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Two new high resolution detectors have been installed on the Alcator C-Mod tokamak to measure the neutral density and ionization rate at the edge of the main chamber plasma. Using a silicon detector sensitive to UV light, and a very narrow filter with transmission peaking at 1216 A, the Lyman alpha radiation emanating from neutral deuterium (and hydrogen) is measured. The detectors consist of 20 channel arrays which view the plasma tangentially 12.5 cm below the outer midplane, and 10 cm above the inner midplane. The imaging is performed using a 1 mm×3 mm slit, which gives a nominal radial resolution of 2 and 3 mm, respectively. The local emissivity is obtained via a standard Abel inversion technique. Employing well-known branching ratios, and using measured local electron density and temperature, the neutral density and ionization rate are inferred with similar radial resolution. Details of the setup and sensitivity of the results to plasma conditions are discussed.

I-mode is a high-performance tokamak regime characterized by the formation of a temperature pedestal and enhanced energy confinement, without an accompanying density pedestal or drop in particle and impurity transport. I-mode operation appears to have naturally occurring suppression of large Edge-Localized Modes (ELMs) in addition to its highly favorable scalings of pedestal structure and overall performance. Extensive study of the ELMy H-mode has led to the development of the EPED model, which utilizes calculations of coupled peeling-ballooning MHD modes and kinetic-ballooning mode (KBM) stability limits to predict the pedestal structure preceding an ELM crash. We apply similar tools to the structure and ELM stability of I-mode pedestals. Analysis of I-mode discharges prepared with high-resolution pedestal data from the most recent C-Mod campaign reveals favorable pedestal scalings for extrapolation to large machines—pedestal temperature scales strongly with power per particle P[subscript net] [over [bar over n][subscript e]], and likewise pedestal pressure scales as the net heating power (consistent with weak degradation of confinement with heating power). Matched discharges in current, field, and shaping demonstrate the decoupling of energy and particle transport in I-mode, increasing fueling to span nearly a factor of two in density while maintaining matched temperature pedestals with consistent levels of P[subscript net] [over [bar over n][subscript e]]. This is consistent with targets for increased performance in I-mode, elevating pedestal β [subscript p] and global performance with matched increases in density and heating power. MHD calculations using the ELITE code indicate that I-mode pedestals are strongly stable to edge peeling-ballooning instabilities. Likewise, numerical modeling of the KBM turbulence onset, as well as scalings of the pedestal width with poloidal beta, indicates that I-mode pedestals are not limited by KBM turbulence—both features identified with the trigger for large ELMs, consistent with the observed suppression of large ELMs in I-mode., United States. Dept. of Energy (Agreement DE-FC02-99ER54512), United States. Dept. of Energy (Agreement DE-FG02-99ER54309)

Cross-field particle transport increases sharply with distance into the SOL and plays a dominant role in the `main-chamber recycling' regime in Alcator C-Mod, a regime in which most of the plasma particle efflux recycles on the main-chamber walls rather than flows into the divertor volume. This observation has potentially important implications for a reactor: contrary to the ideal picture of divertor operation, a tightly baffled divertor may not offer control of the neutral density in the main-chamber such that charge exchange heat losses and sputtering of the main-chamber walls can be reduced. The conditions that give rise to the main-chamber recycling regime can be understood by considering the plasma-neutral particle balance: when the flux surface averaged neutral density exceeds a critical value, flows to the divertor can no longer compete with the ionization source and particle fluxes must increase with distance into the SOL. This critical neutral density condition can be recast into a critical cross-field plasma flux condition: particle fluxes must increase with distance into the SOL when the plasma flux crossing a given flux surface exceeds a critical value. Thus, the existence of the main-chamber recycling regime is intrinsically tied to the level of anomalous cross-field particle transport. Direct measurement of the effective cross-field particle diffusivities Deff in a number of ohmic L mode discharges indicates that Deff near the separatrix strongly increases as plasma collisionality increases. Convected heat fluxes correspondingly increase, implying that there exists a critical plasma density (or perhaps collisionality) beyond which no steady state plasma can be maintained, even in the absence of radiation.

X-ray spectra of 2l-3l' transitions in the elements krypton (Z = 36), zirconium (Z = 40), niobium (Z = 41) and molybdenum (Z = 42) from charge states around neon-like have been observed from Alcator C and Alcator C-Mod plasmas. Accurate wavelengths (±0.5 mA) have been determined with reference to neighbouring aluminium, silicon, sulfur, chlorine and argon lines with relatively well known wavelengths. Line identifications have been made by comparison to ab initio atomic structure calculations, using a fully relativistic, parametric potential code. Calculated wavelengths and oscillator strengths are presented for 2l-3l' transitions in neon-like ions and neighbouring Na-, Mg-, F- and O-like satellites. Electric quadrupole transitions in neon-like ions of the form 2s-3d and 2p-3p are found to be relatively bright, and their measured intensities agree well with the results of collisional-radiative modelling. The 2p-3s magnetic quadrupole transition is also relatively intense in the absence of collisional de-excitation at tokamak densities (1013-1015 cm-3). For krypton, transitions with upper levels up to n = 9 have been observed, and the measured wavelengths and intensities have been compared with calculations. For some neon-like transitions with nearly degenerate upper levels, there can be substantial configuration interaction which alters the line intensities.

