High-Power THz-Waves using gyrotrons: new physics results

Gyrotrons are high-power coherent radiation sources based on the cyclotron maser instability. Since more than three decades the research and development of gyrotrons has been essentially driven from the need of high-power MW-level sources in the THz frequency range (0.1-1 THz) for electron cyclotron resonance heating of magnetically confined plasmas such as in ITER and DEMO. A more recent spin-off research activity has led to the development of lower power (1-100 W) gyrotrons with frequencies belonging to the THz domain. One of the main applications in the THz domain is in the field of Dynamic Nuclear Polarization Nuclear Magnetic Resonance spectroscopy (DNP/NMR). With a THz gyrotron oscillator developped for DNP/NMR spectroscopy and operating at 260 GHz, novel operational regimes have been recently experimentally demonstrated in which the non-linear interaction excites a finite number of side-bands and eventually ends in a chaotic dynamics of the THz radiation field. The route to chaos via a period doubling cascade dynamics is experimentally observed and is supported by numerical simulations. In presence of phase-locked frequency-equidistant side-bands, a novel regime characterized by high-power nanosecond pulses is experimentally observed and is associated to a self-consistent Q-switch mechanism in which the cavity diffraction quality factor dynamically varies by nearly two orders of magnitude on a subnanosecond timescale. This novel regime is also well predicted with the self-consistent TWANG code and may open up new applications for gyrotrons.