Edge Plasma Temperature Research Articles

Optimization of divertor design for Pakistan spherical tokamak

Abstract Handling the power deposition, reducing erosion effects, and plasma configuration are the key factors in the design of a divertor. The design of Pakistan Spherical Tokamak (PST) is based on double-null divertor configuration with actively cooled graphite targets at outer/inner strike point and peak heat flux range capacity of 0.1–0.3 MW/m2. The configuration of PST divertor module is designed with mock-up (used flat type tiles on baffles and dome) and cassette (support PFC and cooling channels) technology. Helium-cooled stage and water-cooled stage are two options for divertor. Therefore, one part of this research is focused on water-cooling system for the divertor. This paper presents the divertor design for PST with cooling channel and material analysis of the divertor, which is carried out in three phases. In the first phase, the plasma edge temperature, density, particle velocity, input power, heat flux, and surface temperature are estimated. In second phase, physics and engineering design of divertor system has been performed. In the third phase, COMSOL simulation has been performed to analyses the material properties, surface temperature rise (∆T °C) at stable heat flux, and thermal hydraulic system for the divertor. It is found from the analysis that the specific heat flux of 0.3 MW/m2 up to 3 s is the safe zone limit. The R & D work ratifies that manufacturing and installation processes are plausible for the proposed divertor design. This design is able to meet the requirement of PST. However, increasing time or specific heat flux beyond these limits would require redesigning of the cooling channel.

Determination of limiter design and material composition of MT-II spherical tokamak

Abstract MT-II is a spherical tokamak with a major radius of 0.15 m and a minor radius of 0.09 m, currently under development at the Pakistan Tokamak Plasma Research Institute. It is designed with a higher elongation of 2.67. This paper presents the design and material analysis of the limiter configuration for the MT-II tokamak, which is being carried out in two phases. In the first phase, theoretical studies and calculations are performed to estimate the plasma edge temperature, density, particle velocity, input power, heat flux, heat load and surface temperature on the limiter tile. In the second phase, computational techniques are applied to analyses the material properties, the maximum/minimum surface temperature rise (∆T °C) at stable heat load and power deposition based on theoretical calculations that will help optimize the design parameters of the limiter. The type of material and the surface temperature of the limiter as well as the general design parameters of MT-II are included in the proposed poloidal limiter. The results suggest that crystalline vein graphite is a suitable candidate for the proposed poloidal limiter. A combination of mechanical and electrical feedthrough techniques are used to improve the performance of the limiter. The proposed limiter is able to meet the requirements of MT-II.

Interstitial hydrogen atoms in W-nitride compounds promoting the formation of atomic vacancies in nuclear fusion reactors

Nitrogen gas seeding has been proposed as a feasible strategy to reduce the edge plasma temperature in nuclear fusion reactors, which could cause the W-nitride compounds to form on the surface layer of the tungsten (W) first wall. Many point defects can be generated in the W-nitride compounds under the extreme fusion environment. By performing first-principles calculations, we investigate the interaction of interstitial hydrogen (H) atoms with point defects in W1N1, W2N1 and W2N3, respectively. It is found that the dissolved H atoms can assist the formation of vacancies in W-nitride compounds. And this correlates with the lattice distortion and charge redistribution induced by interstitial H atoms. In addition, studies on the behavior of H migrating to vacancies show that vacancy-containing W-nitride compounds can effectively trap H atoms, thereby reducing the retention of H atoms in the W first wall. Furthermore, calculation results about the interaction of H with interstitial atoms indicate that the predominant interaction between H and interstitial atoms characterizes the feature of repulsion in the concerned systems. Our work not only reveals the details about the interaction between interstitial H atoms and atomic vacancies in W-nitride compounds, but also provides a fundamental insight into understanding the interactions of intersitial H atoms with vacancy in W-nitride compounds.

Type-I ELM mitigation by continuous lithium granule gravitational injection into the upper tungsten divertor in EAST

