Title:
RADIO-FREQUENCY PLASMA GENERATOR
United States Patent 3663858
Abstract:
A plasma generator which utilizes radio frequency power and is of the kind having a slotted transmission line consisting of a hollow tubular body electrically matched to an RF generator, in which the hollow body makes up a delay line in which the RF energy is caused to propagate with a time delay in the direction of the axis of said hollow body the radiofrequency power, by enabling heating of the plasma independent of the intensity of a magnetic field used for confining the plasma, provides a highly dense plasma.
US Patent References:
Plasma-beam signal generator
Targ et al. - February 1965 - 3171053
Electronic device for generating or amplifying high frequency oscillations
Agdur - November 1963 - 3111604
Electronic signal generator
Mildoon et al. - July 1958 - 2842712
Fast wave transmission line coupled to a plasma
Napoli et al. - April 1968 - 3378723
Electron beam-plasma device operating at multiple harmonics of beam cyclotron frequency
Gruber et al. - January 1968 - 3363138
Plasma-beam signal generator
Targ et al. - February 1965 - 3171053
Electronic device for generating or amplifying high frequency oscillations
Agdur - November 1963 - 3111604
Electronic signal generator
Mildoon et al. - July 1958 - 2842712
Fast wave transmission line coupled to a plasma
Napoli et al. - April 1968 - 3378723
Electron beam-plasma device operating at multiple harmonics of beam cyclotron frequency
Gruber et al. - January 1968 - 3363138
Application Number:
05/087534
Publication Date:
05/16/1972
Filing Date:
11/06/1970
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
376/123, 376/132, 376/140
International Classes:
H05H1/46; H01J7/46; H01J19/80
Field of Search:
315/3.5,39,39.3,111
US Patent References:
Primary Examiner:
Saalbach, Herman Karl
Assistant Examiner:
Chatmon Jr., Saxfield
Claims:
What is claimed is
1. A plasma generator assembly of the type utilizing radiofrequency power for its heating means and wherein a static coaxial magnetic field is used to confine plasma, comprising: an outer container for plasma; a radiofrequency generator means for supplying radiofrequency power to the plasma; a transmission member connected to said generator means and disposed in the outer container for conveying radiofrequency energy to the plasma, the transmission member including a slotted substantially tubular hollow conductive body; and means to produce a static magnetic field to confine the plasma therein, the assembly characterized in that the arrangement of the slotted hollow body of the transmission member enables polarization of a fringing radiofrequency field to be directed relative to the confining static axial magnetic field.
2. A plasma generator assembly as claimed in claim 1 in which said tubular hollow conductive body is helically slotted, the assembly including coaxial connector matching means to match the radiofrequency generator means with the helically slotted conductive body.
3. A plasma generator assembly as claimed in claim 1 in which said tubular hollow conductive body is helically slotted, the assembly including waveguide input connector means to match the radiofrequency generator means with the helically slotted conductive body.
4. An assembly as claimed in Claim 1 in which said outer container is substantially tubular and is coaxial with said slotted substantially tubular hollow body.
5. An assembly as claimed in claim 9 in which at least the slots of the slotted tubular hollow body are filled with an insulating material.
1. A plasma generator assembly of the type utilizing radiofrequency power for its heating means and wherein a static coaxial magnetic field is used to confine plasma, comprising: an outer container for plasma; a radiofrequency generator means for supplying radiofrequency power to the plasma; a transmission member connected to said generator means and disposed in the outer container for conveying radiofrequency energy to the plasma, the transmission member including a slotted substantially tubular hollow conductive body; and means to produce a static magnetic field to confine the plasma therein, the assembly characterized in that the arrangement of the slotted hollow body of the transmission member enables polarization of a fringing radiofrequency field to be directed relative to the confining static axial magnetic field.
2. A plasma generator assembly as claimed in claim 1 in which said tubular hollow conductive body is helically slotted, the assembly including coaxial connector matching means to match the radiofrequency generator means with the helically slotted conductive body.
3. A plasma generator assembly as claimed in claim 1 in which said tubular hollow conductive body is helically slotted, the assembly including waveguide input connector means to match the radiofrequency generator means with the helically slotted conductive body.
4. An assembly as claimed in Claim 1 in which said outer container is substantially tubular and is coaxial with said slotted substantially tubular hollow body.
5. An assembly as claimed in claim 9 in which at least the slots of the slotted tubular hollow body are filled with an insulating material.
