Channeling through plasma to accelerate bright beams for science and medicine

In work that brings the promise of laser driven particle accelerators dramatically closer to reality, with the potential to shrink accelerators from miles in length to meters and open new applications from medicine to high energy physics, researchers at Lawrence Berkeley National Laboratory have produced high quality electron beams in an accelerating structure of only a few millimeters long.

The L'OASIS group (L'OASIS stands for Laser Optics and Accelerator Systems Integrated Studies) uses a technique called plasma-channel guiding to produce tightly focused beams containing billions of electrons, all within a few percent of the same high energy – near 100 million electron volts.

Laser wakefield accelerators send a laser pulse through a gas to form a plasma of dissociated electrons and ions. The radiation pressure of the laser pushes the plasma electrons aside, creating a density modulation, or 'wake'. This changing electron density creates a field that accelerates particles thousands of times more strongly than in conventional machines, accelerating electrons to high energies in short distances. In analogy, "Imagine that the plasma is the ocean, and the laser pulse is a ship moving through it. The electrons are surfers riding the wave created by the ship's wake." says Wim Leemans. The compactness of these accelerators would allow higher energies for the next frontier of fundamental physics and make clinical and laboratory applications of accelerators practical.

Unfortunately, simply punching a laser pulse through a plume of gas makes for a very short trip, since the laser spreads out as it travels. Laser accelerators to date have thus produced diffuse beams, widespread in energy, with less than one percent of the electrons energetic enough to be useful for many applications.

The L'OASIS group uses preliminary laser pulses to open a channel through the plasma before sending the accelerating pulse through. The plasma channel is dense along its walls and less dense in the center, which counteracts the natural tendency of the laser pulse to spread as it propagates. Thus electrons can be accelerated for far longer distances while maintaining a tight focus.

Using the preformed plasma channel, acceleration length was carefully matched to the so-called dephasing length, the distance at which the electrons outrun the wake and stop gaining energy. Not only is particle energy greatest at this optimal length, but the electrons are bunched close together -- with bunches estimated at 10 femtoseconds (10 quadrillionths of a second) in duration -- and all have nearly the same energy, which is critical for applications. The focusablity of the beam is comparable to state of the art conventional accelerators.

Controlled guiding of intense laser pulses as demonstrated in these experiments will be the mechanism for scaling these accelerators efficiently to higher energies, and, eventually, staging them in sequence to reach very high energies. The result will be a new generation of compact, bright, ultrashort electron sources, with applications in highenergy physics and as sources of unique x-ray and infrared radiation.