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Based on the DFT method, the effects of copper and tungsten impurities present in the negative ion source of neutral beams on the cesiated surface were studied, including their effects on the adsorption energy of cesium on the surface and the surface work function. The results indicate that copper impurities significantly increase the average adsorption energy of cesium, whereas tungsten has limited enhancement on the average adsorption energy of cesium and may even reduce it. The work function calculations show that, at cesium coverages below 4/16 θ, copper impurities cause a significant increase in the work function. However, at cesium coverages above 6/16 θ, high-coverage copper impurities lead to a further decrease in the work function, causing the cesium coverage corresponding to the lowest work function to shift towards higher cesium coverage. Under any tungsten impurity and cesium coverage, tungsten impurities can significantly increase the surface work function, with the maximum increase reaching 0.50 eV. The dipole moment density analysis shows that in most cases, impurities significantly reduce the dipole moment density of the cesiated surface, except when the coverage of cesium and copper impurities is above 6/16 θ. The charge transfer results show that the copper impurity layer has more positive charge compared to the tungsten impurity layer, which significantly affects the dipole moment density of the surface system. In addition, adsorbed atoms cause electrons in the molybdenum atomic layer to migrate to the surface, resulting in the molybdenum substrate having a pronounced negative dipole moment.

The impact of H atoms and O impurities on the adsorption stability and work function of cesiated Mo (001) surface are investigated in this paper through DFT calculations. The 53 surface structures with different surface coverages are constructed in this work. Two types of co-deposition structures are considered: Cs-O (or Cs-H) bonds (1) leaning towards and (2) perpendicular to the surface. Our results demonstrate that the minimum work function corresponds to a 0.5 ML coverage of Cs for any surface coverage of H (or O) when the bonds are tilted. Compared to pure Cs deposition, the work function of Cs and H (or O) co-deposition surface increases at low or saturated Cs coverage, while decreases at 0.5 ML. Analysis of the surface dipole moment revealed that the work function changed inversely with the surface dipole moment density. Comparing charge transfer and polarization, it is evident that charge polarization has a significant impact on dipole moment changes. The presence of O impurities significantly enhances the adsorption stability of Cs, with higher coverage of O impurities leading to greater adsorption stability. The average adsorption energy of Cs atoms increases by ∼0.36 eV when co-deposits with O (0.5 θ). The effect of the H atoms on the adsorption stability of Cs atoms is limited. We also find that the vertical co-deposition case did not offer an advantage in terms of adsorption stability and low work function.

Since the last IAEA-FEC in 2021, significant progress on the development of long pulse steady state scenario and its related key physics and technologies have been achieved, including the reproducible 403 s long-pulse steady-state H-mode plasma with pure radio frequency (RF) power heating. A thousand-second time scale (∼1056 s) fully non-inductive plasma with high injected energy up to 1.73 GJ has also been achieved. The EAST operational regime of high β _P has been significantly extended ( H _98y2 > 1.3, β _P ∼ 4.0, β _N ∼ 2.4 and n _e / n _GW ∼ 1.0) using RF and neutral beam injection (NBI). The full edge localized mode suppression using the n = 4 resonant magnetic perturbations has been achieved in ITER-like standard type-I ELMy H-mode plasmas with q _95 ≈ 3.1 on EAST, extrapolating favorably to the ITER baseline scenario. The sustained large ELM control and stable partial detachment have been achieved with Ne seeding. The underlying physics of plasma-beta effect for error field penetration, where toroidal effect dominates, is disclosed by comparing the results in cylindrical theory and MARS-Q simulation in EAST. Breakdown and plasma initiation at low toroidal electric fields (

We studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.

Well focused negative ion beams are required for neutral beam injection systems for heating and current drive in magnetically confined fusion plasma experiments. The control of a single negative ion beamlet divergence is a significant challenge with the use of a cesium-seeded negative ion source, where negative ions are mainly produced by conversion of hydrogen or deuterium atoms on a cesiated surface of a plasma grid. The single negative ion beamlet was found to be made by three-Gaussian components in our previous work. The origins of such multiple components are considered to be related to dynamics in the ion source and extraction processes of negative ions. This work has demonstrated a measurement of the abundance ratio of the three components (41%, 40%, and 19%) based on a full picture of their transverse velocity distributions, which is a powerful technique to investigate the origins of individual components and will contribute to improve the divergence of negative ion beamlet.

Negative ion beam focusing is a key element for advanced applications of negative ion beams such as accelerators for particle physics, compact accelerators for medical fields, and plasma experiments for nuclear fusion because complicated magnetic fields exist both inside of the source plasma and the grid system. In order to understand the beam focusing, phase space structure measurements for a single beamlet have been performed with a research-and-development negative ion source at the National Institute for Fusion Science. A complicated phase space structure is observed in the direction parallel to the filter magnetic field in the vicinity of the plasma grid, while a single-Gaussian beamlet structure is observed in the direction perpendicular to the filter field. Detailed analyses for the phase space structure of the single beamlet reveal that the complicated structure can be identified as a combination of three beam components with different beam axes. The shifts of each axis are also observed to depend on the ratio of the acceleration voltage for the extraction voltage, which may significantly degrade the beamlet focusing.

A radiofrequency oscillation is successfully superimposed on a plasma potential of a filamented source plasma in an ion beam source while maintaining a constant plasma density, in order to investigate effects of oscillating source plasma potential on an extracted ion beam. The experiment is preliminarily performed with a positive argon ion beam source. A class-D amplifier operational over a wide range of a frequency from a few tens of kHz to several MHz is installed; leading the oscillation of the plasma potential in the plasma source for the frequency range being tested. The beam current profile downstream of the extraction grids shows an oscillation of the beam current at the peripheral region of the ion beam; implying that the oscillation of a beam halo is induced by the potential oscillation of the source plasma.

Plasma parameter measurements for negative hydrogen (H ^− ) ion sources have been playing an important role in clarifying fundamental physics related to negative ion production and destruction processes. Measured data of beam properties, such as H ^− ion current density with the co-extracted electron current and the emittance, were correlated to local concentration of charged particles and temperature often characterized by Langmuir probes and optical emission spectrometry. Langmuir probes coupled to pulse lasers quantified local H ^− ion densities from early days of H ^− ion source development, while the cavity ring down photodetachment method removed Langmuir probes from contemporary large-size high power density ion sources. Technological progress has made source plasma diagnostics possible during beam extraction, which has thrown light on the transport of H ^− ions during the application of the extraction electric field. The advancement of plasma diagnostics for high intensity H ^− ion sources are summarized in this report together with recent results from the research and development negative ion source being operated for collaborative research programs at National Institute for Fusion Science.