I. V. Kandaurov

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.

Dissipation of strong Langmuir turbulence in nonisothermal non-Maxwellian plasma

The regime of strong Langmuir turbulence characterized by the plasma nonisothermality and by the presence of an appreciable non-Maxwellian hot-electron component was experimentally studied. Turbulence was excited in the preliminary produced plasma by the relativistic electron beam. Thomson scattering of laser IR radiation served as the main diagnostic method. The spatial spectra of the Langmuir turbulence and of the attendant ion-sound turbulence were studied using Thomson collective scattering. Thomson incoherent scattering was used for studying the plasma electron distribution function and searching for the local dips of plasma density. Stark spectroscopy of turbulent microfields and the method of observation of plasma radiation at the double plasma frequency were also used. Based on the experimental data, the mechanism of Langmuir oscillation damping by plasma electrons was analyzed. The Langmuir wave conversion induced by the ion-sound turbulence is the most probable channel for energy transfer from the turbulence to plasma electrons, the low-frequency fluctuations being the direct consequence of the strong Langmuir turbulence.

Spectra of developed Langmuir turbulence in a nonisothermal magnetized plasma

Strong Langmuir turbulence driven by a relativistic electron beam has been investigated by the laser scattering technique. The special features of the experiments are as follows: (i) in the regime under study, Langmuir turbulence is well developed, and the temporal and spatial scales of the turbulent region far exceed those of a single caviton; (ii) the dispersion of the electron plasma waves is governed by the magnetic field, although ωpe≫ωBe; and (iii) the temperature of plasma electrons is much higher than that of ions, i.e., the damping of ion‐acoustic waves is small. k‐Spectra of electron plasma waves are measured in a broad spectral region by means of CO2‐laser scattering. Criterion of modulational instability of observed spectra is estimated. The role of conversion and scattering of Langmuir waves by ion‐acoustic waves in nonlinear energy transfer is discussed.

Temporal structure of double plasma frequency emission of thin beam-heated plasma

In the work presented here dynamics of spiky microwave emission of a beam-heated plasma near the double plasma frequency in ∼100 GHz band was studied. The plasma is heated by 80 keV, ∼2 MW, sub-ms electron beam that is injected into the multiple-mirror trap GOL-3. The beam-heated plasma diameter is of the order of the emitted wavelength. Modulation of individual emission spikes in the microwave radiation is found. The radiation dynamics observed can be attributed to a small number of compact emitting zones that are periodically distorted.

Strong Langmuir turbulence with and without collapse: experimental study

Experiments on strong Langmuir turbulence (LT) driven by electron beam are reported. The technique of cold high-current relativistic electron beam (REB) permits to set up experimental conditions that are practically important but difficult for theoretical treatment of LT. These conditions include strong kinetic effects of plasma non-Maxwellian electrons, ion-acoustic oscillations which are weakly damped due to plasma non-isothermality and dispersion of Langmuir waves that are considerably modified by external magnetic field. A relatively dense plasma permits the use of the Thomson scattering method for observation of spectra of plasma fluctuations, electron distribution function, and local dynamics of plasma density. LT is studied in two operating modes that are characterized by moderate and increased current of REB. The experimental results with moderate REB current do not support the widely accepted picture when most of the Langmuir oscillations are trapped in density cavities. The energy flow through turbulence to plasma electrons is explained without major contribution of fully developed collapse, whereas, with increased REB current dynamic density cavities of spatial scale much less than the size of turbulent region are directly observed.

Heating of tungsten target by intense pulse electron beam

A test facility for experimental simulation of transient heat loads in ITER divertor with the use of high power electron beam is developed at the Budker Institute of Nuclear Physics. This report presents an experimental study of the absorption of the electron beam with an incident heat power flux of 10-50 GW/m2 on the area of about 2 cm2 of a tungsten target. The electron beam has duration of 0.1-0.3 ms and electron energy of 80-95 keV, Diagnostics for measuring of the beam parameters on the target are briefly discussed. Results of measurement of the beam profile and calorimetry of beam energy deposited in tungsten sample at ELM-like heat load are presented.

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.

Microwave Generation During 100 keV Electron Beam Relaxation in GOL-3

First experimental results on microwave radiation at double plasma frequency from plasma heated by ~100 keV, 2-10 MW electron beam of ~100 μs duration in GOL-3 are presented. Measurements were done in 75-200 GHz band. The spectrum was peaked at ~94 GHz at ~3·1013 cm-3 plasma density, the temporal behavior of radiated power was spiky. The generation efficiency reached ~1% of total beam power.

Observation of dust particles ejected from tungsten surface under impact of intense transient heat load

A test facility for experimental simulation of transient heat load expected for ELMs type I events in ITER is developed at BINP SB RAS. Dynamics of tungsten particles in the ablation plume is investigated by small-angle light scattering technique and using fast CCD and ICCD cameras. The threshold of intense particle generation, sizes and velocities of particles ejected from the surface are estimated.