Measurements of profiles of soft X ray emissivity with 1.5 mm radial resolution are combined with high resolution electron density and temperature measurements in the edge region of the Alcator C-Mod tokamak to facilitate transport analysis of medium-Z impurities during H modes. Results from detailed modelling of the radiation and transport of fluorine are compared with experimental measurements, yielding information about the transport coefficients in the H mode transport barrier region. Evidence is found for a strong inward impurity pinch just inside the separatrix. The region of strong inward pinch agrees very well with the region of strong electron density gradient, suggesting that the inward pinch could be driven by the ion density gradient, as predicted by neoclassical theory. Simulations using the neoclassical impurity convection profile agree very well with experiments. Transport modelling shows that the X ray pedestal width is largely determined by the diffusion coefficient in the transport barrier. This allows diagnosis of changes in the edge diffusion coefficient on the basis of observations of X ray pedestal width changes. Significant differences in the edge diffusion coefficient are seen between different types of H mode. Several scalings for the edge diffusion coefficient in the enhanced Dα H mode are also identified. This may help elucidate the physical processes responsible for this attractive confinement mode.

Neutral particle densities and energy losses have been measured in the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Their effect on the formation and evolution of the edge barrier which accompanies the enhanced confinement regime are discussed. The neutrals can enter the edge dynamics through the particle, momentum, and energy balance. Neutral densities of up to 5×1016 m−3 have been measured in the edge barrier region. Neutrals enter the local dynamics around most of the periphery, not just at the X-point. High resolution measurements of the ionization profile have been obtained for the region near the separatrix. The profile shifts inside the separatrix as the plasma is making a transition from low-to high-mode confinement (H-mode) regimes, partly accounting for the dramatic rise in edge density. The measured neutral density is large enough to affect the bulk ion momentum by charge exchange, and thereby introduces a negative radial electric field at the edge. At the same time, significant edge heat flux, carried by the neutrals, contributes to the measured power loss. At very high edge densities, this loss mechanism could contribute to quenching H-modes.

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.

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...

In the present study recycling and transport in the edge plasma of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is modeled and analyzed with the multi-fluid code UEDGE [T. D. Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)]. Matching the experimental plasma density profiles in the scrape-off layer (SOL) requires a spatially dependent effective anomalous diffusion coefficient D⊥ growing rapidly towards the wall. The midplane pressure of neutral gas, Pmid, is a key parameter that reflects the magnitude of anomalous transport of plasma from the core. Recycling of plasma on the main chamber wall appears to be quite significant, especially in the case of high Pmid∼0.3 mTorr when the main wall provides ∼70% of recycling neutrals in the main chamber. In the upper SOL (well above the x point) draining of particles by the poloidal flow is weak and thus the particle balance is predominantly radial. For the radial heat transport it is found that energy flux carried by radial plasma conv...

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.

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.

Regimes of high-confinement mode have been studied in the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Plasmas with no edge localized modes (ELM-free) have been compared in detail to a new regime, enhanced Dα (EDA). EDA discharges have only slightly lower energy confinement than comparable ELM-free ones, but show markedly reduced impurity confinement. Thus EDA discharges do not accumulate impurities and typically have a lower fraction of radiated power. The edge gradients in EDA seem to be relaxed by a continuous process rather than an intermittent one as is the case for standard ELMy discharges and thus do not present the first wall with large periodic heat loads. This process is probably related to fluctuations seen in the plasma edge. EDA plasmas are more likely at low plasma current (q>3.7), for moderate plasma shaping, (triangularity ∼0.35–0.55), and for high neutral pressures. As observed in soft x-ray emission, the pedestal width is found to scale with the same parameters that determine the EDA/ELM-free boundary.

Impurity transport has been investigated in the reactor-like conditions of the Alcator C-Mod divertor (divertor plasma density ⩾1021 m−3, parallel heat flux in the SOL ⩽0.5 GW m−2). Trace amounts of the recycling impurity gases helium, neon, argon, and krypton were injected into plasmas. A residual gas analyzer that samples the private flux region of the divertor was used for divertor impurity neutral density measurements. Compression ratios (Cz) of up to 400 and enrichment factors (ηz) of up to 20 have been observed for argon in Alcator C-Mod discharges. Compression ratios increase and enrichment values are constant or slightly increasing with line-averaged density for argon in Ohmic discharges. Above the divertor detachment threshold density, Cz and ηz both decrease. During H-mode operation, compression and enrichment ratios for argon decrease relative to their Ohmic values. For similar plasma conditions, impurity compression ratios and enrichment values increase with the mass of the impurity. This mass scaling may be partially explained by the decrease in the ionization mean-free-path with increasing impurity mass.

Experimental observations show that when a divertor plasma detaches, radiation regions (fronts) move towards the X-point. This indicates that the localization of the fronts in the divertor is unstable. We study theoretically the stability of detached divertor operation and compare our results with experimental data. We show that the impurity radiation and plasma recombination effects play a crucial role in the evolution of detached plasma parameters. In the tokamak SOL these effects result in the saturation of upstream plasma density and temperature as the total number of particles (ions and neutrals) in the SOL is increased, and cause both impurity radiation and ionization–recombination fronts propagation towards the X-point. These conclusions are supported by experimental data from Alcator C-Mod.

