DEVICE AND METHOD FOR GENERATING COHERENT TERAHERTZ RADIATION

Abstract

The invention relates to a method and a device (1) for producing coherent Smith-Purcell electromagnetic radiation comprising: an electrically conductive diffraction grating (5), delimited laterally by two electrically conductive external side walls (51, 52); a source (7) for generating an electron beam (11) above the diffraction grating (5), the electron beam being configured to interact with the diffraction grating so as to generate coherent Smith-Purcell electromagnetic radiation (3) directed upstream of the diffraction grating (5) in a predetermined direction; and a mirror (9) configured to recover all of the electromagnetic radiation and to redirect it parallel to the diffraction grating (5) towards an output (15) downstream of the diffraction grating (5). Figure for abstract: Figure 1.

Description

Title of the invention: DEVICE AND METHOD FOR GENERATING COHERENT TERAHERTZ RADIATION Technical field

[0001] The present invention relates to the field of coherent electromagnetic sources in the terahertz THz range, in particular coherent Smith-Purcell radiation generators. STATE OF THE PRIOR ART

[0002] Currently, there are two coherent electromagnetic source technologies in the THz domain.

[0003] A first technology concerns equipment generating high-frequency electromagnetic radiation by stimulated resonance of free electrons moving through a powerful magnetic field. This equipment can generally deliver powers greater than one Watt at a frequency of 1 THz but is relatively expensive and not very compact. The Gyrotron is a piece of equipment of this type of technology which requires very strong magnetic fields, produced by superconducting magnets inserted in cryostats. This very expensive and bulky equipment can deliver very high power of the order of kW continuously but at a single frequency.

[0004] A second technology concerns relatively compact quantum cascade lasers (QCLs) capable of generating a frequency of 1 THz but with an emitted power that does not exceed 1 mW. In addition, these laser sources are not tunable, i.e. it is necessary to change the laser source to change frequency.

[0005] There are relatively compact devices in the prior art that can deliver high-power THz radiation. This type of device exploits the three-dimensional properties of a diffraction grating to generate highly efficient coherent Smith-Purcell radiation. Such devices or methods are described in the article by J.T. Donohue and J. Gardelle, “Simulation of a Smith-Purcell free-electron laser with sidewalls: Copions emission at the fundamental frequency,” Applied Physics Letters 99 (2011), 161112-1, as well as in French patents FR 3004294 and FR 2980923.

[0006] The objective of the present invention is to propose a device and a method for generating coherent Smith-Purcell radiation, making it possible to improve the devices described in the patents cited above by further simplifying implementation, reducing costs and optimizing compactness.

[0007] Another objective of the present invention is to provide a production device

[0008]

[0009]

[0010]

[0011]

[0012] tunable coherent Smith-Purcell radiation, allowing the radiation frequency to be varied. Statement of the invention This objective is achieved with a coherent Smith-Purcell electromagnetic radiation production device comprising: - an electrically conductive diffraction grating, delimited in width by two electrically conductive external side walls; - a source for emitting an electron beam and propagating it above the diffraction grating, the electron beam being configured to interact with the diffraction grating so as to generate coherent Smith-Purcell electromagnetic radiation directed upstream of the diffraction grating in a predetermined direction; and - a mirror configured to recover most of the electromagnetic radiation in order to redirect it parallel to the diffraction grating towards an output downstream of the diffraction grating. Thus, the device according to the invention is compact, economical and simple to implement while generating high-power coherent electromagnetic radiation in the THz range. Advantageously, the mirror comprises a reflective surface whose geometric shape is determined on the basis of the radiation pattern of the diffraction grating, taking into account the emission lobes. The shape of the mirror is thus optimized to return the emitted power to the output of the device. Advantageously, the characteristics of the electron beam include: - a sufficiently low electron speed so that, in a dispersion diagram, where the frequency is expressed as a function of the wave number, a straight line representing the frequency of the electron beam as a function of its wave number, and a curve representing, in the first Brillouin zone, the three-dimensional dispersion relation corresponding to the fundamental mode of the diffraction grating, intersect at a point located outside an isosceles triangle whose base coincides with the abscissa axis of the dispersion diagram and one side of which is a segment of slope C / passing through the origin of said diagram, where c is the speed of light in a vacuum; and - a current density high enough to efficiently excite the fundamental mode of the diffraction grating. This allows us to obtain highly directional electromagnetic radiation directly resulting from the fundamental mode of interaction between the electron beam and the diffraction grating.

