We claim:
1. A molecular beam source comprising:
a porous substrate having a constant diameter central cavity with at least one opening to the exterior of the substrate;
working material nearly saturating said substrate such that a thin liquid layer of said working material covers the surface of the central cavity; and
means for maintaining the temperature of said substrate slightly above the melting point of said working material.
2. The molecular beam source of claim 1, wherein said central cavity comprises an axial bore with two ends, the first end being closed and the second end being open.
3. The molecular beam source of claim 1, wherein the temperature means is disposed such that the temperature of said substrate away from said at least one opening is maintained at a temperature higher than the temperature of said substrate near said at least one opening.
4. The molecular beam source of claim 1, wherein said substrate is thicker away from said at least one opening than near said at least one opening.
5. The molecular beam source of claim 1, wherein the pores of said porous substrate are larger away from said at least one opening than near said at least one opening.
6. The molecular beam source of claim 1, wherein said substrate is selected from the group consisting of tungsten, molybdenum, stainless steel, nickel, copper and the alumina silicates.
7. The molecular beam source of claim 1, wherein said working material is cesium, said substrate is sintered tungsten, and said temperature means is disposed such that the temperature of said substrate near said single opening is maintained at about 30.degree. C. and the temperature of said substrate away from said single opening is maintained between about 80.degree. and 120.degree. C.
8. The molecular beam source of claim 2, wherein the axial length of said bore is substantially greater than its diameter.
9. A molecular beam source comprising:
a porous wick with a non-porous external casing, said wick having a constant diameter axial bore extending from a first end of said wick to a second end of said wick;
an opening at said second end opening said bore to the exterior of said wick and casing;
source material nearly saturating said wick;
a heat source for maintaining the temperature of said source material above its melting point.
10. The molecular beam source of claim 9, wherein said heat source is so disposed as to maintain the temperature of said second end just above said melting point, while maintaining the temperature of said first end further above said melting point.
11. The molecular beam source of claim 9, wherein the external diameter of said wick tapers from said first end to said second end.
12. The molecular beam source of claim 9, wherein the pores of said wick are larger at said first end than at said second end.
13. The molecular beam source of claim 9, wherein said source material is a metal, alkali metal, organic compound or water.
14. The molecular beam source of claim 9, wherein said wick is selected from the group consisting of sintered tungsten, sintered molybdenum, sintered stainless steel and cloth gauze.
15. The process of generating a molecular beam, comprising:
providing a porous substrate having a constant diameter central axial bore, said bore being closed at one end and open at the other with the exterior of said substrate in a non-porous casing;
nearly saturating said substrate with the material of which the beam is to be comprised;
maintaining the temperature of said substrate and said material slightly above the melting point of said material.
16. The process of claim 15, further comprising:
raising the temperature of said substrate and said material at said closed end further above said melting point, while maintaining the temperature at said open end slightly above said melting point.
17. The process of claim 15, wherein said porous substrate is provided by
drilling said axial bore into a block of the material of which said substrate is comprised; and
etching the surface of the bore to re-open the pores therein.
18. The process of claim 17, further comprising:
shaping the outside of said block such that it tapers from said closed end to said open end.