To optimize their performance — and to prepare for next-generation facilities that will push these extremes further — scientists have devised a new tool that can measure how bright these beams are, even for pulses that last only femtoseconds (quadrillionths of a second) or attoseconds (quintillionths of a second). Comparing 1 attosecond to 1 second is like comparing 1 second to 31.7 billion years.

This tool can also measure beam sizes to within a few tens of nanometers (billionths of a meter) — without disrupting experiments that rely on these beams.

The new tool, dubbed a “charge density monitor,” could also provide more precise measures of fundamental physics in high-energy and high-field beam experiments, and help guide R&D efforts that seek to shrink the size and cost of particle collider and accelerator facilities while ramping up their capabilities.

The research using this proposed diagnostic could also impact disciplines ranging from plasma science to atomic physics, and could lead to new applications and reveal new physics.

At the U.S. Department of Energy’s Berkeley Lab Laser Accelerator (BELLA) Center, researchers hope to test this tool by measuring particle properties in the aftermath of an intense laser beam drilling through a jet of gas. In doing so, they hope to learn about the electron beam pulse emerging from this interaction.

“BELLA provides an ideal test bed for evaluating the potential of the beam-measuring method at a state-of-the-art advanced accelerator, since we aim at producing the brightest possible ultrashort bursts of electrons with our compact accelerator technology,” said Wim Leemans, director of the BELLA Center and the Accelerator Technology & Applied Physics Division at Lawrence Berkeley National Laboratory (Berkeley Lab).

“It would provide a powerful tool for measuring and improving BELLA’s beams.”

Leemans led the Berkeley Lab team of contributors as part of an international team in a technical study detailing the new method, published in the May 10 issue of the journal Physical Review X.

Roxana Tarkeshian, a researcher at the University of Bern and previously at the Paul Scherrer Institute, served as the lead author of the study and has pursued the new diagnostic method since 2015, with support from Thomas Feurer, a professor at the University of Bern and an expert in laser-based technology and space physics.

“Its ultrasensitive measurements at high resolution, and its low cost and compactness are among its assets,” Tarkeshian said.

The study details how intense particle beams can barrel through a low-density neutral gas, stripping away electrons from gas atoms through the strong electric fields associated with intense particle beams. An ionized (charged) cloud of matter known as a plasma — containing ions and electrons — forms in the process.

The technique’s “unprecedented” resolution for the duration and size of individual pulses for both electron beams and positron beams relates to an effect in which small changes in beam brightness of just a few percent to tens of percent can result in tens to hundreds of times more ions generated in the presence of an electric field, Tarkeshian noted.

The process is similar to what happens when a very intense, focused laser beam or X-ray pulse interacts with a gas and ionizes the atoms. But there are important differences in the physics of this ionization process for beams of light (photons) vs. other types of particle beams.

With beams of light, electrons and ions (charged particles) are produced throughout the beam’s footprint, and the plasma-associated electrons have a relatively low velocity and tend to hang around the column of ions until they are pulled away by an external electric field. Ions with positive charges then drift in the opposite direction and can be measured.

For electron (negatively charged) or positron (positively charged) particle beams, the shape of the electric field resembles a doughnut and produces a ring-shaped plasma column, with no ions initially left in the beam path — the hole of the doughnut. These particle beams can supply a powerful kick to electrons, which can leave a ring-shaped column of ions behind. And those ions can be guided away by an electric field to a detector that measures the number of ions, their speed, and their charged state.

The latest study shows that the new measurement tool can also glean more information about the beam itself from this “ion doughnut” under the right operating conditions — with the right density and mix of gases, for example.

The team carried out sophisticated simulations using a Berkeley Lab-refined computer code known as WARP and another code known as VSim. Researchers modeled the interaction of particle and photon beams with gases and the ensuing plasma-related dynamics.

“The simulations allowed us to zoom in space and time — from the centimeter scale down to the submicron size of the beam, and to follow the dynamics and distributions of electrons and ions at different timescales,” said Jean-Luc Vay, a senior scientist at Berkeley Lab who contributed to the WARP code and leads the Accelerator Modeling Program in the Lab’s ATAP Division.

Vay noted that aspects of the code proved key in the accurate modeling and understanding of differences between the effects of particle beams versus photon beams, and in finding the best way to tune and operate the system.

Once the full diagnostic system is implemented at accelerator systems, simulations will help to reality-check the actual measurements in experiments and help to develop a path for optimizing beam performance.

“Small changes could be resolved very precisely,” she said, based on measurements of individual beam pulses.

The proposed technique also opens up the possibility to study charge-induced dynamics in matter, and may provide more insight into timescales of fundamental atomic or molecular processes being studied with attosecond photon pulses, she said, including a property known as quantum tunneling in which a particle can appear to spontaneously “tunnel” through the potential barrier of the atom in defiance of classical physics.

