Plasma Recombination Research Articles

Plasma detachment in linear devices

Plasma detachment research in linear devices, sometimes called divertor plasma simulators, is reviewed. Pioneering works exploring the concept of plasma detachment were conducted in linear devices. Linear devices have contributed greatly to the basic understanding of plasma detachment such as volume plasma recombination processes, detached plasma structure associated with particle and energy transport, and other related issues including enhancement of convective plasma transport, dynamic response of plasma detachment, plasma flow reversal, and magnetic field effect. The importance of plasma detachment research using linear devices will be highlighted aimed at the design of future DEMO.

Double threshold behavior in a resonance-controlled ZnO random laser

We observed unusual lasing characteristics, such as double thresholds and blue-shift of lasing peak, in a resonance-controlled ZnO random laser. From the analysis of lasing threshold carrier density, we found that the lasing at 1st and 2nd thresholds possibly arises from different mechanisms; the lasing at 1st threshold involves exciton recombination, whereas the lasing at 2nd threshold is caused by electron-hole plasma recombination, which is the typical origin of conventional random lasers. These phenomena are very similar to the transition from polariton lasing to photon lasing observed in a well-defined cavity laser.

Stimulated emission via electron-hole plasma recombination in fully strained single InGaN/GaN heterostructures

The optical properties of fully coherently grown single InGaN/GaN heterostructures for 12<In%<17 were investigated under low and high density excitations. At lower density, S-shape temperature dependence of the main emission peak, associated with localized exciton recombination was observed. The activation energy of the localized excitons remains invariable in the given range of In-contents. Most interestingly, under high density pulse-excitations, stimulated emission by electron-hole plasma recombination was observed for temperatures up to 295 K.

Low threshold, single-mode laser based on individual CdS nanoribbons in dielectric DBR microcavity

Abstract Single-mode lasers with low threshold are attractive for their potential applications in many areas, such as optical communication, signal processing and displays. Here we report a nanoscale single-mode laser with CdS nanoribbons (NRs) sandwiched between two dielectric distributed Bragg reflectors (DBRs). Under optical pumping, the band edge emission of CdS ribbons can be effectively confined and give lasing in the DBR microcavity, with the lasing threshold as low as ~8 μJ/cm 2 , which is one order lower than those of bare CdS ribbons for comparison. More importantly, the CdS-DBR laser can realize single-mode emission, for the length of the resonance cavity can support a mode spacing larger than the bandwidth of the optical gain. Light polarization measurements demonstrate the single-mode lasing has a polarization degree as high as 97.4%. Time-resolved photoluminescence (TRPL) measurements further reveal that the lasing action comes from the electron-hole plasma recombination process. Moreover, the wavelength of the lasing mode can be broadly tuned by changing the thickness of CdS NRs. This low-threshold single-mode laser may find applications in highly integrated photonics devices and systems.

Comparative aeronomy: Molecular ionospheres at Earth and Mars

AbstractThe ionospheres in our solar system vary not only in their electron densities but also in the dominance of atomic versus molecular ions at their altitudes of peak plasma density. With the exception of Earth's F layer composed of atomic oxygen ions and electrons, all other planets have their peak ionospheric layers composed of molecular ions and electrons embedded in a dense neutral atmosphere. At Mars, both of its ionospheric layers have molecular ions, with the M1 layer at a lower altitude than the more robust M2 layer above it. The terrestrial ionosphere has a prominent region of molecular ions (the E layer) below the dominant F layer. In this paper, we explore the production and loss of molecular ion layers observed under the same solar irradiance conditions at Mars and Earth. We compare observations of M1 and M2 electron densities with terrestrial ionosonde data for the peak densities of the E and F layers during low, moderate, and high solar flux conditions. The subsolar peak densities of molecular ion layers have high correlations at each planet, as well as between planets, even though they are produced by separate portions of the solar spectrum. We use photochemical‐equilibrium theory for layers produced by soft X‐rays (M1 and E) versus the M2 layer produced by extreme ultraviolet (EUV) to identify the key parameters that cause similarities and differences. The yield of our comparative study points to the roles of secondary ionization and temperature‐dependent plasma recombination rates as areas most in need of further study at each planet.

