First Pump-Probe Experiment at Linac Coherent Light Source Completed

(PhysOrg.com) -- The first experiment using the Linac Coherent Light Source to illuminate molecules via a "pump-probe" technique has been completed by an international team of more than 30 scientists from institutions including Lawrence Berkeley National Laboratory, LCLS and the joint SLAC/Stanford PULSE Institute. Ryan Coffee, physicist with the LCLS Laser Group, presented initial results in a seminar at SLAC on Wednesday, November 18.

Pump-probe experiments use one laser pulse, in this case an infrared pulse, to pump energy into a sample and then probe it with another laser pulse, in this case an LCLS X-ray pulse. Such experiments are ideal for looking at atomic and molecular interactions, which take place in tiny fractions of a second. The LCLS probe pulses were as short as a few quadrillionths of a second and a billion times brighter than any X-ray source produced in a laboratory.
Coffee and his colleagues looked at the quantum behavior of electrons in nitrogen molecules, N2. The results represent a step toward a fundamental understanding of how nature converts light into chemical energy and might one day help revolutionize solar power, Coffee said.
Nitrogen atoms distribute their electrons between a lower and a higher energy shell. Using X-rays, the team picked off two electrons from the lower level, allowing a higher shell electron to descend and fill the vacancy. The energy released during this downward plunge ejected another electron from the atom, a phenomenon known as the Auger effect.
The team wanted to study how the nitrogen molecules' orientation affected this reaction. To do this, they used the infrared laser to line up the nitrogen molecules so that they were all facing the same direction.
"In a sense, we tried to make the gas act a little bit like a crystal," Coffee said.
After hitting the nitrogen with X-rays, the researchers detected electrons flying off and measured how the molecules' alignment with respect to the X-rays influenced the Auger effect. They observed numerous features that had strong dependence on the molecules’ direction. The results are currently being prepared for publication.
Future work will focus on how atomic bonds change as molecules either break apart or rearrange. Coffee thinks such work will lead to a deeper understanding of how nature converts light into energy. Ultimately, he hopes the results will lead to technology that will help humans generate power from the sun.
"I'm going for the solar power revolution, though I don't know where it will come from," he said. His gut feeling is that the important atoms to look at are carbon, nitrogen and oxygen.
"That's where energy in nature comes from," he said.
Coffee added that the team owes a debt of gratitude to the LCLS Controls, Accelerator, and Laser Groups, who made the experiment's success possible.

Laser creates record-breaking protons

An international group of physicists working at the Los Alamos Laboratory in the US has used a laser to generate 67.5 MeV protons – the highest-energy protons yet produced in this way. Their work points the way to new laser-based devices for proton therapy, which would be far smaller and cheaper than existing particle-accelerator sources.

When a high-energy proton beam travels through the human body it deposits most of its energy within a small volume, the size and location of which can be calculated to great precision. As a result, protons offer a distinct advantage over other forms of radiation used to destroy tumour cells because they cause less damage to surrounding healthy tissue. Unfortunately, the accelerators needed to generate the protons can cover thousands of square metres and cost some $100m. This has limited the number of proton-therapy facilities available and patients often have to travel considerable distances to be treated in this way.

Some physicists believe that a laser-based proton generator could be made for about one tenth of the cost of a conventional accelerator and be small enough to be contained within a classroom-sized laboratory. The idea is that ultra-powerful laser pulses knock electrons out of the atoms within a tiny target, causing the electrons to accumulate on the target's rear surface. This sets up an electric field across the target, accelerating the resultant ions and forcing them to leave the material as a very high-energy beam.

Energy is a problem

In practice, however, some of the world's most powerful petawatt (10^15 W) lasers have only been able to generate protons with a maximum energy of about 58 megaelectronvolts (MeV). While tumours of the eye can be treated using protons of 60–70 MeV, deeper tumours require energies of about 300 MeV.

The latest breakthrough was carried out by Kirk Flippo of Los Alamos, Sandrine Gaillard of the Forschungszentrum Dresden–Rossendorf research centre (FZD) in Germany and colleagues, who used Los Alamos' Trident laser to generate 67.5 MeV protons. The work relies on a novel target design – an anvil-shaped piece of copper comprising a cone around 100 µm long with a 100 µm flat disc across perched on its tip. Flippo's team directed the laser beam to the inside of the cone, liberating electrons that were guided to the tip and which set up an electric field that accelerated protons away from the disc. The researchers claim that this arrangement is far more efficient than the thin films used in previous experiments – they used 80 J laser pulses, whereas the previous record of 58 MeV involved 450 J laser pulses.