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.

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.

Experiments were performed on Alcator C-Mod with electron cyclotron resonance (ECR) plasmas to help determine their applicabilty to a fusion reactor. Strong radial inhomogeneity of the plasma density was measured, decreasing by a factor of ten a few centimeters inside the resonance location, but remaining approximately constant ( n e ≈10 16 m −3 ) outside the resonance location. Electron temperature remained mostly constant outside the resonance location, T e ≈10 eV ; ion temperature increased outside the resonance location from T i ≈2 eV to 10 eV. Toroidal asymmetries in ion saturation current density were observed, indicating local toroidal plasma flow. The ECR plasma was used to remove a diamond-like carbon coating from a stainless-steel sample. Removal rates peaked at 4.2±0.4 nm/h with the sample a few centimeters outside the resonance location. Removal rates decreased inside and further outside the resonance location. The plasma did not remove the carbon from the sample uniformly, possibly due to plasma flow. Yields were calculated ( Y ≈10 −3 ) to be lower than other published results for chemical sputtering of deuterium ions on carbon, possibly due to toroidally asymmetric plasma conditions.

A method for evaluating the volume recombination rate within a cold ( T e ≲1 eV), relatively dense ( n e ≳1×10 21 m −3 ) plasma is described. These conditions often exist in the detached divertor plasmas of the Alcator C-Mod Tokamak. Temperatures ≲1 eV are measured spectroscopically. Densities in the 0.4 × 10 21 –2.5 × 10 21 m −3 range are measured using the Stark-broadened D 0 Balmer series lines. D 0 line emissions in both the Lyman and Balmer series show that the D 0 excited state populations ( n ≳3) in the recombining regions are close to those given by the Saha–Boltzmann distribution and are essentially independent of the ground state density. The intensities of lines from these excited states are related to the recombination rate. The number of recombinations per D 0 photon is a function of opacity of the Lyman series lines, T e and n e . Determinations of the recombination rates corresponding to the line intensities are made by using a collisional radiative model describing the level populations, ionization and recombination of D 0 and a model which accounts for the radiation transfer of the Lyman lines. The effects of the trapping of Lyman photons on the `recombinations per photon' curves have been calculated using both modelling which considers only spatial diffusion of the photons and modelling by a radiative transfer code which calculates photon transport in space and frequency. Subject to imperfect knowledge of the 2D profiles of opacity and temperature, total recombination rates in the detached divertor plasma are determined. In the Alcator C-Mod divertor region, volume recombination can be a significant sink for ions under detached conditions.

A new radiative cooling curve for molybdenum, determined from the Hebrew University, Jerusalem - Lawrence Livermore atomic code (HULLAC) has been used to estimate the radiative power losses from various ion cyclotron resonance frequency (ICRF) and ohmically heated Alcator C-Mod tokamak plasmas and ohmically heated Frascati tokamak upgrade (FTU) plasmas. This cooling curve has been found to accurately predict the radiative losses from molybdenum ions in plasma. Radiative losses from molybdenum computed by using the HULLAC cooling curve could typically account for greater than 60% of the total radiative losses as measured by bolometry in Alcator C-Mod tokamak plasmas. The molybdenum density was found to be as high as particles in ICRF heated plasmas. Plasmas after botonization of the plasma facing surfaces had the lowest molybdenum density and radiated power of all the plasmas examined; ICRF heated plasmas had the highest. The molybdenum density and radiative power losses were found to increase roughly linearly with input ICRF power during different heating schemes in plasmas with both H-mode and L-mode confinement. Although the molybdenum densities were found to decrease with increasing electron density, the radiative power losses did not change significantly for all types of plasmas studied in Alcator C-Mod and FTU: diverted plasmas heated with ICRF and both limited and diverted plasmas heated ohmically.

Recent breakthroughs in silicon detector technology now permit measurement of radiated power over a wide range in photon energies. These detectors (also known as AXUV photodiodes) have a flat spectral power response from ultraviolet to x-ray energies, and with a slightly reduced efficiency all the way down to visible wavelengths. Since they can be made small, multichannel detectors allow high spatial resolution to be combined with an intrinsic high temporal resolution, which can reach the microsecond range, depending on the application. Additional features include ease of use and installation, and relatively low cost compared to other techniques. A combination of two multichannel toroidally viewing systems has been recently installed on the Alcator C-Mod tokamak. The first array, which is composed of 16 channels, sees tangentially the outer-half of the plasma at the midplane, and is used to measure the total power radiated. The second array, also located at the midplane, consists of 19 channels and views ...

