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." - By Phillip Gentry


[Contact: Phillip Gentry]

06-Mar-2002