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Proceedings of the 9Th International Symposium on Materials in a Space 313 ATOMIC OXYGEN BEAM SOURCES: A CRITICAL OVERVIEW J. Kleiman, Z. Iskanderova, Y. Gudimenko, S. Horodetsky Integrity Testing Laboratory Inc, 80 Esna Park Drive, Units 7-9, Markham, Ontario, L3R 2R7, Canada, Email: [email protected] ABSTRACT Since this review deals mainly with the design and use of AO sources to simulate the LEO space environment, An attempt is made to review the major methods of a brief excursion into the chemistry and physics of the producing the atomic oxygen (AO) beams, based on oxygen atom will be made here. their creation methods and their delivery methods. The paper will present an updated brief overview of the The normal form of molecular oxygen is O2 that is a existing operational facilities and will attempt to colorless paramagnetic gas. It has an unusual electronic summarize the major properties of the systems. structure, which is responsible for both its unusual magnetic properties and the slow rates of its reactions. The paramagnetic behavior of molecular 1. INTRODUCTION oxygen (meaning that when placed in a magnetic field it will tend to move to regions where the magnetic field is Atomic oxygen and atomic oxygen-induced processes strongest) reveals an important aspect of the bonding are responsible for the most significant forms of that determines the existence of O . Paramagnetism is deterioration and failure of polymers and other carbon• 2 associated with the presence of unpaired electrons, based materials in low Earth orbit (LEO). Motivated by meaning that the bonding in O cannot be described in demands for product and process improvements and by 2 terms of the Octet Rule or even just Lewis structures, increasingly stringent restrictions on materials use in since the simplest versions of both these models are LEO, ground-based atomic-oxygen testing systems have based upon the assumption of full electron pairing. been developed to address and solve scores of material Although various more sophisticated bonding models problems. Over the last decade industrial and research can be used to successfully describe O , it is organizations have shown an increased interest in 2 conventional to use molecular orbital theory to provide utilizing facilities that can utilize a pure atomic oxygen a simple rationalization of its paramagnetism [1]. source, a VUV radiation source, and thermal cycling. At the same time, this approach implies that the atomic Once present in the atmosphere, O can be converted to oxygen source itself should produce neutral, directed, 2 atomic oxygen, O, and trioxygen or ozone, O , though high-flux beams of energetic, or hyper-thermal (E~5eV) 3 both of these species must be regarded as high-energy oxygen atoms without uncontrolled ultraviolet (UV, allotropes of oxygen. The bond dissociation energy of VUV) radiation or any other by-products that can -1 O2 at 493.4 kJ mol is considerable (compare to that for complicate the evaluation of the testing results. -1 N2 of 945.4, H2 of 432.0 and F2 of 158.8 kJ mol ) and only at very high temperatures or in the presence of 2. LOW EARTH ORBIT ENVIRONMENT high-energy ultra-violet radiation (<250 nm) present in CONDITIONS: ATOMIC OXYGEN the upper atmosphere is it possible to generate significant amounts of atomic oxygen. The electron The low Earth orbit environment can be characterized configuration of 1s22s22p4 for O means that it contains by many factors. Among the major ones that affect the unpaired electrons in its valence shell and is materials and structures one can name: a) atomic paramagnetic, a property that it shares with O2. Atomic oxygen (AO) with fluxes ranging between (1• oxygen is highly reactive and one of its important 14 -2 -1 5)·10 atoms·cm ·s at energies of ~5eV; b) vacuum reactions is that with O2 to form the rather more readily ultraviolet (VUV) with its most intensive irradiation in studied allotrope, ozone, O3. the range 120–200 nm; c) the thermal cycling (TC) that all objects in LEO undergo and that can range between Various inelastic collision processes involving electrons -100 °C - + 100 °C every 90 minutes; d) the hazards of create chemically reactive species in low-pressure the debris and micrometeoroids; e) irradiation by X• oxygen plasma. Typical emission spectra of the oxygen rays, electrons and proton particles, etc. Among all plasma have been observed in VUV-visible region at factors, AO, by far, plays the most important role in the 130.4, 394.7, 436.8, 777.2, and 844.6 nm [2]. The erosion processes of organic materials. intensity of the emission lines depends on the method of the oxygen plasma production, input power, gas pressure, and can be substantially higher (three orders of ———————————————————————————————————— Proceedings of the 9th International Symposium on Materials in a Space Environment Noordwijk, The Netherlands, 16-20 June 2003 (ESA SP-540, September 2003) 314 