US3912930A

US3912930A - Electron beam focusing system

Info

Publication number: US3912930A
Application number: US400962A
Authority: US
Inventors: John Michael Creedon; Sidney Darwin Putnam
Original assignee: Physics International Co
Current assignee: Physics International Co
Priority date: 1973-09-26
Filing date: 1973-09-26
Publication date: 1975-10-14
Anticipated expiration: 1992-10-14

Abstract

An apparatus for focusing electrons incorporates a cathode and an anode in which electrons are caused to flow along equipotential lines or surfaces that converge toward a common point at or near the anode with the end of the cathode facing the anode having a cavity and with a wire or conductor connected to the anode or an independent electron beam injected along the axis to establish a conducting path to the anode through the focal point of the electron flow pattern.

Description

United States Patent [191 Creedon et a1.

[ ELECTRON BEAM FOCUSING SYSTEM [75] Inventors: John Michael Creedon; Sidney Darwin Putnam, both of Berkeley,

Calif.

[73] Assignee: Physics International Company, San

Leandro, Calif.

[22] Filed: Sept. 26, 1973 [21] Appl. No.: 400,962

[51] Int. Cl. HOIJ 3/14 [58] Field of Search 250/251, 396, 397, 398

[56] References Cited UNITED STATES PATENTS 2,533,790 12/1950 Grivet 250/396 2,665,384 1/1954 Yockey.... 250/396 2,759,117 8/1956 Hasbrouk 250/396 2,771,568 1 1/1956 Steigerwald 250/396 3,209,147 9/1965 Dupouy et a1. 250/396 [451 Oct. 14, 1975 3,532,879 10/ 1970 Braunstein et al 250/251 3,558,877 1/1971 Pressman 3,659,097 4/1972 Bassett et a1..

3,731,094 5/1973 Le Poole 3,778,612 12/1973 Ashkin 250/251 Primary Examiner.lames W. Lawrence Assistant ExaminerB. C. Anderson Attorney, Agent, or FirmRobert R. Tipton [57] ABSTRACT An apparatus for focusing electrons incorporates a cathode and an anode in which electrons are caused to flow along equipotential lines or surfaces that converge toward a common point at or near the anode with the end of the cathode facing the anode having a cavity and with a wire or conductor connected to the anode or an independent electron beam injected along the axis to establish a conducting path to the anode through the focal point of the electron flow pattern.

7 Claims, 3 Drawing Figures U08 Patent Oct. 14, 1975 Sheet 2 of2 3,912,930

ELECTRON BEAM FOCUSING SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to electrical discharge devices and in particular to devices that focus electrons travelling between an anode and a cathode.

Generally, the devices of the prior art currently in use to focus electrons, that is, to cause electrons to converge toward a common point or focus, all employ a cathode for emitting electrons and an anode to which is applied a voltage for accelerating the electrons from the cathode to the anode and use either or both magnetic or electrostatic fields disposed between the anode and the cathode to deflect the electrons and cause them to converge to a common point, for example, the devices used to focus electrons in a cathode ray tube or electron microscope.

Such devices are limited in their ability to reach the high beam currents that are required in high energy physics devices.

SUMMARY OF THE INVENTION In the concept of the present invention, at least one cathode is arranged in a spaced apart relationship to an anode and raised to an electrical potential with its end shaped to help create an electromagnetic field in which the equipotential lines or surfaces of the electric field all converge toward a common point. A narrow path to the anode is used to establish an additional magnetic field for concentrating, funneling or focusing the flow of electrons toward this common point on the anode.

It is, therefore, an object of the present invention to provide an apparatus for focusing electrons.

It is another object of the present invention to provide an apparatus for creating an electromagnetic field in which the equipotential surfaces of the electric field converge toward a common point.

It is a further object of the present invention to provide an apparatus for creating an electromagnetic field in which electrons emitted by the cathode flow generally along the equipotential surfaces of the electric field toward the common point on the anode.

It is still another object of the present invention to provide an apparatus in which the flow of electrons from the cathode to the anode is, in part, parapotential.

It is still a further object of the present invention to provide an apparatus for focusing electrons using internal electric and magnetic fields.

These and other objects of the present invention will be manifest upon study of the following detailed description when taken together with the drawings.

BRIEF DESCRIPTION OF THE DILAWINGS FIG. 1 is an elevational sectional view of the basic apparatus of the present invention showing cathode shape, equipotential lines or surfaces and typical electron trajectories.

