Mirror-confined Plasma Research Articles

Confinement time and ambipolar potential in a relativistic mirror-confined plasma

Advanced aneutronic fusion fuels such as proton-Boron11 tend to require much higher temperatures than conventional fuels like deuterium–tritium. For electrons, the bulk plasma temperature can approach a substantial fraction of the rest mass. In a mirror confinement system, where the electrons are confined by an ambipolar potential of at least five electron temperatures, the tail electrons which can escape the potential are fully relativistic, which must be taken into account in calculating their confinement. In this paper, simple estimates are employed to extend the scaling of the confinement time into the relativistic regime. By asymptotically matching this scaling to known solutions in the non-relativistic limit, accurate forms for the confinement time (and thus, the ambipolar potential) are obtained. These forms are verified using finite-element-based Fokker–Planck simulations over a wide range of parameters. Comparing relativistic and nonrelativistic mirror-confined plasmas with the same ratio of confining potential |eϕ| to electron temperature Te and the same mirror ratio R, the net result is a decrease in the confinement time due to relativistic effects by a factor of S≡(1+15Te/8mec2)/(1+2|eϕ|/mec2).

Kinetic whistler instability in a mirror-confined plasma of a continuous ECR ion source

Kinetic instabilities in a dense plasma of a continuous electron cyclotron resonance (ECR) discharge in a mirror magnetic trap at the Gasdynamic Ion Source for Multipurpose Operation (GISMO) setup are studied. We experimentally define unstable regimes and corresponding plasma parameters, where the excitation of electromagnetic emission is observed, accompanied by the precipitation of energetic electrons from the magnetic trap. A comprehensive experimental study of the precipitating electron energy distribution and plasma electromagnetic emission spectra, together with theoretical estimates of the cyclotron instability increment proves that under the experimental conditions, the observed instability is related to the excitation of whistler-mode waves, which are a driver of losses of energetic electrons from the magnetic trap. The results of this study are important for the further development of the GISMO electron cyclotron resonance ion source facility and for the improvement of its parameters as an ion source. Also, this research on plasma kinetic instabilities is of fundamental interest and provides experimental tools to simultaneously study plasma electromagnetic activity and corresponding changes in a resonant electron energy distribution.

Possibilities of a laboratory experiment on investigation of auroral kilometric radiation in the near-Earth plasma

Observations are reported of electromagnetic emissions at frequencies above the electron gyrofrequency in a mirror-confined electron cyclotron resonance plasma. These emissions are interpreted as the excitation of the fast extraordinary wave at the fundamental harmonic of the electron gyrofrequency, similar to the excitation of the auroral kilometric radiation (AKR) of the Earth. The paper proposes a method for studying the temporal characteristics of physical processes occurring in the auroral electron acceleration region, which is applied to the AKR measurements onboard INTERBALL-2 spacecraft and adopted to the measurements of laboratory plasma emissions. The joint data analysis of satellite measurements and the results of laboratory experiments showed similarities in the cyclotron instability development.

Observation of electron cyclotron harmonic emissions due to electrostatic instabilities in mirror-confined plasma

The authors present observations of electromagnetic emissions near electron cyclotron harmonics from mirror-confined laboratory plasma during electron cyclotron resonance heating by microwaves, and attribute them to electron cyclotron instabilities.

Zebra-like patterns in whistler wave emission spectra from nonequilibrium mirror-confined laboratory plasma

Zebra-like patterns have been observed in the electron cyclotron emission spectra from strongly nonequilibrium plasma confined in a table-top mirror magnetic trap. The analysis of the experimental data suggests that the formation of zebra-like patterns could eventually be related to the modulation of the whistler waves by the ion-acoustic waves excited during the abrupt ejection of electrons into a loss cone caused by the development of the whistler instability under the electron cyclotron resonance condition.

Electron-cyclotron heating and kinetic instabilities of a mirror-confined plasma: the quasilinear theory revised

After a basic introduction to the quasilinear theory of electron-cyclotron interaction in a mirror magnetic field, we develop a unified model that describes the formation of a non-thermal electron distribution function under the combined action of resonant plasma heating from external mono- or bi-chromatic waves and kinetic instabilities in the electron-cyclotron frequency range.

