Radio electric wave generator for ultra-high frequencies

A generator is provided based on an interaction of the cyclotronic type between an electron beam propagating between an electron gun and a collector and a high frequency electromagnetic field in a resonating structure. It is principally characterized in that the electron beam moves along a cycloidal path in a transverse magnetic field under the effect of a drift velocity created by a continuous electric field.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a wave generator for ultra-high 
frequencies, more particularly a millimetric and sub-millimetric wave 
generator of the cyclotronic resonance maser type. 
2. Description of the Prior Art 
As generators of this type the generators called gyrotrons are known in 
particular. In these generators, an electron beam coming from an electron 
gun propagates along helicoidal paths while being guided by a uniform 
magnetic field directed along the axis of the helix. The beam passes then 
through an electromagnetic cavity resonating at a frequency f.sub.o close 
to a multiple of the cyclotronic frequency, in which cavity the transverse 
velocity components of the electrons interact with a transverse electric 
field component of the wave so as to give up their energy thereto. In this 
case, the beam is propagated substantially parallel to the magnetic field. 
Since the interaction takes place with the transverse velocity component 
v.perp. of the electrons, the parallel velocity component v.parallel. 
corresponds then to energy unused in the interaction. Efforts are 
therefore in general made to limit the value of this parallel velocity. 
However in the generators of the above type, it is not possible to operate 
with low values of v.parallel., for in this case an unstable electron beam 
is obtained. Consequently, the values of v.perp./v.parallel. must be 
chosen so that 
EQU 0.5&lt;v.perp./v.parallel.&lt;2 
SUMMARY OF THE INVENTION 
The aim of the present invention is therefore to remedy this drawback by 
providing a new generator of the cyclotronic resonance maser type in which 
the parallel velocity component of the electrons may be equal to zero or 
substantially equal to zero. 
The present invention provides therefore a wave generator for ultra-high 
frequencies based on an interaction of the cyclotronic type between an 
electron beam propagating between an electron gun and a collector and a 
high frequency electromagnetic field in a resonating structure, wherein 
the electron beam moves along a cycloidal path in a transverse magnetic 
field under the effect of a drift velocity created by a continuous 
electric field. 
The present invention also relates to new resonating structures and new 
collectors for this type of generator. 
Thus, according to a preferred embodiment, the resonating structure is 
formed by two electrodes facing each other between which the electron beam 
passes transversely, the two electrodes being brought to different DC 
potentials and being, at least in their central part, spaced apart by a 
distance H, slightly greater than n.lambda./2, n being a whole number and 
.lambda. the wave length corresponding to the cyclotron resonance 
frequency. 
Similarly, the collector is formed by a curved reflector brought to the 
potential of the upper electrode of the resonating structure and 
positioned as an extension of the lower electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiment of the radioelectric wave generator for ultra-high 
frequencies shown in FIG. 1 is formed essentially by an electron gun 1 
providing an electron beam moving under the effect of a continuous 
electrostatic field E.sub.c, perpendicularly to a constant magnetic field 
B the longitudinal direction x, following a cycloidal path, a structure 2 
resonating at a frequency f.sub.o equal to a multiple of the cyclotronic 
frequency and a collector assembly 3 for receiving and removing the 
electrons at the output of the resonating structure. 
The electron gun 1 is an electron gun of the type described in the patent 
application Ser. No. 595,973 in the name of the applicant filed on the 
same day as the present application. It comprises essentially two 
electrodes which face each other, one of which, namely the anode 10 is 
brought to a positive potential whereas the other, namely the sole 11 is 
brought to a negative or zero potential and a cathode 12 brought to the 
potential of the sole and situated in the plane thereof. Anode 10 and sole 
11 have a curved profile divergent from left to right in FIG. 1, so that 
the continuous electric field E.sub.c created between the two electrodes 
10, 11 decreases in this direction. When the assembly is placed in a 
constant magnetic field B directed perpendicularly to the plane of the 
figure, the electrons from the cathode move in direction x perpendicular 
to the magnetic field under the effect of the drift velocity due to the 
continuous electric field between the two electrodes 10, 11 along a 
cycloidal path around the longitudinal x axis. 
The electron beam 13, moving with the cycloidal movement in direction x, is 
then fed into a resonating structure 2. This structure 2 is formed by two 
electrodes 20, 21 facing each other, brought to different DC potentials 
for providing between the electrodes a continuous electric field E.sub.c. 
This structure contains high frequency electromagnetic energy 
corresponding to an oscillation at a frequency f.sub.o close to a multiple 
of the cyclotron frequency f.sub.c. So that the waves at frequencies 
f.sub.o may oscillate in the resonating structure, the distance H between 
the two electrodes, 20, 21 is chosen so as to be, at least in the central 
part of the plates, slightly greater than a whole number of half length 
waves. In addition, the length L of the electrodes is chosen equal to a 
few wave lengths, the dimension along the magnetic field B depending on 
the corresponding dimension of the anode which may be large with respect 
to the other dimensions. 
