Axial Plasma Density Research Articles

Stabilization of magneto-Rayleigh-Taylor instability with non-uniform density and magnetic field profiles in Cartesian Geometry

Managing the Rayleigh–Taylor instability growth rate is one of the main challenges in inertial fusion. Among the proposed methods to tackle this issue, the application of a pre-embedded axial magnetic field and plasma density gradient of layers can be mentioned. Recent experimental evidence in Z-pinch devices shows that it is possible to suppress the growth rate of the Rayleigh-Taylor instability by applying a power law density gradient profile with exponent 2. In this work, in the framework of Cartesian coordinates, for a confined plasma between two fixed planes, the dispersion relation of a magnetized plasma has been obtained using linearization of the ideal MHD equations. Here, plasma with a power-law density profile of ∝r3 and magnetic field with the exponential profile is used. It is shown that for the relative strength of the magnetic field with a value of, ωf*=0.4 and more, the growth rate of instability appears only in a limited range of the range of reduced wave numbers, k*. By increasing this ratio, stability conditions are much improved. On the other hand, recent experimental studies show that self-generated plasma rotation is produced in an imploding Z-pinch device with an initial axial magnetic field. In the second part, the effect of plasma rotation on plasma stability conditions is investigated. Again, a new dispersion relation of the rotating magnetized plasma was derived. It is shown that in the non-magnetized case, a large relative angular velocity, Ω, is necessary to suppress the instability. However, for relative magnetic field strength, ωf*=0.4 and more, stable conditions can be achieved with a relatively lower angular velocity of about 5.

Helicon waves in a converging-diverging magnetoplasma

Waves propagating along a converging-diverging rf magnetoplasma having the characteristics of a bounded m = 0 helicon mode are reported and characterised. The discharge features a 30cm separation between the region of radiofrequency energy deposition by a single loop antenna and the region of maximum magnetic field applied by a pair of coils. With 200W of rf input power, up to a five-fold increase in axial plasma density between the antenna and the magnetic mirror throat is observed together with an Ar II blue-mode. Two dimensional B-dot probe measurements show that the rf magnetic fields are closely guided by the converging-diverging geometry. The wave is characterised as a m = 0 mode satisfying the helicon dispersion relation on-axis with radial boundary conditions approximately matching the radii of the plasma column. Analysis of the wave phase velocity and wave axial damping failed to identify collisionless or collisional wave-plasma coupling mechanisms. Instead, the wave axial amplitude variations can be explained by local wave resonances and possible reflections from localised rapid changes of the refractive index. A Venturi-like effect owing to the funnel-shaped magnetoplasma and conservation of the wave energy may also explain some level of amplitude variations.

Characterization of a new variable magnetic field linear plasma device

A radio frequency plasma device is presented in which the regions of plasma creation and maximum plasma magnetization can be separated along a 1.5-m tube. Measurements of the plasma density, plasma potential, and electron temperature in the device successfully reproduce previously reported plasma features. These validate the ability of the experiment to continue the investigation of a regime of operation in which the axial plasma density follows the profile of the applied magnetic field, as long as the ions are magnetized under the antenna. The density is shown to increase on axis owing to the decreasing cross section of the converging magnetic funnel connecting the antenna region to the solenoids. When the funnel pinching is increased, stronger magnetic fields are required to inhibit cross-field diffusion and to bring the density on axis in the expected 1012 cm−3 range. Collisionless transport of hot electron populations is observed along the field lines which intersect the area under the antenna and coincides with the presence of high-density conics more than 0.5 m away from the antenna for magnetic fields ≥600 G.

