CROSS-REFERENCE TO RELATED APPLICATION 
     This is a National Phase Application pursuant to 37 C.F.R. §371 of International Application No. PCT/EP2007/006645, filed Jul. 26, 2007, claiming priority from German Application No. DE 10 2006 034 988.1, filed Jul. 28, 2006, the entire disclosures of both of which are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ion source for generating negatively charged ions, comprising a closure plate, with an outlet opening provided therein, a wall surrounding a combustion chamber, wherein the wall has a tubular section, which extends from the outlet opening and is formed from an insulator material, and a rear wall, wherein the rear wall is arranged at that end of the tubular section which lies opposite the outlet opening, and closes off the combustion chamber, a coupling coil, the windings of which are arranged around the tubular section of the wall outside the combustion chamber, and a filter field magnet, arranged adjacent to the outlet opening, wherein the filter field magnet is configured in such a way that the field lines of the filter field magnet run transversely with respect to the extension direction of the tubular section. 
     2. Discussion of the Prior Art 
     Ion sources are generally employed whenever atoms are intended to be accelerated with the aid of electromagnetic fields. This is the case firstly in accelerator installations for scientific purposes, where high-energy ions are caused to collide with one another or with stationary targets in the field of nuclear and particle physics. 
     Secondly, in the field of semiconductor technology, materials are doped, that is to say impurity atoms are introduced into them, in such a way that the desired species of atoms is “implanted” into the material in the required amount. For this purpose, the corresponding atoms are shot into the material by means of an accelerator. This is associated with the advantage that the defined energy of the atoms can also be used to define the depth to which the atoms penetrate into the material. 
     In any case, it is necessary, however, for the atoms outwardly to be charged rather than neutral, in order that an electromagnetic field can exert an accelerating force on the atoms. It is necessary, therefore, for the atoms that are to be accelerated to be ionized, and thus for the electron shell not to contain the number of electrons which corresponds to the atomic number of the respective atom. Rather, the number of electrons must deviate from this in order that the charge of the atomic nucleus and that of the electron shell do not mutually compensate for one another. 
     In the simplest form of ionization, some of the electrons of the shell are removed from the respective atoms by a plasma in an ion source, such that the residual ions are then positively charged and can be extracted by a simultaneously applied electric field. 
     In specific accelerator installations, so-called tandem accelerators, in which the center of an acceleration path is at positive potential, it is necessary, however, firstly to generate negatively charged ions, which are then accelerated by the potential to the center of the acceleration path in order to be “stripped” there, that is to say to lose some of the electrons, and thus to be subjected to charge reversal from negative to positive ions. The potential, which now has the effect of repelling the ions, accelerates the latter again. In this case, the principle of the tandem accelerator is associated with the advantage that the acceleration potential is multiply utilized. 
     This means that it is necessary in this case to ensure in the ion source that the atoms have at least one electron more in their shell than is the case in the normal state. With regard to the ion source in tandem accelerators it should furthermore be taken into account that such an accelerator installation is generally set in such a way that only those ions which have a specific charge state are led further to the target after the acceleration process. However, the charge reversal is a statistical process and not all of the ions injected into the accelerator have the desired charge state after the charge reversal, rather a distribution arises which is dependent on the parameters during stripping. 
     This in turn has the effect that a considerable loss of beam current occurs as a result of the charge reversal, such that it is necessary to inject a very high beam current of negatively charged ions into the tandem accelerator in order consequently to obtain a sufficiently high current of positive ions having the desired charge state. This in turn presupposes that the ion source disposed upstream of the accelerator for generating negatively charged ions supplies a highest possible yield. 
     U.S. 2004/0104683, from which the present invention proceeds, discloses for this purpose an ion source comprising a closure plate with an outlet opening provided therein, and comprising a wall surrounding a combustion chamber, wherein the wall has a tubular section, which extends from the outlet opening and is formed from an insulator material, and a rear wall. The rear wall is arranged at that end of the tubular section which lies opposite the outlet opening, wherein the rear wall closes off the combustion chamber. In addition, a coupling coil is provided, the windings of which are arranged around the tubular section of the wall outside the combustion chamber in order to couple a radio-frequency field into the combustion chamber. Finally, a filter field magnet is provided, which is arranged adjacent to the outlet opening and serves to prevent energetic electrons from the plasma from leaving the combustion chamber through the outlet opening. Low-energy electrons can diffuse through the field, however, wherein the filter field magnet is configured in such a way that the field lines of the filter field magnet run transversely with respect to the extension direction of the tubular section. What is disadvantageous about an ion source of this type, however, is that the yield of negative ions is insufficient. 
