Improvement of Gyrotron Beam Quality by Suppression of Parasitic Low-Frequency Oscillations
the methods for suppressing parasitic low-frequency oscillations (LFOs) in the
beam compression region by optimization of the electric and magnetic field
distributions are described. As a result of the optimization, stable operation
of a 4-mm experimental gyrotron in the absence of LFOs at pitch factor values
higher than 1.5 has been achieved.
The efficiency of a gyrotron can be enhanced by
increasing the pitch factor (and rotation energy) of the helical electron beam
(HEB). The main obstacle for the operation of gyrotrons at high pitch factor
parasitic low-frequency (f ~ 100 MHz)
oscillations (LFOs) which develop in the electron space charge accumulated in
the trap between the gun and the magnetic mirror , . The RF field of the
oscillations causes an increase of velocity and energy spreads, as well as an
electron bombardment of the cathode surface resulting in the appearance of
secondary electrons. These factors lead to
beam quality and the
efficiency of energy transformation in gyrotrons.
Thus, the increase
HEB rotation energy can result in an enhancement of the gyrotron efficiency
only in combination with reducing velocity spread or/and with suppressing
parasitic trap oscillations.
This report presents the data on suppressing LFOs by optimization of electric field distribution in the gun region and magnetic field distribution in the HEB compression region.
The measurements were performed with the experimental 74.2 GHz / 100 kW gyrotron at SPbSPU . The tube is equipped a room-temperature pulse magnetic system and operates in the regime of 30 – 60 μs single pulses.
To study the effect of electric field
distribution in the gun region on gyrotron performance, the measurements were
standard cathode system having the inclination angle of 35 degrees all along
the conical part of the cathode and with the modified
cathode system with the angle increased up to 50 degrees for the region
above the emissive strip. According to the calculations performed at IAP RAS
(Russia), such a modification of the cathode system allows to decrease electron
reflection from the magnetic mirror and therefore to reduce LFOs amplitude.
The magnetic field in the compression region was modified with a special control coil.
In the experiments, we used three LaB6 and two impregnated W-Ba cathodes differing in the azimuthal inhomogeneity of electron emission dje.
III. Results and Conclusion
The replacement of the standard cathode system by the modified one is accompanied by a significant decrease of the LFOs amplitude and an increase of the threshold pitch factor athr corresponding to LFOs appearance. In the case of the cathode with 30 % emission inhomogeneities installed in the modified cathode system, the parasitic oscillations appeared at athr @ 1.4. For comparison, the pitch factor athr was equal to 1.31 for a more homogeneous cathode with dje = 20 % installed in the standard system.
For further decrease of the oscillation amplitude and the broadening of the zone of stable gyrotron operation toward high pitch factor regime, we optimized the magnetic field distribution in the HEB compression region for the gyrotron with modified cathode system. As a result of this optimization, the pitch factor athr increased up to 1.56.
The data obtained give an evidence of possible stable operation of the gyrotron at high pitch factor (> 1.5) in the conditions of high quality electron beam with high uniformity of cathode emission and with suppressed parasitic low-frequency oscillations by optimization of electric and magnetic fields distributions.
This work was supported in part by INTAS under Grant 03-51-3861 and in part RFBR under Grant 05-02-08024.
 Sh.E. Tsimring, “Gyrotron electron beams: velocity and energy spread and beam instabilities”, Int. J. Infrared Millimeter Waves, vol. 22, pp. 1433-1468, Oct. 2001.
 O.I. Louksha, B. Piosczyk, G.G. Sominski, M. Thumm, D.B. Samsonov, “On potentials of gyrotron efficiency enhancement: measurements and simulations on a 4 mm gyrotron”, IEEE Trans. Plasma Sci., vol. 34, no. 3, pp. 502-511, June 2006.