[0013] Advantageously, the device comprises:

[0014] - a tube whose axis is parallel to the propagation of the electron beam, said tube being connected to the source on one end and terminated by an outlet porthole on the other end, and

[0015] - a support arranged inside the tube between the source and the outlet porthole on to which the diffraction grating and the mirror are removably fixed.

[0016] Thus, the mirror and / or the network can be replaced by other mirrors and / or networks on the support in order to modify the emission frequency of the electromagnetic radiation according to the parameters desired by the operator.

[0017] Advantageously, the support comprises a first part of circular surface perpendicular to the axis of the tube and provided with a circular orifice to allow the electron beam to pass and a second part of rectangular surface parallel to the axis of the tube, the mirror being fixed on the circular part of the support while the diffraction grating is fixed on the rectangular part of the support.

[0018] This makes it easier to remove the support to change the mirror and / or the network.

[0019] Advantageously, the device comprises a beam stopping element fixed to the rectangular part of the support downstream of the diffraction grating, said beam stopping element being intended to absorb the beam downstream of the grating.

[0020] This allows the beam to be stopped as soon as the electrons have finished interacting with the lattice.

[0021] Advantageously, the beam stopping element has a geometric shape configured to distribute the current density of the electron beam over the maximum surface area while allowing the electromagnetic radiation to pass through the exit porthole.

[0022] Advantageously, the tube provided with the source and the outlet porthole form a vacuum enclosure inside which the pressure is of the order of 108 mbar to 107 mbar.

[0023] Advantageously, the device comprises magnetic focusing and guiding elements configured to focus, guide and maintain the electron beam in an area located above the diffraction grating.

[0024] The invention also relates to a method of producing coherent Smith-Purcell electromagnetic radiation in which: - an electrically conductive diffraction grating is used, laterally delimited by two electrically conductive external side walls; - an electron beam is propagated above the diffraction grating, the electron beam being configured to interact with the diffraction grating in such a way as to generate Smith electromagnetic radiation- coherent Purcell directed upstream of the diffraction grating in a predetermined direction; and - a mirror is used, configured to recover all of the electromagnetic radiation and to redirect it parallel to the diffraction grating towards an output downstream of the diffraction grating. Brief description of the drawings

[0025] The present invention will be better understood on reading the description of exemplary embodiments given purely for informational purposes and in no way limiting, with reference to the appended drawings among which:

[0026] [Fig.lA] schematically represents a device for generating electromagnetic radiation, according to one embodiment of the invention;

[0027] [Fig.lB] is a schematic perspective view of a diffraction grating usable in the device of [Fig.lA];

[0028] [Fig.2] illustrates an example of the dispersion diagram of the diffraction grating, according to one embodiment of the invention;

[0029] [Fig.3] schematically represents a propagation module of the device illustrating the interchangeability of its removable elements, according to one embodiment of the invention; and

[0030] [Fig.4A]

[0031] [Fig.4B]

[0032] [Fig.4C]

[0033] [Fig.4D] illustrate the dimensioning of the mirror according to the radiation diagram, according to an embodiment of the invention.

[0034] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0035] [Fig.lA] schematically represents a device for generating electromagnetic radiation, according to one embodiment of the invention. Furthermore, [Fig.lB] is a schematic perspective view of a diffraction grating usable in the device of [Fig.lA].

[0036] This device 1 is intended to generate coherent Smith-Purcell radiation 3 with high efficiency. It comprises a diffraction grating 5, an electron source 7 and a mirror 9.

[0037] The diffraction grating 5 is electrically conductive and preferably non-magnetic. It is delimited laterally by two walls 51 and 52 which are also electrically conductive and preferably non-magnetic.

[0038] The source 7 is an electron gun (for example, a thermionic gun) intended to produce an electron beam 11 which propagates along an axis Z, above the diffraction grating 5, between the two walls 51 and 52.

[0039] The electron beam 11 is configured to interact with the diffraction grating 5 so as to produce coherent Smith-Purcell electromagnetic radiation 3.

[0040] The diffraction grating 5 has a series of grooves 53 with a rectangular profile and parallel to each other. This is then referred to as a lamellar diffraction grating 5. Other types of profile, for example triangular or sinusoidal, may also be considered without departing from the scope of the present invention.

[0041] The Z axis, along which the electron beam 11 propagates, is perpendicular to the grooves 53. And we further define an X axis which is parallel to the grooves 53, and therefore perpendicular to the Z axis, as well as a Y axis which is perpendicular to the X and Z axes as seen in [Fig.lB]. The three axes X, Y and Z intersect at a point O thus forming a Cartesian coordinate system.

