D. Pappas

A study of molybdenum influxes and transport in Alcator C-Mod

A characterization is presented of Mo sources ΓMo, core Mo content NMo and their dependences on the Alcator C-Mod operational regimes. The primary impurity source locations are the divertor, the inner wall and the ICRF antenna limiters. Boronization is used to coat the first wall surfaces with a thin layer of B, which greatly reduces the Mo sources for a period of time, eroding away with an e folding period of 15-50 discharges. The penetration of Mo into the core plasma under different conditions is analysed using the concept of penetration factor, PF = NMo/ΓMo (s). In general, the inner wall Mo source is large (~1018s-1), but is found to be relatively uncorrelated with the core Mo content in diverted plasmas. The outer divertor source is of a similar order to that of the inner wall and has a penetration factor in the range 10-5 - 2 × 10-3 s depending on density and confinement mode. The antenna limiter Mo sources are typically smaller, but with higher penetration factors of 10-3 - 2 × 10-2 s. The behaviour of the antenna limiter sources is consistent with physical sputtering due to the influence of RF sheath rectification. The measurements of the plasma potential on field lines connected to the antenna are very high when the antennas are energized, often reaching hundreds of volts.

Molybdenum erosion measurements in Alcator C-Mod

Erosion of molybdenum was measured on a set of 21 tiles after a run campaign of 1090 shots in the Alcator C-Mod tokamak. The net erosion of molybdenum was determined from changes in the depth of a thin chromium marker layer measured by Rutherford backscattering. Net Mo erosion was found to be approximately 150 nm near the outer divertor strikepoint and much less everywhere else. Gross erosion rates by sputtering were estimated using ion energies and fluxes obtained from Langmuir probe measurements of edge-plasma conditions. Predicted net erosion using calculated gross erosion with prompt redeposition agrees with measured net erosion within a factor of three. Sputtering by impurities, mainly boron, dominates erosion.

Lower hybrid heating in the Alcator A tokamak

The results and interpretation of the modest-power (~90 kW) lower-hybrid-heating experiment on Alcator A are presented. The expected results from linear waveguide-plasma coupling theory are outlined, and the possible effects of parametric instabilities, scattering from density fluctuations, and imperfect energetic ion confinement are addressed. It is found experimentally that good coupling and the absence of RF breakdown are achieved with a double waveguide array at available RF power densities PRF ≤ 4.5 kW.cm·−2, the waveguide vacuum windows being outside the toroidal field magnets; a waveguide array having vacuum windows near the waveguide mouth so that the ω = ωce layer can be pressurized shows no breakdown at PRP > 8 kW/cm2 when a single waveguide is energized. Energetic ion production and a factor-of-50 increase in the fusion neutron rate are observed to take place at well defined values of central plasma density; below these densities electron heating occurs. The ion tail production is found to be independent of the relative phase of the double waveguide array employed. This ion heating occurs at a lower density than theoretically expected; together with the electron heating this indicates waves having n|| ~5 being absorbed near the plasma centre. Probes at the plasma edge observe a frequency-down-shifted and broadened RF pump signal that is strongly attenuated as the plasma density increases through the neutron production band. These anomalous heating results and probe signals can be explained by a parametric decay of the pump wave into higher n|| lower hybrid waves near the plasma edge. An alternate qualitative explanation would be the poloidal scattering of the lower hybrid waves at the plasma periphery due to density fluctuations; the n|| of the scattered lower hybrid waves would then increase as they propagated inward because of magnetic shear. The neutron rate decay times imply that the RF creates ion tails having a substantial fraction of their energy above 50 keV. The neutron decay times and rates strongly depend on plasma current and indicate the expected influence of ion confinement on RF heating efficiencies. Finally, the RF heating efficiencies are assessed.

The role of particle sinks and sources in Alcator C-Mod detached divertor discharges

Detailed measurements of the magnitude and location of volumetric recombination occurring in the detached divertor of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The drop in divertor plate ion current during detachment is due to two mechanisms: (1) volumetric recombination in the divertor plasma; and (2) reductions in the divertor ion source. Depending on plasma conditions, each of these can be the primary mechanism for the observed ion current reduction in detachment. The ion source during detachment is inferred and its magnitude is consistent with the measured divertor power flow. A scaling of the density in the divertor recombining region for L- (low confinement) mode plasmas is found, ne,r∝ne0.8⋅PSOL2/7. A model based on pressure variation along a flux surface during detachment is consistent with the main features of this scaling.Detailed measurements of the magnitude and location of volumetric recombination occurring in the detached divertor of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The drop in divertor plate ion current during detachment is due to two mechanisms: (1) volumetric recombination in the divertor plasma; and (2) reductions in the divertor ion source. Depending on plasma conditions, each of these can be the primary mechanism for the observed ion current reduction in detachment. The ion source during detachment is inferred and its magnitude is consistent with the measured divertor power flow. A scaling of the density in the divertor recombining region for L- (low confinement) mode plasmas is found, ne,r∝ne0.8⋅PSOL2/7. A model based on pressure variation along a flux surface during detachment is consistent with the main features of this scaling.

