Search Results - (Author, Cooperation:J. R. Wilson)

2018-02-20

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2018-09-14

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2018-02-20

Wiley-Blackwell

1942-2466

Geography

Geosciences

2015-05-02

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

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Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

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2018-04-14

BMJ Publishing

2044-6055

Medicine

Open access, Neurology

1089-7674

AIP Digital Archive

Physics

The nonlinear behavior of the toroidal Alfvén eigenmode (TAE) driven unstable by energetic ions in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 1, 1560 (1994)] is studied. The evolution of instabilities can take on several scenarios: a single mode or several modes can be driven unstable at the same time, the spectrum can be steady or pulsating, and there can be negligible or anomalous loss associated with the instability. This paper presents a comparison between experimental results and recently developed nonlinear theory. Many features observed in experiment are compatible with the consequences of the nonlinear theory. Examples include the structure of the saturated pulse that emerges from the onset of instability of a single mode, and the decrease, but persistence, of TAE signals when the applied rf power is reduced or shut off. © 1997 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

The accuracy of standard finite Larmor radius (FLR) models for wave propagation in the ion cyclotron range of frequencies (ICRF) is compared against full hot plasma models. For multiple ion species plasmas, the FLR model is shown to predict the presence of a spurious second harmonic ion–ion type resonance between the second harmonic cyclotron layers of two ion species. It is shown explicitly here that the spurious resonance is an artifact of the FLR models and that no absorption occurs in the plasma as a result of this "resonance.'' © 1994 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Experiments with plasma heating by waves at the ion cyclotron resonance of a minority species have shown that the heating efficiency degrades above a certain power threshold. It is found that this threshold is due to the destabilization of a branch of shear Alfvén waves, the Energetic Particle Modes, which causes a diffusive loss of fast ions. These modes not only play a fundamental role in the transport of the fast ions, but appear closely related to the formation of giant sawteeth. © 1999 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Recent radio-frequency heating experiments on the Tokamak Fusion Test Reactor (TFTR) [Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] have focused on developing tools for both pressure and current profile control in deuterium–tritium (DT) plasmas. A new antenna was added to investigate pressure profile control utilizing direct ion Bernstein wave (IBW) heating. This was the first time direct IBW heating was explored on TFTR. Plasma heating and driven poloidal flows are observed. Previously heating and current drive via mode-converted IBW waves had been demonstrated in non-DT plasmas but efforts in DT plasmas had been unsuccessful. This lack of success had been ascribed to the presence of a small 7Li minority ion population. In the most recent experiments 6Li was used exclusively for machine conditioning and mode-conversion heating consistent with theory is now observed in DT plasmas. © 1998 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Modes have been observed near the frequency of the second Alfvén gap during off-axis H-minority heating experiments in the circular cross-section Tokamak Fusion Test Reactor. The observation of these modes is surprising in that the second gap, which is generally opened with ellipticity, is expected to be small, of order (r/R)2. A model is proposed in which the second gap is opened by the fast ion beta, which is shown to be able to introduce mode coupling, much as toroidal effects introduce mode coupling for Toroidal Alfvén Eigenmodes (TAE). With the low inferred energy of the fast ion tail (30–50 keV), the fast ion bounce resonance condition is assumed to drive the modes. The modes are seen with and without accompanying TAE mode activity. © 2000 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Recent experiments in the ion cyclotron range of frequencies (ICRF) in the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)] are discussed. These experiments include mode conversion heating and current drive, fast wave current drive, and heating of low (L)- mode deuterium–tritium (D–T) plasmas in both the hydrogen minority and second harmonic tritium regimes. In mode conversion heating, a central electron temperature of 10 keV was attained with 3.3 MW of radio-frequency power. In mode conversion current drive experiments, up to 130 kA of current was noninductively driven, on and off axis, and the current profiles were modified. Fast wave current drive experiments have produced 70–80 kA of noninductively driven current. Heating of L-mode deuterium and D–T plasmas by hydrogen minority ICRF has been compared. Finally, heating of L-mode D–T plasmas at the second harmonic of the tritium cyclotron frequency has been demonstrated. © 1996 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

