The Influence of the Fluctuation Amplitude on the Probe Characteristic

The time-averaged probe current is calculated by using a sampling method for sinusoidal and sawtooth-like oscillations of the space potential. The effect of oscillations on the measured plasma parameters obtained with the aid of the single probe method, double probe method and the method of the second derivative of the probe characteristic is discussed, with the electron saturation current being taken into account. In the ion current the values r_p/λ_D, λ/r_p and M_i characterizing the working regime are varied. The calculated results are checked by corresponding measurements in a beam generated plasma.     

Effect of neutral pressure on the He plasma flow measurement in a linear device

Axial and azimuthal flow velocities have been measured in a linear plasma device called NAGDIS-II (NAGoya DIvertor Simulator-II), along with plasma density and electron temperature, using a vector Mach probe composed of two Mach probes, one of which is for the axial flow, and the other is for the azimuthal flow. To study the effect of neutral pressure on the deduction of the Mach numbers, the ratio of upstream to downstream currents are measured by changing the neutral pressure for the deduction of flow velocities. Helium plasma was generated with pressure of 2–35 mTorr. Since the ion gyro-radius at the magnetic flux of 300 G is larger than the probe size, an unmagnetized collisionless Mach probe theory was used for the deduction of Mach numbers and their variations. In order to check the range of collisionality, plasma density (n_e = 10¹⁰–10¹¹ cm^?3) and electron temperature (T_e = 2–9 eV) are measured by a single electric probe using a conventional collisionless probe theory. Variations of Mach number, electron temperature and plasma density with collisionless models are to be compared with those using collisional models for different pressures where ionization and ion-neutral collision are included. Mach numbers by the collisionless model are found to be overestimated by 120% for the maximum difference even in weakly collisional plasmas. A clear flow reversal exists in the axial direction with higher pressure plasma, even in the linear machine. Azimuthal flows are also measured simultaneously along with axial flows, yet they seem to be very small in the present cold ion plasma (T_i/T_e << 1).     

New Applications of Langmuir Probes

In this work, two new possibilities for standard probe diagnostics are described. The first one can be used to study isotropic, collisionless low-pressure plasma in which the electron energy distribution function is close to a Maxwellian one. In such plasmas, the Boltzmann law, Bohm effect, and 3/2 power law are valid. Use of corresponding system of equations for cylindrical Langmuir probes allowed for measurements of probe sheath thicknesses and the mean ion mass. The solution of this task was provided by accurate probe diagnostics of inductive xenon plasma at pressure p = 2 mTorr that resulted in the determination of the Bohm coefficient C_BCyl = 1.22. The second possibility of probe diagnostics includes a method and device for evaluation of ion current density to a wall under a floating potential using a radially movable plane wall Langmuir probe simulator. This measurement in the same xenon plasma served as the basis for development of an ion source in which the given wall was represented by an ion extracting electrode of the ion extraction grid system.     

The neutron electric dipole moment as a test of the superweak interaction theory

Theoretical calculations of the neutron electric dipole moment D_n are reviewed for various theories of CP violation. It is shown that for the superweak interaction theory D_n is less than 10^?29e · cm in contrast to values of 10^?23 to 10^?24 predicted by many but not all milliweak theories. It is concluded that prospective measurements of D_n may provide decisive evidence against or significant evidence in favour of the superweak theory.     

The Influence of Collisions in the Space Charge Sheath on the Ion current Collected by a Langmuir Probe

The current/voltage characteristics of a cylindrical Langmuir probe have been studied in Ar⁺/electron afterglow plasmas in helium carrier gas under truly thermal conditions at 300 K using our flowing afterglow/Langmuir probe (FALP) apparatus. The orbital motion limited (oml) ion and electron current regions of the probe characteristics have been explored over a wide range of the reduced probe voltage (up to ～ 100) and over a wide range of electron (n_e) and ion (n₊) number densities (1.6 × 10⁷ to 1.5 × 10¹⁰ cm^?3) at a constant pressure of the He carrier gas of 1.2 Torr. The observed increase of the probe ion currents above those predicted by collisionless oml theory, resulting in an apparent increase of the measured ion number density above n_e in the plasma, is explained by the enhancement in the ion current collection efficiency due to collisions of ions with neutral gas atoms in the space charge sheath surrounding the probe. The continuous change in the exponent, χ, of the power-law dependence,i₊ ∞ V of the ion current, i₊, on the probe voltage, V_p from 0.5 at the highest n₊ (smallest sheath) towards 1.0 at the lowest n₊ (large sheath) indicates that the ion current collection from the plasma changes from the oml current regime at the high n₊ to the continuum regime at the low n₊ when the ions undergo multiple collisions with the helium atoms in the space charge sheath and thus “drift” towards the probe.     