The dynamics of coherent modes of gradient drift instabilities in a small magnetron discharge plasma

We report on the dynamic behavior of gradient-driven drift waves in a strongly obstructed magnetron discharge. The magnetron has a magnetic topology that results in a toroidal plasma within the gap and supports the development of very coherent modes of rotating plasma structures. The modes and their rotation are present over a wide range of conditions, and the rotation is retrograde to the usual externally imposed E×B direction. This feature seems to be unique to this device and is attributed to a field reversal due to the strong anode-directed electron diffusion that arises from large axial plasma density gradients. A multi-fluid model is proposed, and a Fourier analysis of the linearized equations results in the identification of conditions that support the growth of these instabilities and their transitions across mode symmetries, controlled experimentally by varying the discharge voltage. The model also provides insight on the possible mechanism driving cross-field particle transport. Experiments are carried out with a segmented anode to confirm the localized current flow concomitant with the presence of a coherent structure. These segment currents together with high speed videography unambiguously confirm the direction of plasma rotation and reveal the existence of a stochastic regime between voltage-controlled mode transitions. An analysis of the segment currents in this regime indicates that the lower frequency state decays into a spectrum of coherent higher frequency states that exhibit features consistent with a three-wave nonlinear parametric mixing process.

Simulations of electromagnetic effects in large-area high-frequency capacitively coupled plasmas with symmetric electrodes: Different axial plasma density profiles

In this paper, the electromagnetic effects are investigated in large-area high-frequency symmetric capacitive discharges by solving the Maxwell equations under two different axial plasma density profiles, i.e., the spatially uniform and the axial non-uniform density profiles. Simulation results show that the spatially uniform density profile underestimates the standing wave effect and overestimates the skin effect. Moreover, the electromagnetic effects are significantly affected by the frequency, sheath width, electron-neutral momentum transfer frequency, and plasma density. As the frequency increases, the surface wavelength is significantly reduced, and the standing wave effect becomes pronounced. An opposite result is obtained by increasing the sheath width. As the electron-neutral momentum transfer frequency increases, the radial damping of surface waves when they propagate from the radial edge to the center becomes prominent. As the plasma density increases, the discharge is dominated by the skin effect instead of the standing wave effect, due to the reduced skin depth.

THE MODEL OF GAS DISCHARGE SUSTAINED BY THE QUADRUPOLE ELECTROMAGNETIC WAVE IN WAVEGUIDE STRUCTURE WITH VARYING RADIUS OF METAL ENCLOSURE

This article presents the results of the theoretical study of stationary state of gas discharge sustained by the electromagnetic wave with azimuth wavenumber m = −2 in three component magnetized plasma-metal waveguide structure of slightly varying radius of metal enclosure in the framework of electromagnetic model. It was studied the influence of the external magnetic field value, the electron effective collision frequency and other parameters on the phase characteristics, spatial attenuation, discharge stability and axial plasma density profile in the structure with constant radius of metal waveguide and with radius that varies slightly along the discharge.

Non-local plasma generation in a magnetic nozzle

Axial plasma density measurements in a 1.5 m long plasma chamber are presented for when the regions of high magnetic field and radio frequency heating are progressively separated using a movable solenoid pair. The results show that the operating regime changes based on the degree of ion magnetisation under the antenna. When ions are magnetized, electrons heated under the antenna are efficiently transported to the solenoids along a column defined by the magnetic field lines which connect to the antenna region. The cross section of this column decreases due to the converging magnetic field geometry, thereby increasing the density of electrons on the axis. This results in a density profile which is singly peaked and centered on the location of maximum magnetic field strength. When the ions are unmagnetised under the antenna, the flux of positive charges to the wall there is increased. Electrons streaming along field lines that intersect the radial wall in the antenna region are then more attracted to the antenna region to balance this flux. This affects the equilibrium conditions along the entire magnetic field line and results in less efficient transport of electrons heated by the antenna to the region of high magnetic field strength. In this regime, there is a global decrease in plasma density and the axial density profile is doubly peaked.

