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 )/(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 In the next section of this paper an attempt will be made The selection of systems in Table 1 represents a variety to present a retrospective overview of fast atomic of options that have been attempted in the development oxygen beam facilities developed for more then a of high-flux hyper-thermal atomic oxygen beam decade worldwide, and state-of-the-art review of the facilities. Those sources can be divided by the principles sources used up to date for ground based simulation of of AO flux formation from neutral or charged AO LEO environmental conditions. The intent of this study particles, or their mixture in plasma, and methods of is to try to address some of the variables outlined in an acceleration or deceleration to the required ~5eV energy attempt to summarize the major properties of the system range. They can be also divided on the principles of the and their possible influence on the results, as well as to operation mode, as continuous or pulsed AO beam emphasize on ‘hard-to-solve’ problems. All sources will sources. One of the most important clan of sources be classified based on the method of AO creation and presented in Table 1 is the gasdynamic sources. FAO beams formation and delivery to the sample. The method of the creation of AO beam, i.e. continuous or The most successful approach, in gasdynamic sources, pulsed, will also serve as criteria in the classification. had been shown using mixtures of light and heavy gas components with a low relative concentration of the heavy components, then in the gasdynamic beam that is 4. OVERVIEW OF FAO SOURCES obtained the heavy particles move with a velocity DEVELOPMENT AND corresponding to the average mass. CHARACTERIZATION In this way it is possible to obtain heavy particles of The Atomic Oxygen Simulators that are used around the higher energy than in the case of a beam containing only world, depending on the energy of the atomic oxygen the pure heavy gas [19]. The maximum attainable fluxes, can be divided into thermal and hyper-thermal particle energy in a gasdynamic beam is determined by [5-8]. Most of them may be grouped broadly into the temperature of the gas in the nozzle chamber, since plasma and neutral beam sources. These two broad the expansion is accompanied by the conversion of the classes, in turn can be divided into few categories enthalpy of the gas into the energy of the molecular according to the basic method of development of the beam. For example, if the gas temperature in the nozzle AO beam as follows: gasdynamic, electrophysical, chamber is 3000K the particle energy reaches 0.5 eV. thermal, high-temperature electrical discharges, ion To increase further the energy of the particles in the beams, and laser breakdown. The basic physical beam it is necessary to use methods of heating the gas processes used for those sources are supersonic that do not cause destruction of materials of the source expansion, laser detonation, ion neutralization, electron• components. The majority of high-flux gasdynamic simulated desorption, and photo-dissociation. Based on sources continuous or pulsed have been operable in the the method of creation of the atomic oxygen flux a energy range ~1-1.5 eV [4, 8]. So, the concept of AO variety of byproducts, as mentioned above, can evolve gasdynamic sources is to highly excite a gas by rf, dc, or that are making the comparison of the results quite microwave sources and subsequently expand the gas ambiguous and not straightforward. through a freejet or hypersonic nozzle, “converting the heat to velocity”. To maximize the expanded velocity, A few examples of FAO beam sources that have been in helium is the gas of choice for excitation, with a few development stage or have been developed in early percentage points of oxygen added downstream. The 1990s based on various principles [6, 10, 12-20] are expanded beam is then composed of a mix of oxygen presented in Table 1. The main emphasis was on atoms and molecules dilute in helium. These are high• formation of neutral, high-flux AO directional beams in flux devices, allowing oxygen atom brightness of 1018 to the required energy range to imitate as close as possible 19 -2. -1 10 atoms/cm s but are limited to O atom energies the LEO conditions and to have the ground based below 3 eV, primarily by material limitations. testing significantly accelerated for the materials test experiments to be done in reasonable time frame. 