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An undulator-based superradiant oscillator as a source of intense terahertz pulses is investigated in an open resonating cavity. A confocal configuration for the optical cavity is found to be favorable for electron-radiation coupling. Constructive accumulation of superradiance over successive bunches of electrons is achieved if the bunch period is an even multiple of the round-trip time of the radiation pulse. Numerical simulations show that the bunch slippage relative to the radiation pulse determines the pulse width and the corresponding bandwidth. Automatic bunch-pulse positioning with which energy extraction is optimized is also observed, as was first observed in a closed-cavity environment.

The undulator superradiance of periodic electron bunches in an optical cavity excites many cavity modes over an ultra-wide band. The interaction dynamics involves a coherent potential well constructed by many tiny ponderomotive potentials provided by individual modes. Decoherence between these tiny potentials is caused mainly by the slippage between the bunch and the radiation pulse. The overall influence of the ultra-wide band, however, does not alter the scaling law for the energy-conversion efficiency established in the single-mode interaction.

Coherent undulator synchrotron emission of a short electron bunch may contain many transverse modes of the optical cavity when the emission occurs at an off-grazing resonance. However, their accumulation over periodic bunches can be targeted to a single designated mode, owing to the sensitivity of the phase-matching condition of accumulation. An analysis of mode accumulation, along with a numerical simulation result, is presented.

The coherence effect [1,2] describes emission of radiation from a group of sources whose emission phases are such that the fields of individual sources may be added constructively. While the power of usual incoherent radiation grows in proportion to the number of sources, that of the coherent radiation grows in proportion to its square. A collection of free electrons in a short bunch is an example of such a group of sources. Recent advances in rf accelerator technology have provided us with intense electron bunches that are short enough for inducing coherent effects in the THz frequency range [3, 4]. Coherent THz radiation utilizing those bunches has been demonstrated through various emission processes, such as synchrotron radiation [5], transition radiation [6], and undulator radiation [7]. Recently, Doria et al. proposed that an appropriate energy-phase correlation may enhance the output power of coherent undulator radiation significantly [8,9]. Their claim is that the proposed correlation induces constructive interference of the individual radiation fields emitted by each electron when the electron distribution satisfies the phase-matching condition. This condition is derived from the mode amplitude at the undulator exit [8]