Parameter study of ICRH wave propagation in IShTAR

A crude first assessment of how waves behave is commonly made relying on decoupled dispersion equation roots. In the low density, low temperature region behind the last closed flux surface in a tokamak where the density decays exponentially and where the lower hybrid resonance is crossed but where the thermal velocity is small enough to justify dropping kinetic (hot plasma) effects the study of the wave behaviour requires the roots of the full cold plasma dispersion equation. The IShTAR (Ion cyclotron Sheath Test ARrangement) device will be adopted in the coming years to shed light on the dynamics of wave plasma interactions close to radio frequency (RF) launchers and in particular on the impact of the waves on the density and their role in the formation of RF sheaths close to metallic objects. As IShTAR is incapable of mimicking the actual conditions reigning close to launchers in tokamaks; a parameter range needs to be identified for the test stand to permit highlighting of the relevant wave physics. Studying the coupled dispersion equation roots allowed us to find a suitable operation domain for performing experiments

2D modeling of electromagnetic waves in cold plasmas

The consequences of sheath (rectified) electric fields, resulting from the different mobility of electrons and ions as a response to radio frequency (RF) fields, are a concern for RF antenna design as it can cause damage to antenna parts, limiters and other in-vessel components. As a first step to a more complete description, the usual cold plasma dielectric description has been adopted, and the density profile was assumed to be known as input. Ultimately, the relevant equations describing the wave-particle interaction both on the fast and slow timescale will need to be tackled but prior to doing so was felt as a necessity to get a feeling of the wave dynamics involved. Maxwell's equations are solved for a cold plasma in a 2D antenna box with strongly varying density profiles crossing also lower hybrid and ion-ion hybrid resonance layers. Numerical modelling quickly becomes demanding on computer power, since a fine grid spacing is required to capture the small wavelengths effects of strongly evanescent modes

Antenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devicesAntenna near-field wave studies in magnetized plasmas as part of the optimization of auxiliary heating in fusion devices

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A new approach to ICRF antennas modeling based on coupling the surface impedance matrix of the plasma to commercial antenna codes

Although modern commercial antenna codes can handle the complex 3D geometry of ion cyclotron resonance frequency (ICRF) antennas they still can not correctly describe hot fusion plasmas. In view of the impact the plasma has on the antenna-near fields and hence the need to use a sensible mock-up for the plasma behaviour, ICRF antenna modeling is currently mostly done by substituting the plasma with suitably chosen dielectric [1,2]. One of the limitations of this approach is the incorrect evaluation of the fields on the plasma surface. In this work a theoretical basis is given and a practical implementation is shown for coupling the spectral plasma surface impedance matrix [3] to modern commercial antenna codes for self-consistent correct calculation of the fields and scattering (‘S’) parameters of the ICRF antennas, hereby allowing to interface the antenna coupling code with a much more realistic model for capturing the subtleties of magnetized plasmas. The approach uses subsequent application of induction and uniqueness theorems of electromagnetism. In a first step the fields of the antenna in vacuum are computed. Once these incident fields are known one can use the surface impedance of the plasma to calculate the total electric and magnetic fields on the plasma surface and the power flow into the plasma. The evaluation of the S-parameters of the antenna requires a second step. We use the obtained tangential electric field on the plasma surface as a necessary boundary condition to solve the equivalent problem and find the Sparameters of the antenna and all the fields around it. This new approach is similar in physics potential to the TOPICA code [4] for its application to antenna design. Moreover, in the new approach it is possible to simulate the presence of cold low density plasma in the antenna box, which is needed for the correct evaluation of the fields and for addressing the sheath effect. The here presented, new approach is numerically more efficient and user-friendly than codes that attempt to directly incorporate the plasma response in the antenna computation. The paper also compares results obtained using the new approach with those obtained by other modeling methods. A new approach to the problem of the minimization of the toroidal electric field of the ICRH antennas is also proposed

Modelling RF sheath dynamics for fusion applications

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Modelling of sheath effects on radio-frequency antennas

The large voltages on radio frequency (RF) antennas that are used for heating of fusion plasmas, can create a thin sheath layer with largely negative potential and thus strong electric near-fields that attract and accelerate positively charged ions. The possible damage to antenna and in-vessel components due to local overheating and sputtering, is one of the main concerns for high power antennas in future fusion reactors. Good predictive simulation tools that take these sheath effects into account are still lacking. A practical implementation for modelling codes was proposed in [1], where sheath properties are introduced by means of a non-linear sheath boundary condition (SBC) on antenna surfaces. The sheath is represented by a scalar dielectric medium with relative permittivity sh = 1 + ish, i.e. a lossy vacuum layer. It is assumed that the electrons are inertia-free and therefore accelerated immediately into the metal surface, and that the power lost in the sheath is purely coming from ions accelerated in the rectified sheath potential. The sheath width (sh) is determined by the Child-Langmuir law, and the sheath potential depends on the electric field component normal to the surface. Continuity of the normal component of the displacement vector at the sheath plasma interface leads to the general description of the sheath as boundary condition Et = t ((sh/sh) n·pl·E) = t ((sh/sh) Dn), where Et is the tangential component of electric field and Dn the normal component of the displacement vector, all with respect to the sheath surface. For pl a cold plasma [2] description is used. Due to the Dn dependence of the sheath width the SBC is a non-linear equation, preventing a direct inversion of the underlying set of equations. A hybrid implementation of a SBC in the TOPICA code [3] was reported in [4], plasma properties were introduced for the calculation of the sheath parameters (sh, pl and sh), but the wave propagation was calculated using a vacuum Green's function. In the present paper a realistic finite density plasma is assumed to surround the antenna, and a cold plasma description assesses the impact of a magnetized dielectric medium on the antenna near-fields. The COMSOL Multiphysics [5] package was used for the RF modelling

PROPAGATIVE AND EVANESCENT BEHAVIOUR OF ELECTROMAGNETIC WAVES IN COLD PLASMAS

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