Demonstration of a Compact CW 263-GHz Extended Interaction Klystron Frequency Upconverter

To realize the miniaturization of RF sources for applications in dynamic nuclear polarization–nuclear magnetic resonance (DNP–NMR) spectroscopy, a compact continuous-wave (CW) 263-GHz extended interaction klystron (EIK) frequency upconverter is designed and proposed in this article, including the high-frequency circuit and a complete compact beam optical system (BOS) based on hybrid permanent magnets. This G-band EIK could be driven by easily accessible W-band signal with power level less than 1 W. A local magnetic field enhancement technology is used in the BOS to allow the beam to be further compressed in the tiny drifting tunnel of output cavity, achieving full cylindrical-beam transmission along the whole hybrid circuit. Particle-in-cell (PIC) simulation results show that the saturated output power of the designed EIK could reach over 250 W at 263 GHz, with an operating beam voltage of 24 kV and a beam current of 0.3 A. In addition, its instantaneous bandwidth is broad enough to meet the requirements of DNP–NMR power source where no instability is observed. The key specifications demonstrate that the designed EIK could stably operate in CW mode. This study is expected to provide new approach for the design of cost-efficient sub-THz EIKs in a compact profile, aiming for DNP–NMR applications and beyond.