Patent Number: 051606941
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a fusion reactor having a reaction zone surrounded by a magnetic field with magnetic flux lines which viewed from the reaction zone are curved in a convex manner. THE PRIOR ART Such a magnetic field, surrounding the reaction zone and having magnetic flux lines curved in a convex manner when viewed from the reaction zone, had already been proposed for controlled nuclear fusion by the doyen of nuclear fusion, Edward Teller, during the Sherwood Conference in Princeton U.S.A. in October 1954 because only with such a magnetic field is it possible to achieve stability during the containment--essential for eliminating contacts between reactants and material reactor parts in the vicinity of the reaction zone--of the high-energy reactants in the reaction zone ("Project Sherwood", Addison-Wesley Publishing Company Inc., Reading, Mass., U.S.A., by A. S. Bishop, p. 85-87). At the same conference, J. L. Tuck put forward a proposal, which fulfilled E. Teller's stability criterion and became known as the "picket fence concept", for the basic construction of a fusion reactor of the type mentioned initially in which the magnetic field is generated by magnetic flux lines, which are curved in a convex manner towards the reaction zone, of several circulating currents which flow parallel to one another and axially apart from one another, each in the opposite current direction to the adjacent circulating current, about the same axis and surround the reaction zone or the plasma of high-energy reactants with the magnetic field they generate ("Project Sherwood", p. 86-89, in particular p. 89). Teller's contribution to the 1954 Sherwood Conference led during the conference itself to an in-depth discussion of the stability problems of containing high-energy plasmas for, apart from the picket fence concept first submitted at this conference, there was not a single concept for a fusion reactor which met Teller's stability criterion, i.e. where the reaction zone was surrounded by a magnetic field with magnetic flux lines curved, when viewed from the reaction zone, in a convex manner, rather all the concepts put forward at that time and all the experimental reactors under development--and especially the various embodiments of the Stellarator and the so-called Mirror machines--were provided with a magnetic field with magnetic flux lines curved in a concave manner viewed from the reaction zone and were, according to Teller's stability criterion, unstable. Nevertheless, during this conference the hope still prevailed that instabilities owing to non-fulfilment of Teller's stability criterion would arise in the experimental reactors under development and especially in the Stellarator and the Mirror machines only at relatively high values of the ratio .beta. of plasma pressure to magnetic pressure upon the plasma and that non-fulfilment of Teller's stability criterion was not an effective barrier to continued development of these experimental reactors. In the ensuring period, however, theoretical studies conducted in particular by E. A. Frieman, H. Grad and C. L. Longmire have demonstrated that Teller's stability criterion is universally valid so that the Stellarator and the Mirror machines are unstable, and are so not only at high but also at low values of the ratio of plasma pressure to magnetic pressure upon the plasma ("Project Sherwood", p. 85-88). Despite these findings, however, the Stellarator and Mirror programmes were continued ("Project Sherwood", p. 106-131), initially probably for the main reason that a great deal of money had already been invested in experimental reactors and there was a wish to use the, in some cases, almost completed experimental reactors at least to check out the, hitherto only theoretically predicted, instabilities by conducting practical stability tests. But otherwise these findings led, not for example to a rethink in the development of fusion reactors on the basis of stable plasma containment as a priority, but on the contrary to a second series of pinch concepts (fast pinch, B.sub.z -stabilized pinch, srew-dynamic pinch, triaxial pinch; "Project Sherwood", p. 90-105) aimed no longer at achieving stability but the fastest possible heating of the plasma to fusion temperatures within the period up to the occurrence of instabilities and the maximum extension of this period. However, because of the abandonment of stability, these concepts even if successful could at best have led to pulsed-mode operation fusion reactors and consequently to an adverse energy balance so that the actual aim of producing energy by nuclear fusion would have been no nearer achievement. Whereas, even after the discovery that all the experimental reactors of the Stellarator, Mirror and Pinch programmes were unstable, practical testing was still carried out for an extended period both with earlier developed experimental reactors and particularly with new developements (e.g. within the framework of the above-mentioned second series of Pinch concepts) although on the basis of this discovery it was already established that these means could not bring us any nearer to achieving the aim of producing energy by nuclear fusion, an aim achievable only with the precondition of continuous reactor operation and hence stable plasma containment, the only concept to guarantee stability and hence at least not to rule out from the start the possibility of achieving the aim of producing energy by nuclear fusion, i.e. the picket fence concept, was after only a short time dropped on the basis of a theoretical study by a small group of scientists in Los Alamos on the grounds that this concept could in fact achieve stability but not containment of a plasma, that particularly in the mid-planes between the circulating currents generating the magnetic field for containing the plasma a high particle loss could be expected, in other words that such a system was not tight ("Project Sherwood", p. 90-91). Although another group working in New York under the direction of H. Grad was also able from theoretical studies to prove in respect of a simplified modification of the picket fence concept, known as cusped geometry and having only two circulating currents generating the magnetic field for plasma containment, that at least in the case of high .beta. values or relatively high plasma pressure in relation to the magnetic pressure upon the plasma such high particle losses were not to be expected, this only concept to guarantee stability was never practically realised despite completion of a series of theoretical preliminary studies for a fusion reactor suitable for energy production based on this cusped geometry concept because the programme was discontinued in the concluding phase shortly before practical realisation and those working on the programme were assigned to different tasks, in particular to theoretical investigation of a modification to the Stellarator which held out the promise of stability in continuous reactor operation ("Project Sherwood", p. 139-142). This modification to the Stellarator consisted, according to a proposal by L. Spitzer, of superimposing upon the original confining field having magnetic flux lines extending within the discharge tube substantially parallel to the axis thereof a stabilising magnetic field with magnetic flux lines extending within the discharge tube substantially perpendicular to the axis thereof, said stabilising