Large edge-localized modes (ELMs) were mitigated by gravitational injection of lithium granules into the upper X-point region of the experimental advanced superconducting tokamak (EAST) device with tungsten plasma-facing components. The maximum ELM size was reduced by ∼70% in high β N H-mode plasmas. Large ELM stabilization was sustained for up to about 40 energy confinement times, with constant core radiated power and no evidence of high-Z or low-Z impurity accumulation. The lithium granules injection reduced the edge plasma pedestal density and temperature and their gradients, due to increased edge radiation and reduced recycling from the plasma-facing components. Ideal stability calculations using the ELITE code indicate that the stabilization of large ELMs correlates with improved stability of intermediate-n peeling-ballooning modes, due to reduced edge current resulting from the profile changes. The pedestal pressure reduction was partially offset by a core density increase, which resulted in a modest ∼7% drop in core stored energy and normalized energy confinement time. We surmise that the remnant small ELMs are triggered by the penetration of multiple Li granules just past the separatrix, similar to small ELMs triggered by deuterium pellet Futatani et al (2014 Nucl. Fusion 54 073008). This study extends previous ELM elimination with Li powder injection Maingi et al (2018 Nucl. Fusion 58 024003) in EAST because (1) use of small, dust-like powder and the related potential health hazards were eliminated, and (2) use of macroscopic granules should be more applicable to future devices, due to deeper penetration than dust particles, e.g. inside the separatrix with velocities ∼10 m s−1 in EAST.

Influence of nitrogen on deuterium retention in tungsten under sequential and simultaneous irradiation

Nitrogen is a candidate for impurity seeding to reduce the edge plasma temperature for ITER’s tungsten divertor. Radiation characteristics and plasma performance are improved with N compared to other candidates neon and argon, however questions remain in terms of how the introduction of N might impact deuterium retention in W. The current study compares the influence of N on D retention in W with sequential (SEQ) and simultaneous (SIM) irradiation of D-3%N at 300–700 K with energies 500 eV/D+ and 1000 eV/N+. Thermal desorption spectroscopy (TDS) is used to measure the D retained, and X-ray photoelectron spectroscopy (XPS) is used to investigate nitride formation at different implantation temperatures. The XPS results show that for the beam composition of this study, the removal of N by D is the dominant interaction, working against N retention in W. Differences found between the N layer with D/N co-bombardment vs. N pre-irradiation might be worth considering when extrapolating sequential experiments to reactor conditions. The observed effect of the N-containing layer on the temperature dependence of deuterium release during TDS supports the XPS findings, suggesting that the phases of the W:N layer produced are different at different temperatures. It is found that SIM irradiation resulted in an overall increase (up to a factor of ∼4) in total D retention compared to SEQ and D-only experiments above 500 K. At 300–500 K, the D retention was not significantly changed by nitrogen pre-, post- or simultaneous irradiation.

The mechanism on retention of hydrogen in three representative tungsten nitride compounds in nuclear fusion reactors

Nitrogen gas seeding is considered as an effective strategy to reduce the edge plasma temperature. The tungsten nitride compounds, which could form on the top of tungsten (W) first wall due to nitrogen gas seeding, may act as a barrier to hinder the diffusion of hydrogen (H) from the tungsten nitride layer to W first wall. By performing first-principles calculations, we investigate the dissolution, diffusion and retention behaviors of H in W2N1, W1N1 and W2N3, respectively. It is found that a H atom prefers to dissolve in tungsten nitride compounds than in W crystal. Hence the compounds can act as a barrier against H diffusion to W first wall, and probably reducing the hydrogen retention in W first wall. All of these behaviors are essentially correlated with the formation of strong N-H bonding in the compounds once H atoms dissolve in them. Moreover, our calculations indcate that H atoms do not form hydrogen molecules even within a vacancy, but stay separately from each other due to the formation of strong N-H bonding. Our work not only proposes that tungsten nitride compounds can significantly reduce hydrogen retention in W first wall, but provides a fundamental insight in understanding the interactions of H with tungsten nitride compounds.

Effect of Biasing Voltage on Hydrogen (Hβ) Temperature in IR-T1 Tokamak, Using Spectroscopy

In this research, the influence of the biasing voltage on hydrogen temperature and plasma confinement is investigated in the IR-T1 tokamak. The applied voltage was in the range of − 250 to + 250 V. Hydrogen (Hβ) temperature was estimated using measurements provided by the optical system based on a two-channel AvaSpec-ULS3648TEC-2-RS-USB2 visible light spectrometer on IR-T1 tokamak. The FWHM of Hβ spectral line was determined by normalizing and fitting the Gaussian distribution function to the experimental data. The hydrogen temperature was calculated in each state (Vbiasing = − 250 V and Vbiasing = + 250 V) at the plasma edge of IR-TI tokamak, using the effects of Doppler and instrument broadenings. Finally, the results were compared with the normal state (Vbiasing = 0 V). The spectroscopic results show a significant decrease in the spectral emission and subsequently improvement in confinement for positive biasing and significant decrease in plasma-edge temperature for negative biasing. These results illustrate the effect of radial induced electric field on plasma confinement.

