Research On Probe Optical Coding Source Method For Z-pinch Plasma Density Diagnosis

Laser probe is a commonly used method for high-temperature and dense plasma electron density diagnostic, especially in the Z-pinch plasma diagnosis. This paper focuses on the methods of density measurement and inversion based on laser wavefront distortion in the Z-pinch experiment. Coded-source-imaging method is proposed and demonstrated.Based on the refractive index formula of laser propagating in plasma, the magnetic field effect is analyzed according to the characteristics of Z-pinch experiments. The main model and mathematical relationships used in numerical calculation and data inversion are summarized. The geometrical optics approximation can describe the laser propagation in plasma and the canonical equation is used in ray tracing; For the axially symmetric density distribution, Abel transformation is used to obtain the distribution of the plasma density according to the deflection angle or wavefront phase distortion when laser passing through the plasma, and the estimation of deviation of the paraxial approximation is analyzed.For the shearing interferometer system, numerical simulations of the interference pattern for Gaussian profile of density distribution are carried out to illustrate the influence of shearing amount and wedge angle. To calculate radial density distribution from interference pattern, we use fringe centerline method or Fourier transform method to extract interferometric phase, and then use polynomial fitting to reconstruct wavefront, and phase distortion inversion to the density distribution is performed. The error sources and distribution are also discussed.Based on the existing probe diagnostics in theory and experiment, this paper focuses on the discussion of a new kind of coded-source-imaging-based laser probing diagnostics. This method put a linear fringe-pattern mask in front of the target to change the wavefront distribution of incident laser, and use deflection method to measure the plasma density distribution. The simulation imaging shows that we need to select the appropriate object plane to improve the imaging quality. The extraction of deflection angle lies on the object-image correspondence and the stripe matching. The areal density distribution can be obtained directly by the integral of deflection angle. Cubic spline interpolation of deflection angles and piecewise integral of Abel transform give out the density distribution, which is in good agreement with the initial predefined density profile. This method has an adjustable sensitivity and about 100 ?m spatial resolution?A coded-source-imaging system is designed in laboratory demonstrating the measurement of density distribution of the flame-gun-plasma. The laser wavelength is 532nm and we use a 4-f lens group for image transmission and ICCD for image recording. This system layout is simple, and doesn't need high performance parameters for light source and optical device, thus it has a good measurement adaptability. For the image processing, we map the sine of fringe-intensity-fitting-plane tangent angle to a gray scale image for picking up fringe skeletons, and the deflection angle distribution information is then obtained, from which the refractivity distribution is calculated. The composition of the plasma torch is analysed with the state equation and the Saha equation, leading to an estimation of the core electron density in the range of 10¹⁶?10¹⁷cm^-3.