WO2020084261A1

WO2020084261A1 - Double wehnelt-electrode electron source for selective additive manufacturing apparatus

Info

Publication number: WO2020084261A1
Application number: PCT/FR2019/052537
Authority: WO
Inventors: Gilbert Durand; Bruno Robin; Gilles WALRAND; Etienne BLANCHET; Cédric Carlavan; Vincent SOUBIES; Camille DELORME
Original assignee: Addup; Leptons-Technologies
Priority date: 2018-10-23
Filing date: 2019-10-23
Publication date: 2020-04-30
Also published as: FR3087577B1; FR3087577A1

Abstract

The invention relates to a source of electron beams which is suitable for selective additive manufacturing, comprising a cathode, the cathode comprising an emission region suitable for emitting electrons, a main Wehnelt electrode, an anode and a set of power supply units suitable for applying an electric voltage to these three parts, the main Wehnelt electrode being positioned between the cathode and the anode, said source comprising an additional Wehnelt electrode and a power supply unit suitable for applying an electric voltage to said additional Wehnelt electrode, said additional Wehnelt electrode being disposed upstream of the main Wehnelt electrode in the direction of propagation of the electron beam in such a way that a first control surface of the additional Wehnelt electrode surrounds an upstream portion of the emission region in the direction of propagation of the electron beam, and a second control surface of the main Wehnelt electrode surrounds a downstream portion of the emission region in the direction of propagation of the electron beam.

Description

DUAL WEHNELT ELECTRON SOURCE FOR SELECTIVE ADDITIVE MANUFACTURING APPARATUS

GENERAL TECHNICAL AREA AND PRIOR ART

The present invention relates to the general field of selective additive manufacturing.

Selective additive manufacturing consists of making three-dimensional objects by consolidating selected areas on successive layers of powdery material (metallic powder, ceramic powder, etc.). The consolidated areas correspond to successive sections of the three-dimensional object. Consolidation is done for example layer by layer, by a total or partial selective fusion carried out with a power source.

Conventionally, high power laser sources or electron beam sources are used as the source for melting the powder layers.

Electron beam sources have the advantage of allowing high manufacturing speeds.

It is known to use an electron beam source composed of a cathode, a wehnelt and an anode.

The electrons are extracted from the cathode and channeled towards the anode using in particular a positive electrical potential difference between the cathode and the anode. The electron beam produced passes through the wehnelt. The potential difference applied between the cathode and the wehnelt is negative, so that the electrons are repelled by the wehnelt. This makes it possible in particular to control the current of the extracted electron beam, as well as the brightness of the electron beam, that is to say the distribution of the electrons in a plane transverse to the direction of propagation of the beam.

There are geometries based on flat or hemispherical cathode, generally used for welding applications.

The diameter of the electron beam in a plane transverse to its direction of propagation and in particular in the plane of powders, that is to say say in the transverse plane in which the electron beam comes into contact with a metallic powder and melts it, is high and increases strongly with the quantity of electrons extracted.

For example, this diameter can be between 150 μm and 250 μm for a current of 20 mA, and increase when the intensity of the current increases with a slope of approximately 30 μm / 10 mA.

This type of geometry cannot simultaneously produce electron beams of an intensity as high as 100 mA while ensuring an electron beam diameter of less than 150 µm for lower current intensities.

There are also geometries based on pointed cathode, generally used for electron microscopy applications. In this case, the diameter of the electron beam at low current can be less than 150 pm but these cathodes are not able to supply currents greater than or equal to 100mA.

To solve the problem of obtaining a diameter of the electron beam less than or equal to 100 μm at low current while being able to supply at least one current of 100 mA, a cathode geometry as represented in FIG. 1 can be envisaged. This type of cathode can operate according to two modes of extraction of electrons.

The first mode of operation known as “cross over” is represented in FIG. 1. In this embodiment, the cathode 1 emits an electron beam 4 which passes through the wehnelt 2 and the anode 3. All of the parts of Figure 1 have an axial symmetry around the axis D shown in dotted lines. The wehnelt 2 and the anode 3 are open and leave the axis D free for the tip of the cathode 1 or the electron beam 4 to pass.

The “cross over” mode corresponds to the situation where the electrons cross between the cathode and the anode, in zone 5 of FIG. 1.

