X-ray Dreams

Scientists of all stripes yearn for the incredibly powerful beacon of an X-ray laser. Charles Rhodes wants one so badly that he's willing to reinvent physics to get it.

By Will Hively|Saturday, July 01, 1995
The day the xenon exploded with X-rays, Charles Rhodes missed all the fun. In fact, he nearly called off the show. Rhodes, director of the Laboratory for Atomic, Molecular, and Radiation Physics at the University of Illinois at Chicago, was expecting a fizzle, not fireworks. It was Armon McPherson who had a hunch the xenon was poised to do something strange. McPherson, who actually runs most of the experiments, wanted to go ahead and zap the xenon with a trillion-watt laser. Rhodes thought the X-ray response would be feeble and wanted to wait until they had a more sensitive detector to pick it up. Charlie told me I’d be wasting my time, McPherson recalls. After Rhodes went home, McPherson went ahead and touched off the xenon.

Both he and Rhodes will be living with the fallout for a good many years, and they couldn’t be more delighted. The torrents of X-rays McPherson unleashed, Rhodes is now saying, may lead to the brightest source of light ever produced at any wavelength--a new kind of X-ray laser. Used in microscopes, this light would give biologists a new mode of seeing. Conventional microscopes cannot see anything smaller than the wavelength of visible light, which is a thousand times longer than that of X-rays. Electron microscopes approach X-rays in their potential to distinguish detail, but they look only at tissue stained with a metal dye and mounted, dead, on a slide. With an X-ray laser microscope, biologists could penetrate living cells. They could take holographic 3-D snapshots of structures suspended in the cell’s plasma, with details resolved to a billionth of a meter. They might even zoom down to the scale of molecules, pick out some bit of DNA, and find out how it orchestrates the chemistry of life. You wouldn’t worry about what you’d look at initially, says Rhodes. You’d just look, and you’d see something new.

Biology is only one application. X-ray lasers might also etch electronic circuits a thousand times smaller than those of today, turning a pocket calculator into a supercomputer. An X-ray beam as a communications carrier could hold a thousand bits of data in the space one bit now occupies on a conventional laser beam wending its way down an optical fiber. Because each X-ray photon packs a thousand times more energy than a photon of visible light, if you put X-ray photons in the laser beams used now for welding, cutting, and drilling, they would become powerful, penetrating weapons.

When a practical X-ray laser hits the market, says Jack Davis, a physicist at the U.S. Naval Research Laboratory, it truly is going to revolutionize everything. Davis says when, not if. The only question in his mind is who will get there first. Teams in the United States, Great Britain, France, Germany, Russia, China, and Japan have been tinkering for years with various schemes.

X-ray lasers already exist, but they are not yet practical. They come in two models. The first one was, in its heyday, the key Star Wars weapon. In 1982 Edward Teller, director emeritus of Lawrence Livermore National Laboratory in California, proposed setting off atomic bombs in space to power orbiting X-ray lasers. They would go BOOM zappa, BOOM zappa, BOOM zappa. . . . They would fry holes in approaching nuclear warheads, then themselves vaporize from the heat of their triggering bombs. Researchers actually fired up bomb-powered X-ray lasers during underground nuclear tests in the 1980s. Stephen Libby, the program’s last manager at Livermore, says only that these tests produced a robust X-ray beam, and that’s all I can tell you. Whether these lasers still exist, nobody is saying. It’s probably safe to assume that they were not reusable.

In 1984 another team at Livermore, headed by Dennis Matthews, demonstrated a smaller, laboratory X-ray laser. Zappa Jr. did not start with a thermonuclear boom, but it required the world’s largest non-X-ray laser, which occupies an entire building, to act as its spark plug. The X- ray laser at Livermore today is still of this vintage. Though reusable, it is much too big and expensive to be called practical. Several other groups, in the United States and elsewhere, have built reasonably small tabletop devices that operate at wavelengths two, three, even four times longer than the dictionary definition of X-rays. These soft X-ray lasers may be practical, but they are mere pretenders--they are simply not up to the kinds of jobs that a true X-ray laser could handle.