     A further problem of this source is that the filter field magnet is arranged within the closure plate adjacent to the outlet opening and is thus exposed to the plasma. This can have the effect that the magnet incurs damage during operation of the source and the source has to be serviced after a short operating time. Moreover, said magnet influences the potential of the plasma in the outlet region by drawing electrons from the plasma. 
     Proceeding from the prior art, the object, therefore, is to provide an ion source for generating negatively charged ions which supplies an increased yield and is suitable for generating ions even from substances available only in gaseous form. 
     SUMMARY 
     According to the invention, this object is achieved by virtue of the fact that the rear wall is formed from an insulator material and has an inlet opening for gas, and that the filter field magnet is arranged on that side of the tubular section which is remote from the combustion chamber. 
     What is achieved by the construction according to the invention is that plasma formed within the combustion chamber by the radio-frequency field radiated in, in contrast to the prior art, comes into contact essentially only with parts which are formed from an insulator material. This has the advantage that the particle loss of the plasma is smaller. It is therefore possible to work with a lower radio-frequency power. 
     As a result of the insulation of the plasma, the latter becomes potential-free. There is therefore the possibility of freely arranging electrodes for example in the region of the outlet opening or the inlet opening and, by way of their potential, manipulating the plasma potential and thus the particle currents and concentrations. Aluminum oxide ceramic (Al 2 O 3 ) can preferably be used as insulation material since it has a particularly low sputtering rate. Operation for years is thereby possible. 
     A further advantage of this arrangement is that the filter magnet is arranged completely outside the combustion chamber and therefore no longer has any contact with the plasma. This prevents the magnet from being able to incur damage during operation. It no longer need be specially encapsulated, nor is it exposed to the temporary vacuum. 
     On the other hand, the filter field magnet is arranged in such a way that it can act effectively on the plasma. It shields the region of the outlet opening from energetic electrons by a low diffusion rate and permits the low-energy electrons necessary for H generation to pass through. 
     Since the filter field magnet is arranged on that side of the tubular section which is remote from the combustion chamber, and thus at a distance from the outlet opening, it no longer influences the potential at the outlet opening and does not dissipate any electron current. 
     Overall, therefore, in conjunction with a high yield of negatively charged ions, the resulting requirement for maintenance work is low and the source can also be used for ionizing gaseous substances such as hydrogen, for example, since the latter can be admitted into the combustion chamber via the inlet opening in the rear wall. 
     In one preferred embodiment of the ion source according to the invention, the diameter of the outlet opening is greater than or equal to the diameter of the opening of the tubular section which faces the outlet plate. Furthermore, a collar element having a through hole is provided in the outlet opening. 
     The collar element serves firstly to bring about the generation of negatively charged ions, wherein this process can proceed as follows in the case of hydrogen. From the plasma, excited hydrogen molecules (H* 2 ) pass into the region of the collar element. It additionally happens here that the hydrogen molecule is split up, a low-energy electron is taken up and a neutral hydrogen atom and a negative hydrogen atom remain. Accordingly, the process proceeds in accordance with the equation H 2 *+e − →H 0 +H − . 
     Secondly, the collar element serves to ensure that fewer electrons are extracted from the combustion chamber. In a manner exhibiting further preference, therefore the collar element has a plurality of electrodes which are arranged one behind another as seen in the axial direction of the outlet opening and which can typically be embodied as ring segments and are insulated from one another, wherein the electrodes can be put at electrical potentials deviating from one another. In this way, in the region of the collar element it is possible to generate an electric field that has the effect that electrons from the plasma can pass through the collar element and hence the outlet opening with only a low probability. In contrast thereto, the negative hydrogen ions, on account of their higher mass, are nevertheless able to pass through the outlet opening. 
     Furthermore, at the first half-rings, positive ions (H + ) are converted to neutral but excited molecules (H 2 *), which together with slow electrons likewise form H 0  (neutral hydrogen atoms) and H −  ions. 
     In order to further intensify the filter effect with regard to the electrons and the efficiency during charge reversal, it is particularly preferred for the internal diameter of the through hole of the collar element to decrease with increasing distance from the combustion chamber. 
