Suppression of Alfvén Modes on the National Spherical Torus Experiment Upgrade with Outboard Beam Injection.

In this Letter we present data from experiments on the National Spherical Torus Experiment Upgrade, where it is shown for the first time that small amounts of high pitch-angle beam ions can strongly suppress the counterpropagating global Alfvén eigenmodes (GAE). GAE have been implicated in the redistribution of fast ions and modification of the electron power balance in previous experiments on NSTX. The ability to predict the stability of Alfvén modes, and developing methods to control them, is important for fusion reactors like the International Tokamak Experimental Reactor, which are heated by a large population of nonthermal, super-Alfvénic ions consisting of fusion generated α's and beam ions injected for current profile control. We present a qualitative interpretation of these observations using an analytic model of the Doppler-shifted ion-cyclotron resonance drive responsible for GAE instability which has an important dependence on k_{⊥}ρ_{L}. A quantitative analysis of this data with the hym stability code predicts both the frequencies and instability of the GAE prior to, and suppression of the GAE after the injection of high pitch-angle beam ions.

Compact and multi-view solid state neutral particle analyzer arrays on National Spherical Torus Experiment-Upgrade.

A compact and multi-view solid state neutral particle analyzer (SSNPA) diagnostic based on silicon photodiode arrays has been successfully tested on the National Spherical Torus Experiment-Upgrade. The SSNPA diagnostic provides spatially, temporally, and pitch-angle resolved measurements of fast-ion distribution by detecting fast neutral flux resulting from the charge exchange (CX) reactions. The system consists of three 16-channel subsystems: t-SSNPA viewing the plasma mid-radius and neutral beam (NB) line #2 tangentially, r-SSNPA viewing the plasma core and NB line #1 radially, and p-SSNPA with no intersection with any NB lines. Due to the setup geometry, the active CX signals of t-SSNPA and r-SSNPA are mainly sensitive to passing and trapped particles, respectively. In addition, both t-SSNPA and r-SSNPA utilize three vertically stacked arrays with different filter thicknesses to obtain coarse energy information. The experimental data show that all channels are operational. The signal to noise ratio is typically larger than 10, and the main noise is x-ray induced signal. The active and passive CX signals are clearly observed on t-SSNPA and r-SSNPA during NB modulation. The SSNPA data also indicate significant losses of passing particles during sawteeth, while trapped particles are weakly affected. Fluctuations up to 120 kHz have been observed on SSNPA, and they are strongly correlated with magnetohydrodynamics instabilities.

Coupling of Neutral-Beam-Driven Compressional Alfvén Eigenmodes to Kinetic Alfvén Waves in NSTX Tokamak and Energy Channeling.

An energy-channeling mechanism is proposed to explain flattening of the electron temperature profiles at a high beam power in the beam-heated National Spherical Torus Experiment (NSTX). Results of self-consistent simulations of neutral-beam-driven compressional Alfvén eigenmodes (CAEs) in NSTX are presented that demonstrate strong coupling of CAEs to kinetic Alfvén waves at the Alfvén resonance location. It is suggested that CAEs can channel energy from the beam ions to the location of the resonant mode conversion at the edge of the beam density profile, modifying the energy deposition profile.

Mitigation of Alfvén activity in a tokamak by externally applied static 3D fields.

The application of static magnetic field perturbations to a tokamak plasma is observed to alter the dynamics of high-frequency bursting Alfvén modes that are driven unstable by energetic ions. In response to perturbations with an amplitude of δB/B∼0.01 at the plasma boundary, the mode amplitude is reduced, the bursting frequency is increased, and the frequency chirp is smaller. For modes of weaker bursting character, the magnetic perturbation induces a temporary transition to a saturated continuous mode. Calculations of the perturbed distribution function indicate that the 3D perturbation affects the orbits of fast ions that resonate with the bursting modes. The experimental evidence represents an important demonstration of the possibility of controlling fast-ion instabilities through "phase-space engineering" of the fast-ion distribution function, by means of externally applied perturbation fields.

Formation and stability of impurity "snakes" in tokamak plasmas.

New observations of the formation and dynamics of long-lived impurity-induced helical "snake" modes in tokamak plasmas have recently been carried out on Alcator C-Mod. The snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced density to a large crescent-shaped helical structure inside q<1, with a regularly sawtoothing core. The observations show that the conditions for the formation and persistence of a snake cannot be explained by plasma pressure alone. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.

Managing livestock using animal behavior: mixed-species stocking and flerds.

