Identification of local Alfvén wave resonances with reflectometry as a diagnostic tool in tokamaks

Abstract

Local Alfvén wave (LAW) resonances are excited in tokamak plasmas by an externally driven electromagnetic field, with low frequency ω A =c A k || <<ω ci , where c A is the Alfvén velocity. Recently, wave driven density fluctuations at the LAW resonance m=± 1, N=±2 with few kW power deposition and 4 MHz frequency were detected in TCABR (B t =1.1T, q 0 =1.1, n 0 =1.4-2.0⋅10 19 m -3 ) using a fixed frequency (32.4GHz) O-mode reflectometer. Here, we show that combination of small power deposition in LAW resonances, swept by plasma density variation or scanned with generator frequencies, in combination with detection of the density fluctuations in the LAW resonances by reflectometry can serve as diagnostic tool for identification of the effective ion mass number A ef and q-profile in tokamaks. The idea is based on the simultaneous detection of the position of m=± 1 local AW resonances, which are excited by M/N=±1/±2 antenna modes, and m=0 generated by poloidal mode coupling effect in tokamaks. According to LAW dispersion, the m=0 resonance depends only on the effective ion mass number and does not depend on the q-profile. Then, using these data we can define qvalue at the position of m=± 1 L AW resonances. Using TAE coils with even toroidal modes (N=2, 4,..) and AW generator in the frequency band 0.6-1 MHz, an application of this method to Joint European Torus (B t =2.3T, q 0 =1-1.3, n 0 =5-7⋅10 19 m -3 ) is demonstrated.

Microwave reflectometry for fusion plasma diagnosis

“…They report on Alfvén cascades which can give information on the minimum q. In a similar vein Elfimov and colleagues [15] investigate local Alfvén wave resonances in the TCABR tokamak using fixed frequency reflectometers. When combined with modelling predictions they demonstrate the potential of both passive and actively driven resonances to be used as a diagnostic tool (MHD spectroscopy) for determining the effective mass and q-profile.…”

Comparative electron temperature measurements of Thomson scattering and electron cyclotron emission diagnostics in TCABR plasmas

“…11 AW heating is based on mode conversion of an externally driven rf field into kinetic or electrostatic Alfvén waves at the localized Alfvén resonance layer. Evidence of AW resonant heating has been found at TCABR, thus confirming core heating.…”

Identificação e localização de ondas de Alfvén excitadas no plasma de um tokamak

“…Como ambos os valores são dependentes das impurezas no plasmaé interessante compará-los. A carga efetivaé definida como [40,41]. Tal métodoé chamado de espectroscopia MHD ou diagnóstico MHD [42].…”