‘Flying focus’ laser overcomes key limitation in plasma-based particle accelerators

In a brand new Nature Physics examine, researchers accelerated electrons to greater than twice the power predicted by the normal dephasing restrict for laser-plasma accelerators working over the identical distance. This was made potential by a specifically engineered laser pulse known as a flying focus, which counteracts a longstanding limitation referred to as “dephasing.”

Laser-plasma accelerators use an intense, ultrashort laser pulse to drive a wave of cost by way of a plasma. In precept, this makes it potential to speed up particles to very excessive energies over simply centimeters, quite than kilometers.

The issue is that the accelerating plasma wave can not preserve tempo with the electrons driving it. The electrons transfer at speeds near the pace of sunshine, however the laser pulse driving the wave travels barely slower. Over distance, the electrons handle to outrun the plasma wave. This dephasing causes the electrons to cease gaining power, thereby affecting the quantity of power such accelerators can ship.

Phys.org spoke to Dr. Charlie Arrowsmith, assistant scientist on the College of Rochester’s Laboratory for Laser Energetics (LLE) and first creator of the examine.

“Dephasing is when the ultra-relativistic electrons accelerating in a wakefield begin to meet up with the laser pulse driving the wakefield,” Arrowsmith stated. “When this occurs, the acceleration ends prematurely and the electrons cease gaining power.”

Dephasing electrons

In a laser-plasma accelerator, the ultrashort laser pulse would not immediately speed up electrons. Because it travels by way of a fuel, it first strips electrons off the encompassing atoms, making a plasma. The extreme pulse then pushes these newly freed electrons out of its path, whereas the a lot heavier, positively charged ions are left behind, largely undisturbed.

In what’s referred to as the “blowout regime,” this creates a cavity virtually fully emptied of electrons, trailing simply behind the laser pulse. The displaced electrons curve again round this cavity and pile up in a dense sheath alongside its boundary.

The ensuing separation of constructive and unfavourable cost units up an electrical subject that may exceed 1 GV/cm—orders of magnitude stronger than the fields sustained in typical radio-frequency accelerators. This construction is usually known as a wakefield as a result of it trails the laser pulse very like the wake behind a ship.

Some electrons get swept up into this construction and “trapped,” driving alongside within the area of the sphere that pushes them ahead, gaining power the entire time they continue to be in that section of the wave.

That is the place dephasing is available in. The trapped electrons are ultra-relativistic, shifting near the pace of sunshine. However the wave carrying them ahead cannot go fairly that quick. Its pace is about by the group velocity of the laser pulse driving it, which is at all times barely under the pace of sunshine in plasma. Over distance, the electrons step by step catch as much as and overtake the accelerating area of the wave, drifting out of the candy spot that had been pushing them ahead. As soon as that occurs, they cease gaining power.

Physicists have tried to work round this for years, largely by decreasing plasma density, which extends the space an electron can journey earlier than dephasing happens. That is how present state-of-the-art accelerators have reached 10 GeV in a single stage. However decrease density additionally weakens the accelerating subject, that means increased laser energies are wanted to compensate, and reaching 100 GeV this fashion would require plasmas roughly 10 meters (33 toes) lengthy.

Flying focus

To eradicate dephasing, the workforce turned to a method known as dephasingless laser wakefield acceleration (DLWFA), first proposed theoretically in 2020.

The concept is to decouple the plasma wave’s pace from the group velocity of the laser pulse fully, utilizing a “flying focus.” This laser pulse is engineered in order that its level of peak depth might be made to comb ahead at a selected pace, impartial of how briskly the sunshine itself travels by way of the plasma.

“One of many key insights was determining how you can get the scheme to work theoretically, which was not potential with out the assistance of state-of-the-art simulations,” Arrowsmith stated. “One other key perception was realizing {that a} high-intensity flying focus might be made utilizing extremely specialised optics.”

Working with the optical manufacturing workforce on the LLE, the researchers developed methods to manufacture these optics in-house at low price. The flying focus was produced utilizing a mirror known as an axiparabola, whose focal size varies with radius.

Somewhat than focusing all incoming gentle to a single level, the axiparabola sends gentle hanging it nearer to its heart to a nearer point of interest, and light-weight hanging it farther out to a farther point of interest. This spreads the main focus into an prolonged line, alongside which the purpose of peak depth might be made to comb ahead near the pace of sunshine, set by the mirror’s geometry and the plasma’s group velocity.

For the precise experiment, the researchers used a hydrogen-argon combination. The flying focus ionizes hydrogen simply, forming the plasma wave itself. Argon, however, has extra tightly certain electrons, that are stripped solely on the laser’s peak depth. This injection of recent electrons immediately into the wave at managed positions is called ionization injection.

With the flying focus tuned to drive a wakefield near the pace of sunshine, and electrons injected immediately into it, the setup was in place to check whether or not dephasing might truly be eradicated.

Slim window of density

The plasma density contained in the cell was measured on every shot utilizing interferometry, and the power spectrum of the ensuing electron beam was recorded downstream with a magnetic spectrometer.

The plasma density itself was a vital variable. Researchers discovered that solely a slim vary of densities, roughly 4.5 to five.4 × 10¹⁸ cm⁻³, introduced the wakefield’s velocity shut sufficient to the vacuum pace of sunshine to supply dephasingless acceleration. Outdoors this window, the electrons both failed to realize a lot cost or ultimately dephased anyway, albeit over an extended distance than typical.

The electrons reached energies as much as 396 ± 14 MeV. This was greater than twice the calculated dephasing-limited power of 185 MeV anticipated for a similar circumstances utilizing typical laser wakefield acceleration.

“Experimental scientists typically encounter issues not popping out as anticipated in experiments, however this was a type of thrilling events the place the info began popping out simply as predicted,” Arrowsmith stated.

Future work

The demonstration is described as a proof of idea quite than a completed accelerator design, with additional work wanted to scale the approach to increased energies and enhance beam high quality.

The researchers word that including a second optic, known as an echelon, in future experiments might permit the wakefield’s velocity to extra carefully match the trapped electrons, enabling finer management over precisely the place within the wave the electron bunch sits.

The paper means that reaching 100 GeV would require an accelerator size of underneath a meter (3.3 toes), an roughly 20-fold discount in contrast with what a conventional single-stage laser-plasma accelerator would wish to achieve the identical power.

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