Mixed-species stocking can foster sound landscape management while offering economic and ecological advantages compared with mono-species stocking. Producers contemplating a mixed-species enterprise should reflect on several considerations before implementing this animal management strategy. Factors applicable to a particular producer's landscape must be considered together with goals and economic constraints before implementing mixed-species stocking. A major consideration when using mixed-species stocking is how to deal with predation losses, especially among small ruminants. An approach being adopted in some commercial operations capitalizes on using innate animal behaviors to form cohesive groups of two or more livestock species that consistently remain together under free-ranging conditions. These groups are referred to as flerds. The mixing of a flock of sheep and/or goats with a herd of cattle into a flerd has been shown to protect sheep and goats from coyote predation, as well as offering other husbandry advantages. Some of the added advantages include more efficient conversion of forage into animal protein. Creation of flerds, their maintenance and advantages are discussed.

Techniques for the measurement of disruption halo currents in the National Spherical Torus Experiment.

This paper describes techniques for measuring halo currents, and their associated toroidal peaking, in the National Spherical Torus Experiments [M. Ono et al., Nucl. Fusion 40, 557 (2000)]. The measurements are based on three techniques: (1) measurement of the toroidal field created by the poloidal halo current, either with segmented Rogowski coils or discrete toroidal field sensors, (2) the direct measurement of halo currents into specially instrument tiles, and (3) small Rogowski coils placed on the mechanical supports of in-vessel components. For the segmented Rogowski coils and discrete toroidal field detectors, it is shown that the toroidal peaking factor inferred from the data is significantly less than the peaking factor of the underlying halo current distribution, and a simple model is developed to relate the two. For the array of discrete toroidal field detectors and small Rogowski sensors, the compensation steps that are used to isolate the halo current signal are described. The electrical and mechanical design of compact under-tile resistive shunts and mini-Rogowski coils is described. Example data from the various systems are shown.

Triggered confinement enhancement and pedestal expansion in high-confinement-mode discharges in the national spherical torus experiment.

We report observation of a new high performance regime in discharges in the National Spherical Torus Experiment, where the H mode edge "pedestal" temperature doubles and the energy confinement increases by 50%. The spontaneous transition is triggered by a large edge-localized mode, either natural or externally triggered by 3D fields. The transport barrier grows inward from the edge, with a doubling of both the pedestal pressure width and the spatial extent of steep radial electric field shear. The dynamics suggest that 3D fields could be applied to reduce edge transport in fusion devices.

Correlation between electron transport and shear Alfvén activity in the National Spherical Torus Experiment.

We report the observation of a correlation between shear Alfvén eigenmode activity and electron transport in plasma regimes where the electron temperature gradient is flat, and thus the drive for temperature gradient microinstabilities is absent. Plasmas having rapid central electron transport show intense, broadband global Alfvén eigenmode (GAE) activity in the 0.5-1.1 MHz range, while plasmas with low transport are essentially GAE-free. The first theoretical assessment of a GAE-electron transport connection indicates that overlapping modes can resonantly couple to the bulk thermal electrons and induce their stochastic diffusion.

Fast soft x-ray images of magnetohydrodynamic phenomena in NSTX.

A variety of magnetohydrodynamic (MHD) phenomena have been observed on NSTX. Many of these affect fast particle losses, which are of major concern for future burning plasma experiments. Usual diagnostics for studying these phenomena are arrays of Mirnov coils for magnetic oscillations and p-i-n diode arrays for soft x-ray emission from the plasma core. Data reported here are from a unique fast soft x-ray imaging camera (FSXIC) with a wide-angle (pinhole) tangential view of the entire plasma minor cross section. The camera provides a 64x64 pixel image, on a charge coupled device chip, of light resulting from conversion of soft x rays incident on a phosphor to the visible. We have acquired plasma images at frame rates of 1-500 kHz (300 frames/shot) and have observed a variety of MHD phenomena: disruptions, sawteeth, fishbones, tearing modes, and edge localized modes (ELMs). New data including modes with frequency >90 kHz are also presented. Data analysis and modeling techniques used to interpret the FSXIC data are described and compared, and FSXIC results are compared to Mirnov and p-i-n diode array results.

Edge-localized modes explained as the amplification of scrape-off-layer current coupling.

It is shown that the edge-localized modes (ELMs) observed in tokamak H mode discharges can be explained as external magnetohydrodynamic (MHD) mode amplification due to coupling with scrape-off-layer current. The proposed model offers a new ELM mechanism that produces a sharp onset and initial fast growth of magnetic perturbations even when the underlying equilibrium is only marginally unstable for a MHD mode and also a quick quenching after the bursting peak. The theory also reproduces various other ELM features.

Scrape-off-layer current model for filament structure observed during edge-localized modes in the DIII-D tokamak.

The plasma in tokamaks often exhibits a relaxation oscillation called the edge-localized mode (ELM), which is generally attributed to MHD instability driven by strong gradients at the plasma boundary. It is shown here that field-aligned currents flowing just outside the boundary may also play a role in the ELM process. The poloidal perturbation magnetic field during ELMs in the DIII-D tokamak calculated from measured currents can reproduce prominent observed features, including a narrow magnetic structure at the outboard midplane similar to filaments observed earlier in DIII-D and NSTX.

