Experimental Study of Gyrotron Efficiency Enhancement by Improvement of Electron Beam Quality Oleg I. Louksha, Bernhard Piosczyk, Dmitriy B. Samsonov, Gennadi G. Sominski, and Manfred K. Thumm, Fellow, IEEE
Abstract—The methods for suppressing parasitic lowfrequency oscillations (LFOs) have been applied to improve electron beam quality in a 4-mm 100-kW gyrotron. In the conditions of suppressed LFOs and of high cathode emission uniformity, the stable operation of this gyrotron at pitch factor values high than 1.5 has been achieved. As a result, the efficiency of the gyrotron at the main TE12,3 mode has been increased from 32 % (corresponds to the designed regime with pitch factor of 1.28) up to 42 %. Index Terms—Emission uniformity, gyrotron efficiency, nonuniform fields, space-charge oscillations.
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I. INTRODUCTION
HE efficiency of a gyrotron can be enhanced by increasing the pitch factor (and rotation energy) of the helical electron beam (HEB). The operation of gyrotrons at high pitch factor can be accompanied by the reflection of electrons with largest transverse velocities from the magnetic mirror. This results in the excitation of the parasitic lowfrequency oscillations (LFOs) in the space charge of electrons trapped between the gun and the cavity (e.g., [1], [2]). The RF field of these oscillations causes a decrease of the beam quality and the efficiency of microwave generation through an increase of velocity and energy spreads as well as through an electron bombardment of the cathode surface resulting in the appearance of secondary electrons. 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. The methods for suppressing LFOs by optimization of electric and magnetic fields distributions have been developed
and studied experimentally for the SPbSPU 4-mm gyrotron [2], [3]. One of them implies an increase of cathode inclination angle to the axis in the region above the emissive strip. According to the calculations, such a modification of the cathode system allows to transform velocity distribution function by decreasing the amount of electrons with largest transverse velocity. Another method bases on the influence of the magnetic field distribution in the compression region on the “potential well” profile for the electrons oscillating in the trap. Optimization of this distribution results in suppression of the instability in the ensemble of nonisochronous electron oscillators. To apply these methods allows the broadening of the zone of stable gyrotron operation by increasing the threshold pitch factor corresponding to LFOs appearance. One of the important sources of HEB quality deterioration is inhomogeneous electron emission from thermionic cathodes of magnetron-injection guns [1], [2], [4]–[7]. It was shown in the SPbSPU 4-mm gyrotron that the decrease of emission homogeneity caused an increase of the LFOs amplitude and a reduction of gyrotron efficiency [2], [4]. For this gyrotron, cathode emission inhomogeneity doesn’t influence negatively on electron beam quality against the background of another sources if emission spread is less than approximately 20%. The data obtained gave an evidence of possible stable operation of the SPbSPU gyrotron at high pitch factor (> 1.5) in the conditions of high quality electron beam with high cathode emission homogeneity and with suppressed parasitic LFOs by optimization of electric and magnetic fields distributions. This report presents new data relating to SPbSPU gyrotron performance in the regimes with increased pitch factor and suppressed parasitic LFOs. II. EXPERIMENTAL SETUP AND METHODS
Manuscript received June 18, 2007. This work was supported in part by the INTAS (grant no. 03-51-3861). O. I. Louksha, D. B. Samsonov and G. G. Sominski are with the SaintPetersburg State Polytechnical University (SPbSPU), 29 Polytechnicheskaya Str., St. Petersburg 195251, Russia (e-mail:
[email protected];
[email protected]). B. Piosczyk and M. Thumm are with the Forchungszentrum Karlsruhe, Association EURATOM-FZK, Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), D-76021 Karslruhe, Germany (e-mail:
[email protected];
[email protected]). .
The measurements were made with the experimental 4 mm, 100 kW gyrotron equipped a room-temperature pulse magnetic system [2]–[4]. The main operating-regime design parameters of the tube are summarized in Table 1. This tube includes a number of diagnostics, in particular, the electron energy analyzer, the special probe and antenna to register the low-frequency instabilities in the beam and parasitic lowfrequency oscillations, the anode analyzer to measure the
50
efficiency (%)
40
The authors wish to thank A. N. Kuftin, V. K. Lygin, V. N.
