Reflectometry diagnostics for atmospheric entry applications: state-of-the-art and new developments

Abstract

Insight into the plasma properties of high-speed ionized flows is key for the validation of predictive aerothermodynamic tools for atmospheric entry applications, namely plasma radiation which will be highly dependent on the flow electron densities, which accordingly need to be accurately estimated. We performed an extensive review on past re-entry microwave diagnostics, including both in-flight and ground-test experiments. This work assesses the peculiar characteristics of entry plasmas, and derives functional requirements for microwave diagnostics tailored for such applications. A preliminary analysis, including a simulation of electromagnetic environment for the RAM-C II flight experiment, obtained through a CFD code coupled to an electromagnetic propagation code, is presented. The obtained results outline the adequacy of such diagnostics for improving verification and validation for high-speed entry plasma flows.

VUV to IR Emission Spectroscopy and Interferometry Diagnostics for the European Shock Tube for High-Enthalpy Research

“…The electron density may be inferred either through spectroscopic methods (Stark broadening techniques [24,25]), or through microwave interferometry techniques. Although these are, to date, not as developed as the formerly discussed optical spectroscopy diagnostics, several proofs of concept for such diagnostics have been demonstrated, and one may expect such techniques to be more vigorously deployed in the near future [26].…”

“…Examples of this diagnostic include measurements made in Xenon [81][82][83][84][85], and Hydrogen [86]. A recent review on the applicability of this technique for atmospheric entry applications is provided in [26].…”

“…This latter approach allows the development of very compact instruments and does not limit future extensions of the bands to cover. A bespoke equipment [26], depicted in Figure 17, was developed according to this design philosophy, which is composed of two sections: a back-end signal generator and a front-end frequency multiplier and antenna assembly. The back end is a complete, digitally controlled, signal-swept generator operating at a convenient microwave frequency range, such as 8-12 GHz or 12-18 GHz (or 10-20 GHz).…”

“…We chose to multiply the back-end frequencies by 6 since this brings us to the 70.8-112.8 GHz frequencies (NATO W bands). The choice for this specific front-end was driven by the legacy of the successful application of this sampling technique in the VUT-1 shock tube [26] for lowionization shocked flows. Selecting higher frequencies might be more appropriate for typical atmospheric entry conditions (see Figure 10), as the 70.8-112.8 GHz band is likely to be below the cutoff frequency of most shocked flows of interest.…”