Effects of cis-beta-ocimene, cis-sabinene hydrate, and monoterpene and sesquiterpene mixtures on alfalfa pellet intake by lambs.

The transition of grasslands to shrub-dominated scrubland reduces livestock productivity and contributes to impoverished conditions for humans in arid and semiarid regions worldwide. Many shrubs that are increasing in dominance contain secondary compounds that deter browsing by herbivores. Knowledge concerning the effects of specific compounds in herbivore diets is limited but may provide useful insights into desertification. Flourensia cernua is a dominant shrub in the northern Chihuahuan Desert that contains an abundance of terpenes. Four experiments were conducted to determine the effects of terpenes on intake of alfalfa pellets by lambs. Two individual monoterpenes (cis-beta-ocimene and cis-sabinene hydrate) were examined in Exp. 1 and 2, and mixtures of monoterpenes (borneol, camphene, camphor, 1,8-cineole, limonene, myrcene, and alpha-pinene) and sesquiterpenes (beta-caryophyllene, caryophyllene oxide, alpha-copaene, and alpha-humulene) were examined in Exp. 3 and 4, respectively. Forty-five lambs (9 lambs/treatment) were individually fed treated alfalfa pellets for 20 min each morning for 5 d. Five treatments (0x, 0.5x, 1x, 2x, and 10x; multiples of the concentrations of the same terpenes in F. cernua) were applied to alfalfa pellets (637 g, DM basis) in an ethanol carrier. The experiments were preceded by a 10-d adaptation period of the lambs to untreated pellets. Except during the 20-min test, the lambs were maintained outdoors and fed untreated alfalfa pellets (total mean intake = 4.7% of BW, DM basis). Day x treatment interactions were detected (P < 0.04) in Exp. 1 and 4 because of a greater intake for 0x than for the other treatments on d 1 (Exp. 1) and a lower intake for the 10x treatment on d 1 and 2 (Exp. 4). A trend for decreased intake (g/kg of BW) as the concentration of the sesquiterpene mixture increased was observed in Exp. 3 (P = 0.093 for the linear contrast). Although there was a tendency for the sesquiterpene mixture to decrease intake, cis-beta-ocimene, cis-sabinene hydrate, and the monoterpene mixture did not appear to affect intake by lambs. Thus, sesquiterpenes may exert antiherbivory properties under certain conditions that may contribute to shrub dominance with extended periods of livestock foraging.

Volatile compounds on the leaf surface of intact and regrowth tarbush (Flourensia cernua DC) canopies.

Shrub expansion into desert grasslands is a serious problem resulting in loss of forage and rangeland productivity. Flourensia cernua DC (tarbush) is one such shrub contributing to the decline of Chihuahuan Desert grasslands. Our previous research has shown tarbush consumption by sheep and goats to be negatively related to leaf surface concentration of individual terpenes and epicuticular wax. Concentrations of compounds such as terpenes often change with plant age and phenology. Our objective was to examine the effect of altering the vegetative state of tarbush on volatile chemicals. Ninety tarbush plants were randomly selected, and all biomass within 10 cm of the soil surface was removed from 45 plants during winter dormancy. Leaves were collected the following summer during active growth from the canopy of intact controls and resprouts. Leaf surface volatiles were analyzed by gas chromatography-mass spectroscopy and subjected to univariate analysis of variance and stepwise discriminate analysis. Of the 87 compounds present on tarbush leaves, 35 were greater in canopy samples and 16 were greater in regrowth samples based on univariate analysis (P<0.05). Mean concentration of total volatiles on canopy leaves tended to be less (P=0.062) than that of regrowth (3,642 vs. 4,684 microg/g DM). Nine compounds in the discriminant analysis (alpha-muurolene, iso-borneol, unknown#6, p-cymen-8-ol, unknown#7, sabinene, beta-caryophyllene, delta-cadinene, and alpha-copaene) explained 95% of the variation between canopy and regrowth samples. Lower cumulative concentration of volatile compounds in canopy than regrowth samples suggests repsrouts may be less vulnerable to herbivory than intact tarbush.

Three-wave interactions between fast-ion modes in the National Spherical Torus Experiment.

Simultaneous bursts of energetic particle mode (EPM) and toroidicity-induced Alfvén eigenmode (TAE) activity that correlate with significant fast-ion loss are observed in beam heated plasmas. Three-wave interactions between these modes are conclusively identified, indicating fixed phase relationships. This nonlinear coupling concentrates the energy of the TAEs into a toroidally localized perturbation frozen in the frame of a rigid, toroidally rotating structure formed by the EPMs. This redistribution of energy is significant because it will modify the effect of the TAEs on fast-ion loss.