The effect of isotope on confinement in high-recycling, L-mode plasmas is studied on the Tokamak Fusion Test Reactor (TFTR) [see D. M. Meade, J. Fusion Energy 7, 107 (1988)] by comparing hydrogen and deuterium plasmas with the same magnetic field and similar electron densities and heating power, with both Ohmic and deuterium-neutral-beam heating. Following a long operational period in deuterium, nominally hydrogen plasmas were created through hydrogen glow discharge and hydrogen gas puffing in Ohmic plasmas, which saturated the exposed limiter surface with hydrogen and raised the H/(H+D) ratio from 10±3% to 65±5%. Ohmic deuterium discharges obtained higher stored energy and lower loop voltage than hydrogen discharges with similar limiter conditions. Neutral-beam power scans were conducted in L-mode plasmas at minor radii of 50 and 80 cm, with plasma currents of 0.7 and 1.4 MA. To minimize transport differences from the beam deposition profile and beam heating, deuterium neutral beams were used to heat the plasmas of both isotopes. Total stored energy increased approximately 20% from nominally hydrogen plasmas to deuterium plasmas during auxiliary heating. Of this increase about half can be attributed to purely classical differences in the energy content of unthermalized beam ions. Kinetic measurements indicate a consistent but small increase in central electron temperature and total stored electron energy in deuterium relative to hydrogen plasmas, but no change in total ion stored energy. No significant differences in particle transport, momentum transport, and sawtooth behavior are observed. Overall, only a small improvement (∼10%) in global energy confinement time of the thermal plasma is seen between operation in hydrogen and deuterium. © 1996 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

The first experiments utilizing high-power radio waves in the ion cyclotron range of frequencies to heat deuterium–tritium (D–T) plasmas have been completed on the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)]. Results from the initial series of experiments have demonstrated efficient core second harmonic tritium (2ΩT) heating in parameter regimes approaching those anticipated for the International Thermonuclear Experimental Reactor [D. E. Post, Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 13th International Conference, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239]. Observations are consistent with modeling predictions for these plasmas. Efficient electron heating via mode conversion of fast waves to ion Bernstein waves has been observed in D–T, deuterium-deuterium (D–D), and deuterium–helium-4 (D–4He) plasmas with high concentrations of minority helium-3 (3He) (n3He/ne(approximately-greater-than)10%). Mode conversion current drive in D–T plasmas was simulated with experiments conducted in D–3He–4He plasmas. Results show a directed propagation of the mode converted ion Bernstein waves, in correlation with the antenna phasing. © 1995 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

The prompt loss of neutral beam ions from the National Spherical Torus Experiment is expected to be between 12% and 42% of the total 5 MW of beam power. There may, in addition, be losses of fast ions arising from high harmonic fast wave (HHFW) heating. Most of the lost ions will strike the HHFW antenna or the neutral beam dump. To measure these losses in the 2000 experimental campaign, thermocouples in the antenna, several infrared camera views, and a Faraday cup lost ion probe will be employed. The probe will measure loss of fast ions with E〉1 keV at three radial locations, giving the scrape-off length of the fast ions. © 2001 American Institute of Physics.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Differences in ease of digestion of bundle sheath cell walls of leaf blades of Panicum spp. are shown to be associated with differences in bundle sheath cell wall structure, including the presence or absence of a suberized lamella. These structural differences are correlated with photo-synthetic type (viz. C3, intermediate C3/C4, and the C4 types, PCK, NADP-ME and NAD-ME), as are mesophyll: bundle sheath area ratios.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

This study investigates forage particle breakdown, which is a factor of potential importance for the low intake of forages, particularly tropical grasses. A comparison was made between fresh leaf blades of a tropical grass, green panic (Panicum maximum var. trichoglume) and a temperate grass, Italian ryegrass (Lolium multiflorum). Leaf blades were chopped into 50 mm lengths and fed to cattle with oesophageal and rumen fistulae. Particle size was assessed on chewed samples immediately after eating and after digestion for 6, 12, 24, 48, 96 h and 3 weeks in nylon bags in the rumen.Chewing during eating reduced particle size more in length than in width and more in green panic than in ryegrass. Mean length was decreased 9·2-fold and 4·7-fold for the two species respectively, and mean width 5·4-fold and 2·3-fold. Green panic leaf had a higher cell wall content and higher dry matter content than ryegrass leaf.Digestion caused a substantial reduction in the width of the particles but not in the length. Width reduction occurred more rapidly in the ryegrass which was reduced to narrow fibres within 12–24 h of digestion while in green panic the same extent of degradation took 〈 48 h. After 96 h digestion, mean width of the chewed material had been reduced 40-fold in ryegrass compared with only 165-fold in green panic; all particles of both species were 〈 1 mm in diameter. The mean length of particles after 96 h digestion was similar to that of the chewed feed. Ryegrass was more digestible than green panic, a difference of 150 g kg DM−1 which was maintained from within the first 6 h of digestion up to 96 h digestion. After three weeks the digestibility of ryegrass was c. 50 g kg DM−1 higher than that of green panic but the proportion of cell wall that was ultimately indigestible was the same (0465) for both species.

