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Tokamak operation at negative triangularity has been shown to offer high energy confinement without the typical disadvantages of edge pedestals (Marinoni et al 2021 Nucl. Fusion 61 116010). In this paper, we examine impurity transport in DIII-D diverted negative triangularity experiments. Analysis of charge exchange recombination spectroscopy reveals flat or hollow carbon density profiles in the core, and impurity confinement times consistently shorter than energy confinement times. Bayesian inferences of impurity transport coefficients based on laser blow-off injections and forward modeling via the Aurora package (Sciortino et al 2021 Plasma Phys. Control. Fusion 63 112001) show core cross-field diffusion to be higher in L-mode than in H-mode. Impurity profile shapes remain flat or hollow in all cases. Inferred radial profiles of diffusion and convection are compared to neoclassical, quasilinear gyrofluid, and nonlinear gyrokinetic simulations. Heat transport is observed to be better captured by reduced turbulence models with respect to particle transport. State-of-the-art gyrokinetic modeling compares favorably with measurements across multiple transport channels. Overall, these results suggest that diverted negative triangularity discharges may offer a path to a highly-radiative L-mode scenario with high core performance.

Global and local transport experiments in ohmic, L-mode and H-mode regimes on the Alcator C-Mod tokamak are summarized. For ohmic plasmas, earlier results derived for energy confinement scaling in ...

To introduce this special issue, this paper gives a brief overview of the key themes in the Alcator C-Mod tokamak research program. These include energy, particle and momentum transport, wave-plasm...

An overview of the diagnostics installed on the Alcator C-Mod tokamak is presented. Approximately 25 diagnostic systems are being operated on C-Mod. The compact design of the machine and the cryostat enclosing the vacuum vessel and magnetic field coils make access challenging. Diagnostics are used to study four focus areas: transport, plasma boundary, waves, and macrostability. There is significant overlap between these topics, and they all contribute toward the burning plasma and advanced tokamak thrusts. Several advanced and novel diagnostics contribute to the investigation of C-Mod plasmas, e.g., electron cyclotron emission, phase-contrast imaging, gas puff imaging, probe measurements, and active magnetohydrodynamic antennas.

The Rydberg series (1s2 to 1snp) up to n = 14 of helium-like argon (Z = 18) has been observed from Alcator C-Mod plasmas using a high-resolution X-ray spectrometer array. High-n satellites to these...

H-mode research on Alcator C-Mod is described, with a focus on the edge transport barrier (ETB). ETB pedestals are characterized using several diagnostics, leading to a thorough description of prof...

A high speed (250 kHz), 300 frame charge coupled device camera has been used to image turbulence in the Alcator C-Mod Tokamak. The camera system is described and some of its important characteristics are measured, including time response and uniformity over the field-of-view. The diagnostic has been used in two applications. One uses gas-puff imaging to illuminate the turbulence in the edge/scrape-off-layer region, where D2 gas puffs localize the emission in a plane perpendicular to the magnetic field when viewed by the camera system. The dynamics of the underlying turbulence around and outside the separatrix are detected in this manner. In a second diagnostic application, the light from an injected, ablating, high speed Li pellet is observed radially from the outer midplane, and fast poloidal motion of toroidal striations are seen in the Li+ light well inside the separatrix.

Research on the Alcator C-Mod tokamak [1] is focused on high particle- and power-density plasma regimes to understand particle and energy transport in the core, the dynamics of the H-mode pedestal, and scrape-off layer and divertor physics. The auxiliary heating is provided exclusively by RF waves, and both the physics and technology of RF heating and current drive are studied. The momentum which is manifested in strong toroidal rotation, in the absence of direct momentum input, has been shown to be transported in from the edge of the plasma following the L–H transition, with timescale comparable to that for energy transport. In discharges which develop internal transport barriers, the rotation slows first inside the barrier region, and then subsequently outside of the barrier foot. Heat pulse propagation studies using sawteeth indicate a very narrow region of strongly reduced energy transport, located near r/a = 0.5. Addition of on-axis ICRF heating arrests the buildup of density and impurities, leading to quasi-steady conditions. The quasi-coherent mode associated with enhanced D-Alpha (EDA) H-mode appears to be due to a resistive ballooning instability. As the pedestal pressure gradient and temperature are increased in EDA H-mode, small ELMs appear; detailed modelling indicates that these are due to intermediate n peeling–ballooning modes. Phase contrast imaging has been used to directly detect density fluctuations driven by ICRF waves in the core of the plasma, and mode conversion to an intermediate wavelength ion cyclotron wave has been observed for the first time. The bursty turbulent density fluctuations, observed to drive rapid cross-field particle transport in the edge plasma, appear to play a key role in the dynamics of the density limit. Preparations for quasi-steady-state advanced tokamak studies with lower hybrid current drive are well underway, and time dependent modelling indicates that regimes with high bootstrap fraction can be produced.

