Particle-in-cell (PIC) simulation of a 250GHz gyrotron

In this paper, the design and PIC simulation studies of a sub-millimetre wave, continuous wave (CW) gyrotron for dynamic nuclear polarization/nuclear magnetic resonance (DNP/NMR) spectroscopy applications is reported. The dynamic nuclear polarization could enhance the sensitivity of nuclear magnetic resonance which is a very impressive technique to enhance the spin spectra of biological samples. This particular application requires few tens of watts of CW power in terahertz and sub-millimetre wave regime. Therefore, one of the possible candidates is the fast wave gyrotron oscillator. To achieve better resolution in NMR spectroscopy one has to use higher magnetic field which in turn increase the cyclotron frequency. This ultimately increases the operating or resonant frequency of the device. In the present work, the gyrotron is designed at 250 GHz to operate in higher order TE52 mode, because at high frequency radius of interaction circuit will be less. The radius and length of interaction cavity are calculated as 2.01 mm and 23 mm respectively. The Eigen mode cold analysis is made to calculate the loaded quality factor (Q) as 1.15 × 10⁴ at 249.91 GHz. The required magnetic field has been calculated as 9.1 Tesla. The designed gyrotron, including the RF cavity, down taper and up taper, is modelled and simulated in “CST Particle Studio” for observing the beam-wave interaction in it. In the present simulation ∼ 107 Watts of peak power at the resonating frequency of 249.9 GHz with few GHz bandwidth has been achieved for the weakly relativistic gyrating electron beam of 12 kV, 100 mA. This type of gyrotron operating in sub-millimetre wave region has important applications in the research areas including physics, chemistry, biology, etc‥