Tunable 330 GHz Gyrotron for DNP/NMR

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We propose a program of research to design, build and operate a tunable 330 GHz gyrotron oscillator for CW DNP/NMR research. There are a number of 500/89 magnets in solid-state NMR laboratories around the world and these could be used with a 330 GHz microwave source to perform DNP/NMR. However, all of the known DNP mechanisms require that the magnetic field be adjusted to the peak of the enhancement curve to optimize the signal to noise ratio (S/N) of the experiment. This necessitates either that the magnetic field be swept with a sweep coil on the magnet, or that the microwave frequency be adjusted. In NMR systems using magnets with sweep coils, DNP/NMR research can be conducted with fixed frequency gyrotrons. However, the vast majority of NMR magnets in existence do not have sweep coils and it is impractical to retrofit these systems. Therefore, a tunable gyrotron oscillator appears to be the approach of choice for laboratories with this type of magnet. We propose to design and fabricate a 330 GHz gyrotron that can be tuned over a range of ¿0.75 GHz which will permit coverage, for example, of the TEMPO EPR spectrum where the peaks in the enhancement curve will be separated by about 710 MHz. The tunable gyrotron oscillator will utilize a novel magnetic field tuning approach based on the excitation of high order axial modes of the resonator. Frequency tuning of ¿0.9 GHz has been recently demonstrated on a 250 GHz gyrotron oscillator here at MIT. The yrotron design will, in principle, be scalable to higher frequencies for application to 600 to 900 MHz where wide-bore magnets are currently available.