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Natural Van Allen belts, man-made events

Man-made events primarily

Man-Made Events with short-term irradiation times.

Optical Fibers

2 100 krad, polymer clad slDca, 20 °C, 0.85 nm: 0.02-0.5 dB/m loss (1-2 orders less loss at 1.5 |im).

> 1014 n/bm2 for 0.02-0.5 dB/m loss.

Losses Increase 1-2 orders, depending on dose, dose rate, wavelength and temperature. Nearly complete annealing in 5 24 hrs.

Transmitters

1-10 Mrad (up to 3.0 dB light loss) for LEDs and laser diodes, peak wavelength shifts, threshold current increases, beam pattern distorts, power loss.

1012-1014 n/enrt2 for LEDs (threshold)

1013-i015n/cm2for laser diodes (threshold). Light output loss and peak wavelength shifts.

Ionization induced burnout at 109-101° rads/s.

Pulsed lasers tum-on delays are up to 100 ns.

Power loss, wavelength shifts.

Detectors

Decrease In responsivity of 10-30% at 10 Mrad.

Dark current increase of 1-2ordeisat10-100 Mrads (for Si PIN photodlodes, worse for APDs, better for AlGaAs/GaAs photodlodes)

Displacement damage thresholds of -1014 nfcm2 for SI PIN photodlodes and -1012 for APDs. Dark current increases, responsMty decreases.

Dark current Increases lineally up to -1010 rads/s. False signal generation by radiation pulse. Upset at 2:107 rads/s. Burnout at £ 10® rads/s. APDs much more sensitive than PIN photodlodes.

Opto-modulators

Depends on device and device technology.

Depends on device and device technology.

Depends on device and device technology. Circuit upset and burnout possible.

NOTES: Optical Fibers

• Damage worse and annealing slower for lower temperatures. Losses generally lower for Increasing wavelength (to 1.5 urn)

• Polymer dad silica cores have lowest losses but losses increase below - 20 °C. Max doss usage of -107-108rads

Transmitters

• At higher temperatures, threshold current and peak wavelength increase while output power decreases for laser diodes

• LEDs have better temperature/temporal stability, longer lifetimes, greater reliability and lower cost Detectors

• APDs are predicted to be more sensitive than PIN diodes to total dose, neutrons and dose rate

• AlGaAs/GaAs photodiodes shewn to be more rafiaBon resistant than hard PIN photodiodes

8.2.2 Other Hostile Environments

Laser Weapons. High-power lasers are being developed as potential ground-based or space-based antisatellite weapons. The flux in power per cross-sectional area from these weapons is given by

where P is the average output power, D is the laser objective diameter, Q is the qnality of the laser beam (dimensionless), A is the wavelength of the laser, J is the angular jitter of the beam (in rad), and R is die range from the laser to the target £2=1 indicates a diffraction-limited weapon; laser weapons being developed will have a beam quality of 1.5 to 3.0. Both pulsed and continuous-wave lasers are in development Equation (8-14) is for a continuous-wave laser, but is approximately correct for the average flux from a pulsed laser. The peak flux for a pulsed laser will be much higher.

For engagement ranges of several hundred km, the laser spot sizes will be several meters in diameter and will, in general, completely engulf the target satellite in laser radiation. To damage or kill a satellite at any range, die laser beam must hold steady long enough to achieve a damaging or killing level. Depending on the incident flux level and sensitivity, this dwell time could be several seconds or minutes.

Fragmentation or Pellet Weapons. The former Soviet Union operated an antisatellite weapon using fragmentation pellets that could attack satellites in low-Earth orbit [U.S. Congress, OTA, 1985]. This weapon, launched from ground locations, achieved an orbit with nearly the same elements as those of the target satellite. Hence we call it a co-orbital antisatellite system. Radar or optical guidance brings the weapon close to the target satellite. A high explosive then creates many small fragments which move rapidly toward the target satellite and damage or kill it by impact

High-Power Microwave Weapons. These weapons generate a beam of RF energy intense enough to damage or interfere with a satellite's electronic systems. Their frequencies of operations range from 1 to 90 GHz, thereby covering the commonly used frequencies for command, communication, telemetry, and control of most modem satellites. A satellite's antenna tuned to receive a frequency the weapons radiate will amplify die received radiation. Thus, it could damage RF amplifiers, downconverters, or other devices in die front end of a receiver.

Neutral-Particle-Beam Weapons. Particle accelerators have been used for high-energy nuclear physics research since the early 1930s, so the technology is well developed. Weapons using this technology must be based in space because the particles cannot penetrate the atmosphere. The particles would be accelerated as negative hydrogen or deuterium ions, then neutralized by stripping an electron as they emeige from the accelerator. (The particles must be electrically neutral to avoid being deflected by the Earth's magnetic field).

8.2.3 Spacecraft Hardening

Hardening of a space system's elements is the single most effective action we can take to make it more survivable. Presently, we use hardening to prevent electronics upset or damage from nuclear-weapon effects. In the 2000s, we will see laser hardening in military satellites which must survive hostile attacks. If projected antisatellite weapons are developed and deployed, hardening will help reduce the effects of High-Power Microwave and Neutral-Particle-Beam weapons on satellites.

Figure 8-16 gives approximate upper and lower bounds on the weight required to harden a satellite to nuclear weapons effects. The technology for hardening satellites against nuclear weapons is well developed up to a few tenths of cal/cm2. Above these levels, the hardening weight increases sharply, as Fig. 8-16 illustrates. Figure 8-17 gives rough upper and lower bounds on hardening costs. Comparable cost data may be found in Webb and Kweder [1998].

10"5

10"3 10~2 10"' Hardness Level (cal/cm2)

Fig. 8-16. Weight Required to Harden a Satellite as Percent of Satellite Weight

10"5

10"3 10~2 10"' Hardness Level (cal/cm2)

Fig. 8-16. Weight Required to Harden a Satellite as Percent of Satellite Weight

For X-rays with photon energy below 3 keV, shielding is very effective with almost any convenient material, such as aluminum. At higher photon energies, materials with higher atomic numbers, Z, are more effective than the low-Z materials. A commonly used shielding material is tantalum because of its availability and ease of manufacture. A satellite's external surfaces are particularly vulnerable to crazing, cracking, delam-ination, or micro-melting. Therefore, we must carefully select materials for these surfaces to protect functions such as thermal control, optical transmission, or reflection. In this category are covers for solar cells, optical coatings on lenses and thermal control mirrors, thermal control paints, metal platings, and optical elements made of quartz or glass. The data for typical satellite materials exposed in underground nuclear tests is, in general, classified.

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