C. Boswell

Active and Fast Particle Driven Alfven Eigenmodes in Alcator C-Mod

Alfven eigenmodes sAEsd are studied to assess their stability in high density reactor relevant regimes where Ti < Te and as a diagnostic tool. Stable AEs are excited with active magnetohydrodynamics antennas in the range of the expected AE frequency. Toroidal Alfven eigenmode sTAEd damping rates between 0.5%, g / v , 4.5% have been observed in diverted and limited Ohmic plasmas. Unstable AEs are excited with a fast ion tail driven by H minority ion cyclotron radio frequency sICRFd heating with electron densities in the range of ne = 0.5‐2 3 10 20 m ˛3 . Energetic particle modes or TAEs have been observed to decrease in frequency and mode number with time up to a large sawtooth collapse, indicating the role fast particles play in stabilizing sawteeth. In the current rise phase, unstable modes with frequencies that increase rapidly with time are observed with magnetic pick-up coils at the wall and phase contrast imaging density fluctuation measurements in the core. Modeling of these modes constrains the calculated safety factor profile to be very flat or with slightly reversed shear. AEs are found to be more stable for an inboard than for central or outboard ICRF resonances in qualitative agreement with modeling.

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.

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...

Applications of visible CCD cameras on the Alcator C-Mod tokamak

Five 7 mm diameter remote-head visible charge-coupled device (CCD) cameras are being used on Alcator C-Mod for several different diagnostic purposes. All of the cameras’ detectors and optics are placed inside a magnetic field of up to 4 T. Images of the cameras are recorded simultaneously using two three-channel color framegrabber cards. Two CCD cameras are used typically to generate two-dimensional emissivity profiles of deuterium line radiation from the divertor. Interference filters are used to select the spectral line to be measured. The local emissivity is obtained by inverting the measured brightnesses assuming toroidal symmetry of the emission. Another use of the cameras is the identification and localization of impurity sources generated by the ion cyclotron radio frequency (ICRF) antennas, which supply the auxiliary heating on Alcator C-Mod. The impurities generated by the antennas are identified by correlating in time the injections seen at the cameras with measurements made with core diagnostic...

The effect of divertor baffling on Alcator C-Mod discharges

The effect of divertor baffling on Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] discharges has been investigated using a novel divertor bypass. The bypass allows the in situ variation of the mechanical conductance from the divertor plenum to the main chamber between or even during discharges. The results indicate a strong dependence on baffling for the compression of deuterium and recycling impurity gases in the divertor plenum in a variety of C-Mod discharges, indicating that tight baffling would be desirable in a future tokamak reactor. There is no apparent effect of the divertor baffling for the Ohmic, L-mode and EDA (Enhanced Dα) H-mode discharges in this study on main chamber neutral particle populations, scrape-off-layer flows, nonrecycling impurity screening, global energy confinement or H-mode threshold powers.

Modeling of Alcator C-Mod divertor baffling experiments

A specific Alcator C-Mod discharge from the series of divertor baffling experiments is simulated with the DEGAS 2 Monte Carlo neutral transport code. A simple two-point plasma model is used to describe the plasma variation between Langmuir probe locations. A range of conductances for the bypass between the divertor plenum and the main chamber are considered. The experimentally observed insensitivity of the neutral current flowing through the bypass and of the Dα emissions to the magnitude of the conductance is reproduced. The current of atoms in this regime is being limited by atomic physics processes and not bypass conductance. The simulated trends in divertor pressure, bypass current, and Dα emission agree only qualitatively with the experimental measurements, however. Possible explanations for the quantitative differences are discussed.

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.

On the use of MHD mode analysis as a technique for determination of q-profiles in JET plasmas

JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis, and determination of mode radial position has the potential as a diagnostic for the plasma q profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pickup signals. Cross correlation of magnetic and multiarray signals from ECE, SXR, and reflectometer systems provide the mode localization. The present status of the diagnostic systems used for fluctuation analysis is summarized, with particular emphasis on the fast magnetic pickup system. A new data collection system allows the magnetic signals to be recorded at 2 MHz for up to 32 s in each pulse. An example of q-profile validation from the observation of Alfven eigenmode cascades and a snake is given. At JET, Alfven cascades are used to determine qmin in optimized shear discharges, while a variety of low frequency modes (such as the snake) allow the determination of the radial...

Observations of Cold, High Density Plasma in the Private Flux Region of the Alcator C-Mod Divertor

Significant plasma density has been observed in the private flux zone of the Alcator C-Mod tokamak. The behavior of the D γ emission profiles is consistent with the source of the plasma being due to an E × B drift generated by a poloidal temperature gradient. The plasma flux due to this drift is derived and evaluated. A plasma flux into the private flux zone is inferred by measurements of volumetric recombination using a tangentially viewing CCD camera and several spectroscopic views observing the high n-lines of the D 0 Balmer series. For the case of an attached divertor the inferred plasma flux to the PFZ has a linear scaling with respect to plasma pressure, as is expected from the temperature-gradient-induced, E × B drift.