Line 8 is one of the electric equipotential lines shown in FIG. 1. All the points of an electric equipotential line are at the same electric potential. The strength of the electric field exerted on an electric charge is perpendicular to the equipotential line which passes through the position of said charge.

The other operating mode is said to be without "cross over". In this mode, the trajectories of the electrons do not cross.

This type of geometry, however, shows instability of the beam diameter when switching from modes with and without "cross over". This instability is illustrated in FIG. 2. The transition current corresponds to the current of the electron beam which separates the two modes. For weaker currents the source works with "cross over". The diameter of the beam has a peak for this transition current. When the intensity of the beam is swept increasing around the transition current, the diameter of the beam increases suddenly from 40 pm to 50 pm then decreases suddenly from 50 pm to 10 pm.

Furthermore, whatever the geometry chosen, the characteristics of the beam produced are deteriorated during the time of use without it being possible to correct them. This is due to the deterioration of the cathode which must then be changed. The lifespan of a cathode is between 600 and 800 hours.

Ideally, additive manufacturing would require an electron beam source including:

- The smallest diameter of the electron beam that can be produced in the powder plane, that is to say in the manufacturing plane, is 100 μm for a beam intensity current range of between 1 mA at 50mA; such a small diameter makes it possible to manufacture with greater precision.

- the beam intensity current can reach 100 mA to carry out the preheating and sintering operations;

- The diameter of the electron beam in a plane transverse to its direction of propagation and in particular in the powder plane varies little and always in the same direction depending on the intensity of the beam. - The diameter of the electron beam in the powder plane remains stable or constant as long as possible despite the wear of the cathode.

As indicated above, the electron sources known in the state of the art do not make it possible to fully meet these requirements.

OVERVIEW OF THE INVENTION

A general object of the invention is to overcome the limitations of the prior art.

In particular, it is sought to propose an electron beam source which has a diameter of the electron beam in a plane transverse to its direction of propagation with a size of lOOpm over a range of beam intensity current between 1 mA at 50 mA.

It is also sought to propose an electron beam source whose beam intensity current can reach at least 100 mA for carrying out the preheating and sintering operations.

Another object also of the invention is to propose an electron beam source whose size of the point of impact varies very little as a function of the intensity of the beam.

Another object is to obtain an electron beam source whose size of the point of impact remains stable or constant as long as possible despite the wear of the cathode.

To this end in particular, the invention provides an electron beam source suitable for selective additive manufacturing comprising a cathode, the cathode comprising an emissive zone suitable for emitting electrons, a main wehnelt, an anode and a set of power supplies. electric adapted to put these three parts under electric tension, the main wehnelt being placed between the cathode and the anode, said source comprising an additional wehnelt and a power supply adapted to put said additional wehnelt under electric tension, said additional wehnelt being placed in upstream of the wehnelt main with respect to the direction of propagation of the electron beam so that a first additional wehnelt control surface surrounds an upstream part of the emissive zone with respect to the direction of propagation of the electron beam and that a second surface of main wehnelt control surrounds a downstream part of the emissive zone in relation to the direction of propagation of the electron beam.

Such a source is advantageously supplemented by the following different characteristics taken alone or in combination:

- the set of electrical power supplies adapted to energize the cathode, the main wehnelt and the anode and the electrical power supply adapted to energize the additional wehnelt, are configured so that the source delivers beams of intensity between 1 mA and 50 mA, preferably between 5 mA and 50 mA, even more preferably between 10 mA and 50 mA, and whose diameters in a plane transverse to the direction of propagation of the electron beam, adapted to be a plane where additive manufacturing powder is consolidated, are substantially constant from one intensity to another;

- the distance separating the main wehnelt and the additional wehnelt according to the direction of propagation of the electron beam,

the thickness of said wehnelts in said direction of propagation, the position of said wehnelts in said direction of propagation relative to the cathode and

the surface and the shape of said wehnelts facing the cathode are adapted so that the source, supplied under certain electrical voltages, delivers beams of intensity of 100 mA;

the source extends along an axis of the source, the axis of the source being an axis of symmetry of the cathode, of the main wehnelt, of the anode and additional wehnelt, the main wehnelt, the additional wehnelt and the anode leaving said source axis free, the first control surface and the second control surface extend around the source axis in a circular fashion and at the same distance from the direction of propagation;

the surface and the shape of said wehnelts facing the cathode are adapted so that in case of deterioration of the cathode, the effects of said deterioration on the diameter, intensity and brightness of the electron beam are corrected by a modification the power supply of said source;

- The cathode is a pointed cathode and in that the tip of the cathode is directed towards the anode.

