FR2760898A1 - ELECTRIC FIELD COUPLING PROBE - Google Patents

Abstract

Electric field coupling probe (20), intended to achieve an improved self-sustaining discharge threshold, which comprises an input for receiving a voltage potential and an output forming an electrode (18) disposed in said electric field, the surface (21) of said electrode (18) is provided to reduce (17; 23) the emission of secondary electrons released by the impact of an electron against said surface, so that the electron emission ratio secondary is kept below the unit.

Description

The present invention relates to electric field coupling probes

  and, more particularly, field coupling probes

  electric with improved self-discharge threshold

  maintained, discharge which is also designated by the

  Anglo-Saxon term of "multipaction".

  Antennas for use in space applications require bandpass filters, or BPF according to the initials of the English term band-pass filter, with high efficiency. These filters not only should operate at high radiofrequency, or RF, power, but should also be able to respond to the severe constraints imposed on the structure of the components to prevent self-sustaining disruptive discharge and generation.

passive intermodulation.

  It is known that bandpass filters which operate at high RF power are susceptible to self-sustained discharge between opposite surfaces in an RF cavity, such as the coupling probe and the

column of agreement.

  The self-sustaining discharge effect is an RF resonant discharge maintained by the emission of secondary electrons from the surfaces of various components. There are three conditions for a self-sustained discharge to occur between two sections or electrodes of a satellite component, between which there is a sufficient potential difference. First, the electrodes must be in a very low pressure environment so that the average free path of the electrons, i.e. the average distance traveled by the electrons without collision, is much greater than the distance between the electrodes. Secondly, the energy of the electrons which are accelerated by the electric field must, on their arrival, be such that the emission ratio of secondary electrons is greater than unity; in other words, the number of electrons released by the impact of an electron on the electrode surface must be greater than one. The secondary electron emission ratio strongly depends on the material of the electrode and the surface conditions. The

  third condition for a self-discharge

  maintained happen is that the transit time of the electrons between the electrodes is in resonance with

the frequency of the applied signal.

  A self-sustaining discharge is initiated when free electrons at a first electrode are accelerated to the other electrode and arrive with sufficient energy to release more electrons from the other electrode. If the resonance condition exists, the impact of the primary electrons coincides with the reversal of the electric field, and both the primary and secondary electrons are accelerated back to the first electrode. If conditions are maintained for many cycles of the electric field, the avalanche phenomenon continues

  and becomes a self-sustaining landfill.

  The self-sustained discharge power threshold is therefore a function of the product f x d where f is the operating frequency and d is the separation between the electrodes. The structure of the components of communications satellites capable of operating at high power without the self-sustained discharge effect occurring depends on the possibility of maintaining sufficiently large intervals inside the components and in the transmission lines. This requires a good knowledge of the secondary electron emittance of the materials and an exact evaluation of the intensity of the electric fields inside the components. Self-sustained discharges can be avoided by proper structure of the components and ensuring good quality workmanship, since contaminated electrode surfaces can significantly reduce the threshold for self-sustained discharge power. Since the risk of self-sustained discharge increases with power, designers have tried

  in the past remedy self discharge

  maintained or to increase the self-sustaining discharge power threshold by increasing the dimensions of the coupling probe surface inside the RF cavity. Unfortunately, due to critical structural requirements, the dimensions of the coupling probe are limited by the dimensions of the cavity as well as by the weight distribution between the various components of the filter structure. For example, in a 0.6 inch or 15.24 mm diameter disc configuration coupling probe, self-sustained discharge effects occur at about 300 watts. So far, one of the ways to increase the self-sustaining discharge power threshold is to increase the area and dimensions of the coupling probe. For example, increasing the diameter of the coupling probe from 0.6 inch to 0.8 inch, i.e. from 15.24 to 20.32 mm can significantly improve the auto discharge power threshold -maintained, that is to say raise it by 300

watts to 600 watts.

  Due to other structural and manufacturing considerations, increasing the dimensions of the probe may require a larger size of the bandpass filter, which may cause a component to be larger and heavier, which may not be

desirable.

  Another prior art method for

  improving self-discharge performance

  maintained by one is to increase the intervals between the probe and the opposite surface. Here, too, the qualities of dimensions and masses are affected. There is therefore a need for a coupling probe structure which overcomes the problems associated with

  Prior art bandpass filters.