Simultaneous high resolution measurements of the soft x-ray emission at the top and outboard edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. Impurity density profiles are derived from these measurements. The impurity density varies considerably on a flux surface in the high confinement mode (H-mode) pedestal region. The soft x-ray and impurity density pedestals are not on the same flux surface at the top and outboard edge. The pedestal widths are consistently larger at the outboard edge in the Enhanced D-alpha (EDA) H-mode, suggesting a ballooning-like character of the EDA quasi-coherent mode. The usual separation of time scales between radial and parallel impurity transport does not hold in the H-mode pedestal region of Alcator C-Mod. Thus, one should not necessarily expect that the impurity density be constant on a flux surface in this region.

Double transport barrier plasmas comprised of an edge enhanced D? (EDA) H-mode pedestal and an internal transport barrier (ITB) have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near |r/a|~0.5, provided the target plasma average density is above ~1.4?1020?m-3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity. The ITB foot is located near r/a~0.5, regardless of how the barrier was produced. The ITBs can persist for ~15 energy confinement times (?E), but exhibit a continuous increase of the central electron density, up to values near 1?1021?m-3 (in the absence of an internal particle source), followed by collapse of the barrier. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. Quasi-steady state double barrier plasmas have been maintained for 10?E or longer, with a bootstrap fraction of 0.13 near the ITB foot.

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.

In this paper we summarize the results of recent measurements of the high confinement mode (H-mode) edge transport barrier in Alcator C-Mod. A number of new, high-resolution edge diagnostics provide measurements of plasma parameters (electron density and temperature, and impurity density) in the pedestal region. Pedestal parameters are measured both at the top edge of the plasma and at the midplane. Very narrow (2-5 mm) electron density and temperature and impurity density pedestals are observed. The impurity pedestal is located inside the density and temperature pedestals in the midplane measurements, but coincides with the ne pedestal at the top edge. Correlation of both width and height of the pedestals at the midplane with global plasma parameters and core confinement characteristics is observed. At the same time there are no clear indications of scaling for the electron density and impurity pedestal widths measured at the top of the plasma.

Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjunction with an atomic physics package for individual line emission. During Low-Confinement-Mode (L-mode) plasmas, the transport can be characterized by pure diffusion, with coefficients ∼5000 cm2/s, reflecting the ∼20 ms decay in the x-ray and VUV line brightnesses. During High-Confinement-Modes (H-modes), the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (o...

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.

Routine tritium operation in TFTR has permitted investigations of alpha particle physics in parameter ranges resembling those of a reactor core. ICRF wave physics in a DT plasma and the influence of isotopic mass on supershot confinement have also been studied. Continued progress has been made in optimizing fusion power production in TFTR, using extended machine capability and Li wall conditioning. Performance is currently limited by MHD stability. A new reversed magnetic shear regime is being investigated with reduced core transport and a higher predicted stability limit.

X-ray spectra of 2p-nd transitions with 4\ensuremath{\le}n\ensuremath{\le}18 in molybdenum (Z=42) charge states around neonlike (${\mathrm{Mo}}^{32+}$) have been observed from Alcator C-Mod plasmas. Accurate wavelengths (\ifmmode\pm\else\textpm\fi{}0.1 m\AA{}) have been determined by comparison with neighboring argon and chlorine lines with well-known wavelengths. Line identifications have been made by comparison to ab initio atomic-structure calculations, using a fully relativistic, parametric-potential code, and agreement between measured and theoretical wavelengths is good. Calculated wavelengths and oscillator strengths are presented for the strongest transitions with upper levels n between 4 and 14 to the lower levels n=2 in the four charge states ${\mathrm{Mo}}^{30+}$\char21{}${\mathrm{Mo}}^{33+}$. Effects of configuration interaction have been observed in the intensities of lines with nearly degenerate energy levels.

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm(-1)≤k(θ)≤11 cm(-1) and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.