The Plasma Spectroscopy Group of The Johns Hopkins University is developing diagnostics for spectroscopic characterization of the tokamak plasma scrape-off layer and divertor regions. A far ultraviolet polychromator has been designed for radiative divertor studies at the Alcator C-Mod and D-IIID tokamaks. Local measurements of resonant transitions of lithium- to boron-like ions of intrinsic or seeded low Z impurity elements will be performed along multiple chords around the X point. Planar diffraction gratings and stacked grids will be used for dispersion and angular collimation of radiation. Phosphor wavelength converters coupled to a photomultiplier tube by an optical fiber will be used as detectors. The design provides a wavelength resolution of ≃10 A, a spatial resolution of ⩽2 cm, and an adequate photometric sensitivity. The in-vessel instrument, proposed for the Alcator C-Mod tokamak, will measure intensities of the lines at 1240 (N V), 765, 923 (N IV), and 990 A (N III). The port-mounted polychromator at DIII-D will be able to monitor intensities of 1550 (C IV), 977, 1176 (C III), and 1335 A (C II) lines. This, together with visible and bolometric diagnostics, should enable estimates of power losses, charge state distribution and local transport of the impurity ions in the divertor. A one-channel prototype of the C-Mod and D-IIID instruments is being built for the CDX-U spherical tokamak. Line-integrated brightnesses of the 2s–2p transition at 1550 A will be measured and inverted to obtain C IV emissivity distribution.

The design of two beam-based diagnostic systems for C-Mod, charge-exchange recombination spectroscopy (CXRS) and beam-emission spectroscopy (BES) will be presented. A 50-keV diagnostic neutral beam (DNB) will be installed on C-Mod for these and other diagnostics. By adding BES and CXRS to the set of C-Mod diagnostics, it will be possible to test theory-based models of turbulent transport, which are typically based on marginal stability and the existence/dominance of ion temperature gradient (ITG) modes. Tests that coordinate turbulence measurements with transport measurements are largely unavailable at present.

A new infrared imaging system has been installed on Alcator C-Mod. This system uses an Amber Radiance 1 IR video camera (filtered to the 4.2–4.4 μm band) to view a 30 cm×30 cm region of the lower divertor from above by means of a re-entrant 5-m long ZnSe based periscope. Capture of the standard 30 Hz video frames (8-bit) and camera control are performed remotely over fiber optic links by a Windows 95 PC, using a MuTech MV-1000 video grabber board. Plans are under way to directly capture the 60 Hz, 12-bit, 256×256 pixel images using a digital video camera interface with a fiber optic link from EDT (Beaverton, Oregon). Preliminary results show that during nondisruptive discharges no substantial surface temperature increase is observed on the upper sections of the divertor, with the exception of “hot spots,” although occasionally, increased heating in toroidal bands is seen. Bands can also be observed after disruptions that result in a downward movement of the plasma.

Under detached plasma conditions in the Alcator C-Mod tokamak, the measured spectra show pronounced merging of the Balmer series lines and a photo-recombination continuum edge which is not a sharp step. This phenomenon, known as a smooth discrete-to-continuum (D-C) transition, is typical only for high-density , low-temperature (Te≈1 eV), and recombining plasma. As we will discuss, this type of transition emphasizes the degree of plasma non-ideality. A theoretical model capable of treating spectra from a detached divertor plasma, and those with a smooth D-C transition in particular, has been developed. It is comprised of three parts: (i) a collisional-radiative model for the population densities of the excited states, (ii) atomic structure and collision rates for an atom affected by statistical plasma microfields, and (iii) a model for calculating the line profiles and the extended photo-recombination continuum. The effects of statistical plasma microfields on the population densities of excited states, on the profiles of Balmer series lines, and on the photo-recombination continuum edge are discussed. The changes in spectrum characteristics with plasma parameter variation, leading to the smooth D-C transition, are analysed. The relevance of volumetric plasma recombination to the spectra observed from a detached divertor plasma is discussed. A comparison of the calculated and measured spectra is used to determine the plasma parameters in the recombining plasma region. Along with other properties of measured spectrum, the smooth D-C transition provides evidence in support of the recombining state of the plasma attained under detached divertor conditions.

MARFEs are experimentally studied using spectroscopic techniques to determine their internal characteristics. We find the local density and temperature in the MARFE to be $(2--3)\ifmmode\times\else\texttimes\fi{}{10}^{21}\mathrm{m}{}^{\ensuremath{-}3}$ and 0.7--1.0 eV, respectively. The volume recombination rate is also large in the MARFE, $(5--10)\ifmmode\times\else\texttimes\fi{}{10}^{22}\mathrm{s}{}^{\ensuremath{-}1}$, similar in magnitude to the plasma source rate in the core. Because this plasma sink is located in confined regions of the plasma, the MARFE acts like a limiter; ions ``recycle'' from the MARFE as neutrals. These characteristics have not been predicted by current models.

Neutral density profiles have been derived along the outboard divertor target of Alcator C-Mod and inside the last closed flux surface (LCFS). Local emission rates are obtained by a tomographic inversion of measured brightness profiles. Electron temperatures and densities provided by a divertor Langmuir probe array are then used to calculate the local neutral density, , from the emissivity profile. For points inside the LCFS, electron cyclotron emission (ECE) and interferometry data were used for electron density and temperature, respectively. Along the divertor target, steep gradients in neutral density persist during regular ohmic attached discharges, with . Plasma detachment leads to values of that can reach up to several times in the bottom of the divertor with much shallower pressure gradients to the midplane. Modelling of the parallel plasma momentum decrease due to charge exchange (CX) and elastic ion - neutral collisions indicates that neutral densities well below those observed are sufficient to explain the measured drop in parallel plasma pressure. This suggests that significant momentum is carried by the neutrals in the divertor resulting in decreased efficiency of momentum transfer between ions and neutrals.

