Measurement of the Current-Density Profile and Plasma Dynamics in the Reversed-Field Pinch

Brower, D. L.; Ding, W. X.; Terry, S. D.; Anderson, J. K.; Biewer, T. M.; Chapman, B. E.; Craig, D.; Forest, C. B.; Prager, S. C.; Sarff, J. S.

doi:10.1103/physrevlett.88.185005

Cited by 56 publications

(44 citation statements)

References 9 publications

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“…3͑a͒. Data is presented from the alpha model, 28 a cylindrical equilibrium model based on a Taylor state with the plasma ␤ϵ nkT/B 2 /2 0 treated as a free parameter, the slope model ͑which finds the on axis current from the derivative of the Faraday rotation data near the axis͒, 29 and the functional fit to the polarimetry data. Note that the alpha model does not include polarimetry data but only uses the parameters F and .…”

Section: A Sawtooth Crashmentioning

confidence: 99%

Measurement of current profile dynamics in the Madison Symmetric Torus

et al. 2004

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The current profile and core magnetic field fluctuation amplitudes in a reversed-field pinch are measured by using a high-resolution polarimetry–interferometry system. This paper presents data showing a redistribution of the current during a sawtooth crash. Also, the core magnetic field fluctuation amplitude is observed to increase at a sawtooth crash consistent with the idea of nonlinearly driven dynamo current. In addition, the parallel current density increases in the outer region of the plasma during auxiliary pulsed parallel current drive. This was expected, as the external application of an edge parallel electric field is designed to flatten the current profile providing an equilibrium closer to the minimum energy Taylor state. However, the current density also increases in the core, relative to standard plasmas. This increase can be explained by a reduction of the dynamo (anti)current drive in the core that should accompany the measured reduction of magnetic fluctuations and by a drop in resistivity caused by the increased confinement of fast electrons.

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Section: A Sawtooth Crashmentioning

confidence: 99%

Measurement of current profile dynamics in the Madison Symmetric Torus

et al. 2004

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“…The applied electric field is intended to flatten the J ʈ / B profile by driving parallel current in the edge, thereby providing stabilization to tearing modes. 6 Both the J ʈ and B profiles are modified 7,8 ͑including a substantial central peaking of the parallel current density͒, J ʈ / B undergoes a relatively subtle change but does stabilize the plasma to tearing modes, and the mode amplitudes are observed to decrease dramatically. This narrows the islands to the point where they no longer overlap, and there is an increase in confinement consistent with the restoration of nested flux surfaces in the plasma core.…”

Section: Introductionmentioning

confidence: 99%

Dynamo-free plasma in the reversed-field pinch: Advances in understanding the reversed-field pinch improved confinement mode

et al. 2005

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Generation and sustainment of the reversed field pinch ͑RFP͒ magnetic configuration normally relies on dynamo activity. The externally applied electric field tends to drive the equilibrium away from the relaxed, minimum energy state which is roughly described by a flat normalized parallel current density profile and is at marginal stability to tearing modes. Correlated fluctuations of magnetic field and velocity create a dynamo electric field which broadens the parallel current density profile, supplying the necessary edge current drive. These pervasive magnetic fluctuations are also responsible for destruction of flux surfaces, relegating the standard RFP to a stochastic-magnetic transport-limited device. Application of a tailored electric field profile ͑which matches the relaxed current density profile͒ allows sustainment of the RFP configuration without dynamo-driven edge current. The method used to ascertain that a dynamo-free RFP plasma has been created is reported here in detail. Several confinement improvements during the accompanying periods of low magnetic fluctuations are observed. Namely, the magnetic fluctuation level is reduced to the point where stochastic-magnetic transport is no longer the dominant process in the core and nested flux surfaces are restored in the core of the dynamo-free RFP.

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“…For the probe chords near plasma midplane ðjdj 0:2 mÞ, the FR effect, resulting from the horizontal magnetic field component B R , is approximately proportional to d, the vertical distance between the chord and plasma midplane. 2 In contrast, the CM effect is almost constant over the vertical scan because of the invariant B T contribution and weak density variation. Due to the height invariance of the CM effect, in the absence of the FR effect, approximately no phase variation is expected in the vertical scan.…”

Section: Discussionmentioning

confidence: 99%

Experimental validation of Mueller-Stokes theory and investigation of the influence of the Cotton-Mouton effect on polarimetry in a magnetized fusion plasma

et al. 2013

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Mueller-Stokes theory can be used to calculate the polarization evolution of an electromagnetic (EM) wave as it propagates through a magnetized plasma. Historically, the theory has been used to interpret polarimeter signals from systems operating on fusion plasmas. These interpretations have mostly employed approximations of Mueller-Stokes theory in regimes where either the Faraday rotation (FR) or the Cotton-Mouton (CM) effect is dominant. The current paper presents the first systematic comparison of polarimeter measurements with the predictions of full Mueller-Stokes theory where conditions transition smoothly from a FR-dominant (i.e., weak CM effect) plasma to one where the CM effect plays a significant role. A synthetic diagnostic code, based on Mueller-Stokes theory accurately reproduces the trends evident in the experimentally measured polarimeter phase over this entire operating range, thereby validating Mueller-Stokes theory. The synthetic diagnostic code is then used to investigate the influence of the CM effect on polarimetry measurements. As expected, the measurements are well approximated by the FR effect when the CM effect is predicted to be weak. However, the code shows that as the CM effect increases, it can compete with the FR effect in rotating the polarization of the EM-wave. This results in a reduced polarimeter response to the FR effect, just as observed in the experiment. The code also shows if sufficiently large, the CM effect can even reverse the handedness of a wave launched with circular polarization. This helps to understand the surprising experimental observations that the sensitivity to the FR effect can be nearly eliminated at high enough B T ð2:0 T). The results also suggest that the CM effect on the plasma midplane can be exploited to potentially measure magnetic shear in tokamak plasmas. These results establish increased confidence in the use of such a synthetic diagnostic code to guide future polarimetry design and interpret the resultant experimental data.

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Measurement of the Current-Density Profile and Plasma Dynamics in the Reversed-Field Pinch

Cited by 56 publications

References 9 publications

Measurement of current profile dynamics in the Madison Symmetric Torus

Measurement of current profile dynamics in the Madison Symmetric Torus

Dynamo-free plasma in the reversed-field pinch: Advances in understanding the reversed-field pinch improved confinement mode

Experimental validation of Mueller-Stokes theory and investigation of the influence of the Cotton-Mouton effect on polarimetry in a magnetized fusion plasma

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