Identification of Local Alfven Wave Resonances with Reflectometry as a Diagnostic Tool in Tokamaks

Abstract

Local Alfven wave (LAW) resonances are excited in tokamak plasmas by an externally driven electro-magnetic field, with the frequency below the ion cyclotron frequency, where using of the Alfven waves is assumed. 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 (Bt =1.1T, q0 =1.1, n0=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 Aef 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 q-value 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 (Bt=2.3T, q0 =1-1.3, n0=5-7 10^19/m^3) is demonstrated.