Optimization Of Plasma Parameters Research Articles

Quasicontinuous x-ray laser withλ=10.8nm in Pd-like tungsten using a nanostructured target

A new-generation x-ray laser project is explained. It is based on the transitions in Pd-like ions in nanoplasmas. The gain coefficient is calculated for the $4{d}_{3/2}^{9}5{d}_{3/2}[J=0]\ensuremath{-}4{d}_{3/2}^{9}5{p}_{1/2}[J=1]$ transition ($\ensuremath{\lambda}\ensuremath{\approx}10.8$ nm) in Pd-like tungsten. It is suggested that a cylindrical target made of nanostructured tungsten is pumped in the longitudinal direction by a laser pulse with energy 1--2 keV and duration $~500$ ps. For this pump pulse the target density and dimensions are calculated, as well as the temporal variations of the optimal plasma parameters for attaining $\mathit{gL}$ $~$ 14. The energy yield in the 10.8-nm line is more than ${10}^{20}$ eV.

Optical, electrical and structural properties of SiO : H films prepared from He dilution to the SiH4 plasma

The effect of incorporation of oxygen into the initially crystalline-like Si : H network and its gradual increase up to 20 at% on the optical, electrical and structural properties of the material has been studied systematically. The prime objective of the experiment was to investigate the contribution of He as the diluent to the SiH4 plasma in modifying the SiO : H network when CO2 was used as the source of oxygen, and to extract some ideas that might be extended in the future development of nc-SiO : H network from a similar plasma with suitably modified parameters in RF-PECVD. Incorporation of oxygen invariably widened the optical gap; however, it reduced the electrical conductivity by several orders of magnitude with a significant increase in its activation energy. A sharp increase in the bonded H content (CH) of the film was identified at the initial stage of oxygen incorporation and that occurred mostly due to the gross change in the network structure from the crystalline-like to an amorphous-like phase. However, on further addition of oxygen CH gradually decreased, bonded mostly in the polyhydride configuration and the overall film surface roughness diminished. Systematic reduction in CH identified a dehydrogenation process occurring in the Si network during the gradual inclusion of oxygen, induced by the presence of He as the diluent to the plasma. The result seems opposite to the conventional H2-diluted plasma and appears significantly favourable for the future development of nc-SiO : H materials under optimized plasma parameters.

Non-intrusive plasma diagnostics for the deposition of large area thin film silicon

Plasma diagnostics for large area, industrial RF parallel-plate reactors can be useful for process optimization and monitoring, provided that their implementation is practical and non-intrusive. For instance, Fourier transform infrared (FTIR) absorption spectroscopy and/or time-resolved optical emission spectroscopy (OES) can easily be retro-fitted into the pumping line of a reactor. Both techniques were used to measure the fractional depletion of silane in silane/hydrogen plasmas. By means of a simple analytical plasma chemistry model, it is shown that the silane depletion is related to the silicon thin film properties such as microcrystallinity. Uses of the diagnostics are demonstrated by two examples: (i) the optimal plasma parameters for high deposition rate of microcrystalline silicon, along with efficient gas utilization, are shown to be high input concentration and strong depletion of silane; and (ii) the optimal reactor design, in terms of fast equilibration of the plasma chemistry, is shown to be a closed, directly-pumped showerhead reactor containing a uniform plasma.

Surface functionalization via in situ interaction of plasma-generated free radicals with stable precursor-molecules on cellulose

The surface functionalization process was accomplished in a consecutive 3 step process including: (1) Argon- and oxygen-plasma enhanced generation of free radical sites on cellophane surfaces; (2) “In situ” gas phase derivatization in the absence of plasma using hydrazine, ethylene diamine, or propylene diamine; (3) Second “in situ”, gas phase derivatization in the absence of plasma using oxallyl chloride or “ex situ” derivatization in the presence of glutaraldehyde. The presence of free radical sites on the plasma exposed cellophane surfaces was demonstrated using “in situ” sulfur dioxide and nitric oxide labeling techniques. It was shown that the free radical sites readily react under “in situ” conditions with the stable chain-precursor components and generate the desired spacer-chain molecules. ESCA, ATR-FTIR analysis and dying techniques were used to monitor the cellophane surface changes. A factorial design was used for selecting the optimal plasma parameters. Functionalized cellophane substrates were used for immobilization of α-chymotrypsin in the presence of spacer-chain molecules. The activity of the immobilized α-chymotrypsin was found to be lower in comparison to the activity of the free enzyme and the presence of virgin cellophane in the free enzyme solution also reduced significantly the activity of the enzyme. It is suggested that the swollen state of the cellophane plays a significant role in the decrease of the immobilized enzyme activity.

