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

为了评估利用发散磁场构型双电层效应的紧凑式螺旋波等离子体推力器的离子加速效果，探索了一种双向偏振态激光诱导荧光测量方法来对螺旋波等离子体源近出口端的离子速度分布函数进行测量。实验中采用Ar作为螺旋波等离子体源工质，中心波长为611.662 nm的激光以轴向方式注入等离子体，以激励一价Ar离子获得波长为461.086 nm的诱导荧光光谱。为了消除磁化等离子体中逆塞曼效应对激光诱导荧光光谱带来的分裂影响，通过四分之一波片将入射激光分别调制为左旋和右旋圆偏振态，并对其诱导光谱进行了分别测量，结果发现不同磁场强度下两次测量结果的偏移值与理论高度吻合，证明了双向偏振态激光诱导荧光测量方法的理论可行性。进一步，采用高斯型滤波器反卷积算法从测量光谱中去除自然展宽和能量饱和效应，再通过对两次相反偏振态测量结果进行平移处理消除逆塞曼效应，从而分离得到实际的多普勒效应。测量了射频能量600 W，不同轴向位置、磁场大小以及气体压力下的螺旋波Ar等离子体激光诱导荧光光谱，结果表明在该实验条件下离子并没有因双电层效应而达到期望值的加速效果，离子速度的形成可能只是一种磁约束作用下的双极电场所导致，并不能产生好的推力性能。

Ion Velocity Distribution Function Measurement Based on the Method of Bidirectional Polarized Laser Induced Fluorescence

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.