Microwave impedance of a tunnel junction in the theory of a near-field microscope with atomic resolution

A theory is constructed for a near-field microwave microscope operating under tunneling breakdown between a probe and a conducting sample. Its informative characteristics are determined by the probe impedance, which is formed from the capacitive impedance Z _p of the near-field probe–sample interaction and tunnel junction impedance Z _t. A technique whereby the impedance Z _p is calculated using coaxial geometry of the probe is developed. Some properties of the impedance Z _t defined via the developed theory and published experimental data are investigated.     

Radial dependence of spin-orbit and Λ-doubling parameters in the X2Π ground state of hydroxyl

Experimental spin-orbit and Λ-doubling constants of OH X²Π in the vibrational range v ≤ 10 were inverted to their operator forms. The A(r) radial form derived from the spin-orbit coupling constants has been used to calculate the spin-orbit distortion parameters, A_Dv, in the range v ≤ 4. These A_Dv values have been used to separate the totally correlated γ_v and A_Dv values obtained experimentally. Radial forms of the composite lambda-doubling operators p(r) and q(r) were generated using, for the first time, the generalized inversion procedure proposed by Watson.     

Floating double probe in non‐Maxwellian plasmas: Determination of the electron density and mean electron energy

A theoretical study of the floating double probe based on the Druyvesteyn theory is developed in the case of non‐Maxwellian electron energy distribution functions (EEDFs). It is used to calculate the EEDF in the electron energy range larger than –e(V_f ? V_p) from the I–V double probe characteristics. V_f and V_p are the floating and plasma potential, respectively. The analytical distribution function corresponding to the best fit of EEDF in the energy range larger than e(V_f ? V_p) allows the determination of the total electron density (n_e) and the mean electron energy (<?_e>). The method is detailed and tested in the case of a theoretical Maxwell–Boltzmann distribution function. It is applied for experiments that are performed in expanding microwave plasmas sustained in argon. Analytical EEDFs determined by this method are compared with those measured by means of single probes under the same experimental conditions. A good agreement is observed between single and double probe measurements. Results obtained under different experimental conditions are used to define the best conditions to obtain reliable results by means of the double probe technique.     

Nonlinear oscillatory waveguide propagation in a plasma

The presence of an intense Gaussian laser beam gives rise to a ponderomotive force on electrons in a collisionless plasma, leading to a redistribution of electron density along the wave-front and consequently to an intensity dependent dielectric constant which saturates with increasing intensity. The intensity dependent dielectric constant is responsible for beam propagation in an oscillatory waveguide. It is seen that (i) a beam of radiusr ₀ less thanr _0min (?c/ω _p) cannot be focused in the plasma regardless of its power, (ii) minimum dimension of oscillatory waveguide increases with increasing power of the beam. Similar results are also obtained for collisional plasma where nonlinearity arises due to nonuniform heating and consequent redistribution of carriers.     

Structure functions in two-dimensional turbulence

The “variable range decomposition” mean field type approximation is applied to the enstrophy and energy balance equations in 2D homogeneous, isotropic turbulence. Enstrophy is seen to be transferred to smaller scales, energy to larger scales. The approximate enstrophy balance equation is supplemented by an exact relation between the velocity structure functionD(r) and the vorticity structure functionD _ω(r) to form a closed set of equations that is used to calculateD andD _ω from scale zero up to the input scale.D _ω is found to depend only on viscosity and the enstrophy dissipation ε_ω and tends to the constant ≈15ε _ω ^2/3 in the enstrophy inertial range.D(r) in addition to the well-knownr ²-law has a second power law term ∝r ^4/3, which is important in the intermediate range between the viscous range and the enstrophy inertial range. All numerical constants are calculated.     

