. Plasmas are produced from Hydrogen and Argon by waves at 2:45 GHz, in the EC range. Up to 20

Benoît Labit 49th Annual Meeting DPP / APS – Orlando, FL – 2007, 12-16 Nov. – Session NM4 ... Centre de Recherches en Physique des Plasmas ... A. Diallo, A. Fasoli, I. Furno, L. Villard, S. Brunner, SH Müllera), M. Podestàb), P. Ricci, C. Theiler