Now Rhodes believes he is on the verge of inventing an X-ray laser that produces extremely short--that is, hard--X-rays with far less power than Zappa Jr. And the way it works, he says, is so fundamentally different from previous methods that it requires a new kind of physics to explain it. With his total commitment to new techniques, Rhodes is pulling away from his rivals--or perhaps they are pulling away from him. Despite his claims, Rhodes is definitely on the fringe of the X-ray laser community. Whether he’s at the front or the back depends on whom you ask. Joe Nilsen, a physicist at Livermore, says, There is no way Charles Rhodes is on the threshold of an X-ray laser. Davis, on the other hand, sees Rhodes as leading the pack. The man is a pacesetter, he says. He’s pushing the envelope. He takes very high risks. He’s a rare individual who knows what needs to be done.

Rhodes, in person, lives up to his heroic billing. He has charisma; his staff adores him. When he lowers his voice, he rumbles like John Wayne, and the undertones say Get to the point. At 56, he looks nimble, lean, athletic. Fearless too. He once chased a mugger who had robbed an old woman near his Chicago home. You get the feeling he will pursue an X-ray laser with the same determination. My opponents, he says, wish they had a weaker opponent.

Rhodes’s shoot-from-the-hip style is more than merely colorful; it expresses his attitude toward scientific research. He seems to think that most scientists waste time on trivial facts. If you ask yourself who gets ahead in the world, he says, it’s the guys who can make the right decisions with just a few percent of the information. He’ll take 1 percent more or less--he’s not fussy that way--but he is very particular about how he stores it. No whining, pinwheeling computer hogs space on his desk. Rhodes is proudly computer illiterate, an old-fashioned pencil-and-paper physicist. All his work exists as old-fashioned hard copy, stuffed into rows of metal filing cabinets.

On the day the xenon exploded, Rhodes was being uncharacteristically cautious. He had been groping toward an X-ray laser for more than a decade by following his instincts, relying in equal portions on experiment, hard-nosed analysis, and luck, with theory almost an afterthought. His goal was simple: before making an X-ray laser, he first needed to find a material that would emit copious X-rays when bombarded with a beam from a conventional laser. His experiments with xenon gas, as with the other materials he had tested, were proceeding with no breakthrough in sight until the day Rhodes made a leap of intuition. Why not let the xenon condense first into tiny droplets--clusters of a few dozen atoms hanging loosely together--before zapping them with the laser? The closely spaced atoms, he thought, might somehow stimulate one another to emit more light--both X-ray and visible--than they would otherwise.

But still, he didn’t put that much stock in this idea. According to mainstream physics, xenon clusters shouldn’t emit any more X-rays than individual xenon atoms should. The theory behind this conclusion is ensconced in thick reference books containing data compiled over decades of research. It’s pretty well understood by now, growls Rhodes. Nonetheless, he thought the theory might be wrong. Rhodes suspected that he and McPherson could indeed get more X-rays out of clusters--but only slightly more, not enough for their crude equipment to detect. He thought there was no point in running the experiment until they had improved their techniques of measuring the radiation.

If going by the book meant little to Rhodes, it meant even less to McPherson. He had arrived at Rhodes’s lab a decade before on a one-year appointment, and he never left. Rhodes saw right away that McPherson had a knack for making things work. Even in his spare time, he unwinds with challenging hobbies. For a while he cut gemstones. Now he grows prizewinning orchids. From seeds other people have trouble growing, Rhodes says, he can get almost 100 percent germination. Like Rhodes, McPherson makes decisions by the seat of his pants. I do things a lot of times on instinct, he admits with a shrug. It’s hard to give scientific arguments sometimes as to why I do things in the lab.

So, early on the day the xenon exploded, McPherson began zapping the xenon clusters with a laser, and on the video monitor he saw flashes of light almost too quick to register. The xenon was absorbing energy from the laser pulse and firing some of it back. Both McPherson and Rhodes had expected that to happen, but McPherson thought the xenon was generating far more light than it should have--and he had a hunch it might also be emitting lots of X-rays. I told Charlie, this thing is radiating like a solid, McPherson remembers. When stimulated by a laser, solids shine a thousand times brighter than gases. McPherson suggested trying to capture the flashes on X-ray-sensitive film. Peering over his shoulder at the video monitor, Rhodes argued that he would have to keep shooting all night and all the next day to capture a mere trace of X-rays. You won’t see anything, Rhodes snapped.

