Cylindrical Magnetized Plasma Research Articles

Inward particle transport driven by biased endplate in a cylindrical magnetized plasma

The inward particle transport is associated with the formation of peaked density profiles, which contributes to improve the fusion rate and the realization of steady-state discharge. The active control of inward particle transport is considered as one of the most critical issues of magnetic confinement fusion. Recently, it is realized preliminarily by adding a biased endplate in the Peking University Plasma Test (PPT) device. The results reveal that the inward particle flux increases with the bias voltage of the endplate. It is also found that the profile of radial electric field ( ) shear is flattened by the increased bias voltage. Radial velocity fluctuations affect the inward particle more than density fluctuations, and the frequency of the dominant mode driving inward particle flux increases with the biased voltage applied to the endplate. The experimental results in the PPT device provide a method to actively control the inward particle flux using a biased endplate and enrich the understanding of the relationship between shear and turbulence transport.

Analysis of Turbulence Driven Particle Transport in PANTA by Using Multi-Field Singular Value Decomposition

Multi-field singular value decompositions (SVDs) is applied to turbulence obtained in a cylindrical magnetized plasma, PANTA. This method enables us to obtain the spatial mode structures with common temporal evolution of different physical quantities, such as the fluctuations of density and flows. Turbulence driven particle transport is evaluated by the method. It is shown that only the coupling of the same mode drives the transport, which stems from the orthogonality of the SVD. Thanks to this characteristics, the number of degrees of freedom which plays roles for the transport dynamics could be significantly reduced.

Axisymmetric oscillatory modes in cylindrical magnetized plasma bounded by a conducting wall

A comparison between analytic theory and numerical simulations of axisymmetric modes in magnetically confined, cylindrical plasma with non-circular cross-section bounded by a conducting wall is presented. If the wall is close to the plasma, modes are oscillatory, with frequency scaling with the Alfvén frequency. The two frequencies differ when the plasma cross-section is elongated, but they become equal in the circular limit. The mechanism for oscillatory behavior is a consequence of the currents induced on the perfectly conducting wall when the plasma is displaced from its equilibrium positions. The induced currents exert a restoring force on the plasma, and the oscillation frequency is a combination of the strength of this force and the plasma mass density. An additional parameter, depending on the distance between the wall and the plasma boundary, also affects the oscillation frequency so that the frequency becomes large when the wall-plasma boundary distance approaches zero.

Radially inward particle transport driven by low-frequency instability in cylindrical magnetized plasma

Net radially inward turbulent particle flux was observed in a linear plasma device, the linear experimental advanced device (LEAD). The amplitude of the inward particle flux is comparable with outward particle flux. The inward particle flux peak value locates in the outer E r shear layer. Cross-phase evolution dominates the radial evolution of particle flux from outward to inward. In the frequency domain, a 1 kHz mode contributes most of the inward particle flux. Frequency-resolved energy transfer analysis infers that this mode obtains energy nonlinearly from very low frequency fluctuations below 0.5 kHz. Moreover, turbulence spreading could also transfer energy to the position where the inward flux peak locates from nearby positions, and thus acts as a nonlocal, nonlinear drive for the 1 kHz mode.

Electrostatic Ion-cyclotron Wave Excitation by a Gyrating Ion Beam in a Magnetized Plasma Containing Heavy Positive Ions

In the present paper, we have developed a theoretical model of cylindrical magnetized plasma column consisting of electrons, ions & heavy positive ion species. A gyrating ion beam of potassium having radius ~1.5 cm and energy 10eV is launched through one end of the cylinder parallel to the static magnetic field and the electrostatic ion cyclotron wave is assumed to propagate nearly perpendicular to the magnetic field. The presence of heavy positive ions in the collisionless magnetized plasma decreases the critical drift for the excitation of EIC waves and hence the injected beam drives the electrostatic ion cyclotron (EIC) waves to instability through Cerenkov interaction. Therefore, the plasma is observed to contains two excited EIC wave modes; a (light positive) ion mode and a (heavy positive) ion mode. The frequencies and growth rates for both the positive unstable modes are worked out using fluid theory for the typical existing experimental parameters which are found to increase with the increase in ion beam energy and beam density. It is also observed that the unstable wave frequency and growth rate for both the modes increases with the increase in relative ion concentration of and heavy positive ions respectively. Magnetic field is also one of the important factors which influence the plasma instabilities appreciably. As the magnetic field increases, the frequency of both the ion modes increases linearly.

Investigation on the nonreciprocal properties of one-dimensional cylindrical magnetized plasma photonic crystals.