- the edges of an opening of the additional wehnelt defining the first control surface are folded towards the main wehnelt.

- the set of power supplies adapted to energize the cathode, the main wehnelt and the anode and the power supply adapted to energize the additional wehnelt are configured so that the potential difference with respect to the cathode:

o of the main wehnelt is between 0 and -3800 volts o of the additional wehnelt is between 0 and -3800 volts o the anode is between 0 and -60,000 volts

The invention also relates to an apparatus for selective additive manufacturing of three-dimensional object comprising in an enclosure:

- a support for depositing successive layers of additive manufacturing powder,

a distribution arrangement suitable for applying a layer of powder on said support or on a previously consolidated layer,

- at least one electron beam source adapted to ensure the selective consolidation of a layer of powder applied by the distribution arrangement,

- electronics for controlling said source, said source being an electron beam source as described above in this section.

The invention also relates to a method of selective additive manufacturing of a three-dimensional object comprising the steps of applying a layer of powder of additive manufacturing on a support or onto a previously consolidated layer, of selective consolidation of the layer of powder applied. , the method being implemented by an apparatus as described above in this section.

PRESENTATION OF THE FIGURES

Other characteristics and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and should be read with reference to the appended figures in which:

- Figure 1, already discussed, is a schematic representation of an electron beam source according to the prior art; - Figure 2, already discussed, shows the evolution of the diameter of the electron beam produced by a source according to the prior art as a function of its intensity;

- Figure 3 is a schematic representation of an example of an electron beam source according to one aspect of the invention;

- Figure 4 is a schematic representation of another example of an electron beam source according to one aspect of the invention;

- Figure 5 shows the evolution of the diameter of the electron beam produced by an example of an electron beam source according to one aspect of the invention as a function of its intensity;

FIG. 6 represents the changes in the diameter of the electron beam as a function of its intensity, the beam being produced on the one hand by an example of an electron beam source according to the prior art and on the other hand by a another example of an electron beam source according to one aspect of the invention;

- Figure 7 is a schematic representation of an additive manufacturing apparatus comprising an electron beam source according to a possible embodiment of the invention.

DESCRIPTION OF ONE OR MORE MODES OF IMPLEMENTATION AND IMPLEMENTATION

Electron beam source

FIG. 3 shows an example of an electron beam source. As in FIG. 1, a cathode 11 emits an electron beam 14 which passes through a main wehnelt 12 and the anode 13. The cathode 11, the main wehnelt 12 and the anode 13 have an axial symmetry around the axis D of the source shown in dotted lines. The main wehnelt 12 and the anode 13 are open and leave the axis D free for the passage of the tip of the cathode 11 or of the electron beam 14. The main wehnelt 12 comprises a second control surface 121 which is defined by an opening of the main wehnelt 12. The opening of the main wehnelt allows free passage of the tip of the cathode 11. The second control surface 121 extends around the axis D of the source in a circular manner.

The electron beam 14 has an axial symmetry around the axis of propagation coincident with the axis D of the source.

The cathode 11 comprises an emissive zone 110, the electrons of the electron beam are emitted from the emissive zone 110 towards the outside of the cathode. The emitting area 110 can be divided into an upstream part 111, a downstream part 112 and a central part 113.

The terms upstream and downstream are used with reference to the direction of propagation of the electrons within the electron beam 14, along the axis D of the source from the cathode 11 towards the anode 14.

The upstream part 111 of the cathode 11 is located farther from the anode 14 than the central part 113 of the cathode 11, itself further away from the anode 14 relative to the downstream part 112 of the cathode 11.

The main wehnelt 12 is positioned so as to surround the downstream part 112 of the emissive zone 110 of the cathode 11. This means that, relative to the axis D of the source, the axial position of the second control surface 121 is the same as the axial position of the downstream part 112.

The source comprises a set 15 of power supplies adapted to energize the main wehnelt 12, the cathode 11 and the anode 13.

