This paper outlines the principle of the DNP-NMR technique. The gyrotron, as a very promising microwave source for NMR spectroscopy, is evaluated. Four factors: power stability, power tuning, frequency stability, and frequency tuning determine the usability of the gyrotron device. The causes of instabilities, as well as the methods of overcoming limitations and extending usability are explained with reference to the theory, the numerical and experimental results reported by gyrotron groups.

In this paper, we describe the development and design procedure of the new kind of coaxial TE_m,1 modes generator based on ring resonator with coupling apertures. The generator enables excitation of subsequent TE_m,1 modes in a cylindrical waveguide. The proposed design method allows to obtain high purity TE_m,1 modes. The angular mode number can be chosen by replacing the plate with coupling apertures. Structure and parameters of the generator was optimized using CST-Microwave Studio. The mode generator was fabricated and checked on the test bench in an anechoic chamber. The measured field distributions confirm excitation of the desired TE_m,1 modes. A good agreement between simulations and measurements is obtained. The presented mode generator, operating in non-rotating TE_m,1 modes, is easy to fabricate, and suitable for cold-test experiments of high power components and devices.