Li Pellet Research Articles

Simulation of triggering and evolution of ELM by pellet injection in EAST under BOUT++ framework

A BOUT ++ three-field magnetohydrodynamic model is employed to study the triggering and evolution of edge localized mode (ELM) by Li pellets injected along the outer mid-plane in the EAST configuration. The linear simulation shows that compared with a large deposition on the pedestal top (scenario I), a smaller deposition within the steep-gradient pedestal region (scenario II) can stimulate much larger linear growth rates of all-n peeling-ballooning modes (PBMs). The nonlinear simulation shows that there exists a pellet size threshold for ELM triggering for two deposition locations; the threshold for scenario I predicted in the present study matches the EAST observation well. Comparison of the two scenarios reveals that a smaller deposition is sufficient to trigger an ELM in a much shorter time in scenario II, whose ELM size is comparable to that in scenario I. This conclusion confirms previous DIII-D and ASDEX-Upgrade observations, suggesting that the steep-gradient pedestal region is a favorable deposition location for ELM triggering with minimum pellet size. Simulation analyses also find that the positive radial gradient of the hump-like pressure profile in the outer mid-plane induced by the pellet deposition plays a different role in the two scenarios. In scenario I, the force resulting from the gradient hinders the outflow of core plasmas and in return, the perturbation is suppressed from spreading inwards after ELM crashes. In scenario II, with a sizable deposition, the gradient results in another competitive perturbation growth region during the linear phase, thus dispersing the free energy and reducing the efficiency of destabilizing PBMs by pellet injection. The suppressing effect of saturated zonal flow on other modes, the short ELM fast crash phase, and the restricting transport effect of the positive radial pressure gradient work together to constrain the pedestal energy loss, especially when the pellet deposition amount is high.

Numerical simulation of fueling pellet ablation and transport in the EAST H-mode discharge

To understand the effect of injected deuterium (D) pellets on background plasma, the ablation of D pellets and the transport of D species in both atomic and ionic states in the EAST device are simulated using a modified dynamic neutral gas shield model combined with the edge plasma code SOLPS-ITER. The simulation results show that there is a phenomenon of obvious atomic deposition in the scrape-off layer (SOL) after pellet injection, which depends strongly on the injection velocity. With increasing injection velocity, the atomic density in the SOL decreases evidently and the deposition time is relatively shortened. Possible effects for triggering of edge localized modes (ELMs) by D and Li pellets are also discussed. With the same pellet size and injection velocity, the maximum perturbation pressure caused by D pellets is obviously higher. It is found that the resulting maximum perturbed pressure is remarkably enhanced when the injection velocity is reduced from 300 m/s to 100 m/s for a pellet with a cross section of 1.6 mm, which indicates that the injection velocity is important for ELM pacing. This work can provide reasonable guidance for choosing pellet parameters for fueling and ELM triggering.

Design of a centrifugal acceleration system for high velocity injection of impurity pellet into EAST tokamak

A centrifugal pellet injection system for high velocity impurity pellet injection to control ELMs and research impurity transport has been developed for the Experimental Advanced Superconducting Tokamak (EAST). The system is composed of the dropper, the centrifugal accelerator, vacuum pumping system and the diagnostic unit. The centrifugal accelerator is designed to accelerate the pellet by the rotating arm driven by a KYKY F100 turbomolecular pump with an adjusted rotating velocity from 0 to 42,300 rpm. In order to evaluate the stability of the rotating arm, the stress distribution of the rotating arm at different rotating velocities is analyzed by the stress field simulation, and the result shows the maximum stress of the rotating arm appears at the transition position between the large circular groove and the acceleration groove. By simulation analysis, it is noted that the arm can work steadily at 2500 rad/s (corresponding to about 398 Hz arm rotation frequency) rotating velocity and may be broken at 4000 rad/s (637 Hz). By the bench testing using the Li pellets of 1 mm diameter as injection material, it shows the centrifugal accelerator can work steadily for a long time under the condition that the rotating velocity doesn't exceed 2200 rad/s (350 Hz) and the maximum velocity of the pellet can reach 286 m/s. Based on the results of simulation and bench test, the centrifugal pellet injection system was installed and then injected Li pellets into the plasma steadily and continuously with the rotating velocity <2200 rad/s on EAST.