An additional wehnelt 16 is located upstream of the main wehnelt 12 relative to the direction of propagation of the electron beam 14.

The additional wehnelt 16 has an axial symmetry about the axis D of the source. The additional wehnelt 16 is open and leaves the axis D of the source free for the passage of the cathode tip 11. The additional wehnelt 16 comprises a first control surface 161 which is defined by the opening of the additional wehnelt 16. The first control surface 161 extends around the axis D of the source in a circular manner.

The additional wehnelt 16 is positioned so as to surround the upstream part 111 of the emissive area 110 of the cathode 11. This means that, relative to the axis D of the source, the axial position of the first control surface 161 is the same as the axial position of the upstream part 111.

The first control surface 161 and the second control surface 121 extend around the axis of the source D in a circular manner and at the same distance from the direction of propagation. In other words, the openings of the main wehnelt 12 and the additional wehnelt 16 are circular in shape and have identical diameters.

The source comprises an electrical supply 19 adapted to put the additional wehnelt 16 under electrical voltage. Line 18 is one of the electrical equipotential lines shown in FIG. 3.

The presence of the additional wehnelt 16 modifies the shape of the equipotential lines essentially in the zone 17 located upstream of the main wehnelt 12.

By applying a negative or zero potential difference between the cathode 11 and the additional wehnelt 16, the additional wehnelt 16 becomes repellant for the electrons. In zone 17, the global electric field tightens the field lines around the D axis.

As the additional wehnelt 16 surrounds the upstream part 111 of the emissive zone 110 of the cathode 11, the field lines at the outer periphery of the upstream part 111 are brought closer or even pressed against the cathode.

This allows to :

block certain areas of the cathode so that less electron is extracted from these areas towards the electron beam,

- guide electrons emitted in certain areas of the cathode along the cathode towards the anode, and

reduce the diameter of the electron beam at the level of the main wehnelt, and therefore increase the brightness of the electron beam.

In the absence of the additional wehnelt 16, electrons are emitted in the upstream part 111 of the emissive zone 110 of the cathode 11. These electrons are associated with a greater divergence of the electron beam than the electrons emitted in the downstream part 112. The electrons emitted in the upstream part 111 correspond in fact to positions further from the axis of propagation or to angles of greater displacement with the axis of propagation.

In the presence of the additional wehnelt 16 either these electrons are not emitted by the cathode, or they are channeled along the cathode towards the anode.

The action of the additional electron repellant 16 wehnelt and placed opposite the upstream part 111 makes it possible to reduce the divergence of the electron beam and the emission of the electron beam by the cathode. This action is weighted by the voltage applied to the additional wehnelt. The additional wehnelt 16 provides a degree of freedom controlling the brightness of the source.

These technical effects are provided without making the system more complex with one or more parts located in a vacuum chamber or with one or more delicate or even critical parts to be adjusted, such as for example electromagnetic coils

FIG. 3 shows a situation where the electrons do not cross between the cathode 11 and the anode 13.

The edges of the opening of the additional wehnelt 16 can be folded towards the main wehnelt 12. The technical effect of such edges folded towards the main wehnelt 12 is to increase the effect of the additional wehnelt 16 on the one hand by reducing the extraction of electrons in areas upstream of the cathode and on the other hand guiding the electrons emitted in areas upstream of the cathode along the cathode towards the anode.

Any other shape of the edges of the opening, any other geometry of the additional wehnelt can of course be envisaged.

The cathode 11 can be pointed, the point being directed towards the anode 13. For this particular form of cathode, the increase in brightness due to the effect of the additional wehnelt is greater than with the other forms of cathode . Any other shape of the cathode, such as a planar, hemispherical or stepped shape can of course be envisaged.

Another example of an electron beam source is shown in FIG. 4. A cathode 31 emits an electron beam 34 which passes through a main wehnelt 32 and the anode 33. An additional wehnelt 36 is located upstream of the main wehnelt 32 relative to the direction of propagation of the beam. electrons 34. The cathode 31 has a shape stepped in two stages. The first stage 311 is located behind the second stage 312 with respect to the direction of propagation of the electron beam. The second stage 312 is narrower than the first stage 311.

By applying a negative or zero potential difference between the cathode 31 and the additional wehnelt 36, the additional wehnelt 36 becomes repellant for the electrons. This blocks the first stage 311 so that no electron is extracted from the first stage 311 to the electron beam.

