Methods for Assessing Parameters of the Stationary Plasma Engine Plume Multifractional Model Based on the Results of Probe Measurements

Abstract

Methods are presented for determining parameters of the stationary plasma thruster (SPT) multifractional conical plume model using the electrostatic probing analyzer braking characteristics with retarding potential. Processing is carried out in three stages. At the first stage, braking characteristics are smoothed using the cubic splines. Node initial position is determined by the piecewise linear approximation nodes. Then, optimization problem is being solved, and node position is found, spline minimum deviation from the experimental curve is achieved in the absence of points with positive derivative. At the second stage, plume ions are divided into monoenergetic fractions. Division is carried out by the inverse function method with respect to the plume ions integral distribution function. At the third stage, fraction angular characteristics are smoothed together. To ensure constancy of the fraction current density total values, which could be violated with separate smoothing, special functions are introduced to separate the fractions. These functions divide the plume ions into two parts according to the retarding potential value corresponding to the fraction boundaries. Separation functions are being smoothed, and a set of smoothed angular characteristics of each fraction is obtained using these functions. In this case, the total fraction current density is not changing. Obtained distributions are used as parameters of a plume multifractional conical model suitable for engineering analyzes of the stationary plasma thruster on a spacecraft. Smoothing eliminates irregularity in angular and energy distributions caused by the measurement error and makes it possible to exclude probable artifacts appearing from analyzing results of the stationary plasma thruster plume impact on the spacecraft elements and systems