Effects of gamma-terpinene, terpinolene, alpha-copaene, and alpha-terpinene on consumption of alfalfa pellets by sheep.

Although plant secondary chemistry influences shrub consumption by free-ranging ruminants, the effects of many specific compounds on herbivores have not been examined. We conducted four experiments to examine effects of individual terpenes on alfalfa pellet intake by lambs. Forty-five lambs were individually fed alfalfa pellets sprayed with gamma-terpinene, terpinolene, alpha-copaene, or alpha-terpinene at one of five concentrations in an ethanol carrier. Treatments (0, 0.5, 1, 2, and 10x) were multiples of the concentration (x) of a specific terpene on the leaf surface of Flourensia cernua (a low-preference shrub for domestic ruminants). Terpenes were applied to alfalfa pellets (0.64 kg.lamb(-1).d(-1), DM basis), and consumption was measured during a 20-min interval for 5 d. A day effect was detected for gamma-terpinene on intake (P < 0.001 for both linear and quadratic contrasts). No effect of terpinolene, alpha-copaene, or alpha-terpinene on intake was detected in this study. None of the terpenes tested was strongly related to intake of alfalfa pellets by lambs under the conditions of this study.

Characteristics of the first H-mode discharges in the national spherical torus experiment.

We report observations of the first low-to-high ( L-H) confinement mode transitions in the National Spherical Torus Experiment. The H-mode energy confinement time increased over reference discharges transiently by 100-200%, as high as approximately 100 ms. This confinement time is approximately 2 times higher than predicted by a multimachine scaling. Thus the confinement time of spherical tori has been extended to a record high value, leading to an eventual revision of confinement scalings. Finally, the power threshold for H-mode access is >10x higher than predicted by an international scaling from conventional aspect-ratio tokamaks, which could lead to new understanding of H-mode transition dynamics.

Transport of phenolic compounds from leaf surface of creosotebush and tarbush to soil surface by precipitation.

During the last 100 years, many desert grasslands have been replaced by shrublands. One possible mechanism by which shrubs outcompete grasses is through the release of compounds that interfere with neighboring plants. Our objective was to examine the movement of secondary compounds from the leaf surface of creosotebush and tarbush to surrounding soil surfaces via precipitation. Units consisting of a funnel and bottle were used to collect stemflow, throughfall, and interspace precipitation samples from 20 creosotebush (two morphotypes) and 10 tarbush plants during three summer rainfall events in 1998. Precipitation samples were analyzed for total phenolics (both species) and nordihydroguaiaretic acid (creosotebush only). Phenolics were detected in throughfall and stemflow of both species with stemflow containing greater concentrations than throughfall (0.088 and 0.086 mg/ml for stemflow and 0.022 and 0.014 mg/ml for throughfall in creosotebush morphotypes U and V, respectively: 0.044 and 0.006 mg/ml for tarbush stemflow and throughfall, respectively). Nordihydroguaiaretic acid was not found in any precipitation collections. The results show that phenolic compounds produced by creosotebush and tarbush can be transported to the soil surface by precipitation, but whether concentrations are ecologically significant is uncertain. Nordihydroguaiaretic acid was not present in the runoff from creosotebush.

Effects of four mono- and sesquiterpenes on the consumption of alfalfa pellets by sheep.

Effects of individual terpenes on alfalfa pellet intake by lambs were examined in four experiments. Forty-five lambs (nine lambs/treatment) were individually fed alfalfa pellets sprayed with either camphene, myrcene, caryophyllene oxide, or beta-pinene at one of five concentrations (one terpene per experiment). Treatments (0, 0.5, 1, 2, and 10x) were multiples of the concentration (x) of a specific terpene in tarbush. Terpenes were applied to alfalfa pellets (0.64 kg x lamb(-1) x d(-1), DM basis), and consumption was measured during a 20-min interval for 5 d. Lambs were maintained and fed alfalfa pellets in one group (except during 20-min tests) at a mean total daily intake of 4.7% of BW (DM basis). Camphene and caryophyllene oxide tended to decrease intake (linear contrasts were P = .0651 and P = .0504, respectively), whereas myrcene and beta-pinene exerted no effect on the consumption of alfalfa pellets by lambs. Camphene and caryophyllene oxide may be involved in the differential herbivory of individual tarbush plants by livestock.

Observation of compressional Alfvén modes during neutral-beam heating on the national spherical torus experiment.

Neutral-beam-driven compressional Alfvén eigenmodes at frequencies below the ion cyclotron frequency have been observed and identified for the first time in the National Spherical Torus Experiment. The modes are observed as a broad spectrum of nearly equally spaced peaks in the frequency range from approximately 0.2omega(ci) to approximately 1.2omega(ci). The frequency has a scaling with toroidal field and plasma density consistent with Alfvén waves. The modes have been observed with high bandwidth magnetic pickup coils and with a reflectometer.