Improvement of Gyrotron Beam Quality by Suppression of Parasitic Low-Frequency Oscillations

O. Louksha¹, B. Piosczyk², D. Samsonov¹, G. Sominski¹, M. Thumm²

¹ St. Petersburg State Polytechnical University (SPbSPU), 29 Polytechnicheskaya st., St. Petersburg 195251, Russia

² Forschungszentrum Karlsruhe, Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), Postfach 3640, D-76021, Karlsruhe, Germany

louksha@rphf.spbstu.ru

Abstract-In this this report, 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.

I. Introduction

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 are is 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 [1], [2]. 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 results in a factors lead to decrease of the beam quality and of the efficiency of microwave generation. energy transformation in gyrotrons. Thus, the increase of 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 the electric field distribution in the gun region and the magnetic field distribution in the HEB compression region.

II. Experimental Setup

The measurements were performed with the experimental 74.2 GHz / 100 kW gyrotron at SPbSPU [2]. 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 made with a the standard cathode system having the inclination angle of 35 degrees all along the conical part of the cathode and with a the modified cathode system with the angle increased up to 50 degrees in 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 the amplitude of 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 dj_e.

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 a_thr 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 a_thr @ 1.4. For comparison, the pitch factor a_thr was equal to 1.31 for a more homogeneous cathode with dj_e = 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 a_thr 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.

References

[1] 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.

[2] 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.