Plasma Recombination Research Articles

Theoretical studies of aluminum wire array Z-pinch implosions with varying masses and radii

A one-dimensional radiation magneto-hydrodynamics code coupled to an atomic kinetics code was developed, whose simulation results were compared with the experimental observations. Based on the simulation results, the physical processes and physical scheme of Z-pinch implosions were studied. During the implosion phase, the plasmas are heated and ionized, and before the stagnation, the greatly ionized plasmas of high temperature and high density are created. Then the continuously rising density leads to the recombination of plasmas and the emission of an intense X-ray pulse. Our simulation shows that there are two necessary conditions for producing high yield of K-shell energy. Firstly, highly ionized plasmas must be prepared before the beginning of recombination. Secondly, the density of the plasmas which are in recombination phase should be high enough to make the plasmas recombine with a large rate so that more internal energy can be drawn before stagnation. As another result of our study, the atropic processes play a role in cooling free electrons during the recombination phase.

Effect of plasma–wall recombination on the conductivity in Hall thrusters

The paper deals with the operation of Hall plasma thrusters and the transversal to the magnetic field current in such systems. The problems of the transverse conductivity and related processes are treated in the framework of the well-known Morozov's model; the difference is in the addition of the mechanism of plasma recombination on the walls of the thruster and further volume ionization of the neutrals created at the walls. The distribution functions of the neutrals and secondary electrons generated by ionization are found, as well as the expression for the transverse current. The obtained result contains an additional correcting factor compared to the conventional Morozov's equation for this current and permits a better quantitative agreement of the theoretical expression and experimental results.

Exciton Related Stimulated Emission in ZnO-Based Multiple-Quantum Wells

The mechanism of ultraviolet stimulated emission was investigated in ZnO/ZnMgO multi-quantum wells grown by laser-molecular-beam epitaxy (laser-MBE) on a lattice-matched ScAlMgO4 (SCAM) (0001) substrate. Stimulated emission induced by exciton–exciton scattering occurred throughout a range of temperatures from 5 K to room temperature. At temperatures higher than 160 K, also stimulated emission due to electron–hole plasma recombination was observed with a higher excitation threshold than that of exciton–exciton scattering. Stimulated emission due to the radiative recombination of localized biexcitons has been observed at low temperatures (5 K). By examining the relative energy positions of the emission lines and the dependence of their intensities on the excitation power density it has been verified that theses lines occur due to the recombination of localized excitons and biexcitons and due to the exciton–exciton scattering. The excitation threshold for biexciton emission was significantly lower than that for exciton–exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced by quantum confinement effects.

Plasma resistivity determination in runaway discharges from positive voltage spikes on TCABR tokamak

Plasma resistivity is estimated in runaway discharges on TCABR tokamak from the decay of positive voltage spikes using method developed in I. El Chamaa Neto et al., Phys. Plasmas 7, 2894 (2000). In this regime, the plasma detachment from the limiter is observed experimentally indicating that plasma recombination is the main process of plasma particle losses. In order to investigate if this process is actually taking place, measurements of electron temperature in the plasma are necessary. The electron temperature deduced from the plasma resistivity is in the range 0.2 - 1.5 eV.

Fundamental effects and non-linear Si detector response

Non-linearity in the energy response of a Si p-i-n charged particle detector has been studied for incident particles with Z 1 between 3 and 26, and energies between 0.1 and 0.7 MeV per nucleon. Although the data closely followed a straight line relations, fitting of the data to a third order polynomial revealed that the response exhibited a persistent curvature that acted to reduce the energy interval spanned by a channel as the energy increased. The curvature increased as Z 1 increased from 4 to 8 and then systematically decreased. The curvature is larger and has the opposite energy dependence to the stopping in a dead entrance window and the energy deposited in non-ionising processes within the active layer. The plasma recombination dependence on the average stopping along the plasma column may account for the reduction in curvature as Z 1 increases from 9 to 25 but cannot explain the net effect. The low-energy increase in energy channel span, which has also been reported by others, might be associated with electron excitation in resonant and direct classical quasi-elastic collisions for low-energy ions, or less likely, electronic non-linearity's associated with Z 1 and energy dependent time structure in the current pulse from the detector. Simple interpolation of the window-loss corrected polynomial coefficients is the best approach if the calibration for Z 1 cannot be established directly.

