[0001] This invention relates to linear beam tubes and more particularly to the electron
guns of such tubes.
[0002] The electron gun end of a typical linear beam tube is shown in Figure 1 of the accompanying
drawings.
[0003] Referring to Figure 1 the glass or ceramic envelope of the linear beam tube is represented
at 1; the cathode of its electron gun is represented at 2 and the anode of its electron
gun is represented at 3. The anode 3 is mounted directly upon the first pole piece
4 of a focussing structure at the entrance of a slow wave structure (not represented
but to the right as viewed) of the linear beam tube. Commonly but not necessarily
this first pole piece 4 constitutes also an end wall of the first cavity of the slow
wave structure. In this case the anode 3, mounted as it is on the first pole piece
4, is held at ground potential and the length of the main cathode voltage insulator
(i.e. the length L of the envelope 1 electrically between the cathode 2 and the pole
piece 4) is determined by the voltage stand-off requirements external to the tube.
Where the tube operates in air the length L requires to be longer than would be the
case if the gun end of the tube were to be immersed in a dielectric liquid.
[0004] With the configuration of Figure 1, during high voltage arcs occurring between the
cathode 2 and anode 3 these electrodes are prone to damage and for this reason the
known configuration shown in Figure 2 of the accompanying drawing has found some favour
by virtue of the protection that may be afforded to the electrodes in the face of
such high voltage arcs.
[0005] Referring to Figure 2 in this case the anode electrode 3 is isolated from the pole
piece 4 and is mounted upon a metal cylinder 5 which cylinder is in turn supported
between two insulating lengths of envelope 1' and 1" which are each of length equal
to L.
[0006] A flange 6 by which the cylinder 5 is mounted, and which is sandwiched between the
two lengths of insulator 1' and 1", forms an electrical connection for the anode 3.
Between the electrical connection formed by flange 6 and earth is an external limiting
resistor 7. In practice, and as shown, the end of the resistor 7 remote from.the flange
6 is grounded by being attached to the first pole piece 4. In some cases grounding
is effected not via the first pole piece 4 but via a current sensor.
[0007] With the construction of Figure 2, during normal operation the anode 3 is held close
to ground potential since there is negligible anode current drawn. However when an
anode to cathode arc occurs a relatively large current flows through the limiting
resistor 7 which charges the anode up to cathode potential, thus causing the arc to
be extinguished. In this case the anode is at a potential other than ground potential
only during such arcs.
[0008] Whilst the tube illustrated by Figure 2 is, as regards cathode to anode arcs, a "protected"
tube as opposed to the tube illustrated by Figure 1 which is an "unprotected" tube,
a serious disadvantage arising from the construction of Figure 2 is the added length
of insulating envelope wall, i.e. the two portions 1' and 1", between the cathode
mount and the first pole piece 4 - effectively double that of the construction illustrated
by Figure 1. Whilst this added length is in itself undesirable there is in consequence
also a tendency for the gun to be the more susceptible to vibration. Such vibration
can give rise to electrical noise which is a serious limitation in some systems.
[0009] The present invention seeks to provide an improved linear beam tube in which the
above difficulty is reduced.
[0010] According to this invention an electron beam tube comprises an electron gun having
at one end thereof a cathode and at the other end thereof an anode, said anode being
mounted adjacent to but isolated from an end wall of a structure downstream of said
gun and wherein said anode is supported by means of folded mounting means comprised
of a first part extending back from said anode towards the cathode end of said gun
and a second part extending forward from the end of said first part remote from said
anode towards said end wall, one of said two parts being of insulating material and
having provided on a surface thereof a limiting impedance which is electrically in
series in a path between said anode and an external termination which in operation
may be grounded (directly or indirectly e.g. via a current sensor) whereby to permit
said anode to tend to charge towards cathode potential if an anode to cathode arc
occurs.
[0011] Said structure may for example be a focussing structure; wave propagating structure;
an electron beam tunnel or the like depending upon the type of linear beam tube to
which the invention is applied.