Description:
The subject invention consists of a plasma generator which utilizes radiofrequency power as transferred by means of a transmission line placed internally of a metal tube, in order to obtain a highly dense plasma and to heat it independently from the intensity of the magnetic field used for confining the plasma.
The radiofrequency line is electrically coupled to a generator which produces energy in a frequency band which is extended from a few megacycles to the field of centimetric waves.
Plasma generators utilizing radiofrequency power are well known in the art. They couple radiofrequency power to the plasma by means of a) transmission lines placed internally of a metal tube; b) radiofrequency inductance coils; c) microwave cavity resonators and d) microwave horn radiators.
However, for confining magnetic field strengths higher than those corresponding to ion- or to electron cyclotron resonance, none of said known radiofrequency plasma generators is able to produce: a) high density fully ionized plasma, and b) collisionless heating of otherwise produced high density plasma.
It is the principal object of this invention to provide a simple radiofrequency plasma generator operative at any value of the confining magnetic field strength.
Another object is to provide a plasma generator giving fully ionized plasma for gases of any mass number.
A further object is to provide a high density plasma having a particle number density (N e >10 13 cm - 3 ) of interest for controlled thermonuclear fusion research work.
Further objects of this invention are: a) stabilization of the produced plasma by means of a minimum radiofrequency field configuration; b) easy adaptation of the present radiofrequency plasma source to almost any available device for plasma physics and controlled thermonuclear fusion research work; c) utilization of high levels of radiofrequency power at any available frequency in the frequency range between a few Mc/s and many tens of thousands of Mc/s.
The present invention improves absorption of RF energy by the plasma in such a way that, for a certain RF power level, very high ionization degrees and temperatures for a gas are obtained.
In addition, the present invention permits to obtain discharges of ionized gases in metal tubes which have been filled with gases having various mass numbers and at very low pressure in the order of between 10 - 6 Torr up to the atmospherical pressure. This object can be achieved by virtue of the basic feature of this invention which permits to match high RF power densities per volume unit within tubes having an inside diameter which is much smaller than the wavelength of the matched energy.
The above objects are achieved, according to the invention, by an RF plasma generator which is formed by a hollow cylindrical body having an RF transmission line arranged so as to provide a delay line, so that the RF energy is propagated in the axial direction with a phase velocity which is much lower than the propagation speed of the wave in vacuo.
The hollow body contains a slot of helical configuration, which is wound around the axis of the hollow body and is electrically matched to an RF generator.
The invention is disclosed in the following with the aid of a few practical examples, as shown in the accompanying drawings.
FIGS. 1, 2 and 3 show constructional examples of the invention, with coaxial matching of the input energy, whereas
FIGS. 4 and 5 show constructional examples of the invention with waveguide type input.
The slotted transmission line as disclosed by the present invention is shown in FIGS. 1 to 5. In the present arrangement the transmission line is slotted helically on a tubular cylinder. Radiofrequency power is fed through a matched transition from a coaxial or from a waveguide input connector to one end of the slotted transmission line. The other end of the transmission line may be open- or short-circuited.
The plasma absorption mechanism of the radiofrequency-power fed to the slotted transmission line structure of this invention is believed to depend upon: a) the polarization of the E-field of the radiofrequency power and b) the strength of the fringing E-field across the slots of the slotted transmission line.
In the following, said absorption mechanism will be illustrated describing the distinguishing features of the present slotted-line structure compared with other radiofrequency coils or helical conductors windings used in other devices such as, for example, travelling wave tubes.
Some of said distinguishing characteristics, which are essential properties of the present invention, are:
a. Provision of matched transition from coaxial or wave-guide input connectors to the slotted transmission line of the present wave-coupling devices.
b. Fringing E-Fields of the radiofrequency power travelling along the slotted transmission line have a field strength which is independent of the path of said transmission line.
c. Polarization of the E-Field of the radiofrequency power:
This is very important in any plasma-wave interaction process and can be fixed in a very simple way, in the present invention, by fixing the path of the slotted transmission line. It is thus possible to take account of the particular plasma-wave dispersion relation in order to couple said wave to the plasma. As an example, in the FIGS. 1 to 5 of the present invention, the E-fringing field of the radiofrequency power is mainly directed axially in the same direction of the applied magnetostatic field, like the polarization of an ordinary wave.