magnetic field being generated by six conductors extending like a sextuple thread with a very large lead helically about the discharge tube and carrying the same current ("Project Sherwood", p. 110-113). Since a twisted magnetic field of decreasing twist towards the axis is produced within the discharge tube as a cumulative field from the paraxial confining field and the stabilising magnetic field extending perpendicular to the axis ("Project Sherwood", p. 113) and the individual helical magnetic flux lines of this twisted magnetic field (unlike paraxial magnetic flux lines extending always on the same side of the axis) wind about the axis and hence successively extend now on one side of the axis then below it, now on the other side of the axis and then above it, the original configuration of the discharge tube of the Stellarator in the form of an 8 (which had been chosen so that the magnetic flux lines extending externally in one curve of the 8 extend in the other curve of the 8 internally and hence, so to speak, on the other side of the axis) was no longer necessary so that, with the introduction of the stabilising magnetic field proposed by L. Spitzer, it was simultaneously possible to go over again to toroidal discharge tubes, and this purely secondary effect of the efforts to stabilise the Stellarator in practice became a primary factor determining the whole course of further development and finally resulted in the gradual suspension of experiments with non-toroidal discharge vessels and the adoption of the toroidal discharge vessel as the basic prerequisite so to speak of new developments which, for the following reasons outlined below, must have contributed in no small way to the lack of success experienced up till now with controlled nuclear fusion. The stabilising magnetic field was however unable to fulfil its actual main purpose, namely stable plasma containment, since in the case of a relatively high plasma pressure in relation to the magnetic pressure upon the plasma or in the case of relatively high .beta. values it was already theoretically unsuitable for stabilisation purposes and the possibility held out by theoretical investigations of a stabilisation at relatively low values of less than c. 20% ("Project Sherwood", p. 112, esp. footnote) was at first in any case impossible to realise in practice. The main reason for this was that the modification of the Stellarator originally suggested by L. Spitzer with six currents of equal intensity and the same direction running helically round the discharge tube was, in view of the then standard resistive heating of the plasma by means of a direct voltage pulse induced in the discharge tube and directed in the axial direction of said tube ("Project Sherwood", p. 114, para. 2), so conceived that the stabilisation theoretically possible at .beta. values below 20% only intercepted or confined in the stable region axial plasma streams in the flow direction predetermined by the direct voltage pulse but excluded axial plasma streams in the reverse flow direction from the stable region. For the six currents of equal intensity running helically round the discharge tube, given the same direction, generate within the discharge tube a "stabilising magnetic field" which has magnetic flux lines extending substantially circularly around the tube axis and which, if the intensity of the helical currents is high enough, is more powerful than the magnetic field generated by the axial plasma stream with magnetic flux lines running similarly circularly around the tube axis and so, given the same direction of the helical currents and of the plasma stream, cancels out the magnetic field generated by the plasma stream and replaces it with a magnetic field of inverse magnetic flux direction so that, upon constriction of the plasma stream at one point and the resultant increase in the field intensity of the magnetic field generated by the plasma stream at this point, the field intensity of the cumulative field made up of the stabilising magnetic field and the magnetic field generated by the plasma stream, owing to the greater intensity of the stabilising magnetic field and the opposing field strength directions of the two magnetic fields at the constriction point, does not rise but falls and the magnetic pressure upon the plasma stream consequently also falls at the constriction point and the constriction therefore disappears by itself, whereas the constriction in contrast to this in the absence of the stabilising magnetic field on account of the increasing magnetic field intensity of the plasma stream generated magnetic field at the constriction point and the consequently increasing magnetic pressure upon the plasma stream at the constriction point continues to grow until the stream at the constriction point breaks away, causing a constriction instability or a so-called bulge-type instability to occur. Since the so-called kink instabilities, i.e. the bulging of the plasma stream towards the discharge tube wall with similarly ensuing breaking away of the stream, were practically excluded by the confining field acting as an axial guidance field for the plasma stream, it was hoped that, with the exclusion of constriction instabilities by the stabilising magnetic field, all the essential causes for the occurrence of instabilities in the axial plasma stream had been eliminated. The fact that the constriction instabilities were however only excluded with opposing field intensity directions of the stabilising magnetic field and of the plasma stream generated magnetic field and hence only with identical direction of the helical currents generating the stabilising magnetic field and of the axial plasma stream, while with an opposing direction of the helical currents and of the axial plasma stream and hence identical field strength directions of the stabilising magnetic field and of the plasma stream generated magnetic field no stabilisation but, on the contrary, a destabilisation was anticipated, was at first not at all recognised as a possible disruptive factor because the flow direction of the axial plasma stream seemed to be fixed in advance by the direct voltage pulse driving said stream. However, when the modification suggested by L. Spitzer was put into practice, there arose not only a series of foreseeable problems but also unexpected new problems associated with the stability of the plasma. The foreseeable problems were basically difficulties attributable to the very low plasma pressure permissible with this modification such as, for example, technical difficulties associated with the very high vacuum, necessary on account of the very low plasma pressure, to which the discharge vessel has to be evacuated prior to introduction of the reaction gas forming the plasma, difficulties with the necessary, quite substantial reduction in the plasma contamination arising from gas pockets in the discharge vessel wall which was absolutely essential on the one hand because of the very low plasma pressure and, given a foreign gas quantity, the correspondingly high ratio of foreign gas quantity to plasma gas quantity and on the other hand because of the extended discharge times and the rising quantity, associated with the longer period of release of foreign gases from the discharge vessel wall, of released foreign gases located in the plasma, and difficulties associated with the high radiation losses caused by contamination of the plasma in particular with