DFG-RSF: Generation of Ultrashort Pulses in Millimeter and Submillimeter Bands for Spectroscopy and Diagnostic of Various Media Based on Passive Mode-locking in Electronic Devices with Nonlinear Cyclotron Absorber in the Feedback Loop.

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The project is planned to run in frame of a joint DFG-RSF research collaboration, namely between IHM, KIT, Karlsruhe and IAP-RAS, Nizhny Novgorod. This proposal describes the full project including both, RAS and DFG parts. Significant improvement of functionality of DNP-NMR (sensitivity, resolution, data acquisition speed, etc.), as well as other methods of monitoring the various media (e.g., dense fusion plasma diagnostic) can be achieved using submillimeter range radiation sources with controllable spectrum. The required spectrum controlling not only provides a smooth sweep of the center frequency over a wide range, but also the generation of a periodic sequence of short pulses with a correlated phase and adjustable repetition rate. This project aims for the development of methods for the generation and amplification of powerful wideband coherent signals of millimeter and submillimeter frequency range on the basis of vacuum electron amplifiers (including gyro-resonance amplifiers). For the first time it is proposed to investigate a mechanism of pulse generation similar to the method of passive mode locking (widely used in laser physics) on the basis of vacuum electronics devices. For this purpose, a nonlinear amplitude filter - the element that absorbs microwave signals with small amplitude and transmits signals with a high power - is placed into the feedback loop of the generator. The filter is expected to be implemented on the basis of the non-linear cyclotron absorption mechanism.The theoretical program of the project includes a detailed analysis of a new mechanism to generate ultrashort pulses within the framework of analytical and computer models, as well as a comparison of the results with the known characteristics of laser systems operating in the passive mode locking regime. Based on the theoretical analysis, the development of an experimental prototype of the ultrashort pulse generator will be performed.The experimental part of the project includes the implementation and study of the properties of non-linear amplitude filter based on the effect of the cyclotron absorption, demonstrating the regime of generation of ultrashort pulses when an existing long-mm wavelength gyro-TWT (operating frequency of 30 GHz) is used as the amplifying element. In parallel, the key elements for a helically corrugated gyro-TWT with a frequency of 260 GHz, as well as a non-linear cyclotron absorber appropriate for this frequency range will be developed.