Development of high-efficient gyrotron based complex for industrial applications

CW gyrotrons with operating frequencies from 24 to 28 GHz and an output power of several tens of kilowatts are being considered as the next stage in development of well-known technological complexes [1, 2] for the sintering of composite materials, growing CVD diamonds and other advanced technologies. In this regard, the total efficiency of microwave system plays a key role for real industrial employment. The report examines the possibility of reducing power consumption of magnet by the installation of ferromagnetic screens surrounding the main solenoid [3]. The project of 25 kW / CW / 28 GHz gyrotron based system is presented.


Introduction
CW gyrotrons with operating frequencies from 24 to 28 GHz and an output power of several tens of kilowatts are being considered as the next stage in development of well-known technological complexes [1,2] for the sintering of composite materials, growing CVD diamonds and other advanced technologies.In this regard, the total efficiency of microwave system plays a key role for real industrial employment.The report examines the possibility of reducing power consumption of magnet by the installation of ferromagnetic screens surrounding the main solenoid [3].The project of 25 kW / CW / 28 GHz gyrotron based system is presented.

Development of the magnetic system
To reduce the power of the main coil it was surrounded by the system of disk screens and inner and outer cylindrical screens (fig. 1 a).As a result, after optimization of the screens shape, it was possible to decrease the value of power consumption of the magnetic system in more than 2-2.5 times (see also fig. 1 b).Small correcting coils placed inside main coil volume allowed to adjust the magnetic field distribution Bz(z) in such a manner as to provide its inhomogeneity less than 1% and reach the operating magnetic field 1.03 T in the cavity.
At the same time ferromagnetic screens substantially increases the inhomogeneity of the magnetic field outside the screens, where the initial formation of the helical electron beam (HEB) takes place.So the coefficient of non-adiabaticity H = h/L B (here h is the step of electron trajectory, L B is the scale of the magnetic field) exceeds 2 in the formation region (fig.2).This makes it difficult to use well developed and reliable magnetron-injection gun (MIG) to form HEB with good enough parameters.To smooth the magnetic field distribution the additional cathode coils was installed to the left from the screen region (see fig. 1a).It allowed to form adiabatic distribution of the magnetic field with H < 1 (fig.2) and to employ adiabatic electron optical system (MIG) to minimize effects of misalignment, variation of technical parameters, etc.

Electron-optical system
Initial form and position of MIG was found on the basis of adiabatic theory of MIGs [4].Accelerating voltage was chosen 23 kV, maximum operating current 2.4 A, cathode radius 19 mm.To diminish the influence of the space charge forces on the beam quality the angle of emitter cone with respect to the axis was chosen 30 degrees to form laminar beam.Further optimization of the electrodes shape (fig.3) has shown that HEB parameters (see fig. 4) are acceptable for high efficient (about 50%) microwave generation.

Choice of the operating mode and cavity design
The analysis of the mode spectrum as well as the comparison of value of the coupling factors for some candidates to the operating mode with moderate azimuthal and radial indexes (modes TE 02 , TE 22 , TE 03 TE 13 and TE 13 were considered) and the neighboring competiting modes has shown that the most suitable is TE 13 one.Further preliminary optimization of the output efficiency K of the gyrotron within the model of fixed RF field distribution [6] with taking into account the ohmic losses has shown the possibility to overcome the value K = 50%.Final optimization was performed on the basis of more complete model [7], which takes into account the influence of the own beam space charge on the interaction process (see fig. 5).According to the results of numerical simulation the total gyrotron efficiency is 51%, output power 25 kW.
During the optimization the restrictions on the power consumption of the main coil and its length also were considered.

Conclusion
Technological gyrotrons with hot magnets surrounding by ferromagnetic screens allow to increase the operating value of the magnetic field in 1.5-2 times approximately and thus to use the generation on the first cyclotron harmonic in the frequency range 24-28 GHz with power level some tens kW and at the same time to keep the existing and well mastered coil cooling system.This opens up new opportunities in mastering novel promising technologies based on powerful CW radiation sources.

Fig. 1 .
Fig. 1.The scheme of magnetic system with shielded solenoid (a).Magnetic field lines are shown.The axial magnetic field distributions (b) for the cases without (lower curve) and with installed ferromagnetic screens (upper curve) are shown

Fig. 2 .
Fig. 2. The dependence of the coefficient of non-adiabaticity H on the longitudinal coordinate in the optimized magnetic system.Z = 0 corresponds to the cavity center