foreign gases of a relatively high molecular weight and with the resultant considerable cooling of the plasma and the associated further increase in gas release from the discharge vessel wall ("Project Sherwood", p. 114, para. 2). Over and above these and various other foreseeable problems, in the experiments with the newly created modified Stellarator which was regarded as stable, a new problem which was totally unexpected in view of the hoped-for stability arose however in the form of a new, never before observed type of instability of the plasma containment during heating of the plasma with the above-mentioned direct voltage pulse ("Project Sherwood", p. 116, para. 2). The causes of this new type of instability could not at first be explained with the result that for a long time, while an attempt was made to explain these instabilities, all that there was to go on were suppositions of varying validity. One of these suppositions was that the instabilities were attributable to the so-called runaway electrons arising with the direct voltage pulse ("Project Sherwood", p. 116, para. 3). It was thought that the electrons of the plasma, which were accelerated by the direct voltage pulse over random, relatively long, free path lengths to, in relation to the mean energy of the electrons, very high kinetic energies, were able because of their high energies to "pierce" the magnetic confining field and so reach the discharge vessel wall where they would release their energy, so that throughout the period of heating of the plasma by the direct voltage pulse energy was being conveyed from the plasma to the discharge vessel wall ("Project Sherwood", p. 188, para. 6 in conjunction with p. 114, para. 2). Only much later was it discovered that what caused these instabilitites were axial plasma vibrations which were triggered by the sudden change in the field intensity at the beginning of the axial direct voltage pulse and led to the axial plasma stream in the discharge tube flowing during the axial plasma vibrations temporarily in the opposite direction to the direction of the direct voltage pulse and hence also in the opposite direction to that of the helical currents generating the stabilising magnetic field, causing the stabilising magnetic field during these phases of reverse flow direction to have not a stabilising but a destabilising effect. A modification in the helical currents generating the stabilising magnetic field was then carried out to the effect that, of the original six currents running in the same direction helically around the discharge tube, three currents each offset relative to one another by 120.degree. in a peripheral direction of the discharge tube were driven in the reverse direction so that the remaining three helical currents running in the original direction are responsible for stabilisation when the axial plasma stream is flowing in the same direction as the direct voltage pulse and the three helical currents running in the opposite direction are responsible for stabilisation when the axial plasma stream is flowing in the opposite direction to the direct voltage pulse. This change in the modification of the Stellarator originally suggested by L. Spitzer made it possible to achieve substantial improvements in stability in the form of much longer discharge times throughout which stable plasma containment could be sustained. Similar successes have been achieved with modifications of the Stellarator changed in this manner, especially at the University of Princeton, U.S.A., and the Max-Planck Institute of Plasma Physics in Garching, Germany. These successes with the Stellarator led to the use in other concepts with toroidal discharge tubes, such as, for example, the so-called theta pinch concept (a further development of the original collapse concept described in "Project Sherwood, p. 68-71), of stabilising magnetic fields of a similar type to that used in the Stellarator which were generated by an even number of currents running helically around the discharge tube, with half flowing in one direction and half flowing in the other direction, e.g. in the said theta pinch concept by four currents running helically around the discharge tube, of which two flow in one direction and two in the other direction. However, scant account was taken of the fact that the really effective stabilising effect of these stabilising magnetic fields (generated by two equal-sized groups of currents flowing in opposite directions helically around the discharge tube) was in no way primarily based on the considerations which had led to these stabilising magnetic fields, rather the main reason why these stabilising magnetic fields permitted stable plasma containment over extended discharge times was that they fulfilled Teller's stability criterion and were in principle magnetic fields with a magnetic field configuration similar to that of the picket fence concept (the similarity in the magnetic field configuration is evident from a cross-section through the discharge tube with the currents uniformly distributed along its periphery and flowing alternately out of the sectional plane and into said plane: for if the circular line of circumference with the currents distributed along it is imagined as a straight line, a current and magnetic field configuration is obtained similar to the upper half of the illustration of the picket fence concept in FIG. 9-2 on page 89 of "Project Sherwood"). It therefore emerges that the picket fence concept, largely ignored by specialists in the field and dropped after only a relatively short period of theoretical investigation in 1955/56 as insufficiently promising, in reality was--at any rate until the development of the so-called Tokamac--the only concept with which in practice an effective stabilising effect could actually be achieved and this concept remains to this day the only concept offering the prospect of realising an indefinite period of stable plasma containment and hence of realising a continuous operation fusion reactor suitable for energy production. For the concept known as Tokamac, with which in practice, e.g. in the PLT (Princeton Large Torus) and the ASDEX (Axial-symmetrical Divertor Experiment), similarly effective stabilising effects (PLT: 1978 0.18 s, 60 million .degree.C.; ASDEX: 1980 3 s) have been able to be achieved, is unsuitable for continuous reactor operation and hence also for energy production by nuclear fusion because in this concept the currents generating the stabilising magnetic field are driven by voltages induced in a metal discharge tube wall (and not as in, for example, the Stellarator by a separate direct current source) and these induced voltages must always point in the same direction owing to the unacceptability of a collapse of the stabilising magnetic field and the resultant ban on zero crossings of the currents generating the stabilising magnetic field and voltages of permanently fixed direction cannot be induced for an indefinite length of time because, with a preset voltage to be induced, the size of the magnet core required for induction increases in proportion to the square of the time during which the voltage to be induced must be sustained. That effective stabilising effects have been able to be achieved at all with the Tokamac concept is due to the fact that this concept in principle represents the ideal implementation of the modification of the Stellarator originally proposed by L. Spitzer: for if in this