Multiple beam lasertron

The invention relates to multiple beam lasertrons. The n (n: integer greater than 1) electron beams of the lasertron are obtained from the same laser beam from which n secondary laser beams are extracted, by occultation, which are deflected respectively towards the n photocathodes of the lasertron.

BACKGROUND OF THE INVENTION 
The present invention relates to multiple beam lasertrons. 
Electronic tubes called "lasertrons" are known from articles and from the 
U.S. Pat. No. 4,313,072. 
In these tubes a photocathode is illuminated by a laser beam whose wave 
length is chosen as a function of the output work of the material from 
which the phtocathode is formed. Thus, a laser beam pulsed at the 
frequency F tears packets of electrons from the photocathode at the same 
frequency F. These packets of electrons are then accelerated in an 
electrostatic electric field and thus gain in kinetic energy. They then 
pass through a cavity resonating at frequency F and their kinetic energy 
is transformed into electromagnetic energy at frequency F. The energy is 
taken from the cavity by coupling it to an external user circuit. 
In FIGS. 1 and 2, two embodiments of lasertrons of the prior art have been 
shown schematically in longitudinal section. 
In these FIGS., the references 1, 2 and 3 designate respectively the 
photocathode, the laser beam and the electron beam. 
In the embodiment shown in FIG. 1, the photocathode 1 is illuminated 
obliquely by the laser beam 2 and the electron beam 3 propagates along the 
longitudinal axis XX' of the tube. 
In the embodiment of FIG. 2, the laser beam 2 and the electron beam 3 
propagate along the longitudinal axis XX' of the tube, but in the opposite 
direction. 
The laser beam 2 is therefore normal to the emissive surface of the 
photocathode. 
The electron beam 3 is accelerated by the electrostatic electric field 
created by an anode 4, then penetrates into a cavity 5 resonating at 
frequency F. A collector 6 then receives the electron beam. The 
electromagnetic energy is taken at frequency F from cavity 5 by coupling 
it to an external user circuit by a guide wave 7, associated with a window 
8, as shown in FIG. 1, or by a loop 9, as shown in FIG. 2. 
The advantage of lasertrons is that they are very compact tubes. In 
lasertrons, electron packets are torn from the photocathode at frequency 
F. Whereas in tubes such a klystrons, several cavities must be used for 
distributing the electrons of an initially continuous beam in packets. 
The problem which arises with lasertrons is that they are limited in 
frequency and in power. 
Thus, for example, in order to obtain high powers, a large current must be 
extracted, which requires a cathode with a large surface and the passage 
of a considerable beam through the cavity. The dimensions of the cavity 
must then be sufficient to allow the passage of this beam, which limits 
the operating frequency. In addition, the use of a large sized cavity 
results in poor coupling between the beam and the cavity, which leads to 
poor efficiency. 
The embodiments of lasertrons which are shown in FIGS. 1 and 2 have the 
following drawbacks: 
in the embodiment shown in FIG. 1, the photocathode is illuminated 
obliquely. The result is, on the one hand, poor light efficiency of the 
photocathode and, on the other hand, a laser beam illumination device 
which must be made as compact as possible for housing it in the vicinity 
of high voltage parts; 
in the embodiment shown in FIG. 2, the laser beam and the electron beam 
follow the same path. Consequently, the surface of the photocathode which 
receives the laser beam is limited by the diameter D of the sliding tube 
of cavity 5 which allows these beams to pass. Furthermore, the laser beam 
illumination device is subjected to the bombardment of the electron beam. 
SUMMARY OF THE INVENTION 
The present invention provides a new lasertron structure which avoids the 
drawbacks of known lasertrons. 
According to the present invention, there is provided a lasertron including 
n photocathodes (n: integer greater than 1) receiving in operation a laser 
beam, pulsed at a frequency F, and emitting n electron beams; m (m: 
integer greater than 0) resonating cavities which resonate at the 
frequency F; n sliding tubes allowing the passage of the n electron beams; 
a collector; and director means situated in the vicinity of the 
photocathodes providing, in operation, oblique illumination of the 
photocathodes by the laser beam.

