Molecular Activated Recombination Research Articles

Coexistence of H-MAR and N-MAR in divertor simulation experimental module of GAMMA 10/PDX

In a divertor simulation experimental module (D-module) of GAMMA 10/PDX, additional hydrogen seeding induces molecular-activated recombination (H-MAR). However, when seeding of hydrogen and nitrogen is combined, Nitrogen MAR (N-MAR) is observed in D-module. In this work, we experimentally investigate plasma detachment processes in D-module by seeding hydrogen-only and hydrogen + nitrogen in order to de-couple MARs contribution to the measured particle flux reduction. Nitrogen is seeded after the electron density, measured with Langmuir probes installed in D-module, begins to decrease due to hydrogen seeding. In both cases, the electron temperature near the V-shaped target decreases from ∼23 eV to ∼2 eV. A clear rollover is observed in the ion flux to the V-shaped target. When only hydrogen is seeded, the Balmer lines emission ratio (i.e. the intensity of Hα emission line (IHα)/the intensity of Hβ emission line (IHβ)), an indicator of H-MAR, is observed to increase between V-shaped targets installed in D-module as the amount of hydrogen gas increases. However, when both hydrogen and nitrogen are seeded, IHα/ IHβ increase is suppressed, and the region of the largest IHα/ IHβ signal shifts upstream of D-module, as observed by a high-speed camera. These results show for the first time the transition from H-MAR-dominated phase to N-MAR-dominated phase and the coexistence of both MARs in D-module. It is found that nitrogen seeding increases plasma detachment even at already relatively low Te (∼5 eV). The high-speed camera measurements suggest this effect to be related to the increase of NHx neutral density, in line with previous modeling works.

Hydrogen isotope effects on recombination dominant plasmas in NAGDIS-II

The detachment processes of the hydrogen (H) and deuterium (D) plasmas are comparatively investigated in the linear plasma device NAGDIS-II. The laser Thomson scattering measurements demonstrate that the recombination rate of the H plasma is greater than that of the D plasma as the neutral pressure increases in the molecular activated recombination (MAR) dominant detachment phase. As the recombination process by MAR is strongly dependent on the vibrational and rotationally excited states of the molecule, the rovibrational quantum state populations of the H and D molecules are measured using the Fulcher-α band spectroscopy. The results indicate that the vibrational temperature in the electronic ground state is considerably higher than the rotational temperature during detachment. The reaction rate coefficients for MARs due to charge exchange chains (CX-MAR) and dissociative attachment chains (DA-MAR) are calculated by the collision-radiation model under the measured temperature conditions. It can be observed that the CX-MAR is larger than the DA-MAR for both H and D, and that the CX-MAR of H is larger than the CX-MAR of D at electron temperatures T e above 1 eV. In consideration of the experimentally observed vibrational and rotational excitation temperatures, the reaction rate coefficients of CX-MAR and DA-MAR are increasing in the low T e region. These calculations are in accordance with the experimental results, which indicate that recombination processes due to MAR are more predominant in the H plasma compared to the D plasma. Furthermore, a transition from MAR to electron–ion recombination processes is observed in the D plasma at T e below 0.5 eV.

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms has been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium, indicating a molecular ion density () that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate () for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the content by . By disabling ion isotope mass rescaling: (1) the total ion sinks are more than doubled due to the inclusion of MAR; (2) the additional MAR causes the ion target flux to roll-over during detachment; (3) the total emission in the divertor increases during deep detachment by roughly a factor of four; (4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50% increase in neutral pressure; (5) total hydrogenic power loss is increased by up to 60% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over. Extrapolating simplified scalings of divertor molecular densities (TCV & MAST-U) to reactor-relevant densities suggests the underestimation of molecular charge exchange could strongly impact divertor physics (neutral atom density, ions sinks) and hydrogen emission (which has implications for detachment control) in deeply detached conditions, warranting further study.