FIG. 2 is an elevational sectional view of a multiple cathode configuration of the present invention showing cathode shape, equipotential lines or surfaces and typical electron trajectories.

FIG. 3 is an elevational sectional view of the apparatus of the present invention utilizing an electron beam as the source of current element flowing along the axis used to establish a magnetic field in addition to the magnetic field of the electrons emitted from the cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic configuration of the apparatus of the present invention is shown in FIG. 1 and comprises an anode 10, a generally cylindrical cathode ll spaced apart from anode l0 and having an axis of rotation 12 perpendicular to anode 10, a power supply 14 for creating a potential difference between cathode l1 and anode l0 anda fine wire or conductor 15 connected to anode l0 coincident with axis 12 and a power supply 16 for creating a potential difference between the end of wire 15 and anode 10. The cathode-anode portion of the apparatus is housed in a vacuum tight housing (not shown) common in the art, which can be sufficiently evacuated to permit electrons to flow between the cathode and the anode.

In particular detail, cathode 11 comprises a generally cylindrical body portion 17 in which the axis of rotation 12 of its outer surface 18, as previously noted, is perpendicular to anode 10 with the end of cathode 11 proximate anode 10 shaped in a particular manner to facilitate the maintenance of equipotential surfaces and the flow of electrons to a common point or focus 20.

In this respect, the end of cathode l1 proximate anode 10 further comprises a hole or cavity 22 whose axis of rotation is coincident with axis 12 and with the remainder of cathode 11 shaped to define a frustoconical surface 24 having its axis of rotation also coincident with axis 12 and its imaginary apex coincident with focus or common point 20. Although FIG. 1 shows the end of cathode l l shaped to define a frusto-conical surface, other shapes are possible.

Wire or conductor 15 is arranged coincident with axis 12 and connected at one end to power supply 16 and at the other end, is connected to anode 10 through focus or common point 20.

Because of the relatively high currents flowing in the apparatus of the present invention, the normal mode of operation of the apparatus is by a pulse of energy.

In operation, as a potential difference is applied to anode l0 and cathode l1 and wire 15, electrons are initially caused to flow in conductor 15 as indicated by arrow 25, creating an initial circular magnetic field 26 around wire 15 in which the magnetic lines of force 26a above conductor 15 enter the plane of the drawing away from the viewer, as indicated by a circle containing an X, while the magnetic line of force 26b below conductor 15 leaves the plane of the drawing toward the viewer, as indicated by a circle containing a dot.

As the flow of electrons through conductor 15 increases to a point beyond its current carrying capacity, if the conductor is a wire, it will melt and vaporize to create a narrow conducting path or plasma 28 along which a current 27 will flow.

As the potential difference between anode 10 and cathode 11 is increased, microscopic protrusions on

surfaces

18 and 24 of cathode ll begin emitting current due to field emission. The current which is emitted heats and vaporizes the microscopic protrusions and the resulting plasma covers

surfaces

18 and 24 of cathode l 1. Electron emission from this plasma on

surfaces

18 and 24 will begin.

This flow of electrons produces a space charge cloud in the anode cathode region that depresses the electric field to zero at the cathode and limits the flow of current between the cathode and the anode.

Theoretical considerations indicate that the shape of the frusto-conical surface 24 is one of the configurations that, in association with the current flow, can createequipotential surfaces between cathode 11 and anode which converge conically toward a common point or focus coincident with the intersection of the axis of symmetry 12 and the anode 10.

For very high potentials, that is, at voltages sufficient to accelerate the electrons emitted from the cathode l 1 to relativistic velocities, other effects occur which indicate that the ratio of current flow to anode potential does not follow classic theory.

At the potentials and cathode configurations used in the apparatus of the present invention, the potential difference between the anode and the cathode or cathodes, the current flowing between the cathode and the anode and the charges on the electrons themselves, all combine to create an electromagnetic field in which forces due to the electric component of the electromagnetic field and the magnetic component of the electromagnetic field are balance in such a manner that the guiding centers or trajectories of the electrons converge generally toward a common point. In other words, there is created an electron flow pattern that is parapotential, that is, the electron trajectories are approximately along the equipotential lines or surfaces of the electric field as shown by typical electron trajectories 25.

It is believed that, in theory, as the electrons approach axis 12 near anode 10, without the effect of the magnetic field created by the current along plasma 28, there is a transition from parapotential flow to flow across the equipotential surfaces. Some of the electrons reach anode 10 before reaching focus or common point 20 because the electric and self-magnetic fields change with the resulting forces being very strong in the general direction of the anode.

