starship-design: Unexpected Explosion Keeps Pakhomov "Waiting"

["L. Parker" <lparker@cacaphony.net>]

Unexpected Explosion Keeps Pakhomov "Waiting"


Huntsville - Mar 6, 2002

Dr. Andrew Pakhomov isn't kidding when he says 50 millionths of a second can
seem like "an eternity" in his lab at The University of Alabama in
Huntsville (UAH).

Most of the time, when he, Dr. Don Gregory and their graduate students
conduct laser propulsion experiments, they measure laser pulses and the
reactions that the lasers cause in from a millionth or a few billionths of a
second.

So 50 whole millionths of a second is, well, it's just an e-t-e-r-n-i-t-y.

That's why Pakhomov and graduate student Shane Thompson were so surprised
when lasers firing into a lead target produced a powerful explosion exactly
50 microseconds after the initial blast of ionized particles went away.

Every time.

"This is really a mysterious thing," says Pakhomov, an assistant physics
professor at UAH. "Now we are writing a detective story."

As part of a NASA-funded research project, UAH's Laser Propulsion Group is
studying what may become a new type of rocket engine. They use powerful
lasers firing pulses that last only tenths of nanoseconds -- tenths of
billionths of a second -- at a wide range of target materials.

When the laser hits, the target absorbs some of the laser's energy.

Electrons fly away from energized atoms, turning them into ions which
explode off the face of the target material.

Voila. Now you have a rocket.

And for every pound of fuel, the laser rockets might be 20 to 40 times more
efficient than the most efficient chemical rockets, Pakhomov says.

Part of the research involved testing different materials to see which gives
the most thrust over the longest time. Lead was one of the materials to be
tested. The experiment used two sensors a few inches apart to measure the
speed at which ionized plasma flies away from the target face.

"But there was a second wave form 50 microseconds after the first," said
Thompson. "That's an eternity to us. And it was hitting both sensors at the
same time, which meant it had to be light instead of plasma."

"We decided it would not be a bad idea to look at the target 50 microseconds
after the shot, so we got a camera," said Pakhomov. "At first we saw only
darkness. It kept Shane here for a long night.

"Then we saw it, a phase explosion. The lead goes through some kind of a
phase transfer."

It takes about one microsecond for the ionized lead plasma to leave the
target surface -- at about 20 kilometers per second. Then, like clockwork,
exactly 50 microseconds later the surface of the lead target explodes with a
burst of particles which emit high energy ultraviolet light.

"But what does the energy do for 50 microseconds?" Pakhomov asks. "Where
does (the energy) go? It seems to disappear. We cannot see where it goes.
What form does it take? It's hard to believe there was anything there."

While they have verified that the explosion is real, the UAH scientists are
just beginning to sort through the theories of why it happens and why it
happens so precisely.

Sorting through this unexpected discovery is a secondary concern. The main
goal is testing different materials under different lengths and strengths of
laser pulses to see what might give the most efficient result.

Earlier experiments in laser propulsion used powerful laser pulses in the
microsecond time range to heat air under a metal shroud to the point that it
exploded like lightning, forming a plasma and a shock wave that pushed
against the shroud. The system works, but it isn't very efficient and it
requires that there be air inside the shroud. This could be a problem once a
spacecraft leaves the atmosphere.

Pakhomov and others realized early that firing the laser at the shroud or
some other metallic target and peeling off, or ablating, the target one
layer of ions at a time could be more efficient than firing it at the air.

"Three billion watts of energy per square centimeter can create a breakdown
of the atoms that are in the air," he said. "In metals, you need only a few
millions of watts per square centimeter, a thousand times less energy. They
already have free electrons, so you need much less energy to create an
ablative breakdown. And that means you can deliver it over longer
distances."

Distance is important if the heavy laser system is on the ground firing at a
target on a spacecraft that is moving away at high speed, which is what you
want a spacecraft to do. Early tests of ablative systems, however, didn't
yield the thrust that NASA needs. Those tests used lasers firing
microsecond-length pulses.

Pakhomov's proposed solution is laser pulse lengths measured in the tenths
of nanoseconds -- or less.

"It takes a couple of picoseconds (trillionths of a second) to form a plasma
cloud," he said. "The problem is that when you have a long pulse, that
plasma reflects the rest of the laser. You create this cloud of highly
ionized material that absorbs your energy."

At this point, Pakhomov and the UAH team think the most efficient system
might be to fire a picosecond laser blast, then wait around twiddling their
thumbs for a microsecond while the plasma gets out of the way, then fire
another blast. Wait and repeat.

"By using short pulses, when one pulse comes the plasma from the previous
pulse is already gone," said Pakhomov. "It also appears that short pulses
are much more efficient in the atmosphere. The pulse is so short it just has
no time to react with the air."

Thompson's newest experiment involves splitting a laser beam and putting
half of the beam in a holding pattern for a few hundreds of picoseconds,
maybe as much as a nanosecond.

"We're probing the reflectance of the second pulse, looking at the kinetics
and how it all develops so we can see when to add more energy," Pakhomov
explained. "We want to know the best time periods to use for the length of
the pulse and between pulses."

Early results of their experiments are so encouraging that Pakhomov thinks
ablative laser propulsion systems might be in service within a few years.
The research at UAH is being supported by a $500,000, two-year NASA research
grant.

The first laser rocket systems might use powerful lasers mounted on the
ground to give spacecraft a boost during launch, when a spacecraft has to
fight through the thickest layer of the atmosphere and overcome inertia. A
similar system aboard the space shuttle might help to boost satellites or
probes out of low Earth orbit.

A third potential use might employ ablative plates and small lasers mounted
aboard a satellite or interplanetary probe as a weight-efficient replacement
for the less efficient, caustic chemical rocket steering and pointing
systems that are used today.

"I believe this field will be developing," Pakhomov said. "I'm expecting to
see some in-space applications soon. With development, I think we could look
for orbiting laser systems. And on the moon, where there is no atmosphere to
deflect the beam, it will be much easier to do such launchers."
BEGIN:VCARD
VERSION:2.1
N:Parker;L.
FN:L. Clayton Parker (E-mail)
ORG:SandCastle Contractors, Inc.
TEL;WORK;VOICE:(850) 650-6588
TEL;HOME;VOICE:(850) 654-4773
TEL;CELL;VOICE:(850) 585-5502
TEL;CAR;VOICE:(850) 585-5504
TEL;WORK;FAX:(850) 650-6588
ADR;WORK:;;P.O. Box 1762;Destin;FL;32540-1762
LABEL;WORK;ENCODING=QUOTED-PRINTABLE:P.O. Box 1762=0D=0ADestin, FL 32540-1762
EMAIL;PREF;INTERNET:lparker@cacaphony.net
REV:20011020T212607Z
END:VCARD