In the different Figures the same references designate the same elements 
but, for the sake of clarity, the sizes and proportions of the different 
elements have not been respected. 
MORE DETAILED DESCRIPTION 
FIGS. 1 and 2 have been described in the introduction to the description. 
The invention provides a new lasertron structure, called multibeam 
lasertron. Two embodiments of these lasertrons are shown in longitudinal 
section in FIGS. 3 and 4. 
Multiple beam klystrons are known in the prior art from articles, as well 
as from the French Pat. No. 9 92 853. These kylstrons have also been 
described in the French patent applications Nos. 86 03949 and 86 03950, 
filed on the Mar. 19, 1986 in the name of the applicant and not yet 
published. 
A great advantage of said klystrons is that they are particularly well 
adapted to operation at very high power. In fact, it has been demonstrated 
that for the same high frequency power, the acceleration voltage applied 
between the anode and a cathode of the klystron is much less in a multiple 
beam klystron than in the single beam klystrons. Now, whatever the type of 
klystron, the need to modulate the speed of the electron beam imposes on 
this acceleration voltage the same upper limit from which the beam is no 
longer modulable. Consequently, with a multiple beam klystron a high 
frequency power may be obtained much greater than the one it is possible 
to obtain with a single beam klystron. 
Multiple beam klystrons generally operate in the TM01 mode. 
It is possible to obtain high power multiple beam klystrons, at high 
frequencies, by dimensioning the cavities so that these klystrons operate 
optimally in the TM02 mode. 
Multiple beam lasertrons are obtained in making modifications to single 
beam lasertrons of the same type as those which are made to single beam 
klystrons so as to obtain multiple beam klystrons. 
Thus, in order to obtain a lasretron with n beams, n photocathodes are used 
illuminated by a laser beam. Each photocathode produces an electron beam 
which passes through at least one resonant cavity with n sliding tubes, 
before reaching a collector. 
The advantages obtained by going over to multiple beam lasertrons are 
similar to those obtained by going over from single beam lasertrons to 
multiple beam klystrons. 
With multiple beam lasertrons large high frequency powers may then be 
obtained an when they operate in the TM02 mode, high powers and high 
frequencies can be obtained. 
FIG. 3 shows by way of example the modifications made to the lasertron of 
FIG. 1 so as to obtain a multiple beam lasertron. 
In the case of a lasertron with n beams (n: integer greater than 1), n 
photocathodes, bearing the reference 1, are used and they are illuminated 
by the laser beam 2. 
These n photocathodes 1 produce n electron beams 3 which are accelerated by 
n anodes 4 positively biasaed with respect to the cathodes. 
The n beams 3 pass through a cavity 5 with n sliding tubes 16 and yield up 
their kinetic energy therein in the form of electromagnetic energy before 
being collected in the collector 6. 
The multiple beam lasertron of FIG. 3 still has the drawbacks mentioned in 
the introduction to the description in connection with the single beam 
lasertron of FIG. 1. 
FIG. 4 is a cross sectional view of a multiple beam lasertron of entirely 
new structure which does not have the drawbacks of the lasertrons of FIGS. 
1, 2 and 3. 
This lasertron includes n photocathodes 1 which are spaced evenly apart 
about the longitudinal axis XX' of the tube. 
An incident laser beam 2 arrives on an optical system 10, which may be 
formed by a lens, made from quartz for example. 
Preferably, the incident laser beam is annular. This optical system 10 is 
centered on the axis XX'. It is placed in front of the collector, in the 
direction of propagation of the laser beam, as can be seen in FIG. 4. The 
optical system produces a laser beam which moves parallel to the 
longitudinal axis XX' of the tube. 
The lasertron of FIG. 4 has a single resonance cavity 6, whose walls 12 and 
13, perpendicular to the axis XX', are formed with n orifices 14. These 
orifices allow n laser beams to be obtained during operation. A cooling 
device, not shown, is disposed on the wall 12 of cavity 5 which receives 
the impact of the laser beam and which transforms it into n laser beams, 
thus, a part of the power of the laser is collected. 
The diameter of the orifices 14 allowing the n laser beams to pass is 
chosen, as well as the thickness of the walls 12 and 13 of the cavity, so 
as to limit the leak of electromagnetic energy coming from the cavity. 
After the n laser beams have passed through the cavity, another optical 
system 11 is provided, which may be formed by a lens; this optical system 
deflects the n laser beams so that they illuminate the n photocathodes at 
an angle as little slanting as possible. 
On the side where it is facing the photocathodes, optical system 11 
includes a plate 15 protecting it against different deposits, which may 
result from the evaporation of different constituents of photocathodes. 
The n photocathodes, illuminated by n laser beams, each emit an electron 
beam 3, focused by anodes 4 and which pass through cavity 5 through n 
sliding tubes 16 before falling on the collector 6. In cavity 5, the 
electromagnetic power is taken off by a wave guide 7, through a dielectric 
window 8. Coils 9 provide focusing for the n electron beams. 
The lasertron of FIG. 4, besides the advantages inherent in multiple beam 
lasertrons, has numerous other advantages. 
Thus, contrary to what happens in the embodiment of FIG. 2, the optical 
system which produces the laser beam and which focuses it does not receive 
any electron beam which risks damaging it and making it opaque. 
The two optical systems 10 and 11 are also protected from the electron 
beams. Plate 15 protects lens 11 from the products which may come from the 
photocathodes. 
The laser beams illuminate the photocathodes with a substantially normal 
incidence which improves the light yield of the photocathodes. 
It should be noted that laser beams including several successive cavities, 
generally 2, are known. The invention relates then to multiple beam 
lasertrons, having one or several successive cavities.