Patent Number: 043495052
Section: description

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown schematically a prior art neutral beamline with direct energy recovery of positive ion energy. One typical neutral beamline of this type is described in U.S. Pat. No. 3,713,967, issued Jan. 30, 1973, to Gordon W. Hamilton et al for "Energetic Neutral Particle System for Controlled Fusion Reactor." A light isotopic species positive ion source 11 is operated at a high positive potential, typically +40 kv for a source of hydrogen (H) or deuterium ions, for example. The ion source may be mounted to a vacuum enclosure 13 for the beamline system by means of an electrical insulator and seal assembly 15. The ion beam from the source 11, is accelerated by means of a +40 kv power source 19 connected between ground and the plasma grid 17 of the source 11. A slight decel voltage typically 1 kv negative is applied by means of a -1 kv power source 21 between ground and the extraction grid 23 of the ion source. The exit grid 25 of the source is tied to ground potential. The negative decel voltage applied between the extraction grid 23 and the exit grid 25 prevents electrons generated in the neutralizer cell 27 from drifting back into the ion source 11. The beam of positive ions extracted from the ion source 11 is thus accelerated to ground potential and remain at ground potential through the gas cell neutralizer 27 by connecting the neutralizer cell to ground potential. In the gas cell, some of the positive ions entering the cell are converted to neutral particles with high kinetic energy and travel along the accelerated beam path and into an evacuated drift tube 29 coupled to a neutral beam utilization device, such as a fusion reactor 31 to heat a magnetically confined reactor plasma. Magnet poles 33, located along the beamline a short distance from the exit end of the neutralizer cell 27 are arranged to deflect the positive ions from the neutral beamline into converter cells 35. In the converter cells 35, the electrons contained in the beam for space charge neutralization are blocked by means of electrostatic fields and strong positive fields are used to decelerate and collect the ions. The current produced from the collection may flow through an electrical load 37 or be fed back to reduce the acceleration source 19 power requirements. An alternate scheme has been proposed for energy recovery based on magnetic suppression of electrons, as discussed above which employs a funnel-shaped ion collector encircling the beam downstream of the electrostatic suppressor electrode. This collector is also operated at a high positive decelerating potential and the recovered energy in the form of recovered ion current may be fed back to supplement the ion source current. These prior art systems have inherent disadvantages as pointed out above. According to the present invention, a neutral beam generator with direct positive ion energy recovery based on transverse magnetic field suppression of electrons at the neutralizer tube output will now be described with reference to FIG. 2. It will be understood that the beam must be manipulated within a vacuum containment, such as the vacuum casing 13 shown in FIG. 1. However, in order to simplify the drawing, the vacuum casing is not shown in FIG. 2. Further, it will be obvious that it is necessary to remove the background gases and the deenergized species from which the charge has been collected. This is done as in any conventional beamline by cryocondensing vacuum pumping panels or other suitable vacuum pumping means (not shown). Referring now to FIG. 2, a positive ion source 41, has its plasma grid 43 connected to a positive voltage source 45 of typically +4 kv for providing an acceleration boost voltage (V.sub.boost). The primary ion acceleration voltage (V.sub.accel) is provided by supply 47 which is connected between ground and the neutralizer gas cell 49, which is also connected to the exit grid 51 of the ion source. The acceleration voltage will depend upon the beam energy requirements and the ion source capacity. For the illustration here V.sub.accel is -40 kv for a 60 amp ion current. This is typical for heating plasmas in the Princeton Large Torus (PLT) research fusion reactor. A deceleration voltage (V.sub.decel) supply 53 is connected between the neutralizer tube 49 and the extraction grid 55 of ion source to provide a slightly negative (typically -1 kv) extraction grid voltage relative to the neutralizer tube 49 voltage to prevent the drift of electrons from the neutralizer back into the ion source. This biasing arrangement provides the potential distribution along the beamline as shown in FIG. 3. As will be seen from FIGS. 2 and 3, the ion beam 57 diverted from the neutral beamline 59 is decelerated to ground potential and the ion charge is collected on a grounded collector 61. It will be appreciated that