Real-time measurement device of RF impedance and power for HUST RF negative ion source

Abstract

A radio-frequency (RF) negative ion source is constructed at Huazhong University of Science and Technology (HUST). It is aimed to produce H− beam up to 30 keV and study the beam optics. For the tuning of matching network, and analyzing the RF power coupling efficiency of the ion source driver and the ratio of harmonics in the RF system, it is necessary to measure the RF impedance and power at the input port of the matching network. Therefore, a real-time measurement device of RF impedance and power is developed. Aimed at sampling the RF voltage and current, a kind of non-contacting sampling circuit is adopted. Parametric analyses are performed to evaluate the influence of coaxial cable on the sampled signal transmission, and then a preprocessing circuit is proposed. Based on FPGA, a high-speed signal processing unit is developed with the high-speed ADC acquisition, FFT analysis and discrete spectrum correction function. The experiments are carried out at 1, 3 and 5 MHz respectively to verify the performance of the measurement device. In general, the measurement device is of high precision and speed for RF impedance and power measurement.