[0012] The end of said second part adjacent said end wall may be attached to said end wall
or carried from the envelope of said tube at a position adjacent said end wall.
[0013] Commonly said end wall will be the first pole piece of a focussing structure, in
which case where the end of said second part adjacent to said end wall is attached
to said end wall said pole piece may comprise said external termination.
[0014] Preferably said first and second parts of which said folded mounting means is comprised
are, in any plane transverse to the beam axis over the major portions of their lengths,
generally circular in cross section and co-axial with said beam axis.
[0015] Both of said parts may overall be generally cylindrical and united at their ends
remote from said end wall and said anode by flange means or at least one of said two
parts may be generally conical.
[0016] In one embodiment of the invention said first part comprises a ceramic tubular member
and said second part comprises a conical metallic member which has a flange at its
end remote from said first part which flange extends through the envelope of said
tube adjacent said end wall.
[0017] Preferably with a construction as last described said limiting impedance is provided
on an internal surface of said tubular member comprising said first part, with one
end in contact with said anode and the other in contact with said conical metallic
member.
[0018] In another embodiment of the invention said first part comprises a metallic tubular
member co- axially within a ceramic tubular member which comprises said second part,
said ceramic tubular member being attached at one end to said end wall and the end
of said metallic tubular member remote from said anode being flanged with its flange
attached to the end of said ceramic tubular member remote from said end wall.
[0019] Preferably with a construction as last described said limiting impedance is provided
on an external surface of said tubular member comprising said second part with one
end in contact with said flange and the other electrically connected to said external
termination.
[0020] Said limiting impedance may comprise deposited resistive material preferably deposited
within a groove in the surface of that member upon which said limiting impedance is
provided as aforesaid.
[0021] The invention is further described with reference to Figures 3 and 4 of the accompanying
drawing which illustrates two embodiments of
'i.the present invention.
[0022] In Figures 3 and 4 like references are used for like parts in Figures 1 and 2.
[0023] Referring to Figure 3 as will be seen the anode 3 is isolated from the pole piece
4 and is mounted upon a ceramic cylindrical member 8 which extends back towards the
cathode end of the electron gun.
[0024] The cylindrical ceramic member 8 is supported by means of a flanged conical metallic
member 9 which surrounds the cylindrical member 8. One end 10 of the conical member
9 is fixed to the end of the cylindrical ceramic member 8 remote from the anode 3.
The other flanged end of the conical member 9 extends through the envelope 1 of the
tube to form an external termination 11. The length of envelope between the flange
11 and the mount of the cathode 2 is equal to L.
[0025] On the interior surface of the cylindrical ceramic member 8 is deposited within a
groove 12 resistive material forming the required limiting impedance. The deposited
resistive material extends beyond the groove so that one end 13 of the limiting impedance
is in electrical contact with the anode 3 and the other end extends over the end of
the cylindrical ceramic member 8 to contact end 10 of the conical metallic member
9. As shown, the resistive material is deposited in the base of the groove 12 and
extends only partly up the side walls of the groove.
[0026] Thus the limiting impedance is again in series in a path between the anode 3 and
the external termination 11. If this termination is grounded then as already described
with reference to Figure 2, arcs occurring between the cathode 2 and the anode 3 may
be suppressed. It will be noted however that the overall length of the gun section
of the protected tube illustrated by Figure 3 corresponds more closely to that of
the electron gun of the unprotected tube of Figure 1 than to the lengthy gun section
of the protected tube of Figure 2.
[0027] Because the limiting impedance operates in a vacuum within the tube envelope the
length of ceramic required to hold off the voltage is less than is the case for the
resistor 7 of Figure 2.
[0028] Referring to Figure 4, the anode 3 is again isolated from the pole piece 4. Anode
3 is mounted upon a generally cylindrical metallic member 14 which extends back from
the anode 3 towards the cathode end of the electron gun.
[0029] Member 14 is co-axially within a cylindrical ceramic member 15 which is mounted at
one end on the pole piece 4 and extends back therefrom towards the cathode end of
the electron gun.
