What Can We Do With X-Ray Lasers? Surprises From Extreme Physics
How do lasers work? By having a photon hit an atom with a certain energy, you can cause the atom to emit a photon with that energy in a process called stimulated emission. By repeating this process on a large scale you will get a chain reaction which results in a laser. However, certain quantum catches cause this process to not happen as predicted, with the photon occasionally being absorbed with no emission at all. But to ensure that the max odds of the process will occur, energy levels of the photons are increased and mirrors are placed parallel to the light path to help stray photons reflect back into the game. And with the high energies of X-rays, special physics is uncovered (Buckshaim 69-70).
The Development of the X-ray Laser
In the early 1970s, the X-ray laser seemed to be out of reach as most lasers of the time peaked at 110 nanometers, well short of the largest X-rays of 10 nanometers. This was because of the amount of energy required to get the material stimulated was so high that it needed to be delivered in a quick firing pulse that further complicated the reflective ability needed to have a powerful laser. So scientists looked to plasmas as their new material to stimulate, but they too fell short. A team in 1972 did claim to finally achieve it but when scientists tried to replicate the results it failed too (Hecht).
The 1980s saw a major player enter the efforts: Livermore. Scientists there had been making small but important steps there for years but after the Defense Advanced Research Projects Agency (DARPA) stopped paying for X-ray research, Livermore became the leader. It led the field in several lasers including fusion-based. Also promising was their nuclear weapons program whose high-energy profiles hinted at a possible pulse mechanism. Scientists George Chapline and Lowell Wood first investigated fusion tech for the X-ray lasers in the 1970s then shifted to the nuclear option. Together the two developed such a mechanism and was ready to test on September 13, 1978 but an equipment failure grounded it. But maybe it was for the best. Peter Hagelstein created a different approach after reviewing the previous mechanism and on November 14, 1980 two experiments titled Dauphin proved that the set -up worked! (Ibid)
And it did not take long before the application as a weapon was realized, or as a defense. Yes, harnessing the power of a nuclear weapon into a focused beam is incredible but it could be a way to destroy ICBMs in the air. It would be mobile and easy to use in orbit. We know this program today as the “Star Wars” program. A February 23, 1981 issue of Aviation Week and Space Technology outlined initial tests of the concept including a laser beam sent at a wavelength of 1.4 nanometers that measured several hundred terawatts, with up to 50 targets possibly being targeted at once despite vibrations along the craft (Ibid).
A March 26, 1983 test yielded nothing because of a sensor failure but the Romano test of December 16, 1983 further demonstrated nuclear X-rays. But a few years later on December 28, 1985, the Goldstone test showed that not only were the laser beams not as bright as suspected but that focusing issues were also present. “Star Wars” moved on without the Livermore team (Ibid).
But the Livermore crew also moved on, looking back at the fusion laser. Yes, it wasn’t capable of as high pump energy but it did offer the possibility of multiple experiments a day AND not replace the equipment every time. Hagelstein envisioned a two-step process, with a fusion laser creating a plasma that would release excited photons which would collide with another material’s electrons and cause X-rays to be released as they jumped levels. Several set-ups were tried but finally a manipulation of neon-like ions was the key. The plasma removed electrons until only the 10 inner one remained, where photons then excited them from a 2p to a 3p state and thus releasing a soft X-ray. A July 13, 1984 experiment proved that it was more than a theory when the spectrometer measured strong emissions at the 20.6 and 20.9 nanometers of the selenium (our neon-like ion). The first laboratory X-ray laser, named Novette was born (Hecht, Walter).
Nova and More Children of Nouvette
The follow-up to Novette, this laser was designed by Jim Dunn and had the physical aspects of it verified by Al Osterheld and Slava Shlyaptsev. It first began operations in 1984 and was the largest laser housed at Livermore. Using a brief (about a nanosecond) pulse of high energy light to excite the material to release X-rays, Nova made use of glass amplifiers as well which improve efficiency but also heat up fast, meaning that Nova could only operate 6 times a day between cool-offs. Obviously this makes for testing out science a harder goal. But some work showed that you could fire a picosecond pulse and test many more times a day, so long as the compression is brought back to a nanosecond pulse. Otherwise, the glass amplifier will be destroyed. Of important note is that Nova and other “tabletop” X-ray lasers make soft X-rays, which has a longer wavelength that prevents penetrating many materials but does give insights into fusion and plasma sciences (Walter).
Linac Coherent Light Source (LCLS)
Located at the SLAC National Accelerator Laboratory, specifically at the linear accelerator, this laser makes use of several genius devices to hit targets with hard X-rays. Here are some of the components of LCLS, one of the strongest lasers out there (Buckshaim 68-9):
- -Drive Laser: Creates an ultraviolet pulse which remove electrons from the cathode, a preexisting part of the SLAC accelerator.
