A ribust industrial accelerator window design

~

Pergamon

Radiat. Ph)~. Chem. Vo]. 52, Nos 1 6. pp. 487 489. 1998

PII: S0969-806X(98)00054-1

~" 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0969-806X,98 si9.00 + 0.00

A ROBUST INDUSTRIAL A C C E L E R A T O R W I N D O W DESIGN Marlin N. Schuetz ~and David A. Vroom 2 ~Raychem Corporation, 201 Dickens Road, Fuquay-Farina, NC 27526, USA 2Raychem Corporation, 300 Constitution Drive, Menlo Park, CA 94025, USA

ABSTRACT An improved design for the thin metal foil window associated with high power industrial accelerators has been developed and tested. This design, which employs specifically shaped flanges, greatly reduce the stresses normally present on accelerators windows and has lead to longer window lifetime and a better means of window cooling.

KEY W O R D S Electron accelerator, accelerator window, curved accelerator window, accelerator window cooling, accelerator window water cooling,

INTRODUCTION In the operation of a high power industrial electron beam accelerator, the window through which the beam exits from high vacuum into air has often been an area of weakness. The weakness arises from a combination of the mechanical stresses on the thin metal foil used as the window material and thermal stress arising from deposition of energy in the foil and temperature cycling. In addition, in many industrial applications, a build-up of extraneous material can occur on the air side of the foil that increases the energy deposition in the window and hampers cooling. All of the above problems with conventional windows can lead to premature window failure. As part of an on-going program to improve the operation of Raychem's processing accelerators, we have designed and tested a new window structure that decreases both the mechanical and thermal stresses on the window foil leading to longer window life. In addition the new design tends to minimise, and in some cases eliminate, the build up of extraneous material on the window foil.

CONVENTIONAL W I N D O W DESIGN Conventional industrial accelerator windows are generally made by capturing a thin metal foil, usually made of a titanium alloy, between two flat stainless steel frames. One of these frames serves as the flange on the end of the vacuum system of the accelerator which in most cases is the end of the electron beam scanning device required to spread the electron beam over a large area. This scanning devise is needed to reduce the energy deposited per unit area by the passage of the electron beam through the window foil. Failure to spread the beam will, for high powered beams, result in rapid window failure. It is necessary to have a vacuum sealing means between the window foil and this vacuum chamber flange. The second fiat flume serves as a devise to clamp the window foil and vacuum sealing means to the flange on the end of the vacuum system. When in use, the metal foil window is subjected to a pressure differential from one side to the other of essentially one bar (atmospheric pressure on one side and high vacuum on the other). The pressure difference causes the foil to deform or "pillow" slightly toward the vacuum side creating stress in the window. Part of this deformation results from transverse stretching of the foil. The radius of curvature of the foil resulting from drawing the vacuum 487

488

Marlin N. Schuetz and David A. Vroom

is defined by the amount of transverse stress incurred. The relationship therebetween for a foil of indefinite length (that is neglecting end effects) is given by the following:

and

s~ = p(R/t) = transverse stress where p= differential pressure across the foil R = radius of curvature t = thickness of foil

Equation 1

s~ = s~/2 = axial stress

Equation 2

and the total stress S at any position on the window is given by S = q (sl 2 + s22 )

Equation 3

Non-uniform stresses also exists at the ends of the window where the metal is stretched in two dimensions. These mechanical stresses on the window foil coupled with the thermal stress induced in the window due to heating by the passage of electrons through the material followed by cooling when the electron beam is turned off leads to fatigue and eventual window failure.

I M P R O V E D W I N D O W DESIGN The improved window design developed by Raychem Corporation replaces the flat window frames traditionally used on accelerators with window frames that force the window foil into a semi-tubular shape by means of nested semi-circular ends on the frames. This concept is illustrated in Figure 1. From Equation 1 above it can be seen that the smaller the radius of curvature of the window the less the mechanical stress on the window. Further the use of curved end frames essentially eliminates the non-uniform mechanical stresses generated at the ends of conventional flat windows. The curved window described here, which has a radius of 32 mm, has less than 20 % of the hoop stress generated in a conventional flat window which generally has a radius of curvature induced by the differential pressure of greater than 150 mm. The reduction in mechanical stress in the window material allows thinner foils to be employed. A curved window having only half the thickness of the a flat window will have a greater mechanical safety factor than the conventional window and will absorb only half as much energy per unit area due to passage of the beam. The reduction in absorbed energy allows either more beam power to be extracted through the same window area or will allow the use of a smaller window for the same beam power. In some applications where a smaller window can be employed, use of the curved window allows the overall size of the accelerator system to be reduced thereby reducing the size of the facility and the cost for shielding. The curved window also lends itself to improved methods of window cooling. In conventional flat window systems, large volumes of high velocity air are blown across the outer surface of the window foil to remove the heat generated by passage of the electrons through the material. A significant amount of electrical energy is consumed in the generation of this air flow and in the industrial environment, the cooling air often entrains dust and other foreign materials that can be deposited on the window forming a layer that further increases the amount of heat absorbed by the window. Further the use of the large volumes of air necessary to cool the window precludes the use of an inert atmosphere to eliminate the formation of ozone by the electrons as they travel through air to the product being treated or into a beam stop. With the curved window we have successfully used air streams, atomised streams of water carried in nitrogen gas and continuous streams of water to cool the window. In the case of the atomised stream of water, it has been possible to fill the (small) irradiation zone with nitrogen thereby eliminating the formation of ozone. Further the use of filtered, de-ionised water in a stream of clean nitrogen essentially eliminates any build up of foreign material on the window foil. This is true even when treating filled polymeric materials which emit low molecular weight fragments during processing that can become entrained in the cooling air stream and thereby be deposited on the

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window. Use of the water particles has proven to be a more efficient means of cooling the window foil and has allowed even higher beam currents to be extracted through the curved window. It cases where the material to be processed is a liquid, we have demonstrated that this material can be used to simultaneously cool the window as it is being processed. Here the liquid is formed into a well defined stream, the thickness of which corresponds to about 75% of the range of the electrons in the liquid and is passed at high velocity over the window foil. No corrosion, erosion or material build up on the window foil was seen during extensive testing with the liquids cooling. The improved cooling of the foil so attained allows even higher beam current per unit area to be extracted from the system.

RESULTS Several curved windows have been constructed and tested in our facilities. These windows have had radii ranging from 6 to 38 mm and have employed air, atomised water and liquid water streams for cooling. In all cases, the beam currents per unit area that were attained exceeded those that could be used with conventional fiat windows having the same foil thickness. In the case of water stream cooling, the increase in beam current per unit area is approximately a factor of three. To form the vacuum seal between the curved window flange attached to the end of the vacuum system and the metal foil, both polymeric and aluminium O-rings have successfully been employed. For the water cooled windows, the lifetime of the aluminium seals was found to be less than anticipated due to corrosion of the metal by the water under the conditions encountered in the radiation zone. Polymeric seals were found to have a long lifetime under the conditions present.

REFERENCES The work described above is covered under U. S. patents 5,051,600, 5,416,440 and 5,530,255 and other pending applications.