EP2103198A1

EP2103198A1 - Plasma accelerator arrangement

Info

Publication number: EP2103198A1
Application number: EP07846645A
Authority: EP
Inventors: Hans-Peter Harmann; Norbert Koch; Günter KORNFELD
Original assignee: Thales Electron Devices GmbH
Current assignee: Thales Electronic Systems GmbH
Priority date: 2006-12-15
Filing date: 2007-11-17
Publication date: 2009-09-23
Anticipated expiration: 2027-11-17
Also published as: EP2103198B1; WO2008071287A1; DE102006059264A1

Abstract

The invention relates to a plasma accelerator arrangement with electrostatic acceleration of ions by an electric field running through a plasma chamber essentially parallel to the beam direction with a plasma geometry of the cylindrical type with a preferably circular cross-section. Various measures to reduce divergence of the emitted plasma beam are disclosed, which may be applied individually or advantageously in combination.

Description

Plasma accelerator arrangement

The invention relates to a plasma accelerator arrangement for generating a directed plasma jet.

Plasma accelerator arrangements are of particular importance in spacecraft drives. For example, electrothermal plasma accelerators which emit gas pulses by means of electrical discharges are known. Other pulsed plasma accelerators generate an arc in a chamber penetrated by a magnetic field. Likewise pulsed, plasma patterns operate with magnetic acceleration of a concentrated plasma ring.

An important group among the plasma accelerators are the embodiments with acceleration of ions by an electrostatic field, wherein ions are generated and accelerated in an electrostatic field by ionization of a working gas in an open-ended cavity called a plasma chamber or ionization chamber.

Commonly used are in particular so-called Hall accelerator with annular plasma chamber and a magnetic field which extends substantially radially through the annular plasma chamber, and an electrostatic acceleration field between an anode in the plasma chamber and a cathode disposed outside the plasma chamber, which also serves as an electron source.

Another group of plasma accelerators, in contrast to the annular geometries of the Hall accelerators, has chamber geometries with a simply connected, in particular circular, cross-sectional area in FIG a sectional plane transverse to the beam direction of the plasma jet. The longitudinal axis of the plasma chamber running parallel to the beam direction in the longitudinal direction lies within the cross-sectional area. The plasma jet is formed in an area around the central longitudinal axis of the chamber. The cross-sectional area is typically substantially uniform in the longitudinal direction, which is why accelerators of such geometry are also referred to as cylindrical accelerators.

Among the cylindrical accelerators, the Kaufman type accelerators at the exit of the plasma chamber have gratings spaced apart in the beam direction, between which there is an electrical voltage which accelerates ions passing through the gratings.

One of them fundamentally different cylindrical accelerator is known from US 6,448,721 B2. There is u. a. a magnet arrangement with two magnet coils is provided, which generate a magnetic field in the chamber, which leads in the anode region from the central longitudinal axis of the plasma chamber starting to a circular surrounding the plasma chamber magnetic pole and in the region of the output of the plasma chamber has the characteristic of the FeI- of a toroidal coil.

A similar geometry with a first center magnetic at the foot of a cylindrical plasma chamber and a second wall of the plasma chamber surrounding magnetic pole is made of "Plume Measurements and Miniaturazition of the Hall Thrusters with Circular Cross-sectional Discharge Chambers" of Shi rasaki and Tahara, 29 ^th Electric Propulsion Conf., Princeton, 2005. The annular magnetic pole may be formed by soft iron magnetic shoes or by radially magnetized permanent magnet segments. Yet another embodiment of a cylindrical thruster is described in DE 101 30 464 A1 and has a magnet arrangement at least two consecutive pole changes in the longitudinal direction magnetic ring arrangements surrounding the plasma chamber and / or in the region of the output of the plasma chamber a permanent magnet ring surrounding the plasma chamber with longitudinally spaced magnetic poles on. The permanent magnet ring generates a special form of the magnetic field.

The annular chamber geometries form between the inner and outer chamber wall an annular channel, which is penetrated by a radial magnetic field through which electrons move as annular drift currents. The cylindrical chamber geometries have substantially different magnetic fields and motion patterns of the electrons and ions, so typically the design features between electrostatic thrusters of different chamber geometries are not interchangeable.

