Edge plasma fluctuations and transport in a reversed-field pinch

Abstract

Edge plasma fluctuations are studied with inserted triple Langmuir probes and magnetic coils in the TPE-1RM20 reversed-field pinch [Yo Yagi et al.]. l\vo-point measurements show that density and potential fluctuations have relatively low mode numbers (m <3, n <40). High coherence (y=0.5) with magnetic field fluctuations and similar mode spectra suggest that density and potential fluctuations are mainly caused by electromagnetic turbulence. Broadband magnetic fluctuations are dominated by m =0, low-n modes and internally resonant m = 1 and m =2 modes. A coherent (j=20-30 kHz) m =0, low-n mode is also observed. Particle flux driven by electrostatic electric field fluctuations is 50%-100% of total flux obtained from D a line intensity measurement. Low-frequency fluctuations (j<100 kHz) give the main contribution to the total flux. Electrostatic fluctuation driven electron energy flux is only of the order of 10% of total nonradiative power loss.

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

“…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.…”

“…Hc, 52.35.Ra, 52.40.Hf, 52.70.Ds In recent years, the structure of the radial electric field at the edge of tokamaks and stellarators has been investigated to establish the stabilizing effect on the transport driven by electrostatic fluctuations [1]. Since in most reversed field pinch (RFP) experiments [2] the particle transport is driven mainly by electrostatic fluctuations [3][4][5][6], there is nowadays a growing interest on the structure of the plasma potential and of the radial electric field at the edge to study the possibility of achieving enhanced confinement regimes also in RFPs.A common feature of tokamak and stellarator experiments is a radial electric field pointing inward right inside the last closed flux surface (LCFS) and outward in the scrape off layer (SOL) generated by limiters (see, for example, [7]). In tokamaks, this radial behavior has been interpreted with different mechanisms, including ion losses due to finite Larmor radius (FLR) effects [7,8].…”

“…Hc, 52.35.Ra, 52.40.Hf, 52.70.Ds In recent years, the structure of the radial electric field at the edge of tokamaks and stellarators has been investigated to establish the stabilizing effect on the transport driven by electrostatic fluctuations [1]. Since in most reversed field pinch (RFP) experiments [2] the particle transport is driven mainly by electrostatic fluctuations [3][4][5][6], there is nowadays a growing interest on the structure of the plasma potential and of the radial electric field at the edge to study the possibility of achieving enhanced confinement regimes also in RFPs.…”

Data Analysis Techniques for Microwave Imaging Reflectometry

“…In this paper, we analyze a pulsed poloidal current drive (PPCD) plasma. In this case, the normalized cutoff radius is about 0.8-0.9 m. This region is close to the reverse field surface, and the fluctuation changes rapidly [14,15].…”

Correlation of electrostatic fluctuation and reversal of toroidal field in the reversed-field pinch plasma

“…For example, the improved high theta mode on the TPE-1RM20 device demonstrates high beta and improved energy confinement in the H $ 2 region. 3 The magnetic and=or electrostatic fluctuations on the dynamo event have been investigated in several RFP devices, [4][5][6][7][8][9][10][11] where it has been shown that the RFP transport in the plasma core region mainly depends on the magnetic fluctuation, whereas the electrostatic fluctuation plays a significant role in the edge region. The energy of the core region is carried by a fast electron to the edge region.…”