FR2543368A1 - Transformer of modes - Google Patents

Abstract

The present invention relates to a transformer of modes making it possible to pass from a circular mode TEmn to a circular mode TEmp with p less than n. This transformer consists of two coaxial waveguides G1, G2 of circular cross section. The inner guide G2 occupies the p<th> zone starting from the axis OO' where in the outer guide G1 the tangential electric field vanishes and thus confines the TEop mode. In a first portion, the two guides have a constant cross section and means e, 10 phase shift by pi /2 the energy transported in the gap between the two guides. In a second portion, the two guides have an increasing cross section and coupling orifices 8 enable all the energy to be concentrated in the inner guide G2.

Description

MODES TRANSFORMER

The present invention relates to a mode transformer.

Mode transformers are well known in the prior art.

laughing We know in particular by the article of HG UNGER entitled "Circular electric wave transmission through serpentine bands", published in the review "The Bell System Technical Journal", Volume XXXVI, September 1957, No 5, transformers of modes constituted by circular guides whose section varies periodically O There are two independent parameters, namely the diameter of the

guide, and the amplitude of the variations in the diameter These trans-

trainers make it possible to switch easily from a TE 02 circu-

lar to a circular TEO 1 mode On the other hand, one encounters difficulties to switch for example from a circular TE 05 mode to a circular TE 01 mode In fact, the TE 0 ,, TE 02, TE 03 and TE 04 modes are generated and it is difficult to keep only the mode

TE, 1 having only two parameters.

The problem that the present invention proposes to solve is the development of a mode transformer which makes it possible to switch from a TE mn mode to a T Emp mode, with p less than N and

this even if N is high and equal for example to 5 or 6.

Some tubes, such as for example gyrotrons or free electron lasers, operate in high modes If we take

for example the case of gyrotrons, it is necessary to pass from the dis-

ponible at the exit of the gyrotron to a field which makes it possible to radiate in a thermo-nuclear machine for example, that is to say that it is necessary to pass to an electric field as uniform as possible to

have high radiation.

The mode transformer according to the invention when it is associated with the gyrotrons makes it possible for example to switch from a TE 05 mode

circular at the exit of the gyrotron in a circular TEO 1 mode.

It is in the interest of keeping the circular T Eo O mode on the path between the gyrotron and the thermo-nuclear machine

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because this course is generally several meters and the mode

TE 01 circular presents only very low losses.

Near the machine, we generally switch from circular TE 01 mode to rectangular or square TE 01 mode using a known mode transformer, the section of which is deformed and

progressively passes from a circular section to a rectan-

gular or square In fact, this TE 01 mode has a simpler field map than the circular TE 01 mode and presents an electric field

more uniform.

The invention relates to a mode transformer receiving a TE mn 'mode and characterized in that it consists of two coaxial waveguides having the same section, the internal guide occupying one of the zones o in the external guide the tangential electric field is canceled out and containing a TE mp mode, with p less than n, and in that these two guides have, in a first part a constant section, means phase shifting by 1 </ 2 in this first part I ' energy transported in the interval between the two guides, and in that these two guides have, in a second part, an increasing section, the internal waveguide comprising, in this second part, coupling orifices making it possible to

concentrate all the energy in the internal guide.

Other objects, characteristics and results of the invention

will emerge from the following description, given by way of example not

limiting and illustrated by the appended figures which represent: FIG. 1, a general diagram showing the position of the mode transformer according to the invention in the case where it is associated with a gyrotron; FIGS. 2 and 3, the configurations of the field at the input of the mode transformer and at the input of the output horn;

Figures 4 and 5, two embodiments of a trans-

mode trainer according to the invention.

In the various figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and

proportions of the various elements are not respected.

FIG. 1 is a general diagram showing the position of the mode transformer according to the invention - in the case where it is

associated with a gyrotron.

From left to right in FIG. 1, there is the gyrotron designated by the reference 1, followed by the mode transformer according to the invention designated by the reference 2, then by a mode transformer according to the known art designated by the reference 3 and finally by -the output horn designated by reference 4 Between the gyrotron 1 and the mode transformer according to the invention 2 may be a

more or less long guide section of uniform section.

We have distinguished three parts in the gyrotron, first the electron gun 5, then the cavity 6 and the adapter 7 This adapter ensures the proper charge of the gyrotron cavity by the output circuit It can be formed by a horn an opening of about ten degrees, which does not modify the transverse distribution of the mode, but transforms the standing wave prevailing in cavity 6 into a traveling wave at the outlet of the adapter The angle of the horn is chosen to achieve the desired adaptation The adapter 7 includes the sealed outlet window 8 ensuring the vacuum in the gyrotron and receives in its middle part the electrons after their passage through

cavity 6.

