AWLYSIS OF TRE SIMULATION OF THE SOLAR WIND2 D. E. Zuccaro 1 REZ'ERENCE: Zuccaro, D. E., "Analysis of the Simulation of the Solar Wind, " ASTM/IES/AIAA Space Simulation Conference, 14-16 September 1970. ABSTRACT: This analysis surveys the properties of the solar wind, establishes a set of requirements for solar wind simula- tion, and develops a conceptual design of a simulator system. The significant features of the design are the following. The protons are formed in an r-f excited plasma discharge ion source. A 20' deflection magnetic mass separator is used to purify the proton beam of other ions, energetic charge exchange neutrals, and Lyman alpha photons. The use of a small diameter beam permits differential pumping of the ion source and the sample chamber. The proton beam either can be expanded to flood the sample, or it can be scanned over the sample. KEY WORDS: solar wind, charge exchange, charge neutralization, sputtering, proton sources, mass separators, ion optics, ultra- high vacuum systems. I. INTRODUCTION The development of spacecraft thermal control coatings requires the laboratory evaluation of the coating's resistance to damage induced by solar photon and particulate radiation. As the characteristics of the solar wind (or solar particulate radiation) have been detemined rather recently, various types of simulators, which had different operating and performance characteristics, were fabricated. The Hughes Research Iabora- 'Ion Device Physics Dept. , Hughes Research Laboratories, Malibu, California. 2This work was performed under Contract NAS 2-5585. 261 tories has performed a study of the simulation of the solar wind for the Ames Research Center of the National Aeronautics and Space Administration. The goals of this program were to survey the properties of the solar wind, to establish the re- quirements for a solar wind simulator, to analyze the techni- ques and apparatus to be used in the system, and to establish an optimized system and its operating techniques. This publication is one of a pair that report the results of this study. The companion paper by King(1) analyzes the ion sources, mass separators, and ion optics systems needed to form a proton beam that uniformly irradiates a 10 cm diameter sample. This paper will survey the solar wind properties, the simulator requirements and, using the results of King's analy- sis, establish a conceptual optimized simulation system. 11. SOLAR WIND COMPOSITION The particle environments in space that are of principal interest to the designers of spacecraft are the solar wind and the radiation that exists at synchronous orbit. The latter been summarized in an extensive survey by Stanley and Ryan. (3" The properties of the solar wind, which have been measured by a number of satellite probes, will be summarized in this sec- tion of this paper. The solar wind is the term applied to the streaming plasma that is evolved from the sun. Because the energy of the par- ticles is much greater than that which can be associated with the corona temperatures, it is believed to result from a super- sonic expansion of the corona's charged particles coupled to the sun's magnetic field. The plasma is neutral, having an equal number of positive charges and electrons per unit volume. The ions and electrons have much different velocities. The most abundant type of ion is the hydrogen ion, H+(i.e., a proton). The second most abundant type is the doubly charged helium ion, He2+ (i.e., an alpha particle). The He2+ to ratio ( rl /rlp ) has been measured by the Vela and 3B satel- lites. (37 During a two year period, the ratio varied over the range of 1 to 8$, with an average value of 3.7%. The variation is believed to reflect time changes in the plasma composition. During periods of solar activity, i.e., solar flares, the plas- ma contains a much greater He2+ content. In a recent class 3 flare, an IIa/Qpratio of 0.22 was observed.(4) Under this con- dition nearly half the charge of the solar wind is carried by the He2+ ions. I1)The numbers in parentheses refer to the list of references appended to this paper. 