Iodine as an Alternate Propellent for Xenon

Introduction

Electric propulsion has taken over the chemical propulsion system in space travel due to its high efficiency with less amount of propellent. We are all aware that Xenon is used as a propellent for the plasma thrusters. This article mainly deals with discussing an alternate propellent for Xenon which in this case is iodine, the potential advantages of iodine as a propellent to the hall thrusters replacing xenon is discussed. It includes a brief discussion of Hall thrusters, Iodine propellent properties, Iodine test histories, test procedures, High power test results and applications of iodine as propellent.

Hall thrusters

The hall thruster consists of an anode which also acts as a gas distributer and an external cathode. The electrons from the cathode will drift towards anode and creates an axial electric field and a radial magnetic field is created with the help of the outer magnetic coil. This radial magnetic field and axial electric field, causes some of the electrons from the cathode to whirl along the magnetic field and produce hall current. The gas atoms from the anode hits these electrons and gets ionized and are accelerated due to the electromagnetic field. The hall thrusters are capable of producing a thrust between 10 and 80 km/s. Hall thrusters was first developed and used by USSR in their meteor spacecraft in 1971 and the specific impulses within the range (1000- 2000) s (iepc). The first American HET to fly in space was the Busek BHT-200. BHT- 200 was also the first reported HET to use iodine as a propellent.

Alternative Propellent Properties

The preferred propellant of Hall thrusters is xenon. Xenon has an atomic mass of 131.3 amu and an ionization potential of 12.13 eV. The main disadvantage of xenon is that that it is a rare gas found in nature, which increases the cost for its production at a very high rate. Due to this factor, the hunt for alternative propellent in hall thrusters had started. Krypton with an atomic mass of 83.8 amu and an ionization potential of 14.0 eV, is cheaper. But performance was found to be low due to higher ionization potential. The next choice of nobel gas was Argon but, it’s ioinization potential (15.8 eV) was greater than Krypton. Radon was not considered as it is a radio-active element. Heavy metals could have been a better choice but due to the fact that metals could short-circuit the electrical insulators in the thrusters this is also discarded. Meanwhile, iodine is found abundantly in nature when compared to Xenon, hence available at low cost. It is thus worth examining iodine more closely as a potential propellant for Hall thrusters.

Ionization Properties (monatomic)

The table compares the properties of iodine and Xenon. It is seen that Iodine is lighter than Xenon which makes the electron impact ionization larger but as seen earlier, the ionization potential is less when compares with Xenon. Xe must be stored in high pressure tanks or at cryogenic conditions. I2 stores in the solid phase at approximately three times the density of Xe. The pressure of the I2 reservoir may be 1000 times lower than the pressure in a Xe tank. I2 vapor is generated by heating the solid to a modest temperature, e.g. 80 - 100 degrees C. The temperature of the I2 flow path and gas distributor must be slightly higher to prevent the formation of condensed phases. However, the anode gas distributor does not have to be heated during ordinary thruster operation. Because I2 stores as a low pressure solid, the reservoir may be irregular in shape, conforming to available space. The propellant inside may even be used to shield electronics. A filled reservoir may in theory be stored for long periods of time in unregulated conditions on the shelf or in orbit.

In comparison with Xe, the vapor pressure of I2 is extremely low, e.g. 1.2x10-6Torr at T =-75oC. This makes iodine much easier to pump inside a test facility. This, in turn, means lower background pressure and less test uncertainty. High power testing becomes feasible in facilities that would be completely inadequate for Xe. I2 is also comparatively low cost. I2 at 99.99+ purity, demonstrated with the BHT-8000 (this paper), is several times cheaper than xenon. This is significant for high throughput missions. The only significant disadvantage to iodine is its reactivity, though this may be addressed though materials selection. Safety precautions are not onerous.