Plasma Diasnostic in Tokamaks Using Alfvén Waves

Abstract

magneto-hidrodinâmico, e CASTOR, que calcula os campos perturbados nesse equilíbrio, associados a instabilidades ou modos excitados, dentro do modelo magneto-hidrodinâmico resistivo.Esse estudo foi feito com o objetivo de determinar, de forma consistente, a qualidade do espectro e as auto-funções associadas a TAEs, com diferentes números toroidais n, excitados pelo atual sistema de antenas do JET. Em particular, foi investigado em detalhe o efeito das fases das correntes de alimentação dos diferentes módulos (oito) do sistema de antenas na qualidade do espectro excitado, utilizando um método original, implementado neste trabalho, de utilizar o código v vi CASTOR.Os resultados indicam que embora a excitação de um determinado modo possa ser privilegiado por uma escolha ótima das fases, modos satélites também podem ser excitados com maior amplitude, de modo que a pureza do espectro não é substancialmente melhorada. Este é o principal resultado obtido neste trabalho. AbstractIn this work we investigated the excitation of Alfvén eigenmodes in tokamaks using external antennas to the plasma column.The basic theory of Alfvén waves is revised, including non-ideal eects such as resistivity. Then the theoretical model for excitation Alfvén waves in a cylindrical plasma column, developed by Kurt Appert, is shown in detail, as an introduction to the more complex problem of Alfvén waves in toroidal plasmas.The cylindrical model is implemented in a numerical code, which is used to study the excitation of Global Alfvén Waves (GAWs), below to the so-called Continuum of Alfvén, in TCABR and JET tokamaks, using a realistic description of their antenna systems.In the sequel, it is given a brief description of Toroidal Alfvén eigenmodes (TAEs) that are excited in the gaps of the Continuum of Alfvén created by the periodicity condition of the toroidal conguration. The excitement of these modes in JET tokamak is studied using the codes HELENA, for reconstruction of magneto-hydrodynamic equilibrium, and CASTOR, which calculates the perturbed elds in this equilibrium, coupled with instability or modes excited within the magneto-resistive hydrodynamic model.This study was carried out in order to determine, consistently, the spectrum quality and the eigenmodes associated with TAEs, with dierent toroidal numbers n, excited by the new JET antenna system.In particular, it was investigated in detail the eect of the phases of the supply currents of the dierent modules (eight) of the antenna system in the quality of the excited spectrum, using an original method, implemented in this work, based on the CASTOR code.The results indicate that, although the excitation of a certain mode may be a privivii viii leged by an optimized choice of phases, satellite modes can also be excited with higher amplitude, so that the purity of the spectrum is not substantially improved. This is the main result obtained in this work.

The upgraded JET toroidal Alfvén eigenmode diagnostic system

“…With this data, the same quantity for any other module can be reconstructed by using the corresponding phase coefficient and the amplitude and phase of the current feeding the module. Other details of the procedure are presented in [19].…”

“…This is due to the close spacing of the antenna modules in each unit, which does not allow better mode discrimination. In fact, to achieve improved mode discrimination, at least one more antenna module would be required [19]; however, this is not feasible in the current JET installation. Even so, doubling the amplitude of desired modes with eigenfunctions peaked inside the plasma is already a substantial improvement with regard to tracking the mode evolution during the discharge.…”