If you throw a stone into water faster than the waves spread, a cone of ripples forms behind it. Electrons do the same thing when they fly through gas: they outrun light in that medium and leave a light cone behind, which is Cherenkov radiation. Usually it looks like a blue glow. But if there are many electrons and they fly in a tight group, their light can add up and amplify, like voices in a choir rather than the noise of a crowd.
Until now, such amplification was only achieved for very long radio waves. The reason lies in the size of electron bunches: they were too large, so light from different electrons was out of phase. Now new accelerators can make tiny bunches, a few femtoseconds (millionths of a nanosecond) long and tens of micrometers in radius. That is like assembling a choir of singers standing very close together: their voices merge into one powerful sound.
It turns out that the bunch's shape strongly affects the color and brightness of the light. If the bunch has a smooth shape, the light is reddish. If the edges are sharp, like a cylinder, the light shifts to ultraviolet. From the light spectrum, one can reconstruct the bunch's length and radius without touching it. Previously, this required destroying the beam or using methods with poorer resolution. Now there is a way to measure the bunch's shape simply by looking at its light.
Cherenkov radiation is often called 'blue glow,' but in this case it can extend into the ultraviolet, invisible to the eye. For a beam with energy 1 GeV, the Cherenkov angle in helium is only 0.5 degrees, so the radiation goes almost strictly forward.
In Arthur C. Clarke's novel 'The Light of Other Days,' Cherenkov radiation is used to detect particles in water; the idea of coherent enhancement from dense bunches echoes the description of radiation from relativistic streams in space.