Molecule assisted recombination of divertor plasmas

The recently observed phenomenon of divertor plasma detachment in all presently operating large fusion devices has strongly promoted the hopes for an adequate reduction of thermal and particle fluxes deposited on divertor plates and for a successful solution of the thermal power exhaust problems in fusion reactors. The plasma detachment regime in tokamak divertors and divertor simulators is characterized by a dramatic drop of plasma particle flux onto the divertor plates and a plasma temperature reduction to the eV and sub-eV level near the plates. Both the experimental evidence and theoretical analyses of this phenomenon lead to the conclusion that its appearance and existence are strongly related to volmnetric plasma recombination in the divertor.The theoretical modeling of divertor plasma recombination usually includes the following "standard" recombination mechanisms in the plasma kinetics (in a pure hydrogenic plasma):(1) radiative recombiintion: e + H+ → H(n) + hv;(2) three-body recombination: e + e + H+ → e + H(n);(3) negative-ion mediated recombination: e + H2(v ⩾ 4) → H2- → H + H-, H+ + H- → H + H (n = 3);(4) ion-conversion mediated recombination: H+ + H2(v ⩾ 4) → H + H2+(v'), e + H2+(v') → H + H (n ⩾ 2).The effective (indirect) electron-proton "recombination" in reaction chains (3) and (4) is made possible via the electron and proton reactions with vibrationally excited hydrogen molecules. These two recombination chains are, therefore, called molecule assisted recombination (MAR) mechanisms. A characteristic feature of these two MAR mechanisms is that they require the existence of vibrationally excited molecules in states with vibrational quantum nunbers higher than v = 3, otherwise their efficiency is very low.Analysis of the temperature dependences of the rate coefficients for the reactions involved in the recombination mechanisms (1)–(4) shows that the radiative and three-body recombinations are effective recombination mechanisms for plasma temperatures below ∼ 0.5 eV (and plasma densities above 5 x 1014 cm-3 for three-body recombination). The MAR mechanisms become dominant for temperatures in the range 0.5–2 eV. Above 2–2.5 eV, MAR mechanisms (3) and (4) become ineffective due to the electron impact destruction of H- (electron detachment) and H2+ (dissociative excitation) ions.The modeling of divertor plasma recombination obviously requires an accurate knowledge of the reaction rate coefficients (or cross sections) of all the processes involved in the recombination mechanisms (1)–(4). However, the full description of plasma recombination kinetics also requires inclusion of many other processes involved in the production and destruction of the reactants and products of the above listed reactions. A specific aspect which contributes substantially to the complexity of divertor plasma recombination kinetics is the necessity to consider all the processes of vibrationally excited molecules and molecular ions.The present Topical Issue is devoted to a detailed analysis of the processes involved in volume recombination of divertor plasmas, with emphasis on the processes involved in MAR kinetics. The contributions to this volume address both the general aspects of MAR (its kinetic structure and effectiveness) and the collisional information for the most important processes involved in MAR kinetics. For several of these processes their isotopic variants are also considered. One of the objectives of the contributions devoted to specific atomic and molecular processes, or to entire classes of such processes, was to present the best available quantitative information on the cross sections and/or reaction rate coefficients of the processes considered. By responding to this objective, the present volume not only elucidates the atomic and molecular physics of divertor plasma recombination, but also provides a source of evaluated cross section information for the needs of divertor plasma modeling.

Alternative Mechanisms for Divertor Plasma Recombination

A number of molecular reaction schemes are considered as possible mechanisms for enhancing the volumetric recombination of divertor plasmas. These schemes include collision processes involving Rydberg hydrogen molecules, the H+3 ion and hydrocarbon molecular plasma impurities. Particular emphasis is placed on the catalytic recombination mechanism provided by the hydrocarbon impurities which can represent an alternative to the recombination schemes based on processes with vibrationally excited molecular hydrogen

Collisional Radiative Kinetics of Molecular Assisted Recombination in Edge Plasmas

Recombination of hydrogen plasma in the divertor volume is a simple explanation for divertor detachment phenomena experimentally observed on many tokamaks and divertor plasma simulators. The presence of vibrationally excited molecules in detached plasma provides an additional mechanism for plasma recombination, the so-called molecular assisted recombination (MAR). The cross-section data on atomic and molecular processes relevant for divertor plasma conditions are reviewed. Collisional radiative kinetics for cascade excitation of vibrational and electronic states as well as for the dissociation, recombination, and ionization of hydrogen molecule and molecular ions is studied. The results of calculations on the effective rate coefficient for MAR in detached divertor plasmas are presented. A comparison of the MAR rate coefficient with the rate coefficients for molecular dissociation, ionization, and electron-ion recombination is given. The sensitivity of MAR rate to the accuracy and completeness of atomic physics data is discussed

Dilute Bose-Einstein condensate with large scattering length.

