Analysis and modelling of the magnetic and plasma profiles during PPCD experiments in RFX

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The equilibrium of a finite-β cylindrical RFP plasma in the presence of saturated-amplitude tearing modes is investigated. The singularities arising from the perturbative analysis of the MHD force balance equation J×B=∇p are resolved through a proper regularization of the cylindrical symmetric component of the pressure and parallel current density profiles, by setting to zero their gradient at the modes rational surfaces. An equilibrium model, which satisfies the regularization rule at the various rational surfaces, is developed. The comparison with the experimental data from the Reversed Field eXperiment (RFX) gives encouraging results. The model provides an easy tool to extrapolate the magnetic data, ordinarily measured only at the plasma edge, inside the plasma. As an application, we present the computation of the particle and heat stochastic diffusion coefficient from the reconstructed magnetic field profile.

Section: Application Of the Model: Particle And Heat Transport Followmentioning

confidence: 97%

“…For transport studies, we will consider the shot 11030, that features both standard confinement as well as a temporal window of good PPCD-induced improved confinement, and has been already used for PPCD studies [28]. Figure 15 shows, instead, an example of <dr 2 > from Eq.…”

Section: Transport Coefficients For Some Sample Casesmentioning

confidence: 99%

An equilibrium model for RFP plasmas in the presence of resonant tearing modes

Zanca

Sattin

2002

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“…The next step is to add a non-axisymmetric perturbation to (7). In the gauge A r = 0, where A r is the covariant radial component of the vector potential A, the total magnetic field can be written as…”

Section: First-order Contribution To Equilibriummentioning

confidence: 99%

“…The presence of these modes is intrinsic to the configuration, since they are required to drive poloidal currents in the outer plasma region while the applied electric field is only toroidal (dynamo effect) [4,5]. In the past, chaos reduction was obtained transiently by inductive poloidal current drive [6,7], but a way of obtaining this effect in a stationary fashion is still lacking.…”

Section: Introductionmentioning

confidence: 99%

Equilibrium reconstruction for single helical axis reversed field pinch plasmas

Martines

Lorenzini

Momo

et al. 2011

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Abstract. Single Helical Axis (SHAx) configurations are emerging as the natural state for high current reversed field pinch (RFP) plasmas. These states feature the presence of transport barriers in the core plasma. Here we present a method for computing the equilibrium magnetic surfaces for these states in the force-free approximation, which has been implemented in the SHEq code. The method is based on the superposition of a zeroth order axisymmetric equilibrium and of a first order helical perturbation computed according to Newcomb's equation supplemented with edge magnetic field measurements. The mapping of the measured electron temperature profiles, soft X-ray emission and interferometric density measurements on the computed magnetic surfaces demonstrates the quality of the equilibrium reconstruction. The procedure for computing flux surface averages is illustrated, and applied to the evaluation of the thermal conductivity profile. The consistency of the evaluated equilibria with Ohm's law is also discussed.

“…Oscillating Poloidal Current Drive (OPCD), where poloidal currents, applied to the plasma from the outside, transiently quench the need for dynamo action by the plasma, and hence reduce magnetic turbulence [30,31]. The poloidal currents at the edge affect the toroidal field, hence the reversal parameter F = B φ (wall)/< B φ > is commonly adopted to monitor the driving (see Fig.…”

Section: Ppcd/opcdmentioning

confidence: 99%

Statistical features of edge turbulence in RFX-mod from gas puffing imaging

Sattin

Scarin

Agostini

et al. 2006

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Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.