Isotope Effect for Plasma Detachment in Helium and Hydrogen/Deuterium Mixture Plasmas

To investigate the isotope effect on the plasma detachment in helium (He) and hydrogen (H)/deuterium (D) mixture plasmas, we performed H2, D2 or He gas puffing into He plasma in the linear plasma device NAGDIS-II. Axial distributions of electron density (ne) and electron temperature (Te) were obtained using a movable Langmuir probe. Additionally, optical emission spectroscopy (OES) was applied to measure axial distributions of Balmer lines and He I lines. When the neutral gas pressure was high (ΔPn = 7 ∼10 mTorr), ne distribution in He-D2 mixture plasma was similar to that in pure He plasma, showing a sharp decrease at the downstream region where Te < 1 eV. In contrast, in He-H2 mixture plasma, a decrease in ne was confirmed from upstream region where Te > 1 eV. The upstream Hα/Hγ in He-H2 plasma was significantly larger than the Dα/Dγ in He-D2 plasma at the same Te. This result indicated that molecular activated recombination (MAR) processes significantly occurred in He-H2 plasma, while electron-ion recombination (EIR) processes were dominant in He-D2 and pure He plasmas.

Impact of neutral gas puffing on the divertor power exhaust and particle control in GAMMA 10/PDX by the LINDA‐KNMC code

AbstractThis paper numerically investigates the impact of neutral gas puffing on the divertor power exhaust and particle control in GAMMA 10/PDX applying the plasma fluid code LINDA and the kinetic neutral code KMNC. The LINDA code is applied to GAMMA 10/PDX plasmas for understanding the physics of detachment and energy loss processes. The LINDA code is coupled with a Monte–Carlo neutral code for describing the hydrogen (H and H2) neutral particles. The plasma ion temperature (Ti) is reduced towards the target plate with the increment of H2 Gas‐Puff rate. This paper also numerically investigates the impact of recombination processes on the detached formation. The ionization, charge–exchange, and recombination of H have been included as a particle and an energy sink and source parts of the fluid equations. The reaction rate coefficient of molecular activated recombination (MAR) has been recently introduced in the LINDA code. In this paper, we study numerically the effect of MAR on the detached plasma. The MAR plays a dominant role in making the detached plasma. The simulation clarified that H neutral particles injection can effectively generate the detached plasma by enhancing the plasma‐neutral collisions processes.

B2.5-Eunomia simulations of Magnum-PSI detachment experiments: II. Collisional processes and their relevance

Detachment is achieved in Magnum-PSI by increasing the neutral background pressure in the target chamber using gas puffing. The plasma is studied using the B2.5 multi fluid plasma code B2.5 coupled with Eunomia, a Monte Carlo solver for neutral species. This study focuses on the effect of increasing neutral background pressure to the plasma volumetric loss of particle, momentum and energy. The plasma particle and energy loss almost linearly scale with the increase of neutral background pressure, while the momentum loss does not scale as strongly. Plasma recombination processes include molecular activated recombination (MAR), dissociative attachment, and atomic recombination. Atomic recombination, which includes radiative and three-body recombination, is the most relevant plasma process in reducing the particle flux and, consequently, the heat flux to the target. The low temperature where atomic recombination becomes dominant is achieved by plasma cooling via elastic H+−H2 collisions. The transport of vibrationally excited H2 molecules out of the plasma serves as an additional electron cooling channel with relatively small contribution. Additionally, the transport of highly vibrational H2 has a significant impact in reducing the effective MAR and dissociative attachment collision rates and should be considered properly. The relevancy of MAR and atomic recombination occupy separate electron temperature regimes, respectively, at eV and eV, with dissociative attachment being relevant in the intermediary. Plasma cooling via elastic H+−H2 collisions is effective at eV.

The role of plasma-molecule interactions on power and particle balance during detachment on the TCV tokamak

This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving and possibly D− play an important role as sinks of energy (through hydrogenic radiation as well as dissociation) and particles (ions) during divertor detachment if low target temperatures (<3 eV) are achieved. Both molecular activated recombination (MAR) and ion source reduction due to a power limitation effect are shown to be important in reducing the ion target flux during a density ramp. In contrast, the electron–ion recombination (EIR) ion sink is too small to play an important role in reducing the ion target flux. MAR or EIR do not occur during N2 seeding induced detachment as the target temperatures are not sufficiently low. The impact of is shown to be underestimated in present (vibrationally unresolved) SOLPS-ITER simulations, which could result from an underestimated rate. The converged SOLPS-ITER simulations are post-processed with alternative reaction rates, resulting in considerable contributions of to particle and power losses as well as dissociation below the D2 dissociation area. Those findings are in quantitative agreement with the experimental results.