The intrinsic impurity content of molybdenum and carbon was measured in the Alcator C-Mod tokamak using low resolution, multilayer mirror (MLM) spectroscopy ( Delta lambda ~1-10 AA). Molybdenum was the dominant high-Z impurity and originated from the molybdenum armour tiles covering all of the plasma facing surfaces (including the inner column, the poloidal divertor plates and the ion cyclotron resonant frequency (ICRF) limiter) at Alcator C-Mod. Despite the all metal first wall, a carbon concentration of 1 to 2% existed in the plasma and was the major low-Z impurity in Alcator C-Mod. Thus, the behaviour of intrinsic molybdenum and carbon penetrating into the main plasma and the effect on the plasma must be measured and characterized during various modes of Alcator C-Mod operation. To this end, soft X-ray extreme ultraviolet (XUV) emission lines of charge states, ranging from hydrogen-like to helium-like lines of carbon (radius/minor radius, r/a~1) at the plasma edge to potassium to chlorine-like (0.4

Analysis of the experimental data from tokamaks and linear divertor simulators leads to the conclusion that plasma recombination is a crucial element of plasma detachment. Different mechanisms of plasma recombination relevant to the experimental conditions of the tokamak scrape-off layer (SOL) and divertor simulators are considered. The physics of Molecular Activated Recombination (MAR) involving vibrationally excited molecular hydrogen are discussed. Although conventional Electron–Ion Recombination (EIR) alone can strongly alter the plasma parameters, MAR impact can be substantial for both tokamak SOL plasma and divertor simulators. Investigation of the effects of EIR on the plasma flow in divertor simulators shows that due to the balances of (a) energy transport and electron cooling, and (b) the plasma flow and recombination, that EIR extinguishes the simulator plasma at an electron temperature about 0.15 eV.

A series of experiments has been carried out to determine the relative screening of the low recycling gaseous impurities, nitrogen and methane, in Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] discharges. The impurity density in the core plasma is directly proportional to the rate of impurity injection. Screening was found to vary weakly with plasma density but to be markedly dependent on the poloidal position of injection. Detachment of the plasma results in significantly less screening both in Ohmic and in high confinement (H-mode) discharges, particularly when the gas is injected into the divertor or at the inner vessel wall. Evidence of impurity transport from the scrape-off layer into the divertor using bolometry and visible spectroscopy is presented and modeling of the transport using a two-dimensional Monte Carlo code is described.

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 ...

Enhanced confinement modes up to a toroidal field of BT=8 T have been studied with up to 3.5 MW of radiofrequency (rf) heating power in the ion cyclotron range of frequencies (ICRF) at 80 MHz. H-mode is observed when the edge temperature exceeds a threshold value. The high confinement mode (H-mode) with higher confinement enhancement factors (H) and longer duration became possible after boronization by reducing the radiated power from the main plasma. A quasi-steady state with high confinement (H=2.0), high normalized beta (βN=1.5), low radiated power fraction (Pradmain/Ploss=0.3), and low effective charge (Zeff=1.5) has been obtained in Enhanced Dα H-mode. This type of H-mode has enhanced levels of continuous Dα emission and very little or no edge localized mode (ELM) activity, and reduced core particle confinement time relative to ELM-free H-mode. The pellet enhanced performance (PEP) mode is obtained by combining core fueling with pellet injection and core heating. A highly peaked pressure profile with a central value of 8 atmospheres was observed. The steep pressure gradient drives off-axis bootstrap current, resulting in a shear reversed safety factor (q) profile. Suppression of sawteeth appears to be important in maintaining the highly peaked pressure profile. Lithium pellets were found to be more effective than deuterium pellets in raising q0.

X-ray spectra from Alcator C-Mod plasmas have been collected using a high wavelength resolution, five-spectrometer array during a wide range of operating conditions, providing a large variety of diagnostic information. Each independently scannable von Hamos-type spectrometer has a wavelength range of 2.6–4.1 A and the complete Rydberg series of He- and H-like argon has been observed. Spectra of Δn = 1 ground state transitions and satellites taken along various chords have been simulated using the results from a collisional radiative model and the MIST transport code. Line ratios are used to infer the electron temperature profile and good agreement is found with electron cyclotron emission profiles. Line intensities have been used to obtain absolute argon densities and argon recycling coefficients, and up-down asymmetric density profiles are sometimes observed. The widths of the strongest lines have been used to deduce ion temperature profiles, and wavelength shifts have been used to determine toroidal rotation velocities. Transitions from around n = 9 to the ground state are populated by charge exchange in the outer regions of the plasma and these line intensities have been used to measure neutral hydrogen density profiles. Spectra from helium-like scandium have been obtained during injection experiments and time histories and line intensities have been used to determine impurity transport coefficients. Δn ≥2 ground state transitions in molybdenum charge states around neon-like; have been observed and used to measure the absolute molybdenum density and to test atomic structure calculations.