modification, instead of six conductors running helically around the discharge tube and carrying the same current, so great a number of conductors running directly adjacent to one another helically around the discharge tube and carrying the same current were to be provided that the entire surface of the discharge tube was covered by such conductors running directly adjacent to one another helically around the discharge tube and carrying the same current, then this plurality of conductors may also be replaced by a metal tube if at the same time provision is made for voltages, which have the same characteristic as previously the conductors running adjacent to one another, to be induced in the metal tube. In the Tokamac concept, such helically running voltages are induced in the toroidal metal discharge tube in that the metal torus forming the discharge tube and the annular plasma inside the metal torus each form a secondary winding of a shell-type transformer, which has a transformer core extending along the torus axis and a shell externally enclosing the torus and in whose core a constantly increasing current flowing through the primary winding generates a constantly increasing magnetic flux which in turn induces in the two secondary windings, i.e. in the metal torus and the annular plasma, rotational voltages of the same level and direction running parallel to the tube axis of the torus. The induced rotational voltage sets the plasma moving in the direction of the tube axis of the torus, thereby producing an axial plasma stream which in turn generates a magnetic field which surrounds the plasma and has circular magnetic flux lines concentric to the tube axis of the torus. Super-imposition of this magnetic field generated by the axial plasma stream by the confining field whose magnetic flux lines run parallel to the tube axis of the torus and hence perpendicular to the magnetic flux lines of the magnetic field generated by the axial plasma stream produces a cumulative field with magnetic flux lines running helically around the tube axis of the torus. The individual charge carriers of the plasma set in motion by the induced rotational voltage then follow these helical magnetic flux lines with the result that the axial plasma stream contains, besides its axial component pointing in the direction of the tube axis of the torus, an additional azimuthal component pointing in the direction of rotation about the tube axis of the torus, and the magnetic field generated by this azimuthal component of the axial plasma stream finally induces in the metal torus an annular voltage which runs around the tube axis of the torus and whose superimposition by the rotational voltage induced by the shell-type transformer and running parallel to the tube axis of the torus produces a helical characteristic in the voltages in the metal torus forming the discharge tube. The voltages running helically about the tube axis of the torus in the metal torus in turn drive helical currents in the metal torus which, if the system and its operating parameters are suitably dimensioned, may together be of the same magnitude as the sum of the six individual currents running helically around the discharge tube in the Stellarator and may also have the same characteristic as these. Thus, the Tokamac concept allows the same current and magnetic field configurations as the modification to the Stellarator originally proposed by L. Spitzer and hence also the stability, which according to the theoretical studies of L. Spitzer was to be achievable with such current and magnetic field configurations, but it excludes the possibility, which still exists in practice in this modification of the Stellarator for the reasons mentioned above, of the occurrence of axial plasma vibrations and plasma instabilities caused thereby because in this concept, in contrast to the electrically non-conductive discharge tube of the Stellarator, an electrically conductive metal torus is provided as a discharge tube which, because of the permanent coupling effected by said shell-type transformer between the annular plasma forming the one secondary winding of this transformer and the metal torus forming the other secondary winding of the transformer, acts as a strong damper which is connected in parallel to the annular plasma and does not allow the axial plasma vibrations to occur in the first place. With the Tokamac concept, it is therefore possible to realise in practice the stability which in theory should have already been a feature of the modification to the Stellarator proposed by L. Spitzer and for this reason an effective stabilising effect has in practice also been achievable with the Tokamac concept (and not only with the above-described changed modification of the Stellarator with two equal-sized groups of currents flowing in opposite directions around the discharge tube or the picket fence concept realised therein). The drawback of the Tokamac concept is however that this stabilising effect is in practice limited in time because the currents generating the stabilising magnetic field in the Tokamac concept are driven by the voltages induced in the metal discharge tube wall and the size of the magnet core required to induce these voltages and hence of course also the size and cost of the entire fusion system as already mentioned increase in proportion to the square of the sustenance time of the induced voltages. For since, according to the Lawson criterion, ignition of the plasma can only occur when the product of the sustenance time and the particle density lies above 3.times.10.sup.14 s/cm.sup.3 and the particle density has a ceiling imposed by the maximum achievable magnetic pressure upon the plasma and the maximum permissible .beta. values, ignition of the plasma can in practice only be achieved by increasing the sustenance time, and since the size and cost of the fusion system increase in proportion to the square of the sustenance time, the limit of what is technically and financially feasible here is very quickly reached. This is clearly evident from the size and cost development of experimental reactors for controlled nuclear fusion for, whereas initially the experimental plants still operating according to the Stellarator concept, such as, for example, the Stellarator C developed in Princeton with a size of around 20 m.sup.3 and an outlay of around 10 million U.S.$ or the Wendelstein in Garching with a size of around 100 m.sup.3 and an outlay of around 50 million U.S.$, were still within the budgetary scope of the relevant research institutes or the universities to which the research institutes belonged, the experimental plants operating according to the Tokamac concept, such as the ASDEX constructed in Germany with a size of around 200 m.sup.3 and an outlay in excess of 150 million U.S.$ or the JET currently under construction and jointly financed by the Western European states with a size of around 750 m.sup.3 and a projected cost in excess of 500 million U.S.$ and finally the INTOR joint venture by the U.S.A., U.S.S.R., Japan and Western Europe with a size of over 2500 m.sup.3 and estimated cost in excess of 2000 million U.S.