     The funnel-shaped part that arises in this way can be regarded as a particle converter, at the surface of which H +  ions are converted into H 2 * molecules as a result of the supply of electrons. H −  ions then arise from said molecules in the manner already described. In addition, however, a direct conversion of H +  ions into H −  ions is also possible, in which case this proceeds in accordance with the equation H + +2e − →H − . 
     Furthermore, it is possible for the collar element to be divided along a plane running in the direction of the longitudinal axis of the through hole, wherein, in a manner exhibiting further preference, the separating plane runs parallel to the field lines of the filter field magnet, such that the collar element has a first, left-hand half-shell and a second, right-hand half-shell, and the half-shells are insulated from one another. As a result, an electric field running transversely both with respect to the direction of the hole and with respect to the direction of the magnetic field can be generated in the region of the collar element, which increases the deflection of the electrons and makes it impossible for a relatively large proportion of electrons to pass the collar element. 
     In order to achieve a good concentration of the beam of negative ions from the source, in one preferred embodiment a plasma diaphragm having a hole can be provided in the outlet opening, wherein the diameter of the hole is less than the diameter of the through hole of the collar element. 
     Finally, it has proved to be advantageous if the longitudinal axis of the through hole of the collar element is inclined with respect to the longitudinal axis of the tubular section. This takes account of the deflection of the particles by the magnetic field of the filter field magnet. The heavy H ions, which are not deflected very much, can pass, while the greatly deflected electrons hit the walls of the collar element. 
     Furthermore, it is preferred if the tubular section is held between the closure plate and a holding plate, the closure plate and the holding plate have bearing surfaces which run parallel to the longitudinal axis of the tubular section and which run around the outer circumference of the tubular section, and circumferential seals are provided between the bearing surfaces and the tubular section. 
     In the case of a construction of this type, the tubular section formed from insulation material is held in such a way that although the combustion chamber is sealed from the surroundings, the tubular section can nevertheless expand in the direction of its extension direction. This prevents the occurrence of stresses in the material of the tubular section on account of the high temperatures, which stresses could lead to cracks in the section. Moreover, the seals are arranged in such a way that they can be cooled by the radially outer closure wall or holding plate. The seals can on the one hand be embodied as conventional rubber seals. It is on the other hand also conceivable, however, for metal seals to be used, which are fixed to the outer surface of the tubular section in such a way that firstly a metal layer is applied to the tubular section and a metallic seal element is fixed thereto. 
     In order to increase the efficiency with which the radio-frequency field is coupled into the combustion chamber, it has furthermore proved to be advantageous if the coupling coil has windings having a flattened cross section. 
     In a further preferred embodiment, the ion source according to the invention has an electrode on the outwardly facing side of the tubular section in the region between the coupling coil and the rear wall. This has the effect that an electric field is formed between the coupling coil and the electrode, which electric field in turn has the effect that the plasma is heated further. 
     Finally, the coupling coil can be surrounded by a ferrite sleeve on the side remote from the tubular section, in order to ensure that the radio-frequency field is predominantly radiated into the combustion chamber and not emitted into the surroundings. 
     In a further preferred embodiment, the radio-frequency coupling coil is surrounded by a plurality of first magnets which run in the axial direction of the tubular section and are arranged parallel to one another around the circumference of the tubular section. Furthermore, the dipole axes, that is to say the connecting lines between the North Pole and the South Pole in the case of a permanent magnet, of the first magnets are oriented perpendicular to the longitudinal axis of the tubular section, and adjacent first magnets have a mutually opposite polarity. By means of the first magnets, a magnetic field is formed whose field lines run perpendicular to the longitudinal axis of the tubular section and the plasma can be constricted in the combustion chamber. As a result, it begins only at a certain distance from the wall of the tubular section. 
     In addition, the region of the tubular section between the coupling coil and the rear wall is surrounded by a plurality of second magnets which run in the axial direction of the tubular section and are arranged perpendicular to one another around the circumference of the tubular section, wherein the dipole axes of the second magnets are likewise oriented perpendicular to the longitudinal axis of the tubular section, and wherein adjacent second magnets have a mutually opposite polarity. 
     In this case, it is particularly preferred if the number of second magnets around the circumference of the tubular section is smaller adjacent to the rear wall than adjacent to the coupling coil. As a result, the plasma compressed to the central region of the combustion chamber by the first magnets is kept away from the rear wall. 
