A bakeable electromagnet assembly is made of coils of metal foil either anodized to insulate between turns or separated by a film of Kapton. Cooling plates connected to a source of cooling fluid are used to remove heat. Heat is conducted from the coils to the cooling plates by a powder which is a heat conductor and electrical insulator such as boron nitride. An external shell made of high magnetic permeability material provides a return path for the magnetic field.

FIELD OF THE INVENTION 
This invention pertains to a new structure of electromagnet which is 
bakeable, more particularly to an electromagnet which can be formed on an 
electron beam tube before bakeout of the tube. 
BACKGROUND OF THE INVENTION 
Klystrons and other electron beam devices requiring a magnetic field for 
their operation could be made smaller and lighter if the electromagnet 
could survive the high temperature bakeout required during the fabrication 
of the device. There are many high temperature electrical devices, for 
example, electrical heating elements using ceramic or mica insulation. The 
requirements for a bakeable electromagnet are different from a heating 
element, however, in that the conductor in an electromagnet needs to be 
kept as cool as possible during operation. 
The approach to forming an electromagnet on an electron beam tube can be 
divided into a "wrapped solenoid" approach and a "wound-on magnet" 
approach. In the "wrapped solenoid" approach, the tube is assembled and 
baked and then a solenoid is wrapped on the tube using the tube as a 
spool. In the "wound-on magnet" approach, the electromagnet is a component 
to be assembled with other components to make a complete tube and then 
baked. 
The "wound-on magnet" design has several serious disadvantages. It is 
presumed that the tube is tested first in an ordinary solenoid magnet to 
assure meeting all electrical specifications. Then the device, now 
representing a substantial monetary investment, is mounted in a winding 
fixture for application of the magnet turns. The magnet winding operation 
may or may not be successful. In either case, further testing must be 
carried out. If the magnet does not yield the desired results, then it 
must be unwound and a second attempt made. The technique offers no change 
to check the magnet before it is used. Another disadvantage rests in the 
fact that cutouts, such as those used for passage of the output waveguide, 
are not possible. Still further, a special system of cooling might be 
required to remove coil heat. One system that has been successful with low 
power linear beam tubes make use of coil circulated in contact with the 
coils. A separate oil-water heat exchanger is employed. 
The "bakeable" magnet calls for the use of certain materials that differ 
from those used in conventional solenoids. The coil winding insulation 
must withstand the bakeout temperatures. Metal oxides have been used in 
some attempts in the past, though the history of such units suggest 
trouble from turn-to-turn shorts. 
OBJECT OF THE INVENTION 
It is the object of the invention to describe a structure for an 
electromagnet which will remain operable after high-temperature bakeout. 
SUMMARY OF THE INVENTION 
Coils of metal foil insulated by Kapton film generate the magnetic field. 
Cooling plates of copper are used to remove heat from the coil in 
operation. A heat conducting but electrically insulating powder or fused 
ceramic such as boron nitride is used to conduct heat from the coils to 
the cooling plates. An external shell of high magnetic permeability 
material is used to provide a return path for the magnetic field. 
These and further constructional and operational characteristics of the 
invention will be more evident from the detailed description given 
hereinafter with reference to the accompanying drawing which illustrates 
one preferred embodiment by way of non-limiting example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the FIGURE wherein reference numerals are used to 
designate parts through, there is shown a cross-section of a bakeable 
electromagnet 10 according to the invention. The electron beam tube 12 is 
shown schematically at the center of the electromagnet 10. An external 
shell 14, usually of material of high magnetic permeability to provide a 
return path for the magnetic field, is shown around the electromagnet. The 
shell 14 must be sealed by a method which will withstand the bakeout 
temperature. Coils 16 made from a foil, usually aluminum or copper, 
generate the magnet field. The conductor layers of the coils 16 are 
individually insulated from each other by high temperature epoxy bonded 
Kapton film. Kapton is a polyimide material made by the E. I. DuPont de 
Nemours Company. The epoxy may carbonize during bakeout, but the Kapton 
will survive and the layers will be insulated. In the alternative, the 
insulation can be provided by anodizing the surface of the foil. Copper or 
aluminum cooling plates 18 containing passages for coolant flow are used 
to remove heat from the coils during normal operation. The coolant can be 
water, oil or any other. The coolant passages would be dry during bakeout, 
probably purged with an inert gas or hydrogen to prevent oxidation. An 
electrical insulation layer 20 which is thermally conductive is located 
between each cooling plate 18 and each coil 16. The layer 20 can be a 
powder with a film of Kapton or a fused layer of ceramic which can be 
tested before incorporation into the magnet. Aluminum oxide (Alumina) is 
not an outstanding thermal conductor, but lends itself well to coating the 
cooling plates. Beryllium oxide (Beryllia) would be ideal, were it not for 
its toxicity, since it is an excellent electrical insulator and has the 
highest thermal conductivity of all the ceramics. Boron nitride is the 
preferred material since it is a good insulator and the packed powder has 
very good thermal conduction. It can be applied to cooling plates not only 
by thermal spraying, but by painting and baking as well. A liner 22, 
preferably of stainless steel, is used between the electromagnet coils 16 
and the tube 12. The layer 20 can extend between the liner 22 and the coil 
16 and between the coil 16 and the external shell 14 to fill in the voids 
provide electrical insulation and conduct heat as necessary. The 
connections 24 between the cooling plates 18 and the cooling source can 
all be external, as shown in the FIGURE. In the alternative, some internal 
connections can be used to reduce the number of external connections. 
This invention is not limited to the preferred embodiment heretofore 
described, to which variations and improvements may be made including 
mechanically and electrically equivalent modifications to component parts, 
without departing from the scope of protection of the present patent and 
true spirit of the invention, the characteristics of which are summarized 
in the following claims.