“…The operating magnetic field of these gyrotrons has gone up to 10 T. However, to extend the application ability of DNP-NMR, gyrotron should be operated at further higher magnetic fields, that is, more than 10 T, which ultimately increases superconducting magnets' cost. Gyrotrons are conducted at higher harmonics using the low-cost NbTi superconductive magnets [20]. A series of higher harmonic Terahertz (THz) gyrotrons are successfully demonstrated in various organizations in the USA [21], [22], [23], [24], [25], Russia [26], and Japan [27], [28], [29], [30] to achieve impressive achievements in DNP-NMR applications.…”