Molecular Assisted Recombination Research Articles

Emission of high rovibrational hydrogen molecules under detached plasma conditions by recycling on the tungsten wall

Abstract It is well known that the rate coefficient of molecular-assisted recombination (MAR) varies by several orders of magnitude depending on the rovibrational states of the hydrogen molecules. A molecular dynamics simulation is performed to estimate the rovibrational states of recycled hydrogen molecules at the divertor in the JA-DEMO reactor under detached plasma conditions. The simulation results reveal that molecules in high rovibrational states are released even with low incident energy, which will be the dominant condition under detached plasma conditions. Molecules generated in this way can strongly affect the formation of the detached plasma via a molecular assisted-process such as MAR.

Molecular data needs for advanced collisional-radiative modelling for hydrogen plasmas

Population models for molecular hydrogen (H2) are applicable in various fields of plasma physics and particularly in fusion research: they are necessary for the evaluation of plasma diagnostics (e.g. optical emission spectroscopy) or can be used to evaluate effective reaction rates for molecular processes (e.g. molecular-assisted recombination in divertor plasmas). The accuracy and completeness of population models for molecular hydrogen is strongly linked to the accuracy and availability of molecular reaction data. While there are recently huge improvements in the field of electron impact cross sections, the shortfalls regarding input data availability persist in the area of collisions between heavy particles and H2. An overview of the status of population models for H and H2 based on the Yacora solver is given. The data needs for collisional-radiative modelling are demonstrated by means of three examples comprising different detail levels, namely a purely electronic collisional-radiative model for the singlet system of H2, a corona model for the Fulcher-α band and a vibrationally resolved collisional-radiative model for the electronic ground state X1 of H2.Graphical abstractElectronic energy level diagram of the hydrogen molecule delimiting the population models discussed in this work

Impact of vibrationally and electronically excited H2 on the molecular assisted recombination rate in detached plasma regimes

The vibrational redistribution of H2 molecules via the B1Σu+ and C1Πu states are included in the determination of molecular assisted recombination (MAR) rates and are found to have negligible impact in detached plasma regimes. In high recycling and detached divertor plasma conditions, MAR occurs through the higher levels (v≥4) of vibrationally excited H2 which are primarily populated through direct electron impact excitation or the electronic excitation of H2 molecules and the subsequent radiative decay to another vibrational state. We use the collisional-radiative model CRUMPET to evaluate MAR rate coefficients with electronic transitions included, which are applied to detached plasma parameters found in the linear plasma device Magnum-PSI.

Comparison between SOLPS-ITER and B2.5-Eunomia for simulating Magnum-PSI

The interaction between plasma, neutrals and surfaces in fusion reactors is of immense importance for heat and particle control, especially for the next generation of devices. Heat loads of are expected for steady state operation at ITER and up to in slow transient situations. To study the complex physics appearing between the plasma and the divertor material, as well as techniques for heat flux mitigation, plasma linear devices are employed. Magnum-PSI, located at DIFFER, can reproduce the heat and particle loads expected at ITER. However, due to the complexity of the plasma–wall interaction, numerical models are required to better understand the experiments and to extrapolate the results to a tokamak divertor configuration. For tokamak geometries, SOLPS-ITER (formerly known as B2.5-Eirene) is employed to solve the plasma and neutral distribution in a coupled way. However, the utilization of this code for linear devices is not straightforward. Thus, a neutral module was developed with linear devices in mind, named Eunomia. Nevertheless, there is still a relevant interest in using SOLPS-ITER with linear devices, as it allows to easily transfer knowledge about relevant atomic and molecular processes close to the surface and the effect of different mitigation techniques. This work presents a systematic comparison between the two neutral modules, Eirene and Eunomia, in stand-alone and coupled runs. Special attention is paid to the implementation of plasma-neutral interactions, in which both codes diverge significantly. The sources of particles and energy that are used by B2.5 in a coupled run are analysed. Significant differences in the implementation of electron impact ionization, molecular assisted recombination and proton–molecule elastic collisions lead to disparate sources of particles and energy and, in some cases, differences in the distribution of neutrals achieved by each code. Moreover, a double counting in proton–atom collisions was identified in Eunomia as a result of this analysis, artificially increasing the plasma-neutral sink of energy. This would lead to different plasma evolutions in coupled runs. Nevertheless, additional-free parameters in both coupled code suites leave sufficient freedom to match experimental data. Additional data would be required to further constrain these parameters and the coupled solution.

