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Title: Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch

Abstract

A detailed study of the dynamics and magnetic topological effects of resistive-tearing modes is presented for different operational regimes in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. Soft-x-ray tomography and magnetic measurements, along with numerical reconstruction of magnetic-field lines with the ORBIT code [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)], have been employed. Magnetic-mode dynamics has been investigated in standard plasmas during the transition to the quasi-single helicity state, in which a single mode dominates the mode spectrum. Single helical soft-x-ray structures are studied with tomography in these cases. These structures are associated with magnetic islands, indicating that helical flux surfaces appear in the plasma. Mode dynamics has also been examined during auxiliary inductive current drive, the goal of which is to reduce the tearing-mode amplitudes. In this case the phenomenology of the soft-x-ray structures appearing in the plasma is more complex. In fact, when a quasi-single helicity spectrum occurs, a single island bigger than in the standard case is usually found. On the other hand, when all modes decrease, two helical soft-x-ray structures are observed, with the same helicity as the two innermost resonant modes.more » This constitutes the first direct evidence of magnetic-chaos reduction during auxiliary inductive current drive, which is responsible for the achievement of the best confinement in the reversed-field pinch configuration up to now.« less

Authors:
; ;  [1]
  1. Consorzio RFX, Euratom-ENEA Association, Corso Stati Uniti 4, 35127 Padova (Italy) (and others)
Publication Date:
OSTI Identifier:
20782452
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 1; Other Information: DOI: 10.1063/1.2160519; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLITUDES; CHAOS THEORY; CHARGED-PARTICLE TRANSPORT; HELICITY; MAGNETIC FIELDS; MAGNETIC ISLANDS; MAGNETIC SURFACES; PLASMA; PLASMA CONFINEMENT; PLASMA DIAGNOSTICS; REVERSE-FIELD PINCH; REVERSED-FIELD PINCH DEVICES; SOFT X RADIATION; TEARING INSTABILITY; TOMOGRAPHY; TOPOLOGY; X-RAY SOURCES

Citation Formats

Franz, P., Marrelli, L., and Piovesan, P. Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch. United States: N. p., 2006. Web. doi:10.1063/1.2160519.
Franz, P., Marrelli, L., & Piovesan, P. Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch. United States. doi:10.1063/1.2160519.
Franz, P., Marrelli, L., and Piovesan, P. Sun . "Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch". United States. doi:10.1063/1.2160519.
@article{osti_20782452,
title = {Tomographic imaging of resistive mode dynamics in the Madison Symmetric Torus reversed-field pinch},
author = {Franz, P. and Marrelli, L. and Piovesan, P.},
abstractNote = {A detailed study of the dynamics and magnetic topological effects of resistive-tearing modes is presented for different operational regimes in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. Soft-x-ray tomography and magnetic measurements, along with numerical reconstruction of magnetic-field lines with the ORBIT code [R. B. White and M. S. Chance, Phys. Fluids 27, 2455 (1984)], have been employed. Magnetic-mode dynamics has been investigated in standard plasmas during the transition to the quasi-single helicity state, in which a single mode dominates the mode spectrum. Single helical soft-x-ray structures are studied with tomography in these cases. These structures are associated with magnetic islands, indicating that helical flux surfaces appear in the plasma. Mode dynamics has also been examined during auxiliary inductive current drive, the goal of which is to reduce the tearing-mode amplitudes. In this case the phenomenology of the soft-x-ray structures appearing in the plasma is more complex. In fact, when a quasi-single helicity spectrum occurs, a single island bigger than in the standard case is usually found. On the other hand, when all modes decrease, two helical soft-x-ray structures are observed, with the same helicity as the two innermost resonant modes. This constitutes the first direct evidence of magnetic-chaos reduction during auxiliary inductive current drive, which is responsible for the achievement of the best confinement in the reversed-field pinch configuration up to now.},
doi = {10.1063/1.2160519},
journal = {Physics of Plasmas},
number = 1,
volume = 13,
place = {United States},
year = {Sun Jan 15 00:00:00 EST 2006},
month = {Sun Jan 15 00:00:00 EST 2006}
}
  • The first localized measurements of tearing mode flows in the core of a hot plasma are presented using nonperturbing measurements of the impurity ion flow. Emission from charge exchange recombination is collected by a novel high optical throughput duo spectrometer providing localized ({+-}1 cm) measurements of C{sup +6} impurity ion velocities resolved to <500 m/s with high bandwidth (100 kHz). Poloidal tearing mode flows in the Madison Symmetric Torus reversed-field pinch are observed to be localized to the mode resonant surface with a radial extent much broader than predicted by linear magnetohydrodynamic (MHD) theory but comparable to the magnetic islandmore » width. The relative poloidal flow amplitudes among the dominant core modes do not reflect the proportions of the magnetic amplitudes. The largest correlated flows are associated with modes having smaller magnetic amplitudes resonant near the midradius. The MHD dynamo due to these flows on the magnetic axis is measured to be adequate to balance the mean Ohm's law during reduced tearing activity and is significant but does not exclude other dynamo mechanisms from contributing during a surge in reconnection activity.« less
  • The first period of physics operation of the Madison Symmetric Torus (MST) reversed field pinch ({ital Plasma} {ital Physics} {ital and} {ital Controlled} {ital Nuclear} {ital Fusion} {ital Research} 1988 (IAEA, Vienna, 1989), Vol 2, p. 757) has produced information on sawtooth oscillations, edge magnetic and electrostatic fluctuations, and equilibrium parameters at large plasma size. Sawtooth oscillations are prevalent at all values of pinch parameter and might constitute discrete dynamo events. Both electrostatic and magnetic fluctuations are of sufficient magnitude to be relevant to transport in the reversed field pinch. In the plasmas studied to date (up to a plasmamore » current of 0.5 MA) the poloidal beta value is about 10% or greater.« less
  • Most Madison Symmetric Torus (MST) [Fusion Technol. {bold 19}, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2{endash}5 ms, corresponding to magnetohydrodynamic (MHD) instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent. {copyright} {italmore » 1996 American Institute of Physics.}« less
  • Strong {bold E{times}B} flow shear occurs in the edge of three types of enhanced confinement discharge in the Madison Symmetric Torus [Dexter {ital et al.}, Fusion Technol. {bold 19}, 131 (1991)] reversed-field pinch. Measurements in standard (low confinement) discharges indicate that global magnetic fluctuations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge. This paper explores possible contributions of {bold E{times}B} flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thus, the improved confinement. In one case, shear in the {bold E{times}B} flow occurs when themore » edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two other cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particle and energy confinement improve. {copyright} {ital 1998 American Institute of Physics.}« less
  • We report on the first results of charge exchange recombination spectroscopy (CHERS) and motional Stark effect (MSE) measurements in the Madison Symmetric Torus reversed field pinch. A 30keV, r A neutral H beam is used in combination with visible and ultraviolet spectroscopy to make the measurements. For CHERs, we find that the C VI lin at 3433.69 A yields the largest charge-exchange signal/background ratio and is most clearly resolved from other nearby lines. Equilibrium ion temperature measurements have been made with an existing fast Doppler spectrometer and a higher throughout spectrometer is being designed to do velocity and temperature fluctuationsmore » measurements. MSE measurements are made by recording the Doppler shifted H spectrum emitted by the beam with a charge coupled device and imaging spectrometer. We have observed separation of the TT components of the Stark manifold at magnetic fields of about 0.5 T and are considering options for increasing measurement accuracy.« less