Electron-Beam Functional Converter

B. Z. Kanter

(Presented by the scientific seminar of the Radio Engineering Faculty)

At the present time, the problem often arises of rapidly transforming broadband electrical signals according to a prescribed law. In this case, the signal at the output of the converting device may be a nonlinear function of one or two input signals. In some converter designs proposed in recent years \[1, 2\], complex electron-optical systems must be used to form and deflect a beam with a specified cross-sectional shape and a strictly uniform current-density distribution. In other converter designs, a target must be used whose secondary-emission coefficient varies over its surface in accordance with the specified conversion law.

The difficulties associated with the practical implementation of the proposed designs have led to another conversion method, different from those previously used in electron-beam devices. This method consists in obtaining the converted voltage by passing the beam current, or a current proportional to it, through a larger or smaller part of a target made of a thin conducting layer deposited on an insulating base and placed in the path of the electron beam. In the case where the secondary-emission coefficient of the target is zero, the current through it flows from the grounding point to the point where the beam strikes (technical direction). When connected according to the circuit shown in Fig. 1, the output voltage of the converter, acting on the grid of an amplifier tube, is proportional to the beam current and to the resistance of the target layer between the grounding point and the point where the beam strikes. The shape and position of the electrodes by means of which the target is connected to the external circuit, the shape of the conducting layer of the target, and also the law governing the variation of the target resistance over its surface must be such that the conversion characteristic is realized, i.e., the prescribed dependence between the voltages on the deflecting plates and the voltage between the target electrodes.

In practice it is difficult to obtain a surface with a very low secondary-emission coefficient; however, a regime in which the majority of secondary electrons return to the point where the beam strikes can be created by placing in front of the target a grid or auxiliary

electrode of special shape, to which a voltage negative with respect to the target is applied. If a voltage positive with respect to the target is applied to the auxiliary electrode, then the current through it will depend on the secondary-emission coefficient:

\[ i_{\text{target}} = i_{\text{beam}}(\delta_{\text{target}} - 1). \]

Fig. 1. Circuit diagram for connecting the electron-beam functional converter. 1—conducting layer of the target.

The voltage at the converter output will also vary in proportion to the current:

\[ U_{\text{out}} = i_{\text{beam}}(\delta_{\text{target}} - 1)\cdot r, \]

where \(r\) is the resistance of the target between the grounding point and the point at which the beam falls.

Figure 2 gives a plot of the dependence of the output voltage on the secondary-emission coefficient of the target. It can be seen from it that for \(\delta < 2\) it is necessary to use the converter in a mode in which the secondary electrons return to the target. For \(\delta > 2\) it is advantageous to use the converter in the mode of collecting secondary electrons on the auxiliary electrode, which is at a positive potential with respect to the target. This makes it possible to obtain an increased output signal with an unchanged beam current.

Fig. 2. Dependence of the magnitude of the converter output voltage on the secondary-emission coefficient of the target.

Tests of experimental converter samples in pulsed operation showed that with a total target resistance of about \(3000\ \Omega\), the output voltage reached \(0.3\ \text{V}\). In this case the rise time of the leading edge of the pulse was \(0.1\ \mu\text{s}\).

REFERENCES

1. Cathode parameter converters, Uspekhi fizicheskikh nauk, vol. 52, No. 1, 1954.

2. E. Labin, La formation. L’Onde Electrique, vol. 34, Nos. 327 and 328, 1954.