Ambipolar Trap Research Articles

Ambipolar Trap for Dust Particles in a V-Shaped Homogeneous Positive Column of Glow Discharge at Low and Medium Pressures

The spatial distribution of dust particles in a low-pressure homogeneous positive column of a dc glow discharge in a right-angle curved (V-shaped) glass tube in different spatial orientations was observed and analyzed. It is found that in all cases, the ambipolar field prevents charged dust particles from falling on the wall of the glass tube. As a result, the design of the curved glass tube makes it possible to create in practice a simple and effective ambipolar trap and to implement the collection and control of dust particles in plasma.

Effect of Glow Discharge Parameters on the Mean Interparticle Distance in Dusty Plasma Structures in the Cryogenic–Room Temperature Range

The molecular dynamics method is used for studying the effect of forces acting in a dusty plasma and discharge parameters on the mean interparticle distance in structures in a dc glow discharge in the temperature range 50–300 K. We report on the results of the dependence of the mean interparticle distance on the neutral gas temperature in simplified and modified models of the dusty plasma. The simplified model includes the screened Coulomb potential and the ambipolar trap; in the modified model, the thermophoretic force, the ion shading force, and the neutral shading force are accounted for additionally. All these forces are calculated from the experimental values of the discharge parameters (current, pressure, temperature, and type of gas). The modified model of the dusty plasma system makes it possible to explain the experimental form of the dependence of the mean interparticle distance in dusty structures on the discharge gas temperature. We propose a method for estimating the ion concentration in the discharge gas from the mean interparticle distance in dusty plasma structures.

Comparison of a Deuterium-Helium-3 FRC and Mirror Trap

Field reversed configurations (FRC), as mirror machines have been developed as simple plasma confinement systems. FRC formed by reversing of magnetic field and reconnection and sustained by plasma current. Several mirrors may be linked together with solenoids - tandem mirrors (TM), end losses in the axisymmetric tandem mirror are reduced by ambipolar trapping - ambipolar trap (AT). Power balance of deuterium - helium-3 (D-3He) plasma in a prolate FRC power plant is compared with the power balance for a central cell of D-3He tandem mirror reactor (ambipolar trap). Deuterium - tritium (D-T) ITER, kinetically stabilized tandem mirror (KSTM) and D-3He FRC parameters are demonstrated.

ICR Heating in an Axisymmetric Solenoid

The density and diameter of plasma obtained In the central solenoid of the fully axisymmetric ambipolar mirror trap AMBAL-M are sufficient enough for ioncyclotron heating using fast waves, which provides plasma heating at the axis. At present the experiment on such ICR heating of the solenoid plasma is under preparation. The heating will be carried out two semiloop antennas installed in the end of the solenoid. For protection from the impact of the edge plasma, the antennas are equipped with Faraday shields and graphite limiters with slanted slits.

Recent Progress in Mirror Studies in Novosibirsk

At present, in the Budker Institute of Nuclear Physics there is in operation the most complete set of modern mirror machines based on different principles of plasma confinement. This set includes the multi-mirror system GOL-3 for confinement of dense plasma heated by relativistic electron beam, the gas dynamic trap (GDT) for confinement of collisional plasma and anisotropic fast ions, and the ambipolar trap AMBAL-M.Mirrors have a number of advantages in comparison with the closed magnetic systems like tokamak, stellarator, etc. The most important are the following. The effects of disruptions are not appeared in mirrors. There are no divertor problems in them. Plasma pressure in a mirror device can be comparable with magnetic field pressure. As to the multi-mirror system, in this case, the β value can be even significantly higher than unity (the so called <<wall confinement>>). At last, mirrors are convenient for direct conversion of energy of charged particles leaving out the ends. This circumstance can turn out to be especially important in future <<low neutron>> schemes of fusion reactors.In principle, mirrors are very attractive from the engineering point of view, if the plasma confined in axisymmetric magnetic systems would be MHD stable. At present, the problem of MHD stability has already been solved for all axisymmetric traps designed in Novosibirsk. At least, the value β [approximately or equal to] 0.4 was obtained in these traps without any indications of macroscopic instability development.Some important results were obtained recently in the GOL-3 experiments. A specific mechanism of reduction of longitudinal electron thermal conductivity was detected. Recently, the magnetic system of the GOL-3 device was reconstructed into multi-mirror configuration. In this configuration, a new mechanism of fast ion heating was observed. As a result, the ion temperature was increased from a few eV up to 1 keV and the confinement time significantly increased (up to 1ms). These results were obtained for plasma density of 1021m-3.The experiments on the GDT device are directed to the solution of the problem of creation of a 14 MeV high power neutron source. At present, new powerful neutral beam injectors for this device are under construction. After installing these neutral beams at GDT, the electron temperature of plasma should increase up to 300 eV. It means that calculated neutron flux for the case of D-T reaction will be about 0.5 MW/m2. If this value will be obtained, it will be immediately possible to begin the design of final stage of high power neutron source.In the paper, the status of all mirror traps in Novosibirsk is presented and description of the main experiments is given.

