Laser Plasma Accelerators achieve 100 electron fluxes per second

Laser plasma accelerators can accelerate particles in distances up to 1,000 times shorter than traditional accelerators require. This technology promises compact systems that are very likely to open new accelerator applications, for example, in medicine and industry. However, the current prototype has one drawback. Most are not sufficient for real applications and can accelerate small number of particle bunches per second.

Kaldera, Desy’s new flagship laser, has taken a decisive step. Driving the compact plasma accelerator Magma, it has been shown that an innovative laser accelerates 100 particle bundles per 100 seconds. This increase in repetition rate opens a path to positively stabilize the performance of future plasma accelerators, which brings them to a much closer look to the initial application.

In traditional accelerators, radio frequency waves are fed to the so-called resonator. These waves pass through particles (mostly electrons) passing through them, allowing them to transfer energy to energy. To raise particles to high energy levels, a large number of resonators must be connected in series. This makes the system long and expensive.

A method still under development and requires less space is laser plasma acceleration. Here, the laser lights a short, powerful pulse into a tube filled with small hydrogen in a fraction of just a few minutes, one millimeter in diameter. The so-called plasma cells contain electric charging gases or plasma. When a laser pulse passes through the plasma, waves are created behind it, like a wake formed behind the boat. This plasma wave is very highly charged so that it can accelerate electrons very quickly at distances of just a few millimeters.

This technique can significantly reduce the size of the accelerator. The 100-meter facility can be replaced with machines that fit into the lab in the basement.

“This allows us to build more cost-effective machines, like free electron lasers,” notes Andreas Maier, lead scientist in plasma acceleration at Desy. Several prototypes have already demonstrated the viability of this technology in Desy as well.

“We were the first in the world to successfully execute a laser plasma accelerator for over 24 hours,” explains Maier. “It accelerated electrons every second.” But this certainly isn’t enough for practical applications. Performable operations in FEL require hundreds to thousands of shots per second.

This is a goal pursued by the team of researchers behind the caldera.

“In the future, this new laser will be the one that will make the technology competitive,” said Manuel Kirchen, team leader in high average power laser plasma acceleration in Dessie’s plasma group.

The setup is based on a technique in which a special laser first produces weak but very short light pulses. These pulses are directed at crystals previously charged with energy by a pump laser, and transfer this energy to a light pulse. Then there is an optical amplifier, with each stage increasing the energy of the pulse.

To prevent increasingly powerful pulses from destroying the system, they use special optical systems to extend to several times their original length. The elongated pulses can then be further amplified without damaging the optics. After final amplification, it is squeezed again by a so-called compressor.

“The surface consists of thin layers with a thin lattice structure,” explains Guido Palmer, head of laser development at Dessie’s Plasma Group. “These absorb less heat than gold coatings used in the past, allowing for new compressor designs that are key to the operation of the caldera.”

The resulting laser pulses leaving the device are short and extremely powerful.

With the initiative of Accelerator Division Director Wim Leemans, the team began construction of Kaldera in 2020.

“Already in our first attempt, we were able to accelerate 100 electronic bunches per second,” Kirchen said. “This is the basis for a significant improvement in the quality of the electronic bunch through aggressive stabilization.”

At this time, vibrations, air fluctuations, or instability of the power grid can significantly destroy the bundle of particles and adversely affect its properties.

“When the air moves, the laser beam moves too,” adds Palmer. “This could result in individual electron bunches having different intensity or energy distributions.”

However, adaptive techniques can compensate for these variations. The laser is equipped with a sensor that includes a camera that accurately measures laser pulses. If the image shows deviation due to interference, the computer calculates the correction signal. This signal, for example, controls the mirror and directs the laser pulse in the desired direction. Such correction techniques are only possible if the laser fires frequently enough. It’s like a caldera, like a caldera with 100 shots per second.

“This is how we can also operate large accelerator facilities such as the Desy Storage Ring Petra III,” says Leemans. “These facilities rely heavily on active feedback systems that maintain uniform quality of the electron beam.”

“Desy’s expertise will benefit greatly from running a world-class particle accelerator, its technology portfolio, and Desy’s philosophy of valued attention to detail,” adds Maier. For decades, Desy has developed and operated highly reliable particle accelerators for its users.

The Kaldera team has already begun upgrading its laser energy. This will allow for higher electron beam energy in the future. Further improvements should be possible in the coming months and years as experts gradually implement ideas for feedback and control within the system.

“With these measures, we hope to reach the ultimate goal of the caldera in a few years,” Lehmans says. “We want to realize a laser plasma accelerator that produces 1,000 high-quality electronic bunches per second.”