High fluence charged-particle beams are generated in a vacuum or near vacuum environment. To use these beams in an atmospheric pressure environment, they must pass through some form of transmission window between the two environments. To date, thin single metal foils have been used for these transmission windows. The total practical fluence of such transmitted beams is limited by the ability of the window to dissipate the excess heat deposited in it by the transiting beam. Existing windows have relied only on simple radial heat conduction through the thin foil, radiative cooling from the foil faces, and/or flowing cooling fluids on the high-pressure face of the foil. The present invention, however, proposes to enclose one or more channels within a double foil window and to flow a cooling fluid through such channel(s). The window cooling rate is thus significantly improved over air convection because of fully-developed turbulent flow and a higher cooling mass transport through such channels(s). Calculations show that a 2-3 order-of-magnitude increase in the time-averaged particle beam current density can be realized while maintaining the physical integrity of the foil window by using the so cooled foil window of the present invention.

FIELD OF THE INVENTION 
This invention relates to a cooled particle beam transmission window, 
particularly one that is cooled by fluid flow. 
BACKGROUND OF THE INVENTION 
High fluence particle beams must be generated under vacuum or near-vacuum 
conditions. That is, major mechanisms for generating, ampere-to-kiloampere 
level particle beams must be immersed in a vacuum environment, with 
background gas pressures of, e.g. 0.0001 Torr or less. To transmit the 
particle beam to the open atmosphere or into a gas background at 
atmospheric pressure, it is necessary to pass the beam through some form 
of "interface" or "window", which is both strong enough to withstand a 
14.7 pounds-per-square-inch pressure differential and thin enough to allow 
passage of the beam's particles with minimum degradation of particle 
energy and with minimum scattering. To-date this has been accomplished 
through the use of various thin foils of materials such as aluminum, 
titanium, beryllium, diamond, sapphire or a high tensile strength plastic 
such as polyester, e.g. "Mylar" or polyimide, e.g. "Kapton". Such window 
can typically sustain a steady-state electron beam current density of 10 
microamperes per cm.sup.2 at 150 keV before its ability to dissipate the 
heat influx is surpassed. Unfortunately, such low current densities limit 
practical applications. Electron-beam welding, for example, requires a 
minimum of 5 ma of continuous current. (For in-air welding, expensive 
pumping systems are used to transport the beam through a hole to the 
workpiece.). Thus, any attempt to pass a continuous beam of greater 
current density through existing foil windows results in a heating, 
softening, and rupturing of the foil window which destroys the vacuum 
environment necessary in the beam's generation region. Such failures can 
occur on microsecond timescales depending upon the specific foil material 
and energy deposition rate. 
Attempts have been made in the prior art to cool such transmission window 
by circulating coolant through conduits proximate such window, see for 
example U.S. Pat. No. 5,235,239 to Jacob et al (1993), e.g. FIGS. 2 and 
5A. In each case, coolant is circulated near a transmission window for 
indirect conductive cooling thereof through intervening structural members 
as shown, which limits the cooling effect thereof on such window. Also per 
FIG. 5A, the coolant system is located in a grid of support bars that 
block or cast shadows on the transmission window and absorb a significant 
portion of a particle beam passed therethrough. 
There is thus a need for a cooling system for such transmission window that 
overcomes the above prior art shortcomings. 
There has now been discovered a transmission window cooling system in which 
coolant directly cools the foils of such window, e.g. via convective heat 
transport with minimal absorption of the particle beam transmitted 
therethrough. At the same time per the invention, the coolant system is 
enclosed and pressurized to permit circulation of coolant therethrough for 
more effective cooling of such window. 
SUMMARY OF THE INVENTION 
Broadly the present invention provides a cooled transmission window for a 
particle beam generator which window comprises, 
a) two side-by-side metal foils which are joined together in portions 
thereof and spaced apart in other portions thereof, to define at least one 
cooling channel therebetween. Also provided are b) means for flowing 
coolant or cooling fluid under pressure through such channels for direct 
cooling of the foils and thus said window. 
Preferably the coolant is flowed through the above channels under pressure 
for enhanced cooling of such windows. 
Although the flow of coollant through the foil cooling channel can be 
laminar, it is preferred that such flow be at least slightly turbulant to 
more turbulant, for more effective cooling of the foils of the channel. 
Various embodiments of the double foil window of the invention are 
described below.

DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment of the transmission window of the invention 10 is shown in 
FIGS. 1 and 2, wherein two spaced foil members 12 and 14 (shown in part), 
are mounted in clamps 18 and 20, as shown in FIGS. 1 and 2. The 
transmission window 10 has a plurality of partitions such as partitions 19 
and 21, defining coolant passages 15 therebetween, as shown or indicated 
in FIGS. 2 and 1. The coolant passages 15 communicate with cooling ducts 
16 and 17 which in turn, communicate respectively with inlet duct 24 and 
outlet duct 26, as shown in FIG. 2 and in part in FIG. 1. 
Also as indicated in FIG. 1, the double foil transmission window 10 is 
mounted by clamps 18 and 20 to the housing 30 of a particle beam generator 
32 (not fully shown) as indicated in FIGS. 1 and 2. As shown in FIG. 1, 
the transmission window 10 is drawn toward the vacuum side of the housing 
30 under pressure of the air side of such window per FIG. 1. To prevent 
pinching or scoring of such window 10, adjacent corners of the housings 30 
have chamfered surfaces 34 and 36 as shown in FIG. 1. 
For the purpose of illustrating the relative advantages of this invention, 
consider a uniform electron beam flowing through a 2-cm diameter 
transmission window whose temperature is thereby elevated to 570 degK. As 
previously stated, one surface of the window is typically exposed to only 
vacuum while the other is exposed to air. For a conventional, single-foil 
window, natural air heat convection from one vertical surface will 
dissipate 0.98 W/cm.sup.2 and heat radiation from both its surfaces will 
be 2.56 W/cm.sup.2. This yields a net 3.54 W/cm.sup.2 of air 
convection/radiation cooling for a conventional window which does not have 
the pressurized cooling fluid system of this invention. 
For a window based on this invention, either a gas or liquid may be used as 
the cooling fluid. Helium, for example, has a convective heat transfer 
coefficient of 3 W/cm.sup.2 -degK for a 1-mil diameter channel with a mass 
flow rate of 332.times.10(-9)kg/s, which yields a net cooling of 180 
W/cm.sup.2. Consequently, helium cooling using this invention is 50 times 
greater than natural air convection and radiation. 
Liquid coolants, on the other had, are much more dense than gas and can 
transport significantly more heat per unit volume out of the transmission 
window. Water, for the same diameter tube, has a convective rate 78 
W/cm.sup.2 -degK for a mass flow rate of 21.times.10(-6)kg/s, which yields 
a net cooling of 2.2 kW/cm.sup.2 using this invention. That is 620 times 
greater than natural air convection and radiation. A high temperture oil 
could also be used as the cooling fluid although its thermal conductivity 
is only one-sixth that of water. Its higher temperature partially offsets 
this low conductivity and yields a net connective rate approximately half 
that of water. Oils offer the advantage of not corroding metal components 
as water would. 
Maximum cooling rates are offered by a liquid metal, such as lithium. Its 
convective rate of 548 W/cm.sup.2 at a mass flow rate of 6.8.times.10(-6) 
kg/s yields net cooling of 5 kW/cm.sup.2. However, liquid metals are 
difficult to work with. 
These examples indicate that liquids such as water, a high temperature oil, 
or a liquid metal, all yield cooling rates far superior to natural air 
convection and radiation. Optimization of the cooling channels, material 
properties, operating temperature, and pressure, predicts that cooling 
rates approximately three orders of magnitude above natural convection and 
radiation are possible using this invention compared to a conventional 
single-foil transmission window. 
The above fluid transport preferably occurs through the entire volume of 
the window which can be intersected by the particle beam. The window 
channels need not necessarily impose a uniform fluid flow rate across the 
entire window cross-section. For example, per FIG. 3 herein, it can be 
advantageous to arrange the channels 33 between the partitions 35, such 
that fluid flow rates peak in the window's central region. That is where 
most of the intercepted beam's energy is likely to be deposited if the 
beam density has a typical Gaussian radial profile. 
Numerous methods to achieve a fluid-cooled window configuration are 
possible. These can be arranged into four generic categories; 
barrier/channel sandwich, cut-and-capped channels, capped-ripple channels 
and simple-sandwich. Each of these four general methods for making the 
present invention are described below. 
The first fabrication method, termed "barrier/channel sandwich" method, is 
illustrated in FIG. 4, wherein foil window 38 has partitions or barrier 
strips 40, bonded between two foils, 42 and 44. Note that the spacing, d, 
between the strips 40, need not be uniform. Also, the barrier width, w, 
and the spacing between the foils, D, need not be uniform throughout the 
window. It can be advantageous to fabricate the channels so as to maximize 
the cooling fluid flow rate through the central region of the window where 
most of the particle beam is intercepted. It is also possible to use 
different thicknesses for each of the two foils, to result in an 
"asymmetric" fluid-cooled transmission window 45, as illustrated in FIG. 
