SCIENCE OBSERVER
To Boldly Go (Again)
David Schneider
Transport yourself back to the early 1960s, before the now-famous
television series Star Trek first aired. At that time, only
wild-eyed visionaries would have dared imagine that people of the
23rd century would be taking in the world on gargantuan flat-panel
video screens, talking to one another across the width of the planet
using wireless, flip-top communication devices and instantly
accessing obscure bits of information by making simple queries to
their computers. Yet today these scenes are commonplace. So it is
fitting that two new high-tech devices also have eerie similarities
with fictional technology first presented on that venerable
science-fiction series. Remember how Enterprise landing
parties always carried "phasers," weapons that could
conveniently be set on "stun" when less-than-deadly force
was needed? And do you recall the invisible doors that mysteriously
kept air in the ship's shuttle bay from whooshing rapidly out into
space? Such marvels are now reality—or almost.
The phaser look-alikes expected to be appearing soon are similar in
principle to equipment that law-enforcement officers have long been
able to purchase from an Arizona company named, appropriately
enough, Taser International. Taser guns fire two small darts, each
trailing fine wires. After these projectiles strike their target,
this nonlethal weapon uses the newly established electrical conduits
to give the person caught in the crosshairs an electric shock, which
disables without killing. The newer generation of stun weapons will
dispense with the clumsy darts and wires entirely. In their place,
they will use lasers to generate conductive channels of ionized
air—raylike beams of plasma—through which an electric
shock can be administered.
This general strategy, having a laser induce ionization and thus
allow for the conduction of electricity, has for decades been the
Holy Grail of scientists and engineers seeking to control lightning.
They have longed to be able to generate a conductive path between a
thundercloud and the ground, which could harmlessly drain off the
buildup of charge that otherwise might strike a sensitive facility,
say an electric-power substation or an airport control tower. A less
ambitious goal is to create a shorter "laser lightning
rod" that could be deployed at the flip of a switch. The main
hitch early on was that ordinary lasers do too good a job at
inducing ionization, which then makes the air opaque to the light
beam. But more promising results have been achieved using lasers
that emit extremely short pulses. Last December, for example, Roland
Ackermann of Université Lyon 1 in France and 15 colleagues
reported in the journal Applied Physics Letters their
success in ionizing a path in air that served to channel an
artificial electric discharge. This recent experiment is notable
because it was carried out under conditions of simulated rain. The
length of the laboratory discharge was only about a meter, so this
technology has a ways to go before it can be used to control real
lightning bolts. But the same principles are now being apply in a
phaser-like way.
Just as Ackermann's paper was being published, an Arizona company
named Ionatron demonstrated the use of laser-guided electric
discharges in something it calls a "portal denial system,"
which can be set up in a corridor and switched on to prevent
intruders from passing through. Three beams in this system create a
virtual electric fence that spans the width of a hallway. Steve
McCahon, Ionatron's executive vice president for technology and
engineering, explains that the company's system demonstrated
nonlethal levels of deterrence but "that doesn't mean you
couldn't turn it up."
Are guns next? According to the boldly written claim on its Web
site, "Ionatron intends to use our compact, non-lethal LIPC
[laser-induced plasma-channel] technology to replace guns as the
weapon of choice in close-range defense." McCahon confirms that
"the thrust is extending range." Exactly how much range
Ionatron has been able to achieve with such a weapon is being kept
secret. And it's unclear whether the sophisticated lasers needed
could ever be made very small. Still, it seems reasonable to
anticipate that in the not-so-distant future, military or
law-enforcement officers might be caught uttering the phrase
"phasers on stun" in all seriousness.
Similar physical principles are behind a second Star
Trek-like technology now coming into use, something called the
"plasma window," which is the brainchild of Ady
Hershcovitch, a physicist at Brookhaven National Laboratory.
Hershcovitch conceived of the plasma window to serve in
electron-beam welding, a technique used to fashion metal welds that
are narrower and deeper than what can be accomplished with
conventional tools.
The chief drawback of this technique is that the electrons used for
welding must be accelerated in a vacuum (just like, for example, the
electrons that light up the front of a television picture tube).
Hence the objects being welded together must normally be placed
within a sealed chamber from which the air has been extracted. With
that constraint, one cannot make welds to, say, the deck of a
battleship. Even for small work pieces, pumping down the vacuum
chamber each time an object is inserted is time-consuming, making
this form of welding rather costly.
To get around this difficulty, some have tried a variation of
electron-beam welding that has the electrons accelerated in vacuum
but the welding done at atmospheric pressure. Such systems rely on
bulky, energy-hungry vacuum pumps to maintain the pressure
differential between the source of electrons and the work piece. So
they are awkward and costly to operate. What is more, the electron
beam has a troubling tendency to spread out once it passes into the
air, negating the fundamental advantage of electron-beam welding in
the first place. Last May, Hershcovitch and colleagues at Acceleron,
a company in Connecticut licensing his invention, described in the
journal Physics of Plasmas how to sidestep these problems,
making electron-beam welding that much more practical.
The trick is to send the electrons out of the welder through a
window that is made up of nothing more than an electric discharge
channeled through a length of ionized gas—that is, a plasma.
The temperature of the plasma is searing (about 15,000 kelvins), so
it can counterbalance atmospheric pressure even though its density
is only two percent of normal air. The low-density plasma offers
little resistance to speeding electrons passing through it, making
it the perfect window for an electron-beam welder.
Hershcovitch had this idea more than a decade ago, but only recently
did he realize that a plasma window could also counteract the
nagging tendency for the electron beam to spread out as it moves
through the air. The reason is that the electrical current used to
heat the plasma sets up a sizable magnetic field, one that exerts a
radial force on the electrons in the beam. As chance would have it,
Hershcovitch initially had the polarity of this current set in such
a way that the magnetic field inside the plasma window made the
electron beam spread out more than usual. But he soon realized what
was going on. After asking himself, "How stupid can I
be?," Hershcovitch reversed the polarity of the current and was
delighted to discover that his plasma window acted to focus the beam
tightly, thus circumventing the second roadblock to electron-beam
welding in air. Hershcovitch notes modestly that this is a good
example of a common principle: "It's better to be lucky than to
be smart."
Phaser-like weapons and ethereal vacuum windows will surely not be
the last Star Trek technologies to come to life. What's
next? Myself, I'm looking forward to those anti-gravity handles:
Hand trucks are such a pain to use on stairs.—David Schneider