Disregarding Rhodes’s skepticism, McPherson decided to test the xenon anyway. That evening he flicked a switch, hit some xenon with a shot from the laser, flicked again, hit more xenon. Half an hour later, he guessed the film was exposed well enough; he developed it and hung it up to dry.

The next day Rhodes found his colleague unusually excited. Rhodes scratched his head. McPherson, he suspected, being a genius at getting measurements, had probably found some laboratory trick to coax a few faint X-rays onto the film. He had worked similar magic in the past. But when Rhodes saw the X-ray spectrum, he was, says McPherson, flabbergasted. According to everything they both knew about physics, the film should have been almost perfectly clear, yet here was McPherson holding up a piece of film black from exposure to X-rays. Clearly, says Rhodes, the xenon clusters floating in this vapor were radiating one devil of a lot stronger than they should have been. They had popped off like X-ray supernovas. That meant, says Rhodes, there was something fundamentally new here.

Okay, new physics--Rhodes let it pass. Someday he might try to work out the theory behind it. At that moment he was focused on a narrow goal: his quest to build a record-shattering X-ray laser. He had been at it since 1980. Now, in June 1993, the X-rays he needed had finally, spectacularly, appeared.

Rhodes was soon busy preparing papers and giving talks. Several groups in Europe were already probing clusters of xenon, argon, and other rare gases, and the researchers there were excited by Rhodes’s results. But his rivals at Livermore were less enthusiastic, to say the least. Dennis Matthews, who still heads the X-ray laser program there, first learned of the findings in an August 25, 1994, article in the New York Times, which said Rhodes had discovered a way to produce X-ray laser pulses of almost incredible intensity. To Matthews, none of it made much sense. Later, he says, I got this manuscript from Charlie Rhodes that said that they were looking at xenon clusters. That was a nice scientific paper and showed some good X-ray emission, but there was no mention of lasing.

Matthews had a point. Rhodes had indeed found a way to produce bursts of intense X-rays, but they were shooting off in all directions. Laser light has to be more than merely intense. It must also be coherent, of only one wavelength, and focused in a beam so tight it barely diverges. To make a true laser, he would need to find a way to amplify his X-rays and make them shine in a coherent beam--no trivial task. Otherwise he would have found little more than a very bright flashbulb.

No one knows the problems Rhodes faces better than Dennis Matthews. Pleasant, easygoing, statesmanlike in appearance--you could mint his face on a coin--Matthews is the father of the laboratory X-ray laser, the one you can actually use now. Our X-ray lasers have always been very conventional, he says. They operate just like optical lasers except that they’re in the X-ray wavelength regime.

The conventional approach to lasers has some distinct advantages- -not the least of which is that by now physicists have more than 30 years of experience with such lasers and need no new physics to explain how they work. The magic begins with excited atoms. If you zap an atom with a pulse of energy, one or more electrons will most likely absorb some of that energy and jump to a higher orbit, or shell, farther away from the nucleus. The more energy you pour into an atom, the higher its electrons jump. When these excited electrons fall back into lower shells, pulled by the positive charge of the atom’s nucleus, they release energy in the form of radio waves, light waves, or shorter waves like X-rays, depending on where they fall. If you want electrons to spit out X-rays, you need to make sure they fall into one of the innermost shells.

One way--the usual way, Matthews’s way--of setting up a fall to an inner shell is to indiscriminately clear the atomic decks and remove lots of electrons. This produces an ion with a very strong positive charge. If you ionize the atoms by heating them, as Matthews does, outer electrons leave first, inner ones last. The nucleus then reels them back in. The drawback is that you need vast amounts of heat--stellar temperatures of around 10 million degrees--to boil away enough electrons to reach those in the innermost layers, where X-rays are made. For this you need an atomic bomb or an incredibly powerful laser.

