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We present a computational investigation of the dependence of material erosion on the incident ion angle at rough graphite and silicon carbide divertor surfaces. Ion angle distributions (IADs) for D plasmas at NSTX-U and DIII-D divertors were calculated by an equation-of-motion model that traces the ion trajectories in the sheath. Then, the effective sputtering yields and ion shadowed area fractions were calculated by a Monte Carlo micro-patterning and roughness code that applied the calculated IADs to surface topographic data that were obtained from optical confocal microscopy of rough graphite and SiC divertor surfaces from NSTX-U and DIII-D experiments. The calculations found that the effective sputtering yields, the sputtering pattern, and the shadowed area are determined by the detailed surface topology rather than the root mean square roughness RRMS, which represents deviations from a flat surface. The suppression of the effective sputtering yields for rough surfaces compared to the yield for a smooth surface was accounted for by the change of the mean local incident ion angle (LIIA) ⟨θ′⟩. The mean surface inclination angle distribution (SIAD) ⟨δ⟩ was found to be a useful parameter to estimate the LIIA from the calculated IADs. We report global empirical formulas for the mean LIIA and fraction of the area shadowed from the main ions for D plasmas for rough surfaces with B-field incident angles α = 85°–89° as a function of the mean SIAD ⟨δ⟩. We propose the use of the mean LIIA ⟨θ′⟩ to estimate the sputtering yield for rough surfaces from the angular dependence of the sputtering yield. [ABSTRACT FROM AUTHOR]

Ammonia molecules formed in a nitrogen-seeded D-fueled plasma are shown to catalyze volumetric recombination of hydrogen ions. This Hydronitrogen Molecular Assisted Recombination (HN-MAR) process occurs by charge and ion exchange reactions with ammonia (ND3), leading to subsequent volumetric recombination with ND3+ and ND4+ molecular ions. When the plasma density is raised to the point where volumetric recombination dominates plasma losses to the wall [ne > 1018 m−3], a calibrated electrostatic quadrupole plasma analyzer shows that the ND4+ ammonium ion density fraction drops from 0.55 to 0.11. We show that this is consistent with a 0-d kinetics model that includes the HN-MAR process with rates that are proportional to the electron density. The results suggest that direct injection of ammonia would provide a more efficient pathway to achieve divertor detachment in future tokamak experiments. [ABSTRACT FROM AUTHOR]

The formation mechanisms of ND3+ and ND4+ are investigated in weakly ionized nitrogen-hydrogen plasmas with electron densities ne ∼ 1016 m−3 and electron temperatures Te ∼ 3 eV. The plasmas are created by an inductively coupled RF (13.56 MHz) discharge of 500 W at the total pressure of 10 mTorr in the nitrogen partial pressure ranging from 0.02 to 2.9 mTorr. The ion density fractions are measured by an electrostatic quadrupole plasma analyzer, which is a combination of ion energy analyzer and mass spectrometer, after calibration with neutral gases. A zero-dimensional rate equation model, sometimes called a global model, is used to understand the source and loss processes of each ion and neutral species in the plasma. The ion density fractions calculated by the model show qualitatively good agreement with the experimental results. Model calculations suggest that ND3+ and ND4+ generation is dominated by electron or D+ exchange reactions of deuterium atomic/molecular ions with ND3. These processes are thought to play an important role in the recombination process of D plasma with ammonia formed in the N2 seeded divertor region. [ABSTRACT FROM AUTHOR]

Efficiently yielding a nitrogen-atom endohedral fullerene (N@C60) is demonstrated for the first time using an electron cyclotron resonance (ECR) plasma source with a minimum-B mirror configuration realized by hexapole permanent magnets. Since electrons are effectively confined around a bottom of the minimum-B mirror in the ECR region, the high-temperature electrons can be generated there, and as a result, the dissociation degree of the nitrogen molecules increases drastically. This highly dissociated nitrogen plasma has a number of nitrogen-atom radicals and ions, thus permitting an enhancement of N@C60 yield. [ABSTRACT FROM AUTHOR]

We report the first observation of global recycling coefficient R near 0.5 in the Lithium Tokamak eXperiment-β (LTX-β), significantly below the minimum R previously reported in other devices. In a series of experiments with varied Li wall conditioning, estimates of the recycling coefficient have been made using a Lyman-α array and DEGAS2 modeling. A progressive reduction in Lyman-α emission with increased lithium and an increase in edge electron temperature are observed. It is also observed that with increasing Li coating thickness, the effective particle confinement time τ p * is reduced and approaches TRANSP calculated energy confinement time (τE), with τ p * near τ E , TRANSP for the lowest recycling coefficients. Edge temperatures approaching core plasma temperatures, first reported in LTX, can now be directly connected to estimates of the recycling coefficient and qualitatively agree with previous UEDGE simulations. The particle flux to the limiting surfaces appears to be significantly reduced in comparison with fluid scrape-off layer (SOL) models, indicating that a large fraction of the SOL ions are mirror trapped. SOL collisionality drops more than an order of magnitude below the banana regime boundary, indicating the importance of kinetic effects. Full-f 1x2v gyrokinetic simulations of SOL field lines with the GKEYLL code indicate that the fraction of ions trapped along field lines increases as collisionality drops, as a result of increased lithium evaporation. [ABSTRACT FROM AUTHOR]

It is important to consider a combination of inhomogeneities, which drive and/or damp instabilities in magnetized plasmas. The inhomogeneity of neutral particle density is taken into account for analyzing turbulent states in linear devices by combining two kinds of simulations in this research. One is a simulation for 2D profiles of neutral particles and the other is that for resistive drift wave turbulence. Radial and axial profiles of neutral particles are calculated by the Monte Carlo method with a configuration of linear device PANTA. The ratio for ionization of neutral particles depends on the electron temperature, and the higher temperature in the operation range makes the neutral density smaller near the center of the plasma. Then, the radial profile of the ion-neutral collision frequency is set by introducing these neutral density profiles into resistive drift wave turbulence simulations. Smaller ion-neutral collision frequency makes the instabilities more unstable, and its inhomogeneity enhances mean electrostatic potential formation. The eigenmode structure is changed with the inhomogeneous ion-neutral collision frequency, and convective derivative nonlinearity generates the azimuthal flow near the center of the plasma. Two roles of the neutral particles on the turbulence, the direct change in linear instabilities and formation of background flows, become clear. [ABSTRACT FROM AUTHOR]