Stabilization of the size of the electron beam

FIG. 5 represents the evolution of the diameter of the electron beam produced by an example of an electron beam source like that illustrated in FIG. 3 as a function of its intensity.

The diameter of the electron beam is measured in a plane transverse to its direction of propagation and in particular in the plane of powders. The diameter of the electron beam is measured at a sufficient distance from the source to correspond to the powder plane if the source was placed in an additive manufacturing apparatus.

In this configuration, the potential difference between the cathode 11 and the anode 13 is fixed. Typically this potential difference can be - 60,000 volts.

For each intensity of the electron beam, the voltages which supply the main wehnelt and the additional wehnelt are adjusted so that the evolution of the diameter of the electron beam as a function of its intensity is as monotonous and as small as possible. It is possible to calibrate the source to determine, before using the source for additive manufacturing, how to adjust the voltages that supply the main wehnelt 12 and the additional wehnelt 16.

For a target intensity the of the electron beam, a target diameter De of the electron beam is sought.

During calibration, the diameter of the electron beam can be determined using a method known from the prior art. For example, it is possible to use a slit device and a tomographic reconstruction. A particular example of tomographic reconstruction is the SYRTE method.

In a first calibration step E1, the source is switched on to produce an electron beam. The power supply assembly 15 adapted to energize the cathode 11, the main wehnelt 12 and the anode 13 is started. The power supply 19 adapted to put the additional wehnelt 16 under electrical voltage is also turned on.

In a second calibration step E2, the electrical supply of the additional wehnelt 16 is set at zero voltage, and the main wehnelt 12 is supplied with a voltage so as to obtain a beam intensity equal to the target intensity le.

In a third calibration step E3, the diameter of the electron beam is determined and compared to the target diameter De.

If the diameter measured is different from the target diameter De, the loop E4 of substeps is implemented:

In a first substep E41, the voltage which supplies the additional wehnelt is increased by a few volts and the voltage which supplies the main wehnelt 12 is reduced by the same value.

In a second substep E42, the diameter of the electron beam is determined and compared to the target diameter De.

The loop of substeps is repeated as long as the diameter of the electron beam is different from the target diameter De. Once the diameter of the electron beam is substantially equal to the target diameter De, that is to say equal to 5%, the values of the voltages which supply the additional wehnelt 16 and the main wehnelt 12 are recorded in correspondence. with the couple of values of the target intensity le and the target diameter De. This gives an electrical configuration of the source.

It is possible to do this for a plurality of pairs of values of the target intensity Ie and the target diameter De, and thus to obtain a plurality of electrical configurations of the source. Once the electrical configurations have been stored, it is possible to adjust the source according to one of the stored electrical configurations to obtain an electron beam with target intensity Ie and target diameter De. It should be noted that the source can be adjusted using linear interpolation between two or more stored electrical configurations. Linear interpolations can be programmed before using the source.

It is in particular possible, to carry out the determination of a plurality of electrical configurations with different values of target intensity lc and always the same value of the target diameter De. Once the electrical configurations have been stored, it is possible to adjust the source according to one of the stored electrical configurations to obtain an electron beam of different intensities always having the same target diameter De. It should be noted that the source can be adjusted by using a linear interpolation between two stored electrical configurations. Linear interpolations can be programmed before using the source.

When the current of the electron beam produced by this source thus supplied with voltage is varied, the diameter of the electron beam no longer has a peak.

The diameter is constant when the intensity is less than 50 mA.

The diameter increases with a slope less than or equal to 10 pm/10mA between 0 and 90 mA.

For a current of 100 mA, the diameter of the beam reaches 50 μm, well below 150 μm. The electron beam source as described above makes it possible to work with a dependence on the size of the electron beam as a function of its intensity which is lower than in the prior art.

The voltages of the different parts of the source can be adjusted so that the diameter of the electron beam produced varies little and always in the same direction relative to its intensity.

Wehnelts tension control

Such an electron beam source also makes it possible to produce electron beams of the same intensity but of different diameters.

For this, different voltage differences W1 and W2 can be applied respectively to the main and additional wehnelt, relative to the cathode, all other things being equal.

