Compton effect lasers
D United States Patent 1 [111 3,879,679 Mourier Apr. 22, 1975 [54] COMPTON EFFECT LASERS 3.257.620 6/1966 Roberts 330/56 X 75 I 3.398.370 8/1968 Hirschfield. 330/56 X I 1 nvemor- Gmrges Pam France 3.639.774 2/1972 Wolff 307/883 [73] Assignee: Thomson-CSF, Paris. France OTHER PUBLICATIONS [22] Filed: May 15, 1973 Pantell et al.. IEEE J. of Quantum electronics. Vol. [211 pp No; 360,416. QE-4. No. ll. Nov.. 1968, pp. 905-907.  
  Primary E.\&#39;aminerRobert J. Webster [30] Foreign Application Priority Data Attorney. Agent, or Firm-Roland Plottal, Esq.  
 May 19. 1972 France 7218034 [57] ABSTRACT [52] U.S. Cl 33l/94.5 N; 330/4.3; 307/883; In order to substantially increase the power output of 330/56; 321/69 R; 328/233 Compton effect lasers. the invention provides for the [5 1] Int. Cl. l-l0ls 3/10; HOls 3/08 electron beam to be located inside the cavity contain- [58] Field of Search 331/945; 330/43 56. ing the photon radiation with which it is to interact. In 330/53; 307/883, 321/69 R; 328/233 the Figure. the reference 40 illustrates the cavity and the surface covered with dots, the electron beam; 30 is [56] References Cited the accelerator device. The direction of the horizontal UNITED STATES PATENTS arrow is that taken by the emergent photon radiation.  
 3.177.435 4/1965 Marcusc 330/56 X 6 Claims. 6 Drawing Figures i I 40 T v B B.  
 COMPTON EFFECT LASERS The present invention relates to an improvement in lasers and more particularly in lasers in which the electrons interacting with the photon radiation are in the free state, that is to say are not associated with the atoms of a gas, a liquid or a solid as in industrial lasers of the kind which have been developed in recent years. The electrons in question are the electrons of an electrical convection current. Lasers of this kind have hitherto been confined to an experimental status.  
  Those skilled in the art will be well aware that when electrons of this kind interact with a photon radiation of frequency 11,, this interaction generates a second photon radiation of frequency 11 whilst the electron has its momentum modified. This effect, which is known as the Compton effect, occurs for E E,,, under frequency conditions determined by the relationship:  
 where E, designates the energy of the electron in the rest state, that is to say the product m c where m is the mass of the electron in the rest state, and E the energy of the electron in motion, or mc m being the mass of the moving electron in the relativistic sense.  
  It would appear then that, by means of this effect and through the medium of a change in the momentum of the electron concerned, it is possible to achieve a change in the frequency of the photon radiation, that is to say to obtain from an incident photon radiation a second photon radiation of different frequency. The formula (1) also shows that the conversion of frequency varies as the square of the energy of the electron involved in the interaction.  
  It should be pointed out that the formula 1 is to be read in two senses: read from left to right, it shows that from a photon radiation of frequency 11, it is possible to obtain a photon radiation of frequency v higher than 11,; read from right to left it shows that a photon radiation of frequency 12 less than 11 is obtained from a photon radiation of frequency v The rest energy of an electron being around 0.5 MeV, in accordance with this formula the frequency 11 400 times higher than the frequency v, is obtained with electrons having an energy of 5 MeV. For example for vl=3000 MHz v 1200 GI-Iz. These frequencies correspond respectively to wavelengths of cm and 0.25 mm (infra-red).  
  The interaction in question is similar to that which is produced in lasers utilising electrons associated with the interior of a solid, liquid, or a gas, such lasers having formed the subject of numerous studies in recent years, and enjoys the same quantum explanation thereas; see for example Stimulated Photon Electron Scattering by Pantell et al, IEEE Journal of Quantum Electronics, Vol. QE-4 No. 11, November, 1968.  
  We will confine ourselves here to a reminder that a Compton Effect laser essentially comprises, from what has been said before, a high-energy convection electron beam, in the path of which there is arranged a cavity which is the source of the photon radiation with which the electron beam interacts on passage through said cavity.  
  In the following, the adjective photon will be used to describe any electromagnetic radiation, whatever its frequency, this even if the latter is outside the optical spectrum.  