magnitude for VUV region between 115 and 200 nm) in technique has proved, in recent years, to be the simplest comparison with air mass zero solar spectral irradiance and most universal means of generating beams in the 1• [3]. to 20-eV region. A complementary method is based on plasma heating of 3. PRINCIPLES OF FAST ATOMIC AND the gas inside a nozzle source. Heating of the gas or MOLECULAR BEAMS vapor in the source may also be achieved by the use of a shock wave. This method has the disadvantage of being Fast atomic oxygen beam facilities in the energy range restricted to intermittent operation [6, 9]. An additional ~1-5eV belongs to atomic and molecular beam sources disadvantage, which applies to arc heated nozzle [4-10]. sources as well, arises from the fact that, at the high source temperatures, excitation, dissociation, and Atoms at thermal velocities have energies of a few ionization may significantly affect the final composition tenths of an electron volt [3, 5]. The use of thermal atom of the beam. test methods, based on oxygen plasma ashers, for LEO spacecraft materials selection and screening has been In general, the methods for producing fast beams are critically evaluated in [5]. Atoms with much higher more complicated than those for thermal energy beams energies are desired not just for LEO space environment and make use of experimental results and techniques simulation [6-10], but also in many other applications from various fields of physics (sources for positive and [4, 8]. For instance, the most interesting energy region negative ions, ion optics, charge transfer collisions, for chemistry is the range from about 1 to 20 eV, electron detachment processes, sputtering from solid because the dissociation energies of all chemical bonds surfaces, high-temperature plasmas, etc.). and the activation energies for most reactions lie in this region. The same energy range is also of fundamental 3.2 Principles of Continuous and Pulsed Molecular interest to many fields of physics and technology—for Beams example, the thresholds for ionization, secondary electron emission, and sputtering are found here. This is All AO sources that are used presently and discussed a rather difficult energy region to work in, and no here use either continuous or pulsed beam sources. In universal sources are available. most cases the major decision to use a pulsed source is motivated by the greater beam intensities that may be 3.1 Fast Atomic Beams Generation and Formation achieved. It is possible to estimate the improvement to be expected in operation of an AO pulsed source over a Many techniques have been used to produce beams with continuous by comparing the signal-to-background energies above those obtained thermally. A universal enhancement <S >/S that could be achieved. method, which has been most extensively used, is the p c production of ion fluxes and then of fast neutral atoms In general, the use of pulsed beams is advantageous in by charge exchange [6, 7]. Ions are background-limited experiments. If one would compare accelerated/decelerated to the desired energy, and then the ratios of beam intensity to background partial neutralized, for instance, by resonant charge exchange pressure for the pulsed and continuous beams, using the in a gas target of the same atomic species. Charge steady-state values (Ic/Pbc) for the continuous beam and transfer collisions at hyperthermal energies involve the ratio of instantaneous beam intensity to the average scattering through only very small angles, and the background pressure (Ip/<Pbp>) for the pulsed beam, it energy of the neutral atoms is essentially the same as can be shown that the following may be achieved [11] that for the primary ions. Other sources in the same energy range start with negative ions, using --1 <S >/S =( I /<P >)/(I /P ) = f (1) photodetachment, autodetachment, stripping, or p c p bp c bc collisional detachment for neutralization [6,8]. This signal-to-background enhancement, resulting from the duty factor alone, may easily be a large one. For To produce beams of lower energies than those obtained example, a 100.us pulsed source operated at 10Hz gives by charge exchange, gas-dynamic, freejet or supersonic <Sp>/Sc=103 [11]. expansion nozzle sources are mostly used [4, 8]. The energy range of conventional nozzle sources can be The most important difference between pulsed and extended to higher energies by using the "seeded beam" continuous molecular beams is that the upper limit to technique. By mixing a small fraction of a heavy gas or the pulsed beam intensity and the lower limit on the vapor with a light carrier gas, the heavy atoms attain background partial pressure that can be maintained approximately the same velocity as the light atoms in during a single pulse are not determined by the pumping the expansion. Although the accessible energy range speed of the vacuum system, as with continuous beams.
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