Fusion Science and Technology / Volume 51 / Number 2T / February 2007 / Pages 17-22
Technical Paper / Open Magnetic Systems for Plasma Confinement
At present, in the Budker Institute there exist two full scale axisymmetric mirror type systems for plasma heating and confinement. The first one is a multi mirror system (GOL-3) and the second is gas dynamic trap (GDT). Principle of multi mirror plasma confinement was proposed by Budker, Mirnov and Ryutov in early 70s and the first experiments with a rare alkaline plasma in 1973-75. These experiments have demonstrated correctness of the proposal. According to initial concept the multi mirror reactor should be made on the basis of a dense (order of 1024m-3) plasma placed in strong (order of 10 T) magnetic field and heated by powerful relativistic electron beam (REB). In the case of use of so dense plasma one can fulfill the main requirement of the theory if the reactor has reasonable size (of order of several hundred meters). Under the main requirement we assume that mean free path of plasma particles should be larger than a single mirror cell size but significantly less than total size of the reactor. Unfortunately, even so strong magnetic field as B = 10T can not provide for magnetic confinement of plasma across the magnetic field. That is why so called "wall confinement" of plasma was discussed. One more key problem of multi mirror reactor is plasma heating. From the beginning the REB considered as the most adequate source of energy for that. It should be noted that the REB can deposit the energy into plasma only with the aid of collective processes. The first experiments on study of REB - plasma interaction were begun in 1972. They have demonstrated that indeed plasma can be heated. In the first experiments the total energy of the beam was 50 J only. In the present experiments total energy of REB increased almost by four orders of magnitude. Consequently, many new phenomena were discovered on GOL-3 in recent years. In particular, for the first time an effect of strong (by three orders of magnitude) suppression of longitudinal electron heat conductance was observed. Correspondingly, high electron temperature (of several keV) was reached. In the case of multi mirror configuration ion heating up to two keV was observed. Besides, it turned out that the heating and confinement of ions happened significantly more affectively than it could be provide by Coulomb collisions. These results permit us to look to the problem of multi mirror reactor with greater optimism. Indeed, there exists a mechanism of scattering of outflowing ions which lead to capture of such ions into a separate trap, thus increasing the longitudinal lifetime. One should add that, at present, GOL-3 facility has unique parameters of density of energy in outflowing plasma flux (1-50 MJ/m2). Such flux of plasma with hot electrons can be applied for testing of structural materials for ITER and DEMO.