Radial Profile Of Plasma Density Research Articles

Characteristics of electron cyclotron resonance plasmas for large area ion source application

The characteristics of electron cyclotron resonance (ECR) plasmas at the plug chamber of the “Hanbit” plasma facility have been investigated. The chamber length and diameter are 2.25 m and 1.5 m, respectively, and three pairs of solenoids are used to produce a variety of magnetic field configurations. Two magnetrons of 5 kW maximum output power each are used to produce the ECR plasma, and the microwaves are launched in extraordinary mode (X-mode) and/or right-hand circular polarized mode (R-mode). The dependence of the radial plasma density profile and the energy distribution of end loss ions upon the magnetic field strength and geometry, gas pressure, microwave power, and the mode of microwave are investigated. The plasma parameters are measured by using a position-controllable electrostatic probe, an x-ray spectrometer, and a multigrid ion energy analyzer. Transitions, occurred by threshold magnetron powers between low and high density modes, are found on certain conditions of magnetic field geometry and gas pressure. The density profile and the ion energy distribution are strong functions of magnetic field geometry and gas pressure, respectively.

Cathodic-arc plasma density, radial profile, and total plasma inventory as measured by microwave interferometer and Faraday cup

The total amount of plasma, peak plasma density, and plasma density radial profile are derived from a series of measurements of ion current density (with a Faraday cup) and integrated electron line density (with a microwave interferometer) for a cathodic arc derived plasma. Comparisons are made between cathode material (erbium and titanium), arc current, background gas pressure, and the presence or absence of a series magnetic solenoid around the coaxial anode and cathode.

Dispersion of dipolar electromagnetic waves in a radially inhomogeneous axially magnetized plasma column

The wavelengths and attenuation coefficients of dipolar left and right-hand circularly polarized waves in a radially inhomogeneous axially magnetized plasma column are computed by means of step approximation of the radial plasma density profile. A comparison with theoretical results assuming radial plasma homogeneity shows that the radial plasma inhomogeneity is significant for wave propagation. In particular, it facilitates the occurrence of backward waves and increases by up to one order of magnitude the wave attenuation coefficient compared with a homogeneous plasma column having the same average plasma density.

Two-dimensional effects in a tunnel ionized plasma

An intense short pulse laser of finite spot size propagating through a gas produces plasma via tunnel ionization on a femtosecond time scale. The radial profile of plasma density is strongly peaked on the axis and has a defocusing property. As electron density grows with time, the trailing part of the laser pulse suffers stronger divergence than the leading front, causing severe temporal distortion of the pulse. A self-consistent paraxial ray theory of electron density evolution and defocusing of the laser reveals that a square (in time) laser pulse, after propagating one Rayleigh length, has an order of magnitude difference in the axial intensity at the front and the tail of the pulse.

M = +1 mode helicon wave excitation using solenoid antenna

In most helicon wave experiments, there has been a poor agreement between the theoretical wave power deposition and the plasma density radial profiles. The characteristics of the solenoid antenna employed for the excitation of m = 1 mode helicon waves are investigated. A cylindrical high density plasma column with high intensity ArII emission is observed to be of a cylindrical shell shape. In cross section, the location of the high intensity column nearly coincides with the maximum point of wave power deposition. The wave magnetic field radial profiles agree well with computations. These results show an excellent agreement between the theory and density profile produced by the m = 1 helicon wave.

Plasma properties determined with induction loop probes in a planar inductively coupled plasma source

Electromagnetic fields in a planar rf inductively coupled plasma source, of interest for materials processing, were measured using a two-loop inductive (B-dot) probe. The two loops were oriented to measure the time derivative of the axial and radial components of the magnetic field Ḃz and Ḃr, respectively, at various positions in the r–z plane of the cylindrically symmetric argon discharge. Maxwell’s equations were used with this data to calculate amplitudes of the rf azimuthal electric field Eφ and current density Jφ, as well as the complex permittivity ε of the plasma, from which the electron density ne was calculated. The electron densities calculated using this technique were found to compare favorably to the results of measurements made with Langmuir probes. Electron drift velocities calculated from Jφ and ne were found to be comparable to electron thermal velocities in the region of highest Eφ and thus may contribute to local enhancement of electron impact reactions, thereby affecting process chemistry and uniformity. The peak in the drift velocity moved radially outward as the pressure increased due changes in the radial plasma density profile. This technique is applicable to chemistries where Langmuir probes are not practical.