In FIG. 1, by way of example, the mode transformer 3 is connected to an output horn 4 but the transformer could also for example be connected to a waveguide leading to a

thermo-nuclear machine or any other output circuit.

At the output of the adapter, that is to say at the input of the trans-

mode 2 trainer, the electric field configuration is the same as in cavity 5 of the gyrotron, but is found radially expanded to fill a larger diameter For example, for a 100 -G Hz gyrotron, the diameter of cavity 6 could be about 1.6 cm while the dimensions of the opening of the horn 7 could be 6 x 6 cm We therefore see that these dimensions are much larger than the wavelength which is 3 mm The circuit so can propagate a lot of different modes that is

the interaction with the electrons which must select the chosen mode.

The mode transformers must ensure the passage from the gyrotron cavity to the output circuit without generating undesirable modes in the cavity or in the output circuit.They must also be compatible with the large power reaching for example 200 KV / cm in the cavity and a few kilo-volts per

centimeter at the input of the output circuit.

Figures 2 and 3 show possible configurations of the field at the input of the mode 2 transformer and at the input of the

exit horn 4.

In FIG. 2, a circular TE 03 mode has been represented The circles in solid lines indicate the zones where the electric field is maximum The dotted circles indicate the zones where the field

electric azimuth or tangential is canceled.

In FIG. 3, a T Eo O square mode has been represented.

ity of the electric field is distributed according to a half-sinusoid, that is

that is to say that the field is maximum in the middle of the section and null on

edges.

Figures 4 and 5 show two embodiments of a

mode 2 transformer according to the invention, which in the particular case

cular taken as an example, makes it possible to switch from the circular TE 03 mode prevailing in the adapter 7 of the gyrotron 1 to a TE, circular mode This circular TE 01 mode is transformed into square TE 01 mode by a mode 3 transformer known to the prior art, and which acts as the intermediary between the transformer according to the invention 2 and

the exit horn 4.

On the left part of figure 4, the shape of the electric field lines has been shown and in dotted lines the intersection with the plane of the figure of the cylindrical surfaces o the electric field

azimuthal or tangential vanishes.

The mode transformer according to the invention consists of two coaxial waveguides G 1 and G 2, of circular section The internal guide G 2 occupies one of the zones o in the internal guide G 1

the tangential electric field is canceled out This internal guide G 2 does not

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does not affect the configuration of the fields on condition that its wall is not too thick For example, an internal guide G 2 is used, the thickness of the copper wall of which is 0.5

mm approx.

To obtain a circular TE 01 mode at the output of the transformer according to the invention, the internal guide is placed on the first surface where the tangential electric field is canceled out starting from the axis OO 'The internal waveguide then contains the circular TE 01 mode while the interval between the guides G 1 and G 2

contains the TE 02 circular coaxial mode.

The mode transformer according to the invention in fact comprises two parts In a first part, the two guides have a constant section and means phase shift by - / 2 the energy

transported in the interval between the two guides.

In the example of FIG. 4, the energy transported in the interval between the two guides has been delayed by 7 / E / 2 by using an internal waveguide G 2, the internal diameter of which is smaller in the first part than its smallest internal diameter, at the beginning of the second part and whose thickness remains the same in the first and the second part More generally for guides of any section, the phase shift of 7 C / 2 is achieved using an internal waveguide whose section in the first part of the transformer differs from its most

low at the start of the second part of the transformer.

In a second part, these two guides have an increasing section and coupling orifices 13 made on the internal guide G 2 make it possible to concentrate the energy in the internal guide. The internal guide G 2 can moreover be constituted by a metal grid . It is known that when two coaxial waveguides are available, it is possible to ensure coupling between the two guides by means of a large number of small holes made on the internal guide. guides which causes at certain positions along the axis, the cancellation of

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electromagnetic fields in one of the guides This is what

passes into the second part of the transformer according to the invention.

You have to choose the length of this second part of the trans-

trainer, which is between the planes A and B in Figure 4, as a function of the standardized coupling coefficient <, that is to say as a function of the number and size of the holes 13 of the internal guide G 2, for that at the level of the plane B, at the output of the transformer according to the invention, all the energy which was present in the plane A, at the input of the transformer according to the invention, is concentrated

in the internal waveguide G 1 To make it possible to

center all the energy in the internal waveguide G 2, it is necessary that the waves in the internal waveguide and in the interval between the guides are initially out of phase by 7 C / 2 this is what happens

in the first part of the transformer.

