Patent ID: 12255038

FIG.1shows a cathode mount assembly1, which is comprised of an electron gun cathode mount (cathode mount)2of pyrolytic graphite which secures thermionic cathode3(e.g. lanthanum hexaboride button). Since pyrolytic graphite is anisotropic with respect to thermal conductivity, it should be understood that the planar layers are oriented so as to minimise thermal conduction along the length of the cylindrical structure of the electron gun cathode mount2, i.e. the planar layers are oriented transverse to, preferably perpendicular to, the length of the electron gun cathode mount2. The cathode mount assembly1, and thus electron gun cathode mount2, is mounted into the electron gun body (shown inFIG.2) via attachment member (locating disc)4, which is typically made of stainless steel, the locating disc also functioning as a precise centering/locating means for positioning the electron gun cathode mount2and thermionic cathode3. The locating disc4is rebated (not shown), and the electron gun cathode mount2has a flange (not shown) which mates with the rebate in the locating disc4to form-fit together. The locating disc4and electron gun cathode mount2when fitted together are secured using additional attachment means (a clamping plate5, and tightened clamping screws6). The locating disc4has venting holes7, which ensure no trapped gases when the cathode mount assembly1is subject to vacuum conditions during set up and operation of the electron gun assembly into which it is installed. The heat source (e.g. laser) (not shown) for the thermionic cathode3is directed down the bore of electron gun cathode mount2as shown by arrow8, to impact the back of the thermionic cathode3, which is thereby heated and emits a beam of electrons9. The electron gun cathode mount2in this figure is shown as having a cylindrical structure, which typically has a wall that is as thin as possible to minimise heat transfer, although other cathode holder shape forms are possible, depending upon the specific requirements of the cathode and electron gun.

InFIG.2, the cathode mount assembly1is mounted central to an additional electrode (the gun cathode)10in the electron gun body11. The gun cathode10is held at the same potential as the thermionic cathode3of the cathode mount assembly1(seeFIG.1) for a diode gun, and aids in shaping, controlling and projecting the electron beam. The electron gun body11provides the means for mounting the cathode mount assembly1, and thus the electron gun cathode mount2ofFIG.1, within the greater structure of an electron gun assembly (electron beam processing apparatus).

FIG.3shows an electron gun cathode mount (cathode holder)12having a tubular structure (a conical structure) formed of pyrolytic graphite, with the thermionic cathode3(lanthanum hexaboride) held at the apex. The electron gun cathode mount12is fabricated by machining from a block of pyrolytic graphite. Since pyrolytic graphite is anisotropic with respect to thermal conductivity, it should be understood that the planar graphite layers are oriented so as to minimise thermal conduction along the length of the electron gun cathode mount12, i.e. from the apex of the cone (where the thermionic cathode3is held) to the base (where the cathode holder12contacts and is operable to connect to the attachment member (not shown) via the laterally outwardly extending flange portion). InFIG.3, the planar layers are oriented transverse to, preferably perpendicular to, the central axis of the conical structure.

FIG.4shows a half-section through the electron gun cathode mount (cathode holder)12ofFIG.3, where the cathode holder12holds the thermionic cathode3at its apex. Line13represents the line of symmetry and central axis of the conical structure. As discussed with respect toFIG.3, the planar graphite layers are oriented parallel to the x-z plane.

FIG.5shows the results of a thermal modelling simulation on the electron gun cathode mount (cathode holder)12half section shown inFIG.4(axisymmetric model). In the simulation, the thermionic cathode3is heated by a laser or by an electron beam (not shown) on the back surface to a temperature of 1727° C. (2000·15K) to emit electrons. Steady-state thermal analysis was carried out to determine the heat input required to achieve the 1727° C. temperature in the thermionic cathode3. A heat flux of 20950 mW/mm2was required and, as can be seen when comparing the temperature key13(temperatures in ° C.) to the shades overlaid on the half-section, whilst the temperature at the thermionic cathode3is roughly 2000° C. (2273·15K), the temperature at the base15(laterally outwardly extending flange portion) of the cathode holder12is of the order of hundreds of ° C. The shading indicates that there is good thermal distribution along the z-x axis, but poor thermal distribution along the z-y axis, meaning less thermal energy is conducted along the cathode holder12to where it contacts the attachment member at the base15, which, given that the rest of the cathode holder12is held in space (i.e. a vacuum during operation of the equipment) then thermal bridging is minimised, meaning less thermal flux is required to be applied by the heat source to maintain the thermionic cathode3at the temperature required for the emittance of electrons therefrom.

FIGS.6aand6bshow electron gun cathode mounts (cathode holders) according to the first aspect of the present invention in cylindrical and conical shaped cross-sections, i.e. having cylindrical (6a) and conical (6b) structures, in which a preferential alignment of the planar layers16of pyrolytic graphite to control thermal properties can be seen (scale exaggerated for clarity). AlthoughFIG.6bshows the planar layers16running perpendicular to the central axis of the conical structure, the planar layers16could in some circumstances be oriented parallel to the central axis of the conical structure (or at selected other angles), dependent upon cone angle/shape. Essentially, the planar layers16should be orientated so as to minimise thermal conduction along the electron gun cathode mount. None of the planar layers should extend along the length of the electron gun cathode mount. There should therefore be no direct planar path for heat transfer along the length of the electron gun cathode mount, i.e. there should be no direct planar path for heat transfer along any of the planar layers of the pyrolytic graphite from a thermionic cathode held at the apex of the conical structure (not shown) to the attachment member (not shown). It is noted that the conical structure is advantageous as it generally means that no direct thermal path along the planar layers is presented from the thermionic cathode to the attachment member (locating member) and its interface with the rest of the electron gun assembly.

FIG.7shows an electron gun cathode mount (cathode holder)17according to the first aspect of the invention, fabricated using additive manufacturing (also known as ‘3D printing’) of precursor material followed by pyrolysis. The electron gun cathode mount17has a lattice structure formed of pyrolytic graphite and further comprises an outer layer or sleeve18that encloses the lattice structure (scale/positioning exaggerated for clarity). The sleeve18may either be fabricated from pyrolytic graphite (using additive manufacturing or more traditional construction methods) or an alternative material (e.g. ultra-thin tantalum). A laterally outwardly extending flange portion19for mating to an attachment member (not shown) is also shown, as is the thermionic cathode3.