Interpretation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasma sustained by high-power microwaves

In Viktorov et al (2016 Eur. Phys. Lett. 116 55001) we reported on chirping frequency patterns that have been observed in the electron cyclotron emission from strongly nonequilibrium plasma confined in a table-top mirror magnetic trap. These patterns were interpreted qualitatively as the first experimental evidence for so-called ‘holes and clumps’ mechanism acting at such high frequencies (1–10 GHz). In the present paper, we prove this statement on a more rigorous basis, considering the formation of nonlinear phase space structures in proximity of the wave-particle resonances within the framework of the Berk–Breizman theory adopted for a kinetically unstable mirror-confined plasma.

Dynamic regimes of kinetic instabilities under conditions of double plasma resonance in mirror-confined plasma

The processes of plasma waves excitation under conditions of a double plasma resonance in a non-equilibrium plasma produced by high-power microwave radiation and confined in an open magnetic trap are studied. Two different dynamic regimes of kinetic instability have been experimentally observed: besides the generation of quasiperiodic series of short (∼50 ns) radiation pulses, a new scenario of instability in the form of generation of a single pulse of large (∼25 μs) duration with a slowly varying frequency near the second harmonic of the electron gyrofrequency in the center of the trap has been discovered. It is shown that the dynamic regimes of plasma wave generation observed in the experiment can arise when the processes of wave excitation and induced scattering compete, and the transition from quasi-continuous generation to pulse-periodic generation is determined by the ratio of the energy of non-equilibrium particles and the temperature of the background plasma. Hereinafter, the studied mechanisms will be used to explain similar phenomena in space, for example, sequences of pulses of different duration in the spectra of solar type-IV radio bursts.

Characterization of a medium-sized washer-gun for an axisymmetric mirror.

A new medium-sized washer gun is developed for a plasma start-up in a fully axisymmetric mirror. The gun is positioned at the east end of the Keda Mirror with AXisymmetricity facility and operated in the pulsed mode with an arc discharging time of 1.2 ms and a typical arc current of 8.5 kA with 1.5 kV discharge voltage. To optimize the operation, a systematic scan of the neutral pressure, the arc voltage, the bias voltage on a mesh grid 6 cm in front of the gun and an end electrode located on the west end of mirror, and the mirror ratio was performed. The streaming plasma was measured with triple probes in the three mirror cells and a diamagnetic loop in the central cell. Floating potential measurements suggest that the plasma could be divided into streaming and mirror-confined plasmas. The floating potential for the streaming plasma is negative, with an electric field pointing inwards. The mirror-confined plasma has a typical lifetime of 0.5 ms.

Kinetic instabilities in a mirror-confined plasma sustained by high-power microwave radiation

This paper summarizes the studies of plasma kinetic instabilities in the electron cyclotron frequency range carried out over the last decade at the Institute of Applied Physics in Nizhny Novgorod. We investigate the nonequilibrium plasma created and sustained by high-power microwave radiation of a gyrotron under the electron cyclotron resonance condition. Resonant plasma heating results in the formation of at least two electron components, one of which, more dense and cold, determines the dispersion properties of the high-frequency waves, and the other, a small group of energetic electrons with a highly anisotropic velocity distribution, is responsible for the excitation of unstable waves. Dynamic spectra and the intensity of stimulated electromagnetic emission are studied with high temporal resolution. Interpretation of observed data is based on the cyclotron maser paradigm; in this context, a laboratory modeling of non-stationary wave-particle interaction processes has much in common with similar processes occurring in the magnetosphere of Earth, planets, and solar coronal loops.

Observation of multiple chirping events in electron cyclotron emission of non-equilibrium mirror-confined plasma

Observation of multiple chirping events in electron cyclotron emission of non-equilibrium mirror-confined plasma

Observation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasma

Chirping frequency patterns have been observed in the electron cyclotron emission from strongly nonequilibrium plasma confined in a table-top mirror magnetic trap. Such patterns are typical for the formation of nonlinear phase-space structures in a proximity of the wave-particle resonances of a kinetically unstable plasma, also known as the “holes and clumps” mechanism. Our data provides the first experimental evidence for the acting of this mechanism in the electron cyclotron frequency domain.

Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma

A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves.

Laboratory study of kinetic instabilities in a nonequilibrium mirror-confined plasma

Kinetic instabilities of nonequilibrium plasma heated by powerful radiation of gyrotron in electron cyclotron resonance conditions and confined in a mirror magnetic trap are reported. Instabilities are manifested as the generation of short pulses of electromagnetic radiation accompanied by precipitation of hot electrons from magnetic trap. Measuring electromagnetic field with high temporal resolution allowed to observe various dynamic spectra of electromagnetic radiation related to at least five types of kinetic instabilities. This paper may be of interest in the context of a laboratory modeling of nonstationary wave-particle interaction processes in nonequilibrium space plasma since the observed phenomena have much in common with similar processes occurring in the magnetosphere of the Earth, planets, and in solar coronal loops.

