Nonlinear decay of high-power microwaves into trapped modes in inhomogeneous plasma

Abstract

We present novel experimental evidence of parametric decay instability of microwave beams in the plasma edge of Wendelstein 7-X stellarator. We propose that the instability is sustained by trapping of only one daughter wave in the non-monotonic density profile measured with high spatial resolution within a stationary magnetic island. The power levels and spectral shapes of the detected microwave signal are reproduced by numerical modelling and a theoretical power threshold is predicted around 300 kW, comparable with observations. We predict a fraction of power drained by daughter waves around 4% in the experiments, potentially increasing above 50% for minor modifications of the density bump. Such absorption levels could significantly reduce the efficiency of the microwave heating and current-drive system in tokamaks and stellarators.

On anomalous microwave scattering in the edge transport barrier in O-mode ECRH and CTS experiments

“…Though equation ( 19) is no more valid if the pump power only slightly exceeds the threshold value P 0 P th 0 , we can use it to derive the rough estimate of the PDI power-threshold. To this end we set γ s = 0 in equation (20), which immediately gives the condition determining P th 0 γ 0 P th 0 = (2s + 1)…”

“…The optimum value of the y component of the wavenumber is equal to q ym = 10.99 cm −1 , that corresponds to the local scattering angle of the ordinary Dependence of the growth rate on the pump power. The solid curve is defined by the analytical dependence (20). The scattered circles are the results of numerical solution of equation (17).…”

“…They have been interpreted [14][15][16][17][18] as a consequence of low-threshold para- * Author to whom any correspondence should be addressed. metric decay instability (PDI) of a pump microwave occurring when the latter passes through a local maximum of the density profile, as proved in recent X2-mode ECRH experiments at ASDEX Upgrade [19] and Wendelstein 7-X [20]. In this situation the excitation of trapped plasma waves is possible and leads to suppression of convective losses of decay waves and a drastic decrease in the PDI threshold.…”

“…Figure 2.Dependence of the growth rate on the pump power. The solid curve is defined by the analytical dependence(20). The scattered circles are the results of numerical solution of equation(17).…”

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

“…The PDIs leading to microwave diagnostics damage at ASDEX Upgrade [1] and significant anomalous X-mode ECRH absorption in low-temperature plasma filaments [2] were both of the above type. More recently, an explanation of strong microwave signals observed at the Wendelstein 7-X stellarator in terms of decay of an X-mode ECRH wave into a trapped UH wave and a propagating X-mode wave around the O-point of a divertor island [51,52], has also provided a potential explanation of the strong microwave signals observed at lower densities in TEXTOR [37,38]. Apart from the PDIs driven by X-mode ECRH, PDIs driven by O-mode ECRH, relevant for the full-field ITER scenarios [12], have also been investigated.…”