Irradiation of intense electromagnetic (EM) wave in sub-millimeter wave region iseffective in enhancing the sensitivity solid state nuclear magnetic resonance (NMR)spectroscopy (Dynamic nuclear polarization (DNP) method). It is important to irradiateEM wave with a optimal frequency at stimulated absorption or emission peak.Continuous tunability of frequency is required to obtain the maximum enhancement.The irradiation power needs several Watts for DNP. We have already developed afrequency tunable gyrotron which is of demountable type. The frequency tunable rangewas 1.6 GHz and the output pwer was 10-30 W. DNP-NMR experiment is going onwith this gyrotron. Then, a new sealed-off gyrotron has been developed to improveoutput power, the stability and CW operation.
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