  Generally, too, the electron beam follows a curve trajectory over the greater parts of its length, leaving the direction of the beam emitted by the laser, the cavity occupying a quasi-point position in said trajectory. The trajectory is that in respect of which the electron acquires the energy, for example 5 MeV as explained hereinbefore, necessary for the operation of the device.  
  This arrangement has the drawback that the interaction between the photon radiation and the electron beam takes place only at the intersection between the trajectory followed by the beam and the cavity, with the consequence of a reduction in the probability of interaction between the two since the majority of the electrons of the beam pass through said cavity without having had time to interact with the radiation.  
  The object of the present invention is to overcome this drawback or at any rate to limit it to a greater or lesser extent.  
  The invention will be better understoof from a consideration of the ensuing description and the attached figures in which:  
 FIG. 1 is a schematic view of a prior art device;  
  FIGS. 2 and 3 are schematic views, in plan and section, of Compton effect lasers in accordance with the invention;  
  FIG. 4 is a schematic view of another embodiment of the laser in accordance with the invention.  
  In FIG. 1, inside the envelope 1 of an electron accelerator only part of which has been shown, there can be distinguished an electron beam. indicated by the dotted line, which beam, as indicated by the arrow, passes from a source which has not been shown towards a collector 2; in the trajectory of the beam there is a resonator 3 containing the photon radiation, this being traversed by the electron beam in the zone 4.  
  Taking a frequency 11, equal to that of the foregoing example, that is to say 3000 MHZ, this resonator is then in fact nothing more or less than a microwave cavity. This is why it has been illustrated in the well-known form taken by these cavities in. microwave techniques, namely that of a metal enclosure of re-entrant shape. It is well-known that the re-en trant shape is used in order to create an intense electric field in the re-entrant portion, that is to say in the zone (zone 4 in FIG. 1) through which the electron beam passes, in order thus to promote the exchange of energy between the beam and the electromagnetic wave contained inside the resonator.  
  It will be seen, from a consideration of this example, why the cavity must generally, at the location where it is traversed by the electron beam, have a small dimension in the direction of said beam, making the interaction zone to become a virtually point zone in the electron trajectory.  
  On the wall 10 of the enclosure of the accelerator 1, there are furthermore provided, as the Figures shows, two stubs 5 and 6, located in extension of one another and arranged between two mirrors 7 and 8 forming the resonant cavity of the laser. The two stubs are closed off at those of their ends located opposite the mirrors, by transparent plates 9 and 11 of suitable disposition in a manner known per se, the photon radiation of optical frequency 11 of the foregoing example being picked up beyond these mirrors. As this example shows, the interaction between the electron beam and the resonator is limited to the zone 4 mentioned hereinbefore. This is a drawback for the reason mentioned earlier.  
  To overcome this drawback, the invention provides for the location of the electron beam inside the cavity containing the photon radiation which interacts with the beam; in this fashion, the interaction between the electrons of the beam and the photon radiation takes place along the whole of the length of the trajectory of the electrons and is not reduced simply to a part thereof, as in the prior art. The invention is open to various embodiments. To give a concrete idea of what is involved, three of these possibilities have been quoted hereinafter by way of non-limitative example.  
  These examples form the subject of FIGS. 2a, 2b, 3a, 3b and 4.  
  In the example of FIGS. 2a and 2b the case has been illustrated where the electron beam is that of a storage ring. Those skilled in the art will be aware that storage rings are utilised at the output of certain accelerators {fin order to store the stream of high-energy electrons 1 furnished by these kinds of apparatus. They generally take the form of torroidal systems in which, under given conditions, the electrons are maintained in a circular trajectory.  
  FIGS. 2a and 2b illustrate and embodiment of a Compton effect laser in accordance with the invention, equipped with such a ring.  