     In a particularly preferred embodiment, an ignition source having a gas inlet is provided, wherein the ignition source has a housing surrounding an ignition chamber and having an end face lying opposite the rear wall. Furthermore, the ignition chamber is connected to the combustion chamber via the inlet opening and the end face has an ignition electrode having a hole. The ignition electrode is covered relative to the ignition chamber by a covering plate and is insulated from the rear wall. The ignition source serves to generate electrons which are shot through the inlet opening into the combustion chamber in order to ignite a plasma there, wherein use is made of the fact that a higher pressure is present in the ignition source than in the combustion chamber. As a result, a plasma can easily be generated by applying a voltage to the ignition electrode in the ignition source, wherein the electrons can then be injected into the combustion chamber and reliably ignite the necessary plasma there. 
     Furthermore, the gas inlet of the ignition source can preferably have an inlet valve with a controller, and the controller can be configured to provide an opening and closing of the inlet valve and/or a different through-flow through the inlet valve as a function of time. This makes it possible to alter the pressure relations in the ignition source automatically in a time-dependent manner. 
     In order to be able to better control the pressure relations in the combustion chamber, it is advantageous to have an additional possibility of introducing gas into the combustion chamber without for example influencing the state in the ignition source. For this purpose, it is preferred if the front closure plate has an inlet for gas and the inlet leads through the closure plate into the combustion chamber. 
     Finally, in one preferred embodiment, an extractor is provided on that side of the closure plate which is remote from the combustion chamber, wherein the extractor has a passage opening oriented with respect to the outlet opening. An acceleration voltage can be applied between the extractor and the closure plate, and a magnetic spectrometer is provided behind the extractor as seen from the closure plate. Furthermore, an electron collector is provided, which is arranged adjacent to the magnetic spectrometer, and the electron collector is configured such that an electrical voltage can be applied. 
     The fact that the electron collector can be put at an increased potential avoids the problem that a high electron current from the ion source into the collector can have the effect of destabilizing the extraction voltage present between the extractor and closure plate. If electrons and H ions are extracted from the outlet opening, the two types of particles do not encounter the extractor during normal operation. While the closure plate is operated at a negative high voltage, the extractor is at ground. The electron collector is in turn at a negative potential, which is somewhat lower than that of the closure plate. A retarding field thus builds up at the electron collector with respect to the electron beam deflected by the magnetic field. Therefore, the beam deposits only a low energy in the electron collector, which as a result heats up only slightly. The cooling which is therefore no longer required and the reduced loading of the collector are already a considerable advantage. 
     A further advantage is that the electrons are taken up by a power supply unit with a lower voltage. It is thereby possible to use cost-effective capacitors that can be very large. Such a power supply unit is smaller and does not endanger the extractor and the source. No flashovers occur either, since the distances are very large. The actual power supply unit that generates the high voltage for the source is loaded only by the low H ion current. The capacitance of the capacitor can therefore be lower, which is important since then in the event of flashovers in the extractor and source region less energy is deposited from the capacitor into the short circuit, which is a further advantage of this arrangement. 
     Finally, the closure plate can preferably be fitted at its circumference to a fixing plate formed from an insulator material, such that the distances with respect to the housing can be smaller in comparison with a plastic mount. Overall a smaller structural size in conjunction with high operational reliability is thus possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The present invention is explained below with reference to a drawing, which illustrates only a preferred exemplary embodiment, wherein 
         FIG. 1  shows a section along the longitudinal axis of an embodiment of an ion source according to the invention, 
         FIG. 2  is an enlarged illustration of the front extraction part of the ion source from  FIG. 1 , 
         FIG. 3  is a section along the line A-B from  FIGS. 1 and 2 , 
         FIG. 4  is an enlarged illustration of the rear part of the ion source from  FIG. 1 , and 
         FIGS. 5   a  and  5   b  are circuit diagrams with respect to the ion source according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an embodiment of an ion source  1  according to the invention in longitudinal section, wherein the ion source  1  has a closure plate  2  provided with a central outlet opening  3 . The closure plate  2  is fitted to a fixing plate  4  produced from an insulator material such as, for example, a ceramic, in particular aluminum oxide ceramic (Al 2 O 3 ). The fixing plate  4  is constructed in a ceramic-appropriate manner, that is to say simply and without a multiplicity of holes, grooves, etc. It is thereby possible to fully utilize the advantage of this material over plastic, which rapidly becomes unusable owing to the formation of carbon tracks after flashovers. It is thus possible for the distances with respect to the housing to be chosen to be smaller in comparison with plastic, such that overall a smaller structural size in conjunction with high operational reliability becomes possible. 
