Uniform field solenoid magnet with openings

A solenoidal magnet coil is used to generate an axial field for focusing a beam of electrons through a linear-beam electron tube. In high-power tubes, the coil typically cannot extend over the entire length of the focused electron beam because it would interfere with the waveguide used to carry out the generated wave power. Thus the axial magnetic field strength falls off near the output end, a region in which it would be desirable to have it uniform or even slightly increasing. Very often the coil is foil-wound and its output end has a notch to allow passage of the waveguide. A similar notch 180 degrees away compensates the sideways distortion of field caused by displacement of coil current away from the notch impediment. In the non-notched regions the current spreads throughout the coil cross-section, but there is still a fall-off of field strength on the axis due to current displacement away from the output end. The invention comprises a second pair of notches in the end of the coil opposite the output end and azimuthally spaced between the first pair. These notches deflect the current toward the output, compensating the magnetic field fall-off.

DESCRIPTION 
Background of the Invention 
In most linear-beam electron tubes, such as klystrons and traveling-wave 
tubes, the electron beam is held focused into a cylindrical outline by a 
uniform magnetic field directed along the beam axis. In high-power tubes 
the magnetic field is typically produced by a solenoid coil outside the 
tube and coaxial with the beam. An iron shell encloses the solenoid to 
confine the field to the interaction region of the tube and to make it as 
uniform as possible throughout that region. The diameter of the beam is 
typically much smaller than the solenoid, so the field very close to the 
axis is the only significant part. 
FIG. 1 illustrates a prior-art klystron 10 in its focusing magnet 20. Tube 
10 slides into magnet 20 from the top. Klystron 10 comprises an electron 
gun 11 for producing a convergent beam 12 of electrons. Beam 12 passes 
through a hollow drift-tube 14 where it interacts with the electromagnetic 
fields of resonant cavities 16, 18 to amplify a signal wave fed into input 
cavity 16 through an input transmission line (not shown) which is 
typically a small coaxial cable. 
In the region of cavities 16, 18, beam 12 is held focused in a pencil shape 
by an axial magnetic field produced by solenoid magnet 20. Beyond the 
interaction region it leaves the magnetic field and expands by its 
space-charge repulsion to land on the inner surface of a large collector 
bucket 22. 
Magnet 20 has a ferromagnetic shell comprising an outside cylinder 24 
joined to ferromagnetic end-plates 26. End-plates 26 are in magnetic 
contact with inner polepieces 27 which are an integral part of klystron 
10. Each polepiece 27 has a small central holes 48, 50 for passing beam 
12. Outside of holes 48, 50 the magnetic field falls off rapidly to a 
negligible value. 
Magnet 20 comprises a number of solenoidal coils 30. However, a single long 
solenoidal winding is often used. To obtain a truly uniform field, coils 
30 should extend all the way between iron end-plates 26. However, to carry 
away the high output power of klystron 10, a waveguide 32 must extend from 
a coupling aperture 34 in output cavity 18, through a vacuum-tight 
dielectric window 36 to an external useful load (not shown). Therefore, in 
the prior art coils 30 could extend axially only to the bottom plane 37 of 
waveguide 32, leaving a magnetically un-energized gap 38 adjacent the 
output polepiece 27. 
In the construction shown by FIG. 1 cavities 16, 18 are tuned by tuner 
plates 40 moved in and out by rods 42. Coils 30 are separated by 
non-magnetic plates 44 which provide mechanical support and thermal 
cooling. Plates 44 have passages for tuner rods 42. 
FIG. 2 is a schematic graph of the axial magnetic field strength produced 
by magnet 10 when all coils 30 have the same current density. The field 
has a uniform value 46 over most of the interaction region, falling 
rapidly to almost zero near the entrance aperture 48 and exit aperture 50 
in polepieces 27. Due to the gap 38 beyond coils 30, the flux lines, 
spread out in this region and the axial field strength 52 falls off 
gradually. If the coils 30 were continued, the field 53 would be uniform 
almost to aperture 50. In the output region of a high-power linear-beam 
tube the beam has bunches of high space-charge density and also suffers 
from electromagnetic defocusing forces. Therefore the weakened focusing 
field 52 causes interception of electrons on the interaction structure, 
with consequent loss of power and dangerous heating. 