Experiments with Dense Plasma in the Central Solenoid of Ambal-M

At present the axisymmetric ambipolar mirror trap AMBAL-M consists of a central solenoid which is attached to a plugging and MHD stabilizing end system and is filled from the other end by a plasma stream generated by a gas-discharge source. In the first experiments we obtained the plasma in the solenoid with ~0.4 m diameter, density ~6·1012cm−3, electron temperature ~50 eV, and ion energy ~250 eV. In order to enhance the plasma flow from the source into the solenoid, the distance between the entrance throat of the solenoid and the plasma source was gradually decreased, and the plasma density was increased to ~2·1013 cm−3. Installation of a second source from the opposite end of the machine allowed us to increase the plasma density up to ~2.5·1013 cm−3 in the solenoid and up to ~1.5·1013 cm−3 in the mirror trap of the end system. For better propagation of the plasma stream from the second source into the trap the coil of the MHD-stabilizer semicusp was switched in the same direction as all other coils, thus the magnetic configuration consisted of a series of simple mirrors. However, the plasma remained MHD stable owing to its line-tying to conducting ends. When this line-tying broke during the fast cut-off of the source current, the density profile in the solenoid abruptly rearranged pointing to possible MHD activity, and independently of the initial shape it became almost flat up to the limiter.Further enhancement of the plasma density was achieved using hydrogen puffing into the solenoid plasma while only the first source was functioning. Two methods of gas puffing were used – through a ceramic tube to the solenoid axis and into a gas-box surrounding the plasma. Optimization of the hydrogen puffing rate led to the density increase up to ~ 6·1013 cm−3 without noticeable degradation of the ion temperature which remained at a high level of ~200 eV (the ratio of the plasma pressure to the magnetic field pressure β~0.1), which is provided by stochastic ion heating from electrostatic oscillations when the source is working. The obtained solenoid plasma density is the highest one achieved in ambipolar mirror traps.

Single minimum-B ambipolar trap

A new type of ambipolar trap is proposed, where a minimum-B magnetic field is used to confine the particles radially and to provide plasma macroscopic stability. The particles are confined axially by creating the plug potentials at both end-mirror cells of the ambipolar trap. The plug potential is produced by only electron cyclotron resonance heating, the mechanism of which is proposed.

Experiments on AMBAL-M

Two types of experiments with a gas-box mounted in the mirror trap of the end system AMBAL-M between the central plane and the exit throat have been performed during the last year. In the experiments of the first type the gas-box was used for generation of a quasistationary hot plasma using input of RF-power at a frequency near the ion cyclotron. The RF-power up to 0.5 MW was introduced into the plasma by means of a Nagoya-III antenna located in a transition region between the mirror trap and the semicusp. At optimized hydrogen puffing into the gas-box, the plasma with the density ~4·1012 cm−3, electron temperature ~100 eV, ion temperature ~400 eV, and duration up to 120 ms was obtained in the mirror trap.In experiments of the second type the gas-box was used for hydrogen supply into the hot initial plasma in the mirror trap with the purpose to increase its density. The hot initial plasma in the trap is maintained owing to the trapping from a plasma stream with the developed electrostatic turbulence generated by a gas-discharge source located before the entrance throat. It was found that in addition to the plasma density increase by a factor of 2–3, hydrogen puffing leads to an unexpected diamagnetism increase by a factor of 2. Measurements showed that the gas puffing does not reduce the electron temperature in the trap. Essential for explanation of the observed effect is the fact that at the gas puffing the measured plasma potential in the trap increases. The increase of the plasma potential enhances the trapping of the ion flow entering the trap and increases the average energy of the electron flow entering the trap.Preparation of an experiment on creation and study of a dense and hot plasma in the central solenoid of the completely axisymmetric ambipolar trap is underway. To perform this experiment, the finished part of the central solenoid will be attached to the carefully studied end system. The hot plasma will be produced by a gas-discharge plasma source located before the solenoid throat. Additional enhancement of the plasma parameters will be achieved using RF-power input and hydrogen puffing. As a result of the experiment, an MHD stable 6 m long plasma with the diameter ~30 cm, density 1013 cm−3, ion temperature of hundreds of electronvolts, electron temperature above 100 eV will be obtained in the central solenoid. Magnitudes of transverse particle and energy losses from the obtained hot collisionless plasma will be determined as well, and substantial reduction of longitudinal plasma losses from the solenoid will be provided owing to creation of an ambipolar potential in the mirror trap plasma. At the initial stage of the solenoid filling the ion distribution function over longitudinal velocities will have two peaks and experimental investigation of the corresponding microinstability is possible.