Acoustic perturbations in a pulsedRF-plasma

The response of a stationary weakly ionized plasma to a density perturbation in the neutral gas component was studied in a neon plasma with the following typical properties: electron density ¯N _e≈8×10¹² cm^?3, electron temperature on the axis of the vesselT _e0≈3.0 eV; neutral gas densityN _n≈1×10¹⁷cm^?3 and neutral gas temperatureT _n0≈600 °K. A neutral density perturbation, generated 50 cm apart from the plasma, produces a fluctuation in the ion density and a sharp spike in the differential voltage of a floating double probe. The experimental observations demonstrate the propagation of an ion sheath and of an electric field perturbation together with the neutral density perturbation. An interpretation of the plasma response to acoustic wave pulses has been proposed by Ingard and Schulz in a theory on acoustic wave modes in a weakly ionized gas. The experimental results are in good agreement with the theoretical expectations.     

Steady state and transient self-interaction of a laser beam in a strongly ionized moving plasma

The steady state and transient self-interaction of a laser beam with a strongly ionized plasma flowing transverse to the direction of propagation have been investigated by a phenomenological approach using perturbation theory, WKB and paraxial ray approximations. The effect of the transverse motion of the plasma has been included by a convection term in the energy balance equation and is found to result in the non-symmetrical heating of electrons. As a result the beam is shifted towards the direction of transverse flow of the plasma by an amount that increases with the flow velocity. The extent of asymmetry in self-focusing along the transverse directions is, however very small. In a typical case of 7.6×10⁵ watt laser of ω=10⁴ GHz and initial beam widthr ₀=0.05 cm the transverse shiftx _p=0.1r ₀ is predicted in a distance of propagationz=0.34 cm in a strongly ionized plasma of electron densityN _e=10¹⁶ cm^?3 and transverse flow velocityW ₀=10⁷ cm/sec.     

Electron sink at a probe in the conditions of a collisional near-probe space-charge layer

On the basis of the concept of the sink effect, the dependence of the electron currents on the retaining potential is calculated for probes of arbitrary form, both in the presence of a magnetic field and in its absence. The focus is on the calculation of currents in the conditions of the collisional near-probe space-charge layer. It is shown that, in the case where the layer thickness is small in comparison with the probe radius, expressions obtained within the frame-work of sink-effect theory (when the layer is assumed to be noncollisional) may be used to calculate the current in such conditions. It is also shown that, instead of the requirement r_D for use of the standard method of determining the plasma parameters, satisfaction of the less stringent requirement r_D is adequate; is the energy relaxation length of the electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 74–80, July, 1990.     

The Influence of Ion – Neutral Collisions in the Plasma Sheath on the Ion Current to an Electrostatic Probe: Monte Carlo Simulation

We have carried out Monte Carlo simulation of the motion of Ar⁺ ions in the space charge sheath surrounding a cylindrical Langmuir probe. From these simulations the percentage of ions crossing the sheath boundary that are collected by the probe have been determined and thus the ion currents to the probe have been calculated. It is shown that the collisions of ions with neutral helium gas atoms in the sheath increase the percentage of ions collected by the probe above that predicted by collisionless orbital motion limited current (OMLC) theory and that the exponent, χ, of the power law dependence, i₊～U, of the ion current, i₊, on the probe voltage, U_p, increases above the value 0.5 predicted by OMLC theory. The results of the simulations are compared with recent Langmuir probe measurements made in flowing afterglow plasmas.     