A study of the influence of plasma–molecule interactions on particle balance during detachment

In this work we provide experimental insights into the impact of plasma-molecule interactions on the target ion flux decrease during divertor detachment achieved through a core density ramp in the TCV tokamak. Our improved analysis of the hydrogen Balmer series shows that plasma-molecule processes are strongly contributing to the Balmer series intensities and substantially alter the divertor detachment particle balance. We find that Molecular Activated Recombination (MAR) ion sinks from $H_2^+$ and/or $H^-$ are a factor $\sim$ 5 larger than Electron-Ion Recombination (EIR) and are a significant contributor to the observed reduction in the outer divertor ion target flux. Molecular Activated Ionisation (MAI) may also be significant during detachment. Plasma-molecule interactions enhance the Balmer line series emission strongly near the target as detachment proceeds. This indicates enhancements of the Lyman series, potentially affecting power balance in the divertor. As those enhancements vary spatially in the divertor and are different for different transitions, they are expected to result in a separation of the $Ly\beta$ and $Ly\alpha$ emission regions. This may have implications for the treatment and diagnosis of divertor opacity. The demonstrated enhancement of the Balmer series through plasma-molecule processes potentially poses a challenge to using the Balmer series for understanding and diagnosing detachment based only on atom-plasma processes.

The role of hydrogen molecular effects on detachment in Magnum-PSI

The hydrogen plasma-chemical processes responsible for tokamak divertor detachment are studied experimentally in the linear device Magnum-PSI, with a focus on molecular activated recombination (MAR) in hydrogen plasmas. Hydrogen plasmas with electron densities up to 6×1020 m−3 were created in Magnum-PSI, and hydrogen gas puffing was used to locally enhance plasma–neutral interaction. Thomson scattering and Balmer line spectroscopy measurements show that as neutral pressure is increased, the plasma passes through regimes dominated by ionization, MAR, and electron–ion recombination in turn. Heat and particle fluxes decrease monotonically with pressure. Fulcher band measurements show that in our plasma conditions, a simple model based on Franck–Condon excitation of a thermal vibrational distribution fails to describe the vibrational distribution of the upper state. These results serve as a benchmark for modeling suites that aim to simulate the ITER divertor and motivates their accurate treatment of the discussed processes, particularly MAR.

Reaction processes of molecular activated recombination leading to detachment of divertor simulation plasma in GAMMA 10/PDX

Abstract Reaction processes of molecular activated recombination (MAR) have been investigated using end loss plasma in GAMMA 10/PDX. A tungsten V-shaped target in a divertor simulation experimental module (D-module) is exposed to the end loss plasma. The additional hydrogen gas is supplied into the D-module, leading to the plasma detachment by MAR. The vibrational temperature (Tvib) of H2(X1Σg+) increased up to ∼10,000 K with increasing the hydrogen gas pressure, and the Ha line intensity (IHa) continued to increase even though the electron density decreased after the electron density rollover. The increase in Tvib should be caused mainly by the increase in the number of vibrationally excited molecules which are produced by the molecular ion conversion process in MAR. The continuous increase in IHa is attributed to the dissociative attachment in MAR. When the hydrogen gas pressure was more than 12 Pa, Balmer emissions from highly excited atom suddenly increased and a population inversion with nH(n = 5) larger than nH (n = 4) was observed although Tvib became rather low (∼2,000 K). The formation of population inversion and increase in highly excited Balmer line intensity are considered to be caused by the reaction of mutual neutralization between molecular ion and negative ion, since the cross section of H(n = 5) production is the largest in the reaction. The negative ion should be produced by the resonant ionization process between H(2s), since the negative ion production by the dissociative attachment reaction cannot be expected due to low Tvib.

