Non-existence of normal tokamak equilibria with negative central current

Recent tokamak experiments employing off-axis, non-inductive current drive have found that a large central current hole can be produced. The current density is measured to be approximately zero in this region, though in principle there was sufficient current drive power for the central current density to have gone significantly negative. Recent papers have used a large aspect-ratio expansion to show that normal MHD equilibria (with axisymmetric nested flux surfaces, non-singular fields, and monotonic peaked pressure profiles) can not exist with negative central current. We extend that proof here to arbitrary aspect ratio, using a variant of the virial theorem to derive a relatively simple integral constraint on the equilibrium. However, this constraint does not, by itself, exclude equilibria with non-nested flux surfaces, or equilibria with singular fields and/or hollow pressure profiles that may be spontaneously generated.Comment: 5 pages, 3 figures. Submitted to Physics of Plasmas, Feb. 14, 2003. Revised Feb. 24, 2003. Vers. 2: revised May 29 to clarify points raised by referee, add references to recent work. July 18, accepted for publicatio

Incorporating magnetic equilibrium information in Gaussian process tomography for soft X-ray spectroscopy at WEST

Paper published as part of the Proceedings of the 22nd Topical Conference on High-Temperature Plasma Diagnostics, San Diego, California, April 2018International audienceGaussian process tomography (GPT) [J. Svensson, JET Internal Report EFDA-JET-PR(11)24, 2011 and D. Li, J. Svensson, H. Thomsen, F. Medina, A. Werner, and R. Wolf, Rev. Sci. Instrum. 84, 083506 (2013)] is a recently developed tomography method applied earlier to soft X-ray (SXR) spectroscopy on WEST---Tungsten (W) Environment in Steady-state Tokamak. The short execution time of the algorithm makes GPT an important candidate for providing real-time information on impurity transport and for fast MHD control. In earlier work, GPT has shown its flexibility by providing good reconstruction results without background information about the magnetic equilibrium. On the other hand, information about the magnetic flux surface geometry can in general be useful for additional regularization of the solution. In this paper, we develop a way to take into account the equilibrium information, by constructing a covariance matrix of the prior Gaussian process depending on the flux surface geometry. The GPT method is validated using synthetic SXR emissivity profiles relevant to WEST plasmas and compares favorably with the classical algorithm based on minimization of the Fisher information

Spatial distribution of the neutron flux on the surface of a graphite-lined cavity

Statement of responsibility on title-page reads: J.T. Madell, T.J. Thompson, A.E. Profio, and I. Kaplan"April 1, 1962.""NYO-9657."Includes bibliographical references (leaves 315-316)U.S. Atomic Energy Commission contract AT(30-1)234

Synthetic SXR diagnostic using GEM detectors on WEST: development in the prospect of tungsten monitoring

International audienceWEST (Tungsten Environment in Steady-State Tokamak) will be operating by the end of 2016 as a test bed for the ITER divertor components in long pulse operation. In this context, radiative cooling of highly ionized impurities like tungsten (W) sputtered from Plasma Facing Components (PFC) into the plasma core is a critical issue since even small impurity concentrations below 10-4 degrade significantly plasma performances and can lead to radiative collapse. In the plasma core, tungsten emission is dominant in the Soft X-ray (SXR) range 0.1 keV – 15 keV with complex contributions from line transition, radiative recombination and Bremsstrahlung emission.This paper presents the recent development of a synthetic SXR diagnostic using GEM (Gas Electron Multiplier) detectors. This diagnostic will be used on WEST for W transport studies and will be equipped with two new GEM based poloidal cameras allowing 2D tomographic reconstructions with spectral resolution in energy bands. Thus once GEM response to plasma emissivity is characterized thanks to synthetic diagnostic, it offers new possibilities to disentangle the different SXR contributions in harsh fusion environments like e.g. WEST or ITER with respect to conventional semiconductor diodes working in current mode. Emitted SXR spectrum from the plasma is modelled thanks to ADAS database from given WEST scenario. The synthetic diagnostic includes Lines of Sight (LoS) etendues of the two cameras as well as probability of photoabsorption through filters, photoionization in the detection gas mixture (Ar-CO2), and transport of electron avalanches in the gas through GEM foils as computed with Magboltz. Local SXR emissivity is then retrieved from tomographic inversion using a Minimum Fisher Information (MFI) algorithm