[0030] The end of member 14 remote from the anode 3 is flanged with its flange 16 attached
to the end of the ceramic member remote from the pole piece 4.
[0031] Passing through the envelope wall 1 adjacent to, but spaced from, the pole piece
4 is an annular metallic member 17 which extends inwardly towards the tube axis to
contact the cylindrical ceramic member 15. Member 17 provides an external termination.
[0032] On the exterior surface of cylindrical ceramic member 15 is deposited within a groove
18, resistive material forming the required limiting impedance. The deposited impedance
formed again extends, beyond the groove, from the flange 16 of cylindrical metallic
member 14 to member 17 forming said external termination.
[0033] Thus again the limiting impedance is in series in a path between the anode 3 and
the external termination formed by member 17 and if the last mentioned is grounded
then again as already described with reference to Figure 2 arcs occurring between
the cathode 2 and the anode 3 may be suppressed.
1. An electron beam tube comprising an electron gun having at one end thereof a cathode
and at the other end thereof an anode,said anode being mounted adjacent to but isolated
from an end wall of a structure downstream of said gun and wherein said anode is supported
by means of folded mounting means comprised of a first part extending back from said
anode towards the cathode end of said gun and a second part extending forward from
the end of said first part remote from said anode towards said end wall, one of said
two parts being of insulating material and having provided on a surface thereof a
limiting impedance which is electrically in series in a path between said anode and
an external termination which in operation may be grounded whereby to permit said
anode to tend to charge towards cathode potential if an anode to cathode arc occurs.
2. A tube as claimed in claim 1 and wherein the end of said second part adjacent said
end wall is attached to said end wall.
3. A tube as claimed in claim 1 and wherein the end of said second part adjacent said
erdwall is carried from the envelope of said tube at a position adjacent said end
wall.
4. A tube as claimed in any of the above claims and wherein said end wall is the first
pole piece of a focussing structure.
5. A tube as claimed in claim 4 as dependent upon claim 2 and wherein said pole piece
comprises said external termination.
6. A tube as claimed in any of the above claims and wherein said first and second
parts of which said folded mounting means is comprised are, in any plane transverse
to the beam axis over the major portions of their lengths, generally circular in cross
section and co-axial with said beam axis.
7. A tube as claimed in claim 6 and wherein both of said parts are, overall, cylindrical
and united at their ends remote from said end wall and said anode by flange means.
8. A tube as claimed in claim 6 and wherein both of said parts are, overall, generally
cylindrical and united at their ends remote from said end wall and at least one of
said two parts is generally conical.
9. A tube as claimed in any of claims 1. to 6 and wherein said first part comprises
a ceramic tubular member and said second part comprises a conical metallic member
which has a flange at its end remote from said first part which flange extends through
the envelope of said tube adjacent said end wall.
10. A tube as claimed in claim 9 and wherein said limiting impedance is provided on
an internal surface of said tubular member comprising said first part, with one end
in contact with said anode and the other in contact with said conical metallic member.
11. A tube as claimed in any of claims 1 to 6 and wherein said first part comprises
a metallic tubular member co-axially within a ceramic tubular member which comprises
said second part, said ceramic tubular member being attached at one end to said end
wall and the end of said metallic tubular member remote from said anode being flanged
with its flange attached to the end of said ceramic tubular member remote from said
end wall.
12. A tube as claimed in claim 11 and wherein said limiting impedance is provided
on an external surface of said tubular member comprising said second part with one
end in contact with said flange and the other electrically connected to said external
termination.
13. A tube as claimed in any of the above claims and wherein said limiting impedance
comprises deposited resistive material.
14. A tube as claimed in claim 13 and wherein said resistive material is deposited
in a groove in the surface of that member upon which said limiting impedance is provided.
15. An electron beam tube substantially as herein described with reference to Figure
3 of the accompanying drawings.
16. An electron beam tube substantially as herein described with reference to Figure
4 of the accompanying drawings.