  In these figures, there is a schematic illustration, in section and in plan, of a metal storage ring taking the formof a rectangular-section torus. In this ring, in which a high vacuum is maintained by any suitable prior art vacuum technique, there circulates an electron beam in which the electrons describe circular trajectories; this beam is represented by zones covered with dots. The ring, which is coincidental with the resonant cavity in which prevails the photon radiation, is coupled, by the device 21 and the window 25, to a source of photon radiation at frequency 11,. This source is for example a microwave generator, not shown, supplying photons at a frequency of 3000 MHz in accordancewith the above mentioned example. All the prior art means are utilised to ensure resonance of the ring in a selected mode. In the example shown in the Figures, the photon radiation propagates as illustrated by the arrow 22, the electrons describing their trajectory in the direction of the arrow 23. A window 24, transparent to the photon radiation of frequency 11 makes it possible to extract this in accordance with the horizontal arrow.  
  It will be seen that in the device described, there is permanent interaction between the electrons of the beam and the photon radiation injected from the microwave source into the ring 20.  
  ln the case of this device, it is possible to vary the frequency v, by varying the injected frequency v, using appropriate priorart devices not shown for this reason. It goes without saying, however, that in this case there is&#39;a limitation on what is possible in terms of these variations, imposed by the bandwidth of the microwave cavity which constitutes the storage ring 20.  
  It is also possible to vary the frequency v at fixed frequency v by varying the energy of the electrons. At the frequency v,, the cavity then generally presents a very large number of possible resonance modes which has the effect of ensuring virtually continuous tuning of the frequency v corresponding to a given frequency 11,.  
  FIGS. 3a and 3 b provide another example of a Compton effect laser in accordance with the invention. In the example shown in these figures, acceleration of the electrons is achieved by using a betatron in which, as those skilled in the art will known, under the effect of a magnetic induction of fixed direction and timevariable intensity, the electrons are caused to describe circular orbits about an axis coincidental with said directions, in which orbits they are accelerated by the induced electric field. This machine, a prior art device, comprises the magnetic circuit 30 with, in particular, two polepieces 31 and 32 and two coils 33 and 34 excited by a time-variable current, for example a sinusoidal current. The electron beam is produced in an evacuated chamber 40 from a heated filament 41; the axis XX of the evacuated chamber 40 coincides with that of the polepieces 31 and 32; in this chamber, the electrons describe circular trajectories of axis XX, the beam occupying a torus, represented by the surfaces covered with dots, centered on the same axis.  
  Those skilled in the art will be aware that the radii of these trajectories are given by the formula:  
 where B designates the magnetic induction in Tesla units, E the energy of the electrons in MeV and R the radius in metres. This formula applies with good approximation to values of E in excess of some few MeV. With a magnetic induction of 1 Tesla, the diameter of the trajectory of an electron having an energy of 5 MeV is, according to this formula, around 3.3 cm, corresponding to a betatron of small size. These diemensions are, furthermore, perfectly compatible with those of the resonant cavity whose-section, for a frequency v, of 3000 MHZ is constituted by a rectangle 5cm by 7 cm. The dimensions of the cavity are of the same order in the case of the storage ring of the foregoing example.  
  Those skilled in the art will be aware, furthermore, that with a betatron, the dimensions of the equipment are not imposed by the energy level E which is to be attained: The same level can be achieved with a weaker magnetic induction and a larger electron orbital radius, as the foregoing formula shows, where only the product BR is fixed when the energy level E is fixed. Thus, there is wide latitude in the choice of the dimensions of the equipment. I  
  In the example shown in FIGS. 3a and 3b, the evacuated chamber 40 is made of a dielectric material in the form of a rectangular-section torus 42, covered on its internal wall with a metal film 44 having a sufficiently small thickness to allow the magnetic field of the circuit 30 to penetrate to the interior of the evacuated chamber.  
  The evacuated chamber 40 constitutes the electromagnetic cavity into which the incident photon radiation of frequency v, is injected.  
  As in the foregoing example, this radiation is injected into the resonant cavity by one of the known techniques, through a coupling device 43 and the window 45. Means are also provided, although not illustrated, to ensure that the cavity 40 resonates at the frequency of the photon radiation injected at frequency 11,. An output window, transparent to the photon radiation generated inside the device, is provided as before. This window is marked 50 and the emergent beam indicated by the horizontal arrow. In the present example, however, it may be constituted quite simply by an interruption in the metal film 41. This film, of course, should have a thickness greater than the skin thickness (100 microns) corresponding to the frequency v, of the injected radiation. The arrows 22 and 23 have the same significance as in the example hereinbefore.  