In particular, positive parameters a and b can be defined such that if the expression “aWl + bW2” remains constant then, whatever the choice of the voltages W1 and W2, the intensity of the beam remains constant.

For a certain choice of initial voltages which supply the main and additional wehnelts, a certain intensity of the beam is obtained.

It is then possible by varying W1 and W2 while keeping the quantity "aW1 + bW2" to produce beams of constant intensity but of different diameter.

It is also possible to adjust the value of the torque of voltages W1, W2 for different beam intensities so that the diameter of the beam in a plane transverse to its direction of propagation and in particular in the powder plane remains constant.

FIG. 6 represents the changes 61, 62 in the diameter of the electron beam as a function of its intensity. The beam is either produced by an example of an electron beam source according to the prior art, corresponding to curve 61, or by an example of a proposed electron beam source, corresponding to curve 62.

Curve 61 presents a peak in diameter for a transition current corresponding to the current of the electron beam which separates the modes with and without "cross over". More generally, curve 61 presents significant variations and not always in the same direction of variation for the current range from 0 to 100 mA.

Curve 62 was obtained by adjusting the value of the torque of voltages W1, W2 for different beam intensities so that the diameter of the beam in a transverse plane remains equal to 70 μm. There is no peak in curve 62 and no appreciable variation for the current range from 0 to 100 mA. Compensation for cathode wear

Another advantage of the electron beam source as described above is the compensation for the degradation of the diameter of the electron beam as a function of the wear of the cathode.

As it is used, the source produces a beam whose diameter increases.

In response to this variation in diameter, it is possible to find a new set of voltages Wl and W2 which makes it possible to compensate for the variation in the diameter of the beam.

With the possibility of modifying the set of voltages, the use of the source can be continued and the service life of the cathode can be increased.

An example of such compensation can be given in the following table:

The first line of the table corresponds to the new condition of the cathode. The tension set W1 W2 applied to the wehnelts corresponds to a beam diameter of 100 μm.

The second line of the table corresponds to a worn state of the cathode. The same set of voltages W1 W2 as on the first line this time corresponds to a larger diameter of the beam of 200 μm.

The third line of the table corresponds to the same worn condition of the cathode as the second line. The set of voltages W1 W2 is modified and makes it possible to reduce the diameter of the beam to 100 μm.

The modification of the set of voltages made it possible to compensate for the variation in the diameter of the beam due to wear of the cathode.

Selective additive manufacturing apparatus

The device 21 for selective additive manufacturing of FIG. 7 comprises:

- A support such as a horizontal plate 23 on which are successively deposited the different layers of additive manufacturing powder (metal powder, ceramic powder, etc.) making it possible to manufacture a three-dimensional object (object 22 in the shape of a fir tree in the figure ),

- a powder reservoir 27 located above the plate 23,

an arrangement 24 for the distribution of said metallic powder on the tray, this arrangement 24 comprising, for example, a squeegee 25 and / or a layering roller for spreading the different successive layers of powder (displacement according to the double arrow A),

an assembly 28 comprising at least one source 211 of electron beam for the fusion (total or partial) of the spread thin layers, the electron beam generated by the source 211 comes into contact with the thin layers spread in the plane of powders.

a control unit 29 which controls the various components of the device 21 as a function of pre-stored information (memory M), - A mechanism 210 to allow the support of the plate 23 to descend as the layers are deposited (displacement according to the double arrow B).

In the example described with reference to FIG. 7, the assembly 28 also includes a laser-type source 212.

In the example described with reference to FIG. 7, at least one galvanometric mirror 214 makes it possible to orient and move the laser beam coming from the source 212 relative to the object 22 as a function of the information sent by the unit of control 29. Any other deviation system can of course be envisaged.

Deflection and focusing coils 215 and 216 make it possible to locally deflect and focus the electron beam on the areas of layers to be sintered or fused.

A heat shield T can be interposed between on the one hand the plate 23 and the part produced thereon and on the other hand the electron source 211.

The components of the device 21 are arranged inside a sealed enclosure 217 connected to at least one vacuum pump 218 which maintains a secondary vacuum inside said enclosure 217 (typically around 10-2 / 10- 3 mbar, even 10-4 / 10-6 mbar).