It is probably due to this particular characteristic of the present invention, that the plasma can be produced independent of the strength of the confining magnetostatic field, in agreement with the non-dependence of the propagation of an ordinary wave from the magnetostatic field strength.
d. Provision of a minimum radiofrequency field configuration inside the slotted cylinder:
When the diameter of the slotted cylinder has cut-off dimensions for the wavelength of the radiofrequency power, the only possible radiofrequency field is the fringing field across the slots of the slotted transmission line. Therefore a minimum r.f. - field configuration will be established inside the slotted cylindrical structure, in absence of plasma.
In presence of plasma, the minimum r.f. - field configuration is still established even for a structure diameter larger than that corresponding to cut-off. In fact, at the operating plasma particle number density, N e >10 13 cm - 3 , the plasma frequency is much larger than the frequency of the applied radiofrequency power. The radiofrequency power, being polarized like an ordinary wave, is radiated transversally to the applied magnetic field and is thus cut-off at the plasma-boundary. This establishes again the condition of a minimum r.f.-field configuration in presence of plasma.
e. Stabilization of the produced plasma by said minimum r.f.- field configuration.
The absence of this property would cause (like in other known systems of radiofrequency plasma generators) wall losses and instabilities of the produced plasma.
f. Radiofrequency power capabilities of the present devices:
These are such as to allow the coupling of a large amount of radiofrequency power, of the order of many kilowatts per liter of plasma volume. The absorption mechanism of this r.f. power is believed to be due to an increased resistance of the plasma to the alternative fringing fields across the slots.
At large values of said fringing E-fields, the electron velocities are randomized by the alternating radiofrequency fields, thus producing an "effective" collision frequency much higher than that corresponding to a cold plasma at the same neutral gas pressure. This increased r.f. resistivity, in an otherwise collisionless plasma, is believed to be the main absorption mechanism of the wave launched by this structure.
g. Distribution of the radiofrequency power:
The r.f. power is uniformly distributed all around the surface of the plasma. This very important characteristic of the present invention, avoids the instabilities and the losses of other conventional systems like wave-guide radiators, which inject large amount of r.f. power through a small plasma surface.
The radiofrequency power coupled by the structure disclosed in the present invention is absorbed as it travels along the slotted transmission line. A large plasma surface can be enclosed by using more than one of such structures.
h. Discharge parameters:
The present device has been tested for a wide range of neutral gas pressure p o , magnetic field strength B, radiofrequency power level P, plasma diameter D, plasma frequency ω p and exciting frequency ω rf , or wavelength λ rf .
The following table gives the tested ranges of the discharge, but the limitations are only due to the limitations of available test apparatus and energy, used up to date for these tests.
0 <B <30 kG
10 - 6 <P 0 <10 3 torr
10 - 1 <D/ λ <10
10 11 <N e <10 14 cm - 3
1 <P <2 kw
1 <ωp/ω rf <30
1 <ωp /ωrf <10 2
The plasma generator shown in FIG. 1 contains an RF generator 10, which, through a coaxial line as represented in a diagrammatical fashion, 12, is matched to a system which has a solid cylindrical portion, 14, and an inner cylinder, 16, with helical slots, 18. The outer cylindrical portion, 14, is connected to the external line of the coaxial cable 12, and the inner cylindrical portion is connected to the internal line of the coaxial cable 12. The assembly as shown in FIG. 1, and especially the cylinder 16 with its grooves 18, makes up a delay line by whose means the RF as supplied by the generator, 10, is propagated in the direction of the axis, 20, with a diminished velocity. In the interior of the cylinder, 16, a hot plasma having a high density can be obtained with a high efficiency of RF energy conversion, even under the influence of extreme variations of the pressure of the neutral gas and in the presence of wide variations of the axial magnetic field B. This is because the delay line has a very satisfactory match in a very wide band for matching the RF generator, 10, to the plasma.
The external portion, 14, can be formed by a portion of a vacuum container (not shown in the drawings) to which it is connected by end flanges, 22. The magnetic field can be generated by the coils, 24, which have been very diagrammatically shown in FIG. 1. With a system according to FIG. 1, for which the inside diameter of the slotted cylinder 16 was about 3 cms. and with and RF generator 10, having an output power of about 70 watts at 2 Gigahertz, it has been possible to generate a plasma having a density of a few 10 13 cm - 3 . The degree of ionization has been 30 percent and the electronic temperatures, as measured, were of about 5 to 12 eV. The intensity of the axial magnetic field could be varied between 1.5 kiloGauss and the maximum value available was 4.5 kiloGauss. With radiofrequency powers above 150 watts, it has been possible to achieve a full ionization of a neutral gas, e.g. argon, at working pressures of 10 - 4 Torr.