Direct measurements of electronic ground state ro-vibrationally excited D2 molecules produced on ECR plasma-facing materials by means of VUV-FT absorption spectroscopy

Ro-vibrationally excited molecules of deuterium are involved in non-equilibrium chemical reactions in divertor region of tokamak (see molecular assisted recombination) or in neutral beam injector to produce negative ion (see dissociative electron attachment) for fusion plasmas. These molecules produced both in a plasma volume and on surfaces are no longer in their electronic ground state X1∑g+(v”⩾0,J”) and populate non-uniformly different vibrational (v″) and rotational levels (J″). The high resolution VUV Fourier Transform spectrometer of the DESIRS beam line (SOLEIL synchrotron) is applied to directly scrutinize the ro-vibrationally excited levels of the D2 ground state from v″ = 0 to 10 and J″ up to 8 in an electron cyclotron resonance (ECR) cold plasma. This is performed for different plasma-facing materials, i.e. quartz, tantalum, tungsten, and stainless steel, in order to compare their relative impact in molecular excitation through recombinative desorption from surfaces. A significant effect of these materials on the absolute distribution of the vibrationally states has been found above v″ > 3. The experimental detection limit for Quartz is v″ = 7 whereas tungsten and stainlees steel excite the molecules up to v″ = 9 and tantalum up to v″ = 10. The use of a bare, temperature-controlled surface of Quartz as a reference, compared to metallic surfaces, allows us to determine the relative production of these excited levels. The recombinative desorption rate on tungsten and stainless steel compared to quartz is ~ 2.6 higher and reaches ~ 5 for tantalum.

Molecular dynamics simulation model of hydrogen recycling on carbon divertor for neutral transport analysis in large helical device

AbstractTo perform the neutral‐transport simulation with processes in which hydrogen molecules contribute to the reaction such as molecular assisted recombination, the parameters of emitted neutral particles at the wall such as the energy distributions and the form (atom or molecule) of emitted neutral particles are necessary as a boundary condition of the calculation. Therefore, in order to provide information of recycled hydrogen on the divertor to neutral‐transport code, molecular dynamics simulation of a hydrogen atom injection into a carbon material is performed to obtain the distributions of emission angle and translational energy of emitted hydrogen atoms and molecules. The distributions of rotational and vibrational energies are also investigated in the case of molecular hydrogen emission. Moreover, the quantum rotational state J, and vibrational state v are estimated from the classical value obtained by the simulation.

Hydronitrogen Molecular Assisted Recombination (HN-MAR) process in ammonia seeded deuterium plasmas

Abstract Ammonia molecules, which will be formed in a nitrogen-seeded divertor plasma, show significant catalytic effects for volumetric recombination of hydrogen ions. This new Molecular Assisted Recombination (MAR), named the Hydronitrogen-MAR (HN-MAR), is based on the charge/ion exchange with NH3 and subsequent volumetric recombination of NH3+ and NH4+. In our PISCES-E RF plasma device, Ar is mixed with ND3/D2 gasses to realize a high electron density plasma [ne ∼ 1017 m−3] which allows us to see the importance of the dissociative recombination reactions which become comparable with wall loss reactions in our chamber. As increasing the plasma density, a calibrated Electrostatic Quadrupole Plasma analyzer measured dropping of ND4+ density fractions which can be explained reasonably by the existence of its dissociative recombination process in the plasma as a dominant reaction.

Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI

In this work we investigate the effects induced by the presence of nitrogen in a detached-like hydrogen plasmas in linear plasma machine Magnum-PSI. Detachment has been achieved by increasing the background neutral pressure in the target chamber by means of H2/N2 puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved ne are ITER-relevant i.e. above 1020 m−3 and electron temperatures are in the range 0.8–2 eV. A scan among five different N2/H2+N2 flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A ne decrease while increasing the fraction of N2 has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. Te remains constant among all scans. The peak intensity of NH*(A3∏->X3∑−, ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N2 content, together with the NH3 signal in the RGA. A further dedicated experiment has been carried out by puffing separately H2/N2 and H2/He mixtures, being helium a poorly-reactive atomic species, hence excluding a priori nitrogen-induced molecular assisted recombination. Interestingly, plasma pressure and heat loads to the surface are enhanced when increasing the content of He in the injected gas mixture. In the case of N2, we observe an opposite behavior, indicating that NH species actively contribute to convert ions to neutrals. Recombination is enhanced by the presence of nitrogen. Numerical simulations with two different codes, a global plasma-chemical model and a spatially-resolved Monte Carlo code, address the role of NHx species behaving as electron donor in the ion conversion with H+ by means of what we define here to be N-MAR i.e. NHx+ H+ → NHx++ H, followed by NHx++ e− → NHx-1 + H. Considering the experimental findings and the qualitative results obtained by modelling, N-MAR process is considered to be a possible plasma-chemical mechanism responsible for the observed plasma pressure drop and heat flux reduction. Further studies with a coupled code B2.5-Eunomia are currently ongoing and may provide quantitative insights on the scenarios examined in this paper.

Rovibrationally Resolved Time-Dependent Collisional-Radiative Model of Molecular Hydrogen and Its Application to a Fusion Detached Plasma

A novel rovibrationally resolved collisional-radiative model of molecular hydrogen that includes 4,133 rovibrational levels for electronic states whose united atom principal quantum number is below six is developed. The rovibrational X 1 Σ g + population distribution in a SlimCS fusion demo detached divertor plasma is investigated by solving the model time dependently with an initial 300 K Boltzmann distribution. The effective reaction rate coefficients of molecular assisted recombination and of other processes in which atomic hydrogen is produced are calculated using the obtained time-dependent population distribution.

Effect of the Divertor Leg Length on the Population of Molecular Ions in the Divertor Simulator TPD-SheetIV

In TPD-SheetIV, a simulation experiment on a long leg divertor configuration has been conducted and hydrogen molecular ion formation in detached plasma was observed. Improvements to the simulation geometry used in DEGAS 2 allowed us to study the effect of leg length on neutral transport. This paper also describes a simple ion balance equation and the ion density ratios obtained with it. From these data, it is shown that a long leg divertor enhances molecular ion density in the divertor, which might enhance molecular assisted recombination.

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.

A power pulsed low-pressure argon microwave plasma investigated by Thomson scattering: evidence for molecular assisted recombination

A squared-wave power pulsed low-pressure plasma is investigated by means of Thomson scattering. By this method the values of the electron density and temperature are obtained, directly. The plasma is created by a surfatron launcher in pure argon at gas pressures of 8–70 mbar. Features of the pulse rise and decay are studied with microsecond time resolution. During the pulse rise we observe initial high temperature values, while the density is still rising. At power switch-off we find decay times of the electron density that are smaller than what is expected on the basis of diffusion losses. This implies that the dominant decay mechanism in the studied pressure regime is provided by molecular assisted recombination.

Kinetic divertor modeling

The coupled dynamics and kinetics between gas and plasma in the divertor region is studied by means of a one-dimensional Particle in Cell-Direct Simulation Monte Carlo (PIC-DSMC) model. In particular, the collision-induced vibrational excitation/relaxation of H2 molecules and particle–surface interaction (vibrational relaxation and recombinative desorption) have been considered in detail to estimate the importance of plasma volumetric recombination by molecular assisted reaction (MAR). Spatially resolved results show that MAR processes are effective very close to the divertor plate in a region smaller than 1.5mm from the divertor plate. For regions more distant the ionization of atoms, produced by MAR, starts to make molecular assisted recombination an ineffective reaction.

Evidence for molecular-assisted recombination of He+ from particle balance measurements in helium-hydrogen mixture plasmas in PISCES-A

Particle balance measurements of He+ ions in He∕H mixture plasma are performed by using wall probes, plunging probes, and spectroscopy. The measurements indicate that radial loss to the chamber wall is the dominant ion sink; nevertheless, a missing ion sink is observed as the plasma hydrogen fraction is increased. This sink term is attributed to hydrogen-mediated recombination of He+ ions, so-called molecular-assisted recombination (MAR). Independent evidence for MAR (acting on hydrogen ions, however) is seen in anomalously large Hα∕Hγ ratios at the cold plasma edge.

