Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz 1H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water–glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.
Gyrotrons are typically driven by electron beams produced by magnetron-type thermionic electron guns operating in the regime of temperature limited emission. Very often, the current density in such annular electron beams is azimuthally nonuniform. To describe the effect of this nonuniformity on gyrotron operation, the code MAGY [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)], which is widely used for modeling of slow and fast microwave sources, was properly modified. The results of numerical simulations demonstrate the effect of azimuthal inhomogeneity of the emission on the excitation of low- and high-frequency satellites of the operating mode and on the efficiency degradation. The calculations are done for parameters typical for megawatt-class, long-pulse, millimeter-wave gyrotrons, which are currently under development for electron cyclotron plasma heating and current drive experiments in controlled fusion reactors.
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