  It is therefore a first object of the present invention to provide a coupling probe, to be used in bandpass filters, the self-sustained discharge threshold of which is improved without

  increase the dimensions and mass of the pass filter

bandaged.

  It is a second object of the present invention to provide a coupling probe which is intended to significantly reduce the emittance of secondary electrons, which is known to cause a self-sustaining discharge in structures of

coupling probes.

  It is a third object of the present invention to provide a coupling probe which comprises a perforated plate which significantly reduces the emittance of secondary electrons and therefore increases the self-sustained discharge threshold of the coupling probe. It is a fourth object of the present invention to provide a coupling probe provided with a perforated circular electrode which significantly reduces the emittance of secondary electrons and therefore increases the self-sustained discharge threshold of

the coupling probe.

  To achieve these goals, the present invention provides an electric field coupling probe intended to achieve an improved self-sustained discharge threshold, which comprises an input for receiving a voltage potential and an output forming an electrode arranged in said field. electric, and which is characterized in that the surface of said electrode is provided to reduce the emission of secondary electrons released by the impact of an electron against said surface, so that said secondary electron emission ratio

  be kept below the unit.

  Said surface can be of a wavy configuration for deflecting electrons away from an opposite surface, and it can then be provided in particular that said wavy surface comprises a series of projections of pyramidal configuration, arranged on said surface, and that each protrusion in pyramidal configuration has side walls arranged so as to reflect incoming electrons away from them

of an opposite surface.

  Said surface may comprise a series of perforations, each perforation comprising orifices smaller than the wavelength of the operational frequency at which said electric field coupling probe, and provision may then be made in particular for said perforations to comprise a series orifices arranged transversely over the entire surface of said coupling probe, and that said orifices are of a diameter smaller than the wavelength of the operating frequency of said coupling probe, the area which consists of perforations

  can be at least fifty percent -

  % - of the total area of said probe.

  The advantages of the electric field coupling probe in accordance with the principles of this

  invention are the improved self discharge threshold

  maintained and the increased RF power at which it can operate. For example, the use of a perforated circular plate as an electric field coupling probe offers a self-sustained discharge threshold improvement of three to four dB compared to a conventional non-perforated probe. The presence of a large number of small holes or perforations on the surface of the probe significantly reduces the emittance of secondary electrons, which increases the threshold of self-sustained discharge. The ports are found to have a negligible impact on filter performance

for small RF signals.

  The aims, features and advantages of the present invention set out above as well as

  others will emerge more from the description

  detailed below with reference to the accompanying drawings in which: FIGS. 1a and 1b are cross-sectional views of an RF cavity which illustrate the use of an electric field coupling probe inside the RF cavity; Fig. 2 is a front view of a conventional coupling probe with disc configuration according to the prior art; Fig. 3a is a cross-sectional view of an RF cavity which illustrates the use of an electric field coupling probe according to the present invention; Fig. 3b is a front view of the coupling probe structure according to a first embodiment of the invention; and Figs. 4a and 4b are structures of coupling probes according to other embodiments of the

present invention.

  To facilitate understanding of the present invention, we will first describe, with reference to Figs. 1a, 1b and 2, a prior art electric field coupling probe when used at

inside an RF cavity 11.

  The coupling probe 10 of the prior art, shown in FIG. 1a, has an inlet end 12 and an outlet 13 which forms an electrode arranged in the electric field generated by the RF cavity 11. In the embodiment of FIG. 1a, the coupling probe is positioned at a side wall 14 of the RF cavity 11. In the embodiment of FIG. 1b, the electrode is positioned above the tuning column 15 of the RF cavity 11. The operation of the tuning column 15 is known to those skilled in the art and requires no

description here.

  A front view of the electric field coupling probe 10, shown in FIGS. la and lb is illustrated in FIG. 2. Prior art electric field coupling probes, such as that shown in FIG. 2, are structured so as to form an electrode 16 with a disc configuration capable of being placed at a suitable distance from the upper end or from the upper side of a tuning column as shown in FIGS. la and lb, respectively. Since the surface of the electrode 16 is at a prescribed distance from the surface of the upper end or the upper side of the RF tuner 15, free electrons which are accelerated do not rebound but release, as also shown below. above, electrons

secondary during an impact.

  As noted earlier, when electrodes are used in a very low pressure environment, and the average distance traveled by electrons without collision is much greater than the distance between the electrodes, it can

  occur a self-sustaining discharge effect.