Electron heating via mode converted ion Bernstein waves in the Alcator C-Mod tokamak

Highly localized direct electron heating [full width at half-maximum (FWHM)≅0.2a] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center (r/a⩾0.3) has been observed in H(3He) discharges at B0=(6.0–6.5) T and ne(0)≅1.8×1020 m−3. [Here, the minority ion species is indicated parenthetically.] Off-axis heating (r/a⩾0.5) has also been observed in D(3He) plasmas at B0=7.9 T. The concentration of 3He in these experiments was in the range of n3He/ne≅(0.2–0.3) and the locations of the mode conversion layer and electron heating peak could be controlled by changing the 3He concentration or toroidal magnetic field (B0). The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to ...

High confinement dissipative divertor operation on Alcator C-Mod

Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] has operated a High-confinement-mode (H-mode) plasma together with a dissipative divertor and low core Zeff. The initially attached plasma is characterized by steady-state enhancement factor, HITER89P [P. N. Yushmanov et al., Nucl. Fusion 30, 1999 (1990)], of 1.9, central Zeff of 1.1, and a radiative fraction of ∼50%. Feedback control of a nitrogen gas puff is used to increase radiative losses in both the core/edge and divertor plasmas in almost equal amounts. Simultaneously, the core plasma maintains HITER89P of 1.6 and Zeff of 1.4 in this nearly 100% radiative state. The power and particle flux to the divertor plates have been reduced to very low levels while the core plasma is relatively unchanged by the dissipative nature of the divertor.

Localization of emission through interpretation of observed Zeeman pattern

Observations of spectral line profiles commonly represent the integration of emission along the line of sight. Depending on the number of views and the symmetries involved, one can use techniques ranging from simple Abel inversion to complex tomographic reconstruction to find the spatial distribution emitters. In tokamak experiments, the spatial dependence of the magnetic field is typically available and can be used to gain important insights into the absence of other spatial information. The Zeeman patterns of spectral lines from neutral atoms and low-Z ions in tokamak plasmas can contain enough information to restrict the location of emission to well defined positions along a given line of sight. Simple modeling of observations with high spectral resolution from Alcator C-Mod plasmas demonstrates the application of this technique to the interpretation of experimental data. This localization of emission is not only of interest to spectroscopists and modelers of tokamak edge and divertor regions, it could...

Mode conversion electron heating in Alcator C-Mod: Theory and experiment

Localized electron heating [full width at half maximum of Δ(r/a)≈0.2] by mode converted ion Bernstein waves (IBW) has been observed in the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. These experiments were performed in D(3He) plasmas at high magnetic field (B0=7.9 T), high-plasma density (ne0⩾1.5×1020 m−3), and for 0.05⩽nHe-3/ne⩽0.30. Electron heating profiles of the mode converted IBW were measured using a break in slope analysis of the electron temperature versus time in the presence of rf (radio frequency) modulation. The peak position of electron heating was found to be well-correlated with 3He concentration, in agreement with the predictions of cold plasma theory. Recently, a toroidal full-wave ion cyclotron range of frequencies (ICRF) code TORIC [M. Brambilla, Nucl. Fusion 38, 1805 (1998)] was modified to include the effects of IBW electron Landau damping at (k⊥ρi)2≫1, This model was used in combination with a 1D (one-dimensional) integral wave equation code METS [...

Recombination and ion loss in C-Mod detached divertor discharges

Abstract We present detailed profiles of the magnitude and location of volumetric recombination occurring in the Alcator C-Mod divertor region during detached divertor discharges. The recombination sink (for ions) is compared to the ion current collected at the divertor plates both as a function of location on the plate and across the entire divertor. We find that, depending on plasma conditions, volume recombination can account for the removal of 10–75% of the ions flowing on detached flux surfaces. A similarly important cause of the observed ion current loss appears to be the reduction in divertor ion sources upstream from the plate. Changes in the ion source rate are consistent with changes in power flowing from the SOL into the divertor region. The lowest levels of recombination are found in H-mode discharges where detachment is induced through puffing of N 2 gas. In these cases the observed ion current loss is due almost entirely to decrease in the ion source. This shows that recombination is not a necessary condition for detachment.

A novel tracer-gas injection system for scrape-off layer impurity transport and screening experiments

Abstract The design, operation, and initial results of a novel tracer-gas injection system is described. This system has been developed to diagnose local impurity transport and screening properties as a function of depth into the scrape-off layer (SOL) of Alcator C-Mod. A fast-scanning Langmuir–Mach probe has been outfitted with a capillary gas-feed and an inertially activated valve. Impurity injection plumes lasting 5–10 ms can be formed at any depth into the SOL, up to the separatrix location. The local dispersion of selected charge state impurities in the SOL can be followed by camera imaging and Doppler spectroscopy. In principle, the local screening properties of the SOL can also be directly assessed by injecting gas at different depths into the SOL and observing the resultant impurity concentrations in the core.