Once you manage to generate enough energy to strip an atom of its electrons, you still have to amplify the X-rays. The trick here is to steep the atoms in energy long enough to let nature take its course. When one atom emits a photon of light at an X-ray wavelength, there’s a good chance that it will strike another atom, and if it does so, the laws of quantum mechanics dictate that it will stimulate an electron to decay to the same inner shell. As the vacancy is filled, another X-ray photon shoots out, carrying the process forward. This stimulated light, doubling and redoubling in brightness faster than any competing wavelength, soon swamps the medium, becoming a thousand, a million, a billion times stronger than all the others.

It is not enough, however, simply to let the X-ray photons fly out all over the place. You must amplify them in such a way that they all wind up going in the same direction. With long-wavelength lasers, you stick a mirror at each end of the cavity where the atoms are excited, causing the beam to reflect back and forth, amplifying as it goes. Any light the atoms shoot off in other directions escapes without further ado, while the beam trapped between mirrors keeps getting brighter. All this happens very fast. Within a few billionths of a second, more or less, you get a narrow, bright beam shining through one of the mirrors, which you thoughtfully made semitransparent. Presto, you have a laser.

With X-rays, the last step in this scheme gets ugly in a hurry-- in one picosecond, a mere trillionth of a second. That’s because, in less than that amount of time, most atoms that have been stripped enough to make X-rays decay: their electrons, ripped from the powerful bonds that hold them close to the atom’s nucleus, spontaneously fall back into the lower shells. In a trillionth of a second, light travels less than a millimeter. A beam returning from a mirror would find most atoms ahead of it already decayed, their electrons settled back into their routine orbits, X-ray emission no longer possible. To keep amplifying a beam, you need to keep the atoms ahead of it excited. So you need to keep pumping energy into the atoms, to keep them popping at 10 million degrees. To shorten an X-ray laser’s wavelength, you need even larger amounts of energy--much larger. Using conventional techniques, to go from a wavelength of 10 nanometers (10 billionths of a meter) to 1 nanometer, you need to deliver 1,000 times more energy 10,000 times more quickly. That’s why the soft X-ray wanna-bes, with wavelengths above, say, 20 nanometers, are not almost X-ray lasers.

If you’re designing an X-ray laser, it’s easy to get into a strange frame of mind. All of a sudden the speed of light seems slow. You’re counting the picoseconds it takes to nurse your X-ray beam along, waiting for it to grow bright enough so you can turn off the power--a billion watts, give or take a few zeros. You’re lucky if your X-rays even make it to a mirror. Or maybe not so lucky, because then you’ll need to invent a new kind of mirror. You wanted X-rays, remember, because they penetrate. Now you’re asking them to reflect. Even the troubles seem to amplify.

Matthews knows these problems as well as anyone because his group at Livermore has solved every single one. As a matter of fact, says Matthews, we have built mirrors and have actually bounced X-rays back and forth through the amplifier. Unfortunately, they don’t last very long. Matthews built his mirrors out of alternating layers of silicon and molybdenum, each the thickness of half a desired X-ray wavelength. They reflect the X-rays for a brief instant before debris scattered by the foils, which explode under the intense heat from the laser beam, destroys them.

The laser producing that beam is the most powerful laser in the world, and it goes by the name of Nova. It occupies an entire building, which sits at the center of a 600-acre complex that is dotted with palm trees, crisscrossed by roads, and laced with curving bike paths. Nova spends most of its time soaking up electricity, storing the energy in huge banks of capacitors. Once every hour or so, it comes to life. For a billionth of a second, it fires off as much energy (100,000 joules) in one pulse of green light as the entire United States consumes in that instant. Nova can concentrate that energy into a single beam or divide it among as many as ten, which race off through white pipes toward steel target chambers the size of closets scattered through the building. After each shot, researchers collect their targets, analyze their data, adjust their computer models, and plan new experiments, which queue up to wait for another jolt.