The acceleration and storage of radioactive ions for a neutrino factory

The term beta-beam has been coined for the production of a pure beam of electron neutrinos or their antiparticles through the decay of radioactive ions circulating in a storage ring. This concept requires radioactive ions to be accelerated to a Lorentz gamma of 150 for 6He and 60 for 18Ne. The neutrino source itself consists of a storage ring for this energy range, with long straight sections in line with the experiment(s). Such a decay ring does not exist at CERN today, nor does a high-intensity proton source for the production of the radioactive ions. Nevertheless, the existing CERN accelerator infrastructure could be used as this would still represent an important saving for a beta-beam facility. This paper outlines the first study, while some of the more speculative ideas will need further investigations.Comment: Accepted for publication in proceedings of Nufact02, London, 200

Ion source developments for stable and radioactive ion beams at GANIL

Since now many years, the Ganil ion source team has in charge to develop ion sources with three main purposes. The first one concerns the radioactive ion production that implies high efficiency ion sources as the amount of created exotic atoms is very low (between 10 to 108 particle per second). The second one deals with high intensities of stable metallic ion beams for the injectors of the accelerator while the last one tries to increase the intensities of very high charge state ion beams for atomic physic. Concerning radioactive ion production, the recent results obtained, in collaboration with the ISN Grenoble group, with the 1+/n+ method drove us to develop a new concept of ecr ion source for monocharged ion production. The results of the first tests of this source will be given. This new idea for the construction of ecr ion source can be applied to multicharged ion production. Concerning the high charge state ion beam production, a new source called SUPERSHYPIE has been built that allow to increase by a factor 2 the length of the plasma of an ECR4M source. This new concept has just been started and has produced arround 50 nAe of Ar17+ . The first results of this new source will be presented. Concerning the developments of metallic ion beams, a separated poster will be presented at this workshop

Tomographic capabilities of the new GEM based SXR diagnostic of WEST

International audienceThe tokamak WEST (Tungsten Environment in Steady-State Tokamak) will start operating by the end of 2016 as a test bed for the ITER divertor components in long pulse operation. In this context, radiative cooling of heavy impurities like tungsten (W) in the Soft X-ray (SXR) range [0.1 keV; 20 keV] is a critical issue for the plasma core performances. Thus reliable tools are required to monitor the local impurity density and avoid W accumulation. The WEST SXR diagnostic will be equipped with two new GEM (Gas Electron Multiplier) based poloidal cameras allowing to perform 2D tomographic reconstructions in tunable energy bands. In this paper tomographic capabilities of the Minimum Fisher Information (MFI) algorithm developed for Tore Supra and upgraded for WEST are investigated, in particular through a set of emissivity phantoms and the standard WEST scenario including reconstruction errors, influence of noise as well as computational time

Distributed forcing of the flow past a blunt-based axisymmetric bluff body

In this paper we address the influence of a blowing/suction-type distributed forcing on the flow past a blunt-based axisymmetric bluff body by means of direct numerical simulations. The forcing is applied via consecutive blowing and suction slots azimuthally distributed along the trailing edge of the bluff body. We examine the impact of the forcing wavelength, amplitude and waveform on the drag experienced by the bluff body and on the occurrence of the reflectional symmetry preserving (RSP) and reflectional symmetry breaking (RSB) wake modes, for Reynolds numbers 800 and 1000. We show that forcing the flow at wavelengths inherent to the unforced flow drastically damps drag oscillations associated with the vortex shedding and vorticity bursts, up to their complete suppression. The overall parameter analysis suggests that this damping results from the surplus of streamwise vorticity provided by the forcing, that tends to stabilize the ternary vorticity lobes observed at the aft part of the bluff body. In addition, conversely to a blowing-type or suction-type forcing, the blowing/suction-type forcing involves strong nonlinear interactions between locally decelerated and accelerated regions, severely affecting both the mean drag and the frequencies representative of the vortex shedding and vorticity bursts

Experimental investigation of the vortical activity in the close wake of a simplified military transport aircraft

This paper focuses on the experimental characterization of the vortex structures that develop in the aft fuselage region and in the wake of a simplified geometry of a military transport aircraft. It comes within the framework of the military applications of airflow influence on airdrop operations. This work relies on particle image velocimetry measurements combined with a vortex-tracking approach. Complex vortex dynamics is revealed, in terms of vortex positions, intensities, sizes, shapes and fluctuation levels, for both closed and opened cargo-door and ramp airdrop configurations