In this paper, the equations for the transfer matrix method of one-dimensional cylindrical magnetized plasma photonic crystals are proposed, and its nonreciprocal properties based on the Thue-Morse sequence are also studied. By adding the influence of the magnetic field into Maxwell's equations in the cylindrical coordinate system, the transmission matrix equation of cylindrical wave propagation in the cylindrical medium is obtained and a quasi-periodic structure with the Thue-Morse sequence is designed to study its nonreciprocal features. By considering Maxwell's equations and the equations of motion of charged particles, the relative dielectric function of plasma under the $z$-axis magnetic field is achieved. Considering the influences of this dielectric function in the cylindrical coordinate system, the transmission matrix equations of cylindrical wave propagation in the cylindrical medium are derived, and those equations are used to design a cylindrical structure with two layers of ordinary medium and one layer of plasma satisfying the Thue-Morse sequence. It is concluded that the nonreciprocal phenomenon becomes more and more obvious with the increase of the plasma frequency, the relative dielectric constant of the medium, and the incident angle. But with the increase of ${\omega _c}$, the nonreciprocal propagation is attenuated, and there is no significant change when the collision frequency is enlarging.

Experiment on low-frequency electromagnetic waves propagating in shock-tube-generated magnetized cylindrical enveloping plasma

We propose a method of applying a static magnetic field to reduce the attenuation of the magnetic field component (S H) of low-frequency electromagnetic (LF EM) waves in dense plasma. The principle of this method is to apply a static magnetic field to limit electron movement, thereby increasing the equivalent resistance and thus reducing the induced current and S H. We consider the static magnetic field acting on the plasma of the entire induced current loop rather than on the local plasma, where the induced current is excited by the magnetic field component of LF EM waves. Analytical expressions of S H suitable for magnetized cylindrical enveloping plasma are derived by adopting an equivalent circuit approach, by which S H is calculated with respect to various plasma parameter settings. The results show that S H can be reduced under a static magnetic field and the maximum magnetic field strength that mitigates blackout is less than 0.1 T. Experiments in which LF EM waves propagate in a shock-tube-generated magnetized cylindrical enveloping plasma are also conducted. S H measured under the magnetic field (the magnetic field strength B 0 acting on the magnetic field probe was about 0.06 T) reduces at f = 10 MHz and f = 30 MHz when n e ≈ 1.9 × 1013 cm−3, which is consistent with theoretical results. The verification of the theory thus suggests that applying a static magnetic field with a weak magnetic field has the potential to improve the transmission capacity of LF EM waves in dense plasma.

Nonreciprocal absorption characteristics of one-dimensional cylindrical magnetized plasma photonic crystals

By combining the propagation matrix of planar magnetized plasma photonic crystals with that of the cylindrical non-magnetized medium, the transfer matrix equation of the cylindrical magnetized plasma photonic crystals is obtained. Applying this equation to the cylindrical structure, a quasi-periodic structure conforming to the Pell sequence is designed, which has good nonreciprocal property. Therefore, the absorptance of light propagating forward and backward in this structure are studied respectively. The concept of contrast ratio (D C ), calculated from the absorption of the forward and backward propagation of electromagnetic waves in the structure, is used to compare the degree of difference between the two. It is found that the nonreciprocal propagation becomes larger with the increase of the azimuth modulus and the relative dielectric constant of the medium, increases first, and then decreases with the increase of the incident angle, but does not change obviously when the plasma collision frequency and the cyclotron frequency of the plasma are getting larger. The absorptance decreases with the increase of azimuth modulus, relative dielectric constant, and plasma cyclotron frequency, and becomes better with the increase of the plasma collision frequency. However, when the incident angle gets larger, the absorptance becomes better at first and then gets worse.

Vortex in plasma

Vortices excited in magnetized plasmas are described in this review article. An emphasis is paid on vortex formation in partially ionized plasmas, in which ion and neutral particles coexist. Experimental results on plasma hole (vortex with a density hole), spiral vortex, tripolar vortex, and counter-$${\boldsymbol{E}} \times {\boldsymbol{B}}$$ vortex are presented, all of which have been observed in a magnetized cylindrical plasma. It is shown that there is a new class of vortex induced by material (neutral) flow in the plasma, which exhibits multi-fluid aspect of partially ionized plasma.