316 Table 1: Fast Atomic Oxygen Testing Facilities – Development Efforts Organization Location AO Formation FAO Flux formation/delivery Ref. Jet Propulsion Lab Pasadena, CA Ion source Electrostatic acceleration/ laser 1 [12] photodetachment of electrons Jet Propulsion Lab Pasadena, CA Pulsed laser breakdown in Detonation /blast wave 2 [13] O2/laser sustain plasma expansion through a nozzle Los Alamos National Los Alamos, NM Cont. plasma – induced Detonation /blast wave 3 [14] Laboratory breakdown expansion through a nozzle NASA – Langley Hampton, VA Oxygen dissosiative Electron-stimulated desorption 4 [15] Research Center adsorption/diffusion through Ag NASA – Lewis Cleveland, OH Ion source Ion flux 5 [6] Research Center NASA – Lewis Cleveland, OH Microwave plasma Electrostatic acceleration 6 [16] Research Center Physical Sciences Inc. Andover, MA Pulsed laser breakdown in Detonation /blast wave 7 [17] O2/laser sustained plasma expansion through a nozzle Princeton Plasma Princetion, NJ RF plasma source Electrostatic acceleration/neutral 8 [18] Physics Laboratory on a biased plate Toronto, University of Downsview, Microwave plasma in He/O Expansion through a skimmer 9 2 [19] (Aerospace Institute) Ontario, Canada Zhukovskij Central Moscow RF arc discharge plasma in Expansion through a skimmer 10 [20] Aviation Institute -TsAGi He/O2 Inst. Nuclear Physics, Moscow Oxygen plasma by spark Electrostatic acceleration/ion 11 [10] MSU discharge deflection Perhaps the most popular type of beams that have been complication for positive ion sources. Gaseous charge under development in the ‘90s is the ion source. Here exchange is inefficient because of low cross sections, positive or negative ions are created by either electron and at surface neutralization often it is still a question of bombardment or rf excitation and then are the energy distribution in the reflected neutral beam. electrostatically accelerated and focused to achieve the The negative ion sources have shown some promise proper velocity, at which point the charge is stripped by when laser photodetachment was tried to be utilized for various techniques such as charge exchange or surface the neutralization process [12]. Furthermore, variable neutralization. Such beams, which can readily achieve beam O(1D) to O(3P) concentrations can be achieved by the appropriate velocity, however, are typically limited adjusting the photodetaching laser wavelength to low fluxes because of coulombic repulsion effects. appropriately. For standard ion sources, achievable neutral fluxes as high as l015 cm-2.s-l have been predicted but not yet The technique in ref [15] promised to provide a high• demonstrated at that time. purity oxygen atom beam. In their approach high• pressure chamber and a vacuum chamber are connected Various types of sources based on charged transfer of by a silver membrane. Oxygen molecules introduced accelerated ions have been attempted and here the into the high-pressure chamber interact with the silver, limitations are mostly associated with the high columbic dissociate, diffuse through the membrane, and repulsion by the low energy charged particles. The ultimately are adsorbed on the vacuum-side silver method on charge exchange with fast ions allows one in surface. Electron-stimulated desorption is then used to principle to boost the final energy of the particles of a free and energize the oxygen atoms. The predicted beam to any desired value. However, because of the ultimate oxygen atom flux achievable by this technique divergence of the beam induced by the space charge, the is 1015 cm-2.s-1, however these devices haven been beam intensity rapidly falls off as the energy of the shown to be operational at fluxes in the range 1013 – beam is reduced. Therefore in the majority of 1014 cm-2.s-1 and their disadvantages are wide energy experiments the particle energy in the beam has been spread and diverging beams. above 100 eV. Perhaps one of the most promising technologies for Focusing limitations caused by coulombic repulsion can materials testing applications was shown to be the laser• be eliminated for instance, to some extent by sustained discharge sources [17], where lasers are used neutralizing the beam by electron injection. Indeed, to produce a high-temperature plasma that is ultimate beam neutralization appears to present a major subsequently expanded in a freejet or supersonic nozzle 317 to produce a high-velocity neutral beam. Such sources 5. CURRENT FAO SOURCES IN LEO SPACE have been demonstrated to produce beams of the desired ENVIRONMENT SIMULATION AND velocity of 8 km.s-1 at flux levels of 1017-1018 cm-2.s-1, ACCELERATED TESTING FACILITIES i.e. exhibited both the appropriate energy for LEO simulation and high flux required for accelerated The information