The X-ray targets are mounted squares of foil a foot or two across, made of silver, gold, and many other metals. When a laser pulse from Nova hits one of them, the foil explodes with X-rays. In practice, not one but two laser beams hit the target, and they are focused onto a line rather than a spot. For a billionth of a second, Nova pours on the heat, keeping atoms excited all along that line on the foil. Each atom shoots X- rays in all directions, but only those X-rays that travel along the line bathed in Nova’s beam succeed in finding atoms primed to give off additional X-rays. As the foil explodes and the Nova pulse fades, two X-ray laser beams shoot out in opposite directions.

Back in 1984 Matthews’s laser produced soft X-rays, at a wavelength of about 20 nanometers. In 1987 his group made the first X-ray laser holograms using hard, 4.5-nanometer X-rays. (Once you have the X- rays, the technique for making images is much the same as for optical microscopes: a spherical mirror focuses the light, which passes through the sample and then falls onto a light-sensitive detector; holograms require the addition of a reference beam.) Matthews’s X-ray images reveal details as small as 50 nanometers, which is much larger than molecule size but ten times the resolution of optical microscopes. These X-rays are not good enough for Rhodes, who wants to use extremely short X-rays--about one-tenth of a nanometer--to resolve individual molecules. Matthews, however, believes that his more modest X-rays are sufficient for seeing most of the things scientists want to see. Any shorter, he thinks, and the X-rays might penetrate too well. After all, bones show up in X-ray pictures only because some of the X-rays get blocked. We haven’t been able to figure out, Matthews says, what you could do with very short wavelength X-rays.

At any rate, physicists at Livermore are not likely to generate such X-rays anytime soon. In theory, using a very large power source, Matthews thinks it’s possible to get X-ray wavelengths as short as 1 nanometer. Shorter than that, I don’t know how to do it.

But the Livermore scientists acknowledge a dilemma: the laser they need to ionize the atoms--that is, the pumping laser--is too big and too expensive. Nobody else has yet been able to afford to build a similar device. If the X-ray laser can’t be reproduced economically, scientists will have to continue making the pilgrimage to Livermore after waiting months to get an appointment.

As an alternative, Matthews is trying to wean at least some X-ray lasing from Nova. He is raising money to design and build a commercial X- ray laser small enough to fit in one room. Pumping lasers available now, he says, might be adequate for a modest X-ray laser powerful enough to be useful in the laboratory.

The room-size laser that Matthews envisions sounds a lot like the prototype Rhodes and McPherson are pursuing. There is, however, one important difference: Rhodes has found a far more efficient method of producing the X-rays than Livermore’s brute-force approach. Not only can he produce X-rays more than ten times shorter in wavelength than Livermore’s best, but he can trigger them with a mere one-thousandth the energy of anything Matthews foresees. Indeed, Rhodes finds it ludicrous to calculate, even with pencil and paper, how much more efficient his X-ray laser will be than anything possible with conventional techniques. Provided, of course, he can finish the job.

As Rhodes darts back and forth between theory and experiment--not far, in his lab--he passes a magnificent piece of equipment. Just as Nova dominates Livermore, this instrument dominates everything he does. It’s not a laser and it’s not even large, but it explains the path he has taken, and why he is so eager to invent new techniques. It is an X-ray microscope. It makes three-dimensional holographic images that can be stored in a computer and viewed on a screen. All he needs to start using it is a practical, short-wavelength X-ray laser.

What happened, says Rhodes, was that the cart got ahead of the horse. Way ahead. In the early 1980s Rhodes formed a company to develop the microscope and filed for a patent, which was granted in 1990. All he needed to make it work was a pulse of X-rays that could penetrate deeply, capture a bright, detailed image, and get out before molecules began wiggling from the heat. The scenario worked out like this: a pulse of X- rays would roar through a cell in one very short flash, lasting less than a trillionth of a second. At the end of that pulse, the molecules it touched would already be moving fast enough to blur their image. The X-rays would hit a detector; an image of the living chemistry that ripples through life would eventually show up on-screen. An eternity would pass. Ten-trillionths of a second or so after first being hit, the cell would vaporize.