A diagnostic neutral beam will be installed on Alcator C-Mod for measurement of ni, nz, Tz, vθ, vφ, and j. The beam will be used primarily for the study of thermal transport, Er in H-mode discharges, and to provide critical profile and turbulence data for tests of theoretically based models of turbulence and transport. The beam selected for this work was used on Texas Experimental Tokamak and is rated at 50 kV and 6 A of extracted current in hydrogen. It can produce beams of deuterium or helium as well. It is currently being reconfigured for installation on C-Mod. Capabilities for the intended measurements are described using simulations based on measured C-Mod profiles.

Considerations are presented on the application of a diagnostic neutral beam to the Alcator C-Mod tokamak for active profile measurements of ion temperature, poloidal and toroidal rotation, impurity and minority species densities, and internal magnetic field. Calculations indicate that, for a typical 50 keV, 0.13 MW hydrogen beam, charge exchange recombination measurements for plasmas with line average densities up to 2.5×1020 m−3 will yield, with good signal-to-noise ratio and ∼1 cm spatial resolution, measurements of helium and low Z impurity density profiles with better than 1 ms time resolution, and ion temperature and rotation profiles with ∼1 ms time resolution. Motional Stark effect (MSE) measurements appear feasible over most of the operating density range of the device (at least up to 5×1020 m−3), also with ms time resolution. Spatial resolution of the MSE measurements will depend on the toroidal width of the beam and the angle of the view with respect to the beam; for a 5 cm wide beam with a 60°...

Local measurements of line emission of either intrinsic or gas puffed impurities will be performed around and below the X point. Such spectroscopic measurements, together with divertor Thomson scattering and bolometric imaging will enable estimates of divertor impurity particle distributions and power losses. A proposed multilayer mirror (MLM) diagnostic mounted inside the tokamak vessel will measure XUV (100–200 A) emission from impurities at multiple locations in the divertor region of the C-Mod tokamak. A prototype of this instrument measuring N V at 162.6 A is scheduled to be installed on C-Mod with a single chordal view of the X point. A multichordal/multispectral device using MLMs and gratings has been designed for the DIII-D tokamak. The instrument will measure the resonance emission of C II to C VI from 40–2000 A, thus enabling a more direct and reliable estimate of the impurity content in the divertor. At DIII-D, the device will be mounted on a port below the mid plane avoiding spectroscopic cont...

With fusion device performance hinging on the edge pedestal pressure, it is imperative to experimentally understand the physical mechanism dictating the pedestal characteristics and to validate and improve pedestal predictive models. This Letter reports direct evidence of density and magnetic fluctuations showing the stiff onset of an edge instability leading to the saturation of the pedestal on the Alcator C-Mod tokamak. Edge stability analyses indicate that the pedestal is unstable to both ballooning mode and kinetic ballooning mode in agreement with observations.

Joint experiment/theory/modelling research has led to increased confidence in predictions of the pedestal height in ITER. This work was performed as part of a US Department of Energy Joint Research Target in FY11 to identify physics processes that control the H-mode pedestal structure. The study included experiments on C-Mod, DIII-D and NSTX as well as interpretation of experimental data with theory-based modelling codes. This work provides increased confidence in the ability of models for peeling-ballooning stability, bootstrap current, pedestal width and pedestal height scaling to make correct predictions, with some areas needing further work also being identified. A model for pedestal pressure height has made good predictions in existing machines for a range in pressure of a factor of 20. This provides a solid basis for predicting the maximum pedestal pressure height in ITER, which is found to be an extrapolation of a factor of 3 beyond the existing data set. Models were studied for a number of processes that are proposed to play a role in the pedestal ne and Te profiles. These processes include neoclassical transport, paleoclassical transport, electron temperature gradient turbulence and neutral fuelling. All of these processes may be important, with the importance being dependent on the plasma regime. Studies with several electromagnetic gyrokinetic codes show that the gradients in and on top of the pedestal can drive a number of instabilities. © 2013 IAEA, Vienna.

Application of lower hybrid range of frequencies (LHRF) waves can induce both co- and counter-current directed changes in toroidal rotation in Alcator C-Mod plasmas, depending on the target plasma current, electron density, confinement regime and magnetic shear. For ohmic L-mode discharges with good core LH wave absorption, and significant current drive at a fixed LH power near 0.8 MW, the interior (r/a q95/11.5, and in the co-current direction if ne(1020 m−3) 1, indicating a good correlation with driven current fraction, unifying the results observed on various tokamaks. For high density (ne ≥ 1.2 × 1020 m−3) L-mode target discharges, where core LH wave absorption is low, the rotation change is in the co-current direction, but evolves on a shorter momentum transport time scale, and is seen across the entire spatial profile. For H-mode target plasmas, both co- and counter-current direction increments have been observed with LHRF. The H-mode co-rotation is correlated with the pedestal temperature gradient, which itself is enhanced by the LH waves absorbed in the plasma periphery. The H-mode counter-rotation increment, a flattening of the peaked velocity profile in the core, is consistent with a reduction in the momentum pinch correlated with a steepening of the core density profile. Most of these rotation changes must be due to indirect transport effects of LH waves on various parameters, which modify the momentum flux.

