V. T. Astrelin

STATUS AND PROSPECTS OF GOL-3 MULTIPLE-MIRROR TRAP

Abstract The paper reviews recent experimental results from GOL-3. Currently efforts are focused on further development of a physical database for multiple-mirror confinement systems and also on an upgrade of plasma heating systems of GOL-3 device. In general, current GOL-3 parameters demonstrate good prospects of a multiple-mirror trap as a fusion reactor.

Plasma Heating and Confinement in GOL-3 Multi Mirror Trap

Recent results of the experiments at GOL-3 facility are presented. In present configuration of the device, plasma with a density of 1014[divided by]1016 cm-3 is confined in a 12-meter-long solenoid, which comprises 55 corrugation cells with mirror ratio Bmax/Bmin=4.8/3.2 T. The plasma in the solenoid is heated up to 2-4 keV temperature by a high power relativistic electron beam (˜1 MeV, ˜30 kA, ˜8 μs, ˜120 kJ) injected through one of the ends. Mechanism of experimentally observed fast ion heating, issues of plasma stability and confinement are discussed.

Direct observation of anomalously low longitudinal electron heat conductivity in the course of collective relaxation of a high-current relativistic electron beam in plasma

The experimental results on a multiple-mirror trap GOL-3 with a short section of reduced magnetic field (“magnetic pit”) are presented. The reduced specific energy release from a relativistic electron beam in the pit brings about a region with a temperature several times lower than in the surrounding plasma. The existence of the low-temperature region directly demonstrates that the longitudinal electron heat conductivity is suppressed in the collective electron-beam interaction with plasma.

Study of the mechanism for fast ion heating in the GOL-3 multimirror magnetic confinement system

Results are presented from experimental studies of ion heating in the GOL-3 device. The experiments were carried out in a multimirror configuration with a local magnetic well. It was found that, during the injection of a relativistic electron beam, a decrease in the local density of the beam in a magnetic well, which is proportional to the decrease in the strength of the longitudinal magnetic field, results in the formation of a short plasma region with a low electron temperature. The measured longitudinal gradient of the plasma pressure corresponds to an electron temperature gradient of ∼2–3 keV/m. Axially nonuniform heating of the plasma electrons gives rise to the macroscopic motion of the plasma along the magnetic field in each cell of the multimirror confinement system. The mixing of the counterpropagating plasma flows inside each cell leads to fast ion heating. Under the given experimental conditions, the efficiency of this heating mechanism is higher than that due to binary electron-ion collisions. The collision and mixing of the counterpropagating plasma flows is accompanied by a neutron and γ-ray burst. The measured ratio of the plasma pressure to the vacuum magnetic field pressure in these experiments reaches 0.2.

Concept of fusion reactor based on multiple-mirror trap

Abstract The paper summarizes recent advances in physics of multiple-mirror confinement. GOL-3 in Novosibirsk is the only existing large-scale device of this type. Achieved plasma parameters are: n ~ 1021 m-3, T ~ 2 keV, τE ~ 1 ms. Intense experimental and theoretical studies revealed several new collective phenomena that radically change plasma behavior in the trap as compared to simple classical theory. These phenomena are intrinsically linked to the second major feature of GOL-3, namely, fast plasma heating by a high-power relativistic electron beam. Collective beam-plasma interaction delivers energy to plasma through strong Langmuir turbulence and changes other plasma properties as well. In particular, the turbulent plasma in GOL-3 features suppressed axial heat transport, fast collective heating of ions, limitation of axial particle loss, and MHD stabilization by a magnetic shear. Mentioned phenomena greatly improve prospects of multiple-mirror confinement for fusion reactor applications. An outlook for possible fusion-scale device is presented.

Progress on the Multimirror Trap GOL-3

Main results of researches on plasma heating and confinement of dense plasma in the multimirror trap GOL-3 are presented. Recently magnetic system of the facility was converted into completely multimirror one. This results in further improvement of energy confinement time of plasma with ion temperature ~1 keV. Collective plasma heating by ~120 kJ (~8 ɷs) relativistic electron beam results in Te ~ 2 keV at ~1021 m-3 density. High Te exists for ~10 μs. To this time Ti reaches ~2 keV. Ion temperature keeps at the high level during ~1 ms. The energy confinement time sufficiently increases and a value of nτE = (1.5 [divide] 3)·1018 m-3s.

Role of q Profile for Plasma Confinement in the Multimirror Trap GOL-3

Dense plasma heating by a relativistic electron beam and its confinement are studied in the multiple mirror trap GOL-3. Axial currents, which exist in the system, cause helical structure of the magnetic field. The safety factor q is shown to be below unity on the axis. Experimental data on the distribution and evolution of currents, structure of the magnetic field, and their influence on confinement and on MHD activity are discussed.

Experiments with “Thin” Electron Beam at GOL-3

Abstract The latest experimental campaign at the GOL-3 multiple-mirror trap was mainly aimed at features of heating and stability of the electron-beam-heated turbulent plasma. The discussed experiments feature a reduced-cross-section electron beam with the current decreased down to 1 ÷ 1.5 kA at the current density of ~1 kA/cm2 (the same as in the “full-scale” experiments). The hot plasma cross-section decreased correspondingly. Lowered current of the electron beam became less than the critical vacuum current. This gives the possibility to make a direct comparison of regimes with the beam injection into a neutral or a preliminary ionized deuterium. New experimental results will be presented on the beam relaxation in the plasma and on heating and stability of the reduced-cross-section plasma with low central safety factor q(0) ~ 0.3. Stabilization of some MHD modes by a controlled coupling of the plasma with an exit receiver plate was demonstrated.

Development of Extended Heating Pulse Operation Mode at GOL-3

Novel technology of electron beam generation for plasma heating in GOL-3 was developed and for the first time used in the experiment. The distinctive features of the new beam are non-relativistic energy, medium power and sub-ms duration. The experiments were done at the following beam and plasma parameters: ~100 keV, ~10 MW, >100 μs, ~1020 m-3. The beam was safely transported through the 13-m-long deuterium-filled multiple-mirror solenoid. The plasma was created and then heated by the beam. Main physical task for the reported experiments was to reach quasi-stationary plasma conditions during the long-pulse beam injection.

Novel Injector of Intense Long Pulse Electron Beam for Linear Plasma Devices

A novel high-power (10 MW) sub-millisecond electron beam is developed for injection into the open (linear) plasma devices. The beam is produced by extraction of electrons from a plasma of pulsed arc discharge in hydrogen. The beam is extracted and accelerated with multiaperture diode-type electron optical system with 241 small round apertures, which are arranged in a hexagon-al pattern. The injector prototype was installed into the end plasma tank of GOL-3 multiple mirror trap and tested to produce an electron beam with up to 100 keV electron energy, about 100 A total beam current and 0.7 ms or longer pulse duration. In a series of preliminary experiments the electron beam was injected into the GOL-3 plasma chamber filled with deuterium gas with a density of 1014-1015cm-3 and transported in a corrugated magnetic field ( up to 1.4 T) along the trap at a distance of 12 m.