  Using the betatron device described above, the same advantage is obtained as with the storage ring device described earlier, namely-a constant interaction between the electrons and the photon radiation filling the electromagnetic cavity constituted by the evacuated chamber 40. r  
  In the case of the betatron however, a further important advantage accrues from the device in accordance with the invention. It is well-known, in other words, that with a betatron it is possible to vary within very wide limits the energy E of the electrons by acting upon the magnetic field applied to the electrons, other things being equal, and this indeed in particular at a constant frequency 11,. Thus a bandwidth of between 1 to 2 octaves is obtained, with excellent orbital stability on the part of the electrons. This stability was well-known as one of the major properties of betatron type induced magnetic field accelerators. It has recently been discussed by Abramyan et al in their work A betatron with spiral electron storage Soviet Physics Technical Physics, Vol. 10, No. 4, October 1965.  
  In the foregoing two embodiments of the invention have been referred to but the invention is not limited to these examples and it would be possible to create the device in accordance with the invention by utilising other prior art accelerators such for example as cyclotrons, synchrotons, etc.. In each of these cases of course, the performance levels which can be achieved will vary depending upon the type of accelerator utilised, the common feature of all these devices being the fact that the electron beam is wholly located within the cavity containing the incident photon radiation.  
  As indicated hereinbefore in respect of the formula (I), the laser in accordance with the invention also makes it possible to obtain, from a photon radiation of given frequency, another photon radiation of lower frequency. In this case, the photon radiation utilises generally radiation from the optical spectrum; the resonance cavity, which can take the same form as in the preceding examples, namely that of a rectangularsection torus in FIGS. 2 and 3), must then be given an optical polish on the internal wall along which the photon radiation propgates. It is well-known that this kind of cavity constitutes the limiting case of an assembly of small mirrors arranged along a polygonal line, the number of sides of which polygon would be doubled indefinitely. In this case, the electron beam too must follow a trajectory very close to said wall.  
  FIG. 4 schematically illustrates an example corresponding to this case. In the example shown in the Figure, the electron beam utilised] is that of a storage ring only the plan view of which, this being considered sufficient for an understanding, has been illustrated. It goes without saying, however, that these electrons could also be ones from an accelerator of any of the aforementioned types. As in the preceding Figures, the electron beam is illustrated by a surface covered with dots; the directions of propagation of the electron beam and the photon radiation are those indicated by the respective curved arrows 73 and 72. In this case, the incident radiation represented by the horizontal arrow at the bottom of the Figure is directed towards the cavity which it enters through the window 74. In this Figure, designates the cavity and 75 the wall thereof which has the said optical polish.  
  The emergent radiation, whose direction is that of the top horizontal arrow is picked up at the output of the coupling device 71.  
  The device shown in FIG. 4 makes it possible, using photon radiation of 10.6 microns wavelength, to employ a C0 laser, the emergent radiation being microwave radiation at around 70 gigacycles.  
  The same device is also used to increase the frequency from the same incident optical radiation, in order to obtain emergent radiation of 250 A units wavelength.  
  Of course, the invention is not limited to the embodiment described and shown which was given solely by way of example.  
 What is claimed is:  
  1. A Compton-effect laser for generating photon radiation of frequency u, from photon radiation of frequency v, comprising, an evacuated chamber including a resonant cavity; means for producing a high energy electron beam having an orbit wholly contained within said cavity; means for inserting photon radiation of frequency 11 into said cavity for interaction with said high energy electron beam for producing photon radiation of frequency v and means for extracting said photon radiation of frequency v from said cavity.  
  2. A Compton-effect laser as claimed in claim 1, wherein said cavity is said evacuated chamber.  
  3. A Compton-effect laser as claimed in claim 1, wherein said means for describing a high energy electron beam includes an induced magnetic field accelerator.  
  4. A Compton-effect laser as claimed in claim 1, wherein the energy of the electrons in said high energy electron beam is 5 MeV, and the frequencies 11, and 11 are respectively 3000 MHz and 1200 GI-Iz.  
  5. A Compton-effect laser as claimed in claim 1, wherein&#39;the frequency v, is that of a C0 laser, and the frequency v is 70 GHz.  
  6. A Compton-effect laser as claimed in claim 1, wherein the frequency v, is that of a C0 laser and the frequency 11 is that corresponding to a wavelength of 250 A.