Claims

1. Electron beam source suitable for selective additive manufacturing comprising a cathode (11), the cathode (11) comprising an emissive zone (110) adapted to emit electrons, a main wehnelt (12), an anode (13 ) and a set (15) of power supplies adapted to put these three parts under electric voltage, the main wehnelt (12) being placed between the cathode (11) and the anode (13), characterized in that said source comprises an additional wehnelt (16) and a power supply (19) adapted to put said additional wehnelt (16) under electric voltage, said additional wehnelt (16) being placed upstream of the main wehnelt (12) relative to the direction of propagation of the beam of electrons so that a first control surface (161) of the additional wehnelt surrounds an upstream part (111) of the emissive zone (110) relative to the direction of propagation of the electron beam and that a second s the main wehnelt control surface (121) surrounds a downstream part (112) of the emissive zone (110) relative to the direction of propagation of the electron beam.

2. An electron beam source according to claim 1 in which the assembly (15) of electrical power supplies adapted to energize the cathode (11), the main wehnelt (12) and the anode (13) and the power supply (19) adapted to put the additional wehnelt (16) under electrical voltage, are configured so that the source delivers beams of intensity between 1 mA and 50 mA, preferably between 5 mA and 50 mA, even more preferably between 10 mA and 50 mA, and whose diameters in a plane transverse to the direction of propagation of the electron beam, adapted to be a plane where powder of additive manufacturing is consolidated, are substantially constant with an intensity at the other.

3. Electron beam source according to one of the preceding claims, characterized in that

the distance between the main wehnelt (11) and the additional wehnelt (16) according to the direction of propagation of the electron beam,

the thickness of said wehnelts (11, 16) in said direction of propagation,

the position of said wehnelts (11, 16) in said direction of propagation relative to the cathode and

the surface and the shape of said wehnelts (11, 16) facing the cathode

are adapted so that the source, supplied with certain electrical voltages, delivers beams of intensity of 100 mA.

4. Electron beam source according to one of the preceding claims, in which the source extends along an axis (D) of the source, the axis (D) of the source being an axis of symmetry of the cathode (11), the main wehnelt (12), the anode (13) and the additional wehnelt (16), the main wehnelt (12), the additional wehnelt (16) and the anode (13) leaving said axis free (D) from the source, the first control surface (161) and the second control surface (121) extend around the axis of the source (D) in a circular fashion and at the same distance from the direction of spread.

5. Electron beam source according to one of the preceding claims, characterized in that

the thickness of said wehnelts (11, 16) in said direction of propagation, the position of said wehnelts (11, 16) in said direction of propagation relative to the cathode and

the surface and the shape of said wehnelts (11, 16) facing the cathode

are adapted so that in the event of deterioration of the cathode (11), the effects of said deterioration on the diameter, intensity and brightness of the electron beam are corrected by a modification of the electrical supply of said source.

6. Electron beam source according to one of the preceding claims, characterized in that the cathode (11) is a pointed cathode and in that the tip of the cathode is directed towards the anode (13).

7. electron beam source according to one of the preceding claims, characterized in that the edges of an opening of the additional wehnelt (16) defining the first control surface (161) are folded towards the main wehnelt (12) .

8. electron beam source according to one of the preceding claims, wherein the assembly (15) of power supplies adapted to energize the cathode (11), the main wehnelt (12) and the anode (13) and the power supply (19) adapted to put the additional wehnelt (16) under electric voltage are configured so that the potential difference with respect to the cathode (11):

- the main wehnelt (12) is between 0 and -3800 volts

- additional wehnelt (16) is between 0 and -3800 volts

- the anode (13) is between 0 and -60,000 volts

9. Apparatus (21) for manufacturing a three-dimensional object by selective additive manufacturing comprising in an enclosure: - a support (23) for depositing successive layers of additive manufacturing powder,

- a distribution arrangement (24) adapted to apply a layer of powder on said support or on a previously consolidated layer,

- at least one electron beam source (211) adapted to ensure the selective consolidation of a layer of powder applied by the distribution arrangement,

- electronics for controlling said source, characterized in that said source (211) is an electron beam source according to one of the preceding claims.

10. Selective additive manufacturing process comprising the steps of

- application of a layer of additive manufacturing powder on a support or on a previously consolidated layer,

- selective consolidation of the applied powder layer, characterized in that the method is implemented by an apparatus according to claim 9.