Radius Of Plasma Column Research Articles

Experimental issues of energy balance in open magnetic trap

The paper presents an overview of experimental results of an investigation of different energy loss channels in the gas dynamic trap (GDT), which is a magnetic mirror plasma confinement device in the Budker Institute of Nuclear Physics. Energy losses along magnetic field lines are considered as well as losses onto radial limiters, which restrict the plasma column radius and provide its magnetohydrodynamic stability via the ‘vortex confinement’ mechanism. The losses along the field lines were measured using a set of pyroelectric bolometers on the plasma absorber and the losses onto the limiters were determined with thermistors from their temperature rise. Additionally, the losses due to charge exchange of fast plasma ions on the residual neutral gas in the GDT were measured using a longitudinal array of pyroelectric bolometers mounted on the wall of the central cell. An attempt was made to draw up the energy balance in the GDT in order to identify the predominant loss channels and reduce those losses in the future.

Theoretical and experimental analysis of plasma radius expansion model in EDM: a comprehensive study

Electrical Discharge Machining (EDM) is an established non-conventional process, which is particularly efficient for the processing of hard-to-cut materials, in order to obtain high dimensional accuracy and surface integrity. However, in order to determine the appropriate parameters for machining novel materials, it is necessary to investigate the EDM process in depth, both by experiments and numerical models, taking into consideration the fundamental physical phenomena occurring during this process and be able to predict the surface morphology and microstructural alterations under various conditions. One of the challenging issues of EDM simulation models that still remain open is the representation of the evolution of plasma channel radius, for which various approaches have been proposed such as a linear, power law, or a more complex piecewise relation, in respect to time. Thus, in this work, the effect of different relations for the plasma channel radius evolution on energy absorption coefficient, plasma flushing efficiency (PFE), and crater morphology is compared under various conditions with a numerical model, which is also compared to experimental data. The results indicate that the energy absorption coefficient is dependent on the plasma column radius function, as slower growth of plasma channel leads to lower absorption coefficient and more efficient material removal, whereas a lower variation and different trends under different conditions were observed regarding PFE values, in respect to the power law exponent. Finally, the crater dimensions were shown to be consistently more narrow and deeper with higher exponents; thus, based on actual observations of indicative craters, it was revealed that the appropriate values for the exponent of the power law plasma radius function are below 0.25.

Microwave propagation along nonuniform plasma column as surface plasmon

Formation and propagation of surface plasmons (SPs) of microwaves (MWs) along a dense non-uniform plasma column were investigated by employing the finite-difference time-domain method and evidenced by experiment of argon helicon plasma. The effects of plasma density, incident frequency, and radius of plasma column on the SP features were discussed. Results showed that when the MW frequency and the density distribution of plasma column satisfy suitable conditions, SPs can be excited on the surface of a uniform plasma column when the frequency is 2–5 times of the MW, while inside a nonuniform plasma column when the plasma frequency is only a little higher (about 5%–20%) than the MW. The SP wavelength depends on the plasma density for a given incident MW frequency. The forward scatter MW field will be enhanced significantly when SP appears along the nonuniform plasma column.

Mathematical Model of an Electric Arc in Differential and Integral Forms With the Plasma Column Radius as a State Variable

Mathematical Model of an Electric Arc in Differential and Integral Forms With the Plasma Column Radius as a State Variable

Radiation Losses From a Microwave Surface-Wave Sustained Plasma Source (Surfatron)