Tunable microwave pulse generation using discharge plasmas

The response of a microwave resonant cavity with a plasma discharge tube inside is (continuously or intermittently) filled with a plasma and studied both numerically and experimentally. The resonance frequency of the cavity-plasma system is sensitive to plasma densities from 1016 to 1020 m−3 corresponding to resonant frequencies of 12.3–18.3 GHz. The system is first characterized for its quasi-steady state response using a low frequency plasma discharge at 70 kHz and 125 V RMS. A plasma discharge is then driven with a high voltage pulse of 4 kV and a CW input microwave signal is converted to a pulsed output signal. The microwave pulse delay and pulse width are varied by selecting the input microwave frequency. The microwave input power is set to +20 dBm. The delay of the microwave pulse is also used as a diagnostic tool for measuring the variation of plasma density in time and, with numerical fitting, the discharge plasma recombination coefficient and diffusion timescales are estimated.

Photoionization of Ca XV with high energy features

Photoionization cross sections of (Ca XV + hν → Ca XVI + e), with high energy resonant photo-absorption phenomena, of a large number of bound states, 701 in total with n ≤ 10 and l ≤ 9, are reported. They are obtained using the R-matrix method with a close coupling (CC) wavefunction expansion of 29 states of n = 2,3 complexes of the core ion Ca XVI. Characteristic features found in photoionization of the ion are illustrated with examples. The cross section (σPI) of the ground 2s22p2(3P) state is found to be unaffected by the size of the wavefunction expansion except for weak sparse resonances in high energy region. However, effects on excited states are considerable as the core excitations to n = 3 states are manifested in huge resonant absorption in high energy photoionization. They show existence of prominent high peak resonant features and enhancement in the background that were not studied before for Ca XV. In addition photoionization of the excited states with a single valence electron is dominated by Seaton resonant structures formed by the photo-excitation-of-core in the high energy region. These features will impact other quantities, such as the opacity, electron-ion recombination in high temperature plasmas where the ion exists, and hence will play important role in determination of elemental abundances in the astronomical objects.

Kinetic Modelling of the Plasma Recombination

AbstractImplementation of self‐consistent model of plasma recombination into the BIT1 PIC code and the simulation of detached SOL plasma are described. Our simulations indicate that in a strongly recombining plasma edge the sheath properties do not change qualitatively. The most affected parameter is the sheath heat transmission coefficient, which can increase by order of magnitude.

Kinetic mechanism of plasma recombination in methane, ethane and propane after high-voltage nanosecond discharge

The results of the experimental and numerical study of high-voltage nanosecond discharge afterglow in pure methane, ethane and propane are presented for room temperature and pressures from 2 to 20 Torr. Time-resolved electron density during the plasma decay was measured with a microwave interferometer for initial electron densities in the range between 5 × 1010 and 3 × 1012 cm−3 and the effective recombination coefficients were obtained. Measured effective recombination coefficients increased with gas pressure and were much higher than the recombination coefficients for simple molecular hydrocarbon ions. The properties of plasma in the discharge afterglow were numerically simulated by solving the balance equations for charged particles and electron temperature. Calculations showed that electrons had time to thermalize prior to the recombination. The measured data were interpreted under the assumption that cluster hydrocarbon ions are formed during the plasma decay that is controlled by the dissociative electron recombination with these ions at electron room temperature. Based on the analysis of the experimental data, the rates of three-body formation of cluster ions and recombination coefficients for these ions were estimated.

The Role of “Momentum Removal” in Divertor Detachment

AbstractThe role of “momentum removal” (the drag force on the plasma ion flow) in divertor detachment is considered and analysed in detail. This analysis of the 2D modelling results shows that the drag force cannot reduce the power and particle flux to the target directly. However, it is essential for creating the conditions for efficient radiation and volumetric plasma recombination, which in turn do the job. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of a falling edge on high power microwave pulse combination

This paper presents an explanation of the influence of a microwave falling edge on high-power microwave pulse combination. Through particle-in-cell simulations, we discover that the falling edge is the driving factor that limits the output power of the combined pulses. We demonstrate that the space charge field, which accumulates to become comparable to the E-field at the falling edge of the former pulse, will trap the electrons in the gas layer and decrease its energy to attain a high ionization rate. Hence, avalanche discharge, caused by trapped electrons, makes the plasma density to approach the critical density and cuts off the latter microwave pulse. An X-band combination experiment is conducted with different pulse intervals. This experiment confirms that the high density plasma induced by the falling edge can cut off the latter pulse, and that the time required for plasma recombination in the transmission channel is several microseconds. To ensure a high output power for combined pulses, the latter pulse should be moved ahead of the falling edge of the former one, and consequently, a beat wave with high peak power becomes the output by adding two pulses with normal amplitudes.

