US2539370A - Electrostatic electron lens system - Google Patents

Description

Jan. 23, 1951 B. J. MAYO ELECTROSTATIC ELECTRON LENS SYSTEM Filed Dec. 20, 1949 l hllillr llnvllltln IVIIIIII l l l I ll [five/7751' BERNARD JOSEPH MAYO r P Patented Jan. 23, 1951 UNITED STATES PATENT OFlCE ELECTROSTATIC ELECTRON LENS SYSTEM Bernard Joseph Mayo, Hayes, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application December 20, 1949, Serial No. 134,005 In Great Britain December 10, 1948 4 Ciai'ms.

The invention relates to electrostatic electron lens systems for facuss'm' composite beams of electrons as distinct from lenses for rocussin fasciculated beams. I

Electrostatic electron lenses for focussing composite beams, such as an electron image of an optical object, onto a mosaic target'ele'ctro'de or fluorescent screen are known and have comprised two cylindrical electrodes to which different potentials are applied for setting up an electrostatic focussing held. The focussing field may serve to focus an electron image emitted from a photo-electric cathode, which is disposed adjacent to the electrode maintained at the lower potential, but it is found with these lenses that considerable curvature of the 'focus'sed electron image occurs. Thus, assuming that the photoelectric cathode is plane, it is of course desirable that a plane image be formed on the target electrode or fluorescent-screen. However, with the known form of lens referred to above, the electron ima'g'e, instead-"of being focussed ina plane, is in focus on-an-acutely curved surface which is concave facing therathode withthe result that, if it is desired to employ a plane target electrodeor fluorescent -screen, the image, whilst being in focus on the axis'of the system, is-nevertheless widely out of *focus at positions remote from the axis. In this known type of lens the radius of curvature "of the image with -a magniflcationfacto'r of -lL7 is found to be as much as0.13 of the "radius of the second cylindrical electrode of the lens.

The object-of the present invention-is to .provide an improved electrostatic-electron lens system which does not introduceas much image curvature as in the=known type of-lens referred to above.

tained at a potential which is less than (and preferably less than 0.4 of the'potentialapplied to an electrode of saidsecond lens-which is nearer to said source.

Lens systems in.-accordanc'e with the present invention can be designedtoproduce unity magnification or magnification greater than unity or demagnification.

In order that the said invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 represents diagrammatically the known form of lens referred to above, and,

Figure 2 is a similar view illustrating a lens according to one embodiment of the present in vention.

As shown in Figure 1 of the drawings'the lens comprises a air of tubular electrodes I and 2 which are of circular form'in cross-section, the electrode I being connected to a photoelectric cathode 3 which emits electrons when anoptical image is projected thereon, the lens formed by the electrodes I and '2 being intended to focus the electron image on to a plane surface 4 which may be a target electrode or a fluorescent screen. In operating the lens, the electrode I may be maintained at the potential of the cathode '3, which may be zero potential, and the electrode -2 is maintained at a'positive potential. The maintenance of the two electrodes I and 2 at the different potentials mentioned serves'to set up an electrostatic 'fOCuSSiIlg field, the equi p'otential surfaces of which are indicated by the full lines 5. The dotted lines indicated in Figure l illusstrate pencils of electrons emitted from the photo-electric cathode, and it will be observed from this figure that the electron pencils are rather wide and, furthermore the image points of the axial, zonal and marginal electron pencils indicated ate, I and-'8 respectively show that the curvature'in the image is considerable, so that, whilst the axial e'lectron pencils will'be in focus at the surface --4, the marginal electron pencils will be widely out of focus. In this form of lens the curvature of the image formed on the surface 4 is as high-as'0.l3 of'the radius of the electrode '2.