International audience; Intrinsic rotation has been observed in I-mode plasmas from the C-Mod tokamak, and is found to be similar to that in H mode, both in its edge origin and in the scaling with global pressure. Since both plasmas have similar edge ∇T, but completely different edge ∇n, it may be concluded that the drive of the intrinsic rotation is the edge ∇T rather than ∇P. Evidence suggests that the connection between gradients and rotation is the residual stress, and a scaling for the rotation from conversion of free energy to macroscopic flow is calculated.

The direct measurement of lower hybrid (LH) waves in plasma can provide information on the propagation and absorption of LH waves in tokamak plasma and validate numerical simulations. Here, we propose to utilize the scattering of probe waves (60 GHz) off LH waves (4.6 GHz) to diagnose LH waves in the scrape off layer. We develop an analytic model to describe the scattering process in weakly inhomogeneous plasma based on the three‐wave coupling problem. We then present first experimental results obtained by upgrading an ordinary‐mode reflectometer system in Alcator C‐Mod.

Recent research on TFTR has emphasized optimization of performance in deuterium plasmas, transport studies and studies of energetic ion and fusion product physics in preparation for the D-T experiments that will commence in July of 1993. TFTR has achieved full hardware design parameters, and the best TFTR discharges in deuterium are projected to QDT of 0.3 to 0.5.The physics phenomena that will be studied during the D-T phase will include: tritium particle confinement and fueling, ICRF heating with tritium, species scaling with tritium, collective alpha-particle instabilities, alpha heating of the plasma and helium ash buildup. It is important for the fusion program that these physics issues be addressed to identify regimes of benign alpha behavior, and to develop techniques to actively stabilize or control instabilities driver by collective alpha effects.

Plasma flow drive via ICRF mode conversion (MC) has been demonstrated on Alcator C‐Mod. The toroidal rotation in these D(3He) MC plasmas is typically more than twice above the empirically determined intrinsic rotation scaling in ICRF minority heated plasmas. In L‐mode plasmas at 3 MW ICRF power input, up to 90 km/s toroidal rotation and 2 km/s localized ( r/a∼0.4) poloidal rotation has been observed. The MC ion cyclotron wave (ICW) was detected by a phase contrast imaging system in heterodyne setup. Through TORIC 2‐D full wave simulation, and comparison with other experimental evidence, we hypothesize that the interaction between the MC ICW and the 3He ions may be the mechanism for the observed MC flow drive. TORIC simulation suggests that similar flow drive scenario may be realized on JET D(3He) plasmas. The promising scenarios on ITER are the inverted minority scenario (T)D and high field launch for T‐D‐(3He) plasma. In non‐radioactive phase, these correspond to (3He)‐H and 4He(3He) plasmas respectively.

The effects of a low-energy charge-exchange process between hydrogenlike ions and neutral hydrogen on x-ray spectra of He-like argon ions from the Alcator C tokamak are investigated. Dependences of charge-exchange cross sections on both the principal and angular momentum quantum numbers n and l are estimated from the observed spectra, taking cascade effects into account with the use of a collisional radiative model.

An instrument designed to measure k-filtered plasma fluctuations has been used to look at edge fluctuations in Alcator C-Mod tokamak plasmas. The instrument utilizes a sieve technique, whereby an optical image is split and filtered through two out-of-phase, striped transmission masks, and then imaged onto photodiode detectors. The difference signal is then digitized and analyzed to give the frequency characteristics of the fluctuations at a particular value of k, as determined by the wavelength of the mask stripes. Measurements have been made looking at the edge plasma with hydrogen Balmer-alpha filtering, and k both parallel and perpendicular to the magnetic field, with |k| in the range from 5 to 15 cm−1. Broad band (ν

ICRF mode conversion (MC) in H(3He, D) and D(H) plasmas have been studied in detail in Alcator C‐Mod. In H(3He, D) plasma, the mode converted ion cyclotron wave (MC ICW) was observed in tokamak plasmas for the first time using a phase contrast imaging system. The MC ICW was observed in the low field side of the ion‐ion hybrid layer, and generally had a wavelength in‐between the fast wave and the MC ion Bernstein wave (IBW). Localized mode conversion electron heating (MCEH) has been clearly observed for the first time in D(H) plasmas with moderate hydrogen concentration in Alcator C‐Mod. Both on‐ and off‐axis (high field side) MCEH have been studied. The MCEH profile was obtained from a break in slope analysis of Te signals in the presence of rf shut‐off. The experimental profiles were qualitatively in agreement with the predictions of the two‐dimensional full‐wave poloidal mode code TORIC. The electron heating contributions from MC ICW and MC IBW are examined from the TORIC simulations.

