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When Ronald Reagan proposed building futuristic defences to make nuclear
weapons ‘impotent and obsolete’, he did not mention specific technologies.
However, his aides cited high-energy lasers as one possibility, and this
echo of the laser-wielding combatants in the film Star Wars inspired the
popular nickname for Reagan’s programme. As the hazy idea evolved into the
Strategic Defense Initiative, spending on high-energy laser weapons boomed.
As much as $1 billion a year was spent on developing them between 1986
and 1988, but then the funding started to dry up. The budget for high-energy
laser weapons this year is around $125 million, well below the $200 million
of 1981 when Reagan first took office. Bill Clinton, the new President,
seems unlikely to reverse this downwards trend when he presents his revised
budget in a few days.

Star Wars lasers suffered a series of blows. The technology proved harder
to realise than its advocates had claimed and, in the face of a mounting
budget deficit, Congress began slashing proposed SDI budgets, first stopping
the programme’s rapid growth and then cutting spending to below previous
levels. Together, these factors forced the managers of the weapons programmes
to move toward near-term defence systems that had more appeal to Congress
but little need for futuristic lasers. The final blow was the collapse
of the Soviet Union, and with it the threat of the massive nuclear missile
attack that Star Wars was supposed to stop.

The attraction of powerful lasers is that they emit intense pulses of
energy, travelling at the speed of light, which the military hoped could
be used to destroy or disable enemy missiles, thousands of kilometres away.
Pentagon interest in laser defence against ballistic missiles goes back
to at least 1962, when Air Force General Curtis LeMay suggested that lasers
– invented just two years earlier – might ‘bring about the technological
disarmament of nuclear weapons’. By the 1970s, the US Army, Navy and Air
Force had parallel programmes under way to bring laser weapons to the battlefield.
The Army crammed a 40-kilowatt laser into a tank-like vehicle. The Air Force
put a 400-kilowatt laser into a military version of the Boeing 707, and
the Navy built a huge 2-megawatt laser – firmly planted on the ground –
to study whether lasers could protect battleships from tactical missiles.
But the results were not encouraging. The most embarrassing failure was
in 1981 when the Air Force’s Airborne Laser Laboratory failed to shoot down
a surface-to-air missile during a well-publicised test.

One big problem lay in developing lasers that generated high powers
for long enough to destroy a missile. The vast majority of lasers in existence
today are tiny semiconductor chips, which emit about a milliwatt of infrared
light for use in CD audio players, laser printers and fibre-optic communication
systems. The most powerful commercial lasers are carbon-dioxide types which
emit 10 to 40 kilowatts. At this power, a laser beam can cut most materials,
barring thick metal plates, because the laser is brought very close to the
object. Ballistic missiles, however, would be at least a thousand kilometres
away from the laser, which would have to deliver a beam with an average
power of at least several megawatts to cause any damage.

By the early 1980s, the armed services were becoming increasingly convinced
that high-energy lasers were too bulky, fragile and costly for the battlefield.
These problems did not deter the Pentagon’s Defense Advanced Research Projects
Agency. Its antimissile laser programme was small until late 1979, when
Malcolm Wallop, a Republican Senator from Wyoming, claimed that a satellite
fleet of 18 laser battle stations could almost totally stop a Soviet nuclear
attack by destroying all heavy ground-launched missiles, long-range bombers
and cruise-missile carriers, and nearly all submarine-launched missiles.

At the heart of this proposal was the chemical laser, in which hydrogen
and fluorine react to produce excited hydrogen fluoride molecules, which
emit infrared light at wavelengths of 2.5 to 3 micrometres. Powered by chemical
fuels, a chemical laser needs little or no electricity to generate light
– an important advantage in space, where electric power is at a premium.
Wallop first suggested battle stations with 5-megawatt lasers and 4-metre
mirrors to focus the beams, but in 1982 was proposing 10-megawatt lasers
and 10-metre mirrors.

Wallop argued for a crash programme to deploy laser battle stations,
but DARPA took a more cautious approach, realising that the technical challenges
went far beyond building a big laser to send beams thousands of kilometres
through space. A battle station would have to find targets, track them to
within about a metre, and focus the laser beam onto a small area to destroy
the missile. It would have to verify that it had ‘killed’ each target before
moving on to the next, to make sure that some bombs did not slip through
the nuclear ‘shield’. Finally, the whole system would have to be integrated
into a single package for launching into space.