5. 
Each of the other three window fabrication styles listed below can also 
have an asymmetric manifestation. Further, an alternate configuration of 
this window method can employ the bonding of each barrier strip to only 
one of the opposing foil faces. This can cut the fabrication costs while 
still giving adequate control of turbulant fluid flow between the foils. 
However, this single-sided bond approach reduces the mechanical support 
given to the unbonded foil face and thereby increases the chances for 
mechanical failure. 
In this and the other fabrication methods, the bonding techniques used for 
the partitions and/or channels must provide sufficient structural strength 
to withstand the hoop stress at the design pressure and temperature. This 
is achieved by increasing the surface area and utilizing a bonding 
technique to exceed the hoop stress specification at the design 
temperature. Appropriate techniques include brazing, resistance welding, 
rolling, soldering and plating. 
The second fabrication method, termed the "capped-cut-channel" method is 
illustrated in FIG. 6. In this method, two foils, 50 and 52, are again 
used. Appropriate microfabrication techniques, e.g. masking and chemical 
etching, are used to cut parallel channels 54 into one of the foils 52. 
The two foils are then bonded together fluid-tight in such a way as to cap 
the cut channel, resulting in closed fluid conduits in double foil window 
55. This method requires only single-sided bonding and optimizes 
mechanical support across the entire face of the window 55. As in the 
barrier-channel-sandwich method above, the channel and barrier widths and 
the resultant channel depths need not be uniform. 
The third fabrication method discussed herein is the "ripple-capped" 
method. It is illustrated in FIG. 7. Here, two foils 60 and 62 are also 
used. One of the foils 60 however, is now corrugated with parallel ripples 
64 running along one dimension. One method for achieving the desired 
corrugation is illustrated in FIG. 8. One of the foils is placed over an 
array of parallel fine wires 66, such as commonly used for radiofrequency 
polarization grids. The wire array, in turn, lies on a hard, flat surface 
68. Pressure is then applied to the exposed foil face using, for example, 
a malleable roller (not shown) to push the foil down into the spaces 
between the wires. The resultant corrugated foil is then bonded to the 
second, flat foil along the troughs of the corrugations per FIG. 7. What 
results, once again, are closed linear fluid flow channels. In this method 
also, techniques such as choosing variable wire diameters and spacing in 
the forming process can result in nonuniform channel width and spacing, if 
desired. Also the ripple-formation process described above can be combined 
with a simultaneous pressure-bonding process, to be accomplished in a 
single step. 
Finally, the "simple-sandwich" method for fabricating the fluid-cooled 
transmission window is illustrated in FIGS. 9 and 10. In this method two 
foils 70 and 72 are placed back-to-back with, e.g. a sub-mil of empty 
space between them. The two foils can be of different thicknesses. They 
are bonded at opposing ends to spacer-strips 73, of desired thickness. 
Alternatively, a single foil of double width can be folded in half, in 
which case only the open-side spacer-strip need be bonded securely enough 
to withstand the full fluid pressure. This arrangement effectively forms a 
single, highly elongated channel transmitting nearly the full volume of 
the resulting window. An inlet manifold 74 and an outlet manifold, 76, are 
bonded fluid-tight to the opposing open ends of the elongated channel as 
shown. A predetermined fluid input pressure is maintained by any 
appropriate means, such as a fluid pump (not shown) in the input manifold, 
to ensure adequate fluid flow rate through the window volume. However, 
such envelope is subject to stress due to the pressure differential 
between the pressurized internal cooling fluid and the external vacuum and 
atmospheric surroundings on opposite foil faces. Bulging of each foil face 
at the center can be expected. This bulging increases the net window 
thickness which increases heat absorption but decreases fluid velocity. 
This bulging can also grow to a failure condition, depending upon the 
working pressure parameters, foil thickness, operating temperatures and 
foil material. This hoop stress may well limit use of the simple-sandwich 
method to small diameter (e.g 1-2 mm) windows. 
The use of this invention is illustrated in FIG. 11. The channeled, 
fluid-cooled, double foil window 80 fabricated by any of the methods 
described above, is incorporated as an integral, vacuum-tight, part of the 
particle-beam source vacuum vessel 82. The window must be located 
downstream of the particle beam source 84, contained within that vessel. 
