Catching waves

by Barbara Jewett

Pretend you’re a contestant on Jeopardy! and the only category remaining is physics. You have no choice but to choose it, so you pick the lowest value choice and cross your fingers. Host Alex Trebek reads the answer: A branch of physics that is synonymous with televisions. The answer: What is plasma?

Warren Mori, a professor of theoretical plasma at UCLA, says another term that can be used for plasma is hot or ionized gas. The best example of hot gas, or plasma, is one you see every day: the sun. Because plasma is synonymous with hot gas, plasma is often considered the fourth state of matter, explains Mori. (The other three are solid, liquid, and gas.)

Plasma physics is the study of the behavior and characteristics of matter in the plasma state. Mori and team members Weiming An and Frank Tsung use NCSA’s Blue Waters supercomputer to study plasma physics related to plasma-based acceleration as well as the plasma physics related to lasers propagating in plasmas with respect to a field called inertial confinement fusion.

Their groundbreaking plasma-based acceleration work providing quantitative agreement between experiment and simulation has twice been published in Nature within the last year. In work published last year the research team had shown that boosting the energy of charged particles by having them “surf” a wave of ionized gas, or plasma, works well when accelerating electrons. While this method by itself could lead to smaller accelerators, electrons are only half the equation for future colliders. Now the same team of researchers has achieved another milestone by applying the technique to positrons.

Plasma physics 101

Researchers involved in gaining a better understanding of our world are constantly looking for the tiniest, most minuscule particle of whatever they are studying. To comprehend his team’s work, says Mori, one must first understand the method for discovering these most fundamental, most minuscule, particles in nature.

Those discoveries rely on particle accelerators—the largest and most complex instruments for scientific discovery in the world—to collide particles. It’s well established that the higher the energy of the colliding particles, the more likely they’ll split into their most basic pieces.

“It’s like with cars,” says Mori. “If they collide at 20 mph there’s a dent, but if you collide them at 200 mph they fall apart.”

The Large Hadron Collider (LHC) in Switzerland is the world’s largest particle accelerator, built by the European Organization for Nuclear Research (CERN). In the United States, the largest accelerator is at the Fermi National Accelerator Laboratory (Fermilab) near Chicago.

Accelerators in use today work by pumping a radio wave or a microwave into a cavity, causing the electric field to increase. If it gets too large, however, the electric field could hit its “breakdown” limit, where the electric field breaks down the walls of the accelerator by pulling electrons from the walls’ inside surface; this would decrease the accelerator’s performance and possibly damage it as well. Thus acceleration of charged particles is limited by the size of the electric fields.

To have an accelerator at a higher energy than the LHC using current technology, Mori notes, would require building something even bigger. Given that the LHC is a 17 mile (27 km) tunnel and had a $9 billion price tag, something bigger is unlikely in today’s economy. So, he says, the U.S. Department of Energy has a research and development program looking into what is called advanced accelerator concepts, and the National Science Foundation has also invested in this to a lesser extent. Advanced accelerator concepts research, says Mori, has explored acceleration gradients.

“It’s like you put your foot on the gas pedal. The rate that the ‘car’—which in this case is the particle—gets accelerated is faster. The thought is if we can make that acceleration rate bigger, then the actual size of the accelerator could be smaller. One idea to increase acceleration is based on using plasma,” Mori explains.

Since material in plasmas is already fully ionized there is no limit to how big the electric field could be. Thus scientists are hopeful plasmas will be the key to an affordable and more compact collider in the future.

Acceleration milestone

Mori says researchers already know they can create plasma waves using short particle beams or laser pulses: The particle has electric fields or the laser has radiation pressure that push the plasma electrons away from it, and the ions in the plasma then attract the electrons back after the particle beam or laserhas gone by, causing them to oscillate back and forth creating a plasma wave wake, analogous to the wake a boat creates behind it on a lake.

Most researchers believe accelerating the trailing electrons to high energy and having the trailing bunch efficiently surf on the plasma wake may be the key to successfully colliding particles in the plasma. So the Mori team’s simulations doing exactly that are a milestone.

“We have been simulating an experiment that was the first to demonstrate high gradient [acceleration] and high energy transfer from a drive electron bunch to a trailing electron bunch,” says Mori, bringing researchers a step closer to a viable plasma wakefield accelerator technology. Mori also added that “working in a close collaboration with experiments and simulations increases the rate of progress.”

Another milestone

A study led by the same team of researchers from UCLA and the U.S. Department of Energy’s SLAC National Accelerator Laboratory has demonstrated amore efficient way to accelerate positrons, the antimatter opposites of electrons. The method may help lead to much smaller, but more powerful, linear electron-positron colliders—machines that could be used to understand the properties of nature’s fundamental building blocks.

The team discovered that a single positron bunch can interact with the plasma in such a way that the front of it generates a wake that both accelerates and focuses its trailing end. This occurs after the positrons have traveled about 10 centimeters (about 4 inches) through the plasma.