With the introduction of wire 15 to create a plasma or conducting path 28 and additional magnetic field 26, the electrons flowing parapotentially from cathode 11 are prevented from being drawn prematurely toward anode 10 by the overriding force of the additional magnetic, field 26.

Thus, it can be seen, that elecrons flowing from outer surface l8iand surface 24 are caused to flow toward anode l0 and to be concentrated and focused at common point 20.

An alternate method of operation would be to connect power supply 16 from cathode 11 to the end of wire 15.

The typical parameters of the apparatus of FIG. 1 are shown in Table 1.

With reference to FIG. 2, a second embodiment of the present invention is illustrated and comprises the same elements as shown in FIG. 1, namely, an anode 10, a first cathode spaced apart from anode 10 and having its axis of rotation 12 perpendicular to anode l0, and further comprising a second generally cylindrical cathode 29 spaced apart from anode l0 and first cathode 30 with its axis of rotation coincident with axis of rotation 12, and third generally cylindrical cathode 11 spaced apart from anode l0 and second cathode 29 with its axis of rotation also coincident with axis 12.

To raise each cathode to its proper electrical potential, first power supply 32 is connected between anode 10 and first cathode 30 to establish an electrical potential V( 1) between anode l0 and cathode 30. A second power supply 33 is connected between first cathode 30 and second cathode 29 to establish an electrical potential V(2) between first cathode 30 and second cathode 29, while a third power supply 34 is connected between second cathode 29 and third cathode 11 to establish an electrical potential V(3) between second cathode 29 and third cathode 11. A fourth power supply 31 is connected between anode l0 and the end of wire 15.

It can be seen that the potential difference between second cathode 29 and anode 10 is the sum of the voltages V(l) plus V(2)=V( 1+2), while the potential difference between third cathode l1 and anode 10 is the sum of the voltages V( l plus V(2) plus V(3) -'V( l+2+3). Since the spacing between the anode and any cathode must be sufficient so that a short circuit does not occur, it may be necessary that the spacing between third cathode 11 and the anode 10 be greater than the spacing of second cathode 29 from anode 10, and the spacing of second anode 29 from anode 10 be greater than the spacing of first cathode 30 from anode 10. It will also be noted that the cathode spacing in the drawing is shown generally larger than in practice in order to be able to illustrate the equipotential lines and electron trajectories.

Also, as previously noted for FIG. 1, the cathode and anode assembly of FIG. 2 is enclosed in a vacuum tight housing (not shown) common in the art, which can be suffiently evacuated to permit electrons to flow between the cathode and the anode.

Again, with reference to FIG. 2, the end of each cathodeproximate anode 10 is shaped in the particular manner shown to facilitate the maintenance of equipotential lines or surfaces and the flow of electrons in the direction of common point or focus 20, but it must be noted that the frusto-conical shapes of the cathodes proximate anode 10 are only one of the possible shapes which could be used to cause the electron flow to converge toward the common point or focus 20. r

In particular, third cathode 11 comprises the identical elements as illustrated in FIG. 1 and, for that reason, the same element numbers are used to identify each part.

As in FIG. 1, the end of cathode 11 proximate anode 10 is shaped to define a frusto-conical surface 24 having its axis of rotation coincident with axis 12 and its imaginary apex coincident with focus or common point 20.

Again, referring to FIG. 2, second cathode 29 comprises a generally cylindrical body portion 36 whose axis of rotation is coincident with axis 12 and whose inside surface 37 is spaced apart from outer surface 18 of third cathode 11.

The end of cathode 29 proximate anode 10 is also shaped to define a frusto-conical surface 38 having its axis of rotation coincident with axis 12 and its imaginaryapex coincident with focus or common point 20.

Similarly, first cathode 30 comprises a generally cylindrical body portion 39 whose axis of rotation, as noted above, is coincident with axis 12 and whose inside surface 40 is spaced apart from outer surface 35 of second cathode 29.

The end of first cathode 30 proximate anode is also shaped to define a frusto-conical surface 42 having its axis of rotation coincident with focus or common point 20.

Although FIG. 2 shows the ends of

cathodes

30, 29, and 11 shaped to define frusto-conical surfaces, other shapes are possible.

It will be noted that anode 10 is adapted to enclose and is spaced apart from first cathode outer surface 41.