The shape of the magnetic field is typical of the different modes of operation of the different types. It is stated in US Pat. No. 6,448,721 B2 that a potential gradient between magnetic field lines can be generated by means of intermediate electrodes and such a potential gradient can be placed close to an annular ionization zone at the anode. Furthermore, the toroid around the output should support a focusing of the plasma jet.

US 2,956,666 describes an electrostatic accelerator having an acceleration grid at the exit of a plasma chamber and a magnetic field extending in the beam direction. GB 2 295 485 A describes an ion accelerator arrangement with an acceleration grid, which is followed by a brake grid in the beam direction. That of a plurality of elongated coils on the outside of the ionization chamber magnetic field extends in the ionization chamber of a central magnetic pole at a cathode in the direction of a plasma chamber surrounding the second magnetic pole obliquely outward.

US Pat. No. 5,847,493 shows a Hall plasma accelerator with a magnet arrangement which, in addition to the toroidal coils for generating the essentially radial magnetic field, has a plurality of further coils distributed through the annular chamber on the outer circumference of the chamber, by means of which the rotational symmetry of the magnetic field is specifically disturbed and the mean beam direction of the plasma jet can be influenced.

In an ion source according to US Pat. No. 6,060,836, a short-length plasma chamber is surrounded by permanent magnets whose poles are radially spaced and which produce a cusp field in the plasma chamber. Ions are extracted from the plasma chamber by means of an electrode spanning the output of the plasma chamber.

GB 2 295 485 A shows a cylindrical plasma chamber containing an annular anode at the chamber wall and an acceleration grid spanning the chamber exit. A magnetic field extends obliquely outwardly from an inner pole at the axis of the chamber towards the anode.

US Pat. No. 3,735,591 discloses an arrangement with a coil arrangement around a cylindrical anode forming the wall of a plasma chamber, producing a substantially axial magnetic field in which a central cylinder located within the plasma chamber comprises a pole piece and a ring at the exit the plasma chamber forms another pole piece. In DE 12 22 589 a plasma accelerator is described in which in an ionization chamber an accelerated electron beam is introduced through an anode and passed through an axial magnetic field of a toroidal coil on the axis. An axial electrostatic field accelerates generated ions in the direction of an exit opening of the plasma chamber. An arrangement with an electron beam supplied from the anode side is also known from DE 108 28 704 A1, in which a generation of the beam-guiding magnetic field is also provided by a series of permanent magnet rings with alternating polarity.

US 6,448,721 shows a plasma accelerator having a cylindrical chamber geometry in which a coil arrangement generates a magnetic field leading from an inner magnetic pole at the longitudinal axis of the chamber obliquely outwards to an annular second magnetic pole. Another annular coil surrounding the chamber may be provided to reinforce radial magnetic field components. Acceleration of ions occurs electrostatically in a field between an anode at the bottom of the chamber and a cathode located laterally outside the chamber.

DE 101 30 464 A1 describes a plasma accelerator arrangement in which a multi-stage magnet arrangement is provided with longitudinally successive alternating pole changes, which preferably comprises permanent magnet rings with magnetic poles arranged opposite to one another in the longitudinal direction.

Plasma accelerators with cylindrical chamber geometry are advantageous from the transverse dimensions of the chamber. The present invention has for its object to further improve such a plasma accelerator.

Solutions according to the invention are described in the independent claims. The dependent claims contain advantageous refinements and developments of the invention.

For the description of the invention, instead of the cylindrical chamber geometry, the term of a chamber geometry with a simply coherent cross-sectional area of the plasma chamber is chosen, since advantageous embodiments of the invention also include chamber geometries widening in the beam direction. The simply connected cross-sectional area is preferably a circular area. Single continuous planar cross-sectional areas here have an unbroken boundary line, whereas the annular cross-sectional areas of the central inner-body Hall configurations each have an inner and an outer uninterrupted boundary line.

