Smith–Purcell effect
Introduction
The **Smith–Purcell effect** was the precursor of the [free\-electron laser](/wiki/Free-electron_laser "Free-electron laser") (FEL). It was studied by Steve Smith, a graduate student under the guidance of [Edward Purcell](/wiki/Edward_Mills_Purcell "Edward Mills Purcell"). In their experiment, they sent an energetic beam of [electrons](/wiki/Electron "Electron") very closely parallel to the surface of a ruled optical [diffraction grating](/wiki/Diffraction_grating "Diffraction grating"), and thereby generated [visible light](/wiki/Visible_light "Visible light"). Smith showed there was negligible effect on the trajectory of the inducing electrons. Essentially, this is a form of [Cherenkov radiation](/wiki/Cherenkov_radiation "Cherenkov radiation") where the [phase velocity](/wiki/Phase_velocity "Phase velocity") of the light has been altered by the periodic grating. However, unlike Cherenkov radiation, there is no minimum or threshold particle velocity.

Smith–Purcell radiation is particularly attractive for applications involving non\-destructive beam diagnostics (bunch\-length diagnostics in accelerators for example) and especially as a viable [THz radiation](/wiki/Terahertz_radiation "Terahertz radiation") source, which has further broad\-range uses in diverse and high\-impact fields like materials sciences, biotechnology, security and communications, manufacturing and medicine. Operating at THz frequencies also allows for potentially large accelerating gradients (\~10s GeV/m) to be realised. This, paired with [plasma\-wakefield](/wiki/Plasma_acceleration "Plasma acceleration") acceleration methods under development and [linear accelerator](/wiki/Linear_particle_accelerator "Linear particle accelerator") (linac) technology, could pave the way to next\-generation, compact (and hence cheaper), less prone to [RF](/wiki/Radio_frequency "Radio frequency") breakdown (current limits for surface E fields are of the order of 10s\-100 MV/m), high energy output linacs.


Who discovered the Smith-Purcell effect?