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Laser Induced Fluorescence of the Iodine Ion
AIR FORCE RESEARCH LAB EDWARDS AFB CA AEROSPACE SYSTEMS DIRECTORATE
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Iodine I2 has been discussed seriously as a propellant for Hall effect and other electrostatic thrusters as early as 2000. Atomic iodine has a mass of 126.9 amu, but as a halogen the natural state is diatomic I2 with a molecular mass of 253.8 amu. Atomic iodine I only has an ionization energy of 10.45 eV. The density of iodine solid at room temperature is 4.933 gcc. Iodine also has a relatively high vapor pressure at low temperatures. Vapor pressures of 100 Pa are achieved at only 39 deg C. It appears obvious that the spacecraft community could greatly benefit from a propellant with nearly the atomic characteristics as xenon, but with a lower ionization potential. In addition, iodine is approximately 100 X less expensive and more abundant than xenon while exhibiting better gasification characteristics than other condensable propellants such as bismuth. Iodine appears to offer the benefits of cryogenic propellant storage without the need for active cooling or insulation, and only a moderate heating requirement for gasification 183 deg C yields 1 atm. A propellant storage system for iodine this therefore believed to have mass fractions of less than 1. While laboratory evaluations of iodine in an operational electrostatic plasma thruster are limited, the results are very promising. Performance appears to be on par with xenon without significant modification of the thruster. Intriguingly, performance appears to rise and exceed xenon at higher discharge potentials. The extent to which these behaviors continue above the limits of the initial studies is unknown, but is under active investigation and one active effort is focused on construction of a full system to demonstrate an iodine Hall effect thruster system. All of these issues must be addressed to determine potential performance gains over state of the art xenon thrusters and potential spacecraft interactions.
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