Although Livermore was firing 4.5-nanometer X-ray laser beams by 1987, they were of no use to Rhodes. The wavelength was too long to resolve molecules, and the relatively dim pulse was so long that before the picture was taken, it would fry the cells and other living matter that Rhodes wanted to photograph. Rhodes needed a quicker, brighter burst. It was clear, he says, that we had to invent something new. To produce such a pulse of X-rays, he figured he would need to find a way to excite some material with roughly one watt per atom. That’s a lot of energy. It would require an impossibly large Nova-style pumping laser unless he could figure out some way of getting leverage. He couldn’t just belt the electrons; he would have to control them, choreograph them. Very high power, very fine control--an unlikely combination. You need the strength of a Superman and the grace of a Baryshnikov, says Rhodes. And that’s not easy to do.

Superman came first. In the mid-1980s, a new kind of short-pulse ultraviolet laser named Prometheus gave Rhodes the pumping power he needed.

Once every second or so, when it’s up and running, Prometheus fires a trillion-watt pulse of light. That level of power is difficult to sustain. Each pulse, in fact, lasts only about a trillionth of a second. So the total energy each pulse carries--a trillion divided by a trillion-- amounts to about one joule, which is not much. An ordinary 100-watt lightbulb radiates a joule every hundredth of a second. The difference between Prometheus and a lightbulb is this: a lightbulb spreads energy; the laser compresses it. If you gathered up one joule of a lightbulb’s radiation--after a hundredth of a second, it’s a ball of light the diameter of North America--and squeezed it down to less than a cubic millimeter, you’d have one zap from Prometheus. When one of those zaps hits a target, the energy it carries, focused to a pinpoint, works out to roughly one watt per atom. That’s a high number, says Rhodes. Another way to get one watt per atom would be to funnel the electricity consumed throughout the United States in a year through the filament of a single lightbulb. Anything caught in such a mighty surge of power--tungsten, xenon, anything at all-- would instantly start shining like matter in a star.

Unlike Nova, which basically puts the electrons under a long, slow boil, Prometheus applies a short, powerful punch. With his new laser, Rhodes could for the first time apply more force to electrons in the atoms than the nucleus could oppose. The electron looks around, says Rhodes, and what does he see? He sees this huge gorilla, all of a sudden, and it’s much stronger than anything else he sees. Even so, according to standard theory, the numbers didn’t add up to much. One mighty though little zap from Prometheus, being so little, hits relatively few atoms in a vapor; being so mighty, it triggers a few X-rays. The whole trick, says Rhodes, is to use jujitsu.

Jujitsu physics is how Rhodes describes what happens when he zaps his beloved xenon clusters with a pulse from Prometheus and the clusters respond by sending off X-rays like little supernovas. As usual, electrons do the work.

All the elements whose atoms form clusters are chemically boring. Chemists call them the noble gases because they are mostly inert, meaning they shun other atoms and will not bond to form molecules. Even when you condense the atoms from a noble gas, forcing them close together in microscopic droplets, they do not form molecules; they just cluster together in gobs. The outermost shell of each atom is full of electrons, as full as it can be. Being somewhat far from the nucleus, these outer electrons have a wide latitude. So in a cluster of atoms, you have gobs of outer electrons just milling around, waiting for something to do.

Somehow, in clusters, Rhodes believes, the outer electrons all cooperate to absorb energy from the pumping laser. Somehow, they do this more efficiently than they could in isolated atoms. In the language of quantum mechanics, electrons couple with photons. If you make a bold assumption--that electrons can somehow combine forces--then a giant, clusterwide pseudoelectron would indeed couple like flypaper with a swarm of photons. Unless we think of something else, Rhodes says, we’re sort of stuck with this, at least at the moment.

What happens when a cluster-electron, or whatever it is, soaks up more energy than it should? The standard answer is that the energized atoms are like pots of boiling electrons, which then leap from their atoms, outer ones first. Rhodes, however, thinks that a giant cluster-electron does not boil off. Instead, it sits there like a pressure cooker, soaking up a thousand times more energy than theory says it should. Somehow, this energy then goes straight to the innermost electrons, causing them to start ramming each other, popping up and down, and even jumping from deep inner shells right off the atoms. Exactly how this happens, Rhodes can’t say for certain. Another way to look at it, though, is to think of the ultraviolet light from Prometheus as a series of electromagnetic waves. They wash over the atoms like a tidal wave and make the outer electrons bob violently up and down, knocking out the occasional electron from an inner shell.

