Dec 28, 2012 - T Edlington, A M Edwards, J G Ferreiraf, 5' J Manhood, R O'Connell#,. M A Singleton#, K Stammers, J Toms, D L Trotman and the COMPASS-D ...
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Characteristics of ohmic H-modes in COMPASS-D
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1994 Plasma Phys. Control. Fusion 36 A111 (http://iopscience.iop.org/0741-3335/36/7A/013) View the table of contents for this issue, or go to the journal homepage for more
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Plasma Phys. Conrrol. Fusion 36 (1994) A I 1I-AI 16. Printed in the UK
Characteristics of Ohmic H-modes in COMPASS-D P G Carolan, S J Fielding, S Gerasimovo, J Hugill, A W Morris, D C Robinson; T N Todd, M ValoviE, J Ashall, K B Axon, R A Bamford’, C A Bunting, T Edlington, A M Edwards, J G Ferreiraf, 5’J Manhood, R O’Connell#, M A Singleton#, K Stammers, J T o m s , D L Trotman and the COMPASS-Dteam.
UKAEAIEuratom Association, Culham Laboratory, Abingdon, Oxon OX14 3DB, UK. Russian Research Centre, “Kurchatov Institute”, 123182 Moscow, Russian Federation. University of Essex, Wivenhoe Park, Colchester, Essex, UK. t Dept. of Engineering Science, Univ. of Oxford, Oxon OX1 3PG, UK. Physics Department, University College, Dublin, Ireland. # Physics Department, University College, Cork, Ireland. $
Abstract. The COMPASS-Dtokamak has recently obtained H-mode plasmas in a single-null X-point configuration with no additional heating. Clear H-mode signatures have been observed induding reduction in De light and the unmist&ble appearance of ELMs and ELM-free periods. Recycling and gas puff can play crucial roles in achieving and terminating the H-modes. The emergence of distinct ELMs, from the apparently-random fluctuations, of the badtground D, light, is usually one of progression, taking some 10s of ms rather than a sudden discontinuity. The relatively slow evolution allows for detail examination of important phenomena, such as improvements to confinement and fluid rotation. The H-mode quality improves with density, unless impurity accumulation becomes important. Improvements to the energy and partide confinement are observed and are accompanied by positive increases in the radial electric field at re 4cm inside the last closed flux surface (LCFS). The ohmic powers in discharges with clearly defined H-modes have not been less, so far, than the power obtained from power threshold scalings from larger machines.
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Conditions for achieving H-modes
The confinement in COMPASS-D has been improved recently on achieving an L- to Hmode transition. The plasmas were in deuterium, with a singlenull X-point (SNX) magnetic configuration (R = 0.55m, a = 0.168m and an elongation IE = 1.57). Ohmic H-modes are achieved in the parameter range:- Ip= 150 + 180kA, B, = 1.0 1.4T, line average plasma density & I = (3 12) x 1019m-3. The SNX configuration, as shown in figure 1 is adjusted to have the LCFS approximately 3.5cm inside the outboard limiter. The grad-B ion drift is directed towards the X-point. The vessel is horonised [l]and conditioned regularly with helium glow discharge cleaning (GDC). H-mod& are observed when the gas puff is at a low level, or entirely absent. A series of discharges is shown in figure 2 immediately following GDC in helium. The X-point is fuur formed from 80ms onwards. Throughout most of the period shown the gas puff d v e is closed. As the shot sequence progresses, the wall loading increases and discharge refuelling becomes more recyding dominated. The H-mode quality is highest for shots with high recycling influx, leading to an increase in density. With the addition of a gas puff an H- to L-mode transition can be induced as occurred at t re145ms in discharge #9533.
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Plasma behaviour
The behaviour of a set of the COMPASS-Dparameters in an L- to H-mode discharge is shown in figure 3. The occurrence of H-modes is recognised by the presence of ELMs. The spikes in D , light are accompanied by MHD bursts but not necessarily by sawteeth crashes, as shown in figure 3. As the density rises the frequency of the ELMs gradually decreases and the ELMs becoming more distinct. The ELM period can lengthen gradually during the discharge up to about 4ms where there can be a sudden change to much longer periods (s 18ms). ELM-free periods lasting for about one energy confinement time, rE% Nms, occur at medium to high density (6x + 12 x 10'9m-3). There is no clear dividing line between poor quality H-modes and L-modes and no sudden transition from L-mode to H-mode in contrast to other devices with suddenly-applied additional heating. The reason for this behaviour may be related to the gradual change in plasma conditions obtained with ohmic heating. The B N spectral line (282.lnm), emitting at about 4cm inboard of the LCFS, is monitored with a high resolution fast spectrometer [Z]. Doppler spectroscopy, with opposing sight lines, is used to measure toroidal and poloidal velocities as well as ion temperatures. As may be observed in figure 4 the toroidal velocity changes gradually by A u + , ~ ( r v%) 10 15km s-' in the ion drift direction during ELM free periods. There is a corresponding change of T,,,,y,(is lOOeV*% 115eV). These temperature changes are seen in both the toroidal and poloidal views of the main emission region. (Small changes to the poloidal velocity have been observed but have revealed no discernible correlation with the ELM period.) The change in the plasma radial electric field, inferred from the toroidal velocity changes alone, is outwards (i.e. +ve) z lOkV m-l, in contrast to most observations of H-mode transitions in larger machines, with additional heating [4] [5]. During the ELM-free period the Soft X-ray emission develops a broader profile which reverts to the original distribution when the ELMs reappear. This suggests a broadening of either, or both, the T
