Apparatus Operating With Contact Ionization for the Production of a Beam of Accelerated Ions

The invention relates to apparatus for the production of a beam of accelerated ions by contact ionization of appropriate particles, such as atoms, on a heated surface of an ionization electrode and for acceleration of the ions produced on the contact ionization surface, acceleration taking place in an acceleration zone between the ionization electrode and an acceleration electrode.

Apparatus of this kind, which may briefly be referred to as "thermal surface ion sources" or "contact ionization sources" is described, for example, in the book by R. G. Wilson and G. R. Brewer "Ion Beams", New York 1973, more particularly pages 26 to 36 and pages 72 to 77. Such apparatus is based on the effect that when neutral atoms impinge upon a surface hot enough for the atoms not to be adsorbed on the surface some of the atoms are ionized on leaving the surface. Saha-Langmuir's law applies to the degree of ionization R, i.e. the ratio of the ions to the total number of particles leaving the surface, the degree of ionization R.sub.+ for positive ions being:

and the degree of ionization R.sub.31 for negative ions being:

where

n.sub.+ =number of positive ions leaving the surface

n.sub.- =number of negative ions leaving the surface

n.sub.0 number of neutral atoms leaving the surface

W=electron work function of the surface

I=ionization potential of atoms

E=electron affinity of atoms

T=surface temperature

k=Boltzmann's constant

K.sub.+ and K.sub.- =statistical factors for positive and negative ions respectively (for alkaline metals K.sub.+ =2, for halogens K.sub.+ =4).

If W-I>0.4 eV and E-W>0.4 eV, then R.sub.+ and R.sub.- have almost the same value 1, i.e. practically all the atoms impinging on the surface vaporize as positive and negative ions. For example, cesium vapour (I=3.88 eV) on impinging on a hot (1300K) tungsten surface (W=4.54 eV) is practically completely positively ionized while on the other hand, for example, iodine vapour (E=3.23 eV) impinging on a hot lanthanum hexaboride surface (W=2.70 eV) is practically completely negatively ionized. Similar high degrees of ionization can be obtained for the other alkali metals and halogens and for a number of other atoms.

It is known either to pass the particles for ionization in vapour form from the front on to the hot surface of the ionization electrode of suitable material or diffuse it from the rear through a hot frit of the appropriate material to the then porous surface. The resulting ions are then sucked away from the surface by an electric field produced between the surface and an acceleration electrode disposed at a distance in front of the same. The attainable current densities J are limited in the above cases by Child's space charge law which, for a planar arrangement is as follows:

where

V=acceleration voltage

M=mass number

d=distance between ionizing surface and the acceleration or extraction electrode.

In an apparatus known from DE-PS No. 28 05 273 C3 for the production of a beam of accelerated ions by contact ionization, the space charge limitation of the emission current density is shifted to much higher values than in the case of planar electrodes by giving the ionizing surface a highly convex curvature so that a very high electrical field strength prevails thereon. The emission current density is then limited by the vapour pressure of the element for ionization prevailing between the electrodes. This vapour pressure must be sufficiently low so that no electrical breakdown occurs due to impact ionization in the vapour.

The embodiment to be described below gets round this limitation by the fact that the vapour for ionization is not passed into the space between the acceleration electrode and the highly convex hot ionizing surface of the ionization electrode; instead the latter has a duct preferably in the form of a capillary, such as a bore, and the atoms for ionization are passed in gas or vapour form from the rear from a reservoir, oven or any other suitable source through this duct which, in comparison with the pores of a frit, is macroscopic. On the way through the heated bore or capillary the atoms are ionized on the inner wall and on emerging from the duct exit are immediately engaged and accelerated by the strong electrical field prevailing at the tip of the ionization electrode. The vapour pressure prevailing in the acceleration zone is governed by the small proportion of non-ionized vapour from the capillary and atoms which had impacted the acceleration electrode in the form of ions and are re-sputtered by the subsequent ions. Given the same emission current density as with the above known apparatus of this kind the vapour pressure is much lower or, alternatively, given an appropriate increase in the vapour throughout it is possible to achieve a much higher emission current density than in the known case before any electrical breakdown occurs.

The invention is explained in detail hereinafter with reference to the drawing wherein:

FIG. 1 is a detail and

FIG. 2 is a simplified sectional view of a preferred embodiment of the present invention for the production of a beam of positive cesium ions.

The apparatus shown in the drawing for the production of a beam of accelerated cesium ions comprises an ionization electrode 1 in the form of a thin elongate tube, made from a material having a high electron work function, e.g. tantalum, and an acceleration electrode 3 in the form of an annular disc disposed at a distance d in front of the front end of the ionization electrode 1. The front end of the ionization electrode 1 tapers in the direction of the acceleration electrode 3 and forms an approximately domeshaped tip 2 having a capillary bore, the tip diameter being small in comparison with the distance d from the acceleration electrode 3. The inside diameter of the tubular ionization electrode 1 tapers stepwise towards the tip, as will be seen from FIG. 1.

The front end of the tubular ionization electrode 1 is surrounded by an annular thermionic cathode 6, which is in turn surrounded by a sheet-metal cylinder 7 which acts as a heat shield. That end of the tubular ionization electrode 1 which is remote from the acceleration electrode 3 leads into a reservoir 8 which is partially filled with cesium metal 9 and has a heating jacket 10 by means of which the reservoir 8 can be heated so that the cesium vapour pressure can be brought to a required value.

Typical dimensions are:

Tip diameter=0.4 mm

Capillary diameter=0.1 mm

Distance d=5 mm

In operation the front end of the ionization electrode 1 is heated to the required temperature of about 1300K by electron impact. For this purpose, electrons are emitted from the filament of the thermionic cathode 6, said filament forming an annular surround for the capillary, and are accelerated radially towards the ionization electrode by some kilovolts. Cesium vapour 4 is fed from the heated reservoir 8 through the tubular ionization electrode 1 and in these conditions the cesium vapour is ionized by impacts on the hot inner wall of the ionization electrode and, in particular, the front capillary part. There is a voltage of some kilovolts between the ionization electrode 1 and the acceleration electrode 3, the polarity of this voltage being such that the Cs.sup.+ ions emerging from the capillary are accelerated towards the acceleration electrode 3. A Cs.sup.+ ion beam 5 of some keV energy then emerges through a central opening in the acceleration electrode 3.

The ion beam can be focused by an electrostatic lens 11 (FIG. 2), the front electrode of which is formed by the acceleration electrode 3. The complete system is disposed in a vacuum container 12 shown only partially in FIG. 2. For the sake of clarity FIG. 2 omits the electrode mountings and electrical bushings, which can be of conventional construction.

The contact ionization surface can also be formed by a layer of a suitable material covering at least part of the inner wall of the tubular duct, more particularly the capillary part on the exit side.

The invention can also be used for the production of ions of elements other than cesium, more particularly of other alkali metals. Beams of negative ions, more particularly halogen ions, can also be produced if the ionization electrode or at least the inner wall of the capillary is made from a material of high electron affinity, e.g. lanthanum hexaboride. Of course other heating devices, and other devices delivering the atoms for ionization, can also be used.