  We propose a radiation source based on a magnetic mirror cavity. Relativistic
electrons are simulated entering the cavity and their trajectories and
resulting emission spectra are calculated. The uniformity of the particle
orbits is found to result in a frequency comb in terahertz range, the precise
energies of which are tuneable by varying the electron's $\gamma$-factor. For
very high energy particles radiation friction causes the spectral harmonics to
broaden and we suggest this as a possible way to verify competing classical
equations of motion.