The resulting atoms make very strange ions. At first Rhodes called them hollow atoms. He now calls them Swiss-cheese atoms, because electrons might pop out from anywhere inside. Whatever you call them, removing electrons from the inside first, if it really happens, has two big advantages. First, you save energy. You don’t need to blast away so many electrons just to get down to the inner, X-ray-making shells as you do with brute-force ionization. Second, you save time. You don’t have to boil electrons completely away, then wait for their return.

Jujitsu physics does not defy physical laws. It does not eliminate the need for violence; it just gives Rhodes more leverage. Roughly what this says, Rhodes summarizes, is that if you make the molecule right, it goes boom with X-rays. He still needs to hit the clusters very hard, but then he can stand back and let nature do the rest.

Most theorists find this theory too much to swallow. Charlie’s Swiss-cheese view of hollow atoms is very controversial, says Jack Davis. People have taken exception, not with the results but with the interpretation. They don’t disbelieve the results he gets in the laboratory. That’s what nature gave him. Part of the problem is that not many researchers have the equipment to test these ideas, and the few who do have idiosyncratic lasers, which produce idiosyncratic results. Duplicating someone else’s trillion-watt pulse of light is a difficult proposition at best. One group in England zapped xenon clusters but got textbook results. Another group tried neon and got magnificent X-ray bursts.

Rhodes seems to enjoy the scramble. His theory’s reception, he says, has all the earmarks of something that’s really new. At one talk he gave in Berlin, a leading physicist listened until the end. Finally he just said, Baloney. That makes Rhodes laugh--which he does explosively. There’s always a huge amount of skepticism, he says. You gotta drive it down their throats.

In two key papers published last year, Rhodes applied his theory not only to his own data but also to six other examples of strong radiation, from experiments others had done, that no one had yet explained. I took those six pieces of data, he says, and I found that in every case, without touching anything, everything made sense. His inside-out theory gave numbers matching the experimental results. It was astonishing. A referee who reviewed one of the manuscripts, however, said that he could explain half the cases, right off the bat, with established theory. I can explain everything, Rhodes shot back. Don’t I get more credit?

Rhodes was unconcerned with the popularity of his theory of xenon clusters. He had too much else to worry about--namely, answering those critics who say he has produced little more than an X-ray lightbulb. The Nova laser at Livermore disciplines its X-rays into a coherent beam by zapping its target along a line and letting the X-rays amplify as they work their way down the line. Prometheus doesn’t have enough power to focus on an entire line. It gets a lot of bang for its joule of energy by concentrating all this power to a pinpoint. X-rays explode from this point in every direction. How, Rhodes wondered, could he focus and amplify the X- rays emanating from this tiny point?

Conventionally, says Rhodes, the way I do that is I make a waveguide. A waveguide is some kind of tube or pipe made of reflective material that conducts light or some other electromagnetic wave. Well, at these power levels, it blows up. We did it to check it out. We used glass capillaries. The inside, of course, was just completely blown away. Rhodes launches another explosive laugh. Totally failed. It was fun, though, just to see what would happen; no one expected the waveguides to work. The solution in the end, he says, seems ridiculously simple. You make your own waveguide in the plasma. That is, we’re now telling the electrons what to do. Tell ’em to make a waveguide that keeps the light focused as it moves through the plasma. And we came up with a solution that had beautiful physics in it--a solution worthy of Baryshnikov.

