Wakefield accelerators

[twilyth]

This is very exciting because it could represent the future of particle physics. As the article explains, modern accelerators rely on power hungry methods to focus and accelerate particles. Specifically, klystrons are used to generate the microwaves that are used to boost particles up to nearly the speed of light.

But using a plasma wakefield is much more powerful and efficient and can generate tremendous energies in a very small space. At present, they can't match traditional accelerators but that could change.

They work by using a sort of rebound effect caused by the flow of charged particles in a plasma.

So Hogan and his colleagues began searching for a way to boost collider energies without dramatically scaling up in power and size.

The team created a plasma of hot lithium gas — essentially a soup of atoms with electrons stripped off — in an 11.8-inch long (30 centimeters) chamber.

The team then shot two bursts of tightly focused electrons traveling at near the speed of light into the plasma, "like a machine gun, one after another," said study co-author Michael Litos, who is also a physicist at the SLAC National Accelerator Laboratory.

Because particles of like charge repel each other, the first bunch of electrons pushes the electrons in the plasma out of the way, while the bigger lithium ions are too massive to move and stay in place. These lithium ions then pull the plasma electrons back into place, creating a bubble like the "wake around a boat," Hogan said. The movement of electrons also generates a huge electric field inside the wake.

The second bunch of electrons trails the first by just a hair's breadth, essentially surfing on the wake of the first pack. By positioning the two bunches of electrons just right, the energy put into the plasma by the first bunch of electrons is efficiently sucked out the plasma's electric field by the second group of electrons, Litos said.

 

[twilyth]

Another article on the subject from Fermilab/SLAC - http://www.symmetrymagazine.org/art...ss-toward-plasma-acceleration?email_issue=620

 

[twilyth]

It looks like wakefield accelerators could replace linear electron-positron accelerators.

Existing accelerators send beams of charged particles along evacuated pipes interspersed with cavities filled with electric fields. Each field imparts a small kick to the passing particles. Protons are accelerated in circular machines such as the LHC, meaning they pass through every cavity many times. Electron accelerators are linear. The reason for the difference is that when a charged particle has its course bent (which is done using powerful magnets) it radiates energy away. This radiation is inversely related to a particle’s mass and, since protons weigh 1,836 times as much as electrons, radiative losses are far greater for electrons than protons. Electron accelerators, which physicists still like because they can use them to answer questions which protons have difficulty addressing, thus need particularly long (and therefore expensive) pipes and tunnels. The next planned electron accelerator, the International Linear Collider (ILC), would, were it to be built, extend for 31km but would have less than 3% of the LHC’s oomph. The AWAKE collaboration has therefore turned its attention to building a better electron accelerator.

The crucial difference between a conventional accelerator and a wakefield one is that the pipe through which the particles travel is not evacuated. Instead, it is filled with a thin gas heated to such a temperature that its atoms lose some of their electrons, becoming ions and forming a plasma. This plasma can then be used to create an appropriate electrical field.

The AWAKE collaborators plan to employ a plasma of vaporised rubidium (an element that is easily ionised) held in a 10 metre-long tube. Into it, using one of CERN’s existing circular accelerators, they will inject bunches of fast-moving protons. As these positively charged particles zip through the tube, they will attract the plasma’s free electrons (which are negatively charged), creating a region of concentrated negative charge in their wake. This will, in turn, be surrounded by a region of concentrated positive charge caused by the electron-deficient (and so positively charged) rubidium ions. These adjacent charged regions following in the wake of the passing protons will create a strong electric field (hence the name “wakefield”). Such fields can be used to accelerate bunches of electrons injected into the plasma alternately with the protons.

According to AWAKE’s spokesman, Allen Caldwell of the Max Planck Institute for Physics in Munich, the electric fields in a machine like this will be 100 times stronger than those of the ILC. A wakefield machine of equivalent strength would be about a kilometre long. It would, admittedly, also need a separate, circular accelerator to provide the protons. But that could be an existing machine no longer required for its original purpose.

 

[CAPSLOCKSTUCK]

A picture speaks a thousand words

 

[Sasqui]

A video is worth a million