Чтение общенаучной литературы. Кытманова О.А. - 6 стр.

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7
along AC with speed y. Then if v = ic/Nicdt the electrons in the beam travel at
approximately the same speed as the signal and a continuous interchange of energy
between the electron stream and the circuit is feasible. . ' The principles of the
travelling -wave valve associate the slowly moving signal with the words
"travelling-wave" in contradistinction to the standing wave which exists m a
klystron or a conventional valve amplifier. It should be noted, however, that the
signal in a multiresonator magnetron may be regarded similarly as a travelling
wave, rotating round the anode.
Hot-Cathode Gas-Filled Triodes: Thyratrons
The useful properties of the hot-cathode gas-filled diode immediately suggest the
introduction of a control grid. The most common valve of this type is one which
contains mercury vapour but the filling gas may be helium or argon or a mixture of
inert gases. In fact, electrically, the valve acts as a switch and so has been given the
name thyratron "from the Greek word meaning a door. It opens the way to the
electric current.
The thyratron grid is known to control the flow of electrons from the cathode to
the anode, but only in the sense of an on/off switch. When the grid is sufficiently
negative no electrons can pass through it to the cathode, and so the current is cut
off. When the grid is made less negative there comes a stage at which a certain
number of electrons can pass through it. If the anode is then sufficiently positive
with respect to the cathode, these electrons produce positive
ions by collision with the gas or vapour molecules These
positive ions partially destroy the space charge near the
cathode, the current flowing through the valve freely. When
the current has started, however, the grid can no longer
control it, for making the grid more negative only increases
the potential difference between it and the anode and draws
to it a greater number of positive ions. These positive ions
form a space charge which masks the potential of the grid
wires and allows electrons to pass freely through from the
cathode, since, therefore, the grid looses control of the
current which it has initiated, to render the thyratron
nonconducting, again it is necessary to cut off the positive potential applied to the
anode. When- the current ceases, the grid regains control. The thyratron is not
constructed in the same manner as the conventional high-vacuum valve. The usual
structure is shown diagrammatically by the axial section drawn in Fig. 11, where
the cathode is an indirectly-heated cylinder, K, coated with oxides. The grid, G, is
a cylinder surrounding the cathode and containing an annual disk, D. Opposite the
aperture in the disk, the cup-shaped anode is placed. This old construction is used
for two reasons. First, as in the gas diode, it is unnecessary for the emitting surface
of the cathode and the collecting surface of the anode to be placed opposite each
other, and secondly, since an extremely small primary electron current is sufficient
to start the valve conducting, the grid control must be made as strong as possible.

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