Open magnetic systems for plasma confinement are suitable for a number of nuclear fusion applications. The near term one is a high-power D-T fusion neutron source [1] capable of producing neutron flux of several MW/m2. Such neutron flux is required for full-scale neutron-material interaction research, aimed at design of first wall and other structural elements of future fusion reactors. Furthermore, high-power neutron source can be used as a driver for subcritical fission reactors including devices for burning of long lived radioactive wastes [2]. Finally, a series of studies has shown that open magnetic traps with reasonably improved axial confinement are completely consistent with reactor-level fusion projects with power gain factor Q≫ 1. Improvement of axial confinement can be achieved, for example, through the use of ambipolar plugs [3] or multimirror end-sections [4]. The most attractive from engineering and physical standpoint are axisymmetric magnetic mirrors. The simplicity of design, intrinsic capability of sustaining high beta plasmas (~ 1), natural channel of impurities and thermonuclear ashes removal, possibility of direct power conversion with near-unity efficiency are crucial benefits of open magnetic systems for plasma confinement.
However, lack of experimental data for the electron temperatures suitable for nuclear fusion applications significantly diminishes the advantages of open systems mentioned above. Low electron temperature, most probably, is caused by high level of axial energy losses which was observed in many experiments. This circumstance accounts for today's relatively low level of research activity in the field of open systems for magnetic plasma confinement.
This report presents the first results of auxiliary electron cyclotron plasma heating (ECRH) experiment, which is presently under way on the gas-dynamic trap (GDT) machine – an axisymmetric magnetic mirror with high mirror ratio [5].
The plasma confined in GDT consists of two components with different mean energies: warm ions with isotropic Maxwell velocity distribution and a population of hot ions, which is produced as a result of oblique injection of hydrogen or deuterium neutral beams into the plasma. Suppression of radial losses caused by the development MHD plasma instabilities is accomplished by implementation of an experimentally and theoretically well-grounded method of vortex confinement [6]. Application of this method allows us to obtain value of local plasma beta approaching β = 0.6 in the regions near the turning points of hot ions [7].
Energy confinement times of hot ions as well as their velocity spread are determined mostly by the collisional slowing-down on the bulk electrons. Since the collisional time proportional to T2/3, the electron drag force is rapidly decreasing with increasing electron temperature.
In a framework of project # 11.G34.31.0003 (Government Decree №220 of April, 2010) launching of microwaves with frequency of 54.5 GHz, 400 kW power and operation time of 0.5 ms was realized. It allowed to achieve electron temperature of more than 0.5 keV, which was measured by Thomson scattering in near-axial region of plasma column [8]. Spectral analysis of scattered laser radiation allows us to conclude that microwave power is effectively delivered to thermal electrons.
According to numerical simulations performed in [2] the GDT based neutron source with electron temperature of more than 0.4 keV is sufficiently effective as a driver for subcritical burner of minor actinides. Other study [9] shows that a GDT-type neutron generator for materials testing with electron temperature of at least 0.3 keV is capable of delivering to the test zones neutron flux of more than 0.4 MW/m2.
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4. A. Beklemishev, A. Anikeev, V. Astrelin, et al., Fusion Sci. Technol. 63 1T, 46 (2013).
5. A.A. Ivanov and V.V. Prikhodko, Plasma Phys. Control. Fusion 55, 063001 (2013).
6. A.D. Beklemishev, P.A. Bagryansky, M.S. Chaschin and E.I. Soldatkina, Fusion Sci. Technol. 57, 351 (2010).
7. A.A. Ivanov, A.D. Beklemishev, E.P. Kruglyakov, et al., Fusion Sci. Technol. 57, 320 (2010).
8. P.A. Bagryansky, Yu.V. Kovalenko, V.Ya. Savkin, A.L. Solomakhin and D.V. Yakovlev, First results of an auxiliary electron cyclotron resonance heating experiment in the GDT magnetic mirror. Nucl. Fus. 54, No 8, 082001 (2014)
9. A.A.Ivanov, E.P.Kruglyakov and Yu.A.Tsidulko, J. Nucl. Mater. 307-311, 1701 (2002).