Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge
Abstract
Linear and nonlinear magnetohydrodynamic (MHD) stability of currentdriven modes are studied in the MST reversed field pinch. Measured low frequency (f < 35 kHz) magnetic fluctuations are consistent with the global resistive tearing instabilities predicted by 3D MHD simulations. At frequencies above 35 kHz, the magnetic fluctuations were detected to be localized and externally resonant. Discrete dynamo events, ``sawtooth oscillations,`` have been observed in the experimental RFP plasmas. This phenomenon causes the plasma to become unstable to m = 1 tearing modes. The modes that may be important in different phases of these oscillations are identified. These results then assist in nonlinear studies and also help to interpret the spectral broadening of the measured data during a discrete dynamo event. Threewave nonlinear coupling of spectral Fourier modes is measured in the MST by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 poloidal and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened kspectrum. During the sawtooth formation themore »
 Authors:
 Univ. of Wisconsin, Madison, WI (United States)
 Publication Date:
 Research Org.:
 Wisconsin Univ., Madison, WI (United States). Plasma Physics Research
 Sponsoring Org.:
 USDOE; National Science Foundation (NSF)
 OSTI Identifier:
 10119065
 Report Number(s):
 DOE/ER/53198234
ON: DE94006107; BR: AT051011A/AT0530020
 DOE Contract Number:
 FG0285ER53198
 Resource Type:
 Thesis/Dissertation
 Resource Relation:
 Other Information: TH: Thesis (Ph.D.); PBD: Jan 1994
 Country of Publication:
 United States
 Language:
 English
 Subject:
 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; REVERSEFIELD PINCH; MAGNETIC FIELDS; TEARING INSTABILITY; TURBULENCE; 700370; PLASMA FLUID AND MHD PROPERTIES
Citation Formats
Assadi, Saeed. Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge. United States: N. p., 1994.
Web. doi:10.2172/10119065.
Assadi, Saeed. Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge. United States. doi:10.2172/10119065.
Assadi, Saeed. 1994.
"Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge". United States.
doi:10.2172/10119065. https://www.osti.gov/servlets/purl/10119065.
@article{osti_10119065,
title = {Measurement of magnetic turbulence structure and nonlinear mode coupling of tearing fluctuations in the Madison Symmetric Torus reversed field pinch edge},
author = {Assadi, Saeed},
abstractNote = {Linear and nonlinear magnetohydrodynamic (MHD) stability of currentdriven modes are studied in the MST reversed field pinch. Measured low frequency (f < 35 kHz) magnetic fluctuations are consistent with the global resistive tearing instabilities predicted by 3D MHD simulations. At frequencies above 35 kHz, the magnetic fluctuations were detected to be localized and externally resonant. Discrete dynamo events, ``sawtooth oscillations,`` have been observed in the experimental RFP plasmas. This phenomenon causes the plasma to become unstable to m = 1 tearing modes. The modes that may be important in different phases of these oscillations are identified. These results then assist in nonlinear studies and also help to interpret the spectral broadening of the measured data during a discrete dynamo event. Threewave nonlinear coupling of spectral Fourier modes is measured in the MST by applying bispectral analysis to magnetic fluctuations measured at the plasma edge at 64 toroidal locations and 16 poloidal locations, permitting observation of coupling over 8 poloidal and 32 toroidal modes. Comparison to bispectra predicted by resistive MHD computation indicates reasonably good agreement. However, during the crash phase of the sawtooth oscillation the nonlinear coupling is strongly enhanced, concomitant with a broadened kspectrum. During the sawtooth formation the plasma is undergoing a pure diffusive process. The dynamo only occurs during the sawtooth crash. High frequency activity prior to a sawtooth crash is caused by nonlinear frequency (smallscale) mode coupling. Growth rate and coupling coefficients of toroidal mode spectra are calculated by statistical modeling. Temporal evolution of edge toroidal mode spectra has been predicted by transfer function analysis. The driving sources of electrostatic fields are different than for the magnetic fields. The characteristics of tearing modes can be altered by external field errors and addition of impurities to the plasma.},
doi = {10.2172/10119065},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1994,
month = 1
}

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 reversedfield 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 »

Controlling fluctuations and transport in the reversed field pinch with edge current drive and plasma biasing
Two techniques are employed in the Madison Symmetric Torus (MST) to test and control different aspects of fluctuation induced transport in the Reversed Field Pinch (RFP). Auxiliary edge currents are driven along the magnetic field to modify magnetic fluctuations, and the particle and energy transport associated with them. In addition, strong edge flows are produced by plasma biasing. Their effect on electrostatic fluctuations and the associated particle losses is studied. Both techniques are accomplished using miniature insertable plasma sources that are biased negatively to inject electrons. This type of emissive electrode is shown to reliably produce intense, directional current withoutmore » 
Edge fluctuations in the MST (Madison Symmetric Torus) reversed field pinch
Edge magnetic and electrostatic fluctuations are measured in the Madison Symmetric Torus (MST) reversed field pinch. At low frequency (<25 kHz), the mode number spectra of magnetic fluctuations agree very well with theoretical prediction for nonlinearly saturated tearing fluctuations resonant in the core. At high frequency (50 kHz to 100 kHz) the magnetic spectra broaden and the modes become resonant in the reversal region. Nonlinear phenomena are under experimental investigation. The low frequency fluctuations phaselock together to produce a rotating localized disturbance. Bispectral analysis in frequency also reveals nonlinear threewave modecoupling at low frequency. Electrostatic fluctuations are substantial and domore » 
Measurements of the momentum and current transport from tearing instability in the Madison Symmetric Torus reversedfield pinch
In this paper measurements of momentum and current transport caused by current driven tearing instability are reported. The measurements are done in the Madison Symmetric Torus reversedfield pinch [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 (1991)] in a regime with repetitive bursts of tearing instability causing magnetic field reconnection. It is established that the plasma parallel momentum profile flattens during these reconnection events: The flow decreases in the core and increases at the edge. The momentum relaxation phenomenon is similar in nature to the well established relaxationmore » 
Nonlinear coupling of tearing fluctuations in the Madison Symmetric Torus
Threewave, nonlinear, tearing mode coupling has been measured in the Madison Symmetric Torus (MST) reversedfield pinch (RFP) [Fusion Technol. [bold 19], 131 (1991)] using bispectral analysis of edge magnetic fluctuations resolved in [ital k]space.'' The strength of nonlinear threewave interactions satisfying the sum rules [ital m][sub 1]+[ital m][sub 2]=[ital m][sub 3] and [ital n][sub 1]+[ital n][sub 2]=[ital n][sub 3] is measured by the bicoherency. In the RFP, [ital m]=1, [ital n][similar to]2[ital R]/[ital a] (6 for MST) internally resonant modes are linearly unstable and grow to large amplitude. Large values of bicoherency occur for two [ital m]=1 modes coupled tomore »