Introduction
Radiation damage sustained by the solar panels, magnetrons, and other electronics will be the limiting factor on the lifespan of the SSP satellites; thus, it is important to have a good understanding of effects of radiation damage, and what countermeasures in the form of space hardening can be taken to mitigate these effects.
Types of Radiation Damage
In GEO, the satellites will be subject to many different kinds of radiation damage. We will consider the differences between heavy ion damage, high energy electron damage, and high energy proton damage. X-ray and gamma ray radiation, as well as neutron damage are not nearly as much of an issue in space as they are for nuclear reactors on earth, they can, however, be produced as secondary particles from primary collisions.
Figure 1. Energy Levels and Effects of Radiation Damage
Solar Flares and Coronal Mass Ejections
Solar flares and coronal mass ejections often occur in conjunction with each other, where large amounts of high energy protons are released, sometimes in the direction of the earth. About 90% of the particles from a solar flare are protons, the rest consist of electrons, alpha particles, and heavy ions. These high energy particles reach the earth in as little time as 30 minutes, and such occurrences are difficult to predict. The type of damage pattern caused by a high energy proton flux to a material consists of small damage cascades throughout the material. These large vacancy volumes in most crystalline structures have a tendency to repair themselves by about 95% by recombination of atoms. The energy levels of these protons range from 10 MeV to 1 GeV, and the particle flux densities from a single flare are typically ~3×1010 p/cm2. Shielding can greatly reduce this particle flux, however.
Figure 2. Effects of Shielding for Reducing Particle Flux from a Solar Flare
Galactic Cosmic Rays
Galactic cosmic rays typically are composed of 85% protons, 14% alpha particles, and 1% nuclides with Z>4, but heavy ions of Z>26 (iron) are rare; these particles typically have energies of about 10 GeV, although some particles can have energies of over 1020 eV. Cosmic ray energy density is approximately ~1 eV/cm3. Heavy ions that have had all of their electrons striped away, and are flying through space at high energies approaching the speed of light are, however, the most damaging and most difficult to shield against. Damage from cosmic rays is nearly constant, and is more damaging to materials than the proton from solar mass ejections. Their intensities due vary a little throughout the year, as they are deflected somewhat by both the earth’s and the sun’s magnetic fields. The type of damaged caused by heavy ions consists of large damage cascades, creating a large sink. However, most crystalline structures will recombine, repairing about 95% of the damage caused by the incident ion. Such damage usually occurs at or near the surface, but can penetrate deep within a solid. Because of the type of damage cascade produced, conventional forms of shielding such as lead or tungsten are often less effective then less dense materials such as polyethylene or water, as these materials are better at absorbing the energy without contributing to a damage cascade.
Secondary Emission
Secondary emission of radiation occurs when primary incident particles of sufficient energy passing through a material induce secondary emission of particles. These particles can be electrons, neutrons, gamma rays, or other particles such as positrons. Secondary emission particles will always be of less energy then the primary knock on particle. The same shielding used for the other types of radiation should work for secondary emission as well.
Limiting Factors
Solar Panels
The damage to the solar panels will be the limiting factor on the lifespan of each satellite, as these panels must remain exposed to the radiation damage in space. The GaAs thin-film solar panels are, however, among the most radiation resistant photovoltaic currently in use today. The panels will also have a glass layer, protecting the semiconductors for micrometeors and other forms of erosion, though, neutrons and charged particles will be able to penetrate to the semiconductor. Collisions of high energy electrons and protons causes a degradation of the minority-carrier diffusion length in the semiconductor material. Though there is a lot of conflicting data describing when the degradation of the solar panels becomes substantial, but it can be expected to provide nearly full power for at 30 years, and could be used for 50 years with significant degradation.
Figure 3. Layer Structure and Cell Configuration in Cross-Section of GaAs Cell
Magnetrons
Magnetrons are extremely rugged and robust devices, though as with any physical object they will sustain radiation damage from being in space. However, the damage sustained from radiation will be less than the electron damage to the magnetron from anode collisions, which remains the limiting factor. Providing minimal shielding can reduce radiation damage to a negligible level. The lifespan of the magnetrons can be made to be over 60 years before the degradation of the devices becomes substantial.
Supporting Electronics
The supporting electronics for communication, DC to DC conversion, etc., will be as vulnerable to radiation damage as the solar panels, depending on the materials used, perhaps even more so. Though, unlike the panels, the other semiconductor electronics can be protected by additional shielding. Using standard shielding for use in geostationary orbit along with space-hardened electronics, the supporting electronics can be made to last beyond the 30 year lifespan of the solar panels.
References
http://holbert.faculty.asu.edu/eee560/spacerad.html
http://engineering.dartmouth.edu/~Simon_G_Shepherd/research/Shielding/docs/Shinn_94.pdf
https://nepp.nasa.gov/docuploads/D41D389D-04D4-4710-BBCFF24F4529B3B3/Dmg_Space-00.pdf