262 Although there are spectroscopic data indicating the pres- ence of many other elements in the sun, only multi 1 charged oxygen ions O5+, O&, and 07+ have been identifiedP;Y 5 at this time. The heavy ion component could be resolved only under quiet sun conditions. During thi eriod it was about 0.5% (by number ratio) f the proton flux.f57 Other ions are believed to be present,r6) but their low relative abundance and the num- ber of ionic states makes it very difficult to identify them by means of energy per charge detectors. The foil collector experiments made during the recent Apollo flights should pro- vide more information about the heavy ion composition of the solar wind. The solar wind proton energies were first accurately mea- sured by the Mariner 2 satellite(7) in the final third of 19&. The average of the daily average proton energies for the period was 135 eV. The 3 hour averages ranged from a low energy of 540 eV (which is associated with quiet sun conditions) to a high energy of 3100 eV. The Vela 2A and 2B satellites made about 13,000 measurements(8) during a period of minimum solar activity from July 1964 to July 1965. The mean value of the proton energy was 920 eV. The largest number of cases was for a 550 eV particle energy, which is associated with the quiet sun condition that was prevalent throughout most of the period. This distribution of proton energies is shown in Fig. 1. The energy of the He2+ ions was 4 times that of the solar wind protons. The highly charged heavy ions (07+, Ob, and 05+), which could be resolved only during quiet sun periods, (5) had energies of about 20 times that of the protons. The solar wind ion flux is about 2 x 108 cm’2 sec-l during quiet sun conditions .( 6, This value is relatively constant, and its variation with solar activity generally less than a factor of 2. The determination of the solar wind electron properties is difficult because the low energy of the electrons means the spacecraft potential can perturb the measurements and the solar induced photoelectrons can cause erroneous measurements. The most accurate measurements are the recent Vela 4B observations@) which showed that electron energy spectrum has a broad maximum in the energy range of 20 to 40 eV. 111. SOLAR WIND SIMULATOR FG3QUIREMENTS The requirements for the laboratory simulation of the solar wind are established mainly by the solar wind properties that have been summarized in the previous section. Additional factors such as the duration of the tests, the cost and com- 263 plexity of the equipment, the need for accelerated testing, and the size and number of samples must be considered. A. Particle Composition The most important parameter to be specified is that of the ion composition. All efforts to date to simulate the solar wind have used either proton beams that were purified by mass separation, or mixed (unpurified) ion beams. The latter con- tain molecular hydrogen ions (H2+ and If3+) as well as other ions. Although it is possible to rule o t total simulation of the solar ion plasma (p,He2+, 07+, O&, and 05') because of the prohibitively expensive and complex apparatus required to produce such a plasma, it is important to note some of the pos- sible effects of these minor constituents. For example, t e sputter yield of oxygen ions is 100 times that of protons.Pl0) Thus, the oxygen ion component would have about the same sput- tering effect as the proton component of the solar wind. The interaction of highly charged ions, such as 07+, O&, and 05+, with the surface results in the formation of x-ray photons or Auger electrons. Both of these can produce secondary ioniza- tion in the target. In this case, the effect of the heavy ions is much greater than that of the protons. Based on the present limitations, simulation of the solar wind is limited to the use of pure proton beams. Nevertheless, one of the first tasks of this optimized simulator will be to determine if the simulation of the minor constituents of the solar plasma is necessary. This could be done with a less com- plex O+ or 02+ source in conbination with an x-ray source. B. Particle Flux T e simulation of the solar wind requires a proton flux of 2 x 108 cm-2 sec-l which corresponds to a ion current density of 3 x A an-'. Because this value is based on satellite data that were obtained during a period of decreasing and mini- mum solar activity, it may be necessary to modify the value for the period of maximum solar activity. The change should be a small one. A much higher flux level may be required to accel- erate the testing rate. This will be particularly true when radiation resistant coatings are developed. At present, accel- erated testing of photon (light) induced damage is performed with 5 to 50 equivalent suns, where limitation is the output of the sources and the reci rocit failure of the samples. A proton beam of 2 x 1011 cm-$ sec-l (ion current density of 3 x lom8A cm-2), which corresponds to a level of 1000 times the normal solar plasma flux, is readily obtained. This is an ac- ceptable design goal as far as the engineering aspects are con- 264 cerned.