Common to the various solution variants of the present invention is the form of the electrostatic acceleration field, which is disposed between a cathode located in the beam direction at or preferably after the exit of the plasma chamber and an anode disposed opposite the exit of the plasma chamber at the foot of the plasma chamber substantially parallel to the plasma chamber extending in the longitudinal direction of the central longitudinal axis of the plasma chamber. Also common to all solution variants is a cusp structure of the magnetic field at a pole, which faces away from the exit of the plasma chamber, of a magnetic ring arrangement arranged in front of the exit of the plasma chamber with magnetic poles spaced apart in the longitudinal direction. In a first solution variant, an advantageous field shaping is provided at the outlet of the plasma chamber with a novel course of a characteristic area designated below as the neutral surface of a magnetic field determined by a magnetic ring arrangement, in particular a permanent magnet ring with longitudinally spaced magnetic poles. It can be shown that such a permanent magnet ring at the outlet of the plasma chamber, as it is already present in DE 101 30 464 A1, generates at its end pointing in the beam direction a magnetic field which is within the plasma chamber, ie radially inside the plasma chamber Magneting, closed magnetic field lines and on the outside of the plasma chamber, ie radially outside the plasma chamber and the magnet ring, closed magnetic field lines and between these two groups of magnetic field lines has a fictitious separation surface which spans the output port of the plasma chamber and in the context of the invention as neutral - Area is designated. This neutral surface strikes the magnetic pole along a line designated below as an entry line or a pole shoe arranged thereon. The entry line is in rotationally symmetrical design of the magnet assembly in a plane perpendicular to the central longitudinal axis of the arrangement plane. In the case of an entry line not lying in a plane, its mean position in the longitudinal direction is assumed to be corresponding. The magnetic field of a single toroid or the radial magnetic field of a Hall thruster with annular chamber geometry does not show such a neutral surface spanning the chamber exit.

While in the arrangement known from DE 101 30 464 A1, such a neutral surface shows a clear bulge against the longitudinal position of the entry line in the jet direction of the plasma jet, according to the present invention it is intended to form the magnetic field in this region in such a way that such a neutral surface is inside a region lying in the longitudinal direction, which in Beam direction maximally 10% of the largest diameter of the entry line offset from the longitudinal position of the entry line offset in the beam direction, ie that the maximum permissible according to this design curvature of the neutral surface in the beam direction compared to the known arrangement is substantially reduced.

It turns out, surprisingly, that such a magnetic field shaping leads to a significantly reduced beam divergence and thus to a higher efficiency of the arrangement as a drive in spacecraft.

Advantageously, the bulging of the neutral surface in the beam direction of the plasma jet is completely canceled. In a preferred embodiment, the neutral surface is retracted against the beam direction against the longitudinal position of the entry line in the plasma chamber, which is hereinafter also referred to as concave course of the neutral surface in contrast to convex course in the known from DE 101 30 464 A1 arrangement. In each case, the passage region of the ejected plasma jet through the neutral surface, in particular the apex of a curvature of the neutral surface, typically lying on the central longitudinal axis of the plasma jet, is decisive.

The magnetic field shaping in the specified manner is the skilled worker with common means, in particular the use of field-shaping pole pieces, variations of the magnetic flux density in the longitudinal direction, etc. possible. Advantageous examples are described with reference to the figures.

Another variant of the solution provides for the plasma chamber in the longitudinal section in front of the exit opening of the plasma chamber to be widened transversely to the beam direction. The chamber geometry can then no longer be considered cylindrical in the strict sense. Surprisingly, leads the expansion of the plasma chamber in the region in front of the exit opening of the chamber does not lead to an expansion, but to a reduction in the divergence of the plasma jet. The widening as an increase in the diameter of the plasma chamber in the longitudinal direction can be linear or non-linear. Advantageously, the cone angle of the expansion in the case of a nonlinear course of the central expansion is at least 5 °, preferably at least 10 ° and at most 30 °, preferably at most 20 °.