on the majority of high flux FAO testing. sources, that are presently in use around the world [21• 43], is presented in Table 2. The type, mode and It was mentioned above that FAO beam sources can be principal of operation, method of AO formation and distinguished by operational mode, i.e. continuous or FAO flux formation/delivery method, the energy and pulsed beams. Continuous discharge sources must flux range of FAO beams are presented. In addition the employ an inert carrier gas to reduce the discharge flux structure including the intensity of O2 or He/O2 or temperature. A 6000 K stagnation temperature is Ar/O2 fluxes as well as levels of VUV radiation are required to obtain a 7.8 km/s beam of pure helium. described where information was found. An extended Seeding the helium with molecular oxygen increases the description of a few major sources and a brief average atomic weight and therefore increases the discussion of their advantages and drawbacks is stagnation temperature required to produce a 7.8 km/s presented below. flow. For this reasons all continuous discharge sources (including cw laser sources) utilize only a small Figure 1 presents in graphic form a summary of the percentage of oxygen to minimize the peak stagnation FAO fluxes and acceleration factors that the systems temperature. An 6000 K continuous helium flux is described in Table 2 are offering presently. Atomic rather difficult to confine and the addition of oxygen oxygen flux levels that are expected for the payload produces a very erosive environment. The reactivity of exposure on ISS are presented in Fig. 1 by the lower atomic oxygen with the materials at elevated (an average for less then 30 days exposure) and upper temperatures generally restricts the operation of present (an average for a one-year exposure) lines [44]. As can conventional DC, RF or microwave discharge sources to be seen from Fig. 1, the facilities that are below the a maximum energy between 1 to 2 eV. Continuous lower estimate line can be considered mostly as tools optical discharge (CW laser) sources can achieve 5 eV for research and LEO simulation studies. For other beam energy however the atomic mass flow rate is facilities, above the upper estimate line, acceleration throughput limited by helium pumping speed factors between 10 and 1000 could be achieved. The restrictions. These devices are generally small area major ground based acceleration testing facilities are and/or low flux sources due to the small percentage of now operating with 1-1.5 orders of magnitude oxygen in the gas flow, and pumping speed limitations acceleration factor. for helium that determine the upper limit of total mass flow through the nozzle. Plasma radiation and thermal 5.1 Examples of high-flux current FAO sources energy accommodation of the inert carrier gas can significantly raise the sample temperature well above The UTIAS Toronto beam facility [31,44] utilizes a ambient. For these reasons continuous sources have microwave power source to excite an O2-He gas mixture some limitations for accelerated LEO material to produce AO that undergoes a supersonic expansion degradation studies. into a vacuum chamber. The system comprises four major components: a microwave-induced plasma torch, An additional important issue for many of FAO beam a sample-skimmer interface, an ultraviolet source, and a sources have been the presence of accompanying vacuum chamber with the associated support UV/VUV radiation associated mostly with AO electronics. The plasma torch is used to generate a dissociation. UV/VUV interaction with materials is one stream of essentially neutral oxygen atoms seeded in a of important components of space environmental effects helium-gas carrier. The sampler-skimmer system strips in LEO environment, with many polymer based off a portion of the lighter carrier gas from the plasma materials being sensitive to VUV radiation. For and produces a diverging AO beam that is then directed instance, for perfluorinated materials highly accelerated into the connecting vacuum chamber. The stainless steel erosion yield was repeatedly confirmed in LEO and chamber maintains the environment at ~10-5 Torr (1 ground based facilities due to synergistic effects of FAO Tor=133.3 Pa), to approximate LEO spacecraft-surface and VUV radiation. Therefore it is highly desirable to conditions. The average FAO energy was measured, have controlled, well defined, independent UV/VUV using time in flight measurements, to be ~2.5-3 eV with source in ground based LEO environmental simulating a comparatively wide distribution that presents some facilities. Unfortunately, many FAO sources produce drawbacks for this type of sources. The strong VUV radiation during their operation, a fact that should advantage is the absence of VUV/UV radiation in the be taken into consideration when comparing results of source that allows an independent VUV source to be testing. installed in the facility. 