An Experimental Facility for Investigation of Gyrotron Cathode Emission Non-Uniformities
Abstract-A high vacuum teststand has been designed and manufactured at SPbSPU for measurements of azimuthal non-uniformities of temperature and emission current density of gyrotron cathodes differing in their configuration, dimension and material of the emitting surface. Characteristics and area of application of this facility are discussed in the report.
One of the important sources of helical electron
beam (HEB) quality deterioration in gyrotrons is inhomogeneous electron
emission from thermionic cathodes of magnetron-injection guns (see, e.g., ).
Typically, these guns operate in the regime of temperature-limited emission.
Therefore, the inhomogeneities of both the work function and the emitter
temperature result in a non-uniform distribution of current density in the HEB
beam spatial non-uniformities can lead to
degradation of the gyrotron efficiency through: (1) the increase of velocity
spread, (2) the excitation of parasitic modes in the cavity, (3) the increase
of energy spread caused by the RF-field of the parasitic space-charge
oscillations in the beam tunnel.
The inhomogeneities of electron emission can result from the imperfections in manufacturing of the large-size conical cathodes used in gyrotrons. Also, the long term performance of the cathode can be affected by several factors, including changes in emission uniformity caused by ion and electron bombardment of the emitter surface. For manufacturers and users of gyrotron cathodes it is therefore very important to have the opportunity of cathode diagnostics on different stage of its operation.
This report describes a high vacuum stand which has been designed and manufactured at SPbSPU for measurement of azimuthal non-uniformities of temperature and emission current density of gyrotron cathodes differing in their configuration, dimension and coating.
The section of the stand is shown in Fig. 1. The method we
use for cathode emission testing is based on the measurement of emission current
density with an immovable anode analyzer in dependence of the azimuthal position
of the cathode. The anode analyzer is insulated from the grounded anode. For a
certain cathode-analyzer distance, the adjustment of cathode and analyzer
voltages allows to provide the operation of local area near the analyzer in the
temperature-limited regime while the whole cathode is space-charge limited. The
stand was designed so as to have the possibility of easy replacement of the
cathode. In the case of the cathodes with different geometry, their anodes have
to differ in geometry also in order to provide the necessary anode-cathode
distance and the uniformity of electric field lines near the emitter. In the
current configuration, this stand allows to test gyrotron cathodes with maximum
diameter up to 150 mm at nominal temperature (up to 1800 oC) in the regime with voltage U
= 0 – 25 kV, current I = 0 – 15 A and
pulse duration t = 5 – 100 μs.
The stand can
be equipped in addition with other cathode
diagnostics and with the instrumentation for modification of the cathode
surface, for example, by ion and electron bombardment.
This work was supported in part by INTAS under Grant 03-51-3861 and in part RFBR under Grant 05-02-08024.
 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.