Possibility of demonstrating a self-sustained fusion reaction in an ambipolar trap

A brief description is given of an axisymmetric mirror trap to demonstrate ignition of a D-T plasma confined by ambipolar barriers. Key problems for this demonstration are ion pumping and suppression of instabilities in the thermal barriers. It is suggested that the fusion-fuel ions can be pumped away from the thermal barriers by exciting trapped-ion bounce oscillations at parametric resonances.

AMBAL-M Status

Investigation of the hot initial plasma created in an axisymmetric end system of the ambipolar trap AMBAL-M has been completed. In the end mirror we obtained the MHD stable plasma with the electron temperature of 50 eV, ion temperature 200 eV, and density about 1013 cm−3. In an MHD anchor – the semicusp a transverse profile of the plasma pressure favorable for the MHD stability was obtained. Pulsed injection of fast atoms with the current of 100 A demonstrated sufficient accumulation rate of the ion population trapped into the initial plasma. The first experiments with ICR-heating of the initial plasma were carried out.Two atomic injectors of the end mirror were prepared for work. In these injectors four quasistationary proton beams were obtained with the energy of 25 keV and current of up to 50 A per beam. After their charge-exchange the atomic beams were passed through an MHD stabilizing shell and the target plasma.Principal vacuum units of the 2-nd stage of the installation were tested and prepaired for assembly. One-half of the magnet-vacuum system of the AMBAL-M central solenoid was assembled and tested for vacuum.

Dissipative trapped particle modes in tandem mirrors

The theory for the dissipative trapped particle modes is developed for a tandem mirror taking into account equilibrium rotational effects, electron temperature gradients, and an axial ambipolar potential, for a broad range of collisional parameters. The electrons are treated as a Maxwellian plasma that occupies the central cell and anchor cell regions. It is assumed that the eigenfunction is piecewise constant with abrupt transitions between the anchor and central cell regions. It is found that when ω/νp≳1, with ω the mode frequency and νp the Pastukhov loss rate, that the energy conservation structure of the collision operator produces important changes to previously developed theories. A solution to the problem is achieved by using the solution for the lifetime of an electron in an ambipolar trap, taking into account the global energy conservation. The energy conservation structure also allows a self-consistent description of dissipative instabilities when thermal gradient and electric fields are present. At very high collision frequency, a new dispersion relation is obtained, which exhibits an axially rotational shear drive coupled to radial temperature gradients producing instability. Numerical studies are presented for some parameters, with the deviation from previous theory highlighted.

Mirror research at Novosibirsk

In the present work the status of the open systen studies at the Novosibirsk Institute ofNuclear Physics (INP) is described.For the heating of dense plasnas in open systens of the multi-mirror type the possibility isbeing studied to use high-current relativistic electron beans (REBS). At ne 3 - 1015 cm-3,jb ~ 2.5 kA/cm2, the experiments indicate that for fields up to B = 5 T the interaction efficiency sbstantially increases with increasing field strength. With the above aim in view, thework being performed at the Institute concerns the construction of high power generators ofmicrosecond REBS. At the generator U-1 a beam with 130 kJ energy content was produced. The construction of the generator U-2 is finished. In this generator, a foilless diode with a band geometry beam has been employed. In a similar geometry, the beams from several generators can beinjected sequentially into the same magnetic track.The results, obtained on a gas-dynamical trap (GDT) with large mirror ratio (R > 80) for thecase when the ion mean free path λii satisfied the inequality: L > λii(lnR/R) (L is thelongitudinal size of the system), have shown that the plasma dynamics in the trap is consistentwith gas-dynamical estimations. At the stage of GDT filling, the plasma in it proves to bestable. At a favourable curvature of the magnetic field lines in expanders the plasma turns outto be MHD stable with the plasma source switched off as well. In the latter case the plasmadecay time is in reasonable agreement with the gas-dynanical estimation: τ ~ LR/vTi.The project of an ambipolar trap (tanden mirror) with axisymmetrical magnetic field has beendeveloped. For MHD stabilization on the outside of the end plugs, two semi cusps with hot ionplasma (T = 15 keV) are planned to be mounted. High mode number MHD modes are assumed to bestabilized by the finite Larmor radius (FLR) effect, while the mode m = 1 will be stabilized bya favourable curvature of the magnetic field. The basic parameters of the central mirror trapare: L = 14 m, B = 0.45 T, ne = 1013 cm-3. The end plugs have the following paraneters:Lp = 1.8 m, R = 4, Bmax = 6 T, ne = 3 × 1013 cm-3.

Model of a fusion reactor based on an adiabatic trap with MHD stabilizers

This article offers a model of a fusion reactor that is intended for the production and study of D-T plasma with fusion parameters and for the solution of the main engineering and technological problems associated with a reactor based on an ambipolar trap. Proposed is the possibility of building a model of a fusion reactor on the basis of an open trap. The margin factor for the plasma confinement time in the trap is the most important quantity for the project of a fusion reactor model and at the same time is the most controversial quantity. The version of parameters presented is oriented at, in particular, the possibility of being varied within certain limits without a change in the geometry and magnetic field of the trap, as well as the stability conditions.