Modeling of Small‐sized ELMs in Detached Divertor Plasmas

AbstractSelf‐consistent modeling of edge plasma transport and deuterium wall inventory including multiple transient events was performed with the multi‐physics 2‐D transport code UEDGE‐MB‐W. In agreement with experimental data trends on DIII‐D, the modeling results show that relatively small‐sized and frequent type‐I Edge Localized Mode (ELM) events, which are typical for high‐power H‐mode discharges with strong deuterium gas‐puff fueling on this tokamak, are not “burning through” the formed detached plasma in the inner and, with further increase in fueling, in the outer divertor. In the latter case, the divertors are filled by sub‐eV, high‐density, strongly‐recombining and highly impurity contaminated plasma. Time‐dependent experimental data supporting the view that volumetric plasma recombination is enhanced during small‐sized ELMs penetrating into the detached plasma are discussed and the confirmatory results from UEDGE‐MB‐W modeling are presented. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Universal Loss Dynamics in a Unitary Bose Gas

The low temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here we first present a theoretical model that describes the dynamic competition between two-body evaporation and three-body re-combination in a harmonically trapped unitary atomic gas above the condensation temperature. We identify a universal magic trap depth where, within some parameter range, evaporative cooling is balanced by recombination heating and the gas temperature stays constant. Our model is developed for the usual three-dimensional evaporation regime as well as the 2D evaporation case. Experiments performed with unitary 133 Cs and 7 Li atoms fully support our predictions and enable quantitative measurements of the 3-body recombination rate in the low temperature domain. In particular, we measure for the first time the Efimov inelasticity parameter $\eta$ * = 0.098(7) for the 47.8-G d-wave Feshbach resonance in 133 Cs. Combined 133 Cs and 7 Li experimental data allow investigations of loss dynamics over two orders of magnitude in temperature and four orders of magnitude in three-body loss. We confirm the 1/T 2 temperature universality law up to the constant $\eta$ *.

Divertor plasma detachment

Regime with the plasma detached from the divertor targets (detached divertor regime) is a natural continuation of the high recycling conditions to higher density and stronger impurity radiation loss. Both the theoretical considerations and experimental data show clearly that the increase of the impurity radiation loss and volumetric plasma recombination causes the rollover of the plasma flux to the target when the density increases, which is the manifestation of detachment. Plasma-neutral friction (neutral viscosity effects), although important for the sustainment of high density/pressure plasma upstream and providing the conditions for efficient recombination and power loss, is not directly involved in the reduction of the plasma flux to the targets. The stability of detachment is also discussed.

Synthesis and in-situ oxygen functionalization of deposited graphene nanoflakes for nanofluid generation

Graphene nanoflakes (GNFs) are a stack of 5–20 layers of highly crystalline graphene sheets having a planar size of approximately 100 by 100 nm and low defect concentration. They are produced here through homogeneous nucleation following the plasma decomposition of a carbon source in an inductively coupled thermal plasma reactor. Following synthesis within the plasma reactor, the GNFs are functionalized downstream in the plasma recombination zone with the addition of oxygen to the main plasma stream. Tunable oxygen functionalization is obtained and values up to 14.2 atomic percent are reached on the surface of the particles. While the non-functionalized GNFs are hydrophobic, the oxygen-functionalized GNFs (O-GNFs) show full stability of all the produced powders when directly dispersed in water or ethanol without any surfactant. The O-GNFs keep their structural integrity even after the implementation of the hydrophilic groups on the surface of the nanoparticles, and maintain their dispersion stability in water and ethanol over a long period of time.

