Search

Brain-derived extracellular vesicles (EVs) play an active role in Alzheimer's disease (AD), relaying important physiological information about their host tissues. Circulating EVs are protected from degradation, making them attractive AD biomarkers. However, it is unclear how circulating EVs relate to EVs isolated from disease-vulnerable brain regions. We developed a novel method for collecting EVs from the hippocampal interstitial fluid (ISF) of live mice. EVs (EVISF) were isolated via ultracentrifugation and characterized by nanoparticle tracking analysis, immunogold labeling, and flow cytometry. Mass spectrometry and proteomic analyses were performed on EVISF cargo. EVISF were 40-150 nm in size and expressed CD63, CD9, and CD81. Using a model of cerebral amyloidosis (e.g. APPswe,PSEN1dE9 mice), we found protein concentration increased but protein diversity decreased with A deposition. Genotype, age, and Aβ deposition modulated proteostasis- and immunometabolic-related pathways. Changes in the microglial EVISF proteome were sexually dimorphic and associated with a differential response of plaque associated microglia. We found that female APP/PS1 mice have more amyloid plaques, less plaque associated microglia, and a less robust- and diverse- EVISF microglial proteome. Thus, in vivo microdialysis is a novel technique for collecting EVISF and offers a unique opportunity to explore the role of EVs in AD.

Introduction Metabolic impairments and sleep disruptions are both recognized as modifiable risk factors in the development of Alzheimer’s disease (AD). A bidirectional relationship exists between sleep and AD where pathology can disrupt sleep, but sleep disturbances can also increase AD-related pathology. This results in a feed-forward cycle of disease progression and further sleep impairments. Sleep fragmentation and poor sleep quality can also cause glucose intolerance and insulin resistance. However, it remains unclear if peripheral metabolic dysfunction alone can impair sleep, particularly in the context of AD-related pathology. Therefore, the goal of this study was to establish mechanisms connecting peripheral metabolic and sleep disruptions and identify potential therapeutic targets for mitigating AD risk. Methods We implanted biosensors measuring interstitial fluid (ISF) levels of glucose and lactate, biomarkers of cerebral metabolism and neuronal activity, respectively, directly into the hippocampus of wildtype and APP/PS1 mice, a model of amyloid-beta (Aβ) overexpression. Biosensors were paired with cortical EEG and EMG recordings for simultaneous sleep/wake analysis. To understand the relationship between metabolic dysfunction and sleep, we altered the peripheral metabolic environment using two paradigms: an acute high-fat, high-sugar diet (HF/HSD) and a pharmacological metabolic rescue. Results We found short-term HF/HSD exposure was sufficient to disrupt ISF glucose and lactate diurnal rhythms, independent of changes to pathology burden. Moreover, we found significant sleep disruptions, particularly decreased delta power, indicating slow wave sleep impairments. These findings were exacerbated in mice with Aβ pathology who have disrupted metabolic and sleep functioning at baseline. Conversely, we found normalizing peripheral glucose tolerance in mice with Aβ pathology was sufficient to rescue impaired ISF glucose and lactate rhythms. We also saw a restoration of normal sleep-wake patterns, specifically within slow wave activity. Conclusion We found modulating peripheral glucose metabolism can bidirectionally alter sleep, particularly slow wave sleep, independent of changes in Aβ burden. This indicates sleep as a mediator in the relationship between metabolic impairments and AD pathophysiology. While impaired sleep increases AD risk, improved sleep slows pathology accumulation. Therefore, this study demonstrates targeting peripheral glucose regulation is a potential avenue for early AD intervention and risk mitigation. Support (if any) F31-AG066302, R01-AG068330, P30-AG072947

