Perturbing microwave beams by plasma density fluctuations

The propagation of microwaves across a turbulent plasma density layer is investigated with full-wave simulations. To properly represent a fusion edge-plasma, drift-wave turbulence is considered based on the Hasegawa-Wakatani model. Scattering and broadening of a microwave beam whose amplitude distribution is of Gaussian shape is studied in detail as a function of certain turbulence properties. Parameters leading to the strongest deterioration of the microwave beam are identified and implications for existing experiments are given

Stochastic effects on phase-space holes and clumps in kinetic systems near marginal stability

The creation and subsequent evolution of marginally-unstable modes have been observed in a wide range of fusion devices. This behaviour has been successfully explained, for a single frequency shifting mode, in terms of phase-space structures known as a `hole' and `clump'. Here, we introduce stochasticity into a 1D kinetic model, affecting the formation and evolution of resonant modes in the system. We find that noise in the fast particle distribution or electric field leads to a shift in the asymptotic behaviour of a chirping resonant mode; this noise heuristically maps onto microturbulence via canonical toroidal momentum scattering, affecting hole and clump formation. The profile of a single bursting event in mode amplitude is shown to be stochastic, with small changes in initial conditions affecting the lifetime of a hole and clump. As an extension to the work of Lang and Fu, we find that an intermediate regime exists where noise serves to decrease the effective collisionality, where microturbulence works against pitch-angle scattering

LOCUST-GPU predictions of fast-ion transport and power loads due to ELM-control coils in ITER

The LOCUST-GPU code has been applied to study the fast-ion transport and loss caused by resonant magnetic perturbations in the high-performance Q= 10 ITER baseline scenario. The unique computational efficiency of the code is exploited to calculate the impact of the application of the ITER ELM-control-coil system on neutral beam heating efficiency, as well as producing detailed predictions of the resulting plasma-facing component power loads, for a variety of operational parameters—the toroidal mode number n0, mode spectrum and absolute toroidal phase of the imposed perturbation. The feasibility of continually rotating the perturbations is assessed and shown to be effective at reducing the time-averaged power loads.Through careful adjustment of the relative phase of the applied perturbation in the three rows of coils, peak power loads are found to correlate with reductions in NBI heating efficiency for n= 3 fields. Adjusting the phase this way can increase total NBI system efficiency by approximately 2-3% and reduce peak power loads by up to 0.43 MWm-2. From the point of view of fast-ion confinement, n= 3 ELM control fields are preferred overall to n= 4 fields.In addition, the implementation of 3D magnetic fields in LOCUST is also verified by comparison with the SPIRAL code for a DIII-D discharge with ITER-similar shaping and n= 3 perturbation

FDTD simulation of a microwave beam propagating across a plasma with density fluctuations

The video shows the absolute value of the wave electric field of an electromagnetic wave propagating across a plasma with density fluctuations. It has been obtained with full-wave simulations using a cold plasma model (for details about the code, see Ref. [1]).<br><br>The geometry was such that a constant background density is taken with half of the cut-off density of the injected microwave. Onto that homogeneous background, density fluctuations are added which are indicated by the white contour lines in the video where each contour line corresponds to an additional increase of 10 % of the cut-off density. Note that only positive density perturbations (with respect the background density) are shown. The density fluctuations are obtained from a Hasegawa-Wakatani drift-wave turbulence model, see Ref. [2]. <br><br>A constant background magnetic field is used, oriented perpendicular to the simulation domain with a strength corresponding to half of the electron cyclotron resonance frequency. An O-mode is injected with a beam radius of 2 times the vacuum wavelength. The number in the lower left corner indicates the wave oscillation periods.<br><br>A quantitative analysis of the beam scattering can be found in Ref. [3].<br><br>[1]: doi <a href="http://dx.doi.org/10.1088/0741-3335/50/8/085018">10.1088/0741-3335/50/8/085018</a><br>[2]: doi <a href="http://dx.doi.org/10.5281/zenodo.47206" target="_blank">10.5281/zenodo.47206</a><br>[3]: <a href="http://arxiv.org/abs/1604.00344">arXiv:1604.00344</a><br><br><br

The deteriorating effect of plasma density fluctuations on microwave beam quality

Turbulent plasma edge density fluctuations can broaden a traversing microwave beam degrading its quality. This can be a problem for scenarios relying on a high spatial localization of the deposition of injected microwave power, like controlling MHD instabilities. Here we present numerical estimations of the scattering of a microwave beam by density fluctuations over a large parameter range, including extrapolations to ITER. Two codes are used, the full-wave code IPF-FDMC and the wave kinetic equation solver WKBeam. A successful comparison between beam broadening obtained from DIII-D experiments and corresponding full-wave simulations is shown