We study a dilute Bose gas of atoms whose scattering length a is large compared to the range of their interaction. We calculate the energy density E of a homogeneous Bose-Einstein condensate (BEC) to second order in the low-density expansion, expressing it in terms of a and a second parameter Lambda* that determines the low-energy observables in the three-body sector. The second-order correction to E has a small imaginary part that reflects the instability due to three-body recombination. In the case of a trapped BEC with large negative a, we calculate the coefficient of the three-body mean-field term in E in terms of a and Lambda*. It can be very large if there is an Efimov state near threshold.

Condensation of electron-hole pairs in bulk GaAs at room temperature under conditions of femtosecond cooperative radiation

The results are given of an experimental investigation of the spectral characteristics of cooperative recombination of a high-density electron-hole plasma in GaAs. It is demonstrated that, under conditions of generation of high-power femtosecond pulses of superradiation, the properties of electrons and holes differ considerably from their properties under conditions of lasing or regular spontaneous recombination. The peak of the cooperative radiation line (ℏω=1.405 to 1.407 eV) is shifted inward into a (nonrenormalized) forbidden band. It is located 20 meV lower on the energy scale than the lasing peak and more than 40 meV below the center of the line of spontaneous recombination at the same pumping level. This corresponds to electron-hole condensation to the bottom of the bands. The properties of cooperative recombination may be defined by the pairing of electrons and holes and by the formation of a short-lived coherent electron-hole BCS state. The estimated value of the order parameter Δ is approximately 2 meV.

Measurements of radiation near an atomic spectral line from the interaction of a 30 GeV electron beam and a long plasma.

Emissions produced or initiated by a 30-GeV electron beam propagating through a approximately 1-m long heat pipe oven containing neutral and partially ionized vapor have been measured near atomic spectral lines in a beam-plasma wakefield experiment. The Cerenkov spatial profile has been studied as a function of oven temperature and pressure, observation wavelength, and ionizing laser intensity and delay. The Cerenkov peak angle is affected by the creation of plasma, and estimates of neutral and plasma density have been extracted. Increases in visible background radiation, consistent with increased plasma recombination emissions due to dissipation of wakefields, were simultaneously measured.

Autoionization phenomena in dense photoionized plasmas

Elementary electron–ion collisional and spontaneous radiative transitions involving autoionizing resonances play important roles in atomic ionization, recombination, and radiative-emission processes in high-temperature astrophysical and laboratory plasmas. When electron–ion collisions are dominant, the ionization structure is usually determined by the steady-state (or dynamical) balance between electron-impact ionization (including autoionization following inner-shell-electron excitation) and radiative recombination combined with dielectronic recombination. The atomic radiative-emission spectrum tends to be dominated by the electron-impact excitation of ordinary spectral lines together with dielectronic satellite lines. In photoionized plasmas, a much higher degree of ionization can be established at a lower electron temperature, particularly as a result of multiple ionization due to Auger transitions following K inner-shell-electron photo-ionization. Radiative and dielectronic recombination can occur predominantly via transitions that are usually not considered to be important, and the unified quantum-mechanical description of the combined electron–ion photo-recombination process may be necessary. The satellite-line emission produced by the radiative decay of multiple-vacancy states can be more prominent, relative to the characteristic-line emission from the decay of single-vacancy states. We have developed a multiple-vacancy-state model for single and multiple ionization, together with characteristic-line and satellite-line emission, resulting from the cascade decay (by spontaneous radiative and Auger transitions) of an arbitrary initial inner-shell-electron vacancy distribution. The initial vacancy distribution may be created by electron or photon impact. We have also developed a unified description of the combined electron–ion photo-recombination process, taking into account the quantum-mechanical interference between radiative and dielectronic recombination. Results of calculations for Fe ions are discussed, which are expected to be valid at low densities. Implications for the detailed kinetics and spectral modeling of dense photoionized plasmas are discussed.