Figure 2 of the drawings illustrates a lens in accordance with the present invention and in this figure the photoelectric'cathode isvindicated by the reference numeral 9 and the plane surface'such as the target electrode or fluorescent screen on which the iinage is to be .focussed is indicated at 4. Adjacent to the cathode 9 is a series of tubular or ring-shaped electrodes I0, II and I2 followed by a longer tubular electrode I3 which together with the electrodes I0, II and I2 form a'flrst'electron lens. The electrode I3 is-follo'wed'by 'afurther tubular electrode I4 the electrodes I3'-an'd I4 forming a second electron lens. In'one specific example of a lens system according -'to the invention, the photo-electric cathode'll may have a diameter of 5 centimetres, the electrode Ii! which is connected to the cathode 9 a diameter of 5 centimetres and a length of 0.4 centimetre, the electrodes II and I2 a diameter of 5 centimetres and each a length of 0.5 centimetre, the electrode It a diameter of 5 centimetres and a length of 8 centimetres and the electrode I4 a diameter of 5 centimetres and a length of 17 centimetres. In operation the electrodes II], II and I2 may be maintained at the same potential as the photo-electric cathode 9 namely at zero potential, the electrode l3 at a positive potential of 5000 volts and the electrode Id at a positive potential which is less than 0.4 of the potential of the electrode I3. Preferably the potential of the electrode I4 is between 500 and 1000 volts, say at 600 volts. The equipotential surfaces set up by the first electron lens are indicated in full lines at 15 and the pencils of electrons emitted by the photo-electric cathode 9 by the dotted line Hi. The equi-potential surfaces set up between the electrodes l3 and I l are indicated in full lines 11. It will be observed from a comparison of Figures 1 and 2 that in the arrangement according to the invention, the pencils of electrons 16 are narrow 'owing to the relatively higher field which is produced at the cathode 9 with the result that all of the rays of the pencil traverse a similar field :and the depth of focus is small and the pencils are converged towards the axis of the system. "The electrons in said pencils are initially slightly (converged by the first electron lens and they then 'move in a nearly field-free region before the electrons in said pencils are converged and focussed by the second lens formed between the electrodes 13 and I l. Hence, by the time the electron pencils reach this last-mentioned lens they are near the axis of the system, so that this latter lens introduces little aberration. The result of the lens system according to the invention is that the curvature of the image formed on the surface 4 is less compared with the lens shown in Figure 1 as is evidenced from the image points of the marginal electron pencils indicated at H! in Figure 2. With a lens system according to the invention with the dimensions of electrodes and potentials as stated above the radius of curvature of the images was found to be 0.3 of the diameter oi t1-h7e electrode M with a magnification factor Although, a stated above, the electrodes l0, H and I2 may be maintained at the same potential as the photo-electric cathode 9, it may, in some cases, be advantageous to maintain the electrodes H and I2 at progressively higher potentials than that of the cathode 9. The size of the various electrodes of the system and the potentials applied thereto control the size of the image formed on the surface 4 and obviously the size of these electrodes and the potentials will be adjusted to produce a desired size of image.

In optics the well known Petzvals formula can be used to determine the curvature of an image and such formula can also be used in electronoptics. Such formula is:

1 N l i' i i (1) where C is the radius of curvature of the image surface at the axis, Nn' the refractive index of the image space, and N N are the indices on either side of the jth refracting surface of radius r For electron optics, since the refractive index is proportional to Vii, we have where V is the potential of the equipotential g. If the surfaces are so selected that the refractive index ivy/N,- is constant at each surface, Formula 1 is simplified to The sign convention for (1) and (2) is as follows:

C and n have the same sign if Vj+1 Vj and-opposite signs if V;i+1 V7'.

It is thus seen that by measuring the axial radius of curvature of each equipotential the image curvature on the axis is easily found, and hence by employing said formula the design of lens according to the invention is facilitated.

I claim:

1. A circuit arrangement comprising an electron discharge device having an electrostatic electron lens system for focussing a composite beam of electrons, a source of electrons for producing a composite electron beam, a plurality of electrodes, means for applying electric potentials to a portion of said electrodes to produce an electron lens for converging pencils of electrons from said source towards the axis of the system, means for applying electric potentials to a portion of said electrodes to produce a second electron lens for focussing said convergent pencils of electrons, said latter means for applying electric potentials being proportioned to maintain an electrode of said second lens which is more remote from said source at a lower potential than that of an electrode of said second lens nearer the source.

2. A circuit arrangement according to claim 1, said source being a plane photoelectric cathode.

3. A circuit arrangement comprising an electron discharge device having an electrostatic electron lens system for focussing a composite beam of electrons, a source of electrons for producing a composite electron beam, a plurality of electrodes, means for applying electric potentials to a portion of said electrodes to produce an electron lens for converging pencils of electrons from said source towards the axis of the system, means for applying electric potentials to a portion of said electrodes to produce a second electron lens for focussing said convergent pencils of electrons, said latter means for applying electric potentials being proportioned to maintain an electrode of said second lens which is more remote from said source at a potential less than 0.4 that of an electrode of said second lens nearer to said source.

i. A circuit arrangement according to claim 3, said source being a plane photoelectric cathode.

BERNARD JOSEPH MAYO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,131,185 Knoll Sept. 2'7, 1938 2,176,974: McGee et al. Oct. 24, 1939 2,185,378 Maloff et al. Jan. 2, 1940 2,189,321 Morton Feb. 6, 1940 2,219,117 Schade Oct. 22, 1940 2,264,709 Nicoll Dec. 2, 1941 2,320,582 Flechsig June 1, 1943

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