The effect of density fluctuations on electron cyclotron beam broadening and implications for ITER

Abstract

We present state-of-the-art computations of propagation and absorption of electron cyclotron waves, retaining the effects of scattering due to electron density fluctuations. In ITER, injected microwaves are foreseen to suppress neoclassical tearing modes (NTMs) by driving current at the q = 2 and q = 3/2 resonant surfaces. Scattering of the beam can spoil the good localization of the absorption and thus impair NTM control capabilities. A novel tool, the WKBeam code, has been employed here in order to investigate this issue. The code is a Monte Carlo solver for the wave kinetic equation and retains diffraction, full axisymmetric tokamak geometry, determination of the absorption profile and an integral form of the scattering operator which describes the effects of turbulent density fluctuations within the limits of the Born scattering approximation. The approach has been benchmarked against the paraxial WKB code TORBEAM and the full-wave code IPF-FDMC. In particular, the Born approximation is found to be valid for ITER parameters. In this paper, we show that the transport in ITER is diffusive unlike in present experiments, thus causing up to a factor of 2 to 4 broadening in the absorption profile. However, the broadening depends strongly on the turbulence model assumed for the density fluctuations, which still has large uncertainties.

Microwave beam broadening due to turbulent plasma density fluctuations within the limit of the Born approximation and beyond

“…at sufficiently low densities or high frequencies. Before concluding this section, it is useful to quote a further result derived in [28], namely that the product Σ α ∆ℓ, where ∆ℓ is the distance travelled in the turbulent region, is found to scale as…”

The deteriorating effect of plasma density fluctuations on microwave beam quality

“…Details about the WKBeam code and the derivation of the underlying equations can be found in Refs. [20,23,27] Since the derivation of the scattering operator relies on the Born approximation, it is expected to become invalid for large fluctuation levels. More precisely, δn e /n e,0 2 1/2 ω 2 pe /ω 2 0 1 (4) needs to be fulfilled for the Born approximation to hold.…”

“…The WKBeam code was deliberately developed to handle this. Estimating the beam broadening due to plasma density fluctuations in ITER using WKBeam is the topic of a paper published earlier this year [20]. The main result of that paper will only be briefly discussed here.…”

“…As mentioned in the beginning of this section, a detailed discussion of the results is found in Ref. [20].…”

“…In the 1980s high-power microwave sources became available and the interaction of injected microwaves and plasma edge density fluctuations in fusion-relevant scenarios was studied with geometrical-optics tools [14,15]. Due to the potentially negative consequences for NTM stabilization in ITER, this topic has gained significant traction since 2009 [16][17][18][19][20][21]. tion and the fluid equation of motion of the electrons on a 2D Cartesian grid.…”