[0001] The present invention relates to the production of extractor grids for use in ion
sources, ion engines and the like.
[0002] The manufacture of electrodes for the extraction and acceleration of ion beams from
plasma ion sources presents a particularly difficult problem because of the high accuracy
required in the finished products if they are to achieve the desired ion extraction
efficiency, electrode lifetime and "optical" quality of the extracted ion beam. Because
of the high temperatures at which they operate, the only suitable materials for the
manufacture of the electrodes are refractory metals such as tungsten or molybdenbum.
[0003] The electrodes take the form of a sheet of refractory metal with an array of holes
formed in it. The holes have to be positioned very accurately relative to one another,
parallel sided and without burrs or deformations at either end. The usual method of
manufacturing such electrodes is by drilling the holes. However, this method of production
is slow and expensive because of the difficulty of drilling holes in the refractory
metals which are used to make the electrodes. Not only are drills blunted readily
by the hardness of the metals, but they tend to wander off line. Also molybdenum in
particular tends to suffer from brittle fracture or even, in rolled sheet form, to
delaminate, so causing ragged edges to the holes where the drill emerges from the
metal sheet. To some extent the problems of drill wander and ragged break-out can
be lessened by clamping the sheet of refractory metal between two sheets of a softer
metal such as mild steel, but this does not solve the problem of the blunting of the
drills and makes the production process even slower as a much greater total amount
of metal has to be drilled.
[0004] According to the present invention there is provided a method of manufacturing a
grid electrode comprising a lamina of refractory metal having an array of holes therein,
wherein the holes are formed by a punching operation.
[0005] The method of manufacture may also include the operations of forming the lamina into
a non-planar shape and/or heating the lamina to relieve stresses set up in the lamina
during earlier operations.
[0006] The process of the invention has a number of advantages compared with drilling and
other hole-producing techniques such as laser drilling, spark erosion or photochemical
etching. Not only does it produce accurately located holes of good quality with parallel
sides and little or no burrs on the break-out side, but it is possible to produce
holes of other than circular cross-section, such as hexagonal or square. Also, the
process is much quicker and cheaper than the other processes previously referred to.
Moreover, the hardness and brittleness of refractory metals and their tendency to
delaminate when in rolled sheet form, which create problems with other hole-producing
technqiues, present no such problems to the process of the present invention.
[0007] By way of example, the production by a method embodying the present invention of
a screen grid for use in an ion engine will now be described with reference to the
accompanying drawings in which:-
Figure 1 shows a lamina of molybdenum prepared for a first stage in the production
of the screen grid
Figure 2 illustrates schematically a second stage in the production process
Figure 3 illustrates another stage in the production process, and
Figure 4 shows the finished article.
[0008] Referring to the drawings, a disk 1 of molybdenum in diameter and 0.25 mm thick was
mounted on a rigid supporting backing piece 2. The assembly 3 was positioned on the
table 4 of a CNC vertical machine, illustrated generally by the outline 5. An hexagonal
array of holes 6 consisting of 9097 holes each of 1.75 mm diameter was formed by punching
through the molybdenum disk 1 and the backing piece 2. The molybdenum disk 1 and the
backing piece 2 were then separated and any minor burrs on the break-out edges of
the holes 6 were removed by water blasting.
[0009] As it was desired that the completed grid should have a dished shape, a stiffening
rim 21 was then formed as shown in Figure 2. After this had been done the grid was
pressed to its approximate final shape. The pressing was carried out progressively
at temperatures of up to 300°C with intervals for stress relieving. The partially
formed grids were then finished to shape and simultaneously stress relieved by stretch
forming against a die 31 at a temperature of 860°C for a period of ten hours.
1 A method of manufacturing a grid electrode comprising a lamina (1) of refractory
metal having an array of holes (6) therein, wherein the holes (6) are formed by a
punching operation.
2 A method according to claim 1 wherein there is included the operation of forming
the lamina (1) into a non-planar shape.
3 A method according to claim 1 or claim 2 wherein there is included a stress-relieving
operation subsequent to the formation of the holes (6) in the lamina (1).