Measurements on impurity pellet ablation trails indicate that the emission from these clouds extends much farther along the magnetic field than across it. For carbon pellets on TFTR, the aspect ratio of this cigar‐shaped plume is of the order of 10. Strong visible line emission from one of the ionization states can be imaged with high time resolution, and since ions should be well confined along the field lines, it should be possible in this way to measure the radial profile of the pitch of the total internal magnetic field. For a pellet injected along a major radius in the plasma midplane, the appropriate viewing geometry consists of imaging the pellet along its flight from behind. A detector scheme capable of accomplishing the measurement, consisting of an array of 1‐D spot‐imaging photodiodes, is described. In order to make meaningful measurements with regard to the tokamak physics of interest, it is highly desirable to measure the field angle profile with a precision of ±10 mrad or better. In order to achieve this for a cigar with aspect ratio of 10, measurement of the mean vertical position for each slice of the cigar must be made with a precision of about 10% of the vertical thickness of the cigar on that slice.

A five‐spectrometer array has been designed to provide high‐wavelength resolution x‐ray spectra from the Alcator C‐Mod plasma. Each independently scannable von Hamos type spectrometer consists of a variable entrance slit, quartz crystal, and position sensitive proportional counter, and has 3 cm spatial resolution with a resolving power of 4000. The wavelength range of each spectrometer is from 2.8 to 4.0 A, which covers the complete Rydberg series of helium‐ and hydrogen‐like argon. Wavelengths are remotely selectable, with 120 mA covered at any one setting. The array will provide single shot ion temperature profiles by measuring the Doppler broadening of the resonance lines of argon. Five proportional counters have been fabricated and all five spectrometers have been constructed. Calibration spectra using cadmium and scandium anode sources have been obtained, and measured performance characteristics are presented.

Studies of potential plasma facing component (PFC) materials for a magnetic fusion reactor generally conclude that tungsten is the best choice due to its low tritium (T) retention, capability to handle high heat fluxes with low erosion, and robustness to nuclear damage and activation. ITER [F. Perkins et al., Nucl. Fusion 39, 2137 (1999)] may operate with all tungsten PFCs to provide the necessary operational experience for a reactor. Alcator C-Mod [I. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] operates with molybdenum (Mo) high-Z PFCs, which have very similar properties to tungsten. The experiments described herein have provided a unique comparison of operation with or without in situ boron coatings applied to the molybdenum PFCs; the latter are likely most relevant to ITER and beyond. ICRF-heated H-modes were readily achieved without boron coatings although the resultant enhancement in energy confinement was typically small (HITER,89∼1). Molybdenum concentrations, nMo∕ne, rise rapidly after the H-...

Measurements of H- and He-like X-ray spectra of elements between Z = 16 and 26 (hv = 2 to 8 keV) are discussed and illustrated with recent results obtained for argon. Examples of instrumental development are presented together with the outlook for future experiments.

Short bursts of aluminum were injected into the Adiabatic Toroidal Compressor (ATC) Tokamak, and measured the time evolution of the radial distributions of highly ionized states of aluminum. The results are compared with a computer code describing neoclassical impurity diffusion and good agreement is found. (auth)

Resultats des premieres mesures radiales des transitions Rx, n=2 a n=1, des elements Z eleves dans des plasmas de deuterium de tokamaks. Les rapports des intensites des raies observees ne peuvent s'expliquer par des processus d'excitation par impact electronique et recombinaison dielectronique. La recombinaison radiative et de transfert de charge est proposee comme mecanisme de population additionnelle. Les raies emises dans la cascade suivant la capture constituent un diagnostic unique de la composante neutre des plasmas

Spectra of the complete series of ground-state transitions in heliumlike Ar/sup 16+/ (1snp-1s/sup 2/, for 2less than or equal ton 0.5), recombination of Ar/sup 17+/ is the dominant population process for upper levels of most transitions, and preferentially populates the n = 2 triplet levels. Impurity transport provides sufficient amounts of hydrogenlike argon at the cooler, outer radii. For the high-n transitions 1s9p-1s/sup 2/ and 1s10p-1s/sup 2/, charge-exchange recombination is the dominant population mechanism at the plasma edge under certain operating conditions. The intensities of these lines have been used to determine neutral-hydrogen density profiles. For ground-state transitions with n>13, charge transfer from excited neutral hydrogen is the only important population process for r/a/sub L/>0.5.

