Methods to generate femto- and attosecond electron and X-ray pulses

Short electromagnetic wave pulses of wavelength λ can be used to modulate the energy of a charged particle beam on a scale of λ2, which in turn can generate particle bunching of dimension ≲λ2, in the usual manner. These short bunches can then radiate correspondingly short photon bunches, including higher energy photons. Four types of devices are discussed which can achieve the requisite energy modulation on a scale of λ2: Vacuum, Cherenkov, resonator, and klystron modulators. The first is the simplest, and for short bunches its performance approaches, or is superior to that of the last two. For broadband short pulses it is generally to be preferred.