At first glance, choreographing any kind of motion in a plasma would seem hopeless. Usually ions and electrons whiz around at random. But that’s because a plasma is usually hot--you rip electrons from atoms by pouring in heat. In a very short pulse, such as the ones Prometheus delivers, the electrons have no time to get hot. At these intensities, Rhodes says, a lot of the electrons are ripped off, but you get weird conditions. It’s a very peculiar plasma. Johndale Solem, a theorist from Los Alamos National Laboratory, joined Rhodes in 1987 for a year to figure out how to organize electrons in this cold plasma. He developed a mathematical model showing that channels could conceivably form in the plasma to guide X-rays. In his model, as the pumping laser pulse passes through, it leaves in its wake a spine of ions. Given the right conditions, electrons that have escaped from these ions will form a tube of negative charge all around the spine. This tube will confine the X-rays by reflecting them, in the same way that the walls of a glass fiber confine optical beams.

All this, of course, was just theory. And there was still a catch. Solem’s calculations showed only that given the right conditions, it was theoretically possible to form a stable tube of electrons in the plasma. They didn’t give a clue as to how to achieve those conditions. Before Rhodes could run experiments, he still needed some technique to produce the channel in the first place. To do this, he needed to create another mathematical model, this one showing what would happen from initial conditions--before his trillion-watt spot of light hit the xenon clusters-- to the moment when the channel was formed. Once it was formed, Solem’s calculations showed, everything would work out fine. But how to get from A to B? This was a moment of great humility for the pencil-and-paper physicist. We’d done all the other stuff analytically, Rhodes says, without a computer. This problem was very different. Only a computer could keep track of what was happening in the plasma from one instant to the next. It would mean crunching millions of numbers.

Rhodes started looking for someone to model this plasma, and fretting about the effort it would take to arrange for time on a supercomputer. People in the United States either weren’t interested or said they were but never followed up. In Moscow, however, Rhodes found theorists with time on their hands and computers that were less than super. Rhodes, of course, liked their style--the way they used efficient codes to make up for less powerful computers. He and the clever Russians started a formal collaboration. They did the calculations, says Rhodes, made a visit over here, and showed me the results--the picture was basically a plot. The day they arrived was as great a moment for Rhodes as the day the xenon exploded.

I was standing in the doorway, Rhodes says. He saw a computer graphic lying on a desk, and immediately, he says, it was absolutely, totally clear the thing would work, and why. On the graph, Rhodes saw a huge spike of energy roaring straight down a channel in the plasma. He already knew such a channel could exist. What he saw now was that the channel would form automatically from initial conditions he could actually create in the plasma with his trillion-watt spot of light. He saw that he could go from A to B. We did the experiments, he says, and we made a match--right on the nose. It turns out to be an astonishingly stable process, and those calculations were absolutely essential for us to understand the channeling. Recent photographs do show channels and bright beams of X-rays. They propagate straight through the plasma, in a line up to 70 times longer than the space first excited by Prometheus. In other words, says Rhodes, gangbusters! Another explosive laugh. It’s a huge, bright streak of X-rays.

Recently Rhodes estimated how bright his beam was and how quickly it achieved peak power. The numbers were thermonuclear. For one-tenth of one-trillionth of a second, these little clusters were radiating X-rays as brightly as a one-megaton bomb.

All Rhodes needs now to achieve his goal of an X-ray laser is to show that he can amplify the beam. Early calculations look promising. His clusters emit X-rays so promptly that they tread on the tail of the pulse from Prometheus. They hit excited atoms before those atoms have time to decay. His beam is actually a very short streak of light, less than a millimeter long, with the pumping pulse at the front and X-rays tagging along at the back. Theoretically, as the X-ray beam travels through the channel it should get stronger and stronger. So far Rhodes has not verified this in the laboratory, but he seems confident that he will.

With all the other stuff fitting, he says, the amplification should follow automatically. All those numbers seem to work out. Pretty soon, he says, they’ll be taking pictures of molecules. He knows they’ll have problems, but he greets them with typical Rhodesian hubris. The first one will be hard, he says. The second will be easier, the third easier yet. By the time you get to the tenth one, it’s routine. A week later you’re taking them every time you turn around. After a month they’ll put on your desk, literally, a bushel basket of pictures. You’ll have so many pictures you won’t know what to do.
Comment on this article

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Collapse bottom bar

Log in to your account

Email address:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »