Re: X-ray lasers

Stewart Eyres [email removed] writes:

> If I understand your posting correctly, you are talking about a
> thermally emitting plasma, i.e. one that is just radiating because it is
> trying v. v. hard to cool down.  I don't think this can be used like a
> laser.  The X-rays are emitted in all directions, so you don't get a
> beam.  The radiation isn't coherent, which means all the peaks and
> troughs of the light waves tend to cancel, reducing the average
> intensity.

No, you can't use it as a laser, but you can use it to pump a laser.
Let me explain (see below, the explanation of nuclear-pumped X-ray lasers is
at the bottom, below the general background on laser technology).

> The idea of a laser is that the light is coherent, so that
> the peaks and troughs add.  Also, a laser relies on mirrors to bounce the
> X-rays back and forth through the lasing-medium, so that each photon has
> plenty of chance to create more photons (leading to the all important
> amplification).  The trouble with X-rays is that they are a bugger to
> reflect, so by conventional Traveller wisdom X-ray lasers appear at TL
> 13.

That's for a "conventional" laser, like the crystalline or gas lasers that
you see in a lab nowadays.  However, some pretty interesting work has been
done by the US Department of Defense on unconventional lasers.  One source
of information is the Congressional Research Office's white paper "Directed
Energy Missile Defense in Space", available from the US Government Printing
Office.  This paper presents the basics of the technology in a clear (if
dry) manner, at a level intended to be intelligable by the average
Congressional staffer.

Let me supply some background.  For my purposes right now, I'll call anything
that emits coherent light a "laser", whether or not it actually relies on
the classic Light Amplification by Stimulated Emission of Radiation
principle.  Physicists in the group can skip the rest; it's "popular" level
science (and may well contain some errors, for which I apologise).

Classic laser technology relies on two things: first, a "population
inversion" - that is, there must be more excited ("charged", or ready
to emit a photon) atoms or molecules of the lasing material than in the
ground state ("uncharged").  Second is the principle of stimulated
emission: if a photon passes near an excited atom, the chances are
relatively high that the excited atom will emit an identical
photon (same frequency, same phase, going in the same direction).  This
creates a sort of an "avalanche effect", building to a pulse of coherent
light of respectable size.  For best effect, mirrors are used to make a
"resonant cavity" some exact multiple of the wavelength of the light.  The
photons bounce back and forth, stimulating the emission of more photons,
until the pulse has reached it's peak power.

You can dispense with the resonant cavity if you are willing to put up with
ridiculously low efficiencies, and can provide a relatively long lasing
element that can somehow get pumped all at once.  Of course, such a laser
will emit a beam from both ends (further reducing efficency, unless you have
a use for both beams).  The stimulated emission "avalanche efect" still
happens (resonant cavity or not), but without the mirrors, a longer run is
needed to build up a pulse of equivalent power.  In addition, a significant
number of excited atoms or molecules won't emit a photon, even though the
"avalanche" passes near them.  If a resonant cavity is present, each of
these gets several chances to emit, as the light is reflected back and forth.
Without it, each one of these non-emitters is a lost opportunity, and wasted
pumping energy.

The very first lasers were crystalline affairs.  Impurities in a crystal
(ruby, as I recall) are the lasing element.  The crystal itself formed the
resonant cavity, and the energy to pump the laser was provided by a
flashtube wound around the crystal.

Gas lasers are similar, except that the energy is usually provided
electrically (by sending an electrical current through the gas, like a neon
sign) instead of optically.  Dye lasers use a dye suspended in a liquid (or
other substance, such as Jell-O) as the lasing element, and energy is
supplied by optical pumping (the flashtube again).

Chemical lasers supply the energy for the laser beam by a chemical reaction.
The lasing element can be almost anything, but is typically a gas resulting
from the chemical reaction, flowing into and through a resonant cavity.

Solid-state lasers (ie: laser diodes) don't rely on mirrors, or on a lasing
medium.  They emit coherent light directly, in much the same way that a
LED (Light Emitting Diode) emits non-coherent light (in fact, some LEDs will
emit coherent light if cooled to very low tempretures).  The energy for
these lasers is electrical, and the laser converts it directly to light.

A free electron laser emits coherent light by arranging for an electron beam
to pass down a "wiggler" - a series of magnets that bend the beam into a
series of up-and-down zigzags.  As the electron beam bends, the electrons
emit coherent light.  By arranging the wiggles just right, the light from
each zig and zag adds up to a powerful coherent beam.

In principle the X-ray detonation lasers used by T:TNE missiles are
pretty well understood even today, and such lasers don't seem to be out
ot the realm of possibility (should their production be made a national
priority) at the present level of technology (TL-8 or so).

The general plan calls for a long, relatively narrow rod of laser material
(exactly what material has the right properties, I don't know, and it's
exact composition is probably a secret even if I did).  The rod, and many
like it, are deployed some distance from a nuclear device, arranged in
such a way that they surround it.

When the nuclear device is detonated, the majority of the energy of the
explosion is released as "soft" X-rays.  If in a planetary atmosphere, the
air would immediately absorb these rays (and heat up immensely in the process)
and form the familliar fireball and mushroom cloud.  In space, the "soft"
X-rays travel unimpeded until they are absorbed by the laser rods, charging
them.  The material in the rods is now ready to lase, which it does
spontaneously and immediately.  If everything is arranged right, the laser
rods emit bursts of coherent X-rays (from both ends of the rod) just before
the expanding fireball (made up of vaporized nuclear device, guidance
system, and so on) reaches the laser rods.

You get a single pulse from each rod, at stupendously low efficiency.  BUT,
you've managed to channel some of the energy of the explosion into a
beam (so it won't spread out too much on the way to the target) of coherent
(much more efficient at causing damage) X-rays (which should be absorbed
efficiently by the hull of the target).

Since the energy source is a nuclear explosion, efficiency really isn't a
problem.  The power of the lasers is limited more by how much energy the
laser rods can absorb without vaporizing themselves.  If we assume that
X-ray mirrors are TL-13 technology, the power of the lasers should about
double at that point, simply by putting mirrors at the back of each rod.

> Sorry to put a damper on low-tech X-ray lasers :-)

Just so long as it isn't a nuclear damper, I think things are still OK ;-)


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