Stochastic magnetic and ambipolar electric fields in the plasma edge region of RFX

Abstract

The edge region of the reversed field pinch experiment RFX has been investigated with Langmuir and calorimetric probes. The energy flux measurements reveal a spatial structure that is consistent with the presence of a superthermal tail in the energy distribution function of the electrons, as expected according to the kinetic dynamo theory (KDT). In the framework of this model, the value of the magnetic field line diffusion coefficient in the edge region has been derived. The radial electric field obtained from the plasma potential gradient is opposite in sign to the ambipolar electric field expected in a stochastic magnetic field. The discrepancy is discussed in terms of particle recycling at the wall.

Plasma Potential Well and Velocity Shear Layer at the Edge of Reversed Field Pinch Plasmas

“…On one hand, the inward directed radial electric field found at the edge could be interpreted as a proof of a nonstochastic behavior of the magnetic field lines. On the other hand, in some experiments the presence of superthermal electrons at the edge has been interpreted as a proof of the extension of this stochastic region up to the wall [9,14,15].In this Letter, we report the edge structure of the plasma potential in the RFX reversed field pinch experiment (R 2 m, a 0.457 m) [16]. Our results suggest that the negative E r at the edge is due to FLR losses.…”

“…In tokamaks, nonambipolar ion losses near the plasma periphery are also proposed as a possible mechanism for the establishment of transport barriers at the edge with scale lengths comparable to the poloidal gyroradius [1]. In RFPs, the structure of the plasma potential at the edge, measured in different devices, reveals that the radial electric field is directed outward in the SOL [3,9] and inward right inside the plasma surface [5,[9][10][11].This behavior was already pointed out to exhibit a surprising analogy to that found in tokamaks and stellarators [11], despite the different physics expected in a RFP configuration. In fact, the large magnetic fluctuation level ͑b͞B ϳ 1%͒ and the wide spectrum of unstable MHD modes characteristic of this configuration result in a wide stochastic region where the higher electron diffusivity should give rise to an outward directed radial ambipolar electric field to restrain the electrons.…”

Total radiation losses and emissivity profiles in RFX

“…Indeed the RFX first wall is made of graphite tiles and therefore the most important impurities are Carbon and Oxigen. Their lower ionization states, which are the most radiative, are normally concentrated in the edge region, where the temperature is low (around 15 eV, [22]). The radiative edge region is highly asymmetric, as the m = 1 component amplitude is comparable to the m = 0 one for optimally centered discharges (horizontal shift of about 1 cm), as can be seen in Fig.…”

“…where T e (a) is assumed to be equal to 15 eV, according to edge measurements [22], and α T is an exponent whose value reasonably ranges in RFX between 3 (peaked profile, as in [23]) and 7 (a flatter profile). This should be considered with some caution, since remarkable uncertainties remain about the actual power dissipation profiles which critically depend on temperature profiles, which are not well diagnosed in the edge region, and on magnetic profiles that are not measured inside the plasma.…”

Application of Neural Networks for the Measurement of Electronic Temperature in Nuclear Fusion Experiments

“…The reconstruction is, however, complicated by the fact that relation (1) is not accurate for high values of V, as can be seen from Fig. 2, showing a single V-I characteristic, corresponding to a single ramp in the applied potential [2,4]. As a consequence, when considering all the sampled pairs (V,I), the value of T e turns out to be over-estimated.…”