M. A. Makarov

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.

Subterahertz Emission at Strong REB-Plasma Interaction in Multimirror Trap GOL-3

Abstract Investigation results for electromagnetic radiation emission induced by strong Langmuir turbulence driven by a microsecond relativistic electron beam are presented. The radiation is associated with a plasmon-plasmon merging process, which generates photons at a double plasma frequency in the range of few hundreds GHz. An original radiometric system for radiation spectrum and power measurements is described.

Diagnostic system for studying generation of subterahertz radiation during beam-plasma interaction in the GOL-3 facility

The design principles and construction of the subterahertz radiometric spectral systems developed for the GOL-3 facility are described. The spectral systems are designed according to the quasi-optical scheme and use multilayer filters based on frequency-selective surfaces. The design and manufacturing technology of such elements are discussed. The results of measuring subterahertz radiation of plasma at the frequency close to the double plasma frequency are presented.

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.

Plasma-Surface Interaction During ITER Type 1 ELMs: Comparison of Simulation with QSPA KH-50 and the GOL-3 Facilities

Abstract The paper presents experimental investigations of plasma-surface interaction and materials behavior under plasma loads relevant to type I ITER ELMs. The experiments were performed with quasi-stationary plasma accelerator QSPA Kh-50 and multi-mirror trap GOL-3 devices located in Kharkov (Ukraine) and Novosibirsk (Russia) respectively. QSPA generated repetitive plasma streams of duration 0.25 ms and the energy density up to 2.5 MJ/m2. In GOL-3 multi-mirror trap plasma was heated up to temperature of 2-4 keV by a high power relativistic electron beam. Energy density in the exhaust plasma stream vary from 0.5 to 30 MJ/m2. Surface patterns of the targets exposed by QSPA and GOL-3 plasma are analyzed. Cracking, development of tungsten surface morphology and droplets splashing are discussed. It is shown that under an applied energy density loads (>1 MJ/m2) the evolution of surface morphology due to plasma irradiation are similar for two devices in spite of the qualitative differences of particles energy of the impact plasma streams. Formation of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm are identified after irradiation of tungsten surface. Experiments show that major cracks (cell size of 1000 μm) are attributed to a ductile-to-brittle transition. The key role of heat loads magnitude on development of surface due to powerful plasma impacts is demonstrated.

Stabilization of Relativistic Electron Beam by Dense Plasma Cloud in GOL-3 Expander

Abstract The paper presents new experimental results from the GOL-3 facility on stabilization of some beam instability modes by controlled conditions at an exit beam receiver. In the experiments the space near the beam collector in the exit expander was filled with krypton. Position and shape of the beam footprint at the exit collector was imaged using the beam bremsstrahlung. The beam shape changes were detected by a set of magnetic probes. Improvement of the beam stability due to krypton puffing was achieved. Possible mechanisms of such stabilization are discussed.

Energy Spectrum of Electrons in Flow from Plasma Column Heated by REB at GOL-3 Facility

Abstract Efficiency of the beam–plasma interaction was investigated for the case of small radial size intense E-beam on the base of the energy spectrum measurements with multifoil analyser. This shows good prospects of such beams for reactor applications.

MHD Activity in GOL-3 at the Stage of Plasma Cooling

Abstract Transient MHD activity in the multiple-mirror trap GOL-3 at the stage of plasma cooling after decay of the net current was studied. Such events are diagnosed as short bursts (generally shorter than 5 cycles) of oscillations of azimuthal magnetic field with the frequency within 0.1 ÷ 1 MHz. Earlier transient events at the plasma decay stage were also observed. At that time an existence of long-lived internal current structures in the GOL-3 plasma was supposed. The paper presents new results from the magnetic diagnostics. Azimuthal and longitudinal mode localization agrees with the assumption that the magnetic field is generated by filaments with the current in the range of 1 ÷ 50 A in nearly uniform azimuthal distribution. Statistical analysis shows that the main oscillation frequencies are grouped near the inversed time of the Alfven wave propagation along the trap. Power dependence of the pulse probability on its amplitude with the index α = -1.87 was found. Relationships of MHD activity on the regime of plasma heating and on a degree of the beam stabilization at the exit receiver are discussed.

Study of Plasma on the GOL-3 Facility by Imaging VUV Spectroscopy

Abstract The series of experiments on plasma heating by the electron beam of reduced diameter was carried out at the GOL-3 facility. To define the size of a high-temperature plasma region the imaging VUV spectrometer was used in these experiments. In the paper the description of the diagnostic system, the results of measurements, the comparison with simulation data of the ionization balance are presented. Estimations of transverse diffusion and macroscopic movements of the beam are discussed.

Formation of high-brightness REB to generate THz radiation in beam-plasma system

Summary form only given. Terahertz emission from plasma with strong Langmuir turbulence excited by high current relativistic electron beam (REB) is experimentally studied at the GOL-3 facility (BINP). The typical parameters of the beam used in the experiments, were the following: 0.6÷0.8 MeV/20 kA/10 μs. In recent experiments the beam current density was about 1÷2 kA/cm2 in a plasma column with the density ~ 2·1014 cm-3 that allowed us to get sub THz-emission with a power of a few kW/cm3 in the frequency range 100÷400 GHz. To extend our experiments on emission to the range of 1 THz we have to inject the beam into the plasma column of higher density 1÷2·1015 cm-3. This level of the plasma density requires the substantial increase of the beam brightness in the plasma. To solve this problem we have performed 2-D self-consistent simulations by numerical code POISSON-2 for the beam formation at different geometries of a magnetically insulated ribbon diode. In addition, the transformation of the generated ribbon beam to a circular one and its compression to the radius ~2 cm in the magnetic field ~4 T in which the plasma column is placed, were also considered in the computer simulations. To determine the brightness of the beam we have measured the beam current density and the angular spread of the beam electrons after the transformation and compression of the beam cross-section. The comparison of the experimental and simulation data has been done. The obtained results are used for the design of a new diode configuration with high brightness of the beam.