Surfatron is an electromagnetic (EM) field applicator yielding surface-wave (SW) discharges (SWDs) within dielectric tubes in a broad range of operating conditions. In this article, the vector wave equation for the electric field is solved numerically in 2-D to determine the influence of plasma parameters, such as electron density, normalized electron–neutral collisions frequency, plasma column length and radius, as well as the length of the surfatron body on the field strength distribution, radiation patterns, and ratio of radiated power to power absorbed in plasma. This analysis allows for estimating the efficiency of plasma generation. The model takes into account the surfatron configuration, reproducing the typical SWD plasma column behavior, namely the fact that the plasma column is “pushed out” of the surfatron interstice as absorbed power increases. The surfatron is assumed to be placed in an air-filled sphere with the surface transparent to the wave. Calculations are performed for a field frequency of 2450 MHz and a fused silica tube, considering plasma parameters typical for both atmospheric- and lower-pressure discharge gases for achieving a parametric survey. It is found that for given plasma parameters, varying the plasma column length from a few up to 30 cm significantly affects the distribution of the electric field around the surfatron and the radiation patterns. It is further shown that the radiated to absorbed power ratio increases with increasing electron density and plasma radius, and decreasing electron–neutral collision frequency. This ratio can exceed 30% for a 30-cm plasma column with a plasma radius of 3 mm and parameters typical for atmospheric and reduced gas pressures. The surfatron length at which radiation losses are minimal is at half the vacuum wavelength.

Single-cycle THz signal accompanying laser wake in photoionized plasmas

Photoionization of an ambient gas by a weakly relativistic, femtosecond laser pulse leaves behind the pulse a flat-top plasma column. The uniform-density core of the column is surrounded by a micron-thin shell, within which the density of plasma species falls down to zero. As the laser pulse ponderomotive force drives the wave of charge separation (the laser wake), electron fluid velocity oscillates along this boundary layer. Coupling velocity oscillations to the transverse density gradient forms an azimuthally polarized rotational current confined within the boundary layer. At each radial offset, this current oscillates with a local (THz-scale) Langmuir frequency, transforming the cylindrical electrostatic plasma wave into a radially evanescent Fourier component of an electromagnetic signal. Summing up these Fourier components yields the time-domain THz signal which is (a) non-local (i.e. not phase-locked to the Langmuir oscillation at any given location), (b) contains the entire frequency band from zero to the Langmuir frequency of the density plateau, and (c) is detectable at a distance orders of magnitude larger than the plasma column radius. A few millimeters away from the column, rapid evanescence of the higher-frequency components transforms the signal into a radially polarized, single-cycle picosecond pulse with a kV m−1-scale electric field. Detecting this pulse with existing (e.g. electro-optical) techniques may bring information on the efficiency of laser coupling to the plasma, the amplitude of wake fields, and the structure of the column boundary.

Multiple parameter space bandgap control of reconfigurable atmospheric plasma photonic crystal

A plane wave expansion method is used to simulate the bandgaps for a square lattice plasma photonic crystal over a parameter space of five independent variables, characteristic of a reconfigurable atmospheric discharge (plasma frequency: 0.056–5.6 × 1012 rad/s, collision frequency: 1–6 × 1012 rad/s, plasma column radius: 0.2–0.4 mm, lattice constant: 1–2 mm, and background dielectric: 1–10). The trends of the first and second bandgaps allow for five-dimensional 4th order polynomial equations to be fitted to the data, defining the boundaries of the first and second bandgaps with 8% and 2% error, respectively. Performance metrics (operational frequency range and frequency sensitivity) of the plasma photonic crystal are defined and evaluated for each controlling variable. The results show that, within the variable space investigated here, the column radius and background dielectric are the most effective controlling variables for the bandgap bandwidth and center frequency, respectively. The maximum frequency range provided by the variable ranges investigated here is 25–400 GHz and 0–250 GHz for the TE1 and TE0 bandgaps, respectively.

Electromagnetic energy rotation along plasma-dielectric interface caused by azimuthal surface waves in isotropic cylindrical metallic waveguide

Electromagnetic energy rotation around the axis of cylindrical metallic waveguides partially filled by isotropic plasma is studied. Angular phase velocity and angular velocity of energy transfer are analyzed as functions of the plasma waveguide parameters: plasma column radius, plasma particle density, azimuthal wavenumber, dielectric layer width and dielectric constant, and collision frequency. This study generalizes the results obtained earlier in the case of the wave propagation in two-component plasma-metal structures (cylindrical metallic waveguides entirely filled by magnetoactive plasma) to the case of three-component plasma-dielectric-metal structures.

