Self-Excited Inductively Coupled Plasma Source Design and Spectral Analysis
LU Si-xian1, LONG Kai-hong2, ZHAN Chen-rui1, LI Ming1*
1. School of Electrical and Control Engineering, North China University of Technology, Beijing 100144，China
2. School of Electrical and Information Engineering, Yiyang Vocational and Technical College, Yiyang 413055, China
Abstract：The inductively coupled plasma (ICP) spectrometer is a widely used elemental analyzer, mainly for qualitative and quantitative elements analysis. The ICP source is the core component of the spectrometer, playing a crucial role. By exciting the target elemental atoms in the sample, the ICP source generates corresponding characteristic spectra, which are then analyzed and measured by the spectrometer, achieving rapid and accurate detection of target elements in the sample. Currently, there are two mainstream types of ICP sources: self-excited and external-excited, each with its advantages and disadvantages. The external-excited ICP source generally consists of Radio Frequency (RF) amplification and impedance matching. The two are designed separately, with a complex circuit and a relatively large volume. The impedance matching uses a mechanical capacitance matching method, with response times in milliseconds or even seconds, resulting in slow matching speeds. At the same time, the design mostly employs Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET) with low power amplification, and the maximum output power is limited. This paper presents a self-excited ICP source based on a gold-metalized silicon n-channel RF power transistor and variable frequency impedance matching. The ICP source uses an integrated design of RF amplification and impedance matching, comparing the phase difference between the frequency signal f1 sampled from the load coil and the amplified frequency phase f0, and controlling the frequency change based on the phase difference to achieve impedance matching. The design uses gold plating technology to amplify power with high-power silicon n-channel RF power transistor, providing greater power density and smaller volume, with a maximum output power exceeding 2 400 W.The paper conducted electrical static tests on the developed ICP source, providing the characteristics of the Phase Detector(PD), Voltage-Controlled Oscillator(VCO), and power output, verifying the performance of each section under corresponding working conditions. Additionally, tests were carried out with standard solutions containing Ba, Na, and Li elements using the spectrometer, obtaining an ICP spectrogram of the target elements. Furthermore, the obtained ICP spectrograms were processed using the Ensemble Empirical Mode Decomposition (EEMD) method to enhance the spectral signals of the target elements effectively. The R2 coefficient of determination (R2) of the signal is increased from 0.97 to 0.99, and the relative RSD Standard Deviation (RSD) is increased from 6.47% to 1.07%, enhancing the signal's accuracy and usability. The self-excited ICP source developed in this paper has been reduced in size through the integrated design of RF amplification and impedance matching. The frequency conversion technology has improved the impedance matching speed from the millisecond level to the nanosecond level, and the MOSFET technology of gold plating technology has increased the maximum output power from 1 800 to 2 400 W, which has laid a foundation for further optimization and application of ICP source and has important scientific research and engineering practice significance.
