For 30 years, high-energy physicists have wondered about the possibility of building a laser electron accelerator that could reveal the secrets of the universe.

It took two decades and billions of Euros to develop CERN’s (The European Organization for Nuclear Research) Large Hadron Collider (LHC). At 27 kilometers in circumference, it is the world’s largest accelerator, big enough to fit around the country of Monaco. The LHC will soon begin testing advanced theories in physics. But given the size and cost of today’s accelerator technologies, physicists are already wondering where to go next.

A group of researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) in Berkeley, California, working with colleagues from the University of Oxford, may have the answer. In 2006, the team accelerated electron beams to 1 GeV (1 giga-electron volts, or 1 billion electron volts) in a distance of just 3.3 centimeters, proving that good beams and high energies can be achieved over small distances. This baby-step approach using lasers may ultimately lead to the development of a particle accelerator no bigger than a football stadium, which could achieve the high energies needed to test the fundamental theories of the universe.

High-energy physicists have been pondering the possibilities of laser accelerators since at least 1979. The biggest hurdle was making a laser that could penetrate a gas to form a plasma with free electrons, without the laser beam dispersing along the way. Shine a flashlight on a spot nearby and the light forms a tight circle. Farther away, however, the spot becomes larger and less defined. Lasers similarly lose their focus beyond a short distance, called the Rayleigh length. If all one needs is a laser pointer, even an out-of-focus laser beam appears to make a point of light several feet away, because the beam starts out so tightly focused. A laser accelerator is more demanding.

A laser beam traveling through a plasma essentially creates a “wake” around it as it moves. Free, negatively charged electrons ride this wake in the plasma “surf” when dissociated from positively charged ions. Getting these electrons up to speeds necessary to test questions in high-energy physics requires that the electrons travel over multiple Rayleigh lengths.

Measuring the speed of a cross-section of a laser beam reveals that the center of the beam is traveling faster than the light around the center. The solution is to slow down the light’s center and speed up the light that is moving in the wake, so that both are traveling closer to the same speed.

Wim Leemans, head of the Laser Optics and Accelerator Systems Integrated Studies (L’OASIS) Program of the Accelerator and Fusion Research Division at Berkeley Lab, began testing laser wakefield acceleration with his graduate students in 1995, after Howard Milchberg from the University of Maryland developed the basic principle in the mid-1990s. Eventually, Leemans and his team took a multiple-laser approach to the problem. The first laser pulse forms a plasma channel in a plume of hydrogen gas. A second pulse from the side heats the plasma and causes it to expand, leaving a void in the center through which the third laser plows its way with a driving intensity of 10 trillion watts of power.

As Leemans explained, “Plasma actually has a lower index of refraction than vacuum, so the wavefront moving through the center of the channel moves slower than at the edges. This flattens the otherwise spherical laser wavefront and extends the distance over which the laser stays intense enough to excite large plasma waves.”

In 2004, Leemans and his Berkeley Lab team were one of three groups, including teams based at Imperial College, London, and the École Polytechnique, Paris, who achieved peak energies of 70 to 200 MeV (mega-electron volts, or million electron volts) with laser wakefield acceleration. One approach to the 1 GeV goal appeared to require using larger spot sizes for the lasers, which would dramatically increase the cost and power requirements.

Leemans and his team found a way to extend their plasma-channel approach through a collaboration with Simon Hooker at Oxford University, whose group had been able to maintain plasma channels inside blocks of synthetic sapphire. “How many laser shots can it take?” Leemans asked. “As many as you like,” Hooker replied. It was a match made in stone.

In October, 2006, the physicists published a report in the journal Nature Physics describing their technique for getting from zero to 1 billion electron volts in 3.3 centimeters. “Billionelectron- volt beams from laser wakefield accelerators open the way to very compact high-energy experiments and superbright free-electron lasers,” Leemans said. The group has proposed to DOE a 10-GeV acceleration module project called BELLA (Berkeley Lab Laser Accelerator), which could be operating in 2013. The goal is to provide a unique user facility for scientists who need advanced light sources and free-electron lasers. “Meanwhile, we’ll be on the way to designing a new generation of powerful accelerators and colliders based on laser wakefield acceleration technology,” Leemans says. “BELLA will help insure that the unique science U.S. Department of Energy has made possible through its leadership in advanced accelerator research will go forward into the future with laserbased technologies.”