SIMS and HR-XPS characterization of lithiated graphite from the magnetic fusion device RFX-mod

Lithium wall conditioning has improved the performance of many magnetic fusion devices. Li conditioning in the RFX-mod device was performed by: (1) a single Li pellet injector and (2) a multi-Li pellet injector during He plasma discharges; (3) a Li evaporator after He glow discharge cleaning and He plasma discharge exposure. This report compares the spatial and depth distributions of Li deposited on polycrystalline graphite witness samples at different locations in RFX-mod and the elemental and chemical compositions of the resulting surfaces. The sample surfaces were analyzed ex situ using secondary ion mass spectrometry (SIMS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS). The results showed that Li pellet injection provided a relatively uniform toroidal coverage while Li evaporation produced highly localized Li deposition. A Li 1s HR-XPS peak at 56.5 eV binding energy (BE) characteristic of lithium-intercalated graphite was only observed with the sample exposed to Li evaporation. All of the samples exhibited a HR-XPS C 1s peak at 285.1–285.2 eV BE that is largely attributed to hydrogenated graphite. This finding suggests that hydrogenation of fresh graphite occurs during He plasma discharge exposures. Our results have implications for density control and the selection of Li conditioning techniques in magnetic fusion devices.

An in-pile experimental loop for the irradiation of tritium breeding ceramics in China Mianyang research reactor (CMRR)

• An in-pile irradiation loop has been established in CMRR for in-situ tritium extraction study. • It allows loading or unloading the breeder pellets online by a special designed pneumatic system. • Several typical experiments have been performed to demonstrate its capability. A new in-pile experimental loop has been established in CMRR for ceramic tritium breeder irradiation. The most typical of this loop is the capability of online reloading of irradiated samples. The irradiation temperature in the pellets bed can be controlled within the range of 300～750 °C, with a spatial inhomogeneity of ±10 °C. Up to now, a series of irradiation experiments have been carried out on three different breeder materials, i.e. Li 4 SiO 4 (7.5 % 6 Li), Li 4 SiO 4 (93 % 6 Li) and Li 2 TiO 3 (7.5 % 6 Li) pellets, indicating the stability of the facility.

Numerical simulation of Li-pellet injection experiments for ELM-pacing in EAST

The injection of light impurity granules into fusion research discharges has shown a great potential for ELM pacing in future nuclear devices. The present work employed a model considering both pellet ablation and transport to systematically study for the first time the evolution of both impurity Li species and target plasma in the edge plasma during single and repeated Li pellet injections, with all initial information extracted from the EAST experiments. The evolutionary distributions of atomic and ionized Li species were presented and show good consistency with experimental CCD camera images. Simulation studies were carried out for single and repeated pellet injections at the outer mid-plane, and statistical uncertainty of ELM pacing rates triggering ELMs is discussed with regard to related injection conditions. It is found that the pressure locally caused by an ELM pacing pellet (with a diameter of 0.6 ∼ 0.7 mm) was 1.8–2.4 times its unperturbed magnitude in the EAST shots of interest. Accumulation from repeated pellet injections results in the rise of pedestal height at a high injection frequency and this effect becomes stronger with increasing pellet size. Pressure changes are also evident in a scenario of multiple small pellets per stroke, which could lead to a nonnegligible, probabilistic ELM triggering rate observed experimentally. The effects of other possible factors are also discussed. With the assumption that the ELM trigger rate of a Li pellet of 0.7 mm diameter is 100%, the predicted trigger rates are in good agreement with those observed in experiments. This work can help understand the statistically probabilistic behavior of ELM triggering efficiency observed in previous studies.

