A variety of Alfvénic eigenmodes have been observed in the magnetic spectra of reversed-field pinch (RFP) plasmas in past years, in machines like RFX-mod [1] and MST [2]. In particular, in the RFX-mod device two branches of long-wavelength Alfvén eigenmodes have been observed. On the one hand, a high-frequency branch (in the range 500?1500 kHz) with two discrete eigenmodes is present during the whole discharge and interpreted as global Alfvén eigenmodes [1]. On the other hand, a lower frequency branch (f < 500 kHz) with three coherent peaks is detected only during helical RFP states, which spontaneously emerge at high plasma current (above 1 MA) and are associated with improved confinement [3].Here, we report the investigation on the modelling study of Alfvén waves in reversed-field pinch plasmas, and the comparison with experimental findings in the RFX-mod device. The nonlinear 3D MHD cylindrical code SpeCyl [4,5] has been used to analyze configurations with increasing level of complexity. First of all, numerical solutions have been compared with analytical ones in the simplest case of a uniform axial magnetic field: an excellent agreement is obtained for both the shear Alfvén wave (SAW) and the compressional Alfvén eigenmodes (CAEs). Then, the RFP configuration has been studied by assuming perturbations with a single space periodicity. Phenomena such as phase mixing of SAW, resonant absorption of CAEs and the appearance of the global Alfvén eigenmode (GAE) are reported. Finally, the fully 3D RFP case with realistic magnetic reconnection events [6] has been investigated, showing for the first time in nonlinear RFP simulations the excitation of Alfvén wavesby magnetic reconnection. Modelling results are in good quantitative agreement with the experimental characterization of the Alfvénic activity observed in RFX-mod [1]. In particular, the two high-frequency eigenmodes observed experimentally are reproducedin nonlinear MHD modelling, and identified as a global and a compressional Alfvén eigenmode. Overall, these findings suggest that reconnection processes can destabilize global Alfvén eigenmodes, as well as compressional Alfvén eigenmodes (normally disregarded in Tokamak plasmas, because of the associated very high frequency).
Alfvén waves in nonlinear 3D MHD modelling of RFP plasmas
Bonfiglio D;Zuin M;Cappello S;
2019
Abstract
A variety of Alfvénic eigenmodes have been observed in the magnetic spectra of reversed-field pinch (RFP) plasmas in past years, in machines like RFX-mod [1] and MST [2]. In particular, in the RFX-mod device two branches of long-wavelength Alfvén eigenmodes have been observed. On the one hand, a high-frequency branch (in the range 500?1500 kHz) with two discrete eigenmodes is present during the whole discharge and interpreted as global Alfvén eigenmodes [1]. On the other hand, a lower frequency branch (f < 500 kHz) with three coherent peaks is detected only during helical RFP states, which spontaneously emerge at high plasma current (above 1 MA) and are associated with improved confinement [3].Here, we report the investigation on the modelling study of Alfvén waves in reversed-field pinch plasmas, and the comparison with experimental findings in the RFX-mod device. The nonlinear 3D MHD cylindrical code SpeCyl [4,5] has been used to analyze configurations with increasing level of complexity. First of all, numerical solutions have been compared with analytical ones in the simplest case of a uniform axial magnetic field: an excellent agreement is obtained for both the shear Alfvén wave (SAW) and the compressional Alfvén eigenmodes (CAEs). Then, the RFP configuration has been studied by assuming perturbations with a single space periodicity. Phenomena such as phase mixing of SAW, resonant absorption of CAEs and the appearance of the global Alfvén eigenmode (GAE) are reported. Finally, the fully 3D RFP case with realistic magnetic reconnection events [6] has been investigated, showing for the first time in nonlinear RFP simulations the excitation of Alfvén wavesby magnetic reconnection. Modelling results are in good quantitative agreement with the experimental characterization of the Alfvénic activity observed in RFX-mod [1]. In particular, the two high-frequency eigenmodes observed experimentally are reproducedin nonlinear MHD modelling, and identified as a global and a compressional Alfvén eigenmode. Overall, these findings suggest that reconnection processes can destabilize global Alfvén eigenmodes, as well as compressional Alfvén eigenmodes (normally disregarded in Tokamak plasmas, because of the associated very high frequency).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