     A seal  5  is provided between the fixing plate  4  and the closure plate  2  and the fixing plate  4  is in turn fitted to an end plate  6 , from which issues the beam tube of an accelerator installation. The ion source  1  is electrically insulated from the end of the beam tube by the insulating fixing plate  4 . 
     A collar element  7  is provided in the outlet opening  3 , the construction of which collar element will be explained in detail later in conjunction with  FIG. 2 . There extends from the outlet opening  3  in the closure plate  2  a tubular section  8 , at whose end lying opposite the outlet opening  3  a rear wall  9  is provided, said rear wall having an inlet opening  10 . Both the tubular section  8  and the rear wall  9  are formed from an insulator material. In this case, the tubular section  8  and the rear wall  9  together form the wall of a combustion chamber  11 , wherein a plasma in the combustion chamber  11  does not come into contact with electrically conductive material, with the exception of the collar element  7 . 
     Furthermore, the outlet opening  3  has a diameter which, in this exemplary embodiment that is preferred to this extent, is equal to the diameter d of the opening of the tubular section  8  that faces the closure plate  2 . That end of the combustion chamber  11  which faces the closure plate  2  is thus formed completely by the collar element  7 . 
     Moreover, the tubular section  8  is held between the closure plate  2  and a holding plate  12 , wherein the holding plate  12  likewise holds the rear wall  9  and is tensioned against the closure plate  2  by means of tensioning struts  13 . In order that the tubular section  8  is not under mechanical stress even in the event of thermal expansion on account of high temperatures in the combustion chamber  11 , the closure plate  2  and the holding plate  12  have bearing surfaces  14  which run parallel to the longitudinal axis of the tubular section  8  and which run around the outer circumference of the tubular section  8  and in which in turn grooves are formed for receiving circumferential sealing rings  15  (also see  FIGS. 2  and  4 ). As a result of this arrangement, although the combustion chamber  11  is sealed from the surroundings by means of the sealing rings  15 , it is nevertheless possible that the tubular section  8  can expand along its longitudinal axis without this influencing the effect of the sealing rings  15 . The sealing rings  15  can on the one hand be embodied as conventional rubber seals, as illustrated. It is on the other hand also conceivable, however, for metal seals to be used, the latter being fixed to the outer surface of the tubular section  8  in such a way that firstly a metal layer is applied to the tubular section  8  in the region of the bearing surfaces  14  and a metallic sealing element is fixed thereto. 
     On that side of the tubular section  8  which is remote from the combustion chamber  11 , a filter field magnet  16  is provided adjacent to the outlet opening  3 , said filter field magnet, in this preferred embodiment, being constructed from two permanent magnets lying opposite one another (see  FIG. 3 ). However, it is also conceivable for an electromagnet to be used instead of permanent magnets. The filter field magnet  16  is configured in such a way that the field lines of the filter magnet  16  run transversely with respect to the extension direction of the tubular section  8 , such that energetic electrons are prevented from leaving the combustion chamber  11  through the collar element  7  (see  FIG. 3 ). 
     Since the filter field magnet  16  is arranged outside the combustion chamber  11 , it cannot have any contact with the plasma therein and cannot be damaged. Moreover, it no longer influences the potential at the outlet opening  3  and does not dissipate any electron current. 
     Furthermore, a coupling coil  17  is provided, the windings of which are arranged around the tubular section  8  of the wall outside the combustion chamber  11 . In this case, the windings of the coupling coil  17  are provided with a flattened cross section, which increases the efficiency with which a radio-frequency field can be coupled into the combustion chamber  11 . The coupling coil  17  is additionally surrounded by ferrite elements that form a ferrite sleeve  18 , which ensures that the radio-frequency field is to a large extent radiated into the combustion chamber  11  and is not emitted into the surroundings, in order thus to minimize the losses. 
     Moreover, the coupling coil  17  is surrounded by a plurality of first magnets  19  which run in the axial direction of the tubular section  8  and are arranged parallel to one another around the circumference of the tubular section  8 . In addition, the dipole axes, that is to say the connecting lines between North Pole and South Pole, of the first magnets  19  are oriented perpendicular to the longitudinal axis of the tubular section  8 , and adjacent first magnets  19  have a mutually opposite polarity. What is thereby achieved is that around the circumference of the tubular section magnetic North Poles and magnetic South Poles are in each case adjacent to one another. As a result, the field lines in the combustion chamber  11  run approximately parabolically in the plane perpendicular to the extension direction of the tubular section  8 . 