The sleep-wake cycle is a master regulator of metabolic and neuronal activity and when altered, can have profound effects on metabolic health and disease. Although consideration is given to how fluctuations in blood glucose affect peripheral physiology and metabolism, less is known about how glucose dysregulation impacts the intrinsic cooperation between brain metabolism and neuronal activity to regulate sleep. To understand the effect of peripheral hyper- and hypoglycemia on these relationships, we paired biosensors measuring hippocampal interstitial fluid (ISF) levels of glucose and lactate with cortical EEG/EMG recordings to produce simultaneous subsecond recordings of ISF glucose, lactate, and sleep-wake states. First, we describe a conserved temporal relationships between ISF glucose and lactate based on their intrinsic oscillations, diurnal rhythms, and sleep/wake cycles. ISF glucose and lactate oscillations are largely anti-correlated but the frequency of their oscillations dictate their power, coherence, and phase. While ISF glucose and lactate both have diurnal fluctuations, only ISF lactate is consistently elevated during wake. During wake, fluctuations in ISF lactate are associated with changes in the EEG power spectrum, suggesting wake-related activity is more closely associated with ISF lactate. Modulation of glucose availability via both hyper- or hypoglycemia disrupts the relationship between peripheral metabolism, brain metabolism, and sleep. Hyper- and hypo-glycemia increase ISF lactate, decrease NREM, and alter EEG spectral activity, again demonstrating ISF lactate drives wake-associated behaviors and disrupts sleep. Taken together, these studies demonstrate that peripheral glucose homeostasis is necessary for maintaining the relationships between brain metabolism, neuronal activity, and sleep-wake patterns and deviations in blood glucose levels are sufficient to disrupt the metabolic signature of sleep-wake states, putting the brain at risk in diseases like type-2-diabetes and Alzheimer’s disease.Graphical Abstract. Peripheral glucose homeostasis directly modifies sleep/wake patterns

Epidemiological studies identified alcohol use disorder (AUD) as a risk factor for Alzheimer's disease (AD), yet there is conflicting evidence on how alcohol use promotes AD pathology. In this study, a 10-week moderate two-bottle choice drinking paradigm was used to identify how chronic ethanol exposure alters amyloid-β (Aβ)-related pathology, metabolism, and behavior. Ethanol-exposed APPswe/PSEN1dE9 (APP/PS1) mice showed increased brain atrophy and an increased number of amyloid plaques. Further analysis revealed that ethanol exposure led to a shift in the distribution of plaque size in the cortex and hippocampus. Ethanol-exposed mice developed a greater number of smaller plaques, potentially setting the stage for increased plaque proliferation in later life. Ethanol drinking APP/PS1 mice also exhibited deficits in nest building, a metric of self-care, as well as increased locomotor activity and central zone exploration in an open field test. Ethanol exposure also led to a diurnal shift in feeding behavior which was associated with changes in glucose homeostasis and glucose intolerance. Complementary in vivo microdialysis experiments were used to measure how acute ethanol directly modulates Aβ in the hippocampal interstitial fluid (ISF). Acute ethanol transiently increased hippocampal ISF glucose levels, suggesting that ethanol directly affects cerebral metabolism. Acute ethanol also selectively increased ISF Aβ40, but not ISF Aβ42, levels during withdrawal. Lastly, chronic ethanol drinking increased N-methyl-d-aspartate receptor (NMDAR) and decreased γ-aminobutyric acid type-A receptor (GABA

Chronic ethanol exposure can increase amyloid-β (Aβ) and tau in rodent models of Alzheimer’s-disease (AD)-like pathology, yet the underlying mechanisms are poorly understood. In this study, a moderate two-bottle choice drinking paradigm was used to identify how chronic ethanol exposure alters Aβ-related pathology, metabolism, and behavior. Complementary in vivo microdialysis experiments were used to measure how acute ethanol directly modulates Aβ in the hippocampal interstitial fluid (ISF). Ethanol-exposed APPswe/PSEN1dE9 (APP/PS1) mice showed increased brain atrophy and an increased number of amyloid plaques. Further analysis revealed that ethanol exposure led to a shift in the distribution of plaque size in the cortex and hippocampus. Ethanol-exposed mice developed a greater number of smaller plaques, potentially setting the stage for increased plaque proliferation in later life. Ethanol also induced changes in N-methyl-D-aspartate and γ-aminobutyric acid type-A receptor (NMDAR and GABAAR, respectively) expression, possibly reflecting changes in the excitatory and inhibitory (E/I) balance in the brain. Ethanol exposure also led to a diurnal shift in feeding behavior which was associated with changes in glucose homeostasis and glucose intolerance. Ethanol exposure also exacerbated alterations in the open-field test and deficits in nest-building behaviors in APP/PS1mice. Lastly, an acute dose of ethanol bidirectionally altered hippocampal ISF Aβ levels – decreasing during the initial exposure and increasing during withdrawal. Acute ethanol exposure increased hippocampal ISF glucose levels, suggesting changes in cerebral glucose metabolism occur in response to ethanol. These experiments indicate that ethanol exacerbates an AD-like phenotype by altering Aβ deposition, behavior, and metabolism. Here, even a moderate drinking paradigm culminates in an interaction between alcohol use and AD-related phenotypes with a potentiation of AD-related pathology, behavioral dysfunction, and metabolic impairment.HighlightsChronic ethanol exposure increases brain atrophy in APP/PS1 mice.Chronic ethanol exposure increased the number of plaques in the brains of APP/PS1 mice.Chronic ethanol exposure led to dysregulated metabolism in APP/PS1 mice.Chronic ethanol exposure altered anxiety- and dementia-related behaviors in APP/PS1 mice.Acute ethanol exposure bidirectionally alters interstitial fluid (ISF) levels of amyloid-β in APP/PS1 mice during exposure and withdrawal.

