Preliminary Scheme of the High Precision RF Impedance Measurement for the Negative Ion Source

A high-power radio-frequency (RF) negative ion source is a key component of the high-energy neutral beam injection (NBI) for the magnetic confinement nuclear fusion facility, in which the driver is the place to produce the plasma using the RF induction heating. To measure the real-time RF impedance of the driver helps us to establish the relationship between the driver equivalent impedance and the plasma parameters. Because the impedance angle of the driver is close to 90° and its variation range is small during the operation of the ion source, it requires the RF impedance measuring device based on V-I detection should be of high phase accuracy. At present, the driver impedance measurement is mainly based on the indirect measurement, but this method needs to establish a complex matching network circuit model, and the calculation is complicated and lacks reliability. Thus a preliminary scheme of RF impedance measurement with high phase accuracy is proposed. For the hardware circuit, a conditioning circuit with low phase noise and high signal-to-noise ratio is designed for the RF voltage and current transformers, and a high-precision sampling circuit with equal intervals of time is designed. For the software design, use all-phase FFT analysis with the phase correction to extract the signal amplitude and phase information. In theory, this scheme can realize the measurement accuracy of 1% amplitude and 0.1° phase at 1 MHz.