A THz gyrotron with a pulse magnet has been designed, constructed and operated in FIR FU. It is developed as one of high frequency gyrotrons included in Gyrotron FU Series. The gyrotron has already achieved the first experimental result for high frequency operations whose radiation frequency exceeds 1 THz. In this paper, the design detail and the operation test results for sub-terahertz to terahertz range are described. The second harmonic operation is confirmed experimentally at the expected frequency of 1.005 THz due to TE 6,11 cavity mode at the magnetic field intensity of 19.0 T.

A high frequency gyrotron with a 15 T superconducting magnet named Gyrotron FU CW VI has achieved continuous frequency tuning through the relatively wide range of 1.5 GHz near 400 GHz. The operation is at the fundamental cyclotron resonance of the TE 06 cavity mode with many higher order axial modes. The output power measured at the end of the circular waveguide system ranges from 10 to 50 watts at the low acceleration voltage of 12 kV for beam electrons. The beam current is also low. It is around 250 mA. This gyrotron is designed as a demountable radiation source for the 600 MHz DNP-NMR spectroscopy. The design and operation results of the gyrotron FU CW VI are presented.

Record-breaking high power coherent radiation at a subterahertz frequency region from a gyrotron utilizing second harmonic resonance modes was attained with a simple cavity. In order to aim at high power and high frequency simultaneously, the oscillation mode was selected carefully enough to realize stable radiation free from mode competition. The cavity radius was determined from the viewpoints of the oscillation frequency, the coupling coefficient between the electron beam, and the rf-electric field. The cavity length was also optimized for the highest perpendicular efficiency. In addition, a new electron gun which is capable of generating a thin laminar beam for a large current was introduced. Consequently, single mode second harmonic radiation with powers of 52 and 37 kW at frequencies of about 349 and 390 GHz, respectively, was achieved.

Gyrotron FU CW II with an 8 T liquid He free superconducting magnet, the second gyrotron of the THz Gyrotron FU CW Series, has been constructed and the operation test was successfully carried out. It will be used for enhancing the sensitivity of 600 MHz proton-NMR by use of Dynamic Nuclear Polarization (DNP). The designed operation mode of the gyrotron is TE2,6 at the second harmonic. The corresponding frequency is 394.6 GHz. The real operation frequency is 394.3 GHz at TE06 mode, because of fabrication error of the diameter of the cavity. The operation is in complete CW at the output power of around 30 W or higher at the TE06 cavity mode. There are many other operation modes at the fundamental and the second harmonic. Typical output power of the fundamental and the second harmonic are higher than 100 W and 20 W, respectively. The highest frequency observed up to the present is 443.5 GHz at the second harmonic operation of TE6,5 mode. The measured results are compared with the theoretical consideration

The new gyrotron series in FIR FU, so-called Gyrotron FU CW Series, is being developed for application to high power THz technologies. The second gyrotron, of the series, Gyrotron FU CW II with an 8 T liquid He-free superconducting magnet has been constructed and the operation test was successfully carried out. It will be used for enhancing the sensitivity of 600 MHz proton-NMR by use of Dynamic Nuclear Polarization (DNP). The designed operation mode of the gyrotron is TE 2,6 at the second harmonic. The corresponding designed frequency is 394.6 GHz. The real operation frequency is 394.3 GHz at TE 06 mode, because of fabrication error of the diameter of the cavity. The operation is in complete CW at the output power of around 30 W or higher at the TE 06 cavity mode. There are many other operation modes at the fundamental and the second harmonics. Typical output power of the fundamental and the second harmonic are higher than 100 W and 20 W, respectively. The third gyrotron, Gyrotron FU CW III with 20 T superconducting magnet has already been constructed and the operation test has been almost completed. The highest frequency is around 980 GHz at the second harmonic operation. This gyrotron demonstrates a possibility of application to higher frequency DNP/NMR up to 1 GHz.