Laser tandem will unite electron wake accelerators into a collider

(ORDO NEWS) — Scientists from the University of Berkeley have created an installation for the simultaneous control of two laser wake accelerators, which will allow “additional acceleration” of the accelerated electrons and push them into each other.

Traditional elementary particle accelerators cannot act on them with an electric field exceeding several tens of megavolts per meter. This limit is one of the reasons for the gigantic size of modern accelerators: when it is exceeded, an electrical breakdown of structures inevitably occurs.

The creation of accelerating fields in a plasma , consisting of free electrons and ions, is able to avoid the limitations associated with dielectric strength.

Without support, the electric fields in it quickly decay, but their “instantaneous” intensity is practically not limited by anything – in the plasma, “everything that is possible has already been broken through.”

The recipe for using giant electric fields in plasma is to use the separation of electric charges before they have time to shift and compensate for the electric field that has arisen between them.

Laser acceleration of elementary particles is based on the separation of charges in plasma under the action of super-powerful femtosecond laser pulses. A femtosecond is one billionth of a millionth of a second, and a typical pulse length of tens of femtoseconds is a few micrometers long.

Laser tandem will unite electron wake accelerators into a collider 1
Simulation of a positive charge bubble in plasma under the action of a driver beam, in the absence of accelerated electrons (left) and with their input (right). Blue shows the electron density in the plasma, and orange shows the intensity of the beam (on the right in each picture), and the density of the accelerated electrons (on the left in the right picture). The graphs show the longitudinal component of the electric field on the beam axis in gigavolts per meter

Electrons are much lighter than protons and atomic nuclei, and respond faster to electromagnetic fields. Getting into the plasma, the laser pulse literally “scatters” electrons from its path.

A positively charged “bubble” with an excess of ions is formed, which attracts the scattered electrons back. Behind the bubble, they converge, creating a region of very dense negative charge.

This bubble, like the charge waves behind it, moves through the plasma following the laser pulse at a speed close to the speed of light.

The electric field between the bubble and its “wake” can reach hundreds of gigavolts per meter, and the electrons that find themselves in the bubble “roll” through the electric field, pushing off the negative charge, like surfers from an ocean wave.

There are many plasma acceleration schemes using laser pulses, charged particle beams, and combinations thereof. Wake accelerators are already capable of accelerating electrons up to several gigaelectronvolts in bench-top facilities, which are hundreds of times smaller and much cheaper than traditional linear accelerators.

But plasma acceleration has “built-in” disadvantages. The process is “superfast” by its nature, and the acceleration region usually does not exceed centimeters in length – then the laser pulse dissipates in the plasma.

Accelerated electrons have a strong spread in energies and directions of flight, and much more precise control of their parameters is required for research in elementary particle physics.

Laser tandem will unite electron wake accelerators into a collider 2
Left: deformable mirrors that control the focusing of laser pulses. Right: The second laser beam line built in the BELLA laboratory.

The employees of the BELLA (Berkeley Lab Laser Accelerator Center) Center of the Lawrence Berkeley National Laboratory, headed by Eric Esarey, took up the improvement of laser acceleration. Their laboratory’s main facility is a petawatt-peak pulsed laser (one petawatt equals a billion megawatts).

In a new press release, the researchers talked about the upgrade of the beam steering facilities and the completion of a second beam feed line that uses part of the pulse from the main laser.

The second line will become an independent source of pulses, the parameters of which can be controlled over a wide range. The use of a pulse from the same laser is necessary in order to more accurately synchronize the operation of the pulses.

It is very difficult to fire two separate lasers with femtosecond precision, but the delay between two parts of the same pulse can be controlled much more precisely, which is what the second line provides.

Thus, instead of one laser accelerator, the laboratory now has two, which can be configured to combine with each other in almost any way.

Modernization allows you to independently control the duration and duration of each pulse, and the interval between them. In addition, mirrors with a deformable surface were added to both lines, making it possible to fine-tune the focusing of laser pulses.

Scientists hope that the upgrade will allow them to assemble plasma accelerators in tandem, as well as build a collider out of them.

In the first case, the task is to pick up an electron bunch emitted from one plasma channel and accelerate it in the second channel. In this case, the parameters of the pulses must be chosen so as to prevent the scattering of electrons.

In the second case, the electrons will fly towards each other, and the control of the trajectory of the “bubbles” with an accuracy of femtoseconds and micrometers will prevent their bunches from missing each other in space and time.

If these tasks can be solved for laser accelerators, over time they can become a compact alternative to some varieties of giant colliders.


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