The role of excited-state ionization on the effective ionization of neutral neon using non-perturbative atomic data

With the aim of providing accurate atomic data for plasma edge temperature and density modeling with neutral neon, new non-perturbative R-Matrix with Pseudostates (RMPS) calculations are performed for both the electron-impact excitation and ionization of neutral neon. The new calculations agree well with past measurements and calculations for both electron-impact excitation and ionization. For ionization, the addition of pseudostates from to allows us to extract new cross sections and rate coefficients for all of the excited states through 2p5 5p. The new RMPS ionization calculation gives a 20% increase in the cross section for the ionization compared with previous non-perturbative calculations and shows classically-expected n4-scaled convergence for the np and nd valence shells. Ionization from these excited states contributes significantly to the effective ionization rate coefficient: a factor of three more than the ground-state ionization rate coefficient at an electron temperature of 25 eV and density of 1014 cm−3. An accurate, non-perturbative set of atomic rates, both LS- and LSJ-resolved, for neutral neon across a wide range of temperatures and densities is compiled.

Probe manipulators for Wendelstein 7-X and their interaction with the magnetic topology

Probe manipulators are a versatile addition to typical plasma edge diagnostics. Equipped with material samples they allow for detailed investigation of plasma–wall interaction processes, such as material erosion, deposition or impurity transport pathways. When combined with electrical probes, a study of scrape-off layer and plasma edge density, temperature and flow profiles as well as magnetic topologies is possible. A mid-plane manipulator is already in operation on Wendelstein 7-X. A system in the divertor region is currently under development. In the present paper we discuss the critical issue of heat and power loads, power redistribution and experimental access to the complex magnetic topology of Wendelstein 7-X. All the aforementioned aspects are of relevance for the design and operation of a probe manipulator in a device like Wendelstein 7-X. A focus is put on the topological region that is accessible for the different coil current configurations at Wendelstein 7-X and the power load on the manipulator with respect to the resulting different magnetic configurations. Qualitative analysis of power loads on plasma-facing components is performed using a numerical tracer particle diffusion tool provided via the Wendelstein 7-X Webservices.

High frequency magnetic fluctuations correlated with the inter-ELM pedestal evolution in ASDEX Upgrade

In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response.

Effects of Nitrogen Concentration on Microstructure of Tungsten Coatings Synthesized by Plasma Sputtering Method

The use of nitrogen seeding to reduce the edge plasma temperature in tokamak raises concern regarding the possible formation of tungsten nitride layers with different properties (melting temperature, hardness, retention, etc.) with respect to the W. Dedicated laboratory experiments are performed to investigate the interaction of W with N plasmas. In this paper, the structure and mechanical performance of WNx coatings with different nitrogen content prepared by rf plasma sputtering have been investigated. The coatings exhibited different phases as a function of the nitrogen content: films with very low N content (<1 atom%) mainly exhibited the W phase, the W and WNx phases were contemporaneously present for N contents in the range 1–5 % and only the WNx phases were observed for nitrogen content higher than 5 %. The mechanical performance of the WNx coatings was strongly influenced by the structure. The coating formed with 5 % of nitrogen showed the highest mechanical performance. The substrate heating during the plasma deposition has also improved the mechanical properties.

Effects of nitrogen on hydrogen retention in tungsten: First-principles calculations

While the use of nitrogen seeding to reduce the edge plasma temperature has been successfully applied in many tokamak experiments, questions remain as to effects of the nitrogen-enriched layer on hydrogen retention in tungsten. In this paper, we investigate the influence of nitrogen on hydrogen dissolution and diffusion behavior in tungsten using the first-principles. The nitrogen has little effect on the dissolution of hydrogen in the interstitial site but significantly impedes the accumulation of hydrogen in the vacancy, leading to the decrease of hydrogen retention in nitrogen-enriched layer. Meanwhile, both the interstitial nitrogen and the vacancy–nitrogen complex can significantly reduce the hydrogen effective diffusivity. This suggests that the nitrogen-enriched layer forming on the tungsten surface can act as a diffusion barrier for the re-emission of implanted hydrogen, enhancing hydrogen diffusion into the bulk, and consequently, increasing of hydrogen retention in bulk. These results provide a sound explanation for the recent experimental results.