Effect of axial electron temperature and plasma density ramp on self-focusing / defocusing of a laser beam in plasma

In the present study, we analyze the self-focusing / defocusing of an intense laser beam in the plasma under combined effect of density ramp and higher order axial electron temperature. The differential equation for beam width parameter is obtained and then solved numerically under paraxial ray approximation. The behavior of beam width parameter with propagation distance is extensively investigated for different values of optimized parameters and is strongly influenced by the higher order axial electron temperature, Tp4. The results indicate that the electron temperature component Tp4 is not of little significance than Tp0 and Tp2. Under the influence of plasma density ramp and higher order axial electron temperature Tp4 the defocusing is reduced and self-focusing becomes stronger, especially for large propagation distance. Furthermore, increase in the density ramp results more reduction in the laser beam spot size and hence, increase in the intensity of the laser which may be useful for various applications including laser-driven fusion.

Optimal design of helicon wave antenna and numerical investigation into power deposition on helicon physics prototype experiment

<sec>Recently, helicon plasma sources have aroused the great interest particularly in plasma-material interaction under fusion conditions. In this paper, the helicon wave antenna in helicon physics prototype experiment (HPPX) is optimized. To reveal the effect of the radial density configuration on wave field and energy flow, Maxwell's equations for a radially nonuniform plasma with standard cold-plasma dielectric tensor are solved. Helicon wave coupling and power deposition are studied under different types of antennas, different antenna lengths and driving frequencies by using HELIC. Through the numerical simulation, the optimal antenna structure and size are obtained, that is, half helix antenna, which works at 13.56 MHz and has a length of 0.4 m, can generate nonaxisymmetric radio frequency energy coupling to excite higher electron density.</sec><sec>The influences of different static magnetic fields and axis plasma densities on power deposition are also analyzed. It is found that the absorbed power of the plasma to the helicon wave has different peak power points in a multiple static magnetic field and axial plasma densities, and the overall coupling trend increases with static magnetic field increasing, but decreases with axis plasma density increasing. According to the simulation results, the ionization mechanism of helicon plasma is discussed. In order to further study the coupling of helicon wave with plasma in HPPX, the induced electromagnetic field and current density distribution are given when the plasma discharges. Under parabolic density distribution, the field intensity of the induced electric field at the edge is large, while neither the induced magnetic field nor current density changes much along the radial direction, the energy is distributed evenly in the whole plasma. Under the Gaussian density distribution, the induced electric field intensity is higher at the edge, while the induced magnetic field and current density in the center are much higher than at the edge. </sec><sec>In this paper studied are the structure and size of helicon wave antenna, the influences of static magnetic field and axial plasma density on plasma power deposition and the distribution of induced electromagnetic field and current density during plasma discharge under different density distributions. This work will provide important theoretical basis for helicon wave antena design and relevant physical experiments on HPPX.</sec>

Experiments on the transportation of a magnetized plasma stream in the GOL-3 facility

The program of the deep upgrade of the GOL-3 multiple-mirror trap is presented. The upgrade is aimed at creating a new GOL-NB open trap located at the GOL-3 site and intended to directly demonstrate the efficiency of using multiple-mirror magnetic cells to improve longitudinal plasma confinement in a gasdynamic open trap. The GOL-NB device will consist of a new central trap, adjoint cells with a multiple-mirror magnetic field, and end tanks (magnetic flux expanders). Plasma in the central trap will be heated by neutral beam injection with a power of up to 1.5 MW and duration of 1 ms. At present, physical experiments directed at developing plasma technologies that are novel for this facility are being carried out using the 6-m-long autonomous part of the GOL-3 solenoid. The aim of this work was to develop a method for filling the central trap with a low-temperature start plasma. Transportation of a plasma stream from an arc source over a distance of 3 m in a uniform magnetic field with an induction of 0.5–4.5 T is demonstrated. In these experiments, the axial plasma density was (1–4) × 1020 m–3 and the mirror ratio varied from 5 to 60. In general, the experiments confirmed the correctness of the adopted decisions for the start plasma source of the GOL-NB device.