High brightness extreme ultraviolet (at 13.5 nm) emission from time-of-flight controlled discharges with coaxial fuel injection

Extreme ultraviolet (EUV) emission from discharge produced plasma with the coaxial injection of fuel vapor (tin and lithium) produced by laser ablation is experimentally studied. Multiple plasma pinches preceding a strong and long recombination radiation of EUV are observed in the first half cycle of a sinusoidal discharge current. Due to the time-of-flight control type of the discharge, the shape of pinch radiation pulses is almost identical. With the coaxial injection of time-of-flight controlled discharges, the highest brightness of EUV emission (maximum extracted energy of 244.3 mJ/2π sr per pulse with the emitter size of ∼1×0.3 mm2 in full width at half maximum) is provided with efficiency exceeding 2% of deposited energy into the plasma (or 1% of dissipated energy in the discharge) due to a much better matching with the optimal plasma parameters in the recombination regime and a decrease in the off-duty factor. Stability of emitting plasma of the repetitive pinches is essentially improved with use of a second laser pulse.

Elongation of extreme ultraviolet (at 13.5 nm) emission with time-of-flight controlled discharges and lateral fuel injection

A way toward a quasicontinuous extreme ultraviolet (EUV) radiation source is proposed and explored. Tin and lithium vapor discharges with the lateral laser-ablation injection are experimentally studied as possible efficient sources of quasicontinuous emission of EUV radiation at a wavelength of 13.5 nm. It is shown that the time-of-flight control of optimal plasma parameters by means of varying ablating laser pulse parameters provides a considerable elongation of maximal-power EUV emission with an overall efficiency of 0.1% and with an energy output exceeding 1% of the energy deposited in the discharge plasma. Along with a high average power and a stable position, such an emitter may have its size small enough to be used in the projection lithography.

Mass spectroscopic measuring of SiCln (n = 0–2) radicals in SiCl4 RF glow discharge plasma

The relative densities of SiCln (n = 0–2) in SiCl4 radio frequency (rf) glow discharge plasma are measured by mass spectrometry. The effects of discharge parameters, i.e., rf power, discharge pressure, substrate temperature, and SiCl4 flow rate on the relative densities of SiCln (n = 0–2) are investigated in detail. An optimum configuration of discharge parameters (low rf power, high discharge pressure, low substrate temperature, and low flow rate), which enhanced the formation of SiCln (n = 0–2) radicals, is searched by a great deal of measurements and discussions. In the optimum configuration of discharge parameters, we measure the spatial distribution of SiCln (n = 0–2) radicals in the most optimized plasma parameters. The experimental results reveal that Si and SiCl may be the dominant precursors in forming the thin film.

Fast equilibration of silane/hydrogen plasmas in large area RF capacitive reactors monitored by optical emission spectroscopy

The optimal plasma parameters for plasma processing, such as deposition of microcrystalline silicon from silane and hydrogen, are generally chosen in steady-state discharge conditions. However, this steady state must be reached in a short time after plasma ignition to avoid significant film deposition in non-optimal conditions during the plasma transient phase.Simple and inexpensive time-resolved optical emission spectroscopy has been used to measure the plasma time evolution from ignition to steady-state conditions in a large area RF capacitive plasma reactor. Absolute values of silane and hydrogen molecular number densities, relative values of electron density, and qualitative information on electron temperature were obtained without the need for absolute intensity calibration. Apart from the experimental verification of constant electron temperature, the particular condition here is that the emission intensities should be followed from the instant of ignition, since the molecular densities are known at this instant.A plasma model for the reactor, and a dispersive axial flow model for the pumping line, were used to show why the plasma chemistry in a well-designed large area reactor generally reaches steady-state conditions in less than one second. The optimal design for fast equilibration is a closed, directly-pumped showerhead reactor with a uniform plasma which fills the whole reactor volume.

The conceptual design for the modification of HL-2A tokamak

HL-2A has achieved parameters of I p = 400 kA, B t = 2.65 T, approaching to its rated operational parameters with I p = 450 kA and B t = 2.8 T so far. Considering present status of HL-2A tokamak and in order to extend experimental research further, the HL-2A will be modified with a more efficient divertor. The objective of modification of HL-2A is to obtain optimized plasma parameters and shape, and an advanced divertor. The plasma configuration will be realized by composite arranging of poloidal field coil (PFC) system. After modification, the plasma current I p will reach 800 kA, the elongation and triangularity of plasma will be above 1.6 and 0.3, respectively.