Various schemes have been devised to reduce the magnetic field distortion. 
Increasing the current density in the upper solenoid section 30 increases 
the field in the output region, but creates an undesirable peak in the 
field before that. U.S. Pat. No. 2,963,616 issued Dec. 6, 1960 to Richard 
B. Nelson and Robert S. Symons describes a means illustrated by FIG. 3, 
which shows only the magnet 20' and output waveguide 32' of klystron 10'. 
Here waveguide 32' is stepped down via an impedance transformer 54 to a 
very shallow waveguide 56 in the region inside focusing magnet 20'. The 
height of the unenergized space 38' is thus reduced, decreasing the 
fall-off of field strength. 
Another prior-art scheme is described in U.S. Pat. No. 2,939,036 issued May 
31, 1960 to Richard B. Nelson. Here a shallow output waveguide is run up 
alongside the collector, parallel to the tube axis instead of outward 
perpendicular to it. Unfortunately this scheme is limited to relatively 
low-power tubes. In high-power tubes the collector is larger than the tube 
body and would interfere with the waveguide. 
The solenoid coils 30 (FIG. 1) are sometimes wound with wire. Another 
useful construction illustrated by FIGS. 4A and 4B, uses coils 30" wound 
spirally of thin metallic foil. Aluminum foil is usually used, insulated 
by an anodized surface. Heat is conducted out of the foil axially via a 
short all-metal path to heat sinks 44" which are for example annular 
copper plates between foil windings 30". With foil coils one can cut out a 
notch 60 in the end coil 62 to allow passage of the output waveguide. 
Notch 60 forces the current flow lines 64 to concentrate below notch 60 by 
adding axial components to the flow. Around the remaining periphery of 
coil 62 the current is free to spread throughout the cross section of coil 
62. A single notch 60 would thus create an asymmetric current pattern 
which would cause a magnetic flux line following the axis to deviate away 
from the axis near the output waveguide end of the magnet. This would bend 
the electron beam. To correct this distortion a second notch 63 is cut in 
coil 62, 180 degrees from notch 60 and shaped to have a 180 degree 
rotational symmetry with notch 60. The resulting current flow lines 64 are 
confined in regions 66 under notches 60, 63 but are free to spread out in 
the intervening regions 68. They form a saddle shape, symmetric with 
respect to a 180 degree rotation about the axis. The magnetic flux lines 
generated by the current have the same symmetry, and the magnetic 
equipotentials are saddle-shaped surfaces. The axial magnetic flux line 
follows the axis throughout. Since the electron beam is much smaller than 
the magnet, the tilted off-axis fields are unimportant. 
The current can spread to the top end of coil 62 in inter-slot regions 68, 
so the fall-off of axial magnetic field strength is not as drastic as in 
the case of FIGS. 1, 2 where the whole coil is cut short. Nevertheless, 
the total current in the top half of coil 62 is less than in the bottom 
half, so there is a substantial fall-off of field. 
In FIG. 5, curve 70 is a graph of axial magnetic field strength as 
experimentally measured for a coil as illustrated by FIG. 4. For 
comparison, curve 71 shows the field for a uniform solenoid extending 
clear to the polepiece, with a small hole in the polepiece. This latter 
would be the ideal condition. Note the rise 72 in the field at a distance 
from the polepiece. This is due to the concentration of current under 
notches 60, 62. 
Summary of the Invention 
The object of the invention is to provide a solenoid magnet which can 
maintain a constant axial field throughout the interaction region of a 
linear-beam electron tube while permitting the outward passage of a 
waveguide through it. 
This object is realized by providing a solenoid coil with oppositely 
located openings near a first end to pass the waveguide and preserve 
symmetry. Near the other end and spaced betwen the openings are regions 
which impede the current flow, forcing current to concentrate near the 
first end in these parts of the periphery. Thus the average currents near 
the two ends can be made equal. Hence the axial magnetic field can be made 
approximately constant. The coil can be wire-wound with the turns having a 
saddle-shaped symmetry. In a foil-wound coil the openings and the impeding 
regions can be portions cut out of the foil winding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be described mainly as embodied in foil-wound magnet 
coils. It will also be shown that it may be embodied in wire-wound coils. 