We describe results from imaging observations of atomic line and continuum emission in the 550.6 nm region on Alcator C-Mod. Both the 550.6 nm neutral molybdenum emission and the adjacent 549 nm continuum emission are imaged separately to isolate line emission. A few complications of using imaging to infer erosion in this wavelength region are discussed including subtraction of continuum emission and determination of an appropriate S/XB coefficient. Diagnostics of surface erosion and thermography using these emissions are briefly reviewed, and used to study phenomenology during ohmic operation, ion cyclotron range of frequencies heating (ICRH), and lower hybrid current drive (LHCD). In addition to broadening of Mo I emission regions in the outer divertor and main limiter during ICRH compared to ohmic operation, mid-plane localized heating of the main limiter associated with fast-ion impact is observed which exceeds the divertor heat flux. During LHCD operation, several localized regions of increased brightness associated with hot spots are interpreted as heating due to localized density peaking, which re-iterates the importance of imaging continuum emission for subtraction. These sources of surface heating exacerbate plasma–material interactions at the device wall and may require additional mitigation if they cannot be avoided in future machines.

A model is developed which constrains heat width, λr based on global power balance, momentum conservation, pedestal stability and sheath heat transmission. The model relies on measurements of the ratio of separatrix to pedestal pressure; a ratio ∼5% is found to be expected for ITER. Applying this model indicates a constraint that the allowed λr ∼ 10–30 mm for ITER if the divertor is in the high-recycling regime as expected (T < 20 eV) while a λr ∼ 1–3 mm requires a separatrix pressure approximately equal to the top pedestal pressure in violation of physical reasoning and the concept of a pedestal. A weaker constraint is applied in the model that upstream separatrix temperature simultaneously satisfies power balance. The constrained model cannot satisfy power balance with λr < 3 mm, and in order to obtain λr ∼ 5 mm requires divertor plasma temperature >100 eV, a condition which would have very negative consequences for the divertor, but has never been observed experimentally., United States. Department of Defense (US DOE award DE-SC00-02060), United States. Department of Defense (contract DE-FC02-99ER54512), United States. Department of Defense (cooperative agreement DE-FC02- 04ER54698)

Growth of tungsten nano-tendrils (“fuzz”) has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The depth of the W fuzz layer (600 ± 150 nm) is consistent with an empirical growth formula from the PISCES experiment. Re-creating the C-Mod exposures as closely as possible in Pilot-PSI experiment can produce nearly-identical nano-tendril morphology and layer thickness at surface temperatures that agree with uncertainties with the C-Mod W probe temperature data. Helium concentrations in W fuzz layers are measured at 1–4 at.%, which is lower than expected for the observed sub-surface voids to be filled with several GPa of helium pressure. This possibly indicates that the void formation is not pressure driven., United States. Dept. of Energy (Award DE-SC00-02060)

Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data. It is found that the fluctuation amplitudes in the near scrape-off layer follow a normal distribution while the far scrape-off layer fluctuations are dominated by large amplitude bursts due to radial motion of blob-like structures and have a positively skewed and flattened amplitude probability distribution. Conditional averaging of the time series reveals burst wave forms with a fast rise and slow decay and exponentially distributed burst amplitudes and waiting times. Based on this, a stochastic model of the burst dynamics is constructed. The model predicts that fluctuation amplitudes should follow a Gamma distribution and that there is a parabolic relation between the skewness and the kurtosis moments of the fluctuations. This is shown to compare favorably with the gas puff imaging data over a range of line-averaged plasma densities.

Fully relativistic, ab initio calculations of the rate of dielectronic recombination (DR) have been performed for fluorinelike ${\mathrm{Mo}}^{33+}$ to magnesiumlike ${\mathrm{Mo}}^{30+}$ and chlorinelike ${\mathrm{Mo}}^{25+}$ and argonlike ${\mathrm{Mo}}^{24+}$. Calculations of the rate of excitation autoionization (EA) have been performed for neonlike ${\mathrm{Mo}}^{32+}$ to aluminumlike ${\mathrm{Mo}}^{29+}$ and argonlike ${\mathrm{Mo}}^{24+}$ and potassiumlike ${\mathrm{Mo}}^{23+}$. The detailed calculations of the atomic structure and rate coefficients for charge states in these groups allow interpolation of the DR and EA rates for the more complex ions having 3${\mathit{s}}^{2}$3${\mathit{p}}^{\mathit{k}}$ (k=1--4 for DR and k=2--5 for EA) ground states. The calculations for DR are broken up by different classes of excitation channels; simple, analytic formulas are then fit to the calculations. The effects of configuration interaction on the rates of DR and EA have been studied in detail and are found to have a slight effect on only a small class of the Auger rates needed for the present work. Radiative transitions between energy levels in the continuum are investigated and found to have a moderate effect on the DR rates and a small effect on the EA rates of the ions in this paper. \textcopyright{} 1996 The American Physical Society.