Brain hyperexcitability is a defining feature of AD, where aberrant neuronal activity may be both a cause and consequence of the pathology. Identifying novel targets to modulate cellular excitability is an important treatment strategy. We showed that inward rectifying, ATP-sensitive potassium (K ATP ) channels regulate excitability at the neurovascular unit to impact production and aggregation of amyloid-beta (Aβ), a hallmark of AD. Sulfonylureas such as glyburide (GLY) are antidiabetic medications that inhibit K ATP channels. We demonstrated that systemic treatment with GLY decreased Aβ production and plaque pathology by modulating vascular K ATP channel activity and neuronal excitability. Here, we investigated whether GLY alters cardiac function or pulse wave velocity (PWV) in APPswe, PSEN1dE9 (APP/PS1) mice, a model of Aβ overexpression.Female APP/PS1 mice at 8 months of age with established amyloid plaque pathology were treated for 1 month with GLY (subcutaneous, slow release 2.5mg pellet, ~30μg/GLY/day) or Placebo (n = 5, each group). After 1 month, transthoracic echocardiography was performed (Vevo 2100 LAZR, FUJIFILM/VisualSonics, Inc.; Toronto, Ca) with a 30 MHz linear array transducer. There were no differences in Placebo vs. GLY mice for heart rate, cardiac output, E/e’ ratio, ejection fraction, and fractional shortening. Arterial stiffness, measured as PWV in the aortic arch [Distance (D)/Time (T); D = mm from ascending to descending aorta and T = (R to ascending aorta foot) - (R to descending aorta foot) in ms], was lower in GLY treated mice (5 ± 1 vs. Placebo 9 ± 1 mm/ms; p < 0.04). Wild Type females at 9 months of age (n = 5) reveal similar cardiac function values as Placebo and GLY treated APP/PS1 mice, with a trend for lower PWV (6 ± 1 mm/ms) than the APP/PSI Placebo group. Thus, short-term treatment with GLY does not impact overall cardiac function. Moreover, improved vascular stiffness in GLY-treated APP/PS1 mice may contribute to improved neurovascular coupling independent of changes in Aβ aggregation, since arterial stiffness is associated with increased cerebrovascular pathology in clinical studies. SLM: Bright Focus Fdn, NIA K01AG050719, Donors Cure New Vision; SMD: T32AA007565; Hypertension & Vasc Res Ctr; NCATS UL1TR001420 (Vevo Core)

This essay offers a reflection on the 1996 Duke University rape scandal. In it, the authors argue that the media coverage of the incident followed several normative racial narratives that shaped public perceptions and regarding marginalized v. privileged individuals, and how sports in the United States can galvanize public attention in unique ways.

