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Plasma Chemistry in Divertor Relevant Plasmas
Abstract and Figures
One of the crucial issues to produce energy by nuclear fusion is the power exhaust problem. A significant fraction of the power generated in the plasma core is channeled through the scrape-off-layer and is delivered to the plasma-facing-components in the so-called divertor region. Divertor targets, which will be actively-cooled tungsten monoblocks in ITER, are nominally subjected to extremely high heat and particle loads, but can only tolerate up to 10 MW/m2 from a technological point of view. To achieve acceptable heat and particle flux onto the divertor targets, so-called plasma detachment is essential to be set up and controlled. Detachment is an operational regime where particular plasma conditions lead to ion recombination in the volume phase, resulting in lower particle flux, and is characterized by a plasma pressure drop along the magnetic field lines towards the target. It has been extensively proven, both theoretically and experimentally, that impurity seeding facilitates achieving detachment. However, little is known on the impurity-induced plasma chemical processes occurring in the divertor region during detachment operation. To address that, dedicated experiments in the linear plasma device Magnum-PSI have been carried out. Linear plasma machines, often referred to as divertor simulators, allow the generation of steady-state divertor-relevant plasma scenarios with great diagnostic accessibility.
Plasma detachment has been firstly achieved by H2 gas injection in the target chamber, with neutral background pressure ranging from 0.3 to 16 Pa. In this work, the influence of three different impurities i.e. N2, Ar and He on detachment performance of a hydrogen plasma is evaluated. Those species have been actively puffed in the target chamber, together with H2, at flux ratios of 0, 5, 10, 15 and 20 %. The background neutral pressures were kept fixed at 2 and 4 Pa i.e. divertor-relevant conditions. Results highlight the beneficial role of N2+H2 seeding, decreasing the plasma pressure in front of the target and reducing the heat flux delivered to it. Interestingly, when looking at plasma density and temperature profiles, we observe that electron temperature remains constant among the scans while the electron density decreases with increasing the content of N2 in the seeded mixture. This is a further indication of enhanced recombination taking place, where ions are converted to neutrals in the plasma volume phase. An opposite trend is found concerning He and Ar. In fact, injection of H2+He and H2+Ar gas mixtures led to an increased heat flux in both cases compared to only H2. A Residual-Gas-Analyzer (RGA) has been used to study the conversion efficiency of N2 to ammonia, showing conversions between 3 and 5 %. The molecular emission band at 336 nm, corresponding to the electronic transition of NH*(A3Π→ X3Σ, Δv = 0), has been observed with optical emission spectroscopy and its intensity increases linearly with the flux ratio of N2 in the seeded mixture. Moreover, plasma radiation has been monitored by using a bolometry system. No significant trends as a function of injected nitrogen have been observed, hence excluding any power limitation effects.
Similar experiments have been carried out with GAMMA10/PDX, a linear plasma machine located at the University of Tsukuba. The uniqueness of that machine lays in the capability of achieving high electron temperatures, albeit electron densities lower by two orders of magnitude compared to Magnum-PSI. Those experiments allowed us to investigate the effect of impurities on detached-like plasmas in a wider range of parameters. Results are in line with what has been achieved in Magnum-PSI, highlighting H2+N2 gas as the most favorable mixture to reduce the particle flux to the target.
Numerical simulations are needed to provide a comprehensive understanding of the fundamental atomic and molecular processes occurring in divertor-like environment. A three-step approach has been adopted as follows: at first, global plasma models have been set-up on the basis of Plasimo code.
Global models are spatially-averaged simulations allowing one to implement a large set of plasma chemical equations and to highlight the most relevant processes among them. Extended models have been built up for N2-H2, Ar-H2 and He-H2 plasma scenarios. This study shows two main nitrogen-included recombination reaction paths resulted to be dominant, i.e. the ion conversion of NH followed by dissociative recombination and a proton transfer between H2+ and N2, producing N2H+. These two processes are referred to as N-MAR (nitrogen-molecular activated recombination). Concerning the remaining two cases, no significant ion recombination process seems to be driven by the presence of either Ar and He. The resulting reduced scheme for H2-N2 chemistry has been implemented in Eunomia, a spatially-resolved Monte Carlo code suited for the transport of neutrals in linear plasma devices. Finally, Eunomia has been coupled with B2.5, a fluid code solving plasma equations. Simulation results of all the three cases of study (H2-He, H2-N2, H2-Ar) qualitatively reproduce the favorable effect of N2, while confirming the deteriorating effect of He and Ar on detachment performance. The importance of NH as electron donor is highlighted and N-MAR confirmed as reaction route enhancing the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable.