     Furthermore, the embodiment of an ion source  1  according to the invention has an electrode  20  on the outwardly facing side of the tubular section  8  in the region between the coupling coil  17  and the rear wall  9 . Said electrode  20  has the effect that an electric field is formed between the coupling coil  17  and the electrode  20  if a radio-frequency is applied to the coupling coil  17 . Said electric field in turn has the effect that a plasma in the combustion chamber  11  can heat up further. 
     Moreover, the region of the tubular section  8  between the coupling coil  17  and the rear wall  9  is surrounded by a plurality of second magnets  21  which run in the axial direction of the tubular section  8  and are arranged perpendicular to one another around the circumference of the tubular section  8 . Furthermore, the dipole axes of the second magnets  21  are likewise oriented perpendicular to the longitudinal axis of the tubular section  8  and adjacent second magnets  21  have a mutually opposite polarity. Such a construction, as in the case of the first magnets  19 , leads to a parabolic field profile in the plane perpendicular to the extension direction of the tubular section  8 , such that a plasma in the combustion chamber  11  is compressed to the region around the center axis. 
     The first and the second magnets  19  and  21  and the coupling coil  17  can be embodied in coolable fashion. 
     In order to ensure that a plasma in the combustion chamber  11  is at a distance from the rear wall  9 , in this respect preferred embodiment the number of second magnets  21  adjacent to the rear wall  9  around the circumference of the tubular section  8  is less than the number adjacent to the coupling coil  17 . 
       FIG. 2  illustrates in particular the closure plate  2  with the collar element  7 , which is arranged in the outlet opening  3  and has a through hole  22 , wherein the longitudinal axis of the through hole  22  is inclined with respect to the longitudinal axis of the tubular section  8 . The collar element  7  has a plurality of electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28 , which are arranged one behind another as seen in the axial direction of the outlet opening  3  and which are embodied in ring-shaped fashion and are insulated from one another, such that the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  can be put at electrical potentials that deviate from one another. The electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  have different tasks during the operation of the ion source, which tasks will be explained later. In particular, the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  are configured in such a way that the internal diameter of the through hole  22  of the collar element  7  decreases with increasing distance from the combustion chamber  11 . 
     Moreover, in this preferred embodiment, the collar element  7  and thus the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  are divided along a separating plane running in the direction of the longitudinal axis of the through hole  22  and parallel to the field lines of the filter field magnet  16 , such that it has a first, left-hand half-shell and a second, right-hand half-shell, which are insulated from one another, moreover. As a result, it is possible to apply an electric field between one half-shell and the other half-shell, which electric field runs transversely with respect to the through hole  22  and the magnetic field of the filter field magnet. 
     For focusing the beam of ions emerging from the ion source  1 , a plasma diaphragm  29  having a hole  30  is provided in the outlet opening  3 . In this case, the plasma diaphragm  29  is arranged on that side of the collar element  7  which is remote from the combustion chamber  11 , and the diameter of the hole  30  is than the diameter of the through hole  22 . 
       FIG. 2  furthermore shows that the closure plate  2  has an inlet  31  for gas, which inlet leads through the closure plate  2  into the combustion chamber. Gas can thus be introduced into the combustion chamber  11  through the front inlet  31  in addition to the inlet opening  10 . 
     As shown in  FIG. 4 , the ion source  1  in this embodiment that is preferred to this extent additionally has an ignition source  32  having a gas inlet  33 , wherein the gas inlet  33  has an inlet valve  34  connected to a controller  35  (see  FIG. 1 ). The controller  35  is configured to enable an opening and closing of the inlet valve  34  and/or a different through-flow through the inlet valve  34  as a function of time, such that gas can be introduced into the ignition source  32  in pulsed and/or temporally modulated fashion. 
     The ignition source  32  comprises a housing  37  surrounding an ignition chamber  36  and having an end face  38  lying opposite the rear wall  9 , wherein the ignition chamber  36  is connected to the combustion chamber  11  via the inlet opening  10 . The end face  38  is provided with an ignition electrode  39  having a hole  40 , and the ignition electrode  39  is covered relative to the ignition chamber  36  by a covering plate  41 . Since the end face  38  is formed from an insulator material, the ignition electrode  39  is insulated from the rear wall  9 . The method of operation of the ignition source  32  will be explained in greater detail below. 
