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Title: Pre-sheath density drop induced by ion-neutral friction along plasma blobs and implications for blob velocities

The pre-sheath density drop along the magnetic field in field-aligned, radially propagating plasma blobs is investigated in the TORPEX toroidal experiment [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)]. Using Langmuir probes precisely aligned along the magnetic field, we measure the density n{sub se} at a poloidal limiter, where blobs are connected, and the upstream density n{sub 0} at a location half way to the other end of the blobs. The pre-sheath density drop n{sub se}/n{sub 0} is then computed and its dependence upon the neutral background gas pressure is studied. At low neutral gas pressures, the pre-sheath density drop is ≈0.4, close to the value of 0.5 expected in the collisionless case. In qualitative agreement with a simple model, this value decreases with increasing gas pressure. No significant dependence of the density drop upon the radial distance into the limiter shadow is observed. The effect of reduced blob density near the limiter on the blob radial velocity is measured and compared with predictions from a blob speed-versus-size scaling law [Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)].
Authors:
; ; ;  [1] ;  [2]
  1. Centre de Recherches en Physique des Plasmas-Ecole Polytechnique Fédérale de Lausanne, Association EURATOM-Confédération Suisse, CH-1015 Lausanne (Switzerland)
  2. MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139 (United States)
Publication Date:
OSTI Identifier:
22252130
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 1; Other Information: (c) 2014 Euratom; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; DISTANCE; LANGMUIR PROBE; LIMITERS; MAGNETIC FIELDS; PLASMA; PLASMA DENSITY; RADIAL VELOCITY; SCALING LAWS