Patent Number: 043549985
Section: summary

The present invention is directed in general to means for improving the power gain factor in a tandem mirror fusion reactor, and more particularly, to a method and apparatus for passively clearing ions trapped in thermal barrier regions formed in a high temperature plasma. Two types of devices are generally favored for generating and confining plasmas. These are the toroidally shaped magnetic confinement devices and the linear cylindrically shaped tandem mirror magnetic confinement devices. The plasma comprises ionized gases including approximately equal numbers of positively charged ions and free electrons at high temperatures. The goal is to generate a fusion reaction in the plasma such that the energy obtained therefrom exceeds the input energy to the system, thereby providing useful output power. The easiest fusion reaction is generated from the bringing together of the two heavy isotopes of hydrogen, deuterium and tritium. When these two particles fuse together, a helium nucleus (alpha particle), a neutron and 17.6 million electron volts of energy are generated. The difficulty with such fusion reactions is that the plasma must exist at an extremely high temperature over a relatively long period of time to first obtain and then maintain the fusion reaction. Such temperatures are needed to overcome the electrostatic Coulomb repulsion between the deuterium and tritium ions. Magnetic and/or electrical field confinement configurations were found to be required to prevent loss of plasma temperature to adjacent walls of the plasma confinement chamber or cell. In addition, the fusion reaction generally produces highly energized protons, neutrons and other particles. A significant problem in the initial formation and sustained maintenance of a high temperature plasma involving such high energy particles is the problem of excluding impurity particles from the plasma. Such impurities are found to cause substantial and potentially disabling plasma energy losses. These energy losses arise because the contaminants generally have a higher atomic number than hydrogen, and the type of electronic excitation, ion recombination and bremsstrahlung radiation losses produced by their presence in the hydrogen plasma (i.e., hydrogen, deuterium, tritium and mixtures thereof) become increasingly deleterious with increasing atomic number of the contaminant. There are a few principal sources for the above-mentioned impurities. First, contaminants such as oxygen, nitrogen or carbon previously absorbed in the walls of the plasma chamber, enter the plasma due either to the vacuum required for operation of the plasma or as a result of other conditions employed to form the plasma initially. Another principal source of contaminants results from the bombardment of the chamber wall material itself by the above described energetic plasma particles and radiation, which tends to cause sputtering or even melting of the chamber wall. Suitable vacuum techniques and high temperature baking may be employed to minimize the adverse effects of absorbed contaminants, but the problem of contaminants produced by bombardment and erosion of the plasma chamber walls have provided substantial difficulties. Finally, helium "ash" impurities generated by the fusion reactions must also be periodically stripped off from the plasma. In the prior art, complicated magnetic divertor systems have been designed in an effort to remove these impurities, but such divertors have been expensive, complex, and otherwise disadvantageous. Conventional divertor devices function to skim off the most contaminated plasma near the wall of the plasma confinement chamber. Tandem mirror reactors confine the fusion plasma ions in an open ended magnetized cylindrical central cell. Axial confinement (end plugging) of the cell is generated by electrostatic potentials of more dense magnetic "mirror" confined plasmas. In operation, the mirror at each end of the fusion reactor acts as a magnetic bottle to narrow the magnetic field and thereby cause the plasma to turn back on itself. That is, the mirror coils at the end of the central cell constrict the magnetic field lines with new field lines, thereby increasing the magnetic field in this region. These extra magnetic field lines push the normal field lines together around the end, with the result that particles within the cell tend to follow the field lines until they are deflected back as if they had come in contact with a mirror. However, some ion leakage still occurs at the portion of the mirror corresponding to the axis of the cylindrical cell, so that a baseball magnet having a minimum-B field is also needed to turn particles around, thereby maintaining the plasma within the central cell of the fusion reactor with a minumum of end loss. A recent improvement in the design of tandem mirror fusion reactors proposed by Baldwin et al., [Baldwin et al., "An Improved Tandem Mirror Fusion Reactor", Lawrence Livermore Laboratory, UCID-18156, April 1979] provides means for enabling a larger electric potential to be generated in the end plug to thereby increase the efficiency of the mirror apparatus. The difficulty in older systems attempting to increase the end plug electrostatic potential was that either the density of the end plug plasma had to be increased, or the temperature thereof had to be increased. The problem with the former is that it led to unacceptable power losses and a large increase