The critical length of the second part of the transformer

also depends on the ratio of the amplitudes of the two waves.

In the second part of the transformer, all the energy that was present at the input of transformer 2, that is to say in plane A, is gradually concentrated in the internal guide G 1 The section of the guides in the second part so as to maintain a reasonable power density, that is to say of the order of 5 to 10 k W / cm 2, in the internal waveguide.In addition, the diameters of the two guides of

so that the phase velocities of the two waves remain close.

At the end of the second part of the mode transformer according to the invention, there is placed in the space between the two coaxial guides an absorbing body 9 although in principle there should be no power in the interval between the guides at

this place.

In Figure 5, there is shown another embodiment of the phase shifter of the first part of the transformer this phase shifter is constituted by at least one cavity 10 opening into the guide

external wave Any known phase shifter can be used at this location.

Thus, one can use a dielectric for example.

In FIG. 5, the end II of the internal waveguide G 2 has been given a rounded shape to minimize reflections at angular points. Bulges 12 are provided on the wall of the external waveguide G 1 to compensate rounding of the internal waveguide.

The preceding description has been given by way of example in the

case of a transformer making it possible to switch from a TE 3 mode

circular to a TE 01 circular mode.

The transformer according to the invention allows the passage of a

mode T Emn any, that is to say of a mode which is not necessary

sarily circular and which may for example be rectangular, square in a TE mode of the same name, m being any integer, p being less than n, and N and p being numbers

integers that can take large values.

In all cases, the mode transformer according to the invention consists of two coaxial waveguides, having the same section, circular, rectangular, square The internal guide occupies one of the zones o in the external guide the electric field tangential is canceled Thus this guide, while the transformer receives a TE mn mode, contains a TE mode, with p less than n, because it is positioned on the p zone from the axis QO 'o the

tangential electric field is canceled out.

In a first part, the two guides have a constant section and means phase shift by tf / 2 the energy transported in the interval between the two guides / This phase shift can be + -r / 2 or </ 2 depending on the modes

and according to the order of the modes.

In a second part, the two guides have an increasing section, and the internal waveguide has coupling orifices allowing all the energy to be concentrated in the guide.

internal wave.

In the case of rectangular waveguides and for certain modes, it happens that the internal waveguide has two of its walls

formed by part of the walls of the external waveguide.

Claims (9)

1 Mode transformer receiving a circular mode TE mn, characterized in that it consists of two coaxial waveguides (G 1, G 2) having the same section, the internal guide (G 2) occupying one of the zones or in the external guide (G 1) the tangential electric field is canceled and containing a mode TE mpy with p less than n, and in that these two guides have, in a first part, a constant section, means ( e, 10) phase shifting by 7 r / 2 in this first part the energy transported in the interval between the two guides, and in that these two guides have, in a second part, an increasing section, the guide of internal wave (G 2) comprising, in this second part, coupling orifices (13) allowing all the energy to be concentrated in the internal guide. 2 Transformer according to claim 1, characterized in that the phase-shifting means of '/ 2 in the first part the energy transported in the interval between the two guides (G 1, G 2) are constituted by a section of the guide d internal wave (G 2) in the first part of the transformer differing from its most low at the start of the second part of the transformer. 3 Transformer according to claim 1, characterized in that the phase-shifting means of (/ 2 in the first part the energy transported in the interval between the two guides are constituted by at least one cavity (10) opening into the guide external wave. 4 Transformer according to one of claims I to 3, charac- terized in that, at the end of the second part of the mode transformer according to the invention, there is placed in the space between the two coaxial guides an absorbent body (9). Transformer according to one of claims 1 to 4, charac- terized in that the internal waveguide (G 2) consists of a metal grid. 6 Transformer according to one of claims I to 5, charac- terized in that the end (11) of the internal waveguide (G 2) o o ' 9 2543368 has a rounded shape and in that the wall of the waveguide external has bulges (12) to compensate for these roundings. 7 Transformer according to one of claims 1 to 6, charac- terized in that the length of the second part of the transformer is determined as a function of the standardized coupling coefficient (X) between the two guides and as a function of the ratio of the amplitudes of the waves in the internal waveguide (G 2) and in the interval between guides. 8 Transformer according to one of claims 1 to 7, charac- terized in that the dimensions of the guides in the second part are chosen so as to maintain a power density of the order of 5 to 10 k W / cm 2 in the internal waveguide and so that the speeds phase of the waves in the internal waveguide and in the interval between the guides are close. 9 Transformer according to one of claims 1 to 8, charac- terized in that the two guides (G 1, G 2) have a circular section, rectangular or square.