In the applications for discharges at atmospherical pressures, the influence of the magnetic field as regards the priming of the discharge results in very high frequency of the electron-ion collisions. Apparently, the use of an axial magnetic field, in addition to diminishing the losses towards the walls of the plasma generator, lends itself in an outstanding manner to the formation of magnetic nozzles having a controlled rate of flow.
FIG. 2 shows the external portion, 14a and the internal portion, 16a, of a second embodiment of the invention, which differs from that of FIG. 1 only in the method of matching the coaxial line, 12a , the latter being shown in cross-section.
The constructional example of FIG. 3, has, also, a coaxial inlet, 12b , from which start the helical slots 18b , these being symmetrical with respect to a plane containing the center of the axial connector, 12b , and arranged perpendicularly to the axis, 20.
FIG. 4 gives an example of construction of the invention, wherein the energy coming from an RF generator is matched to the inner portion, 16c , by the waveguide 12c , and is terminated at the slot 18c . The inner portion is still shielded by a continuous The constructional example of FIG. 4 is especially suitable for the case where the diameter D of the internal cylinder 16c is greater than the RF wavelength.
FIG. 5 shows a constructional example which is essentially akin to that of FIG. 4. The difference consists in that, whereas the slot 18c of FIG. 4 is a continuous helix fed at the center, the slot 18d of FIG. 5 comprises two helical grooves which are symmetrical with respect to the feeding point and thus have opposite directions of winding relative to the axis, 20.
The constructional examples as described above, can, of course, be modified without departing from the scope of the invention. While the portion 16 should be constructed with a conductive material, at least on its surface, the portion 14 can be made either of a conductive or nonconductive material, and possibly it may even be dispensed with completely. The portions 14 and 16 need not take a cylindrical shape but they can have the shape of conical sector, parabolic sectors prisms, pyramids and others. The slot 18 can be open or can be filled with dielectric material(25).
In the latter instance, the portion 16 can become an integral part of the vacuum system for the discharges at low pressure or it can become an integral part of a portion of the discharge duct for residual gases, for example for discharges at high pressures, in the order of the atmospherical pressures.
The radiofrequency line is electrically coupled to a generator which produces energy in a frequency band which is extended from a few megacycles to the field of centimetric waves.
Plasma generators utilizing radiofrequency power are well known in the art. They couple radiofrequency power to the plasma by means of a) transmission lines placed internally of a metal tube; b) radiofrequency inductance coils; c) microwave cavity resonators and d) microwave horn radiators.
However, for confining magnetic field strengths higher than those corresponding to ion- or to electron cyclotron resonance, none of said known radiofrequency plasma generators is able to produce: a) high density fully ionized plasma, and b) collisionless heating of otherwise produced high density plasma.
It is the principal object of this invention to provide a simple radiofrequency plasma generator operative at any value of the confining magnetic field strength.
Another object is to provide a plasma generator giving fully ionized plasma for gases of any mass number.
A further object is to provide a high density plasma having a particle number density (N e >10 13 cm - 3 ) of interest for controlled thermonuclear fusion research work.
Further objects of this invention are: a) stabilization of the produced plasma by means of a minimum radiofrequency field configuration; b) easy adaptation of the present radiofrequency plasma source to almost any available device for plasma physics and controlled thermonuclear fusion research work; c) utilization of high levels of radiofrequency power at any available frequency in the frequency range between a few Mc/s and many tens of thousands of Mc/s.
The present invention improves absorption of RF energy by the plasma in such a way that, for a certain RF power level, very high ionization degrees and temperatures for a gas are obtained.
In addition, the present invention permits to obtain discharges of ionized gases in metal tubes which have been filled with gases having various mass numbers and at very low pressure in the order of between 10 - 6 Torr up to the atmospherical pressure. This object can be achieved by virtue of the basic feature of this invention which permits to match high RF power densities per volume unit within tubes having an inside diameter which is much smaller than the wavelength of the matched energy.
The above objects are achieved, according to the invention, by an RF plasma generator which is formed by a hollow cylindrical body having an RF transmission line arranged so as to provide a delay line, so that the RF energy is propagated in the axial direction with a phase velocity which is much lower than the propagation speed of the wave in vacuo.