The widening extends in the longitudinal direction advantageously only over a part of the longitudinal extent of the plasma chamber. Advantageously, the widening in the longitudinal direction extends over at least the predominant part of the spacing of the magnet poles of the magnet ring arrangement at the exit of the plasma chamber, in particular at least over the entire distance of the magnet poles. In the longitudinal direction of the multistage magnetic ring arrangement around the plasma chamber, the longitudinal region of the widening of the plasma chamber can also continue counter to the beam direction via the magnetic ring stage arranged at the exit of the plasma chamber into the next magnetic ring stage in the direction of the anode.

According to a further advantageous approach, it is provided to form the magnetic field in the plasma chamber such that in a longitudinal region between the two magnetic poles the magnetic ring arrangement arranged at the outlet of the plasma chamber, in which the longitudinal component of the magnetic field predominates over the radial component, over the chamber cross section averaged magnetic flux density decreases asymmetrically against a longitudinal mean longitudinal position. The magnetic field expands vividly in the longitudinal direction. It is surprisingly found that such a widening of the magnetic field leads to a lower divergence of the ejected plasma jet. Preferably, the considered mean longitudinal Area of both magnetic poles of the magnetic ring arrangement spaced by 20% of the length of the magnetic ring assembly of both magnetic poles.

Possibilities for such a design of the magnetic field are known to those skilled in the art. Individual examples are illustrated by the illustrations.

According to a further solution variant, it is provided to make the magnetic flux outside the plasma chamber within the magnetic ring arrangement decreasing in the beam direction. The magnetic ring arrangement preferably contains a permanent magnet ring with longitudinally oppositely directed magnetic poles. Possibilities for such a design of the internal magnetic flux are familiar to the person skilled in the art and can in particular an inhomogeneous magnetization of a permanent magnetic material and / or decreasing in the beam direction cross-sectional area of a permanent magnet ring as internal properties of a permanent magnet ring, but also a magnetic shielding device on the outside of the magnetic ring arrangement and or include a magnetic shorting arrangement on the outside of the magnet assembly, each having a longitudinally varying action.

The invention is illustrated below with reference to preferred embodiments with reference to the figures still in detail. Showing:

1 shows a cross section through a known arrangement,

2 shows an embodiment with reduced bulging of the neutral surface of the magnetic field, Fig. 3 is a counter to the beam direction vaulted course of

Neutral area

4 a widening chamber geometry,

5 shows a ring magnet with magnetic shield,

6 shows a magnetic field widening in the plasma chamber,

Fig. 7 shows another embodiment.

In Fig. 1, an arrangement known from DE 101 30464 A1 is schematically sketched as a sectional image in a cutting plane containing the central longitudinal axis of the plasma chamber, wherein due to the rotational symmetry of the plasma chamber PK with the chamber wall KW and the magnet arrangement about the central longitudinal axis ML for clarity only one Half of the sectional image in the figure is drawn to the right of the central longitudinal axis.

The central longitudinal axis ML coincides with an indicated z-axis, which indicates the beam direction of the ejected plasma jet. The plasma jet is spatially distributed around the z-axis and diverges after the exit of the plasma chamber at zA.

Outside the plasma chamber, a common cathode KA is indicated, which serves as a source of primary electrons for igniting the plasma and for neutralizing the ejected plasma jet. Between the cathode KA and at the foot of the plasma chamber the output opposite anode arranged AN exists an electrostatic field, which within the Plasma chamber in a known manner is substantially parallel to the z-axis.

The magnet arrangement arranged outside the chamber wall KW is constructed in three stages with a first magnetic ring MR1, a second magnetic ring MR2 and a third magnetic ring MR3, which surround the plasma chamber and whose magnetic poles are offset from each other in the z-direction. The pole alignment of the successive magnetic rings made of permanent magnetic material is alternately set opposite, so that in each case the same poles NN, SS immediately adjacent magnetic rings facing each other, resulting in the areas between the first and second magnetic ring and between the second and third magnetic ring respectively cusp structures of the magnetic field in the chamber. Advantageously, pole pieces P12 or P23 can be inserted between successive magnet rings. The magnet rings can be magnetized in the same or different strengths.

