A simple way to tune highly stable electron beams through a wide range of energies for tabletop accelerators has been discovered by researchers in the LOASIS program at the US Department of Energy’s Lawrence Berkeley National Laboratory. The team has demonstrated high-quality beams up to a billion electron volts in a mere 3.3 cm, but now the same accelerating structure has been modified to tune stable, high-quality beams from 100 million to 400 million electron volts. A laser pulse through a capillary filled with hydrogen plasma creates a wake that can accelerate an electron beam to a billion electron volts in just 3.3 cm. The same LOASIS accelerating structure has been modified to tune stable, high-quality beams from 100 to 400 million electron volts. (Images: Roy Kaltschmidt, Berkeley Lab Public Affairs) This breakthrough not only could affect compact high-energy colliders for fundamental physics, but also diminutive sources of intensely bright beams of light, spanning the spectrum from microwaves to gamma rays — a new kind of ultrafast light source for investigating new materials, biological structures and green chemistry. And, compared with today’s giant science facilities, tabletop laser plasma accelerators eventually may be able to do equally powerful research with minimal environmental impact at a fraction of the cost. To describe how the laser plasma accelerator works, Wim Leemans, who heads the LOASIS program, used the analogy of a surfer riding a wave. “The surfers are the electrons themselves. The waves form when a laser pulse plows through a plasma.” A laser plasma accelerator uses a laser pulse (red and blue disks, extreme right) to create a wake through a plasma, creating strong electric fields. As with surfers on a wave, free electrons ride the wake and are accelerated to high energies. Only the electron bunch propelled by the first wave (white glow) is shown in this simulation. (Simulation by Jean-Luc Vay and Cameron Geddes) In a plasma, atomic nuclei (ions) are separated from electrons, and immensely strong electric fields can build up between the oppositely charged particles when they are separated by the waves behind a powerful laser pulse. Some of the electrons in the plasma are swept up by the waves and are quickly accelerated to high energy. “In this case, the wave is a tsunami, and it doesn’t much matter what the surfers do; they’ll be carried along,” Leemans said. “That’s called self-trapping. But there are other ways a surfer can catch a wave. Real surfers can gauge the size and speed of an oncoming wave and start paddling to match its momentum.” A supersonic jet of helium gas (red, upper panel) greatly increases plasma density upstream of the hydrogen-filled capillary (blue). In the lower panel, the black curve shows density increasing, then falling off downstream. The blue curve indicates the phase velocity of the wake, slowing where the laser pulse (pink) is focused, then increasing again as density decreases. Attempts to create tunable electron beams through momentum-matching have been tried by injecting electrons into the accelerating field — first giving them a boost using colliding laser pulses to catch the wave, then using a different drive-laser pulse to excite a wave on which those surfing electrons can be accelerated to high energies. It’s an approach that demands sophisticated timing and synchronization and, along with other tuning methods for one-stage accelerators, requires electron injection that is localized in space and time. Leemans, however, found a third way of helping the electrons catch a wave — a two-stage process. The researchers modified the same 3.3-cm LOASIS accelerator and the same 40 trillion W peak-power drive laser, dubbed TREX, that they used to produce the first billion-electron-volt beam. The accelerator is a block of titanium sapphire with a narrow capillary through it, filled with hydrogen gas that is ionized to a plasma by a jolt of electricity, just before the drive-laser pulse enters. Slowing the laser wake and then speeding it up requires controlling the wake’s phase velocity. To modify the LOASIS system for two-stage, tunable acceleration, the researchers introduced a supersonic jet of helium gas that passes through the accelerator’s hydrogen-filled capillary at the upstream front end. This sharply increases the density of electrons in the subsequent plasma. The plasma density then falls off rapidly downstream. “The extra density itself serves as a lens to focus the laser to higher intensity, and the laser is focused right where the extra density is beginning to decrease,” Leemans said. Here, at the edge of the “density downramp,” the slower waves trap electrons more readily. “The waves in the wake are falling farther behind the laser pulse as it enters the region of lower density.” Density control is only one way to control wave velocity, however. Another method is through laser intensity — an unexpected gift from Albert Einstein’s Special Theory of Relativity. Leemans explained, “The particles in the plasma waves have slowed because of the increased density, but they’re still moving relativistically near the speed of light.” By tailoring plasma density in the two zones over the length of the accelerator, the LOASIS researchers tuned the energy of the electron beams over a range from 100 million to 400 million electron volts, while maintaining energy stability to within a few percent. “Tailoring plasma density longitudinally this way is a concept that shows a new path to the level of sophisticated tuning for accelerators and light sources that users of conventional facilities just take for granted,” Leemans said. “It’s a major step toward perfecting the laser plasma light sources and accelerators of the future.” The study was published in Nature Physics. For more information, visit: www.lbl.gov