Possible mechanism responsible for observed impurity outward flow under radio frequency heating

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that in the presence of in-out asymmetric impurity populations the zero-flux impurity density gradient (the so-called peaking factor) is significantly reduced. A sign change in the impurity flux may occur if the asymmetry is sufficiently large. This may be a contributing reason for the observed outward convection of impurities in the presence of radio frequency heating. The effect of in-out asymmetry is most significant in regions with low temperature gradients. In the trapped electron mode dominated case also an up-down asymmetry can lead to a sign change in the peaking factor from positive to negative. The effect of ion parallel compressibility on the peaking factor is significant, and leads to positive peaking factors in regions with high temperature gradients, even in the presence of in-out asymmetry.Comment: 19 pages, 14 figure

Coherence-imaging approach to time-resolved charge-exchange recombination spectroscopy in high-temperature plasma

A coherence-based, or interferometric approach to spectral analysis of charge-exchange recombination (CXR) emission radiation from high-temperature plasma probed or heated using energetic neutral beams, offers a number of advantages over wavelength-domain instruments. The spectral-line shift and broadening are obtained from measurements of the spectralcoherence at a given fixed time delay. The coherence is monitored by first approximately isolating the spectral line of interest using an interference filter and subsequently imaging the spectral scene using a field-widened electro-optic path-delay-modulated polarization interferometer.Interferometers have the advantage of high-light throughput (no slit aperture). Moreover, because the spectral information is encoded at harmonics of the electro-optic modulation frequency, a single detector suffices to capture the spectral information, thereby opening the possibility for time-resolved two-dimensional spectralimaging. When unwanted spectral features are passed by the interference filter, the interpretation of the coherence phase and amplitude images can become ambiguous. By modulating the particle beam source, however, we show that coherence imaging using a single-delay modulatable interferometer can distinguish and characterize the Doppler-broadened CXR emission component against a significant background of continuum and intrinsic radiation, or pollution from nearby spectral features

Statistical features of edge turbulence in RFX-mod from Gas Puffing Imaging

Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.Comment: 27 pages. Published in PPC

Resonance Lifetimes from Complex Densities

The ab-initio calculation of resonance lifetimes of metastable anions challenges modern quantum-chemical methods. The exact lifetime of the lowest-energy resonance is encoded into a complex "density" that can be obtained via complex-coordinate scaling. We illustrate this with one-electron examples and show how the lifetime can be extracted from the complex density in much the same way as the ground-state energy of bound systems is extracted from its ground-state density

Impur ity behaviour and r adiation patter n in the RFX-mod r ever sed field pinch

Intr oduction RFX-mod is the largest reversed field pinch operating nowadays, with 2 m in major radius, 0.459 m in minor radius and an installed power dimensioned to drive up to 2 MA of plasma current. In the long shutdown period completed at the end of 2004, several new features have been introduced. Above all, a closer resistive shell with external saddle coils has been installed to harness the rich spectrum of MHD modes. This paper describes the behaviour of the impurities in discharges with plasma current of about 600 kA, in terms of influxes from the wall, radiated power and effective charge; first results on the impurity toroidal velocity are also presented. In the former machine configuration, impurities did not represent a major problem. Despite the strong plasma wall interactions the effective charge was kept at reasonable levels (i.e. below 2), especially at high density. In this respect first observations on the restarted machine confirm the trend. In fact, the measured effective charge towards the plasma density is comparable or slightly lower than what found in the former machine. The effect on the influxes and on the emitted radiation of the strong poloidal and toroidal asymmetries associated with the plasma horizontal shift and with the wallmode locking have been investigated. Results and discussion The TV camera pictures of the inner wall in 600kA discharges show strong plasma wall interactions (PWI) in the region where the phase locked MHD modes form a local deformation in the plasma column (se

Impurity transport studies in multiple helicity and enhanced confinement QSH regimes in RFX-mod

The most recent impurity transport studies in RFX-mod are presented. Results from enhanced confinement quasi-single helicity (QSH) and multiple helicity (MH) are compared. The transport parameters are determined by comparing a 1-dimensional collisional radiative simulation with experimental spectroscopic data. Transport parameters obtained for Ni particles injected by the laser blow-off method are described and discussed in relation to results from Ne injection and previous studies

Equilibrium reconstruction for Single Helical Axis reversed field pinch plasmas

Single Helical Axis (SHAx) configurations are emerging as the natural state for high current reversed field pinch (RFP) plasmas. These states feature the presence of transport barriers in the core plasma. Here we present a method for computing the equilibrium magnetic surfaces for these states in the force-free approximation, which has been implemented in the SHEq code. The method is based on the superposition of a zeroth order axisymmetric equilibrium and of a first order helical perturbation computed according to Newcomb's equation supplemented with edge magnetic field measurements. The mapping of the measured electron temperature profiles, soft X-ray emission and interferometric density measurements on the computed magnetic surfaces demonstrates the quality of the equilibrium reconstruction. The procedure for computing flux surface averages is illustrated, and applied to the evaluation of the thermal conductivity profile. The consistency of the evaluated equilibria with Ohm's law is also discussed.Comment: Submitted to Plasma Physics and Controlled Fusio