Characteristics of Self-Focusing of a Gaussian Laser Pulse Under Lateral and Axial Plasma Density Variations

In this paper, we present a paraxial description of relativistic self-focusing of a Gaussian laser beam in a plasma channel with the density distribution as a function of radial and axial variations. We have derived the nonlinear dielectric permittivity of plasma due to ponderomotive force and relativistic effects. The effect of different conditions of the inhomogeneous plasma and the laser pulse on self-focusing and defocusing of Gaussian laser beam propagation in the plasma is studied. We find that with an increase in the value of intensity of the beam, self-focusing enhances and shifts toward lower values of normalized propagation distance. The self-focusing length also increases with an increase in channel width. When the density rapidly increases along the propagation distance, the nonlinearity increases and gives self-focusing at shorter values of $z$ .

One-dimensional time-dependent fluid model of a very high density low-pressure inductively coupled plasma

A time-dependent two-fluid model has been developed to understand axial variations in the plasma parameters in a very high density (peak ne≳5×1019 m−3) argon inductively coupled discharge in a long 1.1 cm radius tube. The model equations are written in 1D with radial losses to the tube walls accounted for by the inclusion of effective particle and energy sink terms. The ambipolar diffusion equation and electron energy equation are solved to find the electron density ne(z,t) and temperature Te(z,t), and the populations of the neutral argon 4s metastable, 4s resonant, and 4p excited state manifolds are calculated to determine the stepwise ionization rate and calculate radiative energy losses. The model has been validated through comparisons with Langmuir probe ion saturation current measurements; close agreement between the simulated and measured axial plasma density profiles and the initial density rise rate at each location was obtained at pAr=30−60 mTorr. We present detailed results from calculations at 60 mTorr, including the time-dependent electron temperature, excited state populations, and energy budget within and downstream of the radiofrequency antenna.

Dynamics of laser beams in inhomogeneous electron–positron–ion plasmas**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274255 and 11305132), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20136203110001), the Natural Science Foundation of Gansu Province, China (Grant No. 2011GS04358), and the Creation of Science and Technology of Northwest Normal University,

Nonlinear interaction of laser and electron–positron–ion plasmas is investigated by invoking the variational principle and numerical simulation, in terms of a nonlinear Schrödinger equation with inhomogeneities effect. It is shown that the plasma inhomogeneity has great influence on the laser beam dynamics. The laser beam can be self-trapped, focused, or defocused depending on the inhomogeneity character. The linearly decreasing axial plasma density makes the laser beam defocus, while the linearly increasing axial plasma density results in self-trapping of the beam. The self-focusing of the trapped beam is found in a high-density region. For the Gaussian types of density distribution, the beam field submits nonlinearly oscillating regime. The results provide an efficient way to manipulate the dynamics of laser beam propagating in plasma.

Intensity distribution of a capillary-discharge 46.9 nm soft X-ray laser

We realize a Ne-like Ar 46.9 nm soft X-ray laser pumped by a capillary discharge. The study of the laserpulse-intensity distribution is important for applications of soft X-ray lasers. The intensity distribution demonstrates the gain distribution, plasma radius, and axial plasma density that contribute to the study of the laser-pulse formation. To measure the intensity in different positions of the X-ray laser spot, we moved transversally an X-ray diode (XRD) assembled with a slit. We obtain the onedimensional intensity distribution. We find a laser divergence (FWHM) of 4.0 mrad. According to the gain-guided model, we calculate the intensity distribution. The measured divergence of 4.0 mrad roughly corresponds to a plasma radius a approximately equal to 230–250 μm and on-axis electron density ne≈ 8.0∙1018−9.0∙1018 cm−3. The results of calculations indicate that the divergence of the intensity distribution increases when the plasma radius decreases and the on-axis electron density increases.