  When the energy of the electrons which are accelerated by the electric field is, on their arrival, such that the emission ratio of secondary electrons is

  greater than unity, a self-discharge effect

  nurtured can also happen. Self-discharge

  maintained between the tuning column 15 and the electrode 13 is launched when electrons at one of the electrodes are accelerated towards the other electrode and arrive there with sufficient energy to release more electrons from the other electrode. If there is a resonance condition and the impact of the primary electrons coincides with the reversal of the electric field, both the primary and secondary electrons are accelerated back to the first electrode. If conditions are maintained for a large number of electrical cycles, the avalanche phenomenon continues and

  becomes a self-sustaining landfill.

  In the embodiment of FIG. 3a, the RF cavity 11 is provided with the coupling probe of the present invention. A top view of the embodiment of a coupling probe used in the

  RF cavity of FIG. 3a is shown in FIG. 3b.

  In this embodiment, several perforations 17 were made in the electrode 18 to allow large quantities of free electrons to pass through the probe through the perforations. The number of holes or perforations as well as the dimensions and the configuration of the holes or perforations can obviously vary. However, it is found that an improvement of 3 to 4 dB is obtained, compared to a conventional non-perforated probe, by using perforations don the area totals fifty percent - 50% - of that of the disc. For example, a 0.6 inch or .24 mm diameter unperforated probe can operate at RF powers up to 300 watts before self-sustained discharge effects appear. On the other hand, an electric field probe modified as shown in FIG. 3b, where at least 50% of the area of the probe is perforated, can operate at RF power levels of 650 watts without self-sustaining discharge effects. So far, the only way to increase RF power for a field probe

  electric, without causing self-discharge effect

  maintained, consisted in increasing the diameter of the probe and / or the interval between probes. With the technique of the present invention, increased RF power may be available without changing the

  dimensions and parameters of an RF cavity.

  The dimensions or diameter of the holes in the coupling probe should generally be smaller than the wavelength of the operating frequency of the probe in order to avoid any change in the electrical performance of the RF cavity and the probe. coupling. Therefore, a wheel configuration structure like that shown in FIG. 4a, in which the openings 19 are smaller than the wavelength of the operating frequency of the coupling probe 20 can be used to obtain a performance similar to that of the perforated disc of FIG. 3b. In the embodiment of FIG. 4b, the disc 21 is not perforated but, on the other hand, has a corrugated surface 22 carrying a series of projections 23 in the form of a pyramid with walls 24 configured at a certain angle, capable of reflecting, at a distance from the opposite electrode surface , all free electrons from electrode 21. The probe can also be square or rectangular, the surface can be concave or convex to reduce the effects of self-sustained discharge, etc. Variants of the preferred embodiments described here will be obvious to those skilled in the art and the embodiment should therefore be considered to illustrate and not limit the invention, the true scope of the invention being set out in the

appended claims.

Claims (6)

  1. Electric field coupling probe (20), intended to achieve an improved self-sustained discharge threshold, which comprises an input for receiving a voltage potential and an output forming an electrode (18) arranged in said electric field and which is characterized in that the surface (21) of said electrode (18) is provided to reduce (17; 23) the emission of secondary electrons released by the impact of an electron against said surface, so that the secondary electron emission ratio is maintained below the unit. 2. Probe (20) according to claim 1, characterized in that said surface (21) is of an undulating configuration (22) for deflecting electrons by them moving away from an opposite surface.   3. Probe (20) according to claim 2, characterized in that said corrugated surface (22) comprises a series of projections (23) of pyramidal configuration, arranged on said surface, and in that each projection (23) of pyramidal configuration has side walls (24) arranged to reflect incoming electrons by them moving away from an opposite surface.   4. Probe (20) according to claim 1, characterized in that said surface comprises a series of perforations (17), and in that each perforation (17) has orifices smaller than the wavelength of the operating frequency to which said probe (20) of   electric field coupling works.   5. Probe (20) according to claim 4, characterized in that said perforations (17) comprise a series of orifices arranged over the entire surface of said coupling probe (20), and in that said orifices (17) are smaller in diameter than the frequency wavelength   of said coupling probe (20).   6. Probe (20) according to claim 5, characterized in that the area which consists of perforations (17) is at least equal to fifty percent (50%) of the   total area of said probe (20).