Problems from the beginning

DARPA started three major programmes in 1982 to demonstrate the feasibility
of building some of the key components of the battle station: the chemical
laser weapon, the optics for focusing large laser beams over long distances,
and a system for pinpointing, tracking and pointing the beam at a target.
All of these became part of SDI, and were scheduled for completion by the
mid-1980s. Technical problems and escalating costs led SDI to cancel the
laser targeting system, known as Talon Gold, although a redesigned programme
is still continuing. It was not until 1987 that the Large Optics Demonstration
Experiment (LODE) demonstrated that it could produce the quality of laser
beam required for focusing on a target a thousand kilometres away. And it
was 1991 before the hydrogen-fluoride laser weapon, known as Alpha, was
reliably generating a megawatt beam – a fifth of the power originally proposed.

There still remains the technical challenge of integrating the laser
with other weapon components and testing them in space. The first step,
planned for 1994, is a ground-based experiment to show that the Alpha and
LODE technologies can be combined with the lightweight 4-metre mirror, developed
for use in space, for directing and focusing a laser beam onto a small spot
on a distant target. The next step is Star Lite, a programme that will integrate
this technology with laser targeting, and test it in space against missiles
in 1997. That is, if the money holds out.

While hydrogen-fluoride chemical lasers generate the most energetic
beams, their long infrared wavelength is a drawback. To destroy a distant
target, the laser energy must be concentrated onto as small a spot as possible.
But the size of the spot is determined by the wavelength of the beam divided
by the size of the output aperture, which in this case is the diameter of
the mirror. With a chemical laser, which emits at wavelengths of 2.5 to
3 micrometres, you need a 4-metre mirror to focus the beam on a 0.9-metre
spot on a missile a thousand kilometres away. The beam from a laser emitting
at shorter wavelengths could be focused on the same spot with a smaller
mirror, which would be cheaper and less bulky in space. Moreover, short-wavelength
pulses of light are better at destroying targets because most materials
absorb more energy at these wavelengths. For example, mechanically polished
aluminium absorbs only 10 per cent of light at 3 micrometres, but about
20 per cent at 1.5 micrometres and 35 per cent at 0.3 micrometres. SDI
plans at least one test this year of new types of chemical lasers which
emit beams at wavelengths of 1.3 to 1.4 micrometres. But, so far, such lasers
fall far short of the power levels of the conventional hydrogen-fluoride
lasers.

Other lasers emit light at even shorter wavelengths, but are too large
and power-hungry to be used in space. Instead, DARPA envisioned putting
such lasers on mountains as part of a ground-based defence system. Connected
to the power grid, the lasers would send visible or near-ultraviolet beams
through the atmosphere to orbiting relay mirrors, where they would be reflected
on to other mirror satellites, and from these onto the targets. However,
the atmospheric absorption at visible wavelengths is enough to bend and
disperse a high-power beam. Also, keeping relay mirrors within range of
the laser poses other problems. A mirror in high, geosynchronous orbit would
stay in the right place, but it would have to be a hundred times larger
than one in low orbit because the beam diameter increases with distance.
Small mirrors in low orbits would be easier and less expensive, but a fleet
of them would be needed to make sure one was always within range of the
laser.

The leading contender for ground-based lasers is the free-electron laser,
invented in the mid-1970s. Instead of extracting energy from a gas, the
free-electron laser extracts light from electrons accelerated to very high
speeds in a beam passing through a magnetic field that varies in intensity
along the path of the beam. The wavelength can be changed by adjusting
electron energy and the distance between peaks in the magnetic field, unlike
other lasers, which emit a fixed wavelength set by the materials that generate
the light. Drawing on well-established particle accelerator technology,
free-electron lasers should be able to generate high powers efficiently.

In 1989, SDI opted for a design of a free-electron laser put forward
by the Los Alamos National Laboratory in New Mexico and Boeing. But plans
for building the massive 1-megawatt demonstration laser at White Sands Missile
Range in New Mexico shrank along with the budget. The first step down was
to a 100-kilowatt Average Power Laser Experiment (APLE). Last year, the
goal was further reduced to a 10-kilowatt laser for 1995. Work continues
at Boeing, but Los Alamos has run out of Star Wars money. Now, with further
budget cuts, the fate of the whole programme is unclear, although SDI is
keen to transfer the whole project to the Army.