[0042] W is the width of the diffraction grating 5 (i.e. the distance between its two side walls 51, 52) and S is the height of the side walls 51, 52. A (respectively H) is the thickness (respectively the depth) of the grooves 53. L is the period of the diffraction grating 5 and N is the number of periods.

[0043] Preferably, the width W = 2L. The period L is chosen according to the operating wavelength of the desired application.

[0044] It is recalled that the Smith-Purcell radiation 3 is emitted at an angle 6 relative to the propagation axis Z of the electron beam, according to the following relationship:

[0045] [Math.l] / t=c / / = Z(l / ^—COS^) / |n| (1).

[0046] where X represents the wavelength of the radiation in a vacuum, f the frequency of this radiation, ç the speed of light in a vacuum, P the ratio v / c (v: speed of the electrons) and n the order of diffraction.

[0047] Formula (1) is sufficient to give the order of magnitude before an exact calculation of the modes of the three-dimensional grating. The present invention is in fact based on the theory of the three-dimensional modes of a diffraction grating 5 of given width W, equipped with side walls 51, 52. On this subject, reference will be made to French patents FR 3004294 and FR 2980923.

[0048] [Fig.2] illustrates an example of a scatter diagram (f, k) of the dif network fraction, according to one embodiment of the invention.

[0049] The dispersion diagram represents the frequency of the radiation f as a function of the wave number k. We limit ourselves to the first Brillouin zone, of length 2tf / 1 L. The line I, whose equation is f = vk / 2æ represents the frequency of the electron beam 11 as a function of the wave number k, is called the "beam line". Curve II represents the dispersion relation of the diffraction grating 5 in two dimensions.

[0050] Lines III and IV correspond respectively to the “front light line” of equation f = ck / 2tt and to the “back light line”.

[0051] Lines III, IV and the abscissa axis (wave number k) delimit a triangle called the “triangle of light”.

[0052] If the point of intersection between the beam line I and the curve representing the dispersion relation of the grating 5 is located inside the light triangle, which is always the case in two dimensions, the device cannot emit in the fundamental mode. The latter is then an evanescent surface wave in the Y direction. Only the harmonics of the fundamental frequency can then be emitted with low efficiency.

[0053] Curve V represents, in the first Brillouin zone, the three-dimensional dispersion relation corresponding to the fundamental mode of the diffraction grating 5. The intersection point P can then be outside the light triangle; and coherent emission on the fundamental mode is then permitted because relation (1) is satisfied.

[0054] In the example described, we therefore choose point P as the operating point of the beam-lattice system. In other words, we choose the speed of the electrons (or, what amounts to the same thing, their kinetic energy) so that the straight line of beam I intersects curve V at point P.

[0055] Generally speaking, in the present invention, the speed of the electrons is chosen to be sufficiently low so that, in the dispersion diagram (k, f) of the grating-beam assembly, the beam line intersects a part of a branch of the dispersion relation, located in the first Brillouin zone and corresponding to the fundamental mode of the diffraction grating 5, at a point P located outside the triangle of light.

[0056] According to a particular embodiment of the present invention, the table Tabl gives the values ​​of the frequency in GHz of the operating point P for a circular electron beam 11 whose kinetic energy is 60 keV interacting with the fundamental mode of the diffraction grating 5 equipped with its side walls 51, 52, of period L in mm. According to this example, the diffraction grating 5 has a rectangular profile of the grooves and its dimensions are H=A=L / 2 and W=2L.

[0057] [Tables 1] L (mm) | / (CHz) 013 $ [ 0J î 1005 î / wrÆIW# . / 7 “.4 “1 / 2 and F” 2L

[0058] Thanks to the side walls 51, 52 located on the sides of the diffraction grating 5, it is possible to extract the fundamental mode of the grating 5. Half of the energy deposited by the electron beam 11 in the diffraction grating 5 is expected in the form of electromagnetic radiation in this fundamental mode. As a reminder, a two-dimensional diffraction grating 5 can only emit Smith-Purcell radiation at the harmonic frequencies of the latter. In any electromagnetic system, the harmonic frequencies always carry less energy than the fundamental frequency.

