A Diagnostic Method For Electron Density Of High-speed Target Plasma

The Experimental Apparatus for Plasma-Electromagnetic Science of Near-Space Hypersonic Targets is a highly efficient ground-based system designed to simulate and test the high-supersonic plasma environment,with its principal goal being the exploration of electromagnetic science challenges specific to this distinctive environmental context.The electron density of high-speed target plasma is a crucial parameter for electromagnetic experiments.The accurate acquisition of plasma parameters is a prerequisite for such experiments,for which this paper carries out a diagnostic study on plasma electron density with high precision and exceptional spatial and temporal resolution.The strategy for achieving precise diagnosis of plasma electron density with high spatial and temporal resolution is outlined as follows: Initially,precise phase determination is attained by considering the influence of pressure,leading to the derivation of high-precision lineaveraged electron density.Subsequently,the electron density distribution is determined by establishing a single probe system,taking into account the influence of pressure on the saturated ion current.The diagnostic results of the langmuir probe are corrected by incorporating findings from the microwave transmission method or laser interferometry method,thereby achieving a high-precision electron density distribution.The primary content and contributions of this paper are outlined as follows:1.A phase detection method is proposed,which is based on the adaptive lattice notch filter with noise-reduction and correlation methods.The device encounters challenges associated with electromagnetic interference,resulting in a decreased signal-to-noise ratio in the received signal.Furthermore,the laser interferometer’s rotating grating displays poor stability,causing incomplete signal sampling and significant errors in phase extraction.To address these issues,this paper proposes a method for detecting phase differences using lattice trapping and correlation analysis.The algorithm is thoroughly analyzed with respect to factors influencing extraction accuracy.A comparative assessment is conducted against the FFT transform method,Hilbert transform method,and correlation analysis method.The findings demonstrate that the proposed algorithm’s accuracy remains unaffected by whether the entire period is sampled or not,with a maximum error of 6.9%.2.This study focuses on acquiring diagnostic correction coefficients for laser interferometers under varying pressures and temperatures.In scenarios of elevated experimental device pressures,the influence of neutral particles on the laser becomes significant.Addressing these concerns,this article initiates its exploration by analyzing the contributions of neutral particles and electrons to the laser refractive index.Subsequently,considering the simulated pressure and temperature conditions of the device,the impact of neutral particles on the current temperature and pressure is taken into account.Correction coefficients are then determined for different pressure and temperature conditions.To enhance the accuracy of the line-averaged electron density measured by the HCN laser interferometer,the distance traveled by the laser through the plasma is calculated through a combination of image edge detection techniques.Simultaneously,the study employs an HCN laser interferometer to derive the average electron density of the plasma,subsequently investigating the varying patterns of plasma electron density under different states.3.A Langmuir probe system was meticulously designed to ascertain the electron density distribution in high-pressure plasma.In order to delineate the radial distribution of plasma electron density within the experimental apparatus without disrupting the flow field,a Langmuir probe system with a spatial resolution of 5mm was implemented in this investigation.Owing to the extensive variations in pressure within this experimental configuration,the probe does not entirely adhere to the collisionless model.Consequently,this study systematically examines the theoretical models of probes under diverse pressure conditions,employs simulations to acquire the pressure and temperature profiles of the experimental device,subsequently computes correction coefficients,thereby facilitating the attainment of high-precision measurements of plasma electron density.Ultimately,a comparative analysis is conducted between the line-averaged electron density obtained from the static electric probe and that acquired by the HCN interferometer,revealing a deviation of 20%.4.This study proposes a diagnostic method for high-precision and high-temporalresolution electron density distribution.The Langmuir probe system is capable of diagnosing the one-dimensional electron density distribution in plasma;however,it exhibits limitations in terms of time resolution and diagnostic accuracy.Both the HCN laser interferometer and microwave transmission method focus on the average value along a specific path of the plasma jet,lacking the ability to obtain electron density at different positions,thus resulting in low spatial resolution.To achieve a high-precision and high-spatiotemporal-resolution electron density distribution,the approach involves initially measuring the line-averaged electron density through microwave transmission or laser interferometer.Subsequently,the probe system is integrated to obtain ion-saturated current,and its average is calculated by comparing it with the line-averaged electron density to derive a coefficient.Finally,this coefficient is multiplied by the ion-saturated current function to obtain the electron density distribution.This method combines the high temporal resolution of laser interferometers or microwave transmission systems(1us)with the high spatial resolution of probes(5mm).Although an exploratory approach,the error falls within an acceptable range,and the diagnostic method presented in this article holds certain engineering practical value.In summary,this paper integrates the microwave transmission method,laser interferometry,and langmuir probe to propose a comprehensive diagnostic approach with high precision and spatio-temporal resolution for electron density distribution.The aim is to obtain a precise radial electron density distribution.The proposed joint diagnostic method not only presents a novel approach for plasma diagnosis but also serves as a valuable electron density distribution model for subsequent electromagnetic experiments.