Progress in the development of integrated advanced ST plasma scenarios in NSTX (Ono et al 2000 Nucl. Fusion 40 557) is reported. Recent high-performance plasmas in NSTX following lithium coating of ...the plasma facing surfaces have achieved higher elongation and lower internal inductance than previously. Analysis of the thermal confinement in these lithiumized discharges shows a stronger plasma current and weaker toroidal field dependence than in previous ST confinement scaling studies; the ITER-98(y, 2) scaling expression describes these scenarios reasonably well. Analysis during periods free of MHD activity has shown that the reconstructed current profile can be understood as the sum of pressure driven, inductive and neutral beam driven currents, without requiring any anomalous fast-ion transport. Non-inductive fractions of 65-70%, and beta(P) > 2, have been achieved at lower plasma current. Some of these low-inductance discharges have a significantly reduced no-wall beta(N) limit, and often have beta(N) at or near the with-wall limit. Coupled m/n = 1/1 + 2/1 kink/tearing modes can limit the sustained beta values when rapidly growing ideal modes are avoided. A beta(N) controller has been commissioned and utilized in sustaining high-performance plasmas. 'Snowflake' divertors compatible with high-performance plasmas have been developed. Scenarios with significantly larger aspect ratios have also been developed, in support of next-step ST devices. Overall, these NSTX plasmas have many characteristics required for next-step ST devices.
Lithium wall coatings have been shown to reduce recycling, improve energy confinement, and suppress edge localized modes in the National Spherical Torus Experiment. Here, we show that these effects ...depend continuously on the amount of predischarge lithium evaporation. We observed a nearly monotonic reduction in recycling, decrease in electron transport, and modification of the edge profiles and stability with increasing lithium. These correlations challenge basic expectations, given that even the smallest coatings exceeded that needed for a nominal thickness of the order of the implantation range.
Previous measurements on the National Spherical Torus Experiment (NSTX) demonstrated peak, perpendicular heat fluxes, qdep,pk 15 MW m−2 with an inter-edge localized mode integral heat flux width, ...during high performance, high power operation (plasma current, Ip = 1.2 MA and injected neutral beam power, PNBI = 6 MW) when magnetically mapped to the outer midplane. Analysis indicates that scales approximately as . The extrapolation of the divertor heat flux and λq for NSTX-U are predicted to be upwards of 24 MW m−2 and 3 mm, respectively assuming a high magnetic flux expansion, fexp ∼ 30, PNBI = 10 MW, balanced double null operation and boronized wall conditioning. While the divertor heat flux has been shown to be mitigated through increased magnetic flux expansion, impurity gas puffing, and innovative divertor configurations on NSTX, the application of evaporative lithium coatings in NSTX has shown reduced peak heat flux from 5 to 2 MW m−2 during similar operation with 150 and 300 mg of pre-discharge lithium evaporation respectively. Measurement of divertor surface temperatures in lithiated NSTX discharges is achieved with a unique dual-band IR thermography system to mitigate the variable surface emissivity introduced by evaporative lithium coatings. This results in a relative increase in divertor radiation as measured by divertor bolometry. While the measured divertor heat flux is reduced with strong lithium evaporation, λq contracts to 3-6 mm at low Ip but remains nearly constant as Ip is increased to 1.2 MA yielding λq's comparable to no lithium discharges at high Ip.
The 3D edge transport code EMC3-EIRENE has been applied for the first time to the NSTX spherical tokamak. A new disconnected double null grid has been developed to allow the simulation of plasma ...where the radial separation of the inner and outer separatrix is less than characteristic widths (e.g. heat flux width) at the midplane. Modelling results are presented for both an axisymmetric case and a case where 3D magnetic field is applied in an n = 3 configuration. In the vacuum approximation, the perturbed field consists of a wide region of destroyed flux surfaces and helical lobes which are a mixture of long and short connection length field lines formed by the separatrix manifolds. This structure is reflected in coupled 3D plasma fluid (EMC3) and kinetic neutral particle (EIRENE) simulations. The helical lobes extending inside of the unperturbed separatrix are filled in by hot plasma from the core. The intersection of the lobes with the divertor results in a striated flux footprint pattern on the target plates. Profiles of divertor heat and particle fluxes are compared with experimental data, and possible sources of discrepancy are discussed.
The application of lithium coatings on plasma facing components has been shown to profoundly affect plasma performance in the National Spherical Torus Experiment, improving energy confinement and ...eliminating edge-localized modes. The edge particle balance during these ELM-free discharges has been studied through 2-D plasma-neutrals modeling, constrained by measurements of the upstream plasma density and temperature profiles and the divertor heat flux and Dα emission. The calculations indicate that the reduction in divertor Dα emission with lithium coatings applied is consistent with a drop in recycling coefficient from R∼0.98 to R∼0.9. The change in recycling is not sufficient to account for the change in edge density profiles: interpretive modeling indicates similar transport coefficients within the edge transport barrier (D/χe∼0.2/1.0m2/s), but a widening of the barrier with lithium.
The application of nonaxisymmetric magnetic fields is shown to destabilize edge-localized modes (ELMs) during otherwise ELM-free periods of discharges in the National Spherical Torus Experiment ...(NSTX). Profile analysis shows the applied fields increased the temperature and pressure gradients, decreasing edge stability. This robust effect was exploited for a new form of ELM control: the triggering of ELMs at will in high performance H mode plasmas enabled by lithium conditioning, yielding high time-averaged energy confinement with reduced core impurity density and radiated power.
We report on a recent set of experiments performed in NSTX to explore the effects of non-axisymmetric magnetic perturbations on the stability of edge-localized modes (ELMs). The application of these ...3D fields in NSTX was found to have a strong effect on ELM stability, including the destabilization of ELMs in H-modes otherwise free of large ELMs. Exploiting the effect of the perturbations, ELMs have been controllably introduced into lithium-enhanced ELM-free H-modes, causing a reduction in impurity accumulation while maintaining high confinement. Although these experiments show the principle of the combined use of lithium coatings and 3D fields, further optimization is required in order to reduce the size of the induced ELMs.
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