Spatiotemporal response of plasma edge density and temperature to non-axisymmetric magnetic perturbations at ASDEX Upgrade

Non-axisymmetric magnetic perturbations (MPs) were successfully applied at ASDEX Upgrade to substantially reduce the plasma energy loss and peak divertor power load that occur concomitant with type-I edge localized modes (ELMs). The response of electron density edge profiles and temperature and pressure pedestal-top values to MPs are reported. ELM mitigation is observed above an edge density threshold and independent of the MPs being resonant or non-resonant with the edge safety factor. The edge electron collisionality appears not to be appropriate to separate mitigated from non-mitigated discharges for the present high-collisionality plasmas. No significant change in the position or gradient of the edge density profile could be observed for the transition into the ELM-mitigated phase, except from the effect of the three-dimensional MP field which leads to an apparent profile shift. An increase in the density and decrease in the temperature at the pedestal-top balance such that the pressure saturates at the value of the pre-mitigated phase. The plasma stored energy, the normalized plasma pressure, and the H-mode quality factor follow closely the evolution of the pedestal-top pressure and thus remain almost unaffected. The temporal evolution of the ion effective charge shows that the impurity content does not increase although flushing through type-I ELMs is missing. The type-I ELMs are replaced in the mitigated phase by small-scale and high-frequency edge perturbations. The effect of the small bursts on the density profile, which is correlated with a transient increase of the divertor thermoelectric current, is small compared with the effect of the type-I ELMs. The residual scatter of the profiles in the mitigated phase is small directly after the transition into the ELM-mitigated phase and increases again when the pressure saturates at the value of the pre-mitigated phase.

ELM as a Trigger Mechanism for the Transition Between Two Edge Regimes

AbstractThe possibility of edge regime transitions under the influence of edge‐localized modes (ELMs) is investigated. A theoretical model of the process is presented and examined numerically. ELMs appear able to trigger irreversible transitions from one equilibrium to another with different edge temperatures. The results correspond to the recent experimental studies on both DIII‐D and JET tokamaks showing type‐I and type‐II ELMs causing a prolonged collapse of the edge plasma temperature (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interaction of high flux deuterium/nitrogen plasmas with beryllium

Before nitrogen can be used as a radiator for edge plasma temperature control in experiments with beryllium as the wall material, the compatibility of nitrogen-containing plasma with beryllium has to be tested. Therefore beryllium samples were exposed to a variety of mixed N2/D2 plasmas in PISCES-B and codeposits from the sputtered material were collected. It was found that introducing N2 to a D2 plasma reduces Be erosion significantly but recovery to the pre-N2 levels is possible in pure D2 plasma. Berylliated vessel walls can be a reservoir for N2 and chemical processes probably play a significant role during nitriding and N2 removal. Nitrided target samples remain conductive and do not lead to additional arcing, but codeposits are insulating. Thermal desorption measurements of nitrided and un-nitrided target samples are comparable, while codeposits show a slightly reduced D retention. However, D release for both target and codeposits is shifted ≈100 K to higher temperatures, above 510 K.

Two-dimensional imaging of edge plasma electron density and temperature by the passive helium emission ratio technique in TJ-II

An intensified visible camera looks tangentially at a poloidal limiter where helium recycles, acting as a wide neutral source, and the atomic line emission due to plasma excitation becomes strongly localized there. It includes a bifurcated coherent bundle, each end with a different interference filter to select helium atomic lines, so that two simultaneous filtered images are captured in one single frame. The object of the proposed technique is to apply the well-known helium-beam line-ratio technique to obtain from selected filtered images the two-dimensional (2D) edge plasma ne and Te. The code EIRENE was used to demonstrate that the helium emission from recycling neutrals dominates the emission for the lines of view passing close above the limiter. Since these chords are nearly parallel to magnetic field lines in the emission region, the images can be approximated to poloidal cuts of the plasma emission within the tolerances discussed in the paper. The absolute radial profiles of Te and ne obtained with the method presented here were checked in the TJ-II stellarator to be in relatively good agreement with other diagnostics within a wide range of plasma parameters for both ECRH and NBI plasmas. The method is finally used to get 2D images of edge plasma Te and ne.