[0059] Furthermore, the continuous and circular electron beam 11 is chosen according to the following characteristics:

[0060] - The current density of the electron beam 11 is sufficiently high to strongly excite the fundamental mode of the diffraction grating 5;

[0061] - The electrons have an energy of 60 keV at the level of the diffraction grating;

[0062] - The electron beam 11 is advantageously shaped by means of of a magnetic transport line 13a, 13b whose maximum axial magnetic field can reach 1.5 Tesla. This magnetic field makes it possible to maintain the electron beam 11 along the diffraction grating 5 and to achieve an electron beam radius of between 20 and 50, thus optimizing the interaction. The stronger the axial magnetic field in the vicinity of the grating, the smaller the equilibrium radius (without transverse oscillations along the grating) of the beam will be.

[0063] For example, using the Smith-Purcell relationship for a grating having a period L of 0.3 mm and for an electron kinetic energy of 60 keV, it is deduced that the radiation is expected at an angle in the vicinity of 138° relative to the direction of propagation of the electron beam 11. The electron kinetic energy of 60 keV ensures a good compromise in terms of compactness, high gain value and reasonable electrical power of the electron beam 11 to have a significant emitted power.

[0064] Thus, according to the present invention, the radiation 3 is directed upstream (i.e. the rear) of the diffraction grating 5 (here, the term upstream is defined in relation to the direction of propagation of the electron beam 11) in a predetermined direction (angle 6).

[0065] In turn, the shape of the mirror 9 is calculated to recover the majority of the electromagnetic radiation 3 and to redirect it parallel to the diffraction grating 5 towards an exit 15 (an exit porthole) downstream (i.e. in front) of the diffraction grating 5.

[0066] Advantageously, the mirror 9 comprises a reflecting surface whose geometric shape is determined on the basis of the complete 3D radiation pattern “DR” of the diffraction grating 5 defining the different modes emitted by the latter (generating the radiation in the direction S but also in the azymuthal direction P). The simulation tool used may be software of the CST® type. The radiation pattern is described in more detail in relation to Figs. 4A-4D.

[0067] Furthermore, the embodiment of [Fig. 1A] shows that the device 1 comprises a tube 17 whose axis of revolution is parallel to the propagation of the electron beam 11 (i.e. in the direction of the Z axis).

[0068] The tube 17 is delimited by the electron source 7 on one end and by an outlet porthole 15 on the other end. Thus, the tube 17 provided with the source 7 and the outlet porthole 15 form a vacuum enclosure inside which the pressure is of the order of 108 mbar to 107 mbar (with 105Pa = 1 bar = 1000 mbar). Preferably, the cylindrical tube 17 is made of non-magnetic metal.

[0069] Furthermore, the device 1 comprises a support 19, arranged inside the tube 17 between the source 7 and the output porthole 15, on which the diffraction grating 5 and the mirror 9 are fixed (by means of screws 20 and / or pins 22) in a removable manner on the Z axis. Preferably, the support 19 is non-magnetic. Advantageously, the support 19 is resistant to more than 300°C.

[0070] According to a particular embodiment of the present invention, the support 19 comprises a first part 19a in the plane (X, Y) of circular surface perpendicular to the axis of the tube 17 (i.e. the Z axis). This first part 19a of the support is provided with a circular orifice 21 to allow the electron beam 11 to pass. The mirror 9 is fixed on this circular part 19a of the support.

[0071] The support 19 comprises a second part 19b in the plane (X, Z) of rectangular surface parallel to the axis Z of the tube 17. The diffraction grating 5 is fixed on this rectangular part 19b of the support so that the electron beam passing through the hole 21 can propagate at grazing incidence along the grating 5. The space between the circular part 19a of the support and the exit porthole 15 forms the propagation chamber 25.

[0072] Furthermore, the device 1 comprises a beam stopping element 23 fixed on the rectangular part 19b of the support 19 located downstream of the diffraction grating 5. The element beam stop 23 is intended to absorb the electron beam 11 downstream of the diffraction grating 5.

[0073] Advantageously, the beam stopping element 23 has a geometric shape configured to distribute the collection of electrons over the largest possible surface area in order to minimize the current density of the electron beam 11 on the stopping element, without obstructing the passage of the radiation 3 which can then pass through the exit porthole 15. Advantageously, the beam stopping element 23 is made of graphite.

[0074] The support 19 as well as the mirror 9, the diffraction grating 5, and the stopping element 23 form a THz propagation module 27 arranged in the propagation chamber 25. The THz propagation module 27 according to the present invention is very compact, its length does not exceed a few tens of mm and is integrated into a vacuum tube 17 with a diameter not exceeding 20 mm,

[0075] Thus, the mirror 9 and the diffraction grating 5 are interchangeable on the support 19 in order to modify the emission frequency of the electromagnetic radiation 3 according to the parameters desired by the operator.

