Generation of terahertz radiation by the methods of vacuum electronics
The terahertz (0.1–10 THz) radiation has many unique features which make it rather attractive for a wide range of fundamental and applied studies in the areas of physics, chemistry, biology, and medicine. Terahertz waves are promising for diagnostics and spectroscopy of various media including development of the methods of high-resolution electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy. High-power terahertz radiation can be used to produce dense plasma and control its parameters (controlled fusion, “point” plasma sources of X-ray radiation, remote sensing of ionizing radiation sources).
The development of the terahertz sources at IAP by the methods of vacuum electronics based on using conventional gyrodevices, which employ extremely strong magnetic fields at the fundamental cyclotron resonance and the second cyclotron harmonic, is supported by the development of gyrodevices operated at higher cyclotron harmonics (large-orbit gyrotrons (LOGs) and gyroamplifiers) and orotrons (generators, whose operation is based on the stimulated Smith-Purcell radiation of rectilinear electron beams in an open cavity).
Pioneering works of IAP researchers (M. I. Petelin, T. B. Pankratova, N. I. Zaitsev, M. M. Ofitserov, A. G. Luchinin, G. S. Nusinovich, V. A. Flyagin) demonstrated already in the 1970–1980s the principle possibility of obtaining high-power continuous-wave and pulsed radiation of the sub-terahertz band (frequencies from 0.33 to 0.65 THz) using gyrotrons. In recent experiments, a gyrotron with a pulse magnetic field of up to 50 T generated a power of 5–0.5 kW at the fundamental cyclotron resonance in single pulses with pulse duration of 50 μs at record-breaking frequencies of 1–1.3 THz (M. Yu. Glyavin, A. G. Luchinin).
IAP is also developing the technology, which, according to calculations, will ensure creation of relatively simple pulse solenoids with a magnetic field of up to 30 T and a pulse repetition rate of up to 0.1 Hz. This will make it possible to produce conventional gyrotrons at the fundamental cyclotron resonance and the second harmonic with a generation frequency of up to 0.8–1.6 THz at a peak power of several hundreds of kilowatts and an average power of about 1 W. The developed gyrotron with an operating frequency of 0.67 THz and a power of 200–300 kW can be used for remotedetection of concealed radioactive materials from distances of several tens of meters.
In order to achieve high frequencies of radiation, along with conventional gyrotrons the so-called large-orbit gyrotrons (LOGs) are also developed at IAP. In LOGs, the use of electron beams with Larmor trajectories of the particles, which gyrate around the axis of the cylindrical electrodynamic system, ensures a higher level of electron mode selection at a high cyclotron harmonic due to resonance electron interaction only with modes whose azimuthal indices are equal to the cyclotron harmonic numbers. At a particle energy of 250–350 keV, generation at high cyclotron harmonics (up to the fifth harmonic) has been achieved in LOGs. The highest generation frequency achieved in these experiments was equal to 0.41 THz for a power of 10–20 kW in 10 μs pulses. In systems with lower voltages of 50–80 kV, the highest-frequency LOGs have been actualized by using an original electron-optical system with a magnetic-field cusp in the near-cathode region. Single-frequency radiation with a power of 0.3–1.8 kW in pulses with a duration of 10 μs was generated in the range from 0.55 to 1.0 THz at the second and third cyclotron harmonics at four resonator cavity modes (V. L. Bratman, Yu. K. Kalynov, V. N. Manuilov). This generator is used successfully for generation of terahertz discharges in gases in the studies aimed at creating a point source of extreme UV radiation. The permanent-magnet LOG with a field of 1.07 T developed at IAP jointly with foreign partners made it possible to achieve generation at the 3rd to 5th harmonics at frequencies up to 0.14 THz (V. E. Zapevalov, M. Yu. Glyavin, et al.).
Continuous-wave terahertz gyrotrons are developed at IAP for the spectroscopy and diagnostics of various media. The gyrotron with a frequency of 0.3 THz and a power of 2.7 kW (V. E. Zapevalov, O. V. Malygin et al.) with a 12 T liquid-helium free cryomagnet manufactured by Sumitomo HI (Japan) was developed by the IAP – GYCOM partnership for the Research Center for Development of Far-Infrared Region, University of Fukui (FIR-FU, Japan). The complex based on this gyrotron is successfully used for various studies.
For the purpose of achieving high resolution in high-field NMR spectroscopy by using dynamic nuclear polarization (DNP/NMR spectroscopy), IAP has created a gyrotron complex on the basis of a highly stable CW gyrotron with a frequency of 0.26 THz and a power of 100 W at the second cyclotron harmonic, which ensures frequency stability and generation power equal to 3×10-6 and 10-2, respectively, during 12 hours of operation (V. E. Zapevalov et al.). The experiments with the use of this generator performed at the Institute of Biophysical Chemistry of Goethe University (Frankfurt-am-Main, Germany) allowed increasing the sensitivity and resolving power of the NMR spectrometer by 80 times.