The magnetic rings MR1, MR2, MR3 are substantially the same length in the longitudinal direction. The length LS1 of the first magnet stage of the magnet arrangement is from the beam pointing pole N of the first magnet ring MR1 at ZE to the center of gravity of the magnetic field lines in the pole piece P12, the length LS2 of the second magnet stage from pole piece P12 to pole piece P23 and the length LS3 the third magnetic stage measured from the pole piece P23 to the anode AN. The magnetic stages M1, M2, M3 are correspondingly assigned to the longitudinal areas of LS1, LS2, LS3. The magnetic poles designated N and S can also be reversed. The plasma chamber is circular cylindrical about the central longitudinal axis with a length LS1 + LS2 + LS3 which is greater than the diameter 2RK of the chamber. The magnetic field generated with such a magnet arrangement shows magnetic fields compared to magnetic fields, z. B. with toroids around the plasma chamber and / or with a central pole in the chamber and an annular pole around the chamber and / or annular chamber geomethe and substantially radial magnetic field some peculiarities which hereinafter with particular reference to the first magnetic stage at the output of Plasma chamber are essential.

The magnetic field of the first magnetic stage extends within the plasma chamber in the cusp structure in the pole piece P12 with high density of the magnetic field lines predominantly transverse to the longitudinal direction z. In a central longitudinal region LM between the opposite poles of the first magnetic stage whose magnetic field extends predominantly parallel to the longitudinal direction z, in particular in a central longitudinal region which is spaced from both ends of the first magnetic stage by about 20% of the length LS1 of the first magnetic stage. In this region, the magnetic flux density typically increases in the radial direction toward the chamber wall. The drawn field lines are not to be understood quantitatively. The magnetic field originating from the magnetic pole N of the first magnetic ring MR1 pointing in the beam direction is partially closed by field lines designated Ml by the plasma chamber radially inside the first magnetic ring and partly by magnetic field lines designated ME outside the plasma chamber radially outside the first magnetic ring. The outside closed magnetic field lines are drawn only in their approach. The spatial regions of the magnetic field lines M1 and the magnetic field lines ME are separated from one another by a fictitious separation surface NF, which is referred to below as the neutral surface. This neutral surface spans the exit opening of the plasma chamber and strikes the magnetic ring along a line designated as entry line EL. Due to the rotational symmetry of the arrangement is the neutral surface NF also rotationally symmetric and the entrance line forms a circular line in the plane of the magnetic pole at zE. The neutral surface NF is curved toward the plane lying perpendicular to the z-axis at zE, in which the entry line EL lies, by a dimension denoted by WS in the beam direction and intersects the longitudinal axis ML at zS, WS = zS-zE. The radius of the entry line EL about the z-axis is denoted by RE. The ratio WS / RE in the known arrangement is typically between 0.5 and 1. The magnetic field of a ring coil commonly used in the prior art does not show such a neutral area spanning the chamber exit.

The explanations and definitions made above with reference to the embodiment of the prior art are taken in the following embodiments of the present invention, unless otherwise stated.

2, a first embodiment of the invention is sketched, in which the bulge of the neutral surface NF denoted by WS in beam direction z is considerably reduced with respect to the plane of the entry line EL of this neutral surface into the magnet arrangement with respect to the field sketched in FIG. It turns out that with such a reduced bulge, which can be continued to a substantially even neutral surface or even to a counter to the beam direction concave curved neutral surface continues, in conjunction with the magnetic field in the output at the magnetic stage a significantly reduced divergence of the ejected plasma jet, without giving up the advantages of the magnetic field arrangement known from the prior art according to FIG. The magnetic field in the magnetic stage in front of the outlet is characterized in particular by two magnetic poles N and S annularly surrounding the plasma chamber PK and spaced apart from each other in the longitudinal direction z, which are preferably provided by a magnet Longitudinal z magnetized ring magnet body MR1 are formed. The magnetic field within the chamber extends in a central region predominantly parallel to the longitudinal direction and extends in the region between the first and the second magnetic stage M1, M2 in the region of the pole piece P12 substantially radially. In particular, the magnetic field between the first magnet stage with magnet ring arrangement MR1 and the second magnet stage with magnet ring arrangement MR2 forms a cusp structure CS, as known per se from the prior art. In the region of the magnetic pole N of the magnet ring arrangement MR1 pointing in the beam direction, the field lines are deflected away from the longitudinal axis parallel to the longitudinal axis ML away from the center and extend in the sketched embodiment with the pole piece on this to substantially radially. The ratio of the size WS in the z-direction to the diameter 2RE of the entry line EL is advantageously at most 0.1. In the case of a concave indentation of the neutral surface NF against the direction of the jet, the value for WS should be regarded as negative, so that regardless of the amount of the concave concavity, the aforementioned relation WS / 2RE <0.1 always applies. For the generation of a magnetic field with the reduced curvature WS described, various measures from the theory and practice of magnetic field shaping are known to the person skilled in the art. In the example outlined, the use of a soft-magnetic pole piece PSA on the magnetic pole N pointing in the beam direction of the magnetic ring arrangement MR1 is provided as an advantageous measure.

