Magnetic fusion reactions, such as between deuterium and tritium combined in a hot plasma, have the potential to provide energy with minimal production of long-lived radioisotopes. It is well known that such reactions take place under very high temperatures conditions at which particle momentum is sufficient to overcome the mutual electrostatic repulsion of the reacting nuclei. Physical walls are not feasible for containing the reactions at the required temperatures, because the walls will cool the reactants, thereby quenching the reaction. As such, devices or apparatuses have been pursued that use non-physical means to containing the fusion reactions. These non-physical means of containment have used inertia, magnetism and electrostatic forces as the means for containing the reactions. Because the present invention is in the class of devices that use magnetic forces, the following will be confined to a discussion of this class of device.
Hot plasma is an electrically conductive fluid that will flow with little resistance along magnetic field lines, but diffuse slowly across field lines because it is retarded by the magnetic reaction forces resulting from currents induced in the plasma. For this reason, an objective in designing a magnetic containment system is to suspend the plasma in a field with lines which form closed paths within the plasma confinement volume, and therefore do not provide direct escape paths along field lines. Closed current loops in the conductive plasma form closed magnetic flux loops within the plasma. A simple current loop, for example, forms poloidal flux loops that enclose a toroidal volume which surrounds the current loop. Three magnetic confinement schemes using such internal magnetic currents to generate at least a portion of the confining field are tokamaks, spheromaks and theta pinch systems.
Spheromaks are formed by two interlinked plasma current loops generated by inductive and/or coaxial gun conductive means, resulting in a near-spherical toroidal confinement volume with field components in both the poloidal and toroidal directions. Tokamaks are toroidal confinement volumes formed by toroidal field components from an external set of poloidal coils and poloidal field components from a toroidal current loop induced by a time-changing external field. Theta pinch systems are toroidal confinement volumes formed by a single plasma current loop induced by a time-changing external field, with field components primarily in the poloidal direction. Reference also should be made to U.S. Pat. No. 4,436,691; Thomas R. Jarboe, "Review of Spheromak Research", Plasma Physics and Controlled Fusion, Vol. 36, pp. 945-990 (1994); Kenro Miyamoto, Plasma Physics for Nuclear Fusion (Revised Edition), pp. 530-552, The MIT Press, Cambridge, Mass. (1987); and M. Tuszewski, "Field Reversed Configurations", Nuclear Fusion, Vol. 28, No. 11, pp. 2033-2092 (1988).
There are a number of known processes for forming currents in the plasma as briefly discussed below. In one process, an inductive transformer action induces transient current loops in a stationary plasma, using time-changing external fields. This process is simple and does not require conductive electrodes to contact with the plasma. However, it is by nature transient, not steady-state. This principle is used in the tokamak and theta pinch approaches
In another process, a conductive current transfer forms a plasma current between two electrodes, and then the plasma is moved away from the electrodes. The movement causes the current to detach from the electrodes and reconnect as a closed loop. The process may be repeated rapidly to form a sequence of current loops which merge with and sustain a preexisting current loop. See also Y. Ono, A. Morita, M. Katsurai and M. Yamada, "Experimental Investigation of Three-Dimensional Magnetic Reconnection by Use of Two Colliding Spheromaks," Phys. Fluids B, Vol. 5, pp 3691- to 3701 (1993). This principle is used in the coaxial gun spheromak formation approaches referenced above.
In yet another process, an inductive current transfer forms a plasma current in a plasma volume, and then the plasma is moved away from the formation area. The movement causes a process in which magnetic reconnection results in a closed current loop that is no longer inductively linked with the original formation field. As with the conductive current transfer process, this process may be repeated rapidly to form a sequence of current loops which merge with and sustain a preexisting current loop. This principle is used in the inductive and conical theta pinch spheromak formation approaches referenced above.
Particle beams or traveling waves may be used to differentially move the plasma electrons relative to the positive nuclei and create a steady-state current within the plasma. This method is discussed in the tokamak confinement scheme referenced above. It is also used in the rotamak, confinement scheme, a variation of the spheromak. See also P. M. Bellan, "Particle Confinement in Realistic 3D Rotamak Equilibria", Physical Review Letters, Vol. 62, No. 21 pp 2464-2467 (1989).
Radial diffusion of plasma through the poloidal field lines of a tokamak has been observed to generate a toroidal "bootstrap" current which supplements or replaces the inductively generated current (See Kenro Miyamoto, Plasma Physics for Nuclear Fusion (Revised Edition), pp. 225 and 552, The MIT Press, Cambridge, Mass. (1987)). This is a first order electromagnetic dynamo process, and is potentially steady-state, assuming a sustained flow of fresh plasma into the confinement volume. It has the further advantage of being driven by the temperature and pressure differences between the central fusion reaction zone and the outside of the containment volume, and therefore does not consume external electric power.
Magnetic mirror plasma confinement devices generate open solenoidal magnetic fields with stronger mirror fields at the ends to reflect plasma particles back toward the center of the containment volume. Experiments were carried out on various means of inserting plasma into these devices, including shooting high velocity pulses of plasma axially through the mirrors, e.g., see B. W. Johnson and J. G. Siambis, "Injection of a Streaming Plasma Into a Mirror Machine," Plasma Physics, Vol. 15, pp 369 to 374 (1973). The experimental data include the finding that the peak transient plasma density in the inlet throat of the mirror is about 6 times higher than the peak plasma density achieved at the midplane of the device. The authors interpret this high inlet density as the result of a shock wave, and treat it as a problem to be overcome in achieving efficient plasma injection into the mirror machine.
The above described systems, have not progressed to the stage where conditions conducive to fusion reactions have been continuously maintained so that energy can be reliably produced for consumption. As such, there is still need for devices, systems and methods for confining plasmas under conditions conducive to fusion reactions and more particularly a system/method where a steady-state current is formed internal to the plasma.