Optically stimulated luminescence of MgO:Na,Li phosphor prepared using solution combustion method

Abstract The purpose of this study is to synthesize magnesium oxide polycrystals doped with sodium and lithium (MgO:Na,Li) using Solution Combustion Synthesis (SCS) technique for developing a storage material with required Optically Stimulated Luminescence (OSL) properties. In this work, we investigated the dosimetric and luminescence characteristics of the synthesized MgO:Na,Li ceramic in pellet form. The phase identification and morphology of the samples were performed using x-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The radioluminescence (RL) emission of the obtained MgO:Na,Li pellet samples showed two different emissions bands that are dominant at ∼730 nm, and with a weak and broad emission between 300 and 590 nm. The samples characterized by a Thermoluminescence (TL) glow curve having dominant TL peaks at around 80, 155 and 315 °C at a heating rate of 5 °C/s. We carried out a step-annealing procedure to investigate the depth of the trapping centers associated with the OSL signal. The thermally affected OSL traps seems to be originated mostly from practically a single deconvoluted TL peak between 170 and 250 °C with intensity comparable to Al2O3:C when Hoya U-340 filter was used. The stability of the OSL signal was tested for shorth-term and long-term fading in dark. We determined that the total OSL area decayed around 15% after 5 weeks. We evaluated the kinetic parameters of the MgO:Na,Li using the computerized glow curve deconvolution (CGCD), Tm-Tstop method and extended initial rise method (IRM) for the TL traps, and isothermal annealing method for the OSL traps. We presented MgO:Na5%,Li5% as a workable OSL material sensitive to ionizing irradiation with suitable dosimetric properties.

Numerical simulation of lithium granule injection in tokamak edge plasma

To understand how a Li pellet affects the background plasma, the edge plasma code SOLPS combined with a neutral-gas-shield ablation model has been employed to simulate the transport of Li species in the edge plasma during Li pellet injections in a typical EAST H-mode plasma. The simulation results show that a Li pellet 0.5 mm with velocity ∼100 m/s can penetrate the separatrix, and that the electron temperature drops quickly where the pellet passes by while the electron density needs a longer time to respond. The plasma pressure in the pedestal region grows significantly with the pellet size; the pressure gradient changes even more drastically. This work indicates that without accumulative contributions, a pellet 0.7 mm or above in diameter at an injection velocity of 100 m/s is likely to provide a pressure perturbation enough to trigger ELMs in the typical EAST H-mode plasmas used in this work.

Numerical simulation of Li pellet ablation in the H-mode pedestal region

The use of non-fuel pellets is desirable for ITER due to its decoupling ELM pacing from fueling. In this paper, the Neutral Gas Shielding Model (Parks et al., Nucl, Fusion 34(1994)417) has been applied to the ablation of Li pellets in the H-mode pedestal region. The results show that the injection velocity of pellets is a more convenient parameter for one to control the deposition profile and location of the injected species in the pedestal. The information obtained from this work can further be used as input for fluid transport codes to track the trajectories of the impurity species or as input for advanced MHD codes for better understanding the non-linear destabilization of ELMs.

Impact of lithium pellets on plasma performance in the ASDEX Upgrade all-metal-wall tokamak

The impact of lithium (Li) on plasma performance was investigated at the ASDEX Upgrade tokamak, which features a full tungsten wall. Li pellets containing 1.6 × 1020 Li atoms were launched with a speed of 600 m s−1 to achieve deep penetration into the plasma and minimize the impact on the first wall. Homogeneous transient Li concentrations in the plasma of up to 15% were established. The Li sustainment time in the plasma decreased with an increasing heating power from 150 to 40 ms. Due to the pellet rate being restricted to 2 Hz, no Li pile-up could take place. No significant positive impact on plasma properties, as reported from other tokamak devices, could be found; the Li pellets rather caused a small reduction in plasma energy, mainly due to enhanced radiation. Due to pellet injection, a short-lived Li layer was formed on the plasma-facing components, which lasted a few discharges and led to moderately beneficial effects during plasma start-up. Most pellets were found to trigger type-I ELMs, either by their direct local perturbation or indirectly by the altered edge conditions; however, reliability was less than 100%.

Li dust injection experiments into T-10 tokamak

An improved 0D transport model has been applied to model the deuterium, lithium and carbon densities of T-10 plasmas during Li dust injection. The deuterium recycling coefficient is found to drop from 0.92 to 0.80 during Li dust injection at the expense of higher Zeff values. The dust injection results are compared with the alternative techniques, Li pellet injection and Li evaporation. Dust injection provides the best recycling conditions at the lowest levels of Li consumption.

