2D Full-wave Code Research Articles

Influences of plasma density perturbations on ion cyclotron resonance heating

The scattering of waves in the ion cyclotron range by plasma density perturbations in the edge has been previously studied by Zhang et al with the help of antenna code RAPLICASOL (2020, Nucl. Fusion, 60, 096001). The further interesting question is whether the density perturbations have an effect on ion cyclotron resonance heating (ICRH) in the core. In this paper, finite element method based 2D full wave code integrating the core with the edge is used to study this issue. The analytical density perturbations are applied to study the influence of density perturbations on field distribution, power deposition, wave coupling, power partition among different species in the core, and the fraction of energy dissipation in the scrape-off layer. The influence of density perturbations becomes global and significant when the poloidal size of the density blob is comparable to the perpendicular wavelength. In addition, the strength of wave scattering is directly proportional to the amplitude and the radial size of the density blob. Finally, a typical experiment on the EAST is chosen and the influence of realistic density perturbations on ICRH is evaluated.

Full-wave 2D modeling of helicons propagation and absorption in thespherical tokamak Globus-M2

Numerical modeling of propagation and absorption of fast waves(helicons) with frequency 200 MHz in 2D inhomogeneous plasma of thespherical tokamak Globus-M2 was carried out with 2D full-wave code. Toroidal effects, poloidal magnetic field and the actual shape of the fluxsurfaces were taken into account. The full wave electric field and RF powerabsorption profiles were computed by solving plasma wave equation withelectron Landau damping term. The modeling demonstrated a fairly highefficiency of helicons absorption in the bulk plasma within a wide range ofexperimental parameters. The waves propagate to the inner regions of theplasma column and are mainly absorbed there; less than 20% of RF energyreturns back to the plasma periphery. Keywords: high temperature plasma, nuclear fusion, tokamak, high frequency plasma waves, helicons, plasma wave equation, full-wave code.\

Full-wave 2D modeling of helicons propagation and absorption in the spherical tokamak Globus-M2

Numerical modeling of propagation and absorption of fast waves (helicons) with frequency 200MHz in 2D inhomogeneous plasma of the spherical tokamak Globus-M2 was carried out with 2D full-wave code. Toroidal effects, poloidal magnetic field and the actual shape of the flux surfaces were taken into account. The full wave electric field and RF power absorption profiles were computed by solving plasma wave equation with electron Landau damping term. The modeling demonstrated a fairly high efficiency of helicons absorption in the bulk plasma within a wide range of experimental parameters. The waves propagate to the inner regions of the plasma column and are mainly absorbed there; less than 20% of RF energy returns back to the plasma periphery. Keywords: high temperature plasma, nuclear fusion, tokamak, high frequency plasma waves, helicons, plasma wave equation, full-wave code.

Полноволновое двумерное моделирование распространения и поглощения геликонов в плазме сферического токамака Глобус-М2

Numerical modeling of propagation and absorption of fast waves (helicons) with frequency 200 MHz in 2D inhomogeneous plasma of the spherical tokamak Globus-M2 was carried out with 2D full-wave code. Toroidal effects, poloidal magnetic field and the actual shape of the flux surfaces were taken into account. The full wave electric field and RF power absorption profiles were computed by solving plasma wave equation with electron Landau damping term. The modeling demonstrated a fairly high efficiency of helicons absorption in the bulk plasma within a wide range of experimental parameters. The waves propagate to the inner regions of the plasma column and are mainly absorbed there; less than 20% of RF energy returns back to the plasma periphery.

Effect of wall boundary on the scrape-off layer losses of high harmonic fast wave in NSTX and NSTX-U

We perform numerical simulations of high harmonic fast waves (HHFWs) in the scrape-off-layer (SOL) of National Spherical Torus Experiment (NSTX)/NSTX-U using a recently developed 2D full wave code. We particularly show that a realistic NSTX SOL boundary can significantly affect HHFW propagation and power losses in the SOL. In NSTX SOL boundaries, HHFW is easily localized near the antenna and propagates less to the SOL, and thus, less power is lost to the SOL. We also show that the lower SOL power losses occur when the SOL volume is smaller and the distance between the last closed flux surface and the antenna is shorter. We investigate the effect of electron density in front of the antenna and the ambient magnetic field strengths on the SOL power losses as well. Showing consistency with the experiments, SOL losses are minimized when the SOL density is near the critical density where the fast wave cutoff is open, and the plasma is strongly magnetized.

Study of the effects of the perpendicular velocity gradient on a Doppler backscattering system using a 2D full wave code

Doppler backscattering is a powerful diagnostic technique that is used to perform turbulence measurements in magnetic confinement devices. The effects of the perpendicular velocity distribution on a Doppler backscattering system are studied using a 2D full wave code, in which the finite-difference time-domain method is used to solve Maxwell's equations. A constant velocity and several different velocity gradients are considered, along with a wide range of density fluctuation levels. The numerical results show that the Doppler backscattering mainly occurs around the cut-off layer, and that it has a linear response to turbulence when the fluctuation level is low. However, at higher density fluctuation levels, nonlinear effects become important, and the influence of the area outside the cut-off layer becomes significant. The velocity gradient causes the backscattering spectrum to become non-Gaussian and asymmetrical; this effect has also been noticed in experiments in the Experimental Advanced Superconducting Tokamak.

