Photonic Crystal Accelerator

At many universities and national labs, electrons are accelerated to high speeds by electric fields imparted by gusts of microwaves. The cavities in which these microwaves are delivered (by devices called klystrons) will support a main radiation mode and other, wakefield, modes, or overtones, as well. For example, at SLAC, which uses microwaves at a frequency of 3 GHz, the presence of the overtones is not a big problem, but for future machines, such as the prospective Next Linear Collider (30 miles long), problems could arise. If this machine were to operate with superconducting equipment, overtones might eject electrons from the main beam, causing them to smash into the sides of the accelerating channel, causing a loss of superconductivity and the shutdown of the accelerator. Now, however, physicists at MIT have used photonic crystals, material structures which allow the passage of light at some frequencies but not others, to greatly limit overtones in an accelerator cavity. This represents the first time a photonic crystal (also referred to as a photonic bandgap, or PBG) structure has acted as an accelerator. Furthermore, in this case the acceleration gradient, an important measure of an accelerator's efficacy, was 35 MeV/m. This is twice the value one normally obtains at the MIT linac being used for this test, where an electron beam with an energy of 17 MeV was boosted by an additional 1.4 MeV in the photonic-crystal structure, which consists of arrays of tiny rods and operates at a frequency of 17 GHz. The next step, says MIT scientist Evgenya Smirnova (now at Los Alamos, smirnova@lanl.gov, 505-667-5634), is to build a longer accelerator structure and use much more klystron power. With this, a much higher acceleration gradient should be possible. (Smirnova et al., Physical Review Letters, 12 August; lab website: www.psfc.mit.edu/wab/novel-ele.html)