The hollow body contains a slot of helical configuration, which is wound around the axis of the hollow body and is electrically matched to an RF generator.
The invention is disclosed in the following with the aid of a few practical examples, as shown in the accompanying drawings.
FIGS. 1, 2 and 3 show constructional examples of the invention, with coaxial matching of the input energy, whereas
FIGS. 4 and 5 show constructional examples of the invention with waveguide type input.
The slotted transmission line as disclosed by the present invention is shown in FIGS. 1 to 5. In the present arrangement the transmission line is slotted helically on a tubular cylinder. Radiofrequency power is fed through a matched transition from a coaxial or from a waveguide input connector to one end of the slotted transmission line. The other end of the transmission line may be open- or short-circuited.
The plasma absorption mechanism of the radiofrequency-power fed to the slotted transmission line structure of this invention is believed to depend upon: a) the polarization of the E-field of the radiofrequency power and b) the strength of the fringing E-field across the slots of the slotted transmission line.
In the following, said absorption mechanism will be illustrated describing the distinguishing features of the present slotted-line structure compared with other radiofrequency coils or helical conductors windings used in other devices such as, for example, travelling wave tubes.
Some of said distinguishing characteristics, which are essential properties of the present invention, are:
a. Provision of matched transition from coaxial or wave-guide input connectors to the slotted transmission line of the present wave-coupling devices.
b. Fringing E-Fields of the radiofrequency power travelling along the slotted transmission line have a field strength which is independent of the path of said transmission line.
c. Polarization of the E-Field of the radiofrequency power:
This is very important in any plasma-wave interaction process and can be fixed in a very simple way, in the present invention, by fixing the path of the slotted transmission line. It is thus possible to take account of the particular plasma-wave dispersion relation in order to couple said wave to the plasma. As an example, in the FIGS. 1 to 5 of the present invention, the E-fringing field of the radiofrequency power is mainly directed axially in the same direction of the applied magnetostatic field, like the polarization of an ordinary wave.
It is probably due to this particular characteristic of the present invention, that the plasma can be produced independent of the strength of the confining magnetostatic field, in agreement with the non-dependence of the propagation of an ordinary wave from the magnetostatic field strength.
d. Provision of a minimum radiofrequency field configuration inside the slotted cylinder:
When the diameter of the slotted cylinder has cut-off dimensions for the wavelength of the radiofrequency power, the only possible radiofrequency field is the fringing field across the slots of the slotted transmission line. Therefore a minimum r.f. - field configuration will be established inside the slotted cylindrical structure, in absence of plasma.
In presence of plasma, the minimum r.f. - field configuration is still established even for a structure diameter larger than that corresponding to cut-off. In fact, at the operating plasma particle number density, N e >10 13 cm - 3 , the plasma frequency is much larger than the frequency of the applied radiofrequency power. The radiofrequency power, being polarized like an ordinary wave, is radiated transversally to the applied magnetic field and is thus cut-off at the plasma-boundary. This establishes again the condition of a minimum r.f.-field configuration in presence of plasma.
e. Stabilization of the produced plasma by said minimum r.f.- field configuration.
The absence of this property would cause (like in other known systems of radiofrequency plasma generators) wall losses and instabilities of the produced plasma.
f. Radiofrequency power capabilities of the present devices:
These are such as to allow the coupling of a large amount of radiofrequency power, of the order of many kilowatts per liter of plasma volume. The absorption mechanism of this r.f. power is believed to be due to an increased resistance of the plasma to the alternative fringing fields across the slots.
At large values of said fringing E-fields, the electron velocities are randomized by the alternating radiofrequency fields, thus producing an "effective" collision frequency much higher than that corresponding to a cold plasma at the same neutral gas pressure. This increased r.f. resistivity, in an otherwise collisionless plasma, is believed to be the main absorption mechanism of the wave launched by this structure.
g. Distribution of the radiofrequency power:
The r.f. power is uniformly distributed all around the surface of the plasma. This very important characteristic of the present invention, avoids the instabilities and the losses of other conventional systems like wave-guide radiators, which inject large amount of r.f. power through a small plasma surface.
The radiofrequency power coupled by the structure disclosed in the present invention is absorbed as it travels along the slotted transmission line. A large plasma surface can be enclosed by using more than one of such structures.
h. Discharge parameters:
The present device has been tested for a wide range of neutral gas pressure p o , magnetic field strength B, radiofrequency power level P, plasma diameter D, plasma frequency ω p and exciting frequency ω rf , or wavelength λ rf .