Optimization of the microcrystalline silicon deposition efficiency

Cost reduction constraints for microcrystalline silicon thin film photovoltaic solar cells require high deposition rates and high silane gas utilization efficiencies. If the requirements in deposition rate have sometimes been fulfilled, it is generally not the case for the silane utilization. In this work, a reactor-independent methodology has been developed to determine the optimum plasma parameters in terms of deposition rate, silane utilization, and material microstructure. Using this optimization method, a microcrystalline layer has been deposited over a large area at a rate of 10.9Å∕s, with a silane utilization efficiency above 80%.

Cleaning of Wall Paintings and Architectural Surfaces by Plasma

Coating surfaces with a protective layer is a common method to protect artwork from deterioration. These layers consist often of organic substances such as acrylic resins. Due to weathering and ageing polymers usually suffer changes in optical properties and these organic coats also reduce the water-permeability of porous systems which can cause an accelerated decay. Therefore, we need a method to remove organic matter without harming the substance. In the present study, cleaning procedures with different types of artificially soiled surfaces were carried out. Two different atmospheric pressure plasma jets that permit an economic application were investigated to get the optimal plasma parameters for cleaning.

Numerical simulation of the effect of hydrogen on recombination gain in the transition to ground state of Li III

Numerical simulations of recombination gain in the Li III transition to ground state (2→1 at 13.5 nm) are presented. The plasma simulated is a mixture of Li and H ions, and the space-time-dependent gain coefficient is calculated for different mixing ratios and different pumping beam parameters. The numerical model includes the initial optical field ionization of the plasma by an intense 100 fs laser pulse, taking into account residual heating, particle collisions, and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools. We show that the addition of hydrogen to the plasma can lead to higher gain with a less restrictive range of experimental parameters. We analyze the effects of the addition of hydrogen on the gain and point to the optimal plasma and pump parameters to produce gain.

A superpowerful source of far-ultraviolet monochromatic radiation

In several independent experiments investigating the interaction between the optical field of an intense laser pulse and a xenon cluster beam, we recorded an anomalously high quantum yield of the plasma radiation in the region 10–15 nm. In several cases, the conversion efficiency into the hemisphere reached 10% of the pumping pulse energy. The nature of this phenomenon has not yet been adequately explained. A high conversion efficiency is shown to be possible when producing a plasma with optimal parameters for the amplification of spontaneous radiation on Ni-like xenon transitions to be generated. In a collisional-radiative model, we performed detailed atomic-kinetic calculations of the gains and radiation spectra on the transitions with λ ≈ 4, 10, and 11.3 nm and in the region 13–13.9 nm. For each transition, we determined the time dependences of the gains on plasma parameters. The theoretical and experimental values of the optimal plasma parameters and energy yields of the radiation are in close agreement. Using a theoretical model, we propose possible plasma pumping schemes to achieve the maximum yield of the intense, narrowly beamed soft X-ray radiation. At a pumping pulse repetition rate of 104 Hz, the output power for various Ni-like xenon transitions ranges from 100 to 5 × 103 W.

Diagnostics of the collisionally pumped plasma and search of the optimal plasma parameters of x-ray lasing: calculation of electron-collision strengths and rate coefficients for Ne-like plasma

The problem of diagnostics for the collisionally pumped plasma and search of the optimal plasma parameters of x-ray lasing are studied. We present the uniform energy approach, formally based on the quantum electrodynamics with using gauge invariant scheme of generation of the optimal one-electron representation, for the calculation of electron collision strengths and rate coefficients for electron-collisional excitation. The aim is to study, in a uniform manner, elementary processes responsible for emission-line formation in plasmas. The electron collision excitation cross-sections and rate coefficients for some Ne-like ions (Fe, Ba) are calculated. To test the results of calculations we compare them with other authors calculations and with available experimental data.

Time history of gain calculations in radiative-collisional model for X-ray lasers

We have performed the calculations of temporal characteristics of gain g(τ) on the 0-1 (4d-4p) transition and on the optically self-pumping 1-1 (4f-4d) transition of Ni-like krypton, g(τ) dependence on plasma electron density (n e ), temperature (T e ) and diameter (d) is outlined. The optimum plasma parameters: 5×10 17 50 eV. We found that time duration of laser action (τ las ) and g(τ) at maximum depend significantly on d. The effective laser in soft x-ray region is possible in a wide plasma filament with d>1 cm.