H-modes exhibiting improved confinement above the L-mode are achieved in Alcator C-Mod with ICRF and with ohmic heating alone without boronization. Both ELM-free and ELMy H-modes are obtained with total input power from 0.75 to 4.2 MW over a range of densities (0.8 to ) and toroidal fields (3 to 8 T). Type III ELMs are often observed to have coherent, high m and n precursor oscillations with frequencies of 100 - 160 kHz. The threshold power required to achieve the H-mode increases with density and toroidal field, in rough agreement with scalings derived from other tokamaks. The power densities and density times toroidal field products are an order of magnitude larger than in other tokamaks, in the range of values expected for ITER. The L - H and H - L transitions occur at approximately the same edge electron temperature. A low density limit to the H-mode is found at about . A high midplane neutral pressure limit of about 0.6 mTorr is also observed.

Brightness profiles of x-ray and VUV lines from eight molybdenum charge states between and have been measured in Alcator C-Mod plasmas. These spatial profiles agree very well with those predicted by a model which includes ionization, recombination, excitation and transport. Comparison with the profiles of many different charge states provides severe constraints upon the rates used in the model. The charge state density profiles are calculated using measured impurity transport coefficients, measured electron density and temperature profiles and newly calculated ionization and recombination rate coefficients. These new rate coefficients include direct collisional ionization, excitation - autoionization, dielectronic and radiative recombination. Excitation - autoionization is shown to be an important process, since the excellent agreement between the measurements and predictions is obtained only with its inclusion. Fits to newly calculated excitation rate coefficients for the transitions are also presented.

At energies below the threshold for direct electron impact excitation, resonant excitations can make a significant contribution to the total excitation rate of a given energy level. In this paper, the rates of resonant excitation into the levels of the 3{ital d}{sup 9}4{ital s} and 3{ital d}{sup 9}4{ital p} configurations of Mo{sup 14+} have been calculated using a fully relativistic, multiconfiguration atomic structure code and detailed accounting of energy levels. By including the effects of resonant excitations in collisional-radiative models for the spectrum of NiI{endash}like Mo{sup 14+} and (by isoelectronic scaling) Zr{sup 12+}, the ratio of the emissivity of the 3{ital d}{sup 10}-4{ital d}{sup 9}4{ital s} {ital E}2 transitions to the emissivity of the 3{ital d}{sup 10}-3{ital d}{sup 9}4{ital p} {ital E}1 transitions is greatly enhanced, and sensitivity to electron temperature in the ratio is introduced. This ratio is density sensitive for {ital n}{sub {ital e}}{ge}10{sup 13} cm{sup {minus}3}, and therefore, given knowledge of either local temperature or density conditions, the {ital E}2-{ital E}1 ratio can serve as a diagnostic for local conditions in magnetically confined fusion plasmas. The current work demonstrates the need to include resonant excitations in collisional-radiative models of the soft x-ray emission of nickel-like ions. Goodmore » agreement is found between measurements of {ital E}1 and {ital E}2 line brightness ratios made in a tokamak plasma, and the predictions of collisional-radiative models in the present work. {copyright} {ital 1996 The American Physical Society.}« less

The regime of high particle and energy confinement known as the H mode [Phys. Rev. Lett. 49, 1408 (1982)] has been extended to a unique range of operation for divertor tokamaks up to toroidal fields of nearly 8 T, line‐averaged electron densities of 3×1020 m−3, and surface power densities of nearly 0.6 MW/m2 in the compact high‐field tokamak Alcator C Mod [Phys. Plasmas 1, 1511 (1994)]. H modes are achieved in Alcator C Mod with Ion Cyclotron Resonant Frequency (ICRF) heating and with Ohmic heating alone without boronization of the all molybdenum tiled first wall. Large increases in charge exchange flux are observed during the H mode over the entire range of energies from 2 to 10 keV. There appears to be an upper limit to the midplane neutral pressure, of about 0.08 Pa above which no H modes have been observed. The plasmas with the best energy confinement have the lowest midplane neutral pressures, below 0.01 Pa. There is an edge electron temperature threshold such that Te≥280 eV ±40 eV for sustaining the...

The divertor of the Alcator C‐Mod tokamak [Phys. Plasmas 1, 1511 (1994)] routinely radiates a large fraction of the power entering the scrape‐off layer. This dissipative divertor operation occurs whether the divertor is detached or not, and large volumetric radiative emissivities, up to 60 MW m−3 in ion cyclotron range of frequency (ICRF) heated discharges, have been measured using bolometer arrays. An analysis of both Ohmic and ICRF‐heated discharges has demonstrated some of the relative merits of detached divertor operation versus high‐recycling divertor operation. An advantage of detached divertor operation is that the power flux to the divertor plates is decreased even further than its already low value. Some disadvantages are that volumetric losses outside the separatrix in the divertor region are decreased, the neutral compression ratio is decreased, and the penetration efficiency of impurities increases.

Imbalances between the inboard and outboard legs of the single null divertor in tokamak Alcator C-Mod are observed to reverse when the direction of the toroidal field is reversed. These imbalances are measured by embedded probes in the target plates, tomographic reconstructions of bolometry and line radiation, and visible imaging. Density imbalances of about a factor of ten at the targets are observed at moderate density, decreasing as the density is raised until they are almost balanced. The data indicate that the electron pressure is not imbalanced, thus arguing against momentum imbalance as the cause of these drift-induced effects. Instead, power flux imbalance caused by Er V-product B convection, and enhanced by radiation, is suggested as the underlying cause.