Rotation dependence of a phase delay between plasma edge electron density and temperature fields due to a fast rotating, resonant magnetic perturbation field

Abstract

Measurements of the plasma edge electron density n e and temperature T e fields during application of a fast rotating, resonant magnetic perturbation ͑RMP͒ field show a characteristic modulation of both, n e and T e coherent to the rotation frequency of the RMP field. A phase delay ⌽ between the n e ͑t͒ and T e ͑t͒ waveforms is observed and it is demonstrated that this phase delay ⌽ is a function of the radius with ⌽͑r͒ depending on the relative rotation of the RMP field and the toroidal plasma rotation. This provides for the first time direct experimental evidence for a rotation dependent damping of the external RMP field in the edge layer of a resistive high-temperature plasma which breaks down at low rotation and high resonant field amplitudes. ͓doi:10.1063/1.3436614͔In magnetically confined high temperature plasmas for exploration of nuclear fusion as future energy source, resonant magnetic perturbation ͑RMP͒ fields are a promising tool for control of edge localized modes by fine tuning of the particle and energy exhaust. [1][2][3][4] For the extrapolation of this technique from present experiments toward ITER, 5 the understanding of RMP field penetration, the resulting perturbed magnetic topology and the plasma transport is of major importance.The penetration of the external RMP field is determined by plasma response currents due to a nonzero relative rotation frequency f rel between the highly conductive plasma and the external RMP field. These currents-depending on the framework applied-potentially shield 6 or amplify 7 the external field on resonant magnetic flux surfaces. The actual response current depends in turn on the resistivity of the plasma. At low resistivity, an ideal plasma response with strong shielding currents on the resonant flux surfaces is expected. As resistivity increases, one expects a breakdown of this shielding currents and the opening of magnetic islands. 8,9 In this paper we study the rotation dependence of the impact of a fast rotating RMP field on the edge electron density n e ͑r , t͒ and temperature T e ͑r , t͒ fields in a highly resistive ͑Lunquist number S = R / A Ϸ 10 4 with A as Alfvén and R as resistive time͒ edge plasma. We show in this paper three first time observations, which in combination provide for low RMP field amplitudes ͑B r Ͻ 0.2 mT͒ evidence for a shielding of the external field even in the resistive edge of low confinement ͑L-mode͒ plasmas at TEXTOR. 10 The RMP field is generated by a helical RMP coil set, the Dynamic Ergodic Divertor ͑DED͒ 11 generating RMP fields with different toroidal ͑n͒ and poloidal ͑m͒ base mode numbers of m / n =12/ 4, 6/2, 3/1, and the capability for RMP field rotation with high frequencies of DED = 1, 2, 5, and 10 kHz.The analysis presented in this letter for high frequency RMP rotation is based on findings on magnetic topology and transport properties during application of static RMP fields. [12][13][14] Here, experimental electron density and temperature field reactions were compared with the magnetic topology modeled by field lin...

Edge plasma properties with 3D magnetic perturbations in RFX-mod

“…Initial conditions are then varied on a grid ( ) θ φ r , , j k l , and for each θ k the connection length L cw can be represented as a contour plot on the grid ( ) φ r , j l . This definition of L cw is the same as that used with orbit in a previous work on TEXTOR [42], and matches the definition which is more commonly found in the literature [37][38][39][40]. The resulting contour plot is shown in figure 14 for four different poloidal angles θ = 0 , 90 , 186 k and 290 .…”

“…, which is extensively used to characterize the magnetic structure of tokamaks during the application of MPs [37][38][39][40]. In this paper a complete 3D study of L cw for a RFP discharge with a (1, −7) mode is carried out.…”

Supersonic helium beam diagnostic for fluctuation measurements of electron temperature and density at the Tokamak TEXTOR

“…Another relevant application of the SHE diagnostic was the investigation of edge plasma structures during fast rotating resonant magnetic perturbations (RMP). 31 Combined analysis of toroidally and poloidally displaced fast edge diagnostics at TEXTOR, such as the reciprocating probe, reflectometry, and the accelerated lithium beam diagnostic, will lead to a better understanding of plasma edge physics. The supersonic helium beam diagnostic provides essential data for this study.…”

Drifts, Currents, and Radial Electric Field in the Edge Plasma with Impact on Pedestal, Divertor Asymmetry and RMP Consequences

“…Negative electric field and strong screening was observed on DIII-D [100,90] AUG [101] and MAST [87]. Positive electric field was found on TEXTOR [102] and low density L-modes of AUG [103].…”