Chirality-dependent Plasma Density Profile Changes from Helicon Wave Ponderomotive Forces

It is shown that nonresonant helicon-wave-induced transport may result in significant changes in the plasma density radial profile; this is illustrated using parameters appropriate to the cylindrical experiment BASIL and the toroidal experiment SHEILA. Whereas m = +1 helicon waves induce an inward-directed transport and change the density profile to a more centrally peaked one with a higher density on the axis, m = −1 helicon waves induce an outward-directed transport velocity and change the density profile to a hollow one. This may be the clue to the puzzle as to why m = −1 helicon waves are frequently difficult or impossible to excite, as the plasma column is effectively blown off to the discharge chamber walls by the ponderomotive force density of the waves with this chirality (sense of rotation of the wavevector with respect to the axial or toroidal magnetic field).

On the structure of the plasma disk in the Jovian magnetosphere

We have determined the structure of the lowenergy plasma disk in the middle Jovian magnetosphere. First, the shape of the dense plasma structures have been described analytically. We have investigated a distribution of the background plasma along the arbitrary magnetic flux tube in the frame of diffuse equilibrium taking into account the action of centrifugal aad gravitational forces. Using the results of this investigation we have shown that background plasma density reaches the maximum on the surface, consisting of the warped disk between the magnetic and centrifugal equators outside lo's orbit and two mirror symmetrical “petals” in the region of the polar cusps. Second, the radial profile of plasma density in the disk has been studied. We have considered the magnetohydrodyoamic stability of the low-latitude dense plasma disk formed under the action of centrifugal force. The equilibrium radial distribution of the plasma was found for the plasma disk at the threshold of the instability with respect to small-scale MHD perturbations. The effect of the inite ion Larmor radius at the threshold of plasma instability was taken into account. We have estimated the iniuence of the inhomogeneity of the plasma perturbations along the magnetic field on the radial density profile bearing in mind the finite conductivity of the Jovian ionosphere. A relationship between the thickness of the plasma disk and the radial plasma distribution has been pointed out. The results obtained were compared with the known experimental data for the Jovian plasma sheet.

Electron cyclotron resonance plasma reactor for cryogenic etching

Electron cyclotron resonance (ECR) plasma reactors are being used for ultralarge scale integrated circuit fabrication to meet the stringent requirements on submicron feature etching. Three issues are critical for ECR reactor design: plasma uniformity, ion energy control, and wafer temperature control. Plasma uniformity is important for minimizing over etch times and reducing the probability of producing charging damage. Ion energy control is needed to optimize etching rate, anisotropy, and selectivity without compromising device yield. Wafer temperature control is important because large ion currents at low pressure can result in wafer heating and thereby alter the rates of surface chemical processes. An ECR plasma reactor is described that is designed to etch compound semiconductors and Si at low temperatures (−170 to 20 °C), where superior selectivity and linewidth control are achievable. By measuring dc bias, floating potential, and ion saturation current densities it is shown that ion energies in this system can be controlled by applying an rf bias to the sample. To characterize plasma uniformity, the radial ion density profile is measured using a fast injection Langmuir probe. Hollow, peaked, or uniform radial plasma density profiles can be obtained depending on microwave power, pressure, and magnetic field. Plasma density profiles are influenced by microwave absorption and refraction which, in turn, are influenced by both the magnetic field and plasma density profiles. The net result is a strong coupling between wave propagation and charge particle transport. To control wafer temperature a cryogenic electrode capable of maintaining a wafer temperature to ±2.5 °C at −170 °C is used while the wafer is exposed to an electron cyclotron resonance plasma. The sample temperature is monitored using infrared laser interferometric thermometry and the heat flux to the wafer surface in an Ar plasma is measured as a function of operating parameters by monitoring temperature transients as the discharge is gated on and off.

Axial structure of a shielded plasma column sustained by a dipolar electromagnetic wave

The authors consider a low-pressure plasma column contained in a dielectric tube surrounded by vacuum and shielded by a metal enclosure. The plasma is sustained by a travelling, weakly damped electromagnetic dipolar wave. The metal shielding turns out to influence substantially the dispersion characteristics and hence the axial structure of the discharge in contrast to the case of azimuthally symmetric waves. Axial profiles of the electron number density and wave power have been calculated. A comparison with the available experimental data shows the significance of the non-uniform radial plasma density profile for this configuration, which is taken into account by replacing the actual electron-neutral collision frequency for momentum transfer by a greater effective value. It is determined by applying a scaling procedure (which is not a curve fitting) that refers to a single experimental point. Thus a very good agreement between theory and experiment has been obtained.

