Abstract：To evaluate the velocity performance of the accelerated ions in the helicon plasma using the electric double layer effect in a divergent magnetic field, the laser induced fluorescence with a bidirectional polarization method was utilized to measure the ion velocity distribution function near the exit of plasma source. In the discharge experiment, Ar was used as the working medium. The laser with central wavelength of 611.662 nm was injected to the plasma axially, by which the primarily ionized ions were stimulated to a higher level and then emitted the fluorescence with wavelength of 461.086 nm. To remove the splitting of laser induced fluorescence spectrum by inverse Zeeman effect in a magnetized plasma, the injected laser was modulated to the left-hand and the right-hand circular polarization by a quarter-wave plate, and the induced fluorescence spectrum were measured respectively. Results show that the wavelength shifts of the two measurements with different magnetic field matched with the theory rather well, which proves the feasibility of the bidirectional polarization method in this work. Furthermore, a Gaussian inverse filter was used as the deconvolution arithmetic to wipe off the natural broadening and saturation broadening from the measured fluorescence signal, then the inverse Zeeman splitting effect was eliminated and the pure Doppler broadening and shift was obtained after dealing with the results by shifting and averaging the results of the two contrary polarization. Under the condition of injected radio-frequency power 600 W, parameters including the stimulated position, the magnetic field strength and the gas pressure were changed to investigate the regulation of ion velocity distribution function. Results indicate that the near-field ion velocity distribution was symmetrical and matched well with a Gaussian distribution, while the far-field ion velocity distribution was somewhat concentrated to the low-velocity part. The averaged ion velocity arises with the increase of the magnetic field strength, while, with the decrease of the gas pressure. In addition, the velocity reduces with the stimulated position more approaching to the downstream of discharge chamber. Although the ion was exhausted with a definite bulk velocity, the order of the accelerated velocity is lower than the expected values of the electric double layer, which attributes more to the bipolar electric field under the restrain of the magnetic field in plasma. Therefore, the compact helicon wave plasma without any extra ion accelerated method couldn’t gain a good thruster performance.
Key words：Laser induced fluorescence; Ion velocity distribution function; Inverse Zeeman effect; Helicon wave plasma
杨 雄，程谋森，王墨戈. 基于双向偏振态激光诱导荧光方法的离子速度分布函数测量[J]. 光谱学与光谱分析, 2017, 37(08): 2346-2351.
YANG Xiong, CHENG Mou-sen, WANG Mo-ge. Ion Velocity Distribution Function Measurement Based on the Method of Bidirectional Polarized Laser Induced Fluorescence. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2017, 37(08): 2346-2351.
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