[0076] Advantageously, the device 1 comprises focusing 13a and guiding 13b elements (for example, a magnetic transport line) arranged outside the tube 17. These elements 13a, 13b are configured to focus, guide and maintain the electron beam 11 in the area located above the diffraction grating 5.

[0077] [Fig.3] schematically represents the THz propagation module of the device illustrating the interchangeability of these removable elements, according to one embodiment of the invention.

[0078] The implementation of the process of changing the diffraction grating 5 and / or the mirror 7 is relatively simple and rapid. The steps for changing the emission frequency of the device 1 are as follows:

[0079] 1. Closing of an isolation valve (not shown) located between barrel 7 to electrons and the vacuum propagation chamber 25 containing the THz propagation module 27. We will note that the electron gun 7 remains constantly under vacuum;

[0080] 2. Venting the vacuum tube and opening the propagation chamber 25 in removing the exit porthole 15;

[0081] 3. Extraction of the THz propagation module 27 using a self-locking stop pin blocking inserting into the support 19;

[0082] 4. Dismantling of the screws 20 for tightening the network 5 and / or the mirror 9;

[0083] 5. Mounting of the new network 5 and / or the mirror 9;

[0084] 6. Installation of the new THz propagation module 27 in the pro chamber pagation 25 using the pin;

[0085] 7. Closing of the propagation chamber 25 by replacing the outlet porthole 15

[0086] 8. Vacuuming (107 mBar - 108 mBar) and opening of the isolation valve.

[0087] It will be noted that the mirror 9 is inserted inside the vacuum tube 17 in a reduced space above the grating 5. As indicated previously, in order to return all of the emitted power to the output of the device 1, the mirror 9 is sized on the basis of the complete radiation pattern “DR” of the grating 5 generating the radiation in the direction 0 but also in the direction V (spherical coordinates).

[0088] Indeed, Figs. 4A-4D illustrate the dimensioning of the mirror according to the radiation diagram, according to one embodiment of the invention.

[0089] More particularly, [Fig.4A] represents a radiation diagram calculated for a diffraction grating with a period L of 0.3 mm, according to an embodiment of the invention.

[0090] The radiation pattern, observable by plotting at different time intervals, the values ​​of the axial component of the Poynting vector (emitted power density) throughout the space. After a time t > 1ns, the radiation pattern keeps the same shape with only local variations in the field intensity. The emission at the Smith-Purcell angle (138° for a grating with a period L of 0.3 mm) is visible in the YOZ section plane but a large part of the emission extends in the direction

[0091] In a second step, the shape of the mirror 5 is determined using the principles of geometric optics. It is based on the study of the radiation pattern of the grating alone in space and is obtained using CST®. Rather than working on the Poynting vector, one can work with the contours of the Bx component of the magnetic field (along the X direction) which is more suitable for observing the front of the waves coming from the diffraction grating 5. As indicated previously, the emission angle in the YOZ plane is the Smith-Purcell angle of 138°. On the other hand, in the (X, Z) planes, the angles are a little more complex to estimate.

[0092] [Fig.4B] represents precisely the radiation in a plane (X, Z) of the diagram of [Fig.4A]. This type of diagram makes it possible to determine the angles in the planes (X, Z) for different values ​​of Y.

[0093] In particular, [Fig.4B] is an example representing the contour Bx in the plane Y= 5 mm. By knowing the Z coordinate of the position where the mirror 5 can be mechanically placed, we can make the contour Z(X) that the mirror must have at this location correspond to "fitter" to intercept the rays coming from the network and return them parallel to the Z axis.

[0094] [Fig.4C] schematically illustrates the construction of the mirror surface, according to one embodiment of the invention.

[0095] The use of a bisquare polynomial is sufficient to restore the curvature of the mirror at any vertical position Y. Knowing the wavefront curve in this plane under polynomial form, we can draw at each point x0 of this curve two straight lines corresponding to the normal and the tangent. The normal at x0 corresponds to a light ray coming from the diffraction grating 5 which must be sent forward parallel to the OZ axis. The tangent at x0 is used to determine a point on the new curve Z(xM) which will allow us to describe the shape of the mirror 5 in the Y plane considered. Knowing the angle of the ray with respect to the OZ axis, we calculate the angle that the normal must have at the point intercepted by the ray of the new curve to send the ray forward.