The poloidal Ohm's law and a profile constraint in tokamaks

Explicit use of the poloidal Ohm's law, together with the radial plasma velocity (obtained from the distribution of plasma sources), leads to a very general constraint on the possible radial profiles of plasma density and temperature. The constraint does not require any ad hoc assumption; it can place severe restrictions on the allowed profiles and is independent of energy and particle transport; also, it may be the underlying principle of profile consistency. The constraint is discussed in the framework of neoclassical theory, using results from the Frascati tokamak.

Experimental studies of high-power microwave reflection, transmission, and absorption from a plasma-covered plane conducting boundary

Experimental studies of the reflection, transmission, and absorption of high-power microwave pulses from a plasma-covered plane conducting boundary are presented. Under optimum conditions, backscattered rf power is attenuated by more than 30 dB over values measured in the absence of the plasma. Measurements of the radial and axial plasma density profiles and the neutral gas pressure near the plane conductor indicate that collisional absorption processes are not the primary source of the observed attenuation in the backscattered microwave signal, and that the plasma density exceeds the critical density over much of the volume nearest the conductor. The effects of a tenfold reduction in the microwave power density on the reflection and absorption characteristics of the system are also reported.

Radial ion transport measurements in a nonaxisymmetric magnetic mirror

Experimental radial ion transport rates and diffusion coefficients are presented for the Constance-B magnetic mirror [Phys. Rev. Lett. 58, 1853 (1987)]. The transport experiments are performed by measuring steady state equilibrium radial profiles of plasma density, ionization source, end loss current, electric field, electron temperature, and ion temperature. A charge coupled device (CCD) camera system [Rev. Sci. Instrum. 60, 2835 (1989)] is used to measure the two-dimensional radial density, source, and electron temperature profiles. End loss diagnostics including movable Faraday cups, electrostatic end loss analyzers, and an ion time-of-flight analyzer [Rev. Sci. Instrum. 59, 601 (1988)] are used to measure radial profiles of potential and ion temperature. The ion confinement time perpendicular to the magnetic field is found to be an order of magnitude shorter than predicted by classical and neoclassical transport theories. The radial profiles of the perpendicular diffusion coefficient (D⊥) are presented for hydrogen, helium, and argon plasmas. The coefficients are a factor of 10 larger than the maximum classical and neoclassical coefficients in all three plasmas. Plasma fluctuations resulting from whistler mode microinstability [Phys. Rev. Lett. 59, 1821 (1987)] as well as nonaxisymmetric potentials are suggested as possible explanations for the experimentally measured radial transport rate.

Results from a double-sided langmuir probe in T-10, and an extended model of a probe in a streaming magnetized plasma

By means of a JANUS-type Langmuir probe operated in the scrape-off layer (SOL) in T-10 the temporal evolution and radial profiles of plasma density, electron temperature, and toroidal asymmetry of saturation currents were measured in dependence on discharge parameters. Of particular importance turned out to be the B T field direction and the radial probe position relative to the active limiter structure (rail and aperture limiter). The interpretation of the results is based on the local SOL structure in terms of connection lengths to the active limiter and special flow effects in T-10, and makes use of a theoretical probe model by Hutchinson. As to the latter model, a refinement concerning its basic principles is presented independently.

The jet plasma boundary with limiter and X-point discharges

The JET tokamak has been operated in various limiter and magnetic separatrix configurations with ohmic, neutral beam and ion cyclotron heating. Radial profiles of plasma density, temperature and particle flux in the scrape-off layer have been measured and scalings with core plasma parameters have been established for the limiter discharges. In the magnetic separatrix configuration, H-modes with energy confinement times up to 1 s have been obtained with neutral beam heating. There are large, localised fluctuations in the edge parameters which appear to be well correlated with the periodic global particle losses. The main impurities are oxygen and carbon. The only significant metallic impurity is nickel which originates from the ion cyclotron heating antennas.

EXPERIMENTAL INVESTIGATION ON PROPERTIES OF E. C. H. PLASMA AND HOT ELECTRON RING

The ECRH experiment has been carried out on a simple mirror-MM-2 device by use of a 15 GHz high power gyrotron. The experimental results showed that the preionization time will be rapidly shorten following raising the pressure of puffed gas. Duing the "C-mode" operation where the gas pressure is higher, the radial profile of plasma density exhibits a saddle shape. The gas "pressure window" for building a hot electron ring is (0.4-1.2)×10-5 Torr, as the gyrotron output is about 30 kW. Simply by using a movable Langmuire probe accompanied with diamagnetic measurment. it has been determined that the hot electron ring radius is 7 cm, the thickness is about 4 cm and the axial boundary of the ring extends from Z= ±10 cm to Z=±20cm, when the central field is 2.95kg. The hot electron temperature is 140 to 170 keV. The average beta value of hot electron ring is (4-5)%. The radial bursts of electrons due to ring instabilities accompanied with the drops of the diamagnetic signal have also been observed.