the entire grounded enclosure may be used as an ion collector since the ions are decelerated to ground potential immediately at the neutralizer exit and thus may eliminate the need for the specific collector surface 61 to obtain energy recovery. In order to obtain the energy recovery at ground potential, the electrons must be blocked at the neutralizer exit. If the electrons are allowed to go to one of the grounded surface potentials, they would be accelerated across the dotted path (the ion deceleration potential) in FIG. 3 and would collectively give up more energy than could be recovered from the positive ions. In fact, they would overcurrent the high voltage supply 47 and turn off the ion source. To accomplish electron blocking, a magnetic field is provided transverse to the beam in the exit end of the neutralizer tube 49. Referring to FIGS. 2, 4, and 5, it will be seen that the field is provided by means of electromagnetic pole pieces 63a and 63b or equivalent magnetic field-producing means are disposed in juxtaposition across the beam path in the exit end of the neutralizer tube 49. The magnetic field may be varied to obtain the proper field strength to block the exit of the electrons. Also, the magnet pole pieces 63 may be tilted at a 45.degree. angle, as shown, or placed at any convenient angle so long as the neutralizer tube 49 is tapered to conform to the magnets. As shown in FIGS. 2, 4 and 5, the end of the neutralizer tube 49 is tapered to conform to the 45.degree. tilt of the magnet poles orientation and extended into the center of the axial extent of the magnetic field along the beamline provided by the pole pieces to closely couple the magnetic field with the neutralizer end geometry. This ensures a maximum magnetic field for blocking the neutralizer generated electrons. It will be appreciated that other configurations for producing the field transverse to the beam may be employed, especially since the ion beam charge is collected at ground potential. To remove the small portion of electrons, which are not forced back into the neutralizer by the blocking magnetic field, an electron collector ring 65 is mounted around the end opening of the neutralizer tube by means of electrical insulators 67. The collector ring is made of a nonmagnetic electrically conductive material, such as copper, in the form of a collar with the same end geometry as the neutralizer tube 49 end, i.e., tapered at a 45.degree. angle at the tube exit end, and extends about 1.5 centimeters past the neutralizer tube end. A positive voltage supply 69 is connected between the neutralizer 49 and the electron collector collar 65 to apply a slightly positive (approx. 300 V) bias voltage (V.sub.collector, FIG. 3) relative to the neutralizer 49 which is biased highly negative. The operation of the system may best be explained with specific reference to FIGS. 4 and 5. The electrons streaming toward the exit of the neutralizer 49 together with the ions and neutrals generated in the neutralizer gas cell first experience an increasing magnetic field (B) perpendicular to the general direction of electron travel with the beam as well as an accelerating primary electric field (E) generated by the negatively biased neutralizer 49 and the rest of apparatus at ground potential exterior to the neutralizer as they approach the end of the neutralizer tube 49. The electrons are then carried out toward the neutralizer end edge by an E x B drift while gyrating as indicated by the e path illustrated in FIG. 5. Once the electrons pass the edge of the neutralizer tube they are accelerated into the electron collector ring. The ring is preferably coaxially disposed about and spaced from the neutralizer tube exit end and functions as an interposed surface, biased a few hundred volts positive with respect to the neutralizer, that terminates the electrons. The energy loss is only a few hundred electron volts of energy instead of the typically 40 keV (depends on the value of V.sub.accel) they would give up in traveling to one of the grounded surfaces. It should be pointed out here that all the vacuum enclosed surfaces beyond the neutralizer cell 49 are at ground potential including the magnet pole pieces 63. Therefore, the ions coming out of the neutralizer 49 experience both retardation due to the primary electric field and transverse deflection due to the magnetic field. Initially, the ion gyroradius is typically several times greater than the gas cell diameter, but it is reduced as the ions are decelerated within the pole region to a speed corresponding to the accelerator boost potential. When they finally reach the surroundings (the ion collector, 61, vacuum chamber walls or pole faces), they impart only the energy corresponding to the accelerator boost potential. The boost potential 45 is kept as low as possible and lies between 2% and 10% of the V.sub.accel potential 47 depending