A further contribution to reducing the bulge WS of the neutral surface NF can be done by dimensioning the magnetic stage at the outlet of the plasma chamber in such a way that the distance of the magnetic poles or when using pole pieces, the distance of the corresponding longitudinal positions on the pole pieces as the length LS1 of the magnetic stage Differing from the state of the art according to FIG. 1, larger than the diameter of the plasma chamber, preferably greater than 1, 5 times the diameter of the plasma chamber is selected.

FIG. 3 shows a section of an embodiment of the magnetic field in the region of the output of the plasma chamber, in which the described neutral surface NF in the region of the central longitudinal axis ML is set back against the plane of the entry line of the neutral plane at z = zE, counter to the beam direction z = zS , The neutral surface is arched concavely against the beam direction.

4 shows a further measure for the advantageous shaping of the magnetic field, in particular in a central longitudinal region LD between longitudinal positions Z1 and Z2. In accordance with the embodiment according to FIG. 2, a pole piece PSA is arranged in FIG. 4 at the pole N of the magnetic ring arrangement MT1 pointing in the beam direction.

In the longitudinal region LD two longitudinally spaced field positions F1 and F2 are considered. According to an advantageous variant of the present invention, the magnetic field between the longitudinally spaced opposite magnetic poles of the magnetic ring arrangement at the outlet of the plasma chamber in the central longitudinal region LD, for which preferably a longitudinal region at a distance of about 20% of the length of the magnetic stage of the two Magnet poles is considered, in the direction Z1 widening in the longitudinal direction in the longitudinal direction in the sense that the corresponding FeId- lines in the field region F1 relative to the center of the magnetic stage M1 in the longitudinal direction symmetrically to F1 lying field region F2 radially further outward and Thus, the magnetic flux density and the total magnetic flux at least in a predominant radial region of the diameter of the plasma chamber about the central longitudinal axis ML in Strahlrich- decrease weight. It turns out that such a divergence of the magnetic field lines in such a central longitudinal region surprisingly leads to a reduced divergence of the ejected plasma jet.

For the formation of such a magnetic field course, the skilled person in turn, various measures are known. In the embodiment outlined, it is provided to achieve such field shaping by reducing the magnetic flux within the annular body of the permanent magnet in a permanent ring MT1 as magnetic ring arrangement of the magnetic stage at the outlet of the plasma chamber in the longitudinal direction. This can advantageously be done as outlined by the fact that the radial thickness of the magnet ring MT1 decreases in the longitudinal direction of the pole S in the direction of the pole N, wherein a spatially homogeneous magnetization of the permanent ring MT1 was assumed.

In Fig. 5, an arrangement is sketched, which in a central longitudinal region LD has a comparable to Fig. 4 field profile. To achieve the field profile is in this example on the outside of a magnet ring MR1 with a constant radial thickness, a magnetic shield AM z. B. in the form of a soft magnetic material attached, the shielding effect, z. B. by increasing radial thickness, increases in the beam direction z. The longitudinally varying shielding exhibits a similar effect to the decreasing radial thickness of the magnetic ring MT1 according to FIG. 4. The geometry of the permanent magnet ring according to FIG. 3 and the magnetic shielding according to FIG. 5 can be implemented particularly advantageously together.

