Experimental measurements of competition between fundamental and second harmonic emission in a quasi-optical gyrotron

Abstract

A quasi-optical gyrotron (QOG) designed for operation at the fundamental ( fce≂100 GHz) exhibits simultaneous emission at fce and 2fce (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic. The experimental measurements show that the excitation of the second harmonic is only possible when the fundamental is present. The frequency of the second harmonic is locked by the frequency of the fundamental. Experimental evidence shows that when the second harmonic is not excited, total efficiency is enhanced.

A far-infrared free-electron maser oscillator operating at the third cyclotron harmonic

“…For example, in the Fukui University FEM with a magnetic field of 7.34 T, a single mode TE2, 6, 1 at the second cyclotron harmonic with a frequency of 384.07 GHz was excited when the electron beam current was 0.5 A; however, both the TE2, 6, 1 at the second cyclotron harmonic with a frequency of 384.07 GHz and the mode TE2, 3, 1 at the fundamental cyclotron harmonic with a frequency of 196.43 GHz were excited simultaneously when the electron beam current was 0.8 A; finally, as the beam current was increased to 1.1 A, only the operation of the TE2, 3, 1 mode at the fundamental cyclotron harmonic occurred, while the operation of the working mode TE2, 6, 1 at the second cyclotron harmonic was completely suppressed [7,9]. The same phenomenon had been observed in other experiments [6,8],as the beam current was increased. This phenomenon restricts the output power, because the device has to work in quite a low current.…”

“…In order to eliminate this problem, operation at the second cyclotron harmonic is widely adopted since the strength of the magnetic field for second cyclotron harmonic operation is only half of that for the fundamental cyclotron harmonic. It is this reason that the FEM operating at the second cyclotron harmonic has been widely developing in the world [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. In this way the working frequency of a FEM has been extended to the far-infrared range of 300-600 GHz [7,9,11,19].…”