DIII-D research is addressing critical challenges in preparation for ITER and the next generation of fusion devices through focusing on plasma physics fundamentals that underpin key fusion goals, understanding the interaction of disparate core and boundary plasma physics, and developing integrated scenarios for achieving high performance fusion regimes. Fundamental investigations into fusion energy science find that anomalous dissipation of runaway electrons (RE) that arise following a disruption is likely due to interactions with RE-driven kinetic instabilities, some of which have been directly observed, opening a new avenue for RE energy dissipation using naturally excited waves. Dimensionless parameter scaling of intrinsic rotation and gyrokinetic simulations give a predicted ITER rotation profile with significant turbulence stabilization. Coherence imaging spectroscopy confirms near sonic flow throughout the divertor towards the target, which may account for the convection-dominated parallel heat flux. Core-boundary integration studies show that the small angle slot divertor achieves detachment at lower density and extends plasma cooling across the divertor target plate, which is essential for controlling heat flux and erosion. The Super H-mode regime has been extended to high plasma current (2.0 MA) and density to achieve very high pedestal pressures (~30 kPa) and stored energy (3.2 MJ) with H 98y2 ≈ 1.6–2.4. In scenario work, the ITER baseline Q = 10 scenario with zero injected torque is found to have a fusion gain metric independent of current between q 95 = 2.8–3.7, and a lower limit of pedestal rotation for RMP ELM suppression has been found. In the wide pedestal QH-mode regime that exhibits improved performance and no ELMs, the start-up counter torque has been eliminated so that the entire discharge uses ≈0 injected torque and the operating space is more ITER-relevant. Finally, the high- (⩽3.8) hybrid scenario has been extended to the high-density levels necessary for radiating divertor operation, achieving ~40% divertor heat flux reduction using either argon or neon with P tot up to 15 MW.

Recent experimental and molecular-level computational analyses have indicated that fused silica, when subjected to pressures of several tens of GPa, can experience irreversible devitrification and densification. Such changes in the fused-silica molecular-level structure are associated with absorption and/or dissipation of the strain energy acquired by fused silica during high-pressure compression. This finding may have important practical consequences in applications for fused silica such as windshields and windows of military vehicles, portholes in ships, ground vehicles, spacecraft, etc. In the present work, our prior molecular-level computational results pertaining to the response of fused silica to high pressures (and shear stresses) are used to enrich a continuum-type constitutive model (that is, the so-called Johnson-Holmquist-2, JH2, model) for this material. Since the aforementioned devitrification and permanent densification processes modify the response of fused silica to the pressure as well as to the deviatoric part of the stress, changes had to be made in both the JH2 equation of state and the strength model. To assess the potential improvements in respect to the ballistic-penetration resistance of this material brought about by the fused-silica devitrification and permanent densification processes, a series of transient non-linear dynamics finite-element analyses of the transverse impact of a fused-silica test plate with a solid right-circular cylindrical steel projectile were conducted. The results obtained revealed that, provided the projectile incident velocity and, hence, the attendant pressure, is sufficiently high, fused silica can undergo impact-induced devitrification, which improves its ballistic-penetration resistance.

Within the present work, the problem of structural integrity (including potential collapse/survivability) of a model building when subjected to a blast attack by a close-proximity vehicle-borne improvised explosive device (VBIED) is investigated using advanced transient, non-linear dynamics, discrete-element modeling (DEM) and simulation computational methods and tools. Since the DEM approach is highly sensitive to the details of the constitutive-material (and contact-interaction) models, a significant portion of the work is devoted to explaining the formulation and the physical basis for the material models used. In particular, since concrete is the key material used in the construction of a building, a critical assessment is provided of the DEM concrete material model employed. To quantify blast-survivability of the building and, in particular, its key structural components, the concept of the so- called design basis threat, DBT (as quantified by the TNT-equivalent charge mass and the associated VBIED standoff distance) is utilized. To help quantify DBT, a parametric study is carried out involving the following design parameters: (a) TNT-equivalent charge mass; (b) VBIED standoff distance; and (c) the degree of concrete reinforcement with steel. The results obtained in the present work also revealed (in a qualitative fashion) the role that the phenomena such as the interaction of the VBIED-induced soil- borne shock waves with the building underground support structure as well as the interaction of the vehicle fragments and the detonation products with the structural elements of the building play in causing damage (and potential collapse) of the building. Within the present work, advanced transient, non-linear dynamics, discrete-element modeling (DEM) and simulation computational methods and tools are used to investigate potential collapse/survivability of a model building when subjected to a blast attack by a close-proximity improvised explosive device (IED). Thus, the main aspects of the present work include: (a) discrete element modeling and simulation methods and tools; (b) impulse loading resulting from the interaction of shrapnel produced during detonation of IEDs, detonation products, explosive casing fragments and soil/pavement ejecta with stationary target structures such as buildings; and (c) constitutive models for the materials (such as concrete) used in the construction of the key structural elements of the targeted buildings. In the remainder of this section, a brief overview of the first two aspects of the current problem will be presented. Then, in Section II, a more detailed account is given of the third aspect of the present problem. A. The Basics of the Discrete Element Method (DEM) The Discrete Element Method (DEM) is a computational modeling and simulation technique within which a material is treated as an assembly/collection of mobile and interacting discrete elements, and the behavior of the material at the macro- length-scale is deduced from the statistical analysis of the (normal, tangential, rolling and twisting) contact-interactions and motions of these elements (e.g. 1) . The state (i.e. position, velocity, acceleration, force and torque) of each discrete particle in the system and its temporal evolution is governed by the basic physical laws, e.g. Newton"s Second Law, Hooke"s Law, and Coulomb friction law. The DEM offers a new way of inferring the basic macroscopic behavior of a material and the formulation of the appropriate material constitutive model. That is, the behavior of the material is prescribed locally through different laws governing the discrete-element interactions while the macroscopic behavior of the material is simply the outcome of the discrete-element interactions. This approach is sharp contrast to the conventional finite element modeling and simulation approach within which macroscopic behavior/properties of the participating materials are prescribed at the outset and the details related to the local kinematic and deformation response are revealed during computational analysis which involves solving the governing mass, momentum and energy conservation equations. Since detailed accounts of the DEM can be found in many sources (e.g. 2) , only a brief overview of this method will be presented in this section. For improved clarity, the DEM is overviewed below by describing separately each of its key components/constituents.

