The Experimental Tests of THz Range Gyrotron with Pulsed Magnetic Field

Abstract

Summary form only given. Development of compact, simple and reliable sources of submillimeter wave radiation is important for numerous applications, which include plasma diagnostics, spectroscopy, new medical technology, atmospheric monitoring, chemical technologies, and production of high-purity materials. A demountable THz gyrotron tube with a pulse magnet has been designed, constructed and tested at IAP RAS. This work is based on the previous results obtained with gyrotrons using pulsed solenoids and on the development of an improved pulsed solenoid, producing up to 40 T magnetic field. The solenoid is made of a composite cable consisting of a Nb-60%Ti alloy mechanically reinforced in an outer copper shell. For reducing ohmic heating and stabilizing the operation, the solenoid is cooled by liquid nitrogen, which reduces the resistance by a factor of 7 in comparison with the room temperature resistance. The cable is wired directly on a thin stainless steel gyrotron body. This allows for significant reduction of the solenoid clearing hole diameter (up to 6 mm) and the energy required for obtaining the necessary magnetic field. Magnetic field is produced in the course of discharge of a bank of capacitors. The voltage and the coil current in 1.5 ms pulses did not exceed 2.5 kV and 6 kA, respectively (total storage energy was about 5.6 kJ). The pulse-to-pulse reproducibility of the magnetic field was within 0.05%. The pulses were repeated one in a minute. After more than 1000 pulses no signs of solenoid deterioration had been observed. Gyrotron components included the simplest cylindrical cavity (3 mm diameter) and diode type magnetron injection gun (accelerating voltage 20-25 kV, beam current 4-5 A, pulse duration 50 microseconds). First experimental results were obtained for high frequency operation at the fundamental cyclotron harmonic. Frequency measurements of single pulse submillimeter wavelength radiation were based on the mixing of the gyrotron signal with the signal from a millimeter-wave frequency synthesizer. The measured frequency is close to the cyclotron frequency defined by the magnetic field. The detection of microwave power was made by the semiconductor detector and by the dummy load. At several modes with frequencies near 1 THz, the output power close to 10 kW with the efficiency 8-10% was observed.