Comment on Li pellet conditioning in tokamak fusion test reactor

Li pellet conditioning in TFTR results in a reduction of the edge electron density which allows increased neutral beam penetration, central heating, and fueling. Consequently, the temperature profiles became more peaked with higher central Ti, Te, toroidal rotation, and neutron emission rates.

Li experiments on T-11M and T-10 in support of a steady-state tokamak concept with Li closed loop circulation

This paper presents a review of the last T-11M and T-10 tokamak activity in the field of Li plasma facing component (PFC) investigation. Attention is mainly paid to the realization of the concept of closed loop lithium circulation as a solution of the PFC problem of a steady-state DT volumetric neutron source on a tokamak basis. Realization of the Li PFC concept demands the decision of three main tasks: lithium injection into the plasma, Li collection before its deposition on the vacuum vessel and the return of Li to the injection zone from the collector. This emitter–collector concept assumes that the main heat flux from a hot plasma to the PFC (limiters and divertor plates) can be dissipated on the entire vessel wall surface by non-coronal Li radiation, which will smoothen the local heat load PFC. A rail limiter on the basis of a capillary porous system manufactured from tungsten felt and provided with W wings was successfully tested in the last T-11M experiments as a prototype of steady-state Li emitter–collector. A witness-sample analysis showed that the lateral sides of the rail and ring limiters crossing the plasma scrape-off layer can collect a significant (∼80%) part of Li, injected into the plasma during discharges. This can be used in the future for closing Li loop circulation. As was shown by Li pellet injection in T-10, the probability of Li penetration into the hot plasma core from its boundary is lower than that of deuterium by a factor of 5–10. This result can explain the effect of plasma cleaning (Zeff (0) ∼ 1) during T-10 Li experiments. Some different schemes of future lithium circulation loops are discussed.

Room-temperature ferromagnetism in Li-dopedp-type luminescent ZnO nanorods

We have observed ferromagnetism in Li-doped ZnO nanorods with Curie temperature up to 554 K. Li forms shallow acceptor states in substitutional zinc sites giving rise to $p$-type conductivity. An explicit correlation emerges between increase in hole concentration with decrease in magnetization and Curie temperature in ZnO:Li. Occurrence of ferromagnetism at room temperature has been established with observed magnetic domain formation in ZnO:Li pellets in magnetic force microscopy and prominent ferromagnetic resonance signal in electron paramagnetic resonance spectrum. Magnetic ZnO:Li nanorods are luminescent, showing strong near UV emission. Substitutional Li atoms can induce local moments on neighboring oxygen atoms, which when considered in a correlated model for oxygen orbitals with random potentials introduced by dopant atom could explain the observed ferromagnetism and high Curie temperature in ZnO:Li nanorods.

High speed movies of turbulence in Alcator C-Mod

A high speed (250 kHz), 300 frame charge coupled device camera has been used to image turbulence in the Alcator C-Mod Tokamak. The camera system is described and some of its important characteristics are measured, including time response and uniformity over the field-of-view. The diagnostic has been used in two applications. One uses gas-puff imaging to illuminate the turbulence in the edge/scrape-off-layer region, where D2 gas puffs localize the emission in a plane perpendicular to the magnetic field when viewed by the camera system. The dynamics of the underlying turbulence around and outside the separatrix are detected in this manner. In a second diagnostic application, the light from an injected, ablating, high speed Li pellet is observed radially from the outer midplane, and fast poloidal motion of toroidal striations are seen in the Li+ light well inside the separatrix.