$, can only be financed on a national or international scale. Bearing in mind that the size of the biggest planned experimental system, i.e. the INTOR, is around 100 times the size of the Stellarator C dating from the early days of research into nuclear fusion and that such a size ratio in the Tokamac concept only makes possible an increase in the sustenance time by the factor 10, then it becomes clear that the drawback of the stabilising effect in the Tokamac concept being in practice limited in time is serious enough to cast doubt on the ability of the Tokamac concept to achieve the aim of energy production by nuclear fusion. This is also already evident from the results of the planning stage of the INTOR which reveal that with the INTOR nuclear fusion itself and ignition of the plasma should be achievable but that the power gain through nuclear fusion at 5 to 10 MW is only 2.5 to 5% of the power consumption of around 200 MW required to operate the INTOR. And since this fairly negative power balance could only be improved to the extent where an acceptable positive power balance of less than 50% own consumption by the fusion reactor of the power generated by nuclear fusion could be anticipated by increasing the planned containment time for the INTOR of 6 to 12 seconds to a period of several minutes and such an increase in the containment time would make the costs for the fusion reactor soar immeasurably, then even if energy production through nuclear fusion were theoretically attainable with the Tokamac concept, such a solution would be impracticable on the grounds of cost. In practical terms, this means that the Tokamac concept is also ruled out for energy production by nuclear fusion and is ruled out in the end for the same reason that made all the other concepts tried out in the course of nuclear fusion development unsuitable for energy production by nuclear fusion, namely that none of these concepts guarantees the stability of the plasma over periods of indefinite length which is required to sustain the plasma containment for an unlimited time, in other words none of these concepts is inherently stable. The question then arises of the correctness of the decision taken mid-1956 during the initial development phase of nuclear fusion to drop the only inherently stable concept, i.e. the picket fence concept and its modification known as cusped geometry, and instead to pursue other projects which may at the time have appeared more promising but in no way fulfilled Teller's stability criterion. For in view of the fact that in the intervening three decades, despite an enormous scientific and technical input throughout the world, it has proved impossible to find a satisfactory solution to the stability problem with the concepts practically tested in the course of nuclear fusion development, it seems fair to say that an absence of stability in a concept is a problem which cannot in practical terms--at any rate at a reasonable cost--be solved, whereas the question whether the deficiencies of the only inherently stable concept, i.e. mainly the problem of particle loss and the non-tight plasma containment in the picket fence concept and the cusped geometry based thereon, can be satisfactorily eliminated has remained largely unanswered owing to the above-mentioned decision to abandon the concept. Admittedly, during the course of the above-mentioned theoretical studies of cusped geometry, various suggestions have been made to reduce particle loss but the question, whether within the scope of this concept a plasma with a non-decreasing particle number can be achieved for an indefinite length of time and whether the particle loss problem can be completely eliminated, remained unanswered in these studies too. SUMMARY OF THE INVENTION. The aim of the invention was therefore, on the basis of the only inherently stable concept, to provide a fusion reactor of the type mentioned initially with a reaction zone which is surrounded by a magnetic field with magnetic flux lines curved in a convex manner viewed from the reaction zone, in which reactor the particle loss problem is solved and there is always in the reaction zone a sufficient number of reactant ions to sustain the fusion process with sufficient kinetic energy for fusion. This aim is achieved according to the invention with a fusion reactor of the type mentioned initially which is characterised by a potential pot surrounding the reaction zone for the conversion of kinetic energy from ionized reactants escaping from the reaction zone into potential energy thereof and for the subsequent return of the ionized reactants into the reaction zone with reconversion of their potential energy into kinetic energy. The reaction zone expediently lies in the centre of the electric potential pot, to the upper edge of which ionized reactants are supplied and accelerated by the potential difference between edge and centre up to a kinetic energy sufficient for fusion and upon not meeting another reactant in the reaction zone pass the centre at a high speed corresponding to their kinetic energy and at the opposite side of the potential pot to their supply side again run against the potential difference at a decreasing speed towards the edge of the potential pot until their kinetic energy, shortly before reaching the potential pot edge, is again converted into potential energy, so that the process of accelerated movement towards the potential pot centre and the subsequent decelerated movement towards the potential pot edge may be repeated any number of times up to a fusion reaction in the reaction zone and consequently a large portion of the reactants supplied to the potential pot edge may be brought into fusion reaction, with the portions, which extend in the potential pot, of the magnetic flux lines of the magnetic field surrounding the reaction zone in the region between potential pot edge and reaction zone running substantially perpendicular to the equipotential lines of the electric field forming the potential pot and substantially parallel to the field lines of the electric field so that the substantially linear acceleration of the ionized reactants towards the reaction zone is not disrupted by the magnetic field surrounding the reaction zone. The main advantage of the present fusion reactor is that it offers for the first time the possibility of indefinite continuous reactor operation and hence attainment of the goal of energy production by nuclear fusion. In principle, this possibility results from the inherent stability of the plasma containment in nuclear fusion reactors of the type mentioned initially with a reaction zone which is surrounded by a magnetic field having magnetic flux lines curved in a convex manner viewed from the reaction zone, as well as from the ability by means of the electric potential pot of the present fusion reactor to achieve complete elimination of the particle loss problem or the lack of tightness of the plasma containment which, in the known proposals for fusion reactors of the type mentioned initially (picket fence concept, cusped geometry), was regarded as an insoluble problem (Picket fence concept, "Project Sherwood", p. 91) to which partial solutions in the sense of a reduction in particle losses were conceivable only in discontinuous reactor operation (Cusped geometry, "Project Sherwood", p. 410). In the present fusion reactor, this particle loss problem is overcome with the aid of a technical trick in that removal of the particles from the direct vicinity of the reaction zone is deliberately permitted but the removing particles are returned by means of the electric potential pot with the same energy back into the reaction zone, with only one conversion of the kinetic energy of the removing particles into potential