| 1. |
- Mantsinen, M. J., et al.
(författare)
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Localized bulk electron heating with ICRF mode conversion in the JET tokamak
- 2004
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 44:1, s. 33-46
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Tidskriftsartikel (refereegranskat)abstract
- Ion cyclotron resonance frequencies (ICRF) mode conversion has been developed for localized on-axis and off-axis bulk electron heating on the JET tokamak. The fast magnetosonic waves launched from the low-field side ICRF antennas are mode-converted to short-wavelength waves on the high-field side of the He-3 ion cyclotron resonance layer in D and He-4 plasmas and subsequently damped on the bulk electrons. The resulting electron power deposition, measured using ICRF power modulation, is narrow with a typical full-width at half-maximum of approximate to30 cm (i.e. about 30% of the minor radius) and the total deposited power to electrons comprises at least up to 80% of the applied ICRF power. The ICRF mode conversion power deposition has been kept constant using He-3 bleed throughout the ICRF phase with a typical duration of 4-6 s, i.e. 15-40 energy confinement times. Using waves propagating in the counter-current direction minimizes competing ion damping in the presence of co-injected deuterium beam ions.
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| 2. |
- Ongena, J., et al.
(författare)
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Recent progress on JET towards the ITER reference mode of operation at high density
- 2001
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 43, s. A11-A30
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress towards obtaining high density and high confinement in JET as required for the ITER reference scenario at Q = 10 is summarized. Plasmas with simultaneous confinement H-98(y.2) = 1 and densities up to n/n(Gw) similar to 1 are now routinely obtained. This has been possible (i) by using plasmas at high (delta similar to 0.5) and medium (delta similar to 0.3-0.4) triangularity with sufficient heating power to maintain Type I ELMs, (ii) with impurity seeded plasmas at high (delta similar to 0.5) and low (delta less than or equal to 0.2) triangularity, (iii) with an optimized pellet injection sequence, maintaining the energy confinement and raising the density, and (iv) by carefully tuning the gas puff rate leading to plasmas with peaked density profiles and good confinement at long time scales. These high performance discharges exhibit Type I ELMs, with a new and more favourable behaviour observed at high densities, requiring further studies. Techniques for a possible mitigation of these ELMs are discussed, and first promising results are obtained with impurity seeding in discharges at high triangularity. Scaling studies using the new data of this year show a strong dependence of confinement on upper triangularity, density and proximity to the Greenwald limit. Observed MHD instabilities and methods to avoid these in high density and high confinement plasmas are discussed.
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| 3. |
- McDonald, C., et al.
(författare)
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ELMy H-modes in JET helium-4 plasmas
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 46:3, s. 519-534
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Tidskriftsartikel (refereegranskat)abstract
- ELMy H-modes in helium-4 plasmas provide valuable information on ELMy H-mode physics as well as a possible early low activation operational phase for next-step tokamaks, such as ITER. With this in mind, a series of helium-4 H-mode experiments were performed on JET with pure helium-4 NBI auxiliary heating (up to 12 MW). A set of ELMy H-mode plasmas were produced, in both the Type I ELM regime and a second regime, which showed characteristics similar to the deuterium Type III regime, but with a reverse ELM frequency dependence on power. Sawteeth were also observed, and had similar behaviour to those seen in deuterium. Compared with deuterium plasmas, Type I ELMy H-mode confinement is seen to be 28 +/- 6% poorer in helium-4 plasmas and the L-H power threshold 42 +/- 10% larger. This is the opposite of the behaviour predicted by experimental isotope mass scalings from hydrogenic plasmas.Comparison with a wider hydrogenic database, enables the effects of isotopic charge and mass to be studied independently.
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| 4. |
- Zastrow, K. D., et al.
(författare)
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Tritium transport experiments on the JET tokamak
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 46, s. B255-B265
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Tidskriftsartikel (refereegranskat)abstract
- An overview is given of the experimental method, the analysis technique and the results for trace tritium experiments conducted on the JET tokamak in 2003. Observations associated with events such as sawtooth collapses, neo-classical tearing modes and edge localized modes are described. Tritium transport is seen to approach neo-classical levels in the plasma core at high density and low q(95), and in the transport barrier region of internal transport barrier (ITB) discharges. Tritium transport remains well above neo-classical levels in all other cases. The correlation of the measured tritium diffusion coefficient and convection velocity for normalized minor radii r/a = [0.65, 0.80] with the controllable parameters q95 and plasma density are found to be consistent for all operational regimes (ELMy H-mode discharges with or without ion cyclotron frequency resonance heating, hybrid scenario and ITB discharges). Scaling with local physics parameters is best described by gyro-Bohm scaling with an additional inverse beta dependence.
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| 5. |
- Adage, Tiziana, et al.