Magnetized implosions driven by intense ion beams

Intense beams of heavy ions, envisaged for the near future at the Institute for Theoretical and Experimental Physics (Moscow) and Gesellschaft für Schwerionenforschung (Darmstadt), will be well suited for conducting implosion experiments in cylindrical geometry. In such implosions, the initial pressure generated by the direct beam heating can be enhanced by more than a factor of 10. If, in addition, an external magnetic field is introduced, the effect of magnetothermal insulation may allow to reach kilovolt temperatures and significant thermonuclear neutron yields in magnetized implosions driven by the beam heating intensities as low as ε̇≃1 TW/g. It is shown how the combined effect of the electrical resistivity and thermal conductivity sets a lower limit on the product UR (R is the radius, and U is the velocity of an imploding plasma volume) as a necessary condition for the regime of self-sustained magnetized implosion (SSMI). The optimal plasma parameters required for initiation of this regime are evaluated. In cylindrical geometry, the threshold for the SSMI regime is determined by the total driver energy deposition per unit areal density of the cylinder, ρ−1|dEb/dz| (kJ cm2/g). The results of one-dimensional magnetohydrodynamic simulations indicate that the advantages of magnetized implosions begin to manifest themselves at a beam energy level of Eb≈100 kJ.

Constraints on Jovian plasma properties from a dispersion analysis of unducted whistlers in the warm Io torus

The dispersion of unducted lightning‐generated whistlers observed by Voyager 1 in the warm torus around density peaks at L = 5.7 and L = 5.9 are analyzed using the HOTRAY code, which incorporates a newly developed diffusive equilibrium density model for the Io torus. Since the wave propagation characteristics are primarily controlled by electron density, a simplified two‐ion (H+ and O+) model has been used to simulate wave dispersion. The properties of O+ are adjusted to simulate the electron density variation at low latitudes (≤20°), where heavy ions dominate, and a variable H+ component is added to model the electron density at higher latitudes. Both the offset and tilt of the Jovian magnetic dipole are taken into account to determine the electron distribution as a function of System III longitude. The results confirm earlier suggestions that modest therml anisotropies (T⊥ > T‖) of heavy ions are required to match the observed whistler dispersion. Proton concentrations typically lie in the range 5–10%, with larger values in the outer torus. On the basis of these optimum plasma parameters, the observed upper cutoff frequencies (∼ 6 kHz) imply a minimum electron density of about 8 cm−3 at high latitudes along field lines that map into the warm torus. This analysis of unducted whistlers indicates that all observed waves originate in the northern hemisphere rather than the southern hemisphere, as assumed in earlier studies of ducted waves. This new result is consistent with optical lightning events, which were only observed in the northern hemisphere by Voyager 1.

Technical Aspects of the First JET Tritium Experiment

The JET experimental programme has been extended from its former formal closing date, end of 1992, to the end of 1996. The extension allows the study of plasma operation with a pumped divertor to be installed in the JET vacuum vessel during a shutdown in 1992–1993[1]. As a consequence the final phase of JET, which involves the use of tritium to study D-T plasmas, will be delayed to 1996.In view of this delay it was decided to adopt a stepwise approach to the introduction of tritium in JET and to carry out a tritium experiment within limits imposed by restrictions on vessel activation and tritium usage. The objectives were:To establish a firm basis for prediction of the performance of future JET D-T pulses, including the question of fuel mixing;To carry out accounting on tritium utilisation, including the assessment of tritium holdup in various components, especially in-vessel components;To demonstrate the production of about 1MW of fusion power for approximately 1 second.Preparations for the experiment included a physics programme to define the optimum plasma parameters, method of injection and diagnostics requirements, as well as a technical programme to design and install special equipment to inject tritium and recover tritiated exhaust gases. Safety aspects included the preparation of an overall probabilistic safety assessment and obtaining the requisite statutory and other approvals for the first tritium experiment. The experiment took place in two steps, firstly with a very weak tritium mixture to check all system, and secondly with a concentration of about 11% tritium in deuterium. The total fusion releases were 1.7 MW at peak power and 2 MJ of energy. The integrated total neutron yield was 7.2 1017 neutrons. This paper reports the technical and safety aspects of the experiment.