And Tarkeshian points out that the proposed diagnostic could prove useful for existing X-ray free-electron lasers (XFELs) such as the Linac Coherent Light Source (LCLS) X-ray FEL at SLAC National Accelerator Laboratory, the FLASH facility at DESY in Germany, the SwissFEL at the Paul Scherrer Institute (PSI) in Switzerland, among others, and facilities under construction like the LCLS-II at SLAC.

For example, a prototype has been installed at LCLS with the support and contributions of SLAC scientist Patrick Krejcik and a team at PSI to diagnose the ultrashort, high-energy electron bunches produced by its accelerator.

Tarkeshian noted that other tools have been developed to provide measurements of accelerator and XFEL beam properties, but as the beams’ pulses pack more and more intensity and energy into shorter and shorter pulses, new tools will be needed to keep pace with these extreme beams.

She credited some decades-old work on a proposed diagnostic for a test accelerator project at SLAC known as the Final Focus Test Beam, or FFTB, in paving the way for this new design concept.

“In our latest work, we have studied not only the concepts but also have addressed the challenges that this technique may face experimentally,” Tarkeshian said.

“It’s great to revive this unfinished concept from decades ago with new ideas, and with continued support we can realize its potential,” she added. “This is a very open path, and we are just beginning.”

Leemans said, “We think that the practical realization of this innovative technique will ultimately be of broad interest to the international high-energy physics and the general accelerator-driven science communities.”

The work was supported by the DOE Office of Science’s Office of High Energy Physics, the European Union’s Seventh Framework Program, and the Swiss National Science Foundation.

Original Source: https://www.sciencedaily.com/releases/2018/05/180510145942.htm

Original Date: May 10 2018
Written by: DOE/Lawrence Berkeley National Laboratory

The researchers used the X-ray free-electron laser Linac Coherent Light Source LCLS at the SLAC National Accelerator Laboratory in the U.S. to shoot extremely intense and ultra-short flashes of X-rays at a jet of water. “It is not the usual way to boil your water,” said Caleman. “Normally, when you heat water, the molecules will just be shaken stronger and stronger.” On the molecular level, heat is motion — the hotter, the faster the motion of the molecules. This can be achieved, for example, via heat transfer from a stove, or more directly with microwaves that make the water molecules swing back and forth ever faster in step with the electromagnetic field.

“Our heating is fundamentally different,” explained Caleman. “The energetic X-rays punch electrons out of the water molecules, thereby destroying the balance of electric charges. So, suddenly the atoms feel a strong repulsive force and start to move violently.” In less than 75 femtoseconds, that’s 75 millionths of a billionth of a second or 0.000 000 000 000 075 seconds, the water goes through a phase transition from liquid to plasma. A plasma is a state of matter where the electrons have been removed from the atoms, leading to a sort of electrically charged gas.

“But while the water transforms from liquid to plasma, it still remains at the density of liquid water, as the atoms didn’t have time to move significantly yet,” said co-author Olof Jönsson from Uppsala University. This exotic state of matter is nothing that can be found naturally on Earth. “It has similar characteristics as some plasmas in the sun and the gas giant Jupiter, but has a lower density. Meanwhile, it is hotter than Earth’s core.”

The scientists used their measurements to validate simulations of the process. Together, the measurements and simulations allow to study this exotic state of water in order to learn more about water’s general properties. “Water really is an odd liquid, and if it weren’t for its peculiar characteristics, many things on Earth wouldn’t be as they are, particularly life,” Jönsson emphasised. Water displays many anomalies, including its density, heat capacity and thermal conductivity. It it these anomalies that will be investigated within the future Centre for Water Science (CWS) planned at DESY, and the obtained results are of great importance for the acivities there.

Apart from its fundamental significance, the study also has immediate practical significance. X-ray lasers are often used to investigate the atomic structure of tiny samples. “It is important for any experiment involving liquids at X-ray lasers,” said co-author Kenneth Beyerlein from CFEL. “In fact, any sample that you put into the X-ray beam will be destroyed in the way that we observed. If you analyse anything that is not a crystal, you have to consider this.”

The measurements show almost no structural changes in the water up to 25 femtoseconds after the X-ray pulse starts to hit it. But at 75 femtoseconds, changes are already evident. “The study gives us a better understanding of what we do to different samples,” explained co-author Nicusor Timneanu from Uppsala University, one of the key scientist developing the theoretical model used. “Its observations are also important to consider for the development of techniques to image single molecules or other tiny particles with X-ray lasers.”

Original source: https://www.sciencedaily.com/releases/2018/05/180514151923.htm

Original Author: Deutsches Elektronen-Synchrotron DESY

Written Date: May 14 2018