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 time evolution of toroidal rotation velocity profiles has been measured in Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas using a tangentially viewing x-ray spectrometer array. The strong co-current toroidal rotation in enhanced Dα (EDA) high confinement mode (H-mode) plasmas is observed to propagate in from the edge on a time scale similar to the energy confinement time. The ensuing steady state rotation velocity profiles in both Ohmic and ion cyclotron range of frequencies (ICRF) heated EDA H modes, which are generated in the absence of any external momentum input, are found to be relatively flat. These profiles may be simulated by a simple diffusion model with the boundary condition of an edge rotation, which appears during the H-mode period. The observed profiles are well matched by the simulations using a momentum diffusivity of ∼0.1 m2/s, which is much larger than the calculated neo-classical value, and the momentum transport may be regarded as anomalous. The Alcator C-M...

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced Dα) H-mode transition in both Ohmic and ion cyclotron range of frequencies heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a timescale of the order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V ~ 50 km s−1, and the momentum transport can be simulated using a simple diffusion model. Assuming that the L–H transition produces an instantaneous edge source of toroidal torque (which disappears at the H- to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ~0.07 m2 s−1 during EDA H-mode and ~0.2 m2 s−1 in L-mode. These values are large compared to the calculated neo-classical momentum diffusivities, which are of the order of 0.003 m2 s−1. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V(0) exceeding 100 km s−1 in some cases), which suggests the presence of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ~10 m s−1. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the centre, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V(0)~−60 km s−1 in some cases), which may be related to the observed higher H-mode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to cease (V ≤ 5 km s−1).

Internal transport barrier (ITB) plasmas with peak pressures of 0.25 MPa and pressure gradients as large as 2.5 MPa m−1 have been produced in Alcator C-Mod using off-axis ion cyclotron range of frequencies (ICRF) heating. The onset of the ITBs is apparent when the pressure gradient exceeds 1.0 MPa m−1, which is similar to the JET criterion of ρs/L≥0.014. Concommitant with the peaking of the core pressure as the ITB develops is a drop of the toroidal rotation velocity profile inside of the barrier foot; the maximum of the velocity gradient coincides with the peak in the pressure gradient. The quasi-coherent (QC) mode, associated with the enhanced Dα (EDA) H-mode plasmas which evolve the ITBs, breaks up and disappears as the barriers develop, even though the measured edge pedestal parameters remain fixed. The position of the ITB foot has been moved over a range of (1/3) of the plasma minor radius by varying the toroidal magnetic field. The peak in the calculated bootstrap current density profile has correspondingly been regulated over a similar range in plasma minor radius. The location of the density profile foot is found to expand as the toroidal magnetic field is reduced and the ICRF frequency is lowered. The density foot radius is relatively independent of q95, however, in a scan of the plasma current at fixed BT and wave frequency.