Furthermore, the injection and removal of the electron beam into and out of 
the resonating structure 2 are provided by means of elements of the 
sliding tube type 22, 23 having a height h such that: 
EQU (n-1).lambda./2&lt;h&lt;n.lambda./2 with .lambda.=c/f.sub.o 
so as to avoid any undesirable resonance. 
Thus, the electron beam is removed from the resonating structure through 
tube 23 towards the collector part 3. This collector part 3 is formed of a 
curved reflector 30 which extends the lower electrode 21 of the resonating 
structure and which is brought to the potential of the upper electrode 20 
of said structure. This reflector 30 collects the electromagnetic energy 
and radiates it in a substantially vertical direction in FIG. 1, towards a 
vacuum tight transparent window which has not been shown in this Figure. 
As for the operation of this type of generator, it may be stated that the 
behavior of the electrons then is practically identical to that observed 
in gyrotrons. In fact, in the reference system moving at the drift 
velocity, the continuous electric field is suppressed by the continuous 
magnetic field and the high frequency electric field is not modified for 
it is longitudinal to the displacement speed. Thus, by using Lorentz's 
transformation and designating with a prime or apostrophe the magnitudes 
measured in the reference system, we obtain: 
##EQU1## 
However, there exists a transformation of the magnetic fields 
##EQU2## 
This transformation will then cause a correction of the value of the 
magnetic field required for optimum interaction. However, the value of 
this correction remains low. 
Furthermore, as shown in FIG. 2, the magnetic field B is obtained by means 
of two superconducting coils B.sub.1 and B.sub.2 disposed according to 
Helmholz's rule and situated inside two drums T.sub.1 and T.sub.2 filled 
with liquid helium. The two drums T.sub.1 and T.sub.2 are connected 
together by a hollow tube C.sub.1 which also contains the electric 
connections between the two coils. The assembly is fed with liquid helium 
and electric current through a tube C.sub.2. 
The electron gun-resonating structure-collector assembly described above 
with reference to FIG. 1 is contained in a metal enclosure E. This 
enclosure comprises, in the embodiment shown, four insulated outputs 
E.sub.1,E.sub.2,E.sub.3,E.sub.4 connected respectively to the cathode, to 
its heating element, to the anode and possibly to the sole or to the 
negative part of the resonating structure. On the upper part of the 
enclosure is provided the transparent window F, preferably circular, for 
outputting the radiation. 
Enclosure E is placed between the two drums T.sub.1 and T.sub.2 so that the 
electron beam is propagated parallel to the drums, namely in direction x. 
The modifications to be made to the above described embodiment so as to use 
it as an amplifier will now be described with reference to FIGS. 3 and 4. 
FIG. 3 shows schematically a section parallel to the plane zoy in the 
median part of the resonating structure 2, illustrating a particular 
embodiment of the input and output circuits for the signal to be treated. 
As in the embodiment of FIG. 1, the electron beam propagates in direction 
x with a drift velocity V.sub.D and orbits with axis z between the two 
electrodes 20, 21 which contain the electromagnetic energy. 
In the embodiment shown in FIG. 3, electrode 21 is moveable, which allows 
the height H to be adjusted depending on the oscillation frequency f.sub.o 
so that: 
EQU H.gtorsim.n.lambda./2 with .lambda.=c/f.sub.o 
However, in the embodiment shown in FIG. 3, the electromagnetic energy is 
transported in direction z in the form of a travelling wave excited in the 
desired mode by an external high frequency source. This wave passes 
through the input window 26 then is matched to the impedance of the 
resonating cavity formed by the two electrodes 20, 21 by means of horn 24. 
The amplified wave is then fed into a guide, not shown, leading for 
example to an antenna, through a horn 25 and a window 27. In this case, 
the collector 3 is formed by a reflector closed at the upper part of the 
resonating structure. 
The above described amplifier has the disadvantage of being reciprocal with 
respect to the input and the output that is to say that it is electrically 
symmetrical with respect to the direction of propagation and also 
amplifies the signals reflected towards the input of the tube because of 
matching which is always imperfect in the output guide. 
FIG. 4 shows an embodiment for overcoming this drawback. 
In this embodiment, electrodes 20 and 21 are offset by an angle .alpha. 
with respect to direction x so that the electrons accelerated by the gun 
in direction x' have also a drift component in direction z equal to 
v.sub.D sin .alpha.. Since the electromagnetic field remains uniform in 
direction x but varies in phase according to an expression of the type cos 
(.omega.t-kz+.phi.), the resonance condition is no longer given by: 
EQU f.sub.o .perspectiveto.lf.sub.c 
but by 
EQU f.sub.o -kVd/2.pi..perspectiveto.lf.sub.c 
(l being the order of the harmonic of the cyclotronic frequency which 
interacts). 
It follows from this latter equation that the resonance depends on the sing 
of k, i.e. on the propagation direction and that said equation, which must 
be fulfilled for amplification to take place, will favor one of the 
propagation directions. 
In the above described embodiments, the electrodes are made preferably from 
copper and the windows from a dielectric material. 
The generators according to the present invention operate like a gyrotron, 
and have then the same applications as the generators of the prior art for 
millimetric waves. They may be used in particular for heating in plasma 
installations, radar transmission, telecommunications, etc.