FIG. 6 is a perspective view of a foil-wound magnet coil embodying the 
invention. The coil would be the end coil of a solenoid magnet, arranged 
the same as end coil 62 of FIGS. 4 at the output end of a linear-beam 
electron tube. The coil 74 has a notch 60' in its upper end to pass an 
output waveguide (not shown). An opposite, symmetric notch 63' compensates 
for bending the axial field line, as described in connection with FIGS. 4. 
Between notches 60' and 63' and azimuthally disposed between them, are 
another pair of notches 76 in the other end of the coil 74. Notches 60', 
63' force the current flow lines 78 to concentrate beneath them at their 
locations 66' on the coil's perimeter. 
Compensating notches 76 similarly force current lines 78 to concentrate 
above them at their locations 68' on the perimeter. As a result, flow path 
78 traces a saddle-shaped curve, oscillating above and below the midplane 
of coil 74. The resulting magnetic equipotential surfaces are also 
saddle-shaped If the pair of compensating notches 76 are also symmetric 
with respect to a 180 degree rotation about the axis, the magnetic flux 
line on the axis will follow the axis accurately, as described in 
connection with FIGS. 4. The effect of the rotational symmetry can be 
visualized by noting that each vector component of current produces a 
vector component of field at each point on the axis. If the current vector 
is rotated 180 degrees, the field field vector will also be rotated 180 
degrees, maintaining its original angle with the axis. The original vector 
and its rotated image lie in the same plane (containing the axis) so their 
components perpendicular to the axis cancel. The 180 degree rotational 
symmetry of current thus must produce only an axial field component on the 
axis. The addition of compensating slots 76 can balance out the net 
downward displacement of current lines 78 by the needed slots 60', 63'. 
This is seen to be obvious if slots 76 are identical with slots 60', 63', 
making the structure symmetric with respect to an axial inversion plus a 
90 degree rotation. However, there may be structural or thermal reasons to 
make slots 76 of a different shape. Whatever their shape, as long as the 
rotational symmetry is preserved, the axial field will be straight. By 
proper slot dimensions and choice of coil length, the required 
compensation of axial field strength fall-off may be achieved almost 
perfectly. Returning to FIG. 5, curve 92 is a graph of measured axial 
field strength for a coil with compensating notches, showing the great 
improvement over the prior art coil of FIGS. 4 (curve 70). 
FIG. 7 illustrates how the invention may be embodied in a wire-wound coil. 
The wires 80 are wound on the surface of a cylinder 82. They alternately 
rise above and fall below a transverse center-plane 84. The waveguide 86 
would pass through the opening between the coil 80 and the magnet 
end-piece at a point where the wires are removed downward away from the 
end-piece. This coil bears some resemblance to the "baseball" coils used 
in some plasma-confining experiments. It is, however, different in both 
form and function because it produces a uniform field instead of a 
confining magnetic-mirror field. 
FIG. 8 is a schematic cross-section of a foil coil embodying the invention. 
It illustrates that the compensating regions near the bottom of the coil 
need not be identical with the working slots 60", 63" and need not even be 
slots. A pair of holes 90 of any proper symmetrical shape can provide the 
necessary current-shaping impediment. Also, the compensating regions need 
not extend clear through coil 62" radially. Another embodiment is to use 
narrow slots which do not interfere with axial heat flow as much as wide 
ones. Each compensating region may comprise a number of slots, grooves or 
holes. 
It will be obvious to those skilled in the art that many other embodiments 
may be made within the scope of the invention. The embodiments described 
above are exemplary and not limiting. Any means of impeding current flow 
at the proper places will suffice. The saddle-shaped distortion off the 
axis may be reduced by using more than the two pairs of current-impeding 
regions, each having the required symmetry. The use of more than two pairs 
will, of course, increase the electrical resistance of the coil. 
The scope of the invention is to be limited only by the following claims 
and their legal equivalents.