This paper presents advances in the use of high resolution x-ray spectroscopy for the measurements of tokamak plasmas and the study of atomic processes. The measurements refer to H- and He- like spectra of Ar (λ = 3.7 to 4.2 A) obtained with a new crystal spectrometer at the Alcator C tokamak. With record count rates of up to 0.5 MHz, time resolved spectroscopy of individual plasma discharge was demonstrated at the 20 ms time scale. In particular, the time evolution of ion and electron temperatures, Ti(t) and Te(t), was determined and occurrences of transient plasma phenomena were recorded in discharges with deuterium pellet injection. X-ray line emission was for the first time observed along different chords through the plasma so as to determine the radial dependence of the principal plasma parameters Ti(r) and Te(r). These radial scan data are shown to be of interest for identifying manifestations of underlying atomic processes and ion transport effects. On the basis of the present results we assess the future use of x-ray spectroscopy diagnostics to obtain extended information on the temporal and spatial dependences of the main plasma parameters together with new information on transient phenomena as well as on the neutral content of tokamak plasmas.

A system for producing 300‐μsec bursts of 1017 metal atoms with 3‐eV average energy is described. It is shown that this system can be successfully used to introduce impurities into CTR‐oriented tokamaks for transport and confinement studies.

X-ray spectra of He-like argon (Ar/sup +16/) have been obtained for the transitions 1snp..-->..1s/sup 2/, with 3< or =n..Ar/sup +16asterisk/(1snp /sup 1/P)+H/sup +/ are presented and their cross sections are estimated.

The He-like spectrum of 1s2 - 1snp transitions and satellites in Ar16+ have been measured from the plasma of the Alcator C tokamak. In the wavelength region of λ = 3.0 to 3.4 A, three groups of satellite lines 1s22σ - 1s2σ nl are observed which accompany each of the resonance lines emanating from the upper orbitals from n = 4 to n = 10. Results of new calculations are presented and used for identification of the observed spectra and for discussion of line intensities and wavelengths. All satellite lines belonging to the high n states observed (n = 4 to n = 10) have been identified in the calculations.

A technical assessment of the critical issues and problem areas in the field of plasma materials interactions (PMI) in magnetic fusion devices shows these problems to be central for near-term experiments, for intermediate-range reactor devices including D-T burning physics experiments, and for long-term reactor machines. Critical technical issues are ones central to understanding and successful operation of existing and near-term experiments/reactors or devices of great importance for the long run, i.e., ones which will require an extensive, long-term development effort and thus should receive attention now. Four subgroups were formed to assess the critical PMI issues along four major lines: (1) PMI and plasma confinement physics experiments; (2) plasma-edge modelling and theory; (3) surface physics; and (4) materials technology for in-vessel components and the first wall. The report which follows is divided into four major sections, one for each of these topics.

Short bursts of aluminum were injected into the Adiabatic Toroidal Compressor (ATC) Tokamak, and measured the time evolution of the radial distributions of highly ionized states of aluminum. The results are compared with a computer code describing neoclassical impurity diffusion and good agreement is found. (auth)

Precision measurements of the absolute wavelengths of the Lyman- doublet (2p SP/sub 3/2,1/2/ 1s SS/sub 1/2/) in hydrogenlike argon (Ar X ), as well as those of some strong dielectronic satellites, are reported. The Alcator C tokamak plasma was seeded with trace amounts of argon and the emission spectra were taken with a high-resolution, crystal x-ray spectrometer. In situ wavelength calibration was achieved by utilizing the potassium K lines from a KCl fluorescence source. For the Ly 2 line, a precision of 11 ppm was achieved. Comparisons of these data with QED predicted wavelengths show good agreement, yielding a test of the 1s Lamb shift at the 3% level. The precision of these measurements was limited by uncertainties in the wavelength calibration, with the uncertainties due to the influence of unresolved satellites being of secondary importance.

A system for producing 300‐μsec bursts of 1017 metal atoms with 3‐eV average energy is described. It is shown that this system can be successfully used to introduce impurities into CTR‐oriented tokamaks for transport and confinement studies.

An experiment measuring the scaling with changing plasma conditions of a parameter characteristic of the transport of aluminum injected into the ATC tokamak is discussed. This parameter is the time after injection at which the photon signal from Al XI reaches its maximum. It is found that the data are in agreement with the predictions of a computer code which uses neoclassical theory in the Pfirsch-Schluter regime. An approximate model describing impurity transport in ATC is also presented. This model is utilized to calculate the expected scaling with current of the aluminum transport assuming classical, neoclassical and pseudoclassical forms for the diffusion coefficient. The data are in agreement with both the neoclassical and pseudoclassical results from this model.