For steady state high-confinement-mode (H-mode) operation, a relaxation mechanism is required to limit build-up of the edge gradient and impurity content. Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] sees two such mechanisms—EDA (enhanced D-alpha H mode) and grassy ELMs (edge localized modes), but not large type I ELMs. In EDA the edge relaxation is provided by an edge localized quasicoherent (QC) electromagnetic mode that exists at moderate pedestal temperature T 3.5, and does not limit the buildup of the edge pressure gradient. The q boundary of the operational space of the mode depends on plasma shape, with the q95 limit moving down with increasing plasma triangularity. At high edge pressure gradients and temperatures the mode is replaced by broadband fluctuations ( f

Double transport barrier modes (simultaneous core and edge transport barrier) have been observed with off-axis ion cyclotron range of frequencies (ICRF) heating in the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. An internal transport barrier (ITB) is routinely produced in enhanced DαH-mode (EDA) discharges where the minority ion cyclotron resonance layer is at r/a∼|0.5| during the current flat top phase of the discharge. The density profile becomes peaked without the presence of a particle source in the plasma core and continues to peak until the increased core impurity radiation arrests the improved energy confinement, ultimately leading to a barrier collapse. With the addition of moderate (0.6 MW) central ICRF heating, the double barrier mode was maintained for as long as the ICRF power was applied. Modeling shows that the internal thermal barrier was maintained throughout the discharge. The presence of sawteeth throughout most of the ITB discharge allows sawtooth-indu...

The formation of internal transport barriers (ITBs) has been observed in the core region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement to low confinement mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes). Experiments with ion cyclotron range of frequencies power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for ten energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Simi...

High resolution measurements on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the “Enhanced Dα” (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2–5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the L-H transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential, and Bpol, with f0∼50–150 kHz and kθ∼4 cm−1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include δ>0.35, q95>3.5, and L-mode target density ne>1.2×1020 m−3. A reduced q95 limit is found for hydrogen discharges.

The steady-state H-mode regime found at moderate to high density in Alcator C-Mod, known as enhanced Dα (EDA) H-mode, appears to be maintained by a continuous quasi-coherent (QC) mode in the steep edge gradient region. Large amplitude density and magnetic fluctuations with typical frequencies of about 100 kHz are driven by the QC mode. These fluctuations are measured in the steep edge gradient region by inserting a fast-scanning probe containing two poloidally separated Langmuir probes and a poloidal field pick-up coil. As the probe approaches the plasma edge, clear magnetic fluctuations were measured within about 2 cm of the last-closed flux surface (LCFS). The mode amplitude falls off rapidly with distance from the plasma centre with an exponential decay length of kr≈1.5 cm-1, measured 10 cm above the outboard midplane. The root-mean-square amplitude of the fluctuation extrapolated to the LCFS was θ≈5 G. The density fluctuations, on the other hand, were visible on the Langmuir probe only when it was within a few millimetres of the LCFS. The potential and density fluctuations were sufficiently in phase to enhance particle transport at the QC mode frequency. These results show that the QC signature of the EDA H-mode is an electromagnetic mode that appears to be responsible for the enhanced particle transport in the plasma edge.

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ≤ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.

During current ramp-up discharges, highly localized magnetohydrodynamic (MHD) fluctuations were observed on the electron cyclotron emission diagnostics of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. The electron temperature profile was hollow, while the density profile was weakly decreasing. Assuming that the equilibration time was short enough to quickly thermalize ions the pressure profile was also found to be hollow. Using this pressure profile as an additional constraint to the EFIT program, an equilibrium with reversed shear was constructed having a q(0)≫1. The localized MHD activity was observed near the inner q=5 rational surface in this reconstructed equilibrium, where the Mercier criterion for ideal MHD stability was violated because of the reversed pressure gradient (dp/dr>0), q>1 and moderate shear. When kinetic effects were added, the ideal Mercier mode was finite ion Larmor radius stabilized. However, ion Landau damping was found to be strong enough to drive...