It must be placed in such a position so as to be transverse to the 
particle-beam axis and to completely encompass the useful cross-section of 
the particle-beam within its physical extent. The description up to this 
point describes the configuration of any particle-beam transmission 
window, including those available prior to this invention. 
Also novel in this invention is the addition of a fluid-cooling subsystem 
within the transmission window 80. The overall fluid-cooling system starts 
with the cooling fluid reservoir 86, which stores the working fluid. Fluid 
is drawn from the reservoir 86 by the fluid pump 88, and forced at some 
working pressure into a microfine particulate filter 90. The filtering is 
necessary to minimize chances of fluid-borne particles clogging one or 
more of the microchannels running through the window. After filtering, the 
fluid, continues into the inlet manifold of the channeled, fluid-cooled 
window 80. The fluid extracts excess heat from the window as it flows 
through its internal channels. The fluid carries that heat to a heat 
dissipation unit 92, where it transfers the heat to the environment and 
the working fluid returns to its lower input operating temperature. The 
heat dissipation unit can use a conventional liquid-air or liquid-liquid 
heat exchange device (such as a radiator) to transfer the excess fluid 
heat to either a background air flow or to a main water flow. When used 
with the above new fluid cooling system, the particle-beam transmission 
window will transmit a high fluence particle-beam into a beam-application 
area 94, at (or above) atmospheric pressure in a steady-state, continuous 
fashion, which is up to three orders-of-magnitude greater than 
conventionally cooled windows. Although the current density that can be 
transmitted in pulsed mode operation is not affected, the duty cycle will 
be significantly increased up to three orders-of-magnitude because natural 
cooling is augmented by efficient forced cooling. 
Thus the invention provides an improved transmission window for the 
practical extraction of high fluence charged-particle-beams from a vacuum 
or near-vacuum environment into an environment near, at, or above 
atmospheric pressure. These charged-particle beams can be either 
electron-beams or ion-beams, referred to herein as "particlebeams." This 
is important because available high-fluence particle-beam sources function 
best under vacuum or near-vacuum conditions. Therefore, in order to apply 
such beams to real-world uses in the open atmosphere, a practical method 
has been sought for their extraction from the vacuum system. Thus the 
present invention can permit superior electron-beam welding in air. It can 
permit fast feed-rate welding of large structures that cannot fit into the 
vacuum chamber of a traditional electron-beam welding system. In the 
medical field, this invention can permit electron-beam surgery and 
possible particle-beam cancer treatments. Toxic waste remediation concepts 
can benefit from the high beam currents which can significantly enhance 
their system processing rate. For the military, this invention can 
directly contribute, to potential directed-energy weapons concepts. 
An important novelty of the present invention is the direct contact cooling 
of the foil window by a coolant. That is, a cooling fluid is forced 
between the foils that define the double foil window of the invention, 
e.g. in FIG. 11 or through microchannels thereof, e.g. as shown in FIG. 7 
hereof. Such fluid flows at high velocity and high pressure along the foil 
and carries away the heat deposited into the foil by the beam. The 
physical processes exploited are fully-developed turbulence, convective 
heat transfer, and mass transport. As noted above, the internal 
forced-fluid cooling method of the invention promises the ability to 
handle up to 2-3 orders-of-magnitude greater particle beam current density 
as compared to conventional foil cooling techniques without internal 
cooling channels. 
Although the additional foil thickness for the transmission window of the 
present invention increases the energy deposited in the window by 
approximately 25%, over that of a one foil transmission window, it is 
expected that a 2-3 order-of-magnitude increase in the steady-state 
particle beam current density can be realized while maintaining the 
integrity of the foil window, using the cooled transmission windows of the 
present invention. This, for example, can permit the transmission of up to 
30 ma, which exceeds the 5 ma minimum continuous electron beam current 
necessary for practical electron beam welding applications. Hence, high 
feed-rate electron-beam welding in air is possible without expensive 
pumping systems. 
Thus an important advantage of the double foil transmission window of the 
present invention is its ability to transmit a high current density 
particle-beam from its vacuum (or near vacuum) generation source 
environment, out into an atmospheric (or greater) pressure environment. 
This advantage opens up new applications of intense particle beams in 
non-vacuum environments. 
Also the double foil transmission window of the invention, with a single 
flow path or a plurality of flow channels therein, permits direct 
convective fluid-cooling of the window material, prevents mechanical 
failure due to overheating and permits the increase of transmitted 
particle flux by 2-3 orders of magnitude as noted above.