Studying the influence of nitrogen seeding in a detached-like hydrogen plasma by means of numerical simulations

The leading candidate for impurity seeding in ITER is currently nitrogen. To date, there have only been a few studies on the plasma chemistry driven by N2/H2 seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2- or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of the Plasimo code. This study shows that two main nitrogen-including recombination reaction paths are dominant, i.e. the ion conversion of NH followed by dissociative recombination, and the proton transfer between and N2, producing N2H+. These two processes are referred to as N-MAR (nitrogen molecular-activated recombination) and have subsequently been implemented in Eunomia, which is a spatially resolved Monte Carlo code designed to simulate the neutral inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N2 on the overall recombination, three studies have been set up, and from a defined puffing location with a constant total seeding rate of H2 + N2, three N2 ratios were simulated, i.e. 0%, 5% and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor of three. The importance of NH as an electron donor is highlighted, and the N-MAR reaction routes are confirmed to enhance the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards a full understanding of the role of N2-H2 molecules in a detached divertor plasma.

Molecular activated recombination in divertor simulation plasma on GAMMA 10/PDX

In the tandem mirror GAMMA 10/PDX, molecular activated recombination (MAR) leading to plasma detachment has been observed by additional hydrogen gas injection to the divertor simulation plasma (i.e. end loss plasma) which is exposed to the V-shaped target in the divertor simulation experimental module (D-module). The temperature near the corner of the V-shaped target decreased from ∼23eV to ∼2eV as the neutral pressure in the D-module increased. A clear density rollover was observed at ∼2Pa. A position of the density maximum moves to upstream of the plasma with increase in the neutral pressure and the density near the corner of the target decreases to detach the plasma from the target. After the occurrence of the density rollover, the Balmer β intensity decreases as with the density but the Balmer α intensity continues to increase, indicating the dissociative attachment process in MAR is more dominant than the ion conversion process although the rate coefficient of the former process is lower than that of the latter one, which is calculated by using a collisional radiative model. This would be caused by the MAR process related to triatomic hydrogen molecules which significantly contributed to the detachment process.

Investigation of detached recombining deuterium plasma and carbon chemical erosion in the toroidal divertor simulator NAGDIS-T

Detached deuterium recombining plasma has been generated in the toroidal divertor simulator. The electron temperature (0.1–0.4 eV) and density (∼10 18 m −3) in the detached plasmas were evaluated with a spectroscopic method using a series of deuterium Balmer line emission from highly excited levels and the Stark broadening of D(2–12). We have investigated the role of volume plasma recombination through Electron–Ion Recombination (EIR) and Molecular Activated Recombination (MAR) processes. Moreover, the carbon erosion in the detached deuterium plasma has been studied with a weight loss method. It is found that deuterium neutrals generated by EIR process could have strong influence on the carbon chemical erosion.

Effect of Fast Plasma Flows on Volume Recombination including MAR in Detached Divertor Plasma

The effect of fast plasma flows on volume recombination including molecular activated recombination (MAR) in detached divertor plasma is theoretically investigated by solving particle balance equat...

Spatially resolved spectroscopy of detached recombining plasmas in the University of Manchester Linear System divertor simulator

The University of Manchester Linear System (ULS) [M. G. Rusbridge et al., Plasma Phys. Controlled Fusion 42, 588 (2000)] is an experiment in which a steady-state plasma stream is confined along a longitudinal magnetic field. The plasma passes through a tapered orifice into a separate gas target chamber (GTC) where it interacts with neutral gas at pressures of up to 15mTorr. The upstream plasma beam is 6–14mm in diameter with electron temperatures between 2and15eV, and densities in the range of 1017–1019m−3. The primary aim of this study is to investigate physical processes relevant to gas target divertors in tokamaks. Upstream parameters, in terms of electron temperature and density, can be varied in the ULS enabling the investigation of electron-ion recombination (EIR) or molecular-activated recombination (MAR) processes. Detached recombining plasmas have been studied in the GTC using an axially scanning spectroscopic system with a spatial resolution of less than 5mm. We have investigated two distinct plasma regimes having the same electron temperature upstream, but upstream densities that differ by an order of magnitude. Vibrationally excited molecules, which are a necessary prerequisite for MAR, have been detected in both these cases. In the higher density case, which ultimately detaches via EIR processes, these excited molecules are present upstream of the EIR region.