During current rampup discharges with early auxiliary RF heating, reversed magnetic shear profiles have been obtained on Alcator C-Mod. MHD oscillations are often observed on ECE diagnostics, as well as magnetics. Although no direct q profile measurement was available, a hollow current density profile with qmin = 2.5 was reconstructed by the EFIT program. The location of the largest electron temperature fluctuations was close to the outer q = 3 rational surface. On the basis of a resistive linear stability code (MARS), the MHD oscillations were identified as n = 1 resistive `multiple' tearing modes. Since the pressure profile was hollow and q > 1 in the core, a resistive interchange mode was also predicted to be unstable at the inner q = 4 rational surface, despite the low βN.

This is an analysis of the H-mode threshold physics by the International H-mode Threshold Database Working Group. The data from 10 different tokamaks world-wide are combined in an attempt to determine the physics of the H-mode transition as well as the scaling of the plasma parameters relevant to the H-mode transition to future magnetic fusion devices. The input power required to achieve H-mode is fitted with log-linear regressions of global plasma parameters such as density, toroidal magnetic field and plasma size. An attempt to cross-validate this analysis method is made by removing each tokamak in turn and recalculating the fit. Based on these global scalings, the hysteresis in the threshold power can be defined in terms of how far away from the threshold scaling plasma parameters can be obtained and remain in H-mode. Local edge temperatures and densities are also analysed at the H-mode threshold in an attempt to find a scaling with plasma parameters. Changes in divertor geometry also have strong effects on the H-mode threshold, so scalings are compared for open and closed divertors.

High-frequency coherent magnetic fluctuations have been observed with hydrogen minority ion cyclotron resonant frequency (ICRF) heating in Alcator C-Mod in low- or reversed-shear current rise conditions. The mode frequencies (150-450 kHz) are in the range expected for toroidal Alfven eigenmodes (TAEs) with central q values greater than unity. These modes have low n 3 and appear to rotate in the ion diamagnetic drift direction. Under these relatively low-density current rise conditions with low or reversed magnetic shear, TAEs can be destabilized by a predicted fast hydrogen ion tail of just over 100 keV. Calculations with the CASTOR code indicate that a broad eigenmode can exist with a calculated frequency at the measured mode frequency, which falls in a clear TAE gap across the profile. The frequency of the current rise modes can increase or decrease rapidly from 150 to 400 kHz in less than 15 ms, which cannot be explained by linear theory. In some cases, only a single mode is excited with a linearly increasing frequency, in contrast to the multiple modes with decreasing frequency usually observed on other devices.

Key phenomena controlling the transport barrier in H-mode plasmas on the Alcator C-Mod tokamak are discussed. Type I edge localized modes (ELMs) are not observed. Instead the particle transport barrier is continuously degraded giving rise to 'enhanced D α ' operation which avoids the deleterious impurity accumulation observed in 'ELM-free' H-modes. The dependence of operational regime on plasma parameters suggests that the instability responsible for barrier degradation is enhanced by higher pressure and longer field-line connection. Detailed analysis of the magnetic structure of fluctuations associated with the different H-mode regimes shows that these field-aligned fluctuations propagate with large perpendicular velocities believed to represent the large velocity shear responsible for barrier formation. In the enhanced D α regime the fluctuations are generally more incoherent, but have not otherwise yet shown a specific distinctive character.

A series of experiments, examining the confinement properties of ion cyclotron range of frequencies (ICRF) heated H mode plasmas, has been carried out on the Alcator C-Mod tokamak. Alcator C-Mod is a compact tokamak that operates at high particle, power and current densities at toroidal fields up to 8 T. Under these conditions the plasma is essentially thermal with very little contribution to the stored energy from energetic ions (typically no more than 5%) and with Ti~Te. Most of the data were taken with the machine in a single null `closed' divertor configuration with the plasma facing components clad in molybdenum tiles. The data include those taken both before and after the first wall surfaces were coated with boron, with emphasis on the latter. H modes obtained from plasmas run on boronized walls typically had a lower impurity content and radiated power and attained a higher stored energy than those run on bare molybdenum. Confinement enhancement, the energy confinement time normalized to L mode scaling, for discharges with boronized walls, ranged from 1.6 to 2.4. The unique operating regime of the Alcator C-Mod device provided a means for extending the tests of global scaling laws to parameter ranges not previously accessible. For example, the Alcator C-Mod edge localized mode (ELM)-free data were found to be 1.1 to 1.6 times the ITERH93 scaling and the ELMy data almost 2.0 to 2.8 times the ITERH92 ELMy scaling law, suggesting that the size scaling in both scalings may be too strong. While both ELM-free and ELMy discharges were produced, the ELM characteristics were not easily compared with observations on other devices. No large, low frequency ELMs were seen despite the very high edge pressure and temperature gradients that were attained. For all of our H mode discharges, a clear linear relationship between the edge temperature pedestal and the temperature gradient in the core plasma was observed; the discharges with the `best' transport barriers also showing the greatest improvement in core c

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.