Impacts of pellets injected from the low-field side on plasma in ITER

Impacts of pellets injected from the low-field side (LFS) on plasma in ITER are investigated using the 1.5D BALDUR integrated predictive modeling code. In these simulations, the pellet ablation is described using the neutral gas shielding (NGS) model. The pellet ablation model is coupled with the plasma core transport model, which is a combination of the MMM95 anomalous transport model and NCLASS neoclassical transport model. The boundary conditions are assumed to be at the top of the pedestal, in which the pedestal parameters are predicted using a pedestal model based on the theoretical-based pedestal width scaling (either magnetic and flow shear stabilization width scaling, or flow shear stabilization width scaling, or normalized poloidal pressure width scaling) and the infinite-n ballooning mode pressure gradient limit. These pedestal models depend sensitively on the density at the top of the pedestal, which can be strongly influenced by the injection of pellets. The combination of the MMM95 and NCLASS models, together with the pedestal and NGS models, is used to simulate the time evolution of the plasma current, ion and electron temperatures, and density profiles for ITER standard type-I ELMy H-mode discharges during the injection of LFS pellets. It is found that the injection of pellets results in a complicated plasma scenario, especially in the outer region of the plasma and the plasma conditions at the boundary in which the pellet has an impact on increasing the plasma edge density, but reducing the plasma edge temperature. The LFS pellet has a stronger impact on the edge as compared to the center. For fusion performance, the pellet can result in either enhancement or degradation, depending sensitively on the pellet parameters; such as the pellet size, pellet velocity, and pellet frequency. For example, when a series of deuterium pellets with a size of 0.5 cm, velocity of 1 km/s, and frequency of 2 Hz are injected into the ITER plasma from the LFS, the plasma performance, evaluated in terms of Q fusion, can increase to 72% of that before the use of pellets. It is also found that the injection of pellets results in an increase in the ion and electron densities, but does not enhance the central plasma density. On the other hand, it results in the formation of another peak of the plasma density in the outer region near the plasma edge. The formation of the density peak results in the reduction of plasma transports near the edge by decreasing the contributions of ion-temperature-gradient and trapped electron modes, as well as kinetic ballooning modes.

Lower hybrid current drive for the steady-state scenario

Lower hybrid current drive (LHCD) experiments performed at a density close to that required for the steady-state (SS) scenario are reported. On C-Mod, FTU and Tore Supra, a strong decay of the bremsstrahlung emission is observed when the density is increased, much faster that the prediction of LHCD modelling. On JET, LH power deposition is also found to be sensitive to the plasma density: LH power modulation indicates that the power deposition moves to the very edge of the plasma (r/a ∼ 0.9) when the density approaches the requirement of the JET SS scenario. From this experiment but also from the reconstruction of the electron cyclotron emission spectrum, the decrease in the LHCD efficiency with density is also found. From LHCD modelling of different JET pulses performed at different densities and wave parallel refraction indexes, it is concluded that the wave accessibility condition is not the key parameter for explaining the decrease in the efficiency. C-Mod, FTU and Tore Supra experiments indicate that the plasma edge parameters, namely density and temperature but also fluctuations, are affecting the efficiency via loss mechanisms which are likely to be collisional damping (C-Mod), parametric decay instabilities or wave scattering (FTU/Tore Supra).

Rotation dependence of a phase delay between plasma edge electron density and temperature fields due to a fast rotating, resonant magnetic perturbation field

Measurements of the plasma edge electron density ne and temperature Te fields during application of a fast rotating, resonant magnetic perturbation (RMP) field show a characteristic modulation of both, ne and Te coherent to the rotation frequency of the RMP field. A phase delay Φ between the ne(t) and Te(t) waveforms is observed and it is demonstrated that this phase delay Φ is a function of the radius with Φ(r) depending on the relative rotation of the RMP field and the toroidal plasma rotation. This provides for the first time direct experimental evidence for a rotation dependent damping of the external RMP field in the edge layer of a resistive high-temperature plasma which breaks down at low rotation and high resonant field amplitudes.

Interaction of nitrogen plasmas with tungsten

The use of nitrogen seeding to reduce the edge plasma temperature has recently been successfully applied in ASDEX Upgrade. While the plasma performance was significantly improved compared with other seeding species such as Ar or Ne, questions remained as to the interaction of nitrogen with a tungsten first wall. In particular the formation of thick tungsten nitride layers with reduced melting temperature and increased physical sputtering was a concern. Therefore dedicated laboratory experiments have been performed to investigate the interaction of W surfaces with N plasmas. Tungsten coated Si samples were exposed to N ions from plasma and ion gun sources at energies from 20 eV to 10 keV and W surface temperatures from 300 to 750 K. After exposure to the N plasma with fluences of up to several 1023 N m−2 the N content in the samples was measured by nuclear reaction analysis. The sputter erosion was determined by measuring the thickness change of the W layer by Rutherford backscattering. The formation of W-nitride phases was investigated in separate XPS experiments where the samples were implanted in situ with kiloelectronvolt N ions. It was found that only very small fractions of N are accumulated on the W surface and that N is bound in a nitride state. At temperatures above 600 K the nitrides are no longer stable which further reduces the N uptake into the W. Moreover the accumulation of N on the surface leads to a decrease in W physical sputtering due to the lower W concentration at the surface.