     In the preferred embodiment illustrated, the ion source  1  has, on that side of the closure plate  2  which is remote from the combustion chamber  11 , an extractor  42  having a passage opening  43 , said extractor being aligned with the outlet opening  3  and the through hole  22 . An acceleration voltage can be applied between the extractor  42  and the closure plate  2  in order to accelerate the ions from the combustion chamber  11  in the direction of the extractor  42 . In this case, the procedure is such that the extractor  42  is at ground potential, while the closure plate  2  and the combustion chamber  11  are put at an increased potential of for example −35 kV (see  FIG. 5   a ). This is readily possible since the closure plate  2  is insulated from the beam tube. 
     A magnetic spectrometer  44  is provided behind the extractor  40  as seen from the point of view of the closure plate  2 , said magnetic spectrometer serving to deflect electrons extracted by the acceleration voltage, wherein said electrons are collected by an electron collector  45  arranged adjacent to the magnetic spectrometer  44 . In this case, the electron collector  45  is configured such that an electrical voltage can be applied, which reduces the potential difference between closure plate  2  and electron collector  45 , wherein the construction is explained below with reference to  FIGS. 5   a  and  5   b.    
     Firstly, a first power supply unit  46  is provided, by means of which a high voltage (−35 kV) is applied between the extractor  42 , which is at ground potential, and the closure plate  2 . Secondly, a second power supply unit  47  is connected between the closure plate  2 , on the one hand, and the electron collector  45 , on the other hand, such that the electron collector  45  is at +5 kV with respect to the closure plate  2 , such that the energy that an electron deposits in the electron collector is approximately 5 keV and thus comparatively low. 
     The ion source  1  according to the invention is operated in the following manner. Firstly, the gas from which negatively charged ions are intended to be obtained is introduced into the ion source  1  via the gas inlet  33 . In this case, the inlet valve  34  can be driven in a time-dependent manner by means of the controller  35 , such that the gas enters into the ignition source  32  in a pulsed manner, for example. In the ignition source  32 , firstly electrons are generated by a high voltage of approximately 1 kV being applied to the ignition electrode  39 , such that a plasma is formed in the ignition chamber  36 . Electrons from the ignition chamber  36  pass via the hole  40  and the inlet opening  10  into the combustion chamber  11  and serve to ignite the actual plasma in the combustion chamber  11 . In this case, the gas in the combustion chamber  11  is heated by means of the radio-frequency field radiated in. This exploits the fact that on account of the small cross section of the inlet opening  10  and of the hole  40  in the ignition chamber  36  a higher pressure is present than is desired for the actual plasma in the combustion chamber  11 . This higher pressure enables an electrode ignition from the plasma that is thus generated, and it is possible to obtain the required electrons. 
     In order to generate the plasma in the combustion chamber  11 , gas is introduced into the combustion chamber  11  firstly via the ignition source  32  and the inlet opening  10  and secondly via the inlet  31 . Secondly, a radio-frequency field is radiated in the combustion chamber  11  via the coupling coil  17 , wherein the flattened cross section of the turns yields a high efficiency. In the combustion chamber  11 , an electric field additionally builds up between the coupling coil  17  and the electrode  20 , which electric field has the effect that the plasma heats up. The arrangement of the first and second magnets  19  and  21  leads to the plasma being concentrated on the central region of the combustion chamber  11 . 
     Moreover, the tubular section  8  formed from an insulator material and the rear wall  9  likewise formed from insulator material prevents the plasma from being “grounded” and particles thus being lost. The latter would have to be additionally generated, which would necessitate additional radio-frequency power. Since the filter field magnet  16  is furthermore arranged outside the combustion chamber  11 , it can neither “ground” the plasma nor incur damage from the plasma. 
     For the case where hydrogen is introduced as gas into the combustion chamber  11 , excited hydrogen molecules (H* 2 ) pass from the plasma into the region of the collar element  7 . It happens here that the hydrogen molecule is split up, a filtered, slow electron is taken up and a neutral hydrogen atom and a negative hydrogen ion remain, such that the process proceeds in accordance with the equation H 2 *+e − →H 0 +H − . 