2D full-wave simulation of waves in space and tokamak plasmas

Simulation results using a 2D full-wave code (FW2D) for space and NSTX fusion plasmas are presented. The FW2D code solves the cold plasma wave equations using the finite element method. The wave code has been successfully applied to describe low frequency waves in planetary magnetospheres (i.e., dipole geometry) and the results include generation and propagation of externally driven ultra-low frequency waves via mode conversion at Mercury and mode coupling, refraction and reflection of internally driven field-aligned propagating left-handed electromagnetic ion cyclotron (EMIC) waves at Earth. In this paper, global structure of linearly polarized EMIC waves is examined and the result shows such resonant wave modes can be localized near the equatorial plane. We also adopt the FW2D code to tokamak geometry and examine radio frequency (RF) waves in the scape-off layer (SOL) of tokamaks. By adopting the rectangular and limiter boundary, we compare the results with existing AORSA simulations. The FW2D code results for the high harmonic fast wave heating case on NSTX with a rectangular vessel boundary shows excellent agreement with the AORSA code.

Full-wave modeling of the O–X mode conversion in the Pegasus toroidal experiment

The ordinary–extraordinary (O–X) mode conversion is modeled with the aid of a 2D full-wave code in the Pegasus toroidal experiment as a function of the launch angles. It is shown how the shape of the plasma density profile in front of the antenna can significantly influence the mode conversion efficiency and, thus, the generation of electron Bernstein waves (EBWs). It is therefore desirable to control the density profile in front of the antenna for successful operation of an EBW heating and current drive system. On the other hand, the conversion efficiency is shown to be resilient to vertical displacements of the plasma as large as ±10 cm.

Lower hybrid wave excitation and propagation in the spherical tokamak Globus-M

The problem of lower hybrid (LH) wave excitation and current drive (CD) in tokamaks with a small aspect ratio (spherical tokamaks) is discussed. It is proposed to solve this problem by exciting the waves slowed down in the poloidal rather than the toroidal direction. As a result, due to the strong poloidal inhomogeneity of the magnetic field in spherical tokamaks, even the waves with comparatively weak slowing down (N ≈ 3–5) excited by a waveguide antenna in the equatorial plane can penetrate into the dense plasma and be absorbed via the Landau mechanism. This approach was applied for modeling the LHCD experiments in the low aspect ratio tokamak Globus-M (R = 0.36 m, a0 = 0.24 m, B0 = 0.4 T, Ip = 0.25 MA, vertical elongation k = 1.6, operating frequency 2.45 GHz). The modeling was carried out using four independent codes: (i) the self-consistent antenna coupling code GRILL3D, (ii) the ray-tracing code incorporating a specially developed ray-tracing technique including some corrections necessary in strongly inhomogeneous plasma, (iii) the 2D full-wave code WAVETOP2D and (iv) the Fokker–Plank code combined with the ray-tracing code allowing simulation of the driven current density profile. The results of simulations were cross-checked and appeared to be in a good agreement. It was demonstrated that the proposed scenario provides a possibility for the efficient LHCD in a spherical tokamak.

Study of Doppler reflectometry capability to determine the perpendicular velocity and the k-spectrum of the density fluctuations using a 2D full-wave code

The capability of Doppler reflectometry to determine the perpendicular velocity and the perpendicular wave-number spectrum of the density fluctuations has been studied using a two-dimensional full-wave code. The numerical results show that the perpendicular velocity of the density fluctuations can be obtained with high accuracy for broad ranges of antenna tilt angles, turbulence levels and radial correlation lengths of the density fluctuations. The diagnostic potential of Doppler reflectometry to determine the perpendicular wave-number spectrum of the density fluctuations depends on the reflectometer response at different probed wave numbers and different turbulence levels. A linear dependence between the scattered wave amplitude and the turbulence level has been found for low turbulence levels and short radial correlation lengths of the density fluctuations, whereas at high turbulence levels and/or large radial correlation lengths the relationship becomes non-linear. In addition, numerical results show that the reflectometer response depends only very slightly on the probed wave number.

Quantitative density fluctuation measurements utilizing quadrature reflectometers on DIII-D

Fixed-frequency reflectometers utilizing quadrature phase detection are employed on DIII-D to study both coherent and turbulent density fluctuations. For coherent fluctuations, the reconstructed phase information successfully identifies MHD/coherent mode activity, e.g. tearing modes, compressional Alfvén eigenmodes, etc. For an m = 3/n = 2 tearing mode, a basic 1D phase screen model is applied to infer fluctuation levels, and the result is consistent with the beam emission spectroscopy (BES) measurement. For turbulent fluctuation studies, a 2D full-wave code (Valeo E.J., Kramer G.J. and Nazikian R. 2002 Plasma Phys. Control. Fusion 44 L1) is applied to interpret reflectometry data obtained in the H-mode edge pedestal region to deduce density fluctuation levels. The measurement is simulated using realistic geometry, plasma conditions and antenna patterns. Comparison of the fluctuation level deduced from the 2D code with BES measurement shows reasonable agreement.

Simulation of fast Alfvén wave interaction with beam ions over a range of cyclotron harmonics in DIII-D tokamak

To study fast Alfvén wave damping on fast ions, a Monte-Carlo code, ORBIT-RF, has been coupled with a 2D full wave code, TORIC4. The ORBIT-RF/TORIC4 combination has been applied to DIII-D experimental conditions to investigate fast wave (FW) heating of injected beam ions over a range of ion cyclotron harmonics. ORBIT-RF using a single dominant toroidal and poloidal Fourier wave number qualitatively reproduces the strong FW–beam interaction at 60 MHz (4ΩD and 5ΩD) and the much weaker interaction at 116 MHz (8ΩD) in DIII-D L-mode plasmas, consistent with experimental observations. The result at 8ΩD differs from linear theory prediction using a Maxwellian to model the fast ion distribution function, suggesting the importance of finite orbit effect, Coulomb collisions for transport across flux surfaces and details of the non-Maxwellian fast ion distribution.