Alfvén and Helicon Wave Resonances Measured with Demodulator Circuit in TCABR

Abstract

The measurements of the RF driven magnetic field oscillations with the analog demodulator circuitry in the TCABR tokamak are presented. This circuitry works as the synchronous detector that is used to multiply the signal from magnetic probes by the reference signal, and then to filter out the low frequency signal. The output signal carries the information about the amplitude and the phase of the RF driven plasma oscillations, and can be acquired by the TCABR data acquisition system during all RF pulse.The experiments were carried out in the regimes with the low-density plasma n = 1 – 9 × 1016/m3 and magnetic field 0.04T when helicon wave resonances can be excited. The antennae were fed by the RF generators whose frequency was varied in the range 2–5MHz with variable time Δt = 1 – 5ms. The signals acquired with the analog demodulator circuitry were compared with the results obtained after numerical simulations of the data acquired directly with the high frequency oscilloscope. The very good agreement was demonstrated that makes possible to use the demodulator circuit measurements with long RF pulse duration in future experiments.