RESOLVING THE LEDGER
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Add as preferred source In a new Nature Physics study , researchers accelerated electrons to more than twice the energy predicted by the traditional dephasing limit for laser-plasma accelerators operating over the same distance. This was made possible by a specially engineered laser pulse called a flying focus, which counteracts a longstanding limitation known as "dephasing."
Laser-plasma accelerators use an intense, ultrashort laser pulse to drive a wave of charge through a plasma. In principle, this makes it possible to accelerate particles to very high energies over just centimeters, rather than kilometers.
The problem is that the accelerating plasma wave cannot keep pace with the electrons riding it. The electrons move at speeds close to the speed of light, but the laser pulse driving the wave travels slightly slower. Over distance, the electrons manage to outrun the plasma wave. This dephasing causes the electrons to stop gaining energy, thereby affecting the amount of energy such accelerators can deliver.
Phys.org spoke to Dr. Charlie Arrowsmith, assistant scientist at the University of Rochester's Laboratory for Laser Energetics (LLE) and first author of the study.
"Dephasing is when the ultra-relativistic electrons accelerating in a wakefield start to catch up with the laser pulse driving the wakefield," Arrowsmith said. "When this happens, the acceleration ends prematurely and the electrons stop gaining energy."
In a laser-plasma accelerator, the ultrashort laser pulse doesn't directly accelerate electrons. As it travels through a gas, it first strips electrons off the surrounding atoms, creating a plasma. The intense pulse then pushes those newly freed electrons out of its path, while the much heavier, positively charged ions are left behind, largely undisturbed.
In what's known as the "blowout regime," this creates a cavity almost entirely emptied of electrons, trailing just behind the laser pulse. The displaced electrons curve back around this cavity and pile up in a dense sheath along its boundary.
The resulting separation of positive and negative charge sets up an electric field that can exceed 1 GV/cm—orders of magnitude stronger than the fields sustained in conventional radio-frequency accelerators. This structure is often called a wakefield because it trails the laser pulse much like the wake behind a boat.
Some electrons get swept up into this structure and "trapped," riding along in the region of the field that pushes them forward, gaining energy the whole time they remain in that phase of the wave.
This is where dephasing comes in. The trapped electrons are ultra-relativistic, moving close to the speed of light. But the wave carrying them forward can't go quite that fast. Its speed is set by the group velocity of the laser pulse driving it, which is always slightly below the speed of light in plasma. Over distance, the electrons gradually catch up to and overtake the accelerating region of the wave, drifting out of the sweet spot that had been pushing them forward. Once that happens, they stop gaining energy.
Physicists have tried to work around this for years, largely by lowering plasma density, which extends the distance an electron can travel before dephasing occurs. This is how current state-of-the-art accelerators have reached 10 GeV in a single stage. But lower density also weakens the accelerating field, meaning higher laser energies are needed to compensate, and reaching 100 GeV this way would require plasmas roughly 10 meters (33 feet) long.
To eliminate dephasing, the team turned to a technique called dephasingless laser wakefield acceleration (DLWFA), first proposed theoretically in 2020.
The idea is to decouple the plasma wave's speed from the group velocity of the laser pulse entirely, using a "flying focus." This laser pulse is engineered so that its point of peak intensity can be made to sweep forward at a chosen speed, independent of how fast the light itself travels through the plasma.
"One of the key insights was figuring out how to get the scheme to work theoretically, which was not possible without the help of state-of-the-art simulations," Arrowsmith said. "Another key insight was realizing that a high-intensity flying focus can be made using highly specialized optics."
Working with the optical manufacturing team at the LLE, the researchers developed techniques to fabricate these optics in-house at low cost. The flying focus was produced using a mirror called an axiparabola, whose focal length varies with radius.
Rather than focusing all incoming light to a single point, the axiparabola sends light striking it closer to its center to a nearer focal point, and light striking it farther out to a farther focal point. This spreads the focus into an extended line, along which the point of peak intensity can be made to sweep forward close to the speed of light, set by the mirror's geometry and the plasma's group velocity.
For the actual experiment, the researchers used a hydrogen-argon mixture. The flying focus ionizes hydrogen easily, forming the plasma wave itself. Argon, on the other hand, has more tightly bound electrons, which are stripped only at the laser's peak intensity. This injection of new electrons directly into the wave at controlled positions is known as ionization injection.
With the flying focus tuned to drive a wakefield close to the speed of light, and electrons injected directly into it, the setup was in place to test whether dephasing could actually be eliminated.
The plasma density inside the cell was measured on each shot using interferometry, and the energy spectrum of the resulting electron beam was recorded downstream with a magnetic spectrometer.
The plasma density itself was a c...
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