Millimeter-wave beam scattering by edge-plasma density fluctuations in TCV

Abstract

In magnetically confined fusion devices, the use of millimeter waves (mmw) at the electron cyclotron (EC) frequencies ranges from plasma diagnostics to plasma heating, current drive and core confinement preservation. For large tokamaks such as ITER, numerical simulations and analytical estimates suggest that plasma edge turbulence could significantly broaden the EC-beam, possibly preventing tearing modes stabilization at the designed power levels. We report measurements of mmw-beam scattering by plasma turbulence in the TCV tokamak. A mmw-Gaussian beam is injected from the top of the device and the transmitted power is measured at the bottom. We show that the measured plasma density fluctuations in the upper part of the scrape-off layer (SOL) are the cause of fluctuations of the transmitted mmw-power. A full-wave model based on COMSOL multiphysics is presented and compared against the wavekinetic-equation solver WKBeam in a TCV case. Using the SOL turbulence simulations from the GBS code, comparison between the scattering effect on the mmw-beam with both the full-wave simulations and the experiments are ongoing. We also present experimental observations of rapid changes in the transmitted power caused by ELMs in ELMy H-mode plasma.

X-mode beam broadening in turbulent plasma

“…Experimental conformation of the broadened ECRH deposition was observed for DIII-D tokamak [13]. The microwave power scattering off blobs and the edge turbulence was also experimentally examined on TORPEX [14,15] and TCV tokamak [16,17] correspondingly. To take into account the role of the plasma turbulence in the beam propagation several methods were developed.…”

“…dq y 2π n 0,qy 2 e iqy(y (x,y,X,ky i )−y (x,y,X,ky j )) (17) where n qx,qy 2 = d∆xd∆y 4π 2 CCF (∆x, ∆y) e −i∆xqx−i∆yqy is Fourier spectrum of the turbulence. Using the explicit form for y (x, y, x , k y ), the exponential factor in the equation ( 14)…”

Modelling effects of edge density fluctuations on electron-cyclotron current drive used for neoclassical tearing mode stabilization

“…2019), finite elements (Chellai et al. 2018) and wave kinetic solver (Weber, Maj & Poli 2015; Snicker et al. 2017).…”

“…Numerous articles have been devoted to the effect of electron density perturbations on the propagation of tokamak-relevant EC beams, and, for the most part, verified the aspects described above. We refer to a cross-section of these studies (see references therein for the complete amount of work), which involve the majority of asymptotic and full-wave models available in the literature: ray tracing (Hizanidis et al 2010;Peysson et al 2011), quasi-optical beam tracing (Balakin, Bertelli & Westerhof 2011;Sysoeva et al 2015), Helmholtz equation solver (Ram & Hizanidis 2016), finite differences in time domain (Williams et al 2014;Köhn et al 2016) and frequency domain (Papadopoulos et al 2019), finite elements (Chellai et al 2018) and wave kinetic solver Snicker et al 2017). A series of wave effects that may affect the intended beam quality have been identified, like for example partial reflection of the beam, radiation emission by the density perturbations and conversion of the wave polarization.…”

On the stabilisation of locked tearing modes in ITER and other large tokamaks

“…It has been designed to have a fixed toroidal launching angle β of about 20 • on the basis of ray tracing calculations for RM stabilisation via continuous EC injection [6]. More recent experimental evidence indicates that the deposition profile of the ECCD is significantly broadened relative to the predictions of ray tracing codes [20][21][22][23][24]. Theoretical calculations for ITER predict a broadening by a factor of 2.5 to 3.5 at the q = 2 surface due to scattering of the EC beam by edge density fluctuations [25].…”