“In this stable state, about 1 billion positrons gained 5 billion electronvolts of energy over a short distance of only 1.3 meters,” said Sebastien Corde, the study’s first author, a former SLAC researcher who is now at the Ecole Polytechnique in France. “They also did so very efficiently and uniformly, resulting in an accelerated bunch with a well-defined energy.”

The Mori team performed simulations on Blue Waters to understand how the stable state was created. “Based on this understanding, we can now beginto use simulations to look for ways of exciting suitable wakes in an improved, more controlled way. This will also lead to ideas for future experiments,” says Mori.

Blue Waters made it possible

The team’s success relied on simulations run on Blue Waters by team member Weiming An.

For both electron and positron simulations, An used anywhere from 4,000–8,000 processors to simulate the beams for 1 meter long in the plasma. Each simulation took about four hours to run.

An used a three-dimensional particle-in-cell (PIC) code, QuickPIC, to create the simulation of the beamplasma interaction and to follow the complicated trajectories of the plasma electrons. As a particle in the plasma moved, it generated electric and magnetic fields that exerted force on the particles, affecting how the particle moved. This in turn caused the particle to make new electric and magnetic fields, those forces then affected future particle movement, and the process continually repeated.

More work to do

An says there are still many challenges in plasma acceleration simulations, such as simulating how the accelerated particles radiate. “There is not a consistent code for simulating this kind of thing. As the computer technology develops we hope to continue to develop our code and improve its capability. There is still a lot of work to be done.“

Another challenge An faces is efficiently simulating acceleration of a high-quality electron beam in which the size of the particle beam is made smaller while the number of particles does not change.

This requires a very small resolution, he says, and that means having to use more cells in the simulation box. This makes the simulation very large.

“For this kind of simulation we have to use at least 10,000 processors, even 30,000 processors, for each run, and each run may last for one day. It takes a lot of CPU hours. So we have to develop our code to run more efficiently and reduce the computing time to simulate such a problem,” he says.

Laser fusion

The Mori team also uses Blue Waters to explore laser driven inertial confinement fusion. Those efforts, led by Frank Tsung, aid the work of experimental scientists in this field.

According to the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in Livermore, California, home of the world’s largest laserfor inertial confinement fusion, the goal is to create a self-sustaining “burn” of fusion fuel that produces as much or more energy than the energy required toinitiate the fusion reaction—an event called ignition. “Achieving ignition would be an unprecedented, game-changing breakthrough for science and could lead to a new source of boundless clean energy for the world,” says the NIF website.

The NIF laser is in a building about the length of three football fields—three times the length of the National Petascale Computing Facility that houses Blue Waters. The laser generates a pulse that is split and amplified into 192 beam lines aimed at a target. Forignition experiments, says Mori, the target consists of a cylinder with holes at each end, called a hohlraum. The length of the cylinder is about a centimeter or less than an inch. Inside the hohlraum is a tiny sphere of, deuterium and tritium fuel.

The beam lines are split, with half entering the top and half entering the bottom of the hohlraum, striking its inside walls, creating x-rays that compress the fuel capsule to extreme temperatures and densities. When the x-rays hit the target, says Mori, they cause some of the fuel to move outward, and some of the fuel to move inward.

“The sphere then goes from solid density to 1,000 times solid density. And when it does that you get fusion. It’s like a miniature sun. It’s important that when the lasers enter the hole they hit the wall where and when you want them to, because if you take a sphere and you want to compress it, it has to be very symmetrical. If it is not symmetric, as you compress one side other parts squirt out. When the laser beams come through, they go through plasma, so if the laser light is reflected, or bends, or is absorbed by the plasma in a way you didn’t want then the fuel will be compressed improperly,” explains Mori.

Tsung conducts simulations on Blue Waters to study how the laser propagates through the plasma from the hohlraum’s entrance hole toward the wall. He says he’s a long way from the types of simulations being conducted for the plasma acceleration work, however.

“One thing that’s different is that Weiming has been doing these simulations in 3D where he can get quantitative agreement between experiments and simulation. With the laser fusion,” Tsung says, “the timescale is very short and we are doing 2D simulations instead of 3D. So for us, there is a long way to go.”

Future discoveries

Mori and his team enjoy looking at fundamental plasma physics processes, trying to make discoveries in their field. “One of the big grand challenges identified by the National Academy of Engineering was, how do we continue to make the next big tools of scientific discovery?” says Mori. Tools like both the large particle accelerators or a large facility like NIF that makes fusion or big tools for scientific discovery. To successfully make such tools also requires discovery driven research in the plasma physics domain, he notes.

For instance, could plasma-based acceleration be occurring in the universe, potentially involved in some way in the generation of cosmic rays, in relativistic shocks? Could plasma-based accelerators be used to make compact radiation sources? With a better understanding of inertial confinement fusion, some day could one make a commercial fusion reactor?

Everyone on the team is “very much looking forward to the next generation of supercomputers,” says Tsung. “As computer power increases there will bemore discoveries in the field of laser fusion, and we’re trying to use the PIC method to look into that. In the accelerator world the agreement between particle-based experiments and simulation has been great, and a large part of that is because we can do these large-scale simulations on Blue Waters.”

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