A frothy breakthrough at Lawrence Livermore National Laboratory (LLNL), using lasers with a silver metal foam that's as light as air, has created the brightest yet X-ray source ever, twice as brilliant as that of anything previous.
Super-bright X-rays may not be the sort of thing you need around the house (unless you have a very unusual home life), but they have valuable applications in advanced research, including the study of the structure of materials at the atomic level, observing chemical reactions in real-time, imaging biological samples with high detail, and analyzing complex molecules.
The brightest of these are especially important at places like LLNL, which is involved in the bleeding-edge physics of nuclear fusion – not only as pure science, but for the development of practical fusion reactors – and ensuring that the US nuclear weapons stockpile remains safe and reliable.
What makes these specialized X-rays so important is their super-high resolution, which makes them ideal for studying extremely dense matter, including the plasmas generated by inertial confinement fusion where pellets of deuterium and tritium are bombarded by a barrage of high-energy laser beams.
Indeed, the same super-lasers used in fusion research at the National Ignition Facility (NIF) are also used to produce this new super-bright X-ray light.
X-rays in places like dentist's offices are usually created by bombarding a metal target with an electron beam. The electrons interact with the electrons in the metal to throw off a beam of X-rays, like so:
However, the new system swaps out the electron beam with a laser, and the usual metal target with a very special one. In this case, one made out of silver foam.
The foam is in the form of 4-mm-wide cylindrical targets. They're manufactured by taking silver nanowires and suspending them in a special mold. This is subjected to a supercritical drying process to remove the solution – and what's left behind is a silver foam that has a thousandth of the density of normal silver. Indeed, it's about the same density as air.
Okay, that's all very clever, but why?
The point is that such fluffy silver has a much larger volume by weight than solid silver. This means that heat can flow through it much faster than otherwise. This means the entire cylinder can heat uniformly in only 1.5 billionths of a second.
The upshot is an X-ray source with an energy of more than 20,000 electron volts. That's minuscule on an everyday scale, but on the microscopic scale of nuclear physics that is a very large amount indeed.
According to LLNL, the development of this new X-ray light will not only push forward our understanding of fusion processes, but the plasma produced in generating the X-rays will give new insights into bright, hot metal plasmas that are far from thermal equilibrium.
"Going forward, this means we need to rethink our assumptions about heat transport and how we calculate it in these particular metal plasmas," said LLNL scientist Jeff Colvin.
The research was published in Physical Review E.
Source: LLNL