A model of charging microparticles by discharge plasma with account for the electrostatic field

The discharge mechanism of charging microparticles in the charge case of the injector of an electrostatic accelerator is analyzed. The processes of origination and spread of the plasma discharge that occurs as the microparticle approaches the charging electrode are discussed. To determine the charge of the particles obtained as a result of microdischarge the problem of expansion of multi-component plasma under the influence of an electrostatic field is solved. A system of equations that describe the dynamics of the expanding plasma bunch, ionization and recombination processes, taking into account the influence of the electrostatic field on the plasmoid surface current, is solved. The described system of differential equations makes it possible to find the degree of ionization of chemical elements contained in the plasma bunch and the amount of ions as a function of time and chemical composition. The dependences of the degree of ionization of ions on the composition of the plasma bunch and the voltage on the charging electrode are obtained. The dependence of the particle charge on the voltage of the needle electrode is shown.

First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background.

The unimpeded relativistic propagation of cosmological neutrinos prior to recombination of the baryon-photon plasma alters gravitational potentials and therefore the details of the time-dependent gravitational driving of acoustic oscillations. We report here a first detection of the resulting shifts in the temporal phase of the oscillations, which we infer from their signature in the cosmic microwave background temperature power spectrum.

Residual gas entering high density hydrogen plasma: rarefaction due to rapid heating

The interaction of background molecular hydrogen with magnetized (0.4 T) high density (1–5 × 1020 m−3) low temperature (∼3 eV) hydrogen plasma was inferred from the Fulcher band emission in the linear plasma generator Pilot-PSI. In the plasma center, vibrational temperatures reached 1 eV. Rotational temperatures obtained from the Q(v = 1) branch were systematically ∼0.1 eV lower than the Q(v = 0) branch temperatures, which were in the range of 0.4–0.8 eV, typically 60% of the translational temperature (determined from the width of the same spectral lines). The latter is attributed to preferential excitation of translational degrees of freedom in collisions with ions on the timescale of their in-plasma residence time. Doppler shifts revealed co-rotation of the molecules with the plasma at an angular velocity an order of magnitude lower, confirming that the Fulcher emission connects to background molecules. A simple model estimated a factor of 90 rarefaction of the molecular density at the center of the plasma column compared to the residual gas density. Temperature and density information was combined to conclude that ion-conversion molecular assisted recombination dominates plasma recombination at a rate of 1 × 10−15 m3 s−1. The observations illustrate the general significance of rapid molecule heating in high density hydrogen plasma for estimating molecular processes and how this affects Fulcher spectroscopy.

Three-body mechanisms in plasma recombination of H3+and D3+Ions

The low-temperature plasma recombination of H3 + and D3 + ions with electrons partially occurs by three-body assisted (ternary) mechanisms that need to be better understood. Intermediate high Rydberg states are expected to play an important role, but it remains unclear which of their interactions with ambient neutrals and electrons enhance their decay into neutral products. This contribution discusses several proposed models, collisional radiative, collisional dissociative recombination, resonant electron capture, and angular momentum l -mixing. It appears that none of them provides a satisfactory explanation. There is one observation that points to a possibly efficient route to long-lived collision complexes that may play a role: Several of the so far unassigned peaks in storage ring data occur at energies where rotational resonances have long lifetimes. A tentative model, based on complex formation and collisional stabilization, leads to qualitative agreement with experiments.

Generalized atomic processes for interaction of intense femtosecond XUV- and X-ray radiation with solids

Generalized atomic processes are proposed to establish a consistent description from the free-atom approach to the heated and even up to the cold solid. It is based on a rigorous introduction of the Fermi-Dirac statistics, Pauli blocking factors and on the respect of the principle of detailed balance via the introduction of direct and inverse processes. A probability formalism driven by the degeneracy of the free electrons enables to establish a link of atomic rates valid from the heated atom up to the cold solid. This allows to describe photoionization processes in atomic population kinetics and subsequent solid matter heating on a femtosecond time scale. The Auger effect is linked to the 3-body recombination via a generalized 3-body recombination that is identified as a key mechanism, along with the collisional ionization, that follows energy deposition by photoionization of inner shells when short, intense and high-energy radiation interacts with matter. Detailed simulations are carried out for aluminum that highlight the importance of the generalized approach.