Coherent THz Pulses from Linear Accelerators

Coherent THz pulses are being produced at several facilities using relativistic electrons from linear accelerators. The THz pulses produced at the Brookhaven accelerator have pulse energies exceeding 50 μJ and reach a frequency of 2 THz. The high repetition rate of the Jefferson Lab accelerator leads to an average THz power of 20 watts. Possible uses for these high power pulses are discussed. Coherent Synchrotron Radiation As first demonstrated by Nakazato and co-workers[1], short bunches of relativistic electrons experiencing a common acceleration radiate coherently for wavelengths greater than the bunch length. Subsequent studies confirmed the detailed relation between the spectral content and electron bunch longitudinal density[2], and it was recognized that this coherent radiation could be a new source of broadband millimeter and submillimeter waves[3]. The characteristics of these pulses are similar to those produced by ultra-fast lasers and photoconductive switches. The coherent radiation appears as a transform-limited pulse, with an intensity that scales as N 2 (where N is the number of electrons in the bunch), giving rise to a coherent enhancement factor of N when compared to the incoherent case. However, compared to pulses produced with photoconductive switches, the power emitted by relativistic electrons can be significantly larger, even for the same acceleration. This can be seen from Larmor’s formula,