This work represents a further step towards the full understanding of the role of plasma chemical volume processes in a detached divertor plasma.
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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. Keywords: GAMMA 10/PDX, Divertor-simulation experiment, Plasma detachment, Molecular activated recombination, Molecular vibrational temperature, Population inversion
The Magnum-PSI facility is unique in its ability to produce and even exceed the heat and particle fluxes expected in the divertor of a fusion reactor, combined with good access to the plasma-material interaction region for diagnostics and relatively easy sample manipulation. In addition, it is possible to study the effects of transient heat loads on a plasma-facing surface, similar to those expected during so called Edge Localized Modes. By virtue of a newly installed superconducting magnet, Magnum-PSI can now maintain these conditions for hours on end for truly long term tests of candidate plasma facing materials. The electron density and temperature in the plasma beam center as a function of different magnetic fields up to 1.6 T, gas flow and source current are determined: particle fluxes greater than 10 ²⁵ m ⁻² s ⁻¹ and heat fluxes of up to 50 MW m ⁻² are obtained. Linear regression and artificial neural network analysis have been used to gain insight in the general behavior of plasma conditions as a function of these machine settings. The plasma conditions during transient plasma heat loading have also been determined. These capabilities are now being exploited to reach fluence of up to 10 ³⁰ particles m ⁻² at ITER-relevant conditions, equivalent to a significant fraction of the divertor service lifetime for the first time.
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 ∼ 10^17 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.
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 H 2 /N 2 puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n e are ITER-relevant i.e. above 10 20 m −3 and electron temperatures are in the range 0.8-2 eV. A scan among five different N 2 /H 2 +N 2 flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n e decrease while increasing the fraction of N 2 has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. T e remains constant among all scans. The peak intensity of NH*(A 3 ∏->X 3 ∑ − , ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N 2 content, together with the NH 3 signal in the RGA. A further dedicated experiment has been carried out by puffing separately H 2 /N 2 and H 2 /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 N 2 , we observe an opposite behavior , indicating that NeH 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 NH x species behaving as electron donor in the ion conversion with H + by means of what we define here to be N-MAR i.e. NH x + H + → NH x + + H, followed by NH x + + e − → NH x-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.
The process of divertor detachment, whereby heat and particle fluxes to divertor surfaces are strongly reduced, is required to reduce heat loading and erosion in a magnetic fusion reactor. In this thesis the physics leading to the decrease of the divertor ion current (It), or ‘roll-over’, is experimentally explored on the TCV tokamak through characterization of the location, magnitude and role of the various divertor ion sinks and sources including a complete measure of particle and power balance. These first measurements of the profiles of divertor ionisation and hydrogenic radiation along the divertor leg are enabled through the development of a TCV divertor spectrometer, together with careful Stark broadening analysis and novel Balmer line spectroscopic techniques.
Over a range in core plasma conditions (plasma current, impurity-seeding, density) the It roll-over is caused by a drop in the divertor ion source; recombination remains either small or negligible until later in the detachment process. In agreement with simple analytical predictions, this ion source is limited by a reduction in the power available for ionisation, Precl, sometimes characterised as ‘power starvation’. Concurrent increases in the energy required per ionisation, Eion, further reduce the number of ionizations. The detachment threshold is found experimentally (in agreement with analytic model predictions) to be Precl/(It Eion) < 2, corresponding to the target electron temperature, Tt ~ Eion/gamma, where gamma is the sheath transmission coefficient. Target pressure loss, required for target ion current loss, is observed to be delivered by both volumetric momentum loss, as typically assumed, and by a drop of the upstream pressure.
The evolution of measured divertor profiles through detachment of the various ion sources/sinks as well as power losses and charge exchange are quantitatively reproduced through full 2D SOLPS modelling of a ramp of core plasma density through the detachment process.