energy and one reconversion of this potential energy into kinetic energy upon the return of the particles into the reaction zone occurring in the potential pot holding the particles captive. The electric potential pot of the present fusion reactor moreover has the critical advantage of rendering superfluous the compression of the plasma required in the known fusion reactors, because the ionized reactants supplied at the upper edge of the potential pot are compressed towards the reaction zone in the centre of the potential pot in inverse proportion to the cube of the distance from the centre so that, e.g. with a reaction zone diameter of one tenth of the potential pot diameter, "compression" to the level of .times.1000 occurs in the reaction zone. This advantage is of critical importance in so far as, in all known fusion reactors, compression of the plasma is effected by magnetic compression which necessitates a steep increase in the magnetic field containing the plasma or in the current generating said magnetic field and this increase inevitably leads to pulsed-mode operation of the fusion reactor if stability of the compressed plasma contained by the magnetic field as well as tight containment of said plasma are not guaranteed. However, as the stability of the plasma in the known fusion reactors with a toroidal discharge vessel decreases with increasing compression of the plasma because the originally doughnut-shaped plasma is compressed by the magnetic compression into a thin circular plasma thread and such a thin plasma thread naturally is more inclined to break, the thinner it is, magnetic compression in fusion reactors with a toroidal discharge vessel leads perforce to a restriction to pulsed-mode operation and hence to unattainability of the goal of energy production by nuclear fusion. The electric potential pot of the present fusion reactor therefore not only ensures the above-mentioned complete elimination of the particle loss problem as yet unsolved in the proposed fusion reactors of the type mentioned initially (picket fence concept, cusped geometry) but also, owing to the fact that its compression of the reactants in the reaction zone is effected without magnetic compression, fulfils all the other preconditions for continuous reactor operation, i.e. overcomes the pulsed-mode operation previously unavoidable in all the known fusion reactors, including the proposed fusion reactors of the type mentioned initially, on account of magnetic compression of the plasma and eliminates the stability problems occurring in fusion reactors with a toroidal discharge vessel on account of magnetic compression of the plasma, so that only by equipping fusion reactors of the type mentioned initially with such a potential pot will the transition from discontinuous to continuous reactor operation and hence to energy production by nuclear fusion be possible. The non-magnetic compression of the reactants in the reaction zone by the electric potential pot plays a positive role in that the magnetic field containing the plasma in the reaction zone may, because there is no longer any need for magnetic compression, be held constant or left at a constant magnetic field intensity during operation of the reactor, which in conjunction with generation of the magnetic field by means of superconducting coils opens up the possibility of reducing the energy required to sustain the magnetic field containing the plasma during operation of the reactor virtually to zero and so improving the energy balance of the present fusion reactor to the extent that the aim of energy production by nuclear fusion can be achieved. In connection with the high density of reactants in the reaction zone achievable by means of the electric potential pot, it is also an important advantage that the present fusion reactor may be operated with a much higher particle density in the reaction zone than the known fusion reactors of the Stellarator or Tokamac types because, as already mentioned, for stability reasons fusion reactors of the Stellarator or Tokamac types have to be operated with very low values of the ratio .beta. of plasma pressure to magnetic pressure upon the plasma and with correspondingly low particle densities in the reaction zone, whereas fusion reactors of the type mentioned initially, as demonstrated above by the example of cusped geometry, are preferably operated with high .beta. values and correspondingly large particle densities in the reaction zone. The advantage of such a high particle density in the reaction zone or a correspondingly high permissible plasma pressure is the elimination of all the aforementioned problems which arise in Stellarator or Tokamac type fusion reactors on account of the very low plasma pressure permissible in them, in particular the elimination of the cited technical problems occurring in Stellarator or Tokamac type fusion reactors with the very high vacuum required on account of the low permissible plasma pressure and with the correspondingly necessary, extremely low plasma impurities from gas pockets in the discharge vessel wall and the high radiation losses caused by such plasma impurities. In the present fusion reactor, the vacuum which is required is in contrast much lower and serious problems with plasma impurities and associated radiation losses no longer arise. Further advantages of the present fusion reactor accrue from the basic concept of fusion reactors of the type mentioned initially realised in principle in the above-mentioned cusped geometry. In this basic concept, the reaction zone basically takes the shape of a double cone acted upon from outside by magnetic pressure and such a formation is, in a similar fashion to a hollow sphere acted upon from outside by mechanical pressure, largely insensitive to disturbances or changes suddenly occurring inside it, such as an abrupt increase in the nuclear fusion rate, because the effects of such changes suddenly occurring at any point inside it are almost immediately evenly spread over the total interior or the total reaction zone whereas, in the case where a thin circular plasma thread forms the reaction zone as in fusion reactors with a toroidal discharge vessel, the effects of such sudden changes remain localised and the plasma thread is therefore inclined to break at the point of such a sudden change. The basic concept of the present fusion reactor therefore offers not only the inherent static stability provided by the--viewed from the reaction zone--convex curvature of the magnetic flux lines of the magnetic field containing the reaction zone but also inherent dynamic stability of the plasma contained by the magnetic field in the reaction zone. One other property of said basic concept is however of critical importance for the production of nuclear fusion reactions in the reaction zone. This is that, in this basic concept, the magnetic field intensity in the centre of the reaction zone enclosed by the magnetic field is praticularly zero with the result that in the centre of the reaction zone no magnetic forces whatsoever and--since the centre of the reaction zone coincides with the centre of the electric potential pot and the electric field strength in the centre of an electric potential pot is similarly zero--no electrical forces either act upon the ionized reactants so that the reactants in the centre of the reaction zone are freely movable in all three degrees of freedom. Such free mobility of the reactants in all