     Electrical potentials deviating from one another are applied to the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  of the collar element  7 , wherein a voltage is also applied between the half-shells on account of the division of the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28  along the plane in the direction of the longitudinal axis. In this way, an electric field can be generated in the region of the collar element  7 , which electric field has the effect that electrons from the plasma can pass the collar element  7  and thus the outlet opening  3  only with a lower probability. In contrast thereto, the negative hydrogen ions, on account of their higher mass and the associated lower influenceability by fields, are nevertheless able to pass through the outlet opening  3 . 
     Moreover, the charge reversal process is intensified further by the arrangement of the electrodes  23 ,  24 ,  25 ,  26 ,  27 ,  28 . Specifically, the first rings of electrodes  23 ,  24 ,  25  as seen from the direction of the combustion chamber  11  and the potential applied to them serve to convert positive hydrogen ions (H + ) into neutral hydrogen molecules (H 2 *) from which H −  ions are then formed with slow electrons. 
     Moreover, the field of the filter field magnet  16  already prevents the passage of energetic electrons from the plasma in the combustion chamber  11  into the region of the collar element  7 , where they might otherwise destroy the H ions just formed. 
     The H −  ions are then extracted from the source by means of the potential difference by means of the first power supply unit  46  between the closure plate  2  and the extractor  42 , wherein the plasma diaphragm  29  having the hole  30  serves to focus the beam. 
     The magnetic spectrometer  44  directs electrons present in the beam into the electron collector  45 , wherein the latter, in contrast to the extractor  42  is not at ground potential, but rather at an increased potential on account of the second power supply unit  47  (see  FIGS. 5   a  and  5   b ). This has the advantage that high electron currents into the electron collector  45  do not impair the stability of the high voltage between extractor  42  and closure plate  2  and the power loss in the electron collector  45  is small and cooling of the electron collector  45  is not necessary. 
     In particular, the fact that the electron collector  45  can be put at an increased potential avoids the problem that a high electron current from the ion source into the collector  45  can have the effect of destabilizing the extraction voltage (first power supply unit  46 ) present between the extractor  42  and closure plate  2 . If electrons and H −  ions are extracted from the outlet opening  3 , the two types of particles do not encounter the extractor  42  during normal operation. While the closure plate  2  is operated at a negative high voltage, the extractor  42  is at ground. The electron collector  45  is in turn at a negative potential, which is somewhat lower than that of the closure plate  2 . A retarding field thus builds up at the electron collector  45  with respect to the electron beam deflected by the magnetic field of the magnetic spectrometer  44 . Therefore, the beam deposits only a low energy in the electron collector  45 , which as a result heats up only slightly. The cooling which is therefore no longer required and the reduced loading of the collector  45  are already a considerable advantage. 
     Furthermore, the electrons are taken up by the second power supply unit  47  with a lower voltage (see  FIG. 5   b ). It is thereby possible to use cost-effective capacitors in the second power supply unit  47  that can be very large. Such a power supply unit is smaller and does not endanger the extractor  42  and the source. No flashovers occur either, since the distances are very large. The first power supply unit  46 , that generates the high voltage for the source, is loaded only by the low H ion current for example to the target  48 . The capacitance of the capacitor can therefore be lower, which is important since then in the event of flashovers in the extractor  42  and source region less energy is deposited from the capacitor into the short circuit, which is a further advantage of this arrangement. 
     Consequently, a high yield of negatively charged ions can be achieved with the ion source  1  according to the invention, wherein there is a low requirement for maintenance work. Moreover, the source can also be used for the ionization of gaseous substances such as hydrogen, for example, which can be admitted into the combustion chamber  11  via the inlet opening  10  in the rear wall.
An ion source for generating negatively charged ions is presented and described, said ion source having a closure plate which is provided with an outlet opening and which has a wall which surrounds a combustion chamber, wherein the wall has a tubular section, which extends from the outlet opening and is formed from an insulating material, and has a rear wall, wherein the rear wall is arranged at the end of the tubular section which lies opposite the outlet opening and closes off the combustion chamber, having a coupling coil whose windings are arranged around the tubular section of the wall outside the combustion chamber, and having a filter field magnet. The problem of making available an ion source for generating negatively charged ions which has an increased yield and is also suitable for generating ions from substances which are only available in a gaseous form is solved by virtue of the fact that the rear wall is formed from an insulating material and has an inlet opening, and in that the filter field magnet is arranged on that side of the tubular section which faces away from the combustion chamber.