Despite the budget cuts, free-electron lasers are faring better than
a third major laser scheme which helped launch Reagan’s Star Wars speech
– the nuclear X-ray laser. The concept behind this laser evolved in the
late 1970s at the Lawrence Livermore National Laboratory in California,
and was enthusiastically promoted by the physicist Edward Teller, one of
the pioneers of the American hydrogen bomb.

X-rays have a short wavelength – between 0.1 and 10 nanometres – which
could make them lethal weapons in space for destroying missiles or warheads.
However, efforts to make X-ray lasers have failed in the past because it
takes a tremendous peak power to energise the laser. With this in mind,
researchers at Livermore turned to brute force – the intense burst of X-rays
from a small nuclear bomb.

Teller and Lowell Wood, his Livermore protege, envisioned a system in
which a single nuclear bomb could have a few dozen X-ray lasers arranged
around it, rather like the bristles on an angry porcupine. X-rays have too
short a wavelength to penetrate the atmosphere, but Teller argued that antisatellite
weapons could easily destroy orbiting battle stations. Instead, he proposed
that the nuclear X-ray system be kept on missiles in submarines, so it could
be launched quickly into space. Once in space, each X-ray laser would point
at a different target, and the bomb would be detonated to fire lethal X-ray
pulses from the lasers. The idea sounded good to Reagan.

The Achilles heel of the X-ray laser turned out to lie in how tightly
the beam can be focused. In chemical, free-electron and most other lasers,
the light is forced to travel back and forth between a pair of mirrors,
so that it is amplified and forms a tightly directed beam. This doesn’t
work for X-ray lasers; their pulses do not last long enough for the light
to make many round trips between the mirrors, and the energetic X-rays are
so intense that they would destroy the mirrors. Early experiments indicated
that long thin rods of the X-ray emitting material would focus the beam
tightly enough, but later tests revealed that those measurements were wrong.
By the late 1980s, it was clear that the X-ray laser would not work as advertised.
Within a couple of years the programme vanished from the budget.

Laser programmes suffered further as planners shifted attention away
from Reagan’s goal of a leak-proof nuclear shield. Congress ordered an increased
emphasis on limited defence, and in early 1991 SDI redefined its main goal
as Global Protection Against Limited Strikes, or GPALS, in which the goal
is to shoot down only a few missiles aimed at either the United States,
its overseas forces or its allies. To meet this goal, SDI is turning to
kinetic-energy weapons – missiles launched from the ground, air or possibly
space, which home in on targets and destroy them by force of impact.

Grapefruit-sized package

Laser developers, however, have not abandoned hope. The armed services
are quietly developing portable lasers with enough power to ‘blind’ battlefield
sensors – and enemy soldiers (‘Lasers designed to blind’, New ÐÓ°ÉÔ­´´,
8 August 1992). SDI, meanwhile, has begun an Aircraft Based Laser programme
to attack short-range missiles. Planners envision a 1-megawatt weapon with
100-kilometre range, using new types of lasers.

The most likely candidates are crystals doped with the rare earth neodymium,
which when energised emits a powerful beam at 1.06 micrometres. The Lawrence
Livermore Laboratory last year demonstrated a grapefruit-sized package that
emits a 1-kilowatt beam, although it required external cooling (Technology,
8 August 1992). SDI is also reportedly studying Russian solid-state lasers
that have generated much higher powers on the ground.

Planners hope that if the laser works in the air, its beams could destroy
missiles loaded with deadly chemicals or biological weapons so they fell
in the country that launched them. However, there are many problems to overcome,
including transmitting a high-quality beam through the turbulent air surrounding
planes, and building lasers and optical systems that can withstand aircraft
vibrations.

As a concept that fits current plans for limited defences, the Aircraft
Based Laser may survive the budget cuts that Clinton is expected to announce
before the end of the month. However, the remaining space-based chemical
laser programmes are likely to be axed. If they are cut, the main legacy
of the nearly $6 billion spent on high-energy laser research over the past
decade will be mountains of plans for systems never built, and one big,
partly completed demonstration.