in density would produce only a small increase in field potential. Increasing the temperature was also difficult since the hotter electrons would not remain in the end plug, but would diffuse into the rest of the reactor. The Baldwin et al. improvement is accomplished by trapping hotter electrons in each end plug. Baldwin et al. disclosed means for thermally insulating the end plug electrons from contact with those in the solenoid central cell and means, in conjunction therewith, for heating the end plug electrons. Auxiliary heating of the electrons in the end plug may include the use of an electron cyclotron resonance heating unit [ECRH] or other microwave energy source. A key benefit provided by the generation of such an electron temperature differential is that it enables a plasma electrical potential barrier to be generated with a much lower end plug plasma density. In the original tandem mirror concept, the end plug plasma density n.sub.p had to be much greater than the plasma density of the central cell n.sub.c. This new concept enables n.sub.p to be approximately the same as or even less than the cell density n.sub.c. Thus, the large reduction in the required plug plasma density enabled by the higher electron temperature in each end plug both reduces the power consumed in the plugs and opens up the ability to use much simpler and less sophisticated technology in the end plugs. The end plug electron thermal insulation is created by generating a depression in the plasma potential at the entrance to each end plug, which thereby serves as an electron "thermal barrier" between each of the end plugs and the solenoid central cell. The thermal barrier is generated most simply by placing a simple mirror coil at the end of the central cell which serves to throttle down the flow of plasma ions from the central cell as said flow moves towards an adjacent end plug. The density is caused to drop as the plasma expands in cross section as it emerges from the high magnetic field at the throat of the mirror coil, thereby creating a potential depression .phi..sub.b. This depression in the positive potential appears to the negatively charged electrons as a potential barrier and therefore serves as an electron "thermal barrier" between the end plug and the reactor central cell. Thus, so long as the electrons are heated in the end plug at a rate faster than they can escape from the plug by collisions, the electron temperature in the end plug rises relative to the electron temperature in the central cell. Further details of the operation of this thermal barrier are described herein below. Although the thermal barrier concept provides significant potential advantages, a major difficulty is that "passing particles", i.e., ions and electrons crossing the thermal barrier, sometimes collide, causing some of these particles to be trapped in this thermal barrier region between the magnetic mirror and the end plug. In time, because of such collisions, the trapped particle density would grow until the total pressure thereof would equal or exceed the pressure in the central cell of the reactor. Thus, some means is required to pump out or otherwise eliminate these trapped particles from the thermal barrier. One solution for reducing the number of trapped ions would be to use magnetic pumping techniques, i.e., generating a new magnetic oscillating field. Such a field would cause the trapped ions to become more energetic, such that they could ultimately escape from the thermal barrier magnetic well back into the stream of plasma ions energetic enough to be able to freely pass across the thermal barrier. The difficulty with such a magnetic pumping system is that it requires the use of additional power in operation; it does not operative passively. A system for eliminating such trapped ions from the thermal barrier without the requirement that additional power be used therefor would thus have high utility. A further drawback of the above described magnetic pumping system is that it requires the use of copper coils close to the plasma for generation of the oscillating magnetic field. This is also undesirable due to the fact that such coils would tend to react with the plasma, becoming a generator of further impurities. Such coils also would be degraded over time due to the radiation incident thereon. Additionally, many of the ions trapped in the thermal barrier would also be impurity ions, and since control of impurities may become a problem, it would also be desirable to provide means for eliminating such impurities from the reactor, rather than reenergizing them and enabling them to reenter the plasma. Accordingly, it is an object of the present invention to provide a method and apparatus for passively removing ions trapped in a thermal barrier formed in a tandem mirror fusion reactor, to ensure that there is no buildup of such ions therein. It is a further object of the present invention to provide such a method and apparatus whereby trapped impurities are removed from the thermal barrier but not returned to the plasma. A still further object of the present invention is to provide such an apparatus which is simple in construction and which does not otherwise disrupt the flow of the plasma. Still another object of the present invention is to provide a method that is static, not involving pulsed or AC components, and a method that eliminates the need for copper coils or other degradable material in the high energy plasma environment.