- -Accelerator: Gets the electrons to energy levels of 12 billion eVolts by using electric field manipulation. Totals in at half the length of the SLAC compound.
- -Bunch Compressor 1: S-curved shape device that “evens out the arrangement of electrons having different energies.
- -Bunch Compressor 2: Same concept at Bunch 1 but a longer S because of the higher energies encountered.
- -Transport Hall: Makes sure electrons are good to go by focusing the pulses using magnetic fields.
- -Undulator Hall: Composed of magnets which cause electrons to move back and forth, thus generating the X-rays.
- -Beam Dump: Magnet which takes out the electrons but lets the X-rays pass undisturbed.
- -LCLS Experimental Station: Location where science happens aka where destruction occurs.
So what could this laser be used for? It was hinted at earlier that the shorter wavelength can make exploration of difference materials easier, but that’s not the only purpose. When a target is hit by the pulse, it is simply obliterated into its atomic parts with temperatures reaching millions of Kelvin in as little as a trillionth of a second. Wow. And if this weren’t cool enough, the laser causes electrons to be cast off from the inside out. They are not pushed out but repelled! This is because the lowest level of electron orbitals has two of them which are ejected courtesy of the energy the X-rays are supplying. The other orbitals become destabilized as they fall inward and then meet the same fate. The time it takes for an atom to lose all its electrons is on the order of a few femtoseconds. The resulting nucleus doesn’t hang around for long though and decays fast into a plasmic state known as warm dense matter, which is mainly found in nuclear reactors and the cores of large planets. By looking at this we can gain insights into both processes (66).
Another cool property of these X-rays is their application with synchrotrons, or particles accelerated throughout a path. Based on how much energy is required for that path, particles can emit radiation. For example, electrons when excited release X-rays, which happen to have a wavelength about the size of an atom. We could then learn properties of those atoms through the interaction with the X-rays! On top of that, we can alter the energy of the electrons and get different wavelengths of X-rays, allowing a greater depth of analysis. The only catch is that alignment is critical, otherwise our images will be blurry. A laser would be perfect for resolving this because it is coherent light and can be sent in controlled pulses (68).
Biologists have even gotten something out of X-ray lasers. Believe it or not but they can help reveal aspects of photosynthesis previously unknown to science. That is because to barrage a leaf with radiation usually kills it, removing any data on the catalyst or the reaction it undergoes. But those long wavelengths of soft X-rays allow for study without destruction. A nanocrystal injector fires photo-system I, a protein key to photosynthesis, as a beam with green light to activate it. This is intercepted by a laser beam of X-rays which causes the crystal to explode. Sounds like not much gain in this technique, right? Well, with the use of a high-speed camera that records at femtosecond time intervals, we can make a movie of the event before and after and voila, we have femtosecond crystallography (Moskvitch, Frome 64-5).
We need X-rays for this because the image recorded by the camera is the diffraction through the crystal, which will be sharpest in that portion of the spectrum. That diffraction gives us an inside peak at the workings of the crystal, and thus how it operates, but the price we pay is the destruction of the original crystal. If successful, then we can divine secrets from nature and develop artificial photosynthesis may become a reality and boost sustainability and energy projects for years to come (Moskvitch, Frome 65-6).
How about an electron magnet? Scientists found that when they had a xenon atom and iodine-bounded molecules mix hit by a high power X-ray, the atoms had their inner electrons removed, creating a void between the nucleus and the outermost electrons. Forces brought those electrons in but the need for more was so large that electrons from the molecules were also stripped! Normally, this shouldn't happen but because of the suddenness of the removal, a highly charged situation erupts. Scientists think this could have some applications in image processing (Scharping).
Buckshaim, Phillip H. “The Ultimate X-Ray Machine.” Scientific American Jan. 2014: 66, 68-70. Print.
Frome, Petra, and John C.H. Spence. "Split-Second Reactions." Scientific American May 2017. Print. 64-6.
Hecht, Jeff. “The History of the X-Ray Laser.” Osa-opn.org. The Optical Society, May 2008. Web. 21 Jun. 2016.
Moskvitch, Katia. “Artificial Photosynthesis Energy Research Powered by X-ray Lasers.” Feandt.theiet.org. The Institution of Engineering and Technology, 29 Apr. 2015. Web. 26 Jun. 2016.
Scharping, Nathaniel. "X-ray Blast Produces a 'Molecular Black Hole.'" Astronomy.com. Kalmbach Publishing Co., 01 Jun. 2017. Web. 13 Nov. 2017.
Walter, Katie. “The X-ray Laser.” Llnl.gov. Lawrence Livermore National Laboratory, Sept.1998. Web. 22 Jun. 2016.
© 2016 Leonard Kelley