Separate control between geometrical and electrical asymmetry effects in capacitively coupled plasmas

Both geometrical and electrical asymmetry effects in capacitive argon discharges are investigated using a two-dimensional particle-in-cell coupled with Monte Carlo collision model. When changing the ratio of the top and bottom electrode surface areas and the phase shift between the two applied harmonics, the induced self-bias was found to develop separately. By adjusting the ratio between the high and low harmonic amplitudes, the electrical asymmetry effect at a fixed phase shift can be substantially optimized. However, the self-bias caused by the geometrical asymmetry hardly changed. Moreover, the separate control of these two asymmetry effects can also be demonstrated from their power absorption profiles. Both the axial and radial plasma density distributions can be modulated by the electrical asymmetry effect.

Measurement and modelling of a radiofrequency micro-thruster

A capacitively coupled radiofrequency (rf) (13.56 MHz) cylindrical argon micro-discharge expanding into a larger glass tube is studied by performing optical and electrical measurements over a pressure range 0.3–5 Torr and a rf power range 5–40 W. Measurements of the axial and radial plasma density profiles at the Paschen minimum near 1.5 Torr are used to develop a global model of the discharge and estimate neutral heating from ion–neutral charge exchange collisions for micro-propulsion applications.

Tailoring plasma density for laser plasma accelerators

To lower the trapping threshold and partially overcome the dephasing problem, a certain axial plasma density profile is desired. To obtain such a density distribution, a laser heating method is proposed and numerically tested in this paper. By using this method, a density downramp and then upramp can be formed. When a laser wake wave passes through such a downramp density, plasma electrons are trapped into the acceleration phase. Propagating in an upramp density, the energy of the trapped electrons would be considerably enhanced.

Axial plasma density propagation of barrier discharge non-thermal plasma bullets in an atmospheric pressure argon gas stream

The characteristics of volume-averaged plasma density on axial propagation for atmospheric argon (Ar) plasma bullets are experimentally investigated. The non-thermal plasma bullets are ejected through a glass tube into the surrounding ambient air. Taking into consideration the time and space profile of the plasma movement, the plasma propagation is measured using a Rogowski coil. The plasma density is evaluated from the propagation velocity and the current magnitude. The plasma density profiles are presented as functions of the applied voltage and the length of growth. The plasma density is in the order of 1016 m−3 and the propagation velocity is in the order of 105 m s−1. These values are similar to those of weakly ionized non-thermal plasma jets.

A space-charge-neutralizing plasma for beam drift compression

Simultaneous radial focusing and longitudinal compression of intense ion beams are being studied to heat matter to the warm dense matter, or strongly coupled plasma regime. Higher compression ratios can be achieved if the beam compression takes place in a plasma-filled drift region in which the space-charge forces of the ion beam are neutralized. Recently, a system of four cathodic arc plasma sources has been fabricated and the axial plasma density has been measured. A movable plasma probe array has been developed to measure the radial and axial plasma distribution inside and outside of a ∼10-cm-long final focus solenoid (FFS). Measured data show that the plasma forms a thin column of diameter ∼5 mm along the solenoid axis when the FFS is powered with an 8 T field. Measured plasma density of ⩾1×10 13 cm −3 meets the challenge of n p/ Zn b>1, where n p and n b are the plasma and ion beam density, respectively, and Z is the mean ion charge state of the beam ions.

Magnetically tapered plasma channels for laser wakefield accelerators

To partially overcome the dephasing problem for the accelerated electron beam in the laser wakefield accelerators, a certain radial and axial plasma density distribution is desired. To obtain such a two-dimensional density profile, a magnetical tapering scheme is proposed and numerically tested. By using this scheme, a precisely tapered channel can be formed. For given laser parameters, the accelerated electrons would stay in phase with the accelerating field over many dephasing lengths. Our preliminary simulation results show the feasibility of such a magnetically tapered plasma channel, and help to identify potential experimental difficulties. For practical implementation, a combination of externally controlled axial magnetic field distributions and wall temperature quenching is desirable.