Evidence for the importance of radial transport in plasma detachment in the Nagoya University Divertor Simulator (NAGDIS-II)

Measurements of ion and electron temperatures have been performed in detaching helium–hydrogen plasmas in a linear divertor simulator experiment using spectroscopy, a Langmuir probe, and an omegatron mass spectrometer. Detachment in these plasmas is characterized by a significant (≈20×) reduction in the central plasma flux at the target plate as the target region neutral pressure is increased from 2 to 12 mTorr. The data indicate that partially detached gas-target plasmas consist of a hot (Te≈5 eV) core region along the axis of the plasma column, surrounded by a cold (Te≈0.1 eV) halo region of recombining plasma. At Te=5 eV, plasma recombination is negligible compared with ionization; these experiments therefore provide evidence that detachment is primarily caused by radial transport and by a gradual drop in the ionization source as the temperature of the core region drops below 5 eV.

Luminescence of silicon thin film and SiGe multiple quantum wells realized on SOI

Abstract We have investigated photoluminescence (PL) of crystalline silicon-based layers sandwiched between SiO 2 barriers. We fabricate these structures on standard silicon on insulator (SOI) wafers realized by bonding techniques. Firstly, we realize silicon thin layers (50–200 nm) by thermal oxidation of the SOI free surfaces. At low temperature and under UV excitation, luminescence spectra is dominated by electron–hole droplet (EHD) or electron–hole plasma (EHP) recombination. We show that the behavior of the condensed phase is strongly modified by the spatial confinement of the carriers. Secondly, we grow by CVD Si / Si 1−x Ge x (x≈20%) multiple quantum wells (MQW) on SOI wafers. All the samples are capped by thermal SiO 2 layers. The main result is that luminescence from these samples is still observed up to room temperature; in comparison, the quenching temperature for similar Si/Si 1− x Ge x MQW directly grown on silicon substrates is 150 K. This result is due to the presence of the buried SiO 2 layer that avoids the thermal transfer of carriers from SiGe wells to the silicon substrate.

Femtosecond pump-probe spectroscopy and time-resolved photoluminescence of anInxGa1−xN/GaNdouble heterostructure

We report a study of the carrier dynamics in an ${\mathrm{In}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$ thin film photoexcited well above the band gap using nondegenerate pump-probe spectroscopy and time-resolved photoluminescence (TRPL) for carrier densities ranging from ${10}^{17}$ to ${10}^{19} {\mathrm{cm}}^{\ensuremath{-}3}$ at 10 K. At carrier densities greater than $4\ifmmode\times\else\texttimes\fi{}{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3},$ optical gain occurs across the entire band tail region after $\ensuremath{\sim}2.5 \mathrm{ps}$ time delay, when the hot carriers completely fill these states. From TRPL measurements performed in the surface emission geometry, we observed stimulated emission (SE) with a $\ensuremath{\sim}28 \mathrm{ps}$ decay time. Since this SE has a threshold density of $1\ifmmode\times\else\texttimes\fi{}{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3},$ which is larger than the total density of localized states, and the SE spectra at early time delays are quite different from the spontaneous emission spectra, we attribute the SE to the recombination of an electron-hole plasma from renormalized band-to-band transitions.

The effects of oxygen on the detection of mercury using laser-induced breakdown spectroscopy

A systematic study of the processes associated with mercury atomic emission in a laser-induced plasma and the interactions of mercury with oxygen species is presented. At early plasma decay times, on the order of 5–10 μs, no significant variation in mercury atomic emission was observed with the addition of oxygen-containing species. At intermediate and long decay times (10–100 μs), a significant reduction in the 253.7-nm mercury emission intensity was recorded with the introduction of oxygen-containing species. The decrease in mercury emission was temporally coincident with the recombination of atomic oxygen, as measured by the O(I) emission. The decreased mercury emission was not due to thermal effects, based on plasma temperature measurements, and was independent of the molecular source of oxygen, for similar concentrations of oxygen as air, carbon dioxide, and carbon monoxide. Analysis of additional mercury atomic emission lines revealed that the reduction in mercury emission in the presence of oxygen species is limited primarily to the 253.7-nm transition. In concert, the data lead to the conclusion that the 253.7-nm mercury emission line is selectively quenched by oxygen species, primarily O 2 and NO, that are formed during the plasma recombination process. Implications for laser-induced breakdown spectroscopy-based emissions monitoring of mercury species are discussed.