318 Table 2: The Main Types of FAO Sources in LEO Space Environment Simulation Facilities Worldwide Energy Flux Type, mode and FAO Flux Flux Name, AO Formation of density, principle of Formation/ Mode structure, Ref. affiliation Method atomic cm-2 · s-1 operation Delivery % O, eV cont/pulse MSFC Elec.-phys., pulse, RF plasma Elec. accel., plate O -10% [21• 1 (USA) plasma, rech. neutr. / scattering Pulsed 5 5 x 1015 + VUV 23] (-200ES) PSI (USA) Gas-dyn., pulse, Laser breakdown in Detonation/blast ESTEC laser O2/Laser sustained wave in supersonic (Netherlands) plasma. nozzle O /O 2 [24• 2 NASA JPL Pulsed 1-16 5·1015/1017 10/90 28] (USA) (+UV/VUV) CERT-ONERA, (France) Montana State Gas-dynamic, Laser: breakdown Detonation/blast O /O 3 University pulse, laser in O / sustained wave in supersonic Pulsed ~5 ~1014 2 [29] 2 ~60/40 (USA) plasma. nozzle LANL (USA) Gas-dynamic, cont, Laser breakdown Detonation/ blast Ar/O /O 4 laser Ar/O wave in supersonic Cont. 1-3 1016 2 [30] 2 90/7/3 nozzle ITL/UTIAS Gas-dynamic, cont, Microwave plasma, Superonic He/O /O 5 Cont. 1-3 1017 2 [31] (Canada) UHF He/O2 expansion 97/1/2 Zhukovskij Gas-dynamic, cont, RF arc discharge, Supersonic Central HF He/O expansion 2 He/O /O 6 Aviation Cont. 1-5 1016 2 [32] 90/7/3 Institute -TsAGi (Russia) SOREQ NRC Electro-phys, cont, Ion source Electrostatic 7 Cont. 30-50 1014 O - 100 [33] (Israel) ionic accel./decel. Kobe Univ. Gas-dynamic pulse Laser breakdown in Detonation/ blast (0.3 - 6.5) O2/O 8 (Japan) laser O2/Laser sustained wave in supersonic Pulsed 4.5-5.1 14 [34] plasma nozzle x 10 ~55/45 Inst. Nuclear El-phys, cont, Oxygen plasma Electrostatic O /O [35, 9 Physics, MSU plasma + accel./ion Cont. 5-80 1016 2 15/80 36] (Russia) recharging deflection Moscow Phys. Gas-dynamic, Oxygen plasma by Shockwave/superso O /O 10 Inst. pulse, spark spark discharge in nic expansion Pulsed 1-5 5·1015/1018 2 [37] 2/98 (Russia) O2 Phill Lab, Cal. Phys., diffusion - O Electron-stimulated 2 [38, 11 Univ. desorption dissociatoin/diffusi disorption Cont. ~5(4-6) 4.5 x 1013 0-100 39] (USA) on through Ag foil Shanghai Univ, Electro-phys, RF plasma Elec. accel., plate (China) pulse(?), plasma, neutralization/reflec Pulse 12 6-20 2 x 1016 ? [40] (2001) recharging tion (?) Beijing Inst. of Electro-phys, ionic• ECR-based ion Deceleration/neutra Spacecraft Env. decel., recharging source lization Cont. 13 5-10 ? ? [41] Eng. (?) (China) (1999) Lanzhou Inst. of Electro-phys, Microwave plasma Electrostatic Phys. plasma, recharging acceleration/plate Pulsed 0-100(?) 14 5-10 4 x 1015 [42] (China) (1998) neutralization/reflec (?) 0+<0,1% tion 319 Fig.1. FAO Fluxes (left scale) and acceleration factors (right scale) on current FAO beam facilities for LEO environmental space materials accelerated testing and simulation research. Using similar basic principles, an atomic oxygen beam spectrometer, by the retardation potential energy was obtained in Zhukovskij Central Aerohydrodynamic analyzer and double Langmuir probe. Neutrals have Institute, Moscow, by using a nozzle gas jet source [20, been evaluated by torsion balance and bolometer. 32]. The gas mixture in the source was heated by a RF inductively-coupled arc discharge. The stagnation The Atomic Oxygen Beam Facility (AOBF) at Marshal pressure and temperature were equal to 40-160 Torr Space Flight Center (MSFC) produces a 5-eV neutral and 1500-6000 K, respectively. The dependence of the AO beam by placing a metal plate in contact with a dissociataion degree of oxygen on the gas mixture in magnetically (3-4 kG) confined AO plasma [21-23]. the discharge chamber and on the RF power of the The AO plasma is produced by a radio frequency-(RF-) beam source was measured on the center line of the driven lower hybrid plasma source. A magnetron flow at a distance of 40-50 cm from the hypersonic supplies 2 kW of power at a frequency of 2.45 GHz to nozzle by a RF mass spectrometer. At a maximum RF an antenna to produce the plasma. Because of the power (about 36 kW) the degree of oxygen dissociation facility geometry the AO plasma is magnetically in the mixture of 98% He and 2% O2 was equal to confined such that a 1-cm- (0.39-in-) diameter plasma 80%-100% at the position of the polymer sample. column is produced on centerline of the test chamber. The plasma column interacts with an electrically biased The electrophysical method to accelerate