Fig. 6 shows an arrangement with expanding geometry of the plasma chamber. The chamber wall KW is assumed to be cylindrical in an anode-assigning section as in FIG. In the magnetic stage at the output of Plasma chamber widens the chamber wall KW radially from an initial radius RC to a radius RM at the output of the plasma chamber. The radial expansion RM - RC is advantageously in the range between 5% and 75% of RM. It can be seen that the widening of the plasma chamber via a magnetic field M1 at the output of the plasma chamber, in particular at the output of the plasma chamber, contributes to reducing the divergence of the ejected plasma jet. The expansion of the plasma chamber does not necessarily extend over the full length of the magnetic stage M1 at the output of the plasma chamber, but may also continue in the direction of the anode in the magnetic stage M2. Preferably, at least in

Longitudinal portion of the magnet assembly at the anode, according to Fig. 1 LS3 of the magnetic stage M3, the plasma chamber is made substantially cylindrical.

A magnetic ring arrangement MS1 in the magnetic stage at the exit of the plasma chamber is embodied in the example sketched in FIG. 6 as a sequence of magnetic rings with an inner diameter progressing in the longitudinal direction. Such a magnet arrangement can advantageously contribute to the effect of the widening field between the longitudinally-spaced end poles of the magnetic ring arrangement MS1 between regions F2 and F1 as described with reference to FIG. 4. In the arrangement according to FIG. 6, a pole piece PSA is again provided on the terminal pole of the magnetic ring arrangement MS1 pointing in the beam direction.

In Fig. 7 an advantageous embodiment is sketched, in which in conjunction with a widening of the chamber wall, a magnet assembly MV1 is provided, which has a decrease in the magnetic flux within the magnet assembly in the longitudinal direction z in the lying at the output of the plasma chamber magnetic stage. This is achieved in the example outlined in that, similar to FIG. 6, a plurality of magnet rings are arranged sequentially in the z-direction. are net, but which have different, in the z-direction gradually decreasing radial wall thicknesses. The effect of the radial widening of the magnetic field in the central longitudinal region described with reference to FIG. 5 is further enhanced here, and the neutral surface NF shows the concave concavity, as described with respect to FIG. 2, against the jet direction. Instead of the stepped course of the magnetic ring arrangement MS1 in FIG. 6 or MV1 in FIG. 7, a smoothed or continuous course of the inner and / or outer wall surfaces of these magnetic ring arrangements can also be provided.

The features indicated above and in the claims, as well as the features which can be seen in the figures, can be implemented advantageously both individually and in various combinations. The invention is not limited to the exemplary embodiments described, but can be modified in many ways within the scope of expert knowledge. In particular, the various measures can also be realized in an advantageous combination, even in other combinations than indicated in the examples outlined. The various measures can be mutually beneficial complement.

Claims

Claims:

A plasma accelerator arrangement for producing a directed plasma beam having a plasma chamber which has a single coherent

Has a cross-sectional area and includes a longitudinally extending central longitudinal axis of the plasma jet, with an at least predominantly to the central longitudinal axis parallel electrostatic acceleration field and with a magnet arrangement which at least in the output region surrounding the plasma chamber magnetic ring arrangement having longitudinally spaced magnetic poles and a magnetic field in the region of the output of the plasma chamber which has a neutral area spanning the exit of the plasma chamber, which encloses a region of magnetic field lines closed within the plasma chamber and a region closed from outside the plasma chamber

Separates magnetic field lines of the magnetic ring assembly and meets in an entry line to the surface of the magnetic ring assembly, characterized in that the intersection of the neutral surface with the central longitudinal axis in the longitudinal direction within a range that ends maximally 10% of the largest diameter of the entry line offset from the entry line in the beam direction.

2. Arrangement according to claim 1, characterized in that the neutral surface in the region of the passage of the plasma jet in the beam direction is not offset with respect to the entry line in the beam direction.

3. Arrangement according to claim 2, characterized in that the neutral surface in the region of the passage of the plasma jet against the entry line offset offset in the direction of the beam.

4. Arrangement according to one of claims 1 to 3, characterized in that the magnetic ring arrangement includes a permanent magnetic ring.