Langmuir Probe in Non‐Maxwellian Plasma: Correlation Between the Electron Energy Distribution Function,the Ion Energy at the Sheath Edge and the Floating Voltage

This work is devoted to the study of the Langmuir probe in non‐Maxwellian plasma, assuming mono‐energetic singly charged ions and a collisionless sheath. Using a general analytical equation for the Electron Energy Distribution Function (EEDF), we study the effect of the EEDF profile on: a) The ion energy at the sheath edge of a negatively biased collector, b) the I‐V probe characteristic and c) the floating voltage (V_p‐Vf). Different methods are used and compared to determine these parameters or characteristics. A correlation is given between the floating voltage, the ion energy at the sheath edge and the EEDF profile. The study is also extended to distribution functions with several components. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

托卡马克等离子体非平行流马赫探针

在电探针的沿磁场一维流动等离子体鞘理论和现有马赫探针的基础上， 种简化的非平行流马赫探针。     

Some properties of the two-point functions in a dilatationally covariant field theory

The object of our concern are some properties of the two-point functions in a model of dilatationally covariant field theory. We examine the one- and two-dimensional irreducible representations of the dilatation group. For the one-dimensional case we obtain either a massless free field theory or a theory of an interacting field which does not contribute on the mass shell μ=p²=0 and is characterized by a spectral function μ^r₊?1. In the two-dimensional case both fields differ from the free field, their spectral functions ρ_ij(μ) do not vanish identically and are products of two factors, a polynomial of order up to two in ln μ and μ^r₊?-1. The differences between the case of internal symmetry and the case of dilatations are emphasized. The formula for the form factor in the Araki-Haag limit is given.     

Radiation transfer in an anisotropically scattering plane-parallel medium with space-dependent albedo ω(x)

A method of analysis is presented for solving radiation-transfer problems involving space-dependent albedo ω(x) for an absorbing, emitting and anisotropically scattering plane-parallel medium with reflecting boundaries. The albedo is represented in terms of Legendre polynomials in the form ω(x) = Σ^R_r=0D_rP_r(x/L), where x is the optical variable, L is the half optical-thickness of the slab, P_r(x/L) are the Legendre polynomials and D_r are known expansion coefficients. The effects of spatial variation of albedo on the reflectivity and transmissivity of a medium having a slab geometry are examined for the cases of both forward and backward anisotropic scattering over a wide range of system variables. The effects of ω(x) on the angular distribution of radiation are also shown for some representative cases.     

托卡马克等离子体中内部磁扰动的测量研究

热等离子体中内部磁扰动水平可以由逃逸电子输运来确定，逃逸电子输运采用扰动实验和稳态实验等四种不同方法较容易获得某些局域的逃逸扩散系数，首先利用等离子体快速移动实验，测量孔栏上硬X射线通量的变化，获得边缘扩散系数；第二，由微波辐射强度和硬X射线通量(HXR)信号的锯齿振荡的峰值延迟时间得到径向位置某处到孔栏之间平均扩散系数；第三，软X射线(SXR)强度和HXR通量的信号的锯齿振荡的峰值延迟时间给出等离子体芯区外的径向平均扩散系数；第四，由来自孔栏上HXR韧致辐射谱求得逃逸电子平均能量，继而得到逃逸约束时间， 关键词： 逃逸电子输运 扩散系数 内部磁涨落 软X射线(SXR) 硬X射线 (HXR)     

Bonding and composition diagrams for sputtered a-Si : H

The combined influence of H partial pressure (p_H) and deposition rate (R_D) on Si-H bonding and total H content in diode-sputtered a-Si : H is presented in two simple graphs for the case of substrate temperature (T_s) equal to 225°C. Similar to a phase diagram, the graphs predict the H content and Si-H bonding that will result if a deposition is carried out with any prescribed pair of values (p_H, R_D), where 0.04 < p_H < 10 Paand 0.01 < R_D < 1 nm/sec. Well defined regions of Si-H bonding represented by dominant infrared stretching absorptions at 2000, 2090 and 2150 cm^?1 are obvious in the bonding diagram. The absorption at 2090 cm^?1 is the most commonly observed and is obtainable over a wide range of intermediate values of p_H and R_D. The absorption at 2000 cm^?1 is dominant only for the lowest p_H and the highest R_D. The absorption at 2150 cm^?1 is dominant in films deposited at high p_H and low R_D. The composition diagram shows that highest total H content is obtained for low R_D and high p_H, and lowest total H content results for high R_D and low p_H