AO ions by metallic plate. The bias applied to the plate accelerates electromagnetic fields with subsequent charge ions from the plasma to the plate. During the exchange is used in [35, 36]. The method is considered acceleration process the ions gain energy equal to the usually to produce low fluxes at energies less then 100 difference in the plasma potential and the neutralizer eV due to ion space charge effect. Being valid to ion plate bias. Once the ions hit the plate, they collect an beams this limitation can be reduced when ions are electron from the metal lattice and become neutral. accelerated in a magnetic plasma-dynamic accelerator Following collision with the neutralizer plate, the (MPDA) where ions are neutralized by electrons. The atoms are reflected toward the test specimen at a technique allows one to produce intensive plasma fraction of their pre-collision energy flows with ion fluxes up to 1019 cm-2 s-1 in a wide energy range. The AOBF is capable of supplying 5-eV AO atoms in a pulsed fashion for long periods of time. The limiting When accelerated ions undergo charge exchange with factor in the length of a test run is the heating of the RF the intrinsic gas the fast neutral beam is formed. This antenna. During operation of the system, the neutralizer method was used to produce the fast air neutral beam plate collects nearly 4 A of ion current from the with velocity range of 5-20 km s-1 and fluxes up to 1017 plasma. In order to maintain space charge conditions, cm-2 s-1 [35, 36]. The beam diagnostics included the the same amount of electron current must be lost to the monitoring of the charged and neutral particles. The antenna. Heating in the system has been limited by ions have been measured by monopolar mass• 320 operating in a pulsed fashion with a duty cycle 5-15 developed as described in [28]. The main sample percent [21, 22]. holder (SHI) is mounted perpendicular to the AO beam at a distance of 40 cm with an effective AO coverage The AO flux produced by the AOBF system is area of 20 x 20 cm2. Samples on SHI are exposed to approximately 5xl015 atoms/cm2/s. During production both AO and VUV in the main beam. The system was of the AO plasma, the system produces also modified by adding a new sample holder plate (SH2) electromagnetic radiation during the dissociation and on the edge of SHI in an angle of 79 deg in such a way ionization process. Attempts to identify and quantify that SH2 surface was parallel to AO beam. At this the radiation using a photodiode with appropriate angle no VUV'or nonreflected oxygen atoms could narrow band filters indicated that the primary radiation reach the samples attached to SH2. In addition, a quartz line was 130 nm, the AO resonant peak in the VUV plate was placed in an angle of 45 deg to SH2 surface. region. The VUV intensity was determined to be nearly The reflection plate is designed for 1) reflection 100-200 times the Sun's intensity averaged over the oxygen atoms toward the sample holder mounted in duty cycle. Two FAO beam facilities of similar type parallel to the AO beam axis (SH2) and 2) elimination have been designed and developed recently in China, in of UV radiation that could reach the sample holder by Shanghai [40] and in Beijing [41]. absorption. However, the real energy distribution and the flux of reflected FAO species still should be The principle of operation of the laser detonation carefully analyzed. source is based on introducing an O2 gas pulse into a conical nozzle and firing a laser light pulse as the gas 5.2 Principals of FAO Beams Characterization begins to expand into the nozzle. The laser pulse initiates plasma that expands into the conical nozzle, Characterization of the atomic oxygen beams in the forming a detonation/blast wave, and generates a beam, existing space environment simulators is a very which consists mainly of fast neutral oxygen atoms important issue. In order to be able to compare between [24-28]. These atoms have an adjustable average results from different facilities and to correctly identify velocity between 5 and 13 km/s. (The average velocity the proper exposure conditions, extreme care should be of O atoms in space is 8 km/s corresponding to 5eV.) taken in choosing the appropriate beam This source is capable of producing beams containing characterization methods. A number of different O atoms fluxes of more than 1015 atoms/cm2.s at a dis• methods were proposed and used for characterization tance of 50 cm from the edge of the conical nozzle, and and measurement of the major atomic oxygen beam a few major facilities of this type are used now in the parameters with various degrees of success [13, 14, 18, space centers and university laboratories. 