Spatial structure of detached plasmas in the ULS divertor simulator

The UMIST Linear system (ULS) is a linear plasma device designed to study plasma and atomic physics issues relevant to tokamak divertors. The ULS produces a steady state plasma beam which interacts with neutral gas in a target chamber. Dependent on the upstream conditions, either electron-ion recombination (EIR) or molecular activated recombination (MAR) may dominate. Here we report on detailed studies of the plasma spatial structure in both regimes. A specially designed optical spectroscopy probe is used to measure the visible emission, with spatial resolution less than 5mm. Atomic and molecular lines are identified, and the results are interpreted using a collisional-radiative model, enabling the MAR rate to be calculated. One dimensional plasma modelling also demonstrates a transition between EIR and MAR dominated regimes, as the upstream conditions are varied.

Recent Progress of Boundary Plasma Modeling in Tokamak

The numerical modeling of tokamak divertor plasmas and recent related topics have been briefly reviewed. Among the various simulation models, main attention is placed on the fluid model of plasmas and the Monte-Carlo model of neutrals. The following two recent topics are mainly discussed: i) the modeling of excited neutrals and MAR (Molecular Activated Recombination), and ii) plasma flow in the detachment plasma.

Studies of detached plasmas on the ULS divertor simulator

We report on studies of detached plasma operation in the UMIST linear system (ULS). The ULS, designed to study a range of edge physics issues relevant to tokamak divertors, is capable of producing plasmas with electron densities and temperatures in the range 10 17–10 19 m −3 and 2–15 eV, respectively. Previous studies of the interaction between the hydrogen plasma and low-pressure hydrogen gas have identified a regime where molecular activated recombination (MAR) processes dominate plasma losses. Here we report on studies in which the upstream plasma parameters are varied such that three-body and radiative electron–ion recombination (EIR) dominates. Initial modelling of the recombination region is undertaken using a simplified version of the one-dimensional electron energy and continuity equations. We determine the factors that govern the threshold between MAR and EIR dominated detached regimes in terms of upstream plasma parameters and compare this with predictions based on the competing effects of electron cooling and recombination.

Generation and characteristics of detached recombining plasma and its dynamic behaviour—a bridge between fusion plasmas and low-temperature ionized gases

First, the role of a magnetic divertor in fusion devices is introduced, and it is shown that the energy balance in burning International Thermonuclear Experimental Reactor indicates a strong necessity for reduction of plasma heat load on the divertor target plate. Next, the plasma heat flow to a material surface through sheaths is generally given in terms of the energy transmission factor, in which the energy deposition based on the surface recombination is essential in high-density plasmas. The heat load is independent of any cooling of the plasmas. Then, a variety of interesting characteristics of detached recombining plasmas, which are important in reducing the plasma heat flow, are discussed: generation and structure of plasma detachment, light emission originating from a series of highly excited Rydberg states of atoms, contribution of molecular activated recombination (MAR), transition between conventional electron–ion recombination (EIR) and MAR, relation to dust coagulation processes and the experimental verification of plasma detachment in tokamak fusion devices. Finally, the dynamic behaviour of the detached EIR plasma obtained in the linear divertor plasma simulator NAGDIS-II against the ELM-like heat pulse is shown.

Static and dynamic behaviour of plasma detachment in the divertor simulator experiment NAGDIS-II

A comprehensive investigation has been performed of the static and dynamicbehaviour of detached recombining plasmas in the linear divertor plasmasimulator NAGDIS-II. For stationary plasma detachment, the transitionfrom electron-ion recombination (EIR) to molecular activatedrecombination (MAR) has been observed by injecting hydrogen gas intohigh density helium plasmas. The particle loss rate due to MAR is found tobe comparable to that of EIR. Experiments have also been performed bythe injection of a plasma heat pulse produced by RF heating into thedetached helium plasma to demonstrate the dynamic behaviour of volumetricplasma recombination. Negative spikes in the Balmer series lineemission were observed and found to be similar to the so callednegative ELM observed in tokamak divertors. Observed Balmer spectrawere analysed in detail using the collisional-radiative model. A rapidincrease of the ion flux to the target plate was observed associatedwith the re-ionization of the highly excited atoms generated byEIR.