Electron beam exit window

Details Electron beam exit window Electron beam exit window

1995-02-17

1996-10-01

O'Shea, Sandra L.

Electric lamp and discharge devices

Cathode ray tube

Electron permeable window

3133591, H01J 3300

Patent

active

055613420

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to an electron beam exit window through which an electron beam generated in an evacuated electron gun passes out into an area of higher pressure, preferably to atmospheric pressure. Such beam exit windows, also known as Lenard windows, are mainly used in electron beam installations in which an electron beam process, such as, e.g. an electron beam polymerization, is performed in an area under atmospheric pressure. The electron beam can be generated in the form of an axial beam which can be moved by scanners over the beam exit window and can be passed through the beam exit window in the form of a ribbon-like or laminar-generated electron beam.
Numerous differently designed apparatuses are known for passing out electron beams to the free atmosphere. The simplest known constructions comprise a thin, gas-impermeable foil, which separates the beam generating chamber from the free atmosphere in vacuum-tight manner. These foils are preferably made from aluminum, titanium or beryllium alloys. During the passage of the electron beam, the foil is heated due to unavoidable interaction between the electron beam and the foil material. The foils must withstand the pressure difference, but must not be so thick that on the one hand they limit the energy losses of the electron beam to be passed out and on the other the power dissipation to be removed from the foil. This is so that the foil heating remains within a temperature range acceptable for the foil material (U.S. Pat. No. 3,222,558). In the simplest case, a gas flow is used for heat removal purposes.
It is also known to successively arrange in a spaced manner in the beam direction a number of thin foils in such a way that individual zones, sealed against the beam generating chamber and the atmosphere, are formed. Through the zones, a cooling gas is passed in such a way that, between the beam generating chamber and the atmosphere, the pressure difference is divided up over the individual zones, in that the average static pressure increases from one zone to the next. The sum of the thicknesses of the individual foils corresponds at least to the thickness of one foil of a beam exit window having only a single foil (East German Patent document 102,511; U.S. Pat. No. 3,162,749). As the minimum possible foil thickness is limited by the manufacturing capability for vacuum-tight foils, and the absorption of the individual foils is summated, the absorption losses are very high. This is true particularly when working with a relatively low accelerating voltage. There is the further disadvantage that the necessary, considerable curvature of the foils, particularly in the window edge area, leads to higher absorption rates due to the inclined incidence of the beam.
In other known designs, use is made of mechanical supporting structures for limiting tensile stresses in the foil. The recesses in these supporting structures are arranged close together and in part are conically directed towards the vacuum side so that the webs supporting the foil are tapered between the recesses on the vacuum side (East German Patent document 207,521, German Patent document DE-OS 18 00 663). Thus, the electrons striking the surfaces of the supporting structure are reflected without a complete energy loss and, subsequently, at least partly pass out of the window. However, even a supporting structure designed in this manner suffers from the shortcoming that the reduction of the effective electron passage surface and, therefore, the additional power loss of the electron beam due to the supporting structure, can be 30% and higher. There is also the further disadvantage that the thermal loading of the supporting structure is very high and, consequently, high demands are made on the thermal conduction and heat dissipation. Frequently, use is made of supporting structures through which flows cooling water, but these structures require larger supporting lamellas. However, due to the resulting shadows that are cast, a disadvantageous effect can occur on the ho

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IEEE Trans. on Plasma Science entitled "Performance Improvements With Adved Design Foils in High-Current Electron Beam Diodes" by R. Shurter et al., Bd. 19, Nr. 5, Oct. 1991, pp. 846-849.
Nucl. Instrum. and Meth. in Phys. Research entitled "Long-Lif Carbon-Fiber-Supported Carbon Stripper Foils" by M. J. Borden et al., Bd. A303, 1991, pp. 63-68.

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