Vacuum Arc Plasma Centrfuge for Element and Isotope Separation

Vacuum arcs have been studied extensively in the past several decades with applications primarily in the areas of switching, vacuum remelting, and vapor deposition. Application of the vacuum arc for element and isotope separation has been studied recently and is reviewed in this paper. An arc was produced in a 30-cm-diameter 4-m-long cylindrical chamber with coaxially mounted electromagnets providing a 2.6-m-long constant axial magnetic field of up to 6 kG. The vacuum discharge between a solid cathode and a mesh anode was triggered electrically. A pulse-forming network (PFN) of 70-mΩ impedance provided nearly constant-current discharge pulses of several kiloamps and 6-12-ms duration. The magnetized plasma column, flowing axially from the anode with a typical velocity of 106 cm/s, rotated nearly as a solid body. This rotation was due to the E × B drift, produced by the axial magnetic field and the radial electric field across the column. A typical rotation frequency was 105 rad/s. The centrifugal effect due to the rotation caused a radial redistribution of ions within the plasma column, thereby producing elemental and isotope enrichment. The separation was observed to increase exponentially with the square of the radius. Enrichments of up to 300 percent were measured in a Cu-Zn plasma. The radial plasma density profile was found to be roughly Gaussian, with central electron densities of about 1013 cm-3. The radial potential profile across the column was measured and found to be parabolic with radius.

The effect of the poloidal divertor neutralizer geometry on the scrape-off plasma temperature and density

The poloidal magnetic divertor operating in the cool-high density plasma mode represents a good solution to the tokamak first wall erosion and impurity transport problems. It also, in general, improves the energy confinement time in the beam-heated discharges. In the existing experiments, the neutralizer plates are oriented close to normal to the flux surfaces in the poloidal plane, but the planned high-power experiments require that the plate be sharply angled to reduce the power flux density. We have studied the effect of changing the plate angle on the radial plasma temperature and density profiles at the plate. We find that some plate orientations shift the electron flux profile relative to the power flux profile, resulting in unfavorable temperature profiles. The question of existence of a steady-state solution was also addressed. For a steady state to exist, the radial neutral fluxes have to exactly cancel the radial diffusive ion fluxes. Depending on the diffusion coefficients, the plasma density may have to increase with radius to satisfy this condition. The implications of these conclusions for the INTOR, and CIT experiments is also discussed.

Laboratory simulations of controlled energetic electron-beam-plasma interactions in space

Abstract : Recent laboratory investigations of space simulated electron beam plasma interactions have concentrated on the transition from single particle behavior to collective plasma processes with emphasis on the beam plasma discharge. The plasma has been characterized with respect to its spatial distributions in density, temperature and density fluctuation power spectra. The latter results have been further classified in terms of amplitude and spectral distributions. The overall results include: (i) detailed radial profiles of plasma density as a function of beam parameters; (ii) the association of low frequency large amplitude fluctuations with ion acoustic, ion cyclotron and drift wave modes; (iii) the identification and spatial mapping of suprathermal electrons in the beam core; and (iv) the experimental definition of a density dependent criterion for the ignition of the beam plasma discharge. The data provide a synoptic perspective on accumulated rocket observations and meaningfully contribute to planning activities for future space borne applications of artificially injected energetic electron beams.

Transport Scaling Studies for EBT Reactor

Transport simulation and modeling studies for the ELMO bumpy Torus (EBT) reactor are carried out by using 0-D and 1-1/2-D transport calculations. The time-dependent 0-D model is used for global analysis whereas the 1-1/2-D radial transport code is used for accurate determination of density, temperature, and ambipolar potential profiles and of the role of these profiles in reactor plasma performance. Analysis with the 1-1/2-D transport code shows that profile effects near the outer edge of the hot electron ring lead to enhanced confinement by a least a factor of 2 to 5 beyond the simple scaling that is obtained from the global analysis. The radial profiles of core plasma density and temperatures (or core pressure) obtained from 1-1/2-D transport calculations are found to be similar to those theoretically required for stability.