Contribution of hydrogen molecular assisted recombination processes to population of hydrogen atom in divertor simulator MAP-II

Electron temperature and density of a hydrogen plasma are measured using laser Thomson scattering in divertor simulator MAP-II. Population of hydrogen atom calculated using the temperature and density are compared to that obtained from Balmer series emission spectroscopy, showing molecular assisted recombinations (MAR) contribute to populating process. One of the MAR, mutual neutralization following negative ion production is indicated by increase of p=3 excited state, while dissociative recombination of molecular ion (H2+) increases p=4–6 states.

Spectroscopic measurement of atomic and molecular deuterium fluxes in the DIII-D plasma edge

Molecular deuterium fluxes into the edge of deuterium-fuelled L-mode discharges are measured using passive visible spectroscopy of D2 emission lines. Comparison with the atomic deuterium influx measured using Dα emission suggests that a significant fraction of the plasma edge fuelling from the walls is in the form of D2. Molecular deuterium flux is observed in both the divertor and main chamber regions but is roughly a factor 100 smaller near the inner main chamber wall and roughly a factor 1000 smaller near the outer main chamber wall, when compared with the divertor region. Very high levels of molecular D2 excitation are measured, with ground state D2 rotational population temperatures Trot up to 10 000 K and vibrational population temperatures Tvib up to 30 000 K. Comparisons between rotational population temperatures and the local electron density suggest that Trot can be used as a reasonably good indicator of electron density in the D2 line emission region. In recombining, detached divertor operation, estimates of the enhanced volume recombination rate due to the presence of vibrationally-excited D2 suggest that the effect of molecular-assisted volume recombination could be comparable in magnitude to that of normal D+ volume recombination (EIR).

Laser Thomson scattering system applicable to low-temperature plasma in the divertor simulator MAP-II

A laser Thomson scattering system was developed for the measurement of electron temperature in low-temperature recombining plasmas. The system is equipped with a double monochromator instead of an interference filter array to obtain narrow Doppler broadening. The system was applied to electron temperature measurement in the divertor simulator MAP-II. When the electron temperature is lowered to 2 eV by puffing hydrogen gas, a signature of hydrogen molecular-assisted recombination (H2-MAR) is observed.

Experimental study of negative ion profiles in H2-MAR plasmas in divertor simulator MAP-II

The negative hydrogen ion density profile is measured in hydrogen molecular assisted recombination (MAR) plasmas in the steady-state linear divertor/edge plasma simulator material and plasma (MAP)-II device. Fulcher band spectroscopy is also performed for the measurement of the ro-vibrational distribution of hydrogen molecules and the degree of dissociation. We have developed negative ion diagnostics using a combined scheme of an ‘eclipse’ laser photo-detachment system and a double probe. The former is to avoid probe surface ablation phenomena while the latter is to avoid the anomaly in the Langmuir probe characteristics in recombining plasmas, which leads to an overestimation of Te. Negative hydrogen ions are detected in the peripheral regions of the hot plasma column while vibrationally excited molecules exist in the plasma core. Based on the rate equation, the measured negative ion density is compared with the results of model calculations.

Spectroscopic study of hydrogen particle behavior in attached and detached divertor plasmas of JT-60U

Hydrogen particle behavior, especially H2 molecule behavior, has been studied spectroscopically in attached and detached divertor plasmas of JT-60U. The decay lengths of the H2 Fulcher line intensity in attached and detached divertor plasmas were roughly 1cm and 4cm, respectively. The fall in intensity of the H2 Fulcher lines with distance from the divertor plates was reproduced by calculation using a neutral transport and a collisional radiative model code. Molecular assisted recombination was estimated to be as important as H+–e recombination in a detached divertor plasma.

Observation of molecular assisted recombination via negative ions formation in a divertor plasma simulator, TPDSHEET-IV

Molecular assisted recombination (MAR) via negative ions formation has been observed in a linear plasma simulator. A small amount of hydrogen gas puffed into a hydrogen plasma strongly reduced the heat flux to the target and rapidly increased the density of negative ions of hydrogen atom, n H − , in the circumference of the plasma, while the conventional radiative and three-body recombination processes disappeared. n H − is localized in the outer region where electrons exist ( n e∼5×10 17 m −3, T e=3–5 eV). The peak value of n H − is 1.2×10 17 m −3 and the ratio of n H − / n e in the outer region goes up to over 20%. These results can be well explained by taking the mutual neutralization between the negative and positive ions of MAR in the detached plasma into account.