Plasma confinement by magnetic mirror configuration
Plasma parameters above 10 keV and 1021 m-3sec are needed for the realization of the fusion reactor.
When the current on two circular coils arrayed flows in a parallel direction with each other, the magnetic field strength near both coils becomes strong and the magnetic field strength between coils becomes weak. This magnetic field configuration is called as the simple mirror configuration. (as shown below) Charged particles wind around magnetic field lines and are trapped in this magnetic configuration. Particles are reflected at the location with strong magnetic field strength near the coils. This is the principle of the plasma confinement in the magnetic mirror configuration. On the other hand, some particles escape from the open end of the configuration. These particles are called as end-loss particles. And moreover, the magnetic field strength on outer region in the radial direction becomes weaker than in the center. Plasmas escape across the magnetic field line and become unstable. This is called as MHD instability.
An idea of minimum-B configuration is introduced for the stable plasma confinement. When the current flows in a baseball shape coil, the magnetic field strength becomes minimum at the center and becomes strong in all directions from the center. This magnetic configuration is called as the minimum-B configuration. (as shown below) Plasmas in the minimum-B configuration feel stronger magnetic field strength in all directions and are confined stably. The shape of the magnetic field line is axisymmetric in the simple mirror configuration and is non-axisymmetric in the minimum-B configuration.
Improvement of the plasma confinement with tandem mirror devicse
Confinement due to potential
Considering the economic property of nuclear fusion reactor, the suppression of the end-loss is required moreover.
To improve the confinement in the mirror configuration, an idea of the tandem mirror system, in which plasmas are confined by potentials, is proposed. That is, mirror fields are arranged in a line and potentials are formed on both end mirrors due to formation of high density and high temperature plasmas. Plasmas in the central region are confined due to this potential. The principle was demonstrated by GAMMA 6 for the first time and improvement of the confinement has been observed. Later, several tandem mirrors (for example, TMX in US) succeeded to improve their confinement due to the potential formation. Mirror systems have advantages to use these escape particles from the open end for direct energy conversion. This will become more advanced power generation system than other torus systems when DD fusion is realized in the future.
Further improvement of confinement
Tandem mirrors with thermal barrier
It is very important for the realization of fusion reactors to form the potential barrier effectively by using simpler technology.
Thermal barrier is the potential well which is formed in front of positive potential for the ion confinement. The purpose is to reduce interactions between hot electrons in the plug/barrier and warm electrons in the central cells. Hot electrons contribute to form the potential. GAMMA 10 has demonstrated this princeple and improvement of the confinement .
Axisymmetrized tandem mirrors
Next to the improvement of the axial confinement, it will become important to suppress the particle transport in the radial direction. The radial transport occurs due to non-axisymmentric configuration of the minimum-B field (called as neoclassical transport). GAMMA 10 is designed to be an axisymmetrized tandem mirror, in which particles are reflected mainly in the axisymmetric plug/barrier cells. Actually, neoclassical transport in GAMMA 10 has been demonstrated to be small.