EMISSION EQUALIZATION SCHEME IN SEGMENTED CATHODES FOR HIGH-CURRENT ACCELERATORS

Yu. I. Terent’ev

The problem of developing accelerators for high currents is associated with the creation of indirectly heated cathodes of large surface area.

Such cathodes are most easily and technically most expediently made from separate sections. In this case, however, it is necessary to ensure equality of the emission currents from all sections of the cathode and to stabilize them when there are changes in the filament voltage, in the emission capability of the cathode, in the vacuum, etc.

Fig. 1. Scheme for equalizing the current density in high-power electron beams.
EP—electron gun with a cathode of two sections, KUV—silicon controlled valve, Ti—amplifying triodes, VN—high-voltage source, Tr. n.—filament transformer, SLi—signal lamps.

In accordance with the indicated requirements, we have developed a circuit for stabilizing the current from each cathode section, shown in Fig. 1.

A resistance \(R_{ki}\) is introduced into the power-supply circuit of each heater section; its value is calculated so that, for all possible spreads in the resistances of the spirals, and also when an allowable number of turns in the spiral are shorted, the emission current does not exceed the nominal value.

A silicon-controlled rectifier—SCR—is connected in parallel with the resistance \(R_{ki}\).

The voltage of the secondary windings of the filament transformer (Fil. tr.) is taken to be sufficient to maintain constant emission currents when the emission capability decreases, the vacuum deteriorates, or the filament voltage is reduced at the moment when the SCR with the longest turn-on time is operating.

To make normal operation of the circuit possible, a voltage drop equal to the voltage drop across the resistance \(R_{ai}\), when a current equal to the nominal emission current of one cathode section flows through it, is set between points \(1\)—\(2\) of the setting potentiometer \(R_2\).

When the filament is switched on and high voltage is applied to the accelerator, a voltage \(U_{R_{ai}} < U_{1-2}\) will be produced across the resistance \(R_{ai}\), since the SCR is closed and the resistance \(R_{ki}\) is fully inserted into the filament circuit. As a result, a mismatch signal appears on the triode \(T_i\),

\[ \Delta U = U_{1-2} - U_{R_{ai}}, \]

which reduces the resistance in the “emitter–collector” section of the triode. A current begins to flow through the control circuit of the SCR along the circuit “\(+\)” \(R_{ki}, R_a\), section \(1\)—\(2\) of the resistance \(R_2\), “emitter–collector” of \(T_i\), diode \(D_i\), control electrode “\(y\),” cathode “\(K\),” “\(-\)” \(R_{ki}\). The SCR opens and shunts the resistance \(R_{ki}\). The filament current \(I_n\) of the section then becomes greater than the nominal value \(I_{\text{n.n.}}\). As the filament current increases, the cathode temperature and its emission capability increase. When the emission current \(I_e\) becomes equal to its nominal value \(I_{\text{en}}\), the mismatch voltage on the triode disappears and the SCR closes. The filament current decreases to its initial value, causing a decrease in the cathode temperature and in the emission current; \(\Delta U\) again appears on the triode—the SCR opens a second time, etc.

When a disturbing action appears (a decrease in emission, deterioration of the vacuum, etc.), the duration of SCR turn-on changes accordingly (Fig. 2), so that the value of \(I_e\) (Fig. 3) remains, as before, close to the value \(I_{\text{en}}\).

Using the resistance \(R_k\) as the power source for the SCR control circuit considerably simplifies the circuit and protects it from the flow of ballast current, since after the SCR opens the resistance \(R_k\) is shunted.

The diodes \(D\) prevent currents from voltage drops across \(R_k\) of reverse polarity from flowing through the SCR control circuits.

As is evident from the circuit, equality of the emission currents from all sections is ensured by the common setting source (resistances \(R_1, R_2\)).

In order to verify the effectiveness of the principle of emission-current stabilization considered here, a circuit analogous to that discussed was tested, using a silicon-controlled rectifier of type D 235 G, resistant to reverse voltage.

The object of stabilization was the emission current of a kenotron of type 5TsS.

With an artificial change in the resistance of the filament circuit by up to \(+0.3\) of its nominal value, the change in the anode current \(I_0\) of the kenotron in all cases did not exceed \(2\text{–}3\ \mathrm{mA}\) at \(I_a = 250\ \mathrm{mA}\).

Fig. 2. Oscillograms of the voltage drop across the resistance \(R_{ki}\)

1 — in the absence of disturbances; 2 — with deterioration of the vacuum, a drop in the emission capability of the cathode sections, and filament voltage; 3 — in the case of an interturn short circuit in the heater spirals

Fig. 3. Curve of the stabilized emission current

A disadvantage of the circuit is the excessive power of the KUV, which in this case operates only when a voltage of positive polarity is applied to it.

With the introduction of rectifier bridges into the filament circuit of the sectional heaters, this disadvantage is eliminated, but the circuit becomes more complicated.

When one or several heater filaments fail, the total current of the electron beam remains unchanged owing to the automatic increase in the emission currents of the cathode sections that remain in operation. The sections are thereby heated to a temperature exceeding the normal one.

Such a mode should be avoided whenever possible, since at elevated temperatures their service life is substantially reduced.

For this purpose, signal lamps are connected in parallel with the CCUs. When any filament burns out, the corresponding CCU ceases to shunt the resistance \(R_{ki}\), and the light of the lamp changes from flashing to steady.

The circuit considered is expedient for use with electron guns supplied by a constant high voltage, or by a pulsed voltage with rectangular pulses having a duration of not less than \(20\)–\(30\ \mu\text{s}\).

The circuit provides a stabilized electron beam with a current density uniformly distributed over the cross section and substantially increases the service life of the cathode.

REFERENCES

1. E. S. Brovman. Application of Controlled Semiconductor Valves in Industry. Publishing House of the Moscow House of Scientific and Technical Propaganda, 1961.