Collision frequency microwave diagnostics based on beam correction

Abstract

Accurate diagnostics of electron density and collision frequency in large-scale, inhomogeneous plasmas are crucial for research and engineering applications involving extreme physical environments, such as near-space high-speed targets. Traditional microwave transmission diagnostics heavily rely on the assumption of a one-dimensional uniform medium, which severely neglects the three-dimensional spatial refraction effects---namely, the ``gradient-index lens effect''---induced by density gradients. Such spatial defocusing and beam distortion introduce significant systematic errors in parameter inversion, particularly leading to a severe overestimation of the collision frequency. To address this challenge, this paper proposes a Beam Amplitude Correction-Collision Frequency Diagnostic Algorithm (BAC-CFDA) based on three-dimensional wave propagation. By deeply integrating the spatial geometric characteristics of the transceiver antennas and lens systems into the theoretical analysis, this method employs Finite-Difference Time-Domain (FDTD) full-wave simulations to construct a virtually collisionless propagation model. This approach successfully decouples and precisely strips away the extrinsic amplitude loss induced by spatial refraction from the genuine intrinsic absorption of the plasma. Dual validation through numerical simulations and physical experiments utilizing a high-power inductively coupled plasma facility demonstrates that the BAC-CFDA algorithm effectively suppresses spatial mismatch interference under various electron density states. The proposed method significantly enhances the inversion accuracy of the collision frequency, reducing the simulation verification error by up to an order of magnitude. This study breaks the spatial restriction of traditional microwave transmission diagnostics, which conventionally requires the plasma to be strictly positioned at the focal point, thereby providing a highly robust theoretical framework and an adaptive diagnostic strategy for the high-precision diagnosis of inhomogeneous plasmas.