Constraining nonstandard recombination: A worked example

We fit the BOOMERANG, MAXIMA and COBE-DMR measurements of the cosmic microwave background anisotropy in spatially flat cosmological models where departures from standard recombination of the primeval plasma are parametrized through a change in the fine structure constant $\ensuremath{\alpha}$ compared to its present value. In addition to $\ensuremath{\alpha}$ we vary the baryon and dark matter densities, the spectral index of scalar fluctuations, and the Hubble constant. Within the class of models considered, the lack of a prominent second acoustic peak in the measured spectrum can be accommodated either by a relatively large baryon density, by a tilt towards the red in the spectrum of density fluctuations, or by a delay in the time at which neutral hydrogen formed. The ratio between the second and first peak decreases by around 25% either if the baryon density ${\ensuremath{\Omega}}_{b}{h}^{2}$ is increased or the spectral index n decreased by a comparable amount, or if neutral hydrogen formed at a redshift ${z}_{*}$ about 15% smaller than its standard value. We find that the present data are best fitted by a delay in recombination, with a lower baryon density than the best fit if recombination is standard. Our best fit model has ${z}_{*}=900, {\ensuremath{\Omega}}_{b}{h}^{2}=0.024, {\ensuremath{\Omega}}_{m}{h}^{2}=0.14, {H}_{0}=49$ and $n=1.02.$ Compatible with present data at 95% confidence level $780<{z}_{*}<1150, 0.018<{\ensuremath{\Omega}}_{b}{h}^{2}<0.036, 0.07<{\ensuremath{\Omega}}_{m}{h}^{2}<0.3$ and $0.9<n<1.2.$

Hydrogen molecules in the divertor of ASDEX Upgrade

In order to reduce the power load onto the target plates detached divertor conditions are often preferred. These are characterized by volume recombination, i.e. three-body and radiative recombination. Due to low T e (few eV) hydrogen molecules can penetrate into the plasma and may play a role in divertor dynamics. In particular, it was suggested, that molecules may assist the volume recombination process. The role of molecules in the divertor is examined here by a combination of experimental results with plasma edge simulations (B2-EIRENE) and a collisional-radiative model for hydrogen molecules. Spectroscopic diagnostics of the Fulcher transition carried out at the divertor of ASDEX Upgrade yield estimates of molecular hydrogen fluxes and the vibrational population in the ground state in detached and attached hydrogen plasmas. Good agreement with B2-EIRENE is achieved only if vibrational levels are treated as distinct (metastable) particles in the model and if the collisional-radiative model is applied to the electronically excited levels. On this basis the contribution of molecules to plasma recombination was determined to be in the order of a few 10%. The dominant molecular process is the dissociation process via H 2 +. As a consequence initially detached divertor plasmas can even re-attach if vibrationally resolved molecules are properly included in plasma edge models. A set of B2-EIRENE calculations carried out for ASDEX Upgrade is discussed. In particular the threshold upstream density for detachment was found to be up to a factor 1.5 higher than that originally expected due to these molecular effects. The transferability of the results to deuterium will be discussed.

Anomalous slowdown of relaxation in an ultracold plasma

Recent experiments by T.C. Killian et al. [Phys. Rev. Lett. 83, 4776 (1999)], in which an ultracold plasma (Ne∼2×109 cm−3, Te∼0.1K, and Ti∼10 μK) with anomalously long lifetime of ∼100 μs was obtained, are explained based on a previously developed theory. The results of computer simulations of the plasma transition into a metastable state and initial heating of electrons up to several K are presented. An expression earlier obtained for the rate of the metastable plasma recombination agrees with the measured anomalously long lifetime. A conclusion is drawn that the previously predicted new physical object—a metastable overcooled plasma—is realized experimentally.

Stimulated emission induced by exciton–exciton scattering in ZnO/ZnMgO multiquantum wells up to room temperature

The mechanism of ultraviolet stimulated emission was investigated in ZnO/ZnMgO multiquantum wells. Stimulated emission induced by exciton–exciton scattering occurred throughout a range of temperatures from 5 K to room temperature. At temperatures higher than 160 K, stimulated emission due to electron-hole plasma recombination was also observed with a higher excitation threshold than that of exciton–exciton scattering. The exciton binding energies of multiquantum wells were larger than that of bulk ZnO and increased with a decrease in the well widths. This enhancement of exciton binding energy is due to the quantum-confinement effect and is favorable for the stability of exciton states.