B. W. James

Rotational Raman calibration of Thomson scattering

Anti-Stokes rotational Raman scattering from nitrogen gas at near atmospheric pressure and room temperature has been used to calibrate the intensity of ruby laser light Thomson scattered from a plasma in a transverse magnetohydrodynamic shock tube. The main limit to the accuracy of this calibration method is the uncertainty in the available Raman cross section for nitrogen.

The cylindrical DC magnetron discharge: I. Particle-in-cell simulation

The radial structure of a low-pressure cylindrical post-cathode direct-current magnetron discharge is investigated using a one-dimensional, electrostatic particle-in-cell code, incorporating non-periodic boundary conditions and an external circuit. Electron and ion collisions with a background gas of argon are modelled using Monte Carlo techniques. The radial structure of the discharge is examined for a range of operating conditions. Profiles of the electric potential, electric field and space charge density are found to vary systematically with the pressure p and magnetic field strength B, in a way which corresponds to a transition from the usual positive space charge mode at low values of B/p to a higher impedance negative space charge mode at higher values of B/p. This is consistent with a continuous and considerable reduction in the ratio of the electron-to-ion classical cross-field transport coefficients with increasing B/p. Results of a fluid model also predict the transition to the negative space charge mode.

Dust as fine electrostatic probes for plasma diagnostic

Here we present a novel use of fine dust for diagnostic measurements in the sheath of a planar rf discharge and in the inertial electrostatic confinement (IEC) plasma. Since dust charge is a function of a number of plasma parameters, and the dust charge adjusts itself to changes in plasma conditions instantaneously, dust particles may be used as an ideal diagnostic tool. A major advantage of such a diagnostic approach is its simplicity as the only measurement of a position of a dust particle or its motion after a perturbation is necessary. This technique only requires access to a discharge chamber, dust particles, a laser to illuminate them and a camera to capture the motion of the dust. The dependences of the sheath profiles on the discharge pressure and power have been determined. The radial potential profiles in a cylindrically-symmetrical capacitively-coupled rf discharge and in an IEC plasma were obtained. The direction of the ion flux in the radial IEC discharge was found.

The rotation of planar-2 to planar-12 dust clusters in an axial magnetic field

Dust clusters containing from one up to 12 particles arranged in a horizontal plane were formed in an inductively coupled rf plasma. When an axial magnetic field was applied to the dust cluster, the cluster rotated as a rigid body in the left-handed direction with respect to the field. The cluster rotation occurred at a magnetic field of tens of gauss in our experiment, which was two to three orders less than that used in previous experiments and that predicted by the models in the literature. In particular, the angular velocity dependence on magnetic field strength varied with number of particles in the cluster and with the structural configuration of the cluster. Other rotational properties such as cluster radius, angular momentum and threshold magnetic field were measured. The dependence of the radial confinement electric field on the magnetic field was also obtained. Finally, comparisons were made between our experimental results and various existing theoretical models. Possible explanations for angular velocity saturation and periodic pauses for the planar-2 configuration, which were observed in our experiments, are given.

Sheath measurement in rf-discharge plasma with dust grains

Abstract The location of the sheath edge in a planar rf-discharge has been determined using test dust grains. The diagnostic technique is based on measurement of the equilibrium position of fine dust grains levitated above the powered electrode in an rf-discharge. Additionally two independent techniques based on probe measurements and analysis of discharge optical emission have been used. All techniques provide values in good agreement. Using the resulting value of sheath thickness the charge on dust particles in the sheath region has been estimated.

Dust particle alignments and confinement in a radio frequency sheath

Stability of vertical and horizontal confinement of dust particles levitating in a rf sheath was studied experimentally. The experiments were carried out in an argon plasma discharge with micron-sized dust particles. Disruptions of particle arrangements were triggered by changing the discharge controlling parameters (pressure and peak-to-peak voltage) as well as by applying an additional bias to the confining electrode. Where the transition was triggered by changes of the discharge parameters, the transition from the horizontal to the vertical alignment has been found to be more pronounced than the reverse one. Clear hysteretic phenomena were observed for the transition triggered by changes of the confining voltage.

Formation of Vortex Dust Structures in Inhomogeneous Gas-Discharge Plasmas

A generalized analytical model of instabilities in a dusty plasma with a nonzero grain charge gradient in a field of nonelectrostatic forces is considered. A review is given of different experimental observations of the dust self-oscillations that occur in the plasmas of an rf capacitive discharge and a dc glow discharge and whose appearance can be explained in terms of the proposed model. It is shown that the change in the grain charge gives rise to dynamic dust structures in laboratory gas-discharge plasmas. Attention is focused on the analysis of the onset of vortex motion of the dust grains.

The cylindrical DC magnetron discharge: II. The negative space charge mode

Particle-in-cell (PIC) simulations in one spatial dimension of a cylindrical post-cathode direct current magnetron discharge have shown that there is a continuous transition from a positive space charge (PSC) mode of operation at higher pressures to a negative space charge (NSC) mode at lower pressures. In this paper we extend this work using a PIC code to investigate the NSC mode in detail. Profiles of ionization and inelastic collision rate, averaged charged particle energies and energy distributions indicate that the size and location of the region of power dissipation depend strongly on the operating pressure. Langmuir probe measurements of the radial distribution of floating potential in a laboratory magnetron discharge do not exhibit the systematic variation with changes in magnetic field or pressure that are predicted by the simulations. This implies the existence of an anomalous transport mechanism which maintains the PSC mode at low pressures and which cannot be modelled self-consistently by a PIC code in one spatial dimension.

Analysis of Macroparticle Charging in the Near-Electrode Layer of a High-Frequency Capacitive Discharge

Spatial variation of dust particle charges are estimated numerically for typical laboratory experiment conditions in a radio-frequency (rf) capacitive discharge. The surface potentials of macroparticles levitating in the upper part of the near-electrode layer of the rf discharge are measured. A model is proposed for the formation of irregular dust oscillations due to stochastic motion of dust in the bulk of a spatially inhomogeneous plasma (in the presence of a dust charge gradient). This mechanism is used for analyzing the results of measurements of the amplitude of vertical vibrations of dust particles in the near-electrode layer of the rf discharge. It is found that the dust charge gradient may be responsible for the development of such vibrations.

Wake-induced symmetry-breaking of dust particle arrangements in a complex plasma

A symmetry-breaking disruption occurs in a system of two dust particles with a decrease of the particle separation. This disruption is attributed to the formation of the common ion wake in the system. In the experiment, the particles levitate in the sheath of a radio-frequency (rf) discharge at low gas pressures (≤60 mTorr) and their separation is changed by the laser manipulation. The experiment is complemented by molecular dynamics (MD) numerical simulations. The experimental and simulation data agree that the disruption condition corresponds to the common ion wake formation at the interparticle distances less than the electron Debye length.