upon various beamline parameters. The role of the V.sub.boost potential is to ensure the ions have enough energy to strike a grounded surface (energy recovery) rather than deflect back into the neutralizer cell 49 (total energy loss). The exact motion of the ions, however, depends on the electric and magnetic field configuration in the magnetic pole region. Free electrons outside the gas cell, in the magnet region and the surroundings, will not hinder the ion current recovery since they are approximately in the field free-region at ground potential. The deceleration of the full-energy ions to an impact velocity at an energy level equal to the V.sub.boost potential returns most of their kinetic energy (corresponding to the V.sub.accel potential) to the electrical power supply system used to accelerate them. Thus, their main energy content is recovered, and the ions convert back to low energy neutral gas to be pumped out of the vacuum chamber as by cryopumping. In the neutralizer, three different energy groups of ions of the source species are present: full energy (corresponding to the initial kinetic energy provided by V.sub.accel +V.sub.boost), one-half energy, and one-third energy. The three energy groups originate in the atomic, diatomic and triatomic states of the source species. As pointed out above various light isotopic species may be used depending upon the particular reactor application requirements. The full energy ions are of primary interest for the purpose of energy recovery since they represent the primary energy component of the beam (85-90%). However, if the one-half and one-third energy ions exit the neutralizer with the full energy ions they do not have sufficient energy to reach the ground potential surfaces since the potential drop between the gas cell and the surroundings is almost the full-energy potential. These ions tend to bend in smaller orbits due to the magnetic field and are forced to strike either the neutralizer tube or electron collector at which point their energy is lost. If they strike an exterior wall of the neutralizer tube or electron collector, additional energy may be lost due to secondary electrons emitted at impact some of which may travel to a grounded surface. The secondary electrons which are accelerated directly to ground potential detract from the energy recovery efficiency of the full energy ions. This loss can be controlled substantially by proper selection of the magnetic field strength to block the fractional energy ions from exiting the neutralizer. Thus, the fractional energy ions are dumped back into the neutralizer without generating parasitic secondary electrons. EXAMPLE A modified duo PIGatron ion source capable of 40 kv/60 A operation (developed for heating plasmas in the PLT and impurity study experiment) was used to test an experimental configuration as illustrated in FIG. 2. This source is particularly well suited for every recovery investigation due to the high-percentage (approx. 85%) full-energy ion component with a hydrogen beam. An available ion current (I.sub.A) of 18 amps at 40 keV was used. The proof of principle experiment was limited to approximately 20 kV due to electronic problems. The following supply voltages were used: V.sub.boost =800 v PA1 V.sub.accel =20 kV PA1 V.sub.decel =1 kV PA1 V.sub.collector =300 v A conventional neutral beam target was used to measure the neutral beam energy. The recovered ion current I.sub.R was typically 1 amp. The electron leakage current, from full energy electrons impinging upon the grounded ion collector, was typically less than 1 amp. The leakage current (I.sub.e) was determined by calorimetrically measuring the power drain to water cooled, ground potential plates covering the magnet poles and dividing this power by the V.sub.accel potential. The efficiency (n), which may be defined as follows ##EQU1## varied between 20% and 80%. The large error was due to subtracting two large numbers to obtain a small difference. The magnetic field strength was typically 1000 Gauss. In one test run, the magnetic field was turned off, the electron collector current immediately destroyed the 400 amp rated blocking diodes in the collector voltage supply. This illustrates the electron blocking function of the strong magnetic field at the neutralizer exit. Since electron blocking is achieved by a magnetic field, the size and density of the beam are not critical as in electrostatic blocking schemes. Thus, it will be seen that a charged particle recovery system for a neutral ion beam generator based on magnetic blocking of electrons is provided. The blocking magnet in combination with the ion retarding electric field at the beam neutralizer exit end separates the charged ionic particles from the neutral beam and electrons to provide energy recovery of the full energy ion component of the beam. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.