High speed frequency modulation of a 460 GHz gyrotron for application to the 700 MHz DNP enhanced NMR spectroscopy

The high speed frequency modulation of a 460 GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GO-I) was achieved by modulation of acceleration voltage of beam electrons. The modulation speed f_m can be increased up to 10 kHz without decreasing the modulation amplitude δf of frequency. δf was increased almost linearly with the modulation amplitude of acceleration voltage δV_a. At the δV_a=1kV, δf=53 MHz in the case of f_m<;10 kHz. The frequency modulation was observed as both the variation of the IF frequency in the heterodyne detection system measured by an oscilloscope and the width of frequency spectrum measured on a frequency spectrum analyzer. Both results well agree reasonably. The experiment was performed successfully for both a sinusoidal wave and a triangle-wave modulations. By using the frequency modulation, we have a possibility to compensate the decreasing of the enhancement factor in high frequency DNP-NMR spectroscopy for example, 700 MHz. In addition, high speed frequency modulation is useful for frequency stabilization by PID control of acceleration voltage by feeding back of fluctuation of the frequency. The frequency stabilization in long time is also important for application of DNP-NMR spectroscopy to analysis of complicated protein molecules.