three degrees of freedom is however, besides the required temperature of the plasma and the required mean kinetic energy of the reactants, a basic requirement for producing nuclear fusions and this basic requirement has not up till now been fulfilled in any of the practically realised fusion reactors because the reaction zone in all these fusion reactors is permeated by a magnetic field used i.a. for stabilisation and compression of the plasma and the ionized reactants in the reaction zone are therefore freely movable only in the direction of the magnetic flux lines of this magnetic field and hence only in one and not all three degrees of freedom and this may well be one of the main reasons why, despite decades of considerable effort, nuclear fusion has up till now been unattainable with the practically realised fusion reactors. In the present fusion reactor, means may advantageously be provided for supplying the reactor with a reaction gas preferably consisting at least partially of deuterium as well as for ionizing and supplying said gas to the upper edge of the potential pot. The advantage of such supply means is a continuous feed of new reactants which take the place of the reactants which have reacted, and such a continuous feed, while not being essential for nuclear fusion itself, may well be for continuous reactor operation. As means for ionizing and supplying the reaction gas to the reactor, there may advantageously be disposed at the upper edge of the potential pot a glow discharge chamber which is provided with means for supplying ionized reactants to the potential pot in the form of canal rays, preferably with a metal foil designed in the manner of a Lenard tube and permeable to the canal rays as a cathode. The advantage of such a glow discharge chamber as an ion source is a relatively low energy consumption for producing the ionized reactants in conjunction with the desired large-area distribution of the ion source at the upper edge of the potential pot. To achieve relatively high ion concentrations at the upper edge of the potential pot, means for producing a current-intensive glow discharge according to B. Berghaus may be provided in the glow discharge chamber. The electric potential pot may in the present fusion reactor advantageously take the form of a rotationally symmetrical cavity having a cross-section substantially in the form of two opposing sectors of a circle, with the cusps of the two sectors which form the cross-section coinciding with the axis of symmetry of the rotationally symmetrical cavity and a median dividing said two sectors each into two identical parts standing vertically on said axis of symmetry and said upper edge of the electric potential pot lying in the region of the arc of the sectors. The apex angle of the sectors may advantageously be between 10.degree. and 80.degree., preferably between 30.degree. and 50.degree.. The advantage of such a shape for the electric potential pot over other possible shapes, such as a circular disc-shaped cavity, is increased compression of the reactants in the reaction zone. Particularly advantageously, there may be provided, at the substantially cone-shaped side surfaces of the rotationally symmetrical cavity spatially defining the electric potential pot, means for lateral electric screening of the potential pot as well as for achieving a potential profile along the screening which is higher than or approximately the same as the potential profile along said median depending upon the distance from the potential pot centre. This has the advantage that the ionised reactants moving back and forth inside the potential pot from the region of the upper edge through the centre towards the opposite upper edge run in a kind of potential trough and so cannot reach the lateral screening of the potential pot, thereby preventing the ionised reactants from coming into contact with the material walls forming the lateral screening. The means for screening and for achieving said potential profile may advantageously comprise stacked rings made of an electrically conducting material, each of which is substantially in the shape of a short truncated cone and fits on top of the preceding ring in the stack in such a way that the ring edges of all the stacked rings together define at one side one of said substantially cone-shaped side surfaces of the rotationally symmetrical cavity. The rings may advantageously be electrically insulated from one another, preferably by means of electrically non-conducting coatings, and may be individually connected to direct voltage sources each supplying the intended potential of the ring. This has the advantage that the potential profile along the screening is independent of the movement and density of the ionised reactants moving inside the potential pot but does require a not inconsiderable technical effort on account of the individual connection of the rings to direct voltage sources each supplying the intended potential of the ring. This effort may advantageously be avoided in that the rings PG,24 are electrically connected to one another by high-resistance resistors preferably formed by electrically poorly conducting coatings and means are provided for generating a current which flows through the stack and produces at the high-resistance resistors the voltage drops required to achieve said potential profile. The price which has to be paid for the advantage thereby achieved, namely removal of the technical effort associated with individual connection of the rings, is however the energy consumption of the current flowing through the stack and, provided said energy consumption is kept down by a relatively low current flowing through the stack, the dependence of the potential profile along the screening upon voltages induced in the rings as a result of the space charge of the ionised reactants moving in the potential pot. In a preferred embodiment of the present fusion reactor, for generating the magnetic field surrounding the reaction zone, two coils with a substantially triangular winding cross-section are advantageously provided disposed coaxially to the reaction zone and the potential pot and on either side of the reaction zone and potential pot, with currents of at least approximately the same magnitude flowing in opposite directions through said coils. This embodiment has the advantage of the absolute minimum number of coils required to generate a magnetic field with magnetic flux lines curved in a convex manner viewed from the reaction zone and, because of the substantially triangular winding cross-section of the coils, the advantage of optimum adaptation of the profile of the magnetic flux lines of the magnetic field generated by the coils to the profile of the field lines of the electric field in the potential pot. To increase the magnetic field intensity in the reaction zone and in particular between the reaction zone and the material walls surrounding it, in the present thermo-nuclear reactor and in particular in said preferred embodiment thereof, a substantially hollow sphere-shaped reactor shell which encloses the coils and the potential pot and is made of a ferromagnetic material, preferably soft iron, may advantageously be provided, with one side of the substantially triangular winding cross-section of the coils advantageously being adjacent to the reactor shell inner wall and extending approximately parallel thereto and a linear extension of the median between the other two sides