The following table gives the tested ranges of the discharge, but the limitations are only due to the limitations of available test apparatus and energy, used up to date for these tests.
0 <B <30 kG
10 - 6 <P 0 <10 3 torr
10 - 1 <D/ λ <10
10 11 <N e <10 14 cm - 3
1 <P <2 kw
1 <ωp/ω rf <30
1 <ωp /ωrf <10 2
The plasma generator shown in FIG. 1 contains an RF generator 10, which, through a coaxial line as represented in a diagrammatical fashion, 12, is matched to a system which has a solid cylindrical portion, 14, and an inner cylinder, 16, with helical slots, 18. The outer cylindrical portion, 14, is connected to the external line of the coaxial cable 12, and the inner cylindrical portion is connected to the internal line of the coaxial cable 12. The assembly as shown in FIG. 1, and especially the cylinder 16 with its grooves 18, makes up a delay line by whose means the RF as supplied by the generator, 10, is propagated in the direction of the axis, 20, with a diminished velocity. In the interior of the cylinder, 16, a hot plasma having a high density can be obtained with a high efficiency of RF energy conversion, even under the influence of extreme variations of the pressure of the neutral gas and in the presence of wide variations of the axial magnetic field B. This is because the delay line has a very satisfactory match in a very wide band for matching the RF generator, 10, to the plasma.
The external portion, 14, can be formed by a portion of a vacuum container (not shown in the drawings) to which it is connected by end flanges, 22. The magnetic field can be generated by the coils, 24, which have been very diagrammatically shown in FIG. 1. With a system according to FIG. 1, for which the inside diameter of the slotted cylinder 16 was about 3 cms. and with and RF generator 10, having an output power of about 70 watts at 2 Gigahertz, it has been possible to generate a plasma having a density of a few 10 13 cm - 3 . The degree of ionization has been 30 percent and the electronic temperatures, as measured, were of about 5 to 12 eV. The intensity of the axial magnetic field could be varied between 1.5 kiloGauss and the maximum value available was 4.5 kiloGauss. With radiofrequency powers above 150 watts, it has been possible to achieve a full ionization of a neutral gas, e.g. argon, at working pressures of 10 - 4 Torr.
In the applications for discharges at atmospherical pressures, the influence of the magnetic field as regards the priming of the discharge results in very high frequency of the electron-ion collisions. Apparently, the use of an axial magnetic field, in addition to diminishing the losses towards the walls of the plasma generator, lends itself in an outstanding manner to the formation of magnetic nozzles having a controlled rate of flow.
FIG. 2 shows the external portion, 14a and the internal portion, 16a, of a second embodiment of the invention, which differs from that of FIG. 1 only in the method of matching the coaxial line, 12a , the latter being shown in cross-section.
The constructional example of FIG. 3, has, also, a coaxial inlet, 12b , from which start the helical slots 18b , these being symmetrical with respect to a plane containing the center of the axial connector, 12b , and arranged perpendicularly to the axis, 20.
FIG. 4 gives an example of construction of the invention, wherein the energy coming from an RF generator is matched to the inner portion, 16c , by the waveguide 12c , and is terminated at the slot 18c . The inner portion is still shielded by a continuous The constructional example of FIG. 4 is especially suitable for the case where the diameter D of the internal cylinder 16c is greater than the RF wavelength.
FIG. 5 shows a constructional example which is essentially akin to that of FIG. 4. The difference consists in that, whereas the slot 18c of FIG. 4 is a continuous helix fed at the center, the slot 18d of FIG. 5 comprises two helical grooves which are symmetrical with respect to the feeding point and thus have opposite directions of winding relative to the axis, 20.
The constructional examples as described above, can, of course, be modified without departing from the scope of the invention. While the portion 16 should be constructed with a conductive material, at least on its surface, the portion 14 can be made either of a conductive or nonconductive material, and possibly it may even be dispensed with completely. The portions 14 and 16 need not take a cylindrical shape but they can have the shape of conical sector, parabolic sectors prisms, pyramids and others. The slot 18 can be open or can be filled with dielectric material(25).
In the latter instance, the portion 16 can become an integral part of the vacuum system for the discharges at low pressure or it can become an integral part of a portion of the discharge duct for residual gases, for example for discharges at high pressures, in the order of the atmospherical pressures.