1. * GB780044 (A)
Description: GB780044 (A) ? 1957-07-31
Improvements in or relating to reflex klystrons
Description of GB780044 (A)
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CH324328 (A) DE1098626 (B) FR1144145 (A) US2881350 (A)
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The EPO does not accept any responsibility for the accuracy of data
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up-to-date or fit for specific purposes.
PATENT SPECIFICATION
780,044 0 Date of Application and filing Complete Specification: Dec.
23, 1954.
No. 37264/54.
Application made in Germany on Dec. 23, 1953.
Complete Specification Published: July 31, 1957.
Index at acceptance:-Class 39(1), D(16A2: 17A2B: 46A).
International Classification:-H01j.
COMPLETE SPECIFICATION
Improvements in or relating to Reflex Klystrons We, SIEMENS & HALSKE
AKTIENGESELLSCHAFT, a German Company, of Siemensstadt, Berlin,
Germany, and Wittelsbacher Platz 4, Munich 2, Germany, do hereby
declare the invention for which we pray that a patent may be granted
to us and the method by which it is to be performed, to be
particularly described in and by the following statement.This
invention relates to reflex klystrons.

2. With a view to producing a reflex klystron of high output, efforts
have been made to ensure that the electron current is as high as
possible and the accelerating voltage as low as possible. However, a
limit is set on the increase in output in this manner by two factors,
namely the emissive power of the cathode and the effect of the space
charge in the reflector space.
As is known, with constant current strength the space charge
progressively increases as the accelerating voltage decreases and at
the surface of reversal of the electrons in front of the reflector
plate it is theoretically infinitely great so that the electrons
spread out laterally instead of returning to the grid space by the
path along which they arrived. Efforts have been made to prevent this
spreading out at the reflector plate by appropriately shaping the
reflector plate and the parts surrounding it.
The invention consists in a reflex klystron having a cavity resonator,
wherein the ratio of the reflector surface area to the cathode surface
area is more than twice the ratio of the square of the length of the
retarding path in front of the reflector and the square of the length
of the acceleration path in front of the cathode.
Preferably the electron beam is made to diverge radially in all
directions at the end of the acceleration path. At high current
densities, a large outwardly directed force in any case exists also in
the unretarded part of the path owing to the large space charge, and
this force will preferably be utilised to produce the divergent beam.
Sprige, 3s. 6d.] The invention will be more particularly described
with reference to the accompanying drawings, in which two embodiments
of the invention are illustrated diagrammatically in greatly
simplified form, only -the parts essential to an understanding of the
invention being shown.
Figure 1 illustrates a reflex klystron in which a concave cathode 1 of
large surface and having relatively low current density is so employed
in known manner that a narrow beam is formed at a point in the
vicinity of the anode 10. This beam then diverges considerably owing
to the space charge forces described.
The walls of tile resonator, which are designated by 2 and 3, are
connected to the grid electrodes 4 and 5. A concave plate 6 is
provided as the reflector. The electron movement takes place within
the beam represented by the chain lines 7 and 8.
Figure 2 illustrates a further embodiment in which, instead of a
cathode of large surface, a high-power cathode of small cross-section
is employed, which is designated by 9. The said cathode may be, for
example, a storage cathode having high electron emissive power and is
disposed so close to the anode 10 that very high current strengths can
be produced at a relatively low anode voltage. A very divergent beam

3. is thus again produced.
In other respects, like parts are designated by the same references as
in Figure 1.
In order that a reflex klystron may operate in accordance with the
invention, it is necessary to prevent the formation of a virtual
cathode in the reflector space. It has been found by experiment and by
theoretical consideration that if a virtual cathode is formed, the
velocity of the returning electrons is only very vaguely defined,
which results in a very small slope of the characteristic
currentvoltage curve of the collector. This slope is a measure of the
quality of the phase focussing of a reflex klystron, because a small
slope indicates that only few electrons of a, particular velocity
group return into the grid I>.1_7L space, and thus that the
building-up of oscillations at a particular frequency is possible only
with low efficiency. Conversely, a large slope indicates the
building-up of oscillations at a particular frequency with high
efficiency.
The twvo grids of the oscillation circuit are shown concavely curved
towards the entering primary beam in the two figures and they
arepreferably also not disposed exactly at the narrowest point of the
beam, so that the arrangement may have a relatively high grid
capacity. The efficiency of the circuit is decreased by this
arrangement, but this is offset by the increased current strength and
by the improved conditions in the reflector space.
Reflex klystrons are particularly suitable for modulation purposes,
since they have a relatively large band width.
The condition essential to the avoidance of the formation of a virtual
cathode is theoretically exactly known. It is a limitation on the
length of the retarding path in front of the reflector and can be
stated matLiematically as a restriction of this length to 1/v2 of the
length of the acceleration path in front of the cathode, everything
being reduced to a plane arrangement. Accordingly, favourable
dimensions for the arrangement are obtained by conforming to the
following simplified formula:
F a-' 2K k= F being the effective reflector surface area, K the
effective cathode surface area, a the length of the retarding path in
front of the reflector, and k the length of the accelerating path in
front of the cathode.
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