Plasma Effect on the Fast- and Mixed-Wave Modes of Gyrotron Devices

The linear theory of a plasma-loaded gyrotron amplifier is studied in the fast- and mixed-wave modes. The dispersion relation is derived using the Vlasov-Maxwell equations. The effects of beam location, plasma column radius, electron beam parameters, and azimuthal harmonic number on the growth rate for the fast- and mixed-wave modes are investigated. In addition, the radial behavior of the electric field is studied. The results show that the growth rate and bandwidth of the mixed-wave mode are larger than those of the fast-wave mode. It is shown that the bandwidth of this structure is largest for a small value of the axial momentum spread. Moreover, it is indicated that for a lower harmonic number, the axial electric field has its maximum value at the center of the plasma.

Magnetic Measurements at the GDT Facility

Plasma generated electromagnetic oscillations at the frequency an order ion-cyclotron have been investigated in the high energy content regimes of GDT operation. That waves are found to be an Alfvén ion-cyclotron instability. The microinstability threshold scaling law has been defined: with increase of the plasma column radius to Larmor radius ratio the threshold value of the diamagnetism in the midplane decreases. It qualitatively matches the theoretical calculations.

Relativistic cold fluid radial equilibrium of an annular nonneutral plasma

The relativistic radial equilibrium of a nonneutral plasma confined in a cylindrical trap is considered for the general case in which a coaxial inner charged conductor is present. Both rigid and sheared modes of plasma rotation in crossed self-consistent electric and magnetic fields are analyzed. It is found that relativistic effects can limit the outer radius of an annular nonneutral plasma column for the fast sheared rotational mode. For the particular case of a uniform plasma density, the outer radius cannot exceed 3 r i c / | Ω | if the inner radius of the plasma column r i is smaller than 3 c / | Ω | , where Ω is the cyclotron frequency. On the contrary, radially unlimited equilibria are found when r i > 3 c / | Ω | . The Brillouin density limit is also analyzed and a class of sheared equilibria is found where the limit valid for the case of rigid rotation can be overcome.

THE THEORETICAL INVESTIGATION OF THz ELECTROMAGNETIC WAVES IN A ROD DEGENERATE PLASMA-WAVEGUIDE

By using the Fourier components of dielectric tensor elements of cold collisionless degenerate plasmas, the dispersion relation of electromagnetic waves in a cylindrical metallic waveguide with a degenerate plasma column protected by an annular dielectric layer are obtained. The permissible frequency regions for slow and fast waves in E-mode (TM) are presented. Furthermore, the graph of cutoff frequency versus the radius of plasma column for the fast waves is investigated. In addition, the time growth rate for excitation of symmetric slow E-modes (TM) by a thin annular relativistic electron beam (TAREB) is studied. The graph of time growth rate respect to radius of dielectric layer and accelerating voltage are presented.

Analysis of the axial electric field in a plasma-loaded-helix travelling wave tube

A helix type slow wave structure filled with plasma is immersed in a strong longitudinal magnetic field. Taking into account the effect of the plasma and the dielectric, the system is separated radially into three regions. By means of the sheath model and Maxwell equation, the distribution of the electromagnetic field is established. Using the boundary conditions of each region, the dispersion relation of the slow wave structure is derived. The trend of change for the radial profile of the axial electric field is analysed respectively in different plasma densities, plasma column radius and dielectric constant by numerical computation. Some useful results are obtained on the basis of the discussion.

Field profiles of symmetric surface waves in diffusion-controlled regime of a non-isothermal plasma columns

The field distribution of a symmetric surface wave (SW) propagating on a weakly ionized unmagnetized plasma placed in a cylindrical ideal waveguide, in which a quartz tube is surrounded by vacuum, is presented in the diffusion-controlled regime. This is done to better understand the effect of some physical parameters such as electron temperature, SW frequency, etc on the plasma column. The graphs of electric and magnetic fields versus the plasma column radius under the radial electron density profile dependency are obtained in the commercial radio frequency region where spatial dispersion might be important.