[0096] This method is then repeated for different values ​​of Y. For mirror 5 at a frequency of 335 GHz, eight planes (X, Z) were chosen for Y varying from 2 to 9 mm in steps of 1 mm. Eight curves are then obtained whose polynomial coefficients vary with Y and can be refitted to obtain the equation of the surface of mirror 5. As an example, all the calculations and the construction of the surface of mirror 5 can be carried out using MapleR software.

[0097] [Fig.4D] represents an example of a file of points determined according to the method explained above in relation to [Fig.4C] for a network with a period L of 0.3 mm.

[0098] Thus, the present invention proposes a device making it possible to achieve relatively high power levels ranging from a few tens of watts at 100 GHz up to one Watt at 1 THz while being very compact, not exceeding a length of around fifty mm for a diameter of around twenty mm.

Claims

Claims

1. Device (1) for producing coherent Smith-Purcell electromagnetic radiation comprising: - an electrically conductive diffraction grating (5), delimited laterally by two electrically conductive external side walls (51, 52); - a source (7) for emitting an electron beam (11) and propagating it above the diffraction grating (5), the electron beam being configured to interact with the diffraction grating so as to generate coherent Smith-Purcell electromagnetic radiation (3) directed upstream of the diffraction grating (5) in a predetermined direction; and a mirror (9) configured to recover most of the electromagnetic radiation and to redirect it parallel to the diffraction grating (5) towards an output (15) downstream of the diffraction grating (5), said mirror (9) comprising a reflecting surface whose geometric shape is determined on the basis of a radiation diagram of the diffraction grating (5) defining the different modes emitted by the latter.

2. A device according to claim 1, such that the characteristics of the electron beam include: - a sufficiently low electron speed so that, in a dispersion diagram, where the frequency (f) is expressed as a function of the wave number (k), a straight line (I) representing the frequency of the electron beam as a function of its wave number, and a curve (V) representing, in the first Brillouin zone, the three-dimensional dispersion relation corresponding to the fundamental mode of the diffraction grating (5), intersect at a point (P) located outside an isosceles triangle whose base coincides with the abscissa axis of the dispersion diagram and one side (III) of which is a segment of slope Cl'In passing through the origin of said diagram, where c is the speed of light in a vacuum; and - a current density high enough to excite the fundamental mode of the diffraction grating (5).

3. Device according to claim 1 or 2, comprising: - a tube (17) whose axis is parallel to the propagation of the electron beam (11), said tube being provided with the source (7) on one end and an exit porthole (15) on the other end, and - a support (19) arranged inside the tube between the source (7) and the exit porthole (15) and on which the diffraction grating (5) and the mirror (9) are removably fixed.

4. Device according to claim 3, such that the support (19) comprises a first part (19a) of circular surface perpendicular to the axis of the tube (17) and provided with a circular orifice (21) to allow the electron beam (11) to pass and a second part (19b) of rectangular surface parallel to the axis of the tube, the mirror (9) being fixed on the circular part of the support while the diffraction grating (5) is fixed on the rectangular part of the support.

5. Device according to claim 4, comprising a beam stopping element (23) fixed on the rectangular part (19b) of the support downstream of the diffraction grating (5), said beam stopping element being intended to absorb the beam downstream of the grating.

6. Device according to claim 5, such that the beam stopping element (23) has a geometric shape configured to distribute the current density of the electron beam over the maximum surface area while allowing the electromagnetic radiation to pass through the exit porthole (15).

7. Device according to any one of claims 3 to 6, such that the tube provided with the source (7) and the outlet porthole (15) form a vacuum enclosure inside which the pressure is of the order of 108 mbar to 107 mbar.

8. A device according to any preceding claim, comprising focusing and guiding elements configured to focus, guide and maintain the electron beam (11) in an area above the diffraction grating (5).

9. Method for producing coherent Smith-Purcell electromagnetic radiation in which: - an electrically conductive diffraction grating (5) is used, delimited laterally by two side walls external (51, 52) electrically conductive; an electron beam (11) is propagated above the diffraction grating (5), the electron beam being configured to interact with the diffraction grating so as to generate coherent Smith-Purcell electromagnetic radiation (3) directed upstream of the diffraction grating (5) in a predetermined direction; and a mirror (9) is used, configured to recover all of the electromagnetic radiation and to redirect it parallel to the diffraction grating (5) towards an output (15) downstream of the diffraction grating (5), said mirror (9) comprising a reflecting surface whose geometric shape is determined on the basis of a radiation diagram of the diffraction grating (5) defining the different modes emitted by the latter.

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