30 20 20
0
10
22
24
26
28
30
32
0
voltage (kV)
(a) 50
50
efficiency (%)
40
30
30 20
20
10 0 17.0
10
17.5
18.0
18.5
LFOs amplitude (mV)
60
40
III. RESULTS
ACKNOWLEDGMENT
30
10
azimuthal distribution of cathode emission current density. In these experiments, we used a LaB6 cathode characterized by the coefficient of relative spread of emission current density equal to 25%. The improved cathode system [3] with increased inclination angle to the axis was installed in the gyrotron. It was possible to modify the spatial distribution of magnetic field in the compression region with an additional control coil.
The operation of the SPbSPU gyrotron was studied in the conditions of suppressed LFOs due to – optimization of electric field distribution in the gun region by using improved cathode system; – realization of the optimized magnetic field distribution in the beam compression region; – installation of a LaB6 emitter with sufficiently high emission homogeneity. In the case of suppressed LFOs, the increase of accelerating voltage and magnetic compression ratio (both increase the HEB pitch factor) results in the increase of the gyrotron efficiency at the main TE12,3 mode (Fig. 1). The observed enhancement of the efficiency is caused by the increase of the beam energy related to the transverse motion of electrons due to increase of the pitch factor. The maximum value of the efficiency is approximately 42%. For this regime, the pitch factor value was calculated with EGUN code as 1.55. The obtained maximum efficiency is ~ 1.3 times greater than the efficiency for the designed gyrotron regime at the pitch factor of 1.28. Measured values are close to the data of the calculations which give the gyrotron efficiency at α = 1.6 equal to 44% for the transverse velocity spread δv⊥ = 30% and 48% for δv⊥ = 0. Further increase of the pitch factor by increasing voltage and magnetic compression results in the reduction of the gyrotron efficiency (Fig. 1). The regimes with α > 1.55 are characterized by the presence of parasitic LFOs. Their amplitude increases with increasing voltage and magnetic compression. Most likely, the occurrence of LFOs causes the decrease of HEB quality and, therefore, the gyrotron efficiency.
40
LFOs amplitude (mV)
TABLE I MAIN OPERATING-REGIME PARAMETERS OF THE EXPERIMENTAL GYROTRON Accelerating voltage U0 = 30 kV Beam current Ib = 10 A Pulse duration τ = 30 – 60 µs Cavity magnetic field B0 = 2.75 T Cathode magnetic field Bc = 0.152 T Operating mode TE12,3 Operating frequency f0 = 74.2 GHz Average pitch factor (transverse-toα = 1.28 longitudinal velocity ratio)
0 19.0
magnetic compression
(b) Fig. 1. Gyrotron efficiency and amplitude of low-frequency oscillations as functions of accelerating voltage (a) and of magnetic compression ratio (b) (Ib = 7.5 A, B0/Bc = 18.53 (b), U0 = 28 kV (a)).
Manuilov, and V. E. Zapevalov of IAP, Nizhny Novgorod as well as K. A. Poduschnikova and S. A. Fefelov of Ioffe Institute, St. Petersburg for helpful discussions and for assistance in the design, simulations and manufacturing of the tube components. REFERENCES [1] [2]
[3]
[4]
[5]
[6]
[7]
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, and 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. O. Louksha, B. Piosczyk, D. Samsonov, G. Sominski, and M. Thumm, “Improvement of gyrotron beam quality by suppression of parasitic lowfrequency oscillations,” in Digest IRMMW-THz 2006, Shanghai, China, 2006, p. 85. O. I. Louksha, B. Piosczyk, G. G. Sominski, M. Thumm, and D. B. Samsonov, “Effect of electron emission inhomogeneity on electron beam characteristics and output parameters of a 4-mm gyrotron,” in Proc. Int. Workshop “Strong Microwaves in Plasmas”, Nizhny Novgorod, Russia, 2005, pp. 135-140. M. Yu. Glyavin, A. L. Goldenberg, A. N. Kuftin, V. K. Lygin, A. S. Postnikova, and V. E. Zapevalov, “Experimental studies of gyrotron electron beam systems,” IEEE Trans. Plasma Sci., vol. 27, pp. 474-483, Apr. 1999. G. S. Nusinovich, A. N. Vlasov, M. Botton, T. M. Antonsen Jr., S. Cauffman, and K. Felch, “Effect of the azimuthal inhomogeneity of electron emission on gyrotron operation,” Phys. Plasmas, vol. 8, no. 7, pp. 3473-3479, July 2001. J. P. Anderson, R. J. Temkin, and M. A. Shapiro, “Experimental studies of local and global emission uniformity for a magnetron injection gun,” IEEE Trans. Electron Devices, vol. 52, no. 5, pp. 825-828, May 2005.