5. Arrangement according to one of claims 1 to 4, characterized in that at least at the pointing in the beam direction magnetic pole, a pole piece is arranged.

6. Arrangement according to claim 5, characterized in that the pole piece projects radially beyond the magnet arrangement into the plasma chamber.

7. A plasma accelerator arrangement for producing a directed plasma jet having a plasma chamber having a single coherent cross-sectional area and including a longitudinal central longitudinal axis of the plasma jet, with an at least predominantly to the central longitudinal axis parallel electrostatic acceleration field and with a magnet arrangement which at least in the region of the output of the plasma chamber a magnetic ring surrounding this with longitudinally spaced magnetic poles and generates a magnetic field in the region of the output of the plasma chamber, which extends within the plasma chamber in a lying between the spaced magnetic poles and longitudinally spaced from the two magnetic poles longitudinal longitudinal region is substantially parallel to the central longitudinal axis, characterized that in a longitudinal direction between the two spaced poles of the magnetic ring assembly lying central longitudinal region with predominantly parallel to the longitudinal component of the closed within the plasma chamber field lines the magnetic see flux density decreases in the beam direction.

8. Arrangement according to claim 7, characterized in that the magnetic ring arrangement contains at least one permanent magnetic ring.

9. Arrangement according to claim 7 or 8, characterized in that soft magnetic pole pieces are provided on the longitudinally opposite magnetic poles.

10.Anordnung according to any one of claims 7 to 9, characterized in that the central longitudinal region is in each case 20% of the length of the magnetic ring arrangement spaced from both magnetic poles.

11.Anordnung according to any one of claims 7 to 10, characterized in that the length of the magnet assembly is at least equal to the diameter of the plasma chamber.

12. A plasma accelerator arrangement for producing a directed plasma jet having a plasma chamber having a single coherent cross-sectional area and including a longitudinal central longitudinal axis of the plasma jet, with an at least predominantly to the central longitudinal axis parallel electrostatic acceleration field and with a magnet arrangement which at least in the region of the output of the plasma chamber a magnetic ring arrangement surrounding the latter having longitudinally spaced magnetic poles and generating a magnetic field which runs predominantly parallel to the central longitudinal axis within the plasma chamber in a central longitudinal region located between the spaced magnetic poles and spaced longitudinally from both magnetic poles, characterized in that the plasma chamber mer in a longitudinal direction between the spaced magnetic poles lying area extending in the beam direction widening.

13. Arrangement according to claim 12, characterized in that the expansion amounts to between 5% and 75% of the largest diameter of the plasma chamber.

14. Arrangement according to claim 12 or 13, characterized in that the magnet arrangement is multi-stage in the longitudinal direction with alternating Polaus- direction and the widening of the plasma chamber does not extend over all stages.

15. A plasma accelerator arrangement for producing a directed plasma jet having a plasma chamber having a simply contiguous cross-sectional area and including a longitudinal central longitudinal axis of the plasma jet with an electrostatic acceleration field parallel at least predominantly to the longitudinal axis and with a magnet arrangement at least in the region of the exit of the plasma chamber a magnetic ring surrounding this having longitudinally spaced magnetic poles, characterized in that decreases in a longitudinal direction between the spaced magnetic poles and spaced from the two magnetic poles middle longitudinal region of the magnetic flux within the magnetic ring arrangement in the beam direction decreases.

16. The arrangement according to claim 15, characterized in that the ring magnet assembly includes a permanent magnetic ring magnet.

17. Arrangement according to claim 16, characterized in that the magnetic ring has a decreasing wall thickness at least in a partial section in the longitudinal direction with homogeneous magnetization.

18. Arrangement according to one of claims 15 to 17, characterized in that on the outside of the magnetic ring arrangement, a magnetic shielding device is arranged with varying in the longitudinal direction shielding effect.

19. Arrangement according to one of claims 15 to 18, characterized in that on the outside of the magnetic ring arrangement, a magnetic short circuit arrangement is arranged between longitudinally spaced magnetic poles.

20. A plasma accelerator arrangement having a feature combination of at least two of claims 1, 7, 12 and 15.