AAWSAP DIRD, State of the Art and Evolution of High-Energy Lasers, March 2010

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The reasons are that they avoid separate (and usually hazardous) fuels to carry/refuel
and allow propagation at wavelengths that are favorable for use from the earth or in
space. In addition to significant progress in compact, rugged, reliable and efficient
laser systems for spacecraft, major developments in energy generation and storage will
be needed. These lasers might be expected to have an overall "wall plug" efficiency of
25 percent, which requires at least four times the laser output power in prime power to
generate the laser beam. A MW solid-state laser would require at least 4 MW of
electrical power while lasing. If a low duty-cycle were allowable, much of the required
lasing energy could be accumulated from a smaller prime-power source and stored in
batteries, capacitors or flywheels. In addition to the energy required to support lasing,
the residual energy (3 MW from a 25 percent efficient MW laser) in the form of heat
must be stored and then removed from the spacecraft.
Solid-state lasers will be the optimum candidate for power in the sub-MW to MW range
due to their ruggedness and relative simplicity. An FEL would be the prime candidate
for power well above a MW. This type of laser lends itself better to heat removal from
the gain medium than a solid-state laser. This ability of an FEL to be scaled to higher
power comes with considerable additional complexity and a requirement for about a
MW of continuous power to maintain a large cryogenic refrigeration system which cools
the niobium accelerator cavity with liquid helium at 2K. The future development of
appropriate higher temperature superconducting materials may lessen this
requirement. The storage and removal of heat from any of these electrically powered
lasers may prove to be a more stressing task than generation of the required prime
power for lasing.
Table 1 provides an estimate of the irradiance that various potential space-based laser
systems could provide. It assumes a wavelength of 1 µm, a transmitting telescope of 1
meter diameter and a system beam quality of about 1.5. If the transmitting telescope
were increased to 3 meter diameter, the same irradiance would be delivered at 3 times
the range.
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