In operation, the voltages V( l V( 1+2) and V( l+2+3) are selected to create the equipotential lines or surfaces 43, shown dashed in FIGS. 1 and 2, and individually identified by suffix letters-in lower case beginning with line 43a nearest anode 10.

Thus, the spacing of the ends of

cathode

11, 29 and 30 from anode 10 as well as voltages V( 1 V( 1+2) and V( 1+2+3) applied, respectively, to the three cathodes, is critical to achieve equipotential lines or surfaces 43 which, in fact, define concentric cones having their axes of rotation coincident with axis 12 and apexes coincident with focus or common point 20. In FIG. 2, equipotential line 43b is equal to a voltage of V( 1 the potential of first cathode 30 relative to anode 10, while equipotential line 43a is equal to onehalf of V( l Similarly, equipotential line 43d is equal to a voltage of V( 1+2), the potential of second cathode 29 relative to anode 10, while equipotential line 430 is equal to a voltage of V( 1) plus one-half of the voltage V(2).

In a like manner, equipotential line 43f is equal to a voltage V( 1+2+3), the potential of third cathode 11 relative to anode 10 while equipotential line 43a is equal to a voltage V(l+2) plus one-half the voltage V(3).

Thus, it can also be seen that the voltage difference between outer surface 35 of second cathode 29 and inner surface 40 of first cathode 30 is V(2), while the voltage difference between outer surface 18 of third cathode 11 and inner surface 37 of second cathode 29 is V(3).

As described for the configuration of FIG. 1, in FIG. 2, when the voltages V( l V(1+2) and V( l+2+3) are negative with respect to anode 10 and sufficiently large, a field emission initiated plasma forms on the cathode surfaces and the cathodes become copious emitters of electrons, and electrons will be caused to flow from

cathodes

11, 29, and 30 to anode 10.

After this plasma has formed, the current density emitted from the cathodes is determined by the space charge limit. The current density is limited everywhere to the value for which the space charge in the anodecathode region reduces the electric field to zero at the cathode.

For very high potentials, that is, at voltages suflicient to accelerate the electrons to relativistic velocities, other effects occur which indicate that the ratio of current flow to anode potential does not follow classic theory.

In a manner similar to FIG. 1, as the voltage V(4) increases, the current through wire increases to a point where the wire melts and becomes vaporized to now define a conducting path of charged particles or conducting plasma 28 for a beam of electrons 27 creating a funnel or path through the space charge.

Similar to FIG. 1, in FIG. 2, the current flowing along path 28, as indicated by arrow 25, will create a circular magnetic field 26 around wire 15 or path 28 in which the magnetic lines of force 26a above conducting path 28 enter the plane of the drawing away from the viewer, as indicated by a circle containing an X, while the magnetic lines of force 26b below conducting path 27 leave the plane of the drawing toward the viewer, as indicated by a circle containing a dot.

With reference to FIG. 2, it can be seen that the elecrons are emitted primarily from

outer surfaces

18, 35 and 41 of cathodes ll, 29 and 30, and from the

frustoconical surfaces

42, 38 and 24 of these cathodes, respectively, because of accelerating voltages V(3), V(2) and V( l respectively. In view of this, these surfaces could be treated to be better emitters by the use of special coatings.

In the construction and operation of the apparatus of FIG. 2, the magnitude of voltages V( l V(2) and V(3) between cathodes is important because they function to control the magnitude of electron current from each cathode.

Each cathode must emit a certain proportion of the total current flowing to anode 10 in order to achieve optimum focusing.

The amount of current flowing from each cathode is controlled by the potential difference between the electrodes and the geometrical configurations of the cathodes. For the embodiment shown in FIG. 2, the most important parameters of the cathode configuration are the inner radius and the spacing from the anode of each cathode. If the cathode surfaces 43, 38 and 24 are not frusto-conically shaped, then the most important parameter for each cathode is the ratio of the radius to distance from the anode of the area on the cathode surface where this ratio is a maximum.

In the nested diode configuration of FIG. 2 the total current flowing from second cathode 29, third cathode 11, and in wire 15 or plasma 28, creates an additional magnetic field which, in conjunction with the selfmagnetic field of the electrons flowing from first cathode 30, causes a more nearly perfect balance between the forces due to the electric and magnetic fields, acting on the electrons from first cathode 30, so that these electrons are more tightly focused around common point 20. The current flowing from third cathode 11 and the current in wire 15 or plasma 28 provide an additional magnetic field which, in conjunction with the self-magnetic field of electrons flowing from second cathode 29, causes the electrons from second cathode 29 to be more tightly focused around common point 20.