Full power operation of the International Thermonuclear Experimental Reactor (ITER) has been delayed and will now begin in 2035. Delays to the ITER schedule may affect the availability of tritium for subsequent fusion devices, as the global CANDU-type fission reactor fleet begins to phase out over the coming decades. This study provides an up to date account of future tritium availability by incorporating recent uncertainties over the life extension of the global CANDU fleet, as well as considering the potential impact of tritium demand by other fusion efforts. Despite the delays, our projections suggest that CANDU tritium remains sufficient to support the full operation of ITER. However, whether there is tritium available for a DEMO reactor following ITER is largely uncertain, and is subject to numerous uncontrollable externalities. Further tritium demand may come from any number of private sector “compact fusion” start-ups which have emerged in recent years, all of which aim to accelerate the development of fusion energy. If the associated technical challenges can be overcome, compact fusion programmes have the opportunity to use tritium over the next two decades whilst it is readily available, and before full power DT operation on ITER starts in 2035. Assuming a similar level of performance is achievable, a compact fusion development programme, using smaller reactors operating at lower fusion power, would require smaller quantities of tritium than the ITER programme, leaving sufficient tritium available for multiple concepts to be developed concurrently. The development of concurrent fusion concepts increases the chances of success, as it spreads the risk of failure. Additionally, if full tritium breeding capability is not expected to be demonstrated in DEMO until after 2050, an opportunity exists for compact fusion programmes to incorporate tritium breeding technology in nearer-term devices. DD start-up, which avoids the need for external tritium for reactor start-up, is dependent upon full tritium breeding capability, and may be essential for large-scale commercial roll-out of fusion energy. As such, from the standpoint of availability and use of external tritium, a compact route to fusion energy may be more advantageous, as it avoids longer-term complications and uncertainties in the future supply of tritium.
Account of drifts and currents dramatically decreases the accessible time step for the integration of time dependent equations of SOLPS-ITER code for edge modeling. Running the code with sophisticated EIRENE Monte-Carlo model for neutrals and large number of fluid equations for multiple ion species makes the computation time unacceptably long. In the paper the main mechanisms leading to the time step limitations caused by drifts are discussed. Several methods of the suppression of these mechanisms are suggested and the results of numerical scheme tests with the applied corrections are presented. Application of these schemes decreases the time of convergence to steady state solution by more than an order of magnitude.
The reaction of spillover hydrogen (SH) with cyclopropylglycine (cPG) in high-temperature solid-state catalytic hydrogen isotope exchange (HTCIE) has been studied experimentally and using density functional theory (DFT). NMR spectroscopy and mass spectrometry have shown a high regioselectivity and stereoselectivity for the reaction of spillover hydrogen with cPG fragments. The Gly fragment has been shown to be the most reactive in HTCIE, for which high stereoselectivity of substitution of hydrogen by deuterium has been demonstrated.
Predictions for the operation of tokamak divertors are reliant on edge plasma simulations typically consisting of a fluid plasma code in combination with a Monte-Carlo (MC) code for neutral species. Pilot-PSI is a linear device operating with a cascaded arc plasma source that produces plasmas comparable to those expected during the inter-ELM phase in the ITER divertor (T e ∼ 1 eV, n e ∼ 10²⁰ m⁻³). In this study, plasma discharges in Pilot-PSI have been modelled using the Soledge2D fluid plasma code (Bufferand et al 2015 Nucl. Fusion 55) coupled to the Eirene neutral MC code (Reiter et al 2005 Fusion Sci. Technol. 47, 172-186) in order to (a) investigate which phenomena need to be included in the modelling to reproduce experimental trends and (b) provide new insights to the interpretation of experiments. The simulations highlight the key role of ion/molecule elastic collisions in determining the ion flux reaching the target. Recombination is likely to play a role at high molecular background pressure. However, even with the most advanced atomic and molecular model used in this work, T e at the target is overestimated with respect to the measurements using TS and spectroscopy. T e in the simulations appears to saturate at 0.7 eV for a wide range of parameters, while experimentally values of 0.1-0.3 eV are found. As a consequence, in the simulations the volume recombination is underestimated, which is a strong function of T e when it is below 1 eV. Further analysis of simulation results using a two-point formalism shows that inelastic collisions between electrons and neutral background particles remove most of the energy flux, mainly via dissociation of molecules and molecular ions. However this happens mostly in the upstream region of the beam where T e > 1 eV. For T e < 1 eV, there seems to be no significant energy removal mechanism in the simulated cases. The results also indicate that conclusions on the importance of volume processes, e.g. recombination, cannot be solely based on T e or the dominance of certain reaction rate coefficients over others, but rather the complete transport picture, including macroscopic flow, has to be taken into account. In the cases studied here, the plasma is typically advected to the wall too fast for recombination to remove a significant fraction of the particle flux. © 2018 DIFFER - Dutch Institute for Fundamental Energy Research.