Effective temperatures, sawtooth mixing, and stochastic diffusion ripple loss of fast H+ minority ions driven by ion cyclotron heating in the Tokamak Fusion Test Reactor

This paper presents studies of the H+ minority ions driven by Ion Cyclotron Radio Frequency (ICRF) heating in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Phys. Plasmas 5, 1577 (1998)] deuterium plasmas using primarily passive H° flux detection in the energy range of 0.2–1.0 MeV. The measured passive H+ energy spectra are compared with active (Li pellet charge exchange) results. It is shown that in the passive mode the main donors for the neutralization of H+ ions in this energy range are C5+ ions. The measured effective H+ tail temperatures range from 0.15 MeV at an ICRF power of 2 MW to 0.35 MeV at 6 MW. Radial redistribution of ICRF-driven H+ ions was detected when giant sawtooth crashes occurred during the ICRF heating. The redistribution affected ions with energy below 0.7–0.8 MeV. The sawtooth crashes displaces H+ ions outward along the plasma major radius into the stochastic ripple diffusion domain where those ions are lost in ∼10 msec. These observations are consistent with the model of the redistribution of energetic particles developed previously to explain the results of deuterium-tritium (DT) alpha-particle redistribution due to sawtooth oscillations observed in TFTR. The experimental data are also consistent with numerical simulations of H+ stochastic ripple diffusion losses.

The effect of lithium wall conditioning in TFTR on plasma–surface interactions

Five TFTR deuterium supershots with increasing Li pellet injection are analyzed in detail. Five chords of experimental H-α measurements are compared to predictions from a series of computational models. First, experimental data from the discharge is used in the TRANSP plasma transport code to predict the ion flux to the wall. Then a modified version of the DEGAS neutral transport code which includes both reflection, desorption and sputtering of hydrogenic species from the wall is used to determine the neutral density profile across the machine. This data combined with the known density and temperature contours predicts values for the magnitude of H-α light observed for 16 viewing angles of the diagnostic. To match the experimental data, the wall reflection, desorption and sputtering coefficients were altered using data from VFTRIM-3D to include the effect of the added Li. In addition, the first 10 cm of the stainless steel wall adjoining the C inner bumper limiter was treated as C-covered; the highest flux area of the inner wall was treated as a sink; and the lower reflection coefficients for a Li-wall rather than a C-wall were used over an increasingly larger area of the inner wall as the Li concentration in the discharges increased.

Electro- and photoluminescence of the Tm 3+ ion in Tm 3+- and Li +-doped ZnO ceramics. Influence of the sintering temperature

The electro- and photoluminescence properties of ZnO:Tm,Li pellets sintered between 900 and 1300°C have been studied at room temperature. The photoluminescence spectra show the emissions arising from both the Tm 3+ ions and ZnO itself whereas the electroluminescence spectra show only the emission arising from the Tm 3+ ions. The dependence of the current, capacitance and luminescence intensity upon voltage was measured. As for the ZnO:Bi composites, the electro-optical properties of the ZnO:Tm,Li composites depend mainly upon the grain boundaries, but unlike for the ZnO:Bi composites, they cannot be interpreted in terms of a succession of grain boundaries barriers. It is concluded that the grain boundaries act as preferential conduction pathways. The Tm 3+ ions are mainly located between the grains in intergranular layers where their luminescence acts as a probe, which evidences both the current flow and the presence of hot electrons.

Experiments on Li pellet injection into Heliotron E

Li pellets of large size were injected into electron cyclotron resonance (ECR) heated plasmas and neutral beam injection (NBI) heated plasmas of Heliotron E. The discharge behaviour, pellet ablation and wall conditioning were studied. The electron pressure is doubled after injection into the NBI plasma and remains unchanged in the case of ECR heating. This may be due to the energy exchange between the electrons and thermal ions with the fast ions from the neutral beam. The observed discrepancy between the experimental and modelled ablation rates may be caused by both the plasma cooling due to pellet ablatant and the ablation stimulated by the fast ions in the NBI-heated regime and by the fast electrons in the ECR-heated regime. In preliminary experiments on wall conditioning by Li pellet injection, no improvement of plasma performance after Li pellet injection was observed in the divertor or limiter configuration, with the limiter radii .

Recent experiments on Li pellet injection into Heliotron E

For lithium pellet injection into Heliotron E a new design of injector was used. Some innovations in the process of lithium handling and pellet preparation are described. The response of the main plasma parameters to Li pellet injection is studied and the increase in electron pressure after injection into NBI-heated plasmas is discussed. The ablation rate of the Li pellet was measured by the Li(I) and Li(II) lines and compared with the model.