Investigation of Singly Ionized Iodine Spectroscopy in Support of Electrostatic Propulsion Diagnostics Development

Abstract

This effort examines the spectroscopy of the second spectrum of the iodine atom (I II) in order to determine one, or more, useful transitions for laser-induced fluorescence of an accelerated atomic iodine singly charged ion (I + ). While the second spectrum of iodine has been analyzed, it is not particularly well characterized. Nor has it been studied substantially within a plasma such as those of interest to the spacecraft propulsion community. Our goal is to examine the spectral data available in the literature and determine transitions suitable for development into diagnostics tools, such as laserinduced fluorescence (LIF), to examine the plasma acceleration within an electro-static plasma propulsion thruster. While xenon remains the preferred propellant for electrostatic spacecraft propulsion, a number of alternative propellants are being analyzed in various laboratories. Some of the propellants that have been investigated in the recent literature include krypton, bismuth, and iodine. Of these alternative propellant candidates, iodine is the least well investigated. However, due to its close mass (127 versus 131 amu) compared to xenon, it has strong potential for use as an electro-static propulsion propellant. Iodine's benefits include a solid density of 4.9 g/cc, a low boiling point of 183 • C. Compared to xenon storage density of 1.2 g/cc at 2,000 psi, or the bismuth boiling point of 1,564 • C, there appear to be system level advantages to iodine fueled electrostatic spacecraft propulsion. This effort focuses on the development of a laser-induced fluorescence diagnostic tool for the iodine ion.

Laser induced fluorescence diagnostic for velocity distribution functions: applications, physics, methods and developments

“…The I II scheme presented here was first investigated by Hargus et al using emission spectroscopy [33]. The metastable 5D 0 4 state is excited to the 5P 3 state by absorption of a 696.0694±0.0002 nm photon (in air).…”

“…Iodine II. Steinberger and Scime [118] provided the first measurement of single ionized iodine (I II) fluorescence using the LIF scheme proposed by Hargus et al [33]. Because of the 5/2 nuclear spin of iodine, the absorption transitions used include obvious Hyperfine Structures.…”

“…They are increasingly employed in basic plasma physics experiments to study sheath/presheath formation and other physical processes [11][12][13][14][15], in Hall thrusters to monitor IVDFs [16][17][18][19][20][21], in helicon devices to study ion beam driving mechanism [22][23][24][25][26][27][28][29], and in plasma processing to monitor evolution of IVDFs along the presheath [30], among other usages. The LIF diagnostic itself has advanced significantly since its invention: new LIF schemes have been discovered to expand the range of ions and neutral to be measured [7,[31][32][33][34][35], accurate evaluation of hyperfine and isotope effects [36], Zeeman split and atomic reference spectra improves LIF accuracy [37,38], the adaption of diode lasers improves costeffectiveness and operational safety [7,35,37], time-resolved techniques are formulated to observe periodic phenomena, and the physical limitations of LIF are being actively explored [28,39,40]. This review will focus on single-photon, LIF diagnostics of metastable ions and neutrals, and will briefly discuss some of the basic principles of the LIF diagnostic.…”

Laser induced fluorescence of Ar-I metastables in the presence of a magnetic field

“…No viable iodine lines are observed in the near vicinity of any of the wavelengths used in schemes 1-3. Instead, the laser frequency is calibrated using a fixed volume argon Opthos Instruments Inc. cell inserted into an Evenson 2.45 GHz microwave resonant cavity [42,43]. The gas in the closed cell is assumed to have zero mean flow.…”

Photoemission resulting from collisions of I2 with 5–100 eV electrons

“…Experiments provide an essential underpinning for refinement of the theory and simulation methods which are necessary for obtaining cross sections for processes involving unstable intermediates or species that are otherwise not amenable to laboratory techniques. The present study examines the luminescence of excited species resulting from collisions of I 2 with electrons in single-collision conditions, and is motivated by interest in elucidating the processes occurring in iodine-fueled spacecraft ion thrusters [1][2][3][4][5][6][7]. Several configurations of 'steady', plasma-based ion thrusters exist, the common features of which include ionization of a propellant gas via collisions with 20-60 eV electrons and generation of thrust by acceleration of the resulting cations using electrostatic or electromagnetic fields.…”