Team-member Michael Bussmann of the FZD says that this significant step forward in maximum proton energy was also made possible by increasing the intensity of the main part of each pulse relative to the "pre-pulse", which precedes the main pulse and can damage the target.

Not enough protons

However, it might take a decade or more before laser-generated protons can be used to combat cancer. Another major challenge is that Trident and the other more intense lasers simply require too much energy to be able to function at the roughly 10 Hz pulse rate needed to produce enough protons for cancer therapy.

According to Bussmann, reaching the sought-after high production rates will be a matter of getting the target right. One possibility will be some kind of refinement of the anvil shape, he says. Others, however, believe that the answer lies in reducing the size of the target, allowing electrons to be heated and ejected from the target much more quickly and therefore with a more uniform energy distribution, in other words leading to fewer low-energy electrons. "We already have enough energy in our lasers, the question is how can we use it more efficiently," says Bussmann. "Nobody has the final idea right now," he said, "but we are in a position to test all these different theories and see which works best."

Looking beyond cancer therapy, Flippo believes that such proton sources could also be used to create medical isotopes and employed to generate neutrons for research in condensed-matter physics and other areas of science. They might also be used to search for nuclear materials inside cargo, given that the characteristics of a proton beam are altered in a well defined way by radioactive substances.

The research was presented at the annual meeting of the Division of Plasma Physics of the American Physical Society, held in Atlanta on 2–6 November.

Electron self-injection into an evolving plasma bubble

Just five years ago, experimentalists finally demonstrated that such laser-plasma accelerators could produce monoenergetic, collimated electron beams with quality comparable to conventional accelerators. The secret was for the laser to produce a "bubble" almost completely devoid of electrons in its immediate wake that captured electrons from the surrounding plasma and accelerated them in an exceptionally uniform way. Yet the precise mechanism by which the bubble captured these electrons and accelerated them with such uniformity has remained one of the outstanding mysteries of this field.

Now new theoretical work by scientists from the University of Texas and Commissariat à l'Énergie Atomique (CEA, France), to be reported at the 2009 APS Division of Plasma Physics Annual Meeting, has shed light on this mystery. Formation of the exceptional quality electron beam is attributed to the evolution of the bubble shape which, in turn, is directly associated with the nonlinear evolution of the driving laser pulse (nonlinear focusing and defocusing).

The basic premise of this work is that the size of the bubble—the cavity of electron density traveling over the positive ion background with nearly the speed of light—is determined by the spot size of the driving laser pulse. Plasma nonlinearities cause the laser to focus and defocus in the course of propagation. Once the laser diffracts, the bubble expands. Electrons that constitute a dense electron shell surrounding the bubble move with relativistic speeds and thus have high inertia. As a consequence, some of them become too heavy to follow the expanding shell; they fall inside the bubble, stay inside till the end of the plasma (i.e. get trapped) and finally gain multi-GeV energy.

The trapped charge is proportional to the bubble growth rate. Once the laser becomes self-guided, and the spot size oscillations saturate, the injection process clamps. Simultaneously, longitudinal non-uniformity of the accelerating gradient equalizes the trapped electron energy. This scenario of self-injection and monoenergetic bunch formation is discovered and explored in fine detail in the 3-D particle-in-cell simulations. This is fundamentally different from the previous work which concentrated on either one-dimensional models of electron trapping or on the reduced description of transverse plasma wave breaking in planar 2-D geometry.

The discussed mechanism of electron self-injection is very robust in experiments with the high-power laser (tens of terawatts to petawatt). In addition, an appropriate modification of the plasma density (e.g. using a thin dense slab as a nonlinear lens for the laser) may cause the laser to self-focus and defocus faster, which results in a single self-injection event. This kind of laser beam manipulation may lead to the generation of a 2.5 GeV mono-energetic (~1% energy spread) electron bunch containing ~1010 electrons in a future experiment with the recently commissioned Texas Petawatt (TPW) laser - the most powerful laser in the world. Electrons with 2.5 GeV of energy are traveling at 99.999998% of the speed of light. Electron beams with such unique properties are clearly beneficial for medical applications, radiation physics, material science, and homeland security.
Source: American Physical Society