Two methods which together allow sensitivity calibration from 20 A to 430 A are described in detail. The first method, useful up to 120 A, uses a low power source to generate K∝ X-rays which are alternately viewed by an absolute detector (a proportional counter) and the spectrometer. The second method extends that calibration to 430 A. It relies on the 2:1 brightness ratio of bright doublet lines from impurity ions which have a single outer shell electron and which are present in hot, magnetically confined plasmas. It requires that the absolute sensitivity of the spectrometer be known at one wavelength point, and in practice requires a multi-element spectral detector.

Densities of C, O, and Si impurities during the steady-state portion of Alcator-C discharges have been computed from spectroscopic measurements of the absolute brightnesses of visible and uv emission lines in combination with a one-dimensional transport calculation which models the charge-state and emissivity profiles. Profiles of all the charge states of a particular impurity were calculated by utilizing independent measurements of plasma density and temperature and by solving the coupled system of transport and rate equations connecting the ionization states. These profiles were then used to calculate emissivity profiles by solving the matrix equation relating the level populations through collisional excitation, collisional deexcitation, spontaneous emission, inner-shell ionization, and cascades from upper levels. Three different types of limiters (molybdenum, graphite, and SiC-coated graphite) have been used on the Alcator-C. It was observed that the principal impurities in the plasma, under most conditions, were determined by the type of limiter material being used. However, the source of the impurities could be either the wall or the limiters, since it has been shown that the wall becomes coated with limiter material. A significant influx of impurities directly from the limiters was often seen during the application of lower-hybrid-frequency rf power to the plasma.

Internal transport barriers (ITBs) marked by steep density and pressure profiles and reduction of core transport are obtained in Alcator C-Mod. Transient single barriers are observed at the back-transition from H- to L-mode and also when pellet injection is accompanied by ion cyclotron resonance frequency (ICRF) power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: The ions and electrons are tightly coupled by collisions with Ti/Te = 1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atm have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier region, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long-wavelength drift wave turbulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron modes (TEMs) inside the barrier. The TEM ultimately drives sufficient outgoing particle flux to balance the inward pinch and halt further density rise, which allows control of particle and impurity peaking.

Trace nonrecycling impurities (scandium and CaF 2 ) have been injected into Alcator C-Mod plasmas in order to determine impurity transport coefficient profiles in a number of operating regimes. Recycling Ar has also been injected to characterize steady-state impurity density profiles. Subsequent impurity emission has been observed with spatially scanning X-ray and vacuum ultraviolet spectrometer systems, in addition to very high spatial resolution X-ray and bolometer arrays viewing the plasma edge. Measured time-resolved brightness profiles of helium-, lithium-, and beryllium-like transitions have been compared with those calculated from a transport code that includes impurity diffusion and convection, in conjunction with an atomic physics package for individual line emission. Similar modeling has been performed for the edge observations, which are unresolved in energy. The line time histories and the profile shapes put large constraints on the impurity diffusion coefficient and convection velocity profiles. In L-mode plasmas, impurity confinement times are short (∼20 ms), with diffusivities in the range of 0.5 m 2 /s, anomalously large compared to neoclassical values. During Enhanced D α (EDA) H-modes, the impurity confinement times are longer than in L-mode plasmas, and the modeling suggests that there exists inward convection (≤50 m/s) near the plasma edge, with greatly reduced diffusion (of order 0.1 m 2 /s), also in the region of the edge transport barrier. These edge values of the transport coefficients during EDA H-mode are qualitatively similar to the neoclassical values. In edge localized mode-free H-mode discharges, impurity accumulation occurs, dominated by large inward impurity convection in the pedestal region. A scaling of the impurity confinement time with H-factor reveals a very strong exponential dependence. In internal transport barrier discharges, there is significant impurity accumulation inside of the barrier foot, typically at r/a = 0.5. Steady-state impurity density profiles in L-mode plasmas have a large up-down asymmetry near the last closed flux surface. The impurity density enhancement, in the direction opposite to the ion B X VB drift, is consistent with modeling of neoclassical parallel impurity transport.

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input. The magnitude of the rotation ranges from -60 km/s (countercur...

Electron temperature measurements from line ratios of He‐ and H‐like argon in the Alcator C tokamak