BACKGROUND OF THE INVENTION
This invention relates generally to the art of electron beam accelerators
and more particularly to the art of cooling such electron beam accelerators.
Electron beam accelerators are used in a variety of environments.
The most readily known environment is that of cathode ray tubes which in and of
themselves have an extremely diverse variety of applications.
While cathode ray tubes maintain accelerated electrons within the
environment of the tube enclosure, applications exist for electron accelerators
where the electrons must pass through a window for further utilization. Examples
of this are applications such as crosslinking and grafting of polymeric materials,
the curing of coatings and inks, the pasteurization of food stuffs or the sterilization
of medical products wherein the electrons pass through a thin foil window to reach
the product treatment area.
Windows utilized in conventional electron beam accelerators in such
applications are generally formed of a foil which is reasonably transparent to
an electron beam. Generally such foils are formed of titanium or titanium alloys
that are very thin, e.g. approximately 0.4 to 1.5 mils in thickness. In particular,
a preferred alloy is formed of 96% (weight percent) titanium with 3% aluminum
and 1% vanadium.
When such windows are contacted by the accelerated electrons, a significant
portion of the electron energy is imparted to the window resulting in heating thereof.
This heat must be removed from the thin foil window or the window will overheat
and melt resulting in catastrophic failure of the electron beam accelerator.
Currently such windows are cooled by one of two methods. Water cooled
support structures which are either ribbed or perforated cool the window by conduction.
Such conduction cooling, however, has not been totally satisfactory since the windows
have relatively short lives.
A more effective method of window cooling utilizes a high velocity
jet of air which passes over the exterior window surface. Windows cooled in this
manner have long service lives, but there is a problem associated with the production
of ozone when such air is irradiated by the electrons passing therethrough.
There is thus a significant need for improvement with regard to the
cooling of electron beam accelerator windows.
SUMMARY OF THE INVENTION
It is thus an object of this invention to provide a novel electron
beam apparatus window having improved cooling associated therewith.
It is further an object of this invention to provide such an improvement
which reduces or eliminates the air available for ionization during the cooling
of such windows.
These as well as other objects are accomplished by an improvement
to an electron beam accelerator in the form of heat pipes located on the window
structures of such a generator for the cooling of such window by such heat pipes.
The heat pipes eliminate the need for air cooling and thus reduce or eliminate
the production of ozone.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 of the drawings illustrates schematically the operation
of a heat pipe.
Figure 2 of the drawings illustrates schematically an electron beam
accelerator in association with an x-ray target or product to be irradiated.
Figure 3 of the drawings is an isometric view of an electron beam
accelerator window assembly in accordance with this invention,
In accordance with this invention, it has been found that the window
of an electron beam accelerator may be cooled by the placement thereon of heat
pipes for the conduction of heat away therefrom. It is most surprising in accordance
with this invention that heat pipes function without deterioration in the environment
of an electron beam accelerator. Various other advantages and features will become
apparent from a reading of the following description given with reference to the
various figures of drawing.
Heat pipes have been utilized in a variety of applications since
their inception. They have been most utilized in the art of injection molding but
also utilized in the nuclear reactor art. As used herein and shown schematically
in figure 1, a heat pipe is generally a hollow tube having a volatile substance
therein. The heat pipe is normally formed of thermally conductive material and
upon placement of the tube within a heat environment, the volatile liquid within
the heat pipe vaporizes and absorbs the heat energy from the portion of the heat
pipe subjected to the heat load. This causes the liquid at that point to boil and
enter a vapor state. When that happens, the liquid picks up the latent heat of
vaporization in passing from the liquid to vapor state. The gas which then has
a higher pressure moves toward the center of the sealed container and away from
the heat source to a cooler location where it is condensed by external means of
cooling. When the gas is condensed, it gives up the latent heat of vaporization.
The liquid medium is then returned toward the heat source through the heat pipe
The heat pipe thus functions to transfer heat from the input to output
end of the heat pipe with effective thermal conductivities many thousands of times
that of copper heat sinks. For example, a heat pipe is described in U.S. Patent
No. 4,601,331 which utilizes a heat pipe in the environment of a microwave tube
collector. Such patent is hereby incorporated by reference.
Figure 2 of the drawings illustrates an electron beam accelerator
and/or x-ray generator of the type applicable to the current invention. Referring
to figure 2 of the drawings, there is illustrated an electron beam accelerator
1 and an x-ray generator 3. The electron beam accelerator generally has a filament
5 from which electrons are emitted and passed through a high voltage divider network
7 through a plurality of anodes 9 to arrive in a scan coil assembly 11. In passing
through the scan coil assembly, the electrons are scanned through a scanning horn
13 to contact a window 15 such as that described above. An electron 17 is illustrated
as having passed through the window and headed toward a metal x-ray target 21 or
product to be irradiated.
After traversing through the window the high speed electrons interact
with the stream of air being used to cool the window resulting in ionization of
the air molecules and consequently the formation of ozone and NOx products.
Referring to figure 3 of the drawings, a window assembly 31 is illustrated
having a window mounting bracket 33, foil window 35 and a plurality of heat pipes
illustrated here as 37, 39 and 41. Heat pipes 37, 39 and 41 rest upon window 35
for the conducting of heat away therefrom. There is illustrated also as a part
of the window assembly 31, external cooling means 45 which are preferably water
cooling for the heat pipes 37, 39 and 41.
A preferred heat pipe for utilization with this invention is a heat
pipe from Noren Company of Menlow Park, California, which utilizes either mercury
or water as the volatile liquid. Surprisingly, both have been found to operate
in the environment of the electron accelerator.
One of the advantages associated with the use of heat pipes for cooling
an electron window is that the entire electron beam x-ray generator or product
irradiation chamber may be isolated in nitrogen and generally the entire environment
is isolated except for the cooling water. Therefore, there are no adverse affects
to the environment associated with ozone production. The generally inert surrounding
of nitrogen also provides for the likelihood of a longer life of all equipment
in that environment.
It is thus seen that this invention provides a significant improvement
in the cooling of electron beam accelerator windows. It further seen that this
improvement eliminates the production of ozone which has heretofore been associated
with electron beam accelerators. As many variations become apparent to those of
skill in the art from a reading of the foregoing description, such variations
are embodied within the spirit and scope of this invention as defined by the following