Global sound modes in mirror traps with anisotropic pressure

Global oscillations of inhomogeneous plasma with frequencies close to the bounce frequency of ions in mirror traps have been studied. It has been shown that, in some cases, the sound can be reflected from the axial plasma inhomogeneity. The ideal magnetohydrodynamic (MHD) model with Chew-Goldberger-Low approximation has been utilized to determine conditions of existence of the standing waves in the mirror-confined plasma. Linearized wave equation for the longitudinal plasma oscillations in thin anisotropic inhomogeneous plasma with finite β has been derived. The wave equation has been treated numerically. The oscillations are studied for the case of the trap with partially filled loss-cone and the trap with sloshing ions. It has been shown that in cells of the multiple-mirror trap standing waves can exist. The frequency of the wave is of the order of the mean bounce-frequency of ions. In the trap with sloshing ions, the mode supported by the pressure of fast ions could exist. The results of oscillations observation in the experiment on the Gas Dynamic Trap have been presented.

Mirror Confined Plasma in the Large Helical Device

Detailed stracture of the magnetic field in the Large Helical Deveice (LHD) is studied numerically. The connection length of the divertor field line is found to be the order of several km or more. The possibility of the mirror-confined plasma in the chaotic field line region is shown. The divertor plasma model is proposed based on the long connection length and the existence of the mirror-confined plasma in the chaotic field line region. A new ICRF heating scheme (runaway ion heating scheme) for LHD is also proposed based on the existence of the mirror-confined plasma in the core plasma region.

Source of VUV Emission Based on Beam-Heated Plasma of GOL-3-II Facility

New approach to a vacuum ultraviolet (VUV) source based on mirror-confined plasma is presented. A plasma cloud with ne=1016−1017 cm−3 and Te=10–50 eV serves as an active medium. Such plasma can be obtained in GOL-3 facility by means of two-stage heating by relativistic electron beam. Current experiments on GOL-3-II facility are described. Results of recent study of VUV flush from hydrogen and nitrogen plasma with calculations of ionisation balance and radiation power are presented. Transitions in Li-like ions for recombination scheme and in Ne-like ions for collision excitation scheme are considered as a candidates for coherent VUV generation. Prospects of population inversion and gain achievement are discussed.

Spectroscopic study of molecular-hydrogen processes in a mirror-confined plasma.

Visible and near-ultraviolet molecular hydrogen emission from the Tara Tandem Mirror central cell plasma was investigated in order to determine molecular densities, ionization rates, and continuum dissociation rates. Measurements of H{sub 2} {ital G}{r_arrow}{ital B} band emission were used to infer spatial density and ionization profiles, maximum densities of 4{times}10{sup 12} cm{sup {minus}3} at the gas injection port, and a total molecular ionization rate of 254 A. Continuum emission in the near ultraviolet was identified as the H{sub 2} dissociative continuum through its wavelength distribution, time behavior, and intensity. Wavelength-integrated continuum emission measurements were used to obtain the dissociation rate associated with the continuum: 6 A. The power expended in molecular ionization, dissociation, and radiation is estimated to be 8.4 kW out of 300 kW of rf power injected.

Suppression of Alfvén Ion Cyclotron Instability in a Mirror by End Plugging

The effect of end plugging of the ion on the Alfven ion cyclotron instability in a mirror-confined plasma is studied. The dispersion equation in the uniform-plasma approximation is derived by assuming a loss cone distribution of the ion, taking into account the end plugging, and is solved numerically. It is shown that the end plugging effect of filling the loss cone mainly causes the reduction of the temperature anisotropy of the ion, leading to the suppression of the instability.

MODE COUPLING OF INTERCHANGE INSTABILITY IN PLASMAS

Explosive mode coupling of low frequency interchange instability in partially ionized plasmas is studied. The coupled equations describing coherent wave-wave interactions of the interchange mode are derived. The possibilities of explosive instabilities for the interchange mode are found. It is shown that there exist the explosive coupling of positive energy wave and negative energy wave, positive energy wave and negative dissipation, and negative energy wave and positive dissipation, in the partially ionized plasmas. The theoretical results obtained can be applied to the ionospheric plasma and the mirror-confined plasma.