Numerical Simulation and Experimental Identification of Divertor Configuration in the HL-2A Tokamak

Single- and double-null divertor configurations in HL-2A are simulated by SWEQU equilibrium code. Lower divertor discharges in the first physics campaign have been achieved by two kinds of power supply method of multipole-field coils. Single-null divertor configuration has been identified by visible photography, target probe arrays and the reconstructed magnetic surface. Magnetic separatrix and minor radius of plasma column are obtained by a reconstructed code of multiple current filaments using 18 Mirnov signals.

Polarization-Reversal-Induced Damping of Left-Hand Polarized Wave on Electron Cyclotron Resonance

The left-hand polarized wave, which has been believed not to be related to electron cyclotron resonance (ECR), is for the first time demonstrated to be damped near the ECR point in an inhomogeneously magnetized plasma as a result of the polarization reversal to the right-hand polarized wave. The polarization reversal is found to be caused by the existence of the boundary between a plasma and a vacuum region, the local point of which shifts depending on the plasma column radius. This phenomenon is quantitatively explained in terms of the dispersion relation including the effect of the radial boundary condition.

Dispersion characteristics and field structure of surface waves in a warm inhomogeneous plasma column

The electron temperature and the radial inhomogeneity of the plasma density are considered in studying the characteristics of the surface electromagnetic wave in a plasma column. The Maxwell equations are combined with the fluid momentum equation to obtain the governing wave equations. For a uniform plasma the wave equations are analytically solved yielding the dispersion relation in a closed form and they are solved by the finite difference method after being converted to an operator eigenvalue equation for a nonuniform plasma. It is shown that the influence of the thermal electron motion is closely related to the density profile and the plasma column radius in addition to the electron temperature and collision frequency. Finally, the electric field structure is found to be quite different from the one obtained from the cold plasma theory.

Dispersion relation of azimuthal electromagnetic surface waves on a magnetized plasma column in a dielectric lined slow-wave waveguide

The dispersion relation of azimuthal electromagnetic surface waves on a magnetized plasma column surrounded by a metallic cylindrical dielectric lined slow-wave waveguide is obtained. The permissible frequency region for these waves in E- and B-mode is presented. Furthermore, the graphs of frequency spectra and radial dependence of fields on the external magnetic field strength, geometric dimensions of the waveguide, radius of plasma column and thickness of dielectric are investigated.

Axial structure of plasma column sustained by non‐symmetric electromagnetic surface wave

AbstractThis article presents the results of theoretical study of the axial structure of gas discharge that is sustained by a non‐symmetric electromagnetic surface wave (SW) with azimuth wavenumber m = –2. The phase and attenuation characteristics of the wave, and the spatial wave field structure were studied as well. The wave considered propagates along the magnetized slightly nonuniform in axial direction plasma column and sustains it. The influence of external magnetic field value and finite plasma column radius on the discharge stability, on the plasma density axial profile and on the SW properties has been studied in the case of the diffusion regime of gas discharge sustaining.

Quadrupole and Octopole Electromagnetic Waves in Slightly Non-Uniform Magnetized Plasma Column

The paper presents the results of a theoretical study of the dispersion properties, wave field structure and energy characteristics of travelling electromagnetic nonsymmetric surface waves that propagate along a magnetized plasma column surrounded by an infinite vacuum region. The waves with azimuth wave numbers m = ±2, ±3 were considered. An external constant magnetic field parallel with respect to the axis of the waveguide system is applied. The theoretical study is carried out using a hydrodynamic model of a cold, radially uniform plasma. The influence of external magnetic field strength and the finite plasma column radius on the dispersion, attenuation and radial wave field structure is studied in the approximation of weak longitudinal plasma density non-uniformity. It is shown, that the external magnetic field essentially affects the dispersion, radial wave field structure and wave damping. Research carried out allows one to estimate the possibility of quadrupole and octopole modes use for gas discharge sustaining. The analysis of the obtained results makes it possible to estimate the plasma density, that can be sustained in gas discharges on quadrupole and octopole waves.