The current flowing in wire or conductor 15 or plasma 28 provides an additional magnetic field which in conjunction with the self-magnetic field of the electrons emitted from cathode 11, causes the electrons from cathode 11 to be more tightly focused around common point 20.

The equipotential surface 43d of FIG. 1 acts as an intermediate anode for cathode 11, and the current flowing from cathode 11 is determined by the relative configuration of cathode 11 and the equipotential surface 43d as well as the potential difference V(3).

Equipotential surface 43b acts as an intemiediate anode for cathode 29, and the current flowing from cathode 29 is determined by the relative configuration of equipotential surface 43b and cathode 29 as well as the potential difference V(2).

The equipotential surface along anode 10 acts as the anode for cathode 30, and the current from cathode 30 is determined by the relative configuration of anode l and cathode 30 as well as the potential difference V( l For optimum focusing, theoretical considerations indicate that certain fixed relations are required between the currents from

cathodes

30, 29, 11 and in wire 15 or plasma 28.

For the following definitions:

1(30) current from cathode 30 1(29) current from cathode 29 K1 1) current from cathode l l l( 15) current in wire 15 or plasma 28 optimum focusing should occur when and V( l V(2) and V(3) are in megavolts.

The typical parameters of the apparatus of FIG. 2 are shown in Table 2.

TABLE 2 Anode 10 potential O Cathode 30 potential V( l 2 megavolts Cathode 29 potential V( l+2) 4 megavolts Cathode l l potential V( l+2+3) -6 megavolts End of wire l5 potential V(4) =-6 megavolts Cathode to Anode Spacing:

Cathode 30-Anode l0 0.8 cm

Cathode 29-Anode I0 1.6 cm

Cathode ll-Anode 10 2.4 cm Cathode to Cathode Spacing:

Cathode l l-Cathode 29 3.l 1 cm Cathode 29-Cathode 30 6.80 cm Cathode Inner Radius:

Cathode 30 16.8 cm

Cathode 29 5.61 cm Cathode ll 1.22 cm Cathode Current:

Cathode 30 I593 Kiloam Cathode 29 324 Kiloamp.

Cathode 11 66 Kiloamp.

Wire 15 17 Kiloamp.

Total Current 2000 Kiloamp.

Where a conducting wire has been used in the apparatus of FIGS. 1 and 2, in FIG. 3 a beam of electrons is used to illustrate one of several other techniques for creating a current along this axis and thus an additional magnetic field for control of the trajectory of the electrons emitted by the cathodes.

The apparatus of FIG. 3 comprises the same basic elements as in FIG. 1, namely, an anode 10, a generally cylindrical cathode 45 spaced apart from anode 10 and having an axis of rotation 12 perpendicular to anode l0, and with electron accelerator 46 disposed with its axis coincident with axis 12.

Cathode 45 of FIG. 3 comprises a generally cylindrical body portion 48 in which the axis of rotation of its outer surface 49 is coincident with axis 12 and with the end of cathode 45 shaped to define a frusto-conical surface 50.

Although FIG. 3 shows the end of cathode 45 shaped to define a frusto-conical surface, other shapes are possible.

With the exception of the use of an electron beam to establish the current on the axis which generates the additional magnetic field, the operation of the apparatus of FIG. 3 is, in all respects, identical to that of FIG. 1.

Another method of creating a current flow along the axis would be to use a jet of gas emanating from the anode or the cathode.

We claim:

1. An apparatus for focusing electrons comprising means defining an evacuated housing,

an anode,

a first generally cylindrical cathode spaced apart from said anode, said cathode having means defining a hole therein, the axis of said hole being coincident with the axis of rotation of said cathode,

means for producing a first accelerating voltage to accelerate electrons emitted from said first cathode to relativistic velocities from said first cathode to said anode,

means for controlling the magnitude of electron current flow from said first cathode to said anode whereby the electric component of the electromagnetic field of said electon flow from said first cathode and the self-magnetic component of the electromagnetic field of said electron current flow from said first cathode are each of a magnitude and direction to cause said electrons to converge toward a common point generally along equipotential surfaces created thereby, and

means for creating a central electromagnetic field and current of electrons along a narrow path between said first cathode and said anode to control the point of convergence of said emitted electrons, said path being coincident with the axis of rotation of said cylindrical cathode and said hole.

