Laser wakefield acceleration, a process in which electron acceleration is driven by high-powered lasers, is able to produce high-energy beams of electrons in tabletop-scale distances. Now a team of scientists from LLNL and UCLA has performed electron acceleration experiments that revealed new, never-before-seen electron ring formations in addition to the typically observed beams. The research was reported in a Physical Review Letters paper published on July 31.
Using the ultra-short-pulse Callisto laser system at LLNL’s Jupiter Laser Facility, a plasma was produced in a low-density gas cell target. The interaction of the high-intensity laser with the gas created a relativistic plasma wave, which then accelerated some of the electrons in the plasma to more than 100 million electron volt (MeV) energies.
These electron beams are usually directed along the laser axis and have fairly low divergence. In these experiments, the typical beams were observed, but in certain cases they also were accompanied by a second, off-axis beam that had a ring-like shape. This new feature had never before been reported, and its origin was unclear until the UCLA collaborators finished computationally intensive three-dimensional calculations of the experimental conditions.
“The dynamics of the plasma wave are often calculated in simulations, but the small spatial scale and fast timescale of the wakefield process has made direct measurements of many effects difficult or impractical,” said lead author Brad Pollock. “The discovery of new features, such as the electron rings here, allows us to compare with simulations and infer what is going on in the experiments with much greater confidence.”
In the simulations, a ring-like electron structure was produced during the wakefield acceleration process if the plasma was sufficiently long and the total number of electrons was large enough to perturb the plasma wave structure. Under these conditions, the plasma wave structure was modified in such a way as to force some electrons off of the laser axis and into a “pocket” outside of the plasma wave, which then guided some of these electrons through the remainder of the plasma.
“In addition to the diagnostic implications of this particular feature, it may also be possible to tailor the parameters of electron ring-beams for their own applications, including accelerating positively charged particles—positrons, for example,” Pollock added.
LLNL co-authors were Félicie Albert, Arthur Pak and Joseph Ralph, and UCLA co-authors were Frank Tsung, Jessica Shaw, Chris Clayton, Asher Davidson, Nuno Lemos, Ken Marsh, Warren Mori and Chan Joshi. The work was supported by the Laboratory Directed Research and Development (LDRD) Program.
Switching from gold to depleted uranium (DU) hohlraums in NIF high-foot (high initial laser picket) experiments has improved implosion performance and marked a turning point in NIF’s efforts to achieve ignition, LLNL researchers reported in a Physical Review Letters paper published online on July 28.
Thanks to the increased laser picket pulse, high-foot implosions have demonstrated improved resistance to hydrodynamic instability-induced mixing of ablator material into the central hot spot of the deuterium-tritium (DT) fusion fuel. Now the high-foot experiments using DU hohlraums instead of gold hohlraums in cryogenic layered DT implosions have produced NIF’s highest neutron yields to date, the researchers said.
Uranium hohlraums provide a higher albedo (re-direct more soft x-rays to the target capsule) and thus an increased drive—equivalent to an additional 25 terawatts of laser power at the peak of the drive compared to standard gold hohlraums—leading to higher implosion velocity. In addition, the DU hohlraums produce better radiation symmetry and thus an improved hot-spot shape, closer to round. NIF experiments in uranium hohlraums have achieved total yields approaching 1016 (ten quadrillion) neutrons—about 50 percent more than any experiment with a gold hohlraum. More than half of the yield was due to alpha-particle heating of the DT fuel.
The “first DU experiments were a turning point in the high-foot campaign,” the researchers said. “Thanks to the strongly reduced instability growth in the high-foot drive design, the twofold advantage of DU hohlraums”—increased drive and implosion velocity and round implosions at higher peak laser power—“becomes visible. Despite significantly higher target fabrication requirements, all subsequent layered implosion experiments aiming at highest performance have used DU hohlraums.”
Researchers are now exploring alternate hohlraum configurations to allow better control of the implosion shape, as well as modifications of the high-foot drive design to provide higher final fuel compression.
Lead author Tilo Döppner was joined on the paper by researchers from LLNL, the Laboratory for Laser Energetics at the University of Rochester, MIT, Los Alamos National Laboratory, General Atomics, and the UK’s Atomic Weapons Establishment.
Until recently, little experimental data existed about the behavior of beryllium (Be) at extremely high pressures and strain rates, with material models predicting very different behaviors in these regimes. In a successful example of international research collaboration, a team of scientists from LLNL and the Russian Federal Nuclear Center-All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF) has enhanced this field of knowledge.
In a recent cover article in the Journal of Applied Physics, the team showed that at extreme conditions, beryllium has very little strength and that most models over-predict its material strength.