(författare)
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Hypothalamic, metabolic, and behavioral responses to pharmacological inhibition of CNS melanocortin signaling in rats
- 2001
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Ingår i: Journal of Neuroscience. - 0270-6474 .- 1529-2401. ; 21:10, s. 3639-3645
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Tidskriftsartikel (refereegranskat)abstract
- The CNS melanocortin (MC) system is implicated as a mediator of the central effects of leptin, and reduced activity of the CNS MC system promotes obesity in both rodents and humans. Because activation of CNS MC receptors has direct effects on autonomic outflow and metabolism, we hypothesized that food intake- independent mechanisms contribute to development of obesity induced by pharmacological blockade of MC receptors in the brain and that changes in hypothalamic neuropeptidergic systems known to regulate weight gain [i. e., corticotropin-releasing hormone (CRH), cocaine- amphetamine- related transcript (CART), proopiomelanocortin (POMC), and neuropeptide Y (NPY)] would trigger this effect. Relative to vehicle- treated controls, third intracerebroventricular (i3vt) administration of the MC receptor antagonist SHU9119 to rats for 11 d doubled food and water intake (toward the end of treatment) and increased body weight (similar to 14%) and fat content (similar to 90%), hepatic glycogen content (similar to 40%), and plasma levels of cholesterol (similar to 48%), insulin (similar to 259%), glucagon (similar to 80%), and leptin (similar to 490%), whereas spontaneous locomotor activity and body temperature were reduced. Pair- feeding of i3vt SHU9119- treated animals to i3vt vehicle- treated controls normalized plasma levels of insulin, glucagon, and hepatic glycogen content, but only partially reversed the elevations of plasma cholesterol (similar to 31%) and leptin (similar to 104%) and body fat content (similar to 27%). Reductions in body temperature and locomotor activity induced by i3vt SHU9119 were not reversed by pair feeding, but rather were more pronounced. None of the effects found can be explained by peripheral action of the compound. The obesity effects occurred despite a lack in neuropeptide expression responses in the neuroanatomical range selected across the arcuate (i. e., CART, POMC, and NPY) and paraventricular (i. e., CRH) hypothalamus. The results indicate that reduced activity of the CNS MC pathway promotes fat deposition via both food intake- dependent and -independent mechanisms.
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| 6. |
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| 7. |
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| 8. |
- Testa, D., et al.
(författare)
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Alfven mode stability and wave-particle interaction in the JET tokamak : prospects for scenario development and control schemes in burning plasma experiments
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335 .- 1361-6587. ; 46:7, s. S59-S79
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the effect of different ion cyclotron resonance frequency (ICRF) heating schemes, of error field modes, of the plasma shape and edge magnetic shear, and of the ion delB drift direction on the stability of Alfven eigenmodes (AEs). The use of multi-frequency or 2nd harmonic minority ICRF heating at high plasma density gives rise to a lower fast ion pressure gradient in the plasma core and to a reduced mode activity in the Alfven frequency range. Externally excited low-amplitude error fields lead to a much larger AE instability threshold, which we attribute to a moderate radial redistribution of the fast ions. The edge plasma shape has a clear stabilizing effect on high-n, radially localized AEs. The damping rate of n = 1 toroidal AEs is a factor 3 higher when the ion VB drift is directed towards the divertor. These results represent a useful step towards the extrapolation of current scenarios to the inclusion of fusion-born alpha particles in ITER, with possible application for feedback control schemes for the various ITER operating regimes.
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| 9. |
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| 10. |
- Eriksson, L-G, et al.
(författare)
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Destabilization of fast-ion-induced long sawteeth by localized current drive in the JET tokamak.
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 92:23
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Tidskriftsartikel (refereegranskat)abstract
- In a tokamak fusion reactor the energetic alpha particles will transiently stabilize the magnetohydrodynamic activity causing sawtooth oscillations. The crash events terminating long sawtooth free periods can provide seed islands for neoclassical tearing modes [Phys. Rev. Lett. 88, 105001 (2002)]]. To shorten the sawtooth periods localized current drive near the q=1 surface is a possibility. This Letter provides the first experimental evidence for the effectiveness of this method in the different physics regime associated with fast-ion-induced long sawteeth on the JET tokamak.
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| 14. |
- Rotsaert, F. A. J., et al.
(författare)
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Biophysical and structural analysis of a novel heme b iron ligation in the flavocytochrome cellobiose dehydrogenase
- 2003
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Ingår i: Journal of Biological Chemistry. - 0021-9258 .- 1083-351X. ; 278:35, s. 33224-33231
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Tidskriftsartikel (refereegranskat)abstract
- The fungal extracellular flavocytochrome cellobiose dehydrogenase (CDH) participates in lignocellulose degradation. The enzyme has a cytochrome domain connected to a flavin-binding domain by a peptide linker. The cytochrome domain contains a 6-coordinate low spin b-type heme with unusual iron ligands and coordination geometry. Wild type CDH is only the second example of a b-type heme with Met-His ligation, and it is the first example of a Met-His ligation of heme b where the ligands are arranged in a nearly perpendicular orientation. To investigate the ligation further, Met(65) was replaced with a histidine to create a bis-histidyl ligated iron typical of b-type cytochromes. The variant is expressed as a stable 90-kDa protein that retains the flavin domain catalytic reactivity. However, the ability of the mutant to reduce external one-electron acceptors such as cytochrome c is impaired. Electrochemical measurements demonstrate a decrease in the redox midpoint potential of the heme by 210 mV. In contrast to the wild type enzyme, the ferric state of the protoheme displays a mixed low spin/high spin state at room temperature and low spin character at 90 K, as determined by resonance Raman spectroscopy. The wild type cytochrome does not bind CO, but the ferrous state of the variant forms a CO complex, although the association rate is very low. The crystal structure of the M65H cytochrome domain has been determined at 1.9 Angstrom resolution. The variant structure confirms a bis-histidyl ligation but reveals unusual features. As for the wild type enzyme, the ligands have a nearly perpendicular arrangement. Furthermore, the iron is bound by imidazole N-delta1 and N-epsilon2 nitrogen atoms, rather than the typical N-epsilon2/N-epsilon2 coordination encountered in bis-histidyl ligated heme proteins. To our knowledge, this is the first example of a bis-histidyl N-delta1/N-epsilon2-coordinated protoporphyrin IX iron.
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