2. The apparatus as claimed in claim 1 wherein the end of said first cathode proximate said anode defines a frustrum of a cone, with the axis of rotation of said cone being coincident with the axis of rotation of said first cathode, and

wherein said means for controlling the magnitude of electron current flow comprises said first cylindrical cathode having a radius and a spacing from said anode for an electron current flow of a magnitude at which electron flow is generally along equipotential surfaces.

3. The apparatus as claimed in claim 2 further comprising at least a second cathode spaced apart from said anode and disposed concentric within and spaced apart from said first cathode, said second cathode defining a frustrum of a cone with an axis of rotati n perpendicular to said anode and having means defining a hole therein, the axis of said hole being coincident with the axis of rotation of said second cathode and said first cathode, said second cathode claim 1 wherein having a radius and a spacing from said anode persaid means for creating a central electromagnetic mining an electron Current flow of a magnitude at field and current of electrons along a narrow path which electron flow is generally along equipotencomprises a beam f electrons. Surfaces but less the magmtude of e166 5 6. The apparatus for focusing electrons as claimed in tron current flow from said first cathode, and claim 1 wherein means for producing a second acceleratlng voltage to S d m 8 ans for creating a central electromagnetic accelerate electrons emitted from said second cathode to relativistic velocities from said second cathode to said anode. 10

4. The apparatus for focusing electrons as claimed in claim 1 wherein Claim 1 wherein said means for creating a central electromagnetic Said means for creating a central electromagnetic field and r e t of el t along a narrow h field and current of electrons along a narrow path comprises an electrically conductive wire. comprises a conductive gas.

5. The apparatus for focusing electrons as claimed in field and current of electrons along a narrow path comprises a plasma of ionized particles. 7. The apparatus for focusing electrons as claimed in

Claims

1. An apparatus for focusing electrons comprising means defining an evacuated housing, an anode, a first generally cylindrical cathode spaced apart from said anode, said cathode having means defining a hole therein, the axis of said hole being coincident with the axis of rotation of said cathode, means for producing a first accelerating voltage to accelerate electrons emitted from said first cathode to relativistic velocities from said first cathode to said anode, means for controlling the magnitude of electron current flow from said first cathode to said anode whereby the electric component of the electromagnetic field of said electon flow from said first cathode and the self-magnetic component of the electromagnetic field of said electron current flow from said first cathode are each of a magnitude and direction to cause said electrons to converge toward a common point generally along equipotential surfaces created thereby, and means for creating a central electromagnetic field and current of electrons along a narrow path between said first cathode and said anode to control the point of convergence of said emitted electrons, said path being coincident with the axis of rotation of said cylindrical cathode and said hole.

2. The apparatus as claimed in claim 1 wherein the end of said first cathode proximate said anode defines a frustrum of a cone, with the axis of rotation of said cone being coincident with the axis of rotation of said first cathode, and wherein said means for controlling the magnitude of electron current flow comprises said first cylindrical cathode having a radius and a spacing from said anode for an electron current flow of a magnitude at which electron flow is generally along equipotential surfaces.

3. The apparatus as claimed in claim 2 further comprising at least a second cathode spaced apart from said anode and disposed concentric within and spaced apart from said first cathode, said second cathode defining a frustrum of a cone with an axis of rotation perpendicular to said anode and having means defining a hole therein, the axis of said hole being coincident with the axis of rotation of said second cathode and said first cathode, said second cathode having a radius and a spacing from said anode permitting an electron current flow of a magnitude at which electron flow is generally along equipotential surfaces, but less than the magnitude of electron current flow from said first cathode, and means for producing a second accelerating voltage to accelerate electrons emitted from said second cathode to relativistic velocities from said second cathode to said anode.

4. The apparatus for focusing electrons as claimed in claim 1 wherein said means for creating a central electromagnetic field and current of electrons along a narrow path comprises an electrically conductive wire.

5. The apparatus for focusing electrons as claimed in claim 1 wherein said means for creating a central electromagnetic field and current of electrons along a narrow path comprises a beam of electrons.

6. The apparatus for focusing electrons as claimed in claim 1 wherein said means for creating a central electromagnetic field and current of electrons along a narrow path comprises a plasma of ionized particles.

7. The apparatus for focusing electrons as claimed in claim 1 wherein said means for creating a central electromagnetic field and current of electrons along a narrow path comprises a conductive gas.