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.

Results of ICRF heating experiments in Alcator C-Mod during the November 1994 to June 1995 campaign are summarized. Efficient heating of high-density plasmas was demonstrated with high power densities (up to volume averaged, surface averaged). These experiments were carried out with RF powers up to 3.5 MW at 80 MHz, at magnetic fields up to 8 T and plasma currents up to 1.2 MA. For on-axis hydrogen minority heating at 5.3 T, near complete absorption is achieved. Energy confinement in L-mode plasmas was found to be consistent with the ITER89-P scaling. H-mode is routinely observed when the ion drift is directed toward the X-point. The H-mode power threshold was found to scale as low as , which is a factor of two lower than the scaling observed on other tokamaks. PEP modes with highly peaked density and ion temperature profiles and highly enhanced fusion reactivity were obtained with Li pellet injection followed by on-axis ICRF heating at both (H minority heating) and 8 T ( minority heating). In H - plasmas at T highly localized direct electron heating by the mode converted ion Bernstein wave was observed. Nearly complete absorption by electrons in a small volume resulted in an extremely high electron heating power density of with .

The sign of the toroidal magnetic field has a major effect on the divertor in Alcator C-Mod, determining whether the higher recycling is on the inboard or outboard side and leading to inboard/outboard temperature and density differences of up to a factor of ten. A recently published paper (Hutchinson et al 1995 Plasma Phys. Control. Fusion 37 1389) reported in detail on these observations, which are an indication of the importance of plasma particle drifts for understanding and modelling the divertor. The conference presentation covered these published results and also some additional results, documented here, in which the plasma flows associated with the field-direction-dependent asymmetries are measured using a Mach probe and a localized impurity puffing plume analysis technique.

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.

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.

Wall conditioning in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] by injection of lithium pellets into the plasma has resulted in large improvements in deuterium–tritium fusion power production (up to 10.7 MW), the Lawson triple product (up to 1021 m−3 s keV), and energy confinement time (up to 330 ms). The maximum plasma current for access to high‐performance supershots has been increased from 1.9 to 2.7 MA, leading to stable operation at plasma stored energy values greater than 5 MJ. The amount of lithium on the limiter and the effectiveness of its action are maximized through (1) distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and (2) injection of four Li pellets into the Ohmic phase of supershot discharges before neutral‐beam heating is begun.

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...

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.

In the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] a substantial improvement in fusion performance has been realized by combining the enhanced confinement due to tritium fueling with the enhanced confinement due to extensive conditioning of the limiter with lithium. This combination has resulted in not only significantly higher global energy confinement times than have previously been obtained in high current supershots, but also in the highest central ratio of thermonuclear fusion output power to input power observed to date.

A series of transport experiments has been carried out in Alcator‐C‐Mod. [Phys Plasmas 1, 1511 (1994)]. Data from both Ohmic and ICRF (ion cyclotron range of frequencies) heated plasmas can be fitted with an L‐mode (low mode) scaling law. The Ohmic τE’s show no scaling with density in any regime and can reach values of 2–3 times neo‐Alcator. Impurity confinement has been studied with the laser blow‐off technique with τI showing nearly linear scaling with plasma current. Ohmic and ICRF H modes are obtained over a wide range of discharge parameters, extending the range in the international database for nB, by almost a factor of 10. The power threshold for ELM‐free (edge localized mode) discharges is in rough agreement with the scaling P/S=0.044nB. Energy diffusivities of Ohmic and ICRF heated plasmas have been measured from local analysis of plasma profiles and power fluxes. The same analysis produces a value for plasma resistivity which lies between the Spitzer and neoclassical calculations. Analysis of pl...

Detailed measurements and transport analysis of divertor conditions in Alcator C‐Mod [Phys. Plasmas 1, 1511 (1994)] are presented for a range of line‐averaged densities, 0.7