Comparison of probe and narrow-band imaging measurements in a magnetized cylindrical plasma

We analyze similarities and differences between electrostatic probe measurements and high-speed visible light imaging measurements in a low-temperature argon helicon plasma. Electrostatic probes are the usual diagnostics for making fluctuation measurements in low-temperature plasmas, although fast-imaging of visible emission is becoming increasingly common. The two diagnostics are highly complementary, and typical analysis takes fluctuations in both ion saturation current and visible light intensity to be proxies for plasma density fluctuations. The observations presented here indicate that care is needed in applying this approximation. The plasma is produced with a helicon antenna, and we measure fluctuations in ion saturation current, floating potential, and visible emission near wavelengths of 488 nm (argon ions) and 750 nm (argon neutrals). Simultaneous measurements show relatively low correlation values between ion saturation current and ion line emission and an anti-correlation between floating potential and ion line emission. Variations in cross-coherence between ion emission and probe quantities indicate the frequency dependence in the relationship between the measurements. For various frequency bands where probe/imaging coherence is relatively high, fluctuation intensity profiles differ significantly and coherent fluctuations are observed in ion light which are not apparent in the probe measurements. A sensitivity analysis indicates that electron temperature fluctuations, although nearly always neglected when analyzing data from low-temperature plasma experiments, may play a role in the discrepancy between probe and narrow-band imaging measurements.

Simultaneous measurements of turbulent Reynolds stresses and particle flux in both parallel and perpendicular directions in a linear magnetized plasma device.

We report temporally resolved simultaneous measurements of the turbulent Reynolds stresses in both the parallel and perpendicular directions and the corresponding particle fluxes in the fusion relevant cylindrical magnetized plasma device Controlled Shear Decorrelation eXperiment (CSDX). CSDX simulates the plasma conditions of multiple plasma instabilities that can arise in the scrape-off layer of fusion devices. In this study, we designed and used a 6-tip Langmuir probe in a novel yet simple design to simultaneously measure all the three dimensional components (radial, azimuthal, and axial) of fluctuations in velocity from the floating potentials and plasma densities with high temporal resolution. From these, we calculated the parallel and perpendicular Reynolds stress and the particle fluxes in addition to the density and potential spectra and the cross phase between different quantities. We can obtain radial profiles of all the aforementioned plasma quantities, which are extremely useful for studying plasma turbulence due to multiple instabilities. We have also cross-checked the time averaged velocity profiles from the probe with laser induced fluorescence measurements of the mean plasma velocity for some common plasma source parameters.

Electromagnetic fluctuations in the intermediate frequency range originating from a plasma boundary layer

We demonstrate the transition in the waves generated by the electron-ion hybrid instability from a predominantly electrostatic to a predominantly electromagnetic character in a magnetized cylindrical laboratory plasma, in which we have induced sheared electron flow, transverse to the axial magnetic field and localized to a narrow azimuthal region. The transition occurs when the density of the plasma is increased, so that the electron skin depth is reduced to the same order as the wavelength of the waves. In the electromagnetic mode, we observe prominent bursts in the wave activity exhibiting substantial (up to 30%) frequency chirp, randomly occurring at a rate that is highly sensitive to the electric field structure in the boundary layer.

Investigating flow patterns and related dynamics in multi-instability turbulent plasmas using a three-point cross-phase time delay estimation velocimetry scheme

Complexities of flow patterns in the azimuthal cross-section of a cylindrical magnetized helicon plasma and the corresponding plasma dynamics are investigated by means of a novel scheme for time delay estimation velocimetry. The advantage of this introduced method is the capability of calculating the time-averaged 2D velocity fields of propagating wave-like structures and patterns in complex spatiotemporal data. It is able to distinguish and visualize the details of simultaneously present superimposed entangled dynamics and it can be applied to fluid-like systems exhibiting frequently repeating patterns (e.g., waves in plasmas, waves in fluids, dynamics in planetary atmospheres, etc.). The velocity calculations are based on time delay estimation obtained from cross-phase analysis of time series. Each velocity vector is unambiguously calculated from three time series measured at three different non-collinear spatial points. This method, when applied to fast imaging, has been crucial to understand the rich plasma dynamics in the azimuthal cross-section of a cylindrical linear magnetized helicon plasma. The capabilities and the limitations of this velocimetry method are discussed and demonstrated for two completely different plasma regimes, i.e., for quasi-coherent wave dynamics and for complex broadband wave dynamics involving simultaneously present multiple instabilities.