“This finding has important implications for scientists working with technology where beryllium is subject to extreme pressures and strain-rates,” said Marc Henry de Frahan, the paper’s lead author. Henry de Frahan began the research as a summer student with the NIF & Photon Science Directorate and is now a graduate student at the University of Michigan.
The purpose of the experiments was to put Be into regions of stress and strain-rate that are difficult to access with focused experiments by using a technique originally developed at Los Alamos National Laboratory in the early 1970s. The technique has since been used extensively by RFNC-VNIIEF over the past few decades.
“Since Be presents its own unique challenges and the Russians had an experimental capability and experience with this technique, we decided to form a collaboration with them in 2009,” said co-author Rob Cavallo, a physicist in LLNL’s Design Physics Division.
The technique involves setting off a high explosive near the Be. On the side of the Be facing the explosive, the team imposed a sinusoidal “ripple” pattern designed by co-author Jon Belof. When the expanding explosive products push against, or load, the target, the target accelerates. The low-density gas pushing against the higher-density metal subjects the interface to Rayleigh-Taylor hydrodynamic instabilities, and the ripples grow in amplitude as the target accelerates.
If the target had no strength at all, the ripples would grow indefinitely and become turbulent at some point. Be does have strength, so the ripple growth is limited by the strength of the material. The main diagnostic for the experiments is an x-ray image from the side of the target showing the height of the ripples at some time after the explosive loading has occurred. The other diagnostic is velocimetry of the target showing its acceleration profile.
Because the researchers could devise the initial ripple amplitude and could measure the acceleration, they were able to infer the strength of the material using simulations with strength models. The experiment does not directly measure strength in the way a focused experiment might, but it determines the effect of strength, which can constrain the performance of various strength models. Using this technique allowed the team to reach pressures of about 50 GPa (490,000 atmospheres) and strain rates near 1,000,000 per second (a rate of 1 per second under tension means a piece of material would double its length in 1 second; thus 1,000,000 per second indicates a million-fold increase).
“The strength models used to assess the experiments are largely phenomenological, meaning experiments were conducted under some conditions, and then parameters were fit to make the model match the data,” Cavallo said. “When the material is loaded differently than the original experiments that defined the model, the model no longer accurately describes the material behavior. We wanted to determine how well these models would work for Be when the Be is loaded far away from the phase space where they were originally fit.”
The result was that only a new “relaxation” model, designed by co-author Olga Ignotova, came close to matching the data. The challenge for the models is that they are based on the assumption that material response is largely a combination of the material’s equation of state and plastic flow. Be, however, is known to be susceptible to material failure and damage.
“It appears from the much higher growth that was seen in the experiments than was predicted by the models that material failure mechanisms probably need to be incorporated into any material response model that endeavors to describe Be behavior under such conditions,” Cavallo added.
In a companion set of experiments, LLNL provided the Russian team with two hockey puck-shaped targets to do loading and recovery analysis. Since the Rayleigh-Taylor targets essentially disintegrate at late times, they cannot be picked up and examined for material damage signatures. The hockey pucks are loaded slightly differently and are robust enough to be recovered. The experiments showed that under loading conditions similar to the ones the Rayleigh-Taylor targets experienced there is evidence of damage, as well as twinning (bending of the crystal structure into a different orientation), which can carry the loading and mimic plastic strength.
“This collaboration is an excellent example of how experiments really enable us to discriminate material models of complex phenomena that sometimes show significant disagreement,” Belof said. “The theory of plasticity in these materials is extremely difficult, and without this study of beryllium under dynamic conditions it’s unlikely that we would have resolved this.” Belof conducted the research as a postdoctoral researcher and has since converted to a full-time staff member at LLNL, where he serves as a program leader in the Design Physics Division.
In addition to providing the basis for a strong international collaboration, the research also created a conduit for young scientists to gain vital experience—allowing Henry de Frahan to transition from a summer student into a lead author, and Belof to begin a permanent career at the Laboratory.
“My experience at Livermore exposed me to best-in-class scientists and enabled me to expand upon my research at the University of Michigan,” Henry de Frahan said. “These experiences gave me great insight into scientific collaborations and large-scale experimental campaigns.”
Physics of Plasmas (PoP), a peer-reviewed journal publishing original experimental and theoretical contributions in plasma physics, recently released a list of the most-cited papers for January-June 2015.
Out of the 30 most-cited papers on the list, the top five reported on experiments at NIF, all with lead authors from LLNL. Nos. 8, 9, 17, 21, and 24 on the list also were related to NIF, and the lead authors on eight of the 30 papers were from LLNL—more than any other institution on the PoP list. LLNL researchers also served as co-authors on an additional five papers.
The NIF-related papers were:
Other papers with LLNL co-authors were: