Design of Optical Measurements for Electrothermal Plasma Discharges

Abstract

Ablation controlled electrothermal (ET) plasma discharge devices consist of a small diameter capillary through which a large amount of energy is discharged. The high energy in the discharge ablates an inner sleeve material, ionizes the material, and a high energy-density plasma jet accelerates out the open end. ET devices can find applications in internal combustion engines, Tokamak fusion fueling and stabilization, hypervelocity launchers, and propulsion. The ballistic properties of an ET device are highly dependent on the propellant and ablated material. A useful noninvasive technique to characterize a propellant in these types of devices is spectroscopy. The purpose of this study is to design and conduct experiments on the ET facility called PIPE to verify results and assumptions in the ETFLOW simulation code as well as resolve data collection issues such as equipment triggering as spectrometer saturation. Experiments are carried out using an Ocean Optics LIBS2500plus high resolution spectrometer and a Photron FASTCAM SA4 high speed camera. Electron plasma temperatures are estimated using copper peaks in the UV region with the relative line intensity method, and electron plasma density is estimated by measuring the full width at half maximum (FWHM) of the stark broadened H--β line at 486 nm. Electron temperatures between 0.19 eV and 0.49 eV, and electron densities between 4.68*1022 m-3 and 5.75*10²² m⁻³ were measured in the expanding plasma jet about an inch outside the source with values as expected for this region. Velocity measurements of PIPE match well with simulations at around 5333 m/s. This study concluded that the assumption that the propellant Lexan is completely dissociated is a valid assumption, and that the ETFLOW results for electron temperature, density, and bulk plasma velocity match experimental values.