Miniaturizing Particle Accelerators

Professor Chandrasekhar Joshi

Accelerators are the largest and arguably the most expensive scientific devices built by humans to understand matter and forces at their most basic level. The Large Hadron Collider at CERN and the proposed International Linear Collider are both based on fifty-year-old microwave technology. To push the frontier of elementary particle physics beyond these machines, an entirely new paradigm must be invented for building colliders. Now a totally revolutionary technology developed in the Electrical Engineering department by Professor Chandrashekhar Joshi's research group promises to miniaturize these gigantic machines by a factor of a thousand or more. Here the accelerating structure is produced in a diluted plasma and powered by a laser beam or a particle beam.

In a culmination of more than two decades of research, Professor Joshi and his collaborators from the University of Southern California and Stanford Linear Accelerator Center (SLAC) successfully doubled the energy of the electrons from the Stanford Linear Accelerator in a distance of less than one meter using a plasma. The electrons first traveled through three kilometers of the linear accelerator at SLAC gaining 42 billion electron volts (or GeV) energy. Then they passed through a meter long cell of lithium where they excited a space charge density wave or a wakefield, rather like the wake produced by a motorboat as it speeds through water. The electric field of this plasma wave accelerated the electrons in the back of the same beam up to a maximum energy of 85 GeV. The researchers call their scheme a Plasma Afterburner because, like an afterburner on a jet engine, the plasma is used to provide extra thrust.

The experimental results, published in the 2007 February issue of Nature, demonstrated one of the biggest acceleration gradients ever achieved. The gradient is a measure of how quickly particles gain energy. In this case, the electrons zooming through the plasma chamber gained 3000 times more energy per meter than they usually do in the accelerator.