of the triangular winding cross-section extending through the centre of the reaction zone. The advantage of such a reactor shell made of ferromagnetic material is, with a predetermined coil current, an increase in the magnetic field intensity in the reaction zone and, with a predetermined magnetic field intensity in the reaction zone, a reduction in the coil current for generating the magnetic field surrounding the reaction zone. In the present fusion reactor and in particular in said preferred embodiment thereof, the coils may particularly advantageously be superconducting coils comprising tubular windings through which a cooling medium preferably formed by a liquefied gas flows and keeps the current-conducting walls of said windings at a temperature within the superconductivity range of the material of said walls, with means for supplying the cooling medium to the coils being provided and each of the two coils being surrounded by a heat-insulating shell preferably constructed in the manner of a Dewar flask. The advantage of such superconducting coils is that the energy consumption for generating the magnetic field surrounding the reaction zone may be kept so low that it no longer has any significant influence upon the energy balance of the fusion reactor. The substantially triangular winding cross-section of the coils in said preferred embodiment of the present thermo-nuclear reactor may advantageously basically take the form of an equilateral triangle, with the windings of the coils being formed by preferably tubular conductors, the cross-section of which conductors preferably likewise has the external shape of an equilateral triangle, and with the median between the two triangle sides, pointing approximately towards the reaction zone, of the substantially triangular winding cross-section of the coils making an angle in the region of 30.degree. to 50.degree., preferably between 37.degree. and 43.degree., with the axis of the coaxially disposed coils. The advantage of such a winding cross-section section in the form of an equilateral triangle is that the conductor cross-section of the coil windings may also take the form of an equilateral triangle so that almost complete utilisation of the winding space of the coils and consequently a maximum magnetic field intensity in the vicinity of the coils and hence also in the outer regions of the reaction zone may be achieved. For the capture and chemonuclear conversion of neutrons released in nuclear fusion reactions, there may advantageously be provided in the present fusion reactor a blanket which surrounds the reaction zone and the potential pot and in which in liquid lithium flows from a storage tank, disposed in the region of the upper edge of the pot and covering the potential pot in this region, along the side surfaces of the potential pot into the region surrounding the reaction zone and from there approximately in the direction of the axis of the reaction zone and potential pot into a collecting tank, the collecting tank being connected to the storage tank by a separating device, preferably a tritium stripper, and a first heat exchanger as well as by a lithium pump for circulating the liquid lithium through the blanket. Advantageously, the flow cross-section for the liquid lithium may be at least approximately constant in the portions of the blanket extending along the side surfaces of the potential pot and approximately in the direction of the axis of reaction zone and potential pot in order to achieve a substantially constant flow rate of the lithium in said portions of the blanket and the width of the, in said portions of the blanket, annular flow cross-section may for this purpose be at least approximately inversely proportional to the mean diameter of the annular flow cross-section or to the mean distance of the flow cross-section from the axis of reaction zone and potential pot. The blanket surrounding both the reaction zone and the potential pot has the advantage that almost no neutrons can escape from the fusion reactor, and the advantage of a constant flow cross-section for the lithium in the blanket lies in the fact that it allows a combination of a high flow rate with laminar, non-turbulent flow of the lithium and that as a result the width of the annular flow cross-section and consequently the capture effect of the lithium is greatest at the point where the most neutrons are to be captured, i.e. in the vicinity of the reaction zone. Said first heat exchanger in the lithium circuit may give up its heat advantageously to a potassium circuit passing through a second heat exchanger and a potassium turbine, the potassium turbine driving a first generator for generating electric energy, and the second heat exchanger advantageously gives up its heat to a water/steam circuit leading through a steam turbine as well as a condenser and a pump, the steam turbine driving a second generator for generating electric energy. The advantage of such a two-stage heat exchange with a potassium circuit in the first stage is the adaptability of the potassium circuit to the temperature in the lithium circuit. In the present fusion reactor, means may also advantageously be provided for supplying reactants to the reaction zone and for discharging reaction products and excess reaction gas from the reaction zone, said means comprising at least one gas reservoir for gas to be supplied to the reaction zone, supply means, preferably with a supply channel coaxial to the axis of the reactor, for supplying reaction gas from at least one gas reservoir to the reaction zone, discharge means, preferably with a discharge channel coaxial to the axis of the reactor, for carrying reaction products and excess reaction gas away from the reaction zone, a gas separating system, preferably in the form of a gas fractionator, for the gas coming from the reaction zone and a gas pump, preferably a vacuum pump, for conveying gas out of the reaction zone as well as preferably in the circuit through the gas separating system, gas reservoir, supply means, reaction zone and discharge means. Besides carrying reaction products away from the reaction zone, such gas supply and discharge means have the advantage of opening up the possibility of supplying to the reaction zone un-ionised reactants which are ionized in the reaction zone and whose electrons released during said ionization, together with the ions thereby produced and the ionized reactants from the potential pot, form a true plasma of ions and electrons in the reaction zone. A further advantage of such means is the movement of the supplied atoms in the axial direction of the reactor for, since this direction of movement is substantially perpendicular to the direction of movement of the ionized reactants passing from the potential pot into the reaction zone, there is a much greater chance of collisions between ionized reactants from the potential pot and ionized reactants from the supply channel with subsequent fusion reaction than there is of collisions of the ionized reactants from the potential pot with one another because, just as with sustaining an unstable equilibrium, there is virtually no chance of a frontal collision between atomic nuclei moving towards one another in a straight line because atomic nuclei of like electrical charge avoid one another while a similar avoiding process with atomic nuclei moving at right angles to one another quite often does not rule out a collision of the atomic nuclei.