Nonlinear competition of turbulent structures and improved confinement in magnetized cylindrical plasmas

Nonlinear competition of turbulent structures and their roles in transport are investigated by using three-dimensional simulation code of resistive drift wave turbulence in magnetized cylindrical plasmas. Selective formation of zonal flows and streamers has been obtained by controlling the strength of damping of the zonal flow. In addition, there is an energy path from the drift waves to a flute type structure, which is linearly stable, and it becomes effective just below the stability boundary of the zonal flow. The flute structure directly induces transport effectively, and affects the drift waves and the zonal flow. A large amplitude zonal flow is formed selectively even with existence of the flute structure. The property of the particle confinement is investigated by changing the particle source intensity, which controls the strength of driving of the drift waves. The characteristic of the particle confinement changes according to turbulent states, and an improved confinement regime is obtained in the zonal flow dominant state. Study on cylindrical plasmas reveals the fundamental mechanism of improved confinement in the magnetized plasma with influence of turbulent structural formation.

Low-frequency electrostatic waves in a magnetized, current-free, heavy negative ion plasma

AbstractWe report experimental observations of a low-frequency (≪ ion gyrofrequency) electrostatic wave mode in a magnetized cylindrical (Q machine) plasma containing positive ions, very few electrons and a relatively large fraction (n−/ne > 103) of heavy negative ions (m−/m+ ≈ 10), and no magnetic field-aligned current. The waves propagate nearly perpendicular to B with a multiharmonic spectrum. The maximum wave amplitude coincided spatially with the region of largest density gradient suggesting that the waves were excited by a drift instability in a nearly electron-free positive ion–negative ion plasma

Rotating Instability in Low-Temperature Magnetized Plasmas

The formation of a rotating instability associated with an ionization front ("rotating spoke") and driven by a cross-field current in a cylindrical magnetized plasma is shown and explained for the first time on the basis of a fully kinetic simulation. The rotating spoke is a strong double layer (electrostatic sheath) moving towards the higher potential region at a velocity close to the critical ionization velocity, a concept proposed by Alfvén in the context of the formation of the solar system. The mechanisms of cross-field electron transport induced by this instability are analyzed.

Electron-ion hybrid instability experiment upgrades to the Auburn Linear Experiment for Instability Studies

The Auburn Linear EXperiment for Instability Studies (ALEXIS) is a laboratory plasma physics experiment used to study spatially inhomogeneous flows in a magnetized cylindrical plasma column that are driven by crossed electric (E) and magnetic (B) fields. ALEXIS was recently upgraded to include a small, secondary plasma source for a new dual source, interpenetrating plasma experiment. Using two plasma sources allows for highly localized electric fields to be made at the boundary of the two plasmas, inducing strong E × B velocity shear in the plasma, which can give rise to a regime of instabilities that have not previously been studied in ALEXIS. The dual plasma configuration makes it possible to have independent control over the velocity shear and the density gradient. This paper discusses the recent addition of the secondary plasma source to ALEXIS, as well as the plasma diagnostics used to measure electric fields and electron densities.

Transitions between drift waves in a magnetized cylindrical plasma: experiments and fluid model, solved with a spectral method

A bifurcation scenario between collisional drift waves with different azimuthal wavenumbers m in a magnetized plasma column is experimentally studied, and compared with a linear two-fluid model solved with a new spectral method. The control parameter is the potential of an internal metallic tube in the experiments, the electron drift along the axis of the cylinder in the model. By increasing this parameter, we find bifurcations from azimuthal modes m = 5 to m = 1. The linear properties of the model agree well with the experimental observations.

Suppression of drift wave turbulence and zonal flow formation by changing axial boundary conditions in a cylindrical magnetized plasma device

For drift wave turbulence, due to charge conservation, the divergence of the parallel current is coupled to the divergence of the perpendicular polarization current, which determines the effective radial momentum flux, i.e., the Reynolds stress. Changes in the current flow patterns also affect the nonlinear energy transfer from smaller to larger scales. Here, we show that by changing the end plate boundary conditions in a cylindrical plasma device, the radial currents through the plasma and hence the net momentum transport and the nonlinear coupling for the inverse energy transfer are strongly modified. The transition to drift wave turbulence and the formation of low frequency zonal flows can be either suppressed with conducting boundaries or enhanced with insulating boundaries.

Dynamics of particle flux in a cylindrical magnetized plasma

Fluctuation-induced cross-field convective particle flux is studied in the cylindrical magnetized plasma assembly for non-linear turbulence analysis (PANTA). The plasma presents a wave turbulence characterized by a limited number of developing modes. To study the link between mode dynamics and particle transport, frequency-dependent time evolution of transport is obtained by applying complex continuous wavelet decomposition on spatio-temporal data from electrostatic probes. The method is shown to recover transport description consistent with other analysis methods for a weakly turbulent state. For a plasma state featuring a large non-sinusoidal structure, the time evolution of transport on the second harmonic of the main fluctuation shows a slow modulation of azimuthal mode number m = 0 with similarities to a zonal perturbation.