27, 36, 45-47]. In general the atomic oxygen beams are characterized by a number of different parameters, The VUV radiation associated with each atomic the most important of which include: a) atomic oxygen oxygen pulse for the spectral range of 115-180 nm, at a beam energy; b) the ratio between atomic and distance of 40 cm, was measured, for example at molecular oxygen in the beam; c) atomic oxygen beam COSOAR facility [28], as 2.4 x 10-2 W/cm2. Because flux intensity and d) the presence of ion and electron the solar radiance at this spectral range is 2.4 x 10-6 components at the atomic oxygen beam. W/cm2, the samples are exposed to 1.0 x 104 equivalent A brief overview of the most basic methods that are suns (E.S.). So, both most popular, widely used sources used for characterization of the AO beams is given represent not FAO, but more a combined, FAO + below. strong VUV environment, that may be seen often as a drawback. 5.2.1 Atomic oxygen beam energy measurements. One of the ways of removing the VUV radiation from To measure the kinetic energy EAO of oxygen atoms in the AO beam is using a chopper. However, the use of a beams a simple equation can be used: chopper requires synchronization of the whole system 2 and could be complicated and costly. A much simpler EAO=mv /2, (2) and cheaper solution was demonstrated in [28]. The method is based on using a UV quartz plate mounted in where m is the mass of the oxygen atom (m=16) and v a way to reflect the oxygen atoms to a sample holder its velocity. To measure this velocity, the Time-of- mounted in parallel to the AO beam axis, and 2) it Flight (TOF) method is used most commonly [44-47]. should prevent the irradiation of the sample by VUV The basic idea of TOF method is very simple, i.e from formed by the AO source. the measurements of the time t, required for an oxygen atom to travel a distance L, its velocity can be To separate between the AO beam and the VUV determined as radiation associated with it, a reflection system was v=L/t. (3) 321 The substitution of Eq. (3) into Eq. (2) gives: 5.2.2 The ratio between atomic and molecular oxygen in the beam (degree of dissociation) 2 2 EAO=mL /2t (4) The degree of dissociation . is determined from mass The parameter L is a constant for a specific system, spectrometer measurements of the beam [44]. Analysis m=16, so combining these constants we can rewrite of the relative change in oxygen peak heights, equation (4) as follows: originally presented in [49], was modified to include a correction for room air entrained by plasma. This 2 EAO=k/t , (5) parameter is necessary to calculate an atomic oxygen beam flux. where k= mL2/2. 5.2.3 Atomic oxygen beam flux (and fluence) Usually, to measure the time t, beam modulation (or measurements perturbation) methods are used. These methods can be classified into mechanical and electronic. Presently, there are many ways to evaluate and measure the atomic oxygen beam flux. In [45] an Mechanical modulation is achieved using choppers atomic oxygen beam flux was calculated from the (rotating wheels). The wheel is positioned in the dissociation fraction and gas throughput measurements. vacuum chamber along the atomic oxygen beam axis. The gas throughput was measured by recording the rate For good resolution of this system, the chopper gate of change of the pressure P in a bottle of known opening time, ût, must be shorter than the spread in the volume V feeding the source for any given source flight times, therefore, the duty factor ût/tgate must be pressure Ps. The throughput was derived from the chosen such that the time between consecutive gate relation pulses, tgate, is not longer than the longest flight time -1 encountered, tmax(ût/tgate0.01) Q=VûP/ût in Torr l s (4) A large gain in intensity can be achieved by using The atomic oxygen flux FAO was then obtained using multiple pulses during the frame time. Such methods, the relation: therefore, can be classified as single-pulse or multi• 19 pulse methods. The most common multiple-pulse FAO=6.44×10 DQ, (5) method is the use of a pseudorandom modulation . -1 -1 sequence for which TOF distributions may be extracted where FAO is in atoms s , Q is in Torr l s at 300K from measured signals by taking the cross-correlation (gas temperature at source inlet) and D is the function of the chopping function with the measured dissociation fraction: response [47]. 