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

To investigate links between the anatomy of grass leaves and their resistance to breakdown in the rumen, leaf blades of the tropical grass, green panic (Panicum maximum var, trichoglume), and the temperate grass, Italian ryegrass (Lolium multiflorum), were chopped into 50 mm lengths and fed fresh to cattle. Particle size reduction of the chewed feed was studied immediately after eating and after digestion in nylon bags in the rumen for 6, 12, 24, 48, 96 h and 3 weeks. The lengths of fibre elements, separated into groups of different diameters, were measured on samples digested for 96 h. The process of tissue breakdown was studied using light and electron microscopy.Green panic leaves had twice the cross-sectional area of thick-walled tissues, a higher vascular bundle frequency per unit leaf width, and less, but more densely packed mesophyll, than did the ryegrass leaves. Despite the contribution of these characteristics to greater leaf rigidity, green panic was broken down to a greater degree by chewing than was ryegrass.During digestion, width reduction of the chewed leaf particles was faster in ryegrass than in green panic because of two anatomical features: (i) the straight-walled intercostal cells of the epidermis in ryegrass were easily separated allowing the epidermis to split, whereas the sinuous walls in green panic were resistant to splitting, and (ii) the epidermis of ryegrass was linked to the vascular bundles by thin-walled mesophyll cells and was shed when these were digested, whereas in green panic the linkage was via thick-walled bundle sheath cells causing the epidermis to remain attached for much longer. Ryegrass leaf was reduced to isolated fibres within 24 h digestion; this process took 〉48 h in green panic. These fibres all had a high resistance to length reduction by digestion irrespective of their anatomical or species origin. Even after 3 weeks in the rumen there was little digestive disruption to the longitudinal walls of these fibres.The isolated vascular fibres of ryegrass were smooth-surfaced in contrast to those of green panic which were rough owing to attached undigested bundle sheath cells and jagged, broken sections of epidermis; this could influence ease of separation of particles from the digesta mass and flow from the rumen.Anatomical differences between these grasses were, therefore, important in the rate of width reduction of leaf particles during digestion and for the characteristics of the isolated fibre, but not for length reduction of particles during digestion

Electronic Resource

1365-2494

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

The effect of temperature on the nutritive value of Macroptilium atropurpureum cv. Siratro and line 10–105 was determined in a controlled environment at day/night temperatures of 24/18, 30/24 and 36/30°C, and a 13-h daylength. Estimated dry matter digestibility, cell wall and lignin concentrations of leaf, stem and stubble were measured in two 5-week regrowth cuts.Siratro and line 10–105 were similar in digestibility and responded similarly to temperature with whole tops, stem and stubble decreasing by an average of 0.0023, 0.0026 and 0.0056 units of digestibility for each 1°C rise in growth temperature. These changes reflect the increase in cell wall and lignin concentrations with higher temperature.The digestibility of young, just-expanded leaves was increased by increased growth temperature. This effect may be explained by the faster rate of leaf expansion at high temperature which reduced cell wall and lignin concentrations at this stage of leaf development. However, for the bulk leaf fraction digestibility was unaffected by temperature. Leafiness declined at high temperatures.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

A new diagnostic, composed of an array of high-frequency magnetic probes, has been installed in TFTR for the study of waves driven by the ICRF heating antennas and waves generated spontaneously during ohmic and beam-heated discharges. The former is of interest because of the possibility of surface wave generation by the heating antennas, and the latter because emission spectra may be useful as a beam ion or fusion product diagnostic. The array consists of seven fixed probes positioned at various toroidal and poloidal locations in order to obtain information about the corresponding wavenumbers in the edge region. Each probe consists of two orthogonal loops oriented to measure Bφ and Bθ. The array utilizes single-turn loops with areas of 12 cm2 and carefully selected signal processing elements to achieve a usable bandwidth of 1–500 MHz. Reduction of electrostatic pickup is accomplished by differentially combining signals from both ends of a loop using a wideband hybrid junction and by using an alumina shield to reduce capacitive coupling. The system permits simultaneous observation of both the magnetic and electrostatic components of the probe signal, allowing verification of the rejection ratio in a particular experiment. Details of the diagnostic design and experimental plans are discussed. This work was supported by U. S. DOE Contract No. DE-AC02-76-CHO-3073.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

Edge electron density profile measurements, including the scrape-off layer, have been made during ion cyclotron range of frequency (ICRF) heating with the two-frequency differential-phase reflectometer installed on an ICRF antenna on the Tokamak fusion test reactor (TFTR). This system probes the plasma using the extraordinary mode with two signals swept from 90 to 118 GHz, while maintaining a fixed-difference frequency of 125 MHz. The extraordinary mode is used to obtain density profiles in the range of 1×1011–3×1013 cm−3 in high-field (4.5–4.9 T) full-size (R0=2.62 m, a=0.96 m) TFTR plasmas. The reflectometer launcher is located in an ICRF antenna and views the plasma through a small penetration in the center of the Faraday shield. A 26-m-long overmoded waveguide run connects the launcher to the reflectometer microwave electronics. Profile measurements made with this reflectometer system will be presented along with a discussion of the characteristics of this differential phase reflectometer and data analysis. © 1995 American Institute of Physics.

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