Gyrotron Travelling Wave Amplifier for high field Electron Paramagnetic Resonance

Experimental results are presented of the first successful gyrotron backward wave oscillator (gyro-BWO) with continuous frequency tuning near low terahertz region. A helically corrugated interaction region (HCIR) was used to allow efficient interaction over a wide frequency band at the second harmonic of the electron cyclotron frequency without parasitic output. The gyro-BWO generated a maximum output power of 12 kW when driven by a 40 kV, 1.5 A, annular-shaped large-orbit electron beam and achieved a frequency tuning band of 88-102.5 GHz by adjusting the cavity magnetic field. The performance of the gyro-BWO is consistent with 3D particle-in-cell (PIC) numerical simulations.

The first successful operation was achieved of a high power gyro-BWO with a wide frequency tuning capability in the low terahertz frequency range. A novel helically corrugated interaction region and thermionic cusp electron gun were used in the gyro-BWO. Stable single mode output was achieved in a wide frequency tuning range of ~16% with a maximum power of 12 kW and an electronic efficiency of 20%.

In this sub-terahertz gyro-BWO experiment stable output frequency and power were demonstrated due to single mode operation. A maximum output power of 12 kW which corresponds to a 20% interaction efficiency was achieved with a 1.5 A 40 kV large orbit electron beam. A wide frequency tuning range of 88-102.5 GHz was achieved by tuning the cavity magnetic field. Excellent agreement was shown between the simulations of the gyro-BWO using MAGIC and the measured results in the experiments. This method can, in principle, be extended to the higher terahertz range.

This work was given as keynote talks by 6th UK, Europe, China Millimeter Waves and THz Technology Workshop by the Program Committee, Rome, Italy 2013 and the 55th American Physical Society, Division of Plasma Physics Annual Meeting at the 2013 conference in Denver Colorado, USA.