1/2 D=1-[IAO/IO2] [T1/T2] , (6) A TOF experiment involves a number of key components: a chopper or other means of modulation, a where IAO and IO2 are, respectively intensities of mass• flight path, a detector and, finally, the detection and spectra peaks of Atomic and molecular oxygen, and T1 processing electronics. To obtain the ratio ût/tgate0.01 and T2, the respective temperatures inside the plasma this chopper must rotate with a very high speed. For tube. example, in [47], for 150-mm-diameter disk with 1-mm slits, operation at 25,400 rpm was required for a 5 µsec Another way of measuring the atomic oxygen beam resolution. flux was suggested in [44]. For samples that encompass the entire beam, the atomic oxygen beam Many types of detectors are used for atomic oxygen flux at some distance xi between the AO source and the beam TOF experiments. All of them must have a fast sample is given by equation: response, high sensitivity, and low background. 17 2 2 -1 Ionization detection is most commonly used. To ensure FAO=8.15×10 / (xi-0.642) atoms (cm s) (7) fast response, the ionizer must operate below the emission level at which space charge in the ionizer The AO fluence (-AO) is computed simply from the results in long ion storage times. Usually a mass flux and the total exposure time t as spectrometer is required to reduce the influence of the 17 2 . -2 background in inelastic scattering experiments, but for -AO= FAO×t= 8.15×10 ×t/(xi-0.642) atoms cm determination of TOF distributions of incident beams, (8) provided they are not seeded, even a fast ion-gage can be used. 322 5.2.4 The presence of ion and electron components 2. Fozza A.C., Kruse A., Hollander A., Ricard A., in the atomic oxygen beam Wertheimer M.R., “Vacuum Ultraviolet to Visible Emission of Some Pure Gases and Their Mixtures To ensure an accurate measurement of the electron and Used for Plasma Processing”, J. Vac. Sci. Technol. A ion currents from the AO sources, a four greed 16(1), 1998, pp.72-77. assembly was used in [45]. The first three plates consisted of a central wire mesh 25mm in diameter and 3. Dever J., McCracken C., Bruckner E.. “RF Plasma 99% transmission. The final plate that acted as the Asher Vacuum Ultraviolet Radiation Characterization electron and ion collector was made of beryllium• and Effects on Polymer Films”. in: Proceedings of the copper with a mesh attached to the front of it. All four 6th International Space conference ICPMSE-6, eds. J. plates could be biased either positively or negatively Kleiman and Z. Iskanderova, Toronto, Canada, 2003, (to bias ion or electron components respectively). To (in press). bias off electron component an increasing negative voltage was applied. The positive current, reaching a 4. Atomic and Molecular Beam Methods, Ed. Scoles plateau, indicated that all the electrons has been G., Oxford University Press, 1988. rejected. A similar process was repeated to establish the limit for positive ion rejection. 5. Koontz S.L., Albyn K., Leger L.J., “Atomic Oxygen Testing with Thermal Atom Systems: A Critical Evaluation”, Journal of Spacecraft and Rockets, v.28, 6. SUMMARY #3, 1991, pp. 315-332. An attempt was made to review the existing in the 6. Banks B.A., Rutledge S.K., Paulsen P.E., and international space community atomic oxygen Stueber T.J., “Simulation of the Low Earth Orbital simulators and to discuss and generalize them based on Atomic Oxygen Interaction with Materials by Means of the method of producing the atomic oxygen beam and an Oxygen Ion Beam”, NASA TM-101971, 1989. the method of its delivery to the target or sample. The performance and parameters of the existing AO sources 7. Caledonia G.E., “Laboratory Simulations of were compared to the requirements of the LEO orbit Energetic Atom Interactions Occuring in Low Earth where the ISS and most of manned spacecraft are Orbit”, in: “Rarefied Gas Dynamics: Space-Related orbiting. Due to a large variety of methods existing to Studies”, ed. Munts E.O., Weaver D.P., Campbell create the AO beams and to deliver them to the targets, D.H., Vol.116, Progress in Astronautics and as well as due to the fact that while some AO systems Aeronautics, AIAA, 1989, pp. 129-142. are designed as pure accelerating simulators for testing, 8. Kudryavtsev N.N., Mazyar O.A. and Sukhov A.M., qualification and selection of space-bound materials “Apparatus and techniques for the investigation of while others are designed for reproduction of the real methods of generating molecular beams”, Physics- space environment conditions mostly for complex Uspekhi 36(6), June 1993, pp..513-528. studies of interaction of the oxygen beams with materials, it can be predicted that new systems will 9. 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Nelson E, Chow P.P. and Klaasen J., “Study of emission characteristics /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//S =( I /