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- Cecconello, Marco, et al.
(författare)
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Current profile modification experiments in EXTRAP T2R
- 2004
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 46:1, s. 145-161
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Tidskriftsartikel (refereegranskat)abstract
- Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 mus. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.
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- Dieckmann, Mark E, 1969-, et al.
(författare)
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Plasma sounding at the upper hybrid frequency
- 2000
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105:A6, s. 13103-13117
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Tidskriftsartikel (refereegranskat)abstract
- A sounder measures the density of plasmas in various parts of the solar system. The sounder emits wave pulses into the ambient plasma and listens to the response. Intensity peaks in the wave response are typically related to two mechanisms. One is provided by waves that are reflected off plasma inhomogeneities and propagate back to the emitting antenna, where they are then detected. The second is provided by waves propagating with the same group velocity as that of the receiving antenna. In the second case the waves stay close to the antenna and thus yield a long-lasting response. Response peaks to sounding at the upper hybrid (UH) frequency have, in most cases, been related to reflected waves. In this work we examine if accompanying waves can give rise to the UH response peak. We examine quantitatively how the plasma response to sounding at the UH frequency depends on the plasma density, on the electron temperature, and on the emission amplitude. For the first two parameters this is done by solving the linear dispersion relation. The well-known property of the UH waves to change from having a zero group velocity to propagating waves, depending on how the electron density compares to the electron cyclotron frequency, is applied to Alouette sounder data. It is discussed how the change in the group velocity may affect the spectral profile of the UH resonance. We present results from numerical particle in cell (PIC) simulations which show that in the case of nonpropagating UH waves, energy can be coupled into the plasma even though the vanishing group velocity of the UH waves should not allow this. The PIC simulations and sounder data from the Alouette mission show that in the case of propagating UH waves the response duration to sounding may be used to determine the electron temperature. Emission amplitudes that are typical for plasma sounders are also shown to suppress the generation of certain electron cyclotron harmonic waves.
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- Dieckmann, Mark E, 1969-, et al.
(författare)
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Simulating thermal noise
- 2004
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 69:6, s. 456-460
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Tidskriftsartikel (refereegranskat)abstract
- Thermal noise measurements by space-borne antennas are commonly used to determine plasma parameters like the electron density and the plasma temperature from the noise spectra. It would be desirable to have a controlled experiment in which noise from a plasma with known properties is sampled in space and in time and which results can then be used to reproduce the satellite measurements. Here we examine the possibility to use particle-in-cell (PIC) simulations as such an experiment. In this work we present a statistically averaged noise spectrum computed with a PIC code for a simple single-Maxwellian and unmagnetized electron plasma and we compare it to both, the thermal noise spectrum for the corresponding real plasma and the noise spectrum we would anticipate from our numerical scheme. We find that we can produce noise fields with sufficiently low amplitudes to keep the plasma in a linear regime. We show that the simulation noise at low and at large wave numbers differs not only from thermal noise of a physical plasma but also from the numerical noise we would expect from our numerical scheme. We explain the drop of the noise power at low wave numbers by our initial conditions. We find experimentally the relation that connects the theoretical noise spectrum for our simulation code with that we actually measure, provided that the phase velocity of the noise is less than the maximum velocity of the computational particles.
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- Doverskog, M., et al.
(författare)
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Determination of NADH-dependent glutamate synthase (GOGAT) in Spodoptera frugiperda (Sf9) insect cells by a selective H-1/N-15 NMR in vitro assay
- 2000
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Ingår i: Journal of Biotechnology. - 0168-1656 .- 1873-4863. ; 79:1, s. 87-97
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Tidskriftsartikel (refereegranskat)abstract
- This is the second of two papers [Drews, M., Doverskog, M., Qhman, L., Chapman, B.E., Jacobsson, U., Kuchel, P.W., Haggstrom, L., 2000. Pathways of glutamine metabolism in Spodoptera frugiperda (Sf9) insect cells: evidence for the presence of the nitrogen assimilation system, and a metabolic switch by H-1/N-15 NMR. J. Biotechnol. 78, 23-37]. where the general goal has been to determine and characterise the glutamine metabolism in Sf9 cells. The presence of glutamate synthase (GOGAT) activity was investigated in cell-free extracts of S. frugiperda (Sf9) insect cells by modified H-1/N-15 spin-echo and gradient enhanced multiple quantum coherence NMR spectroscopy techniques. Cell-free extracts were prepared from cells cultured in a serum-free medium. The assay conditions were based on conventional spectrophotometric and chromatographic methods. NMR data showed that nitrogen from [5-N-15] glutamine was selectively incorporated into 2-oxoglutarate forming [2-N-15] glutamate with a specific activity of 4.15 +/- 0.21 nmol [2-N-15] glutamate min (-1) (mg total protein)(-1) in the cell-free extracts. The enzyme activity was exclusively dependent on NADH as coenzyme and was completely inhibited by 1 mM azaserine. From the results obtained, we conclude that Sf9 cells possess NADH-GOGAT activity. Furthermore, the high specificity of the NMR method enables distinction of competing reactions from glutaminase and glutamate dehydrogenase.
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- Drews, M., et al.
(författare)
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Pathways of glutamine metabolism in Spodoptera frugiperda (Sf9) insect cells : evidence for the presence of the nitrogen assimilation system, and a metabolic switch by H-1/N-15 NMR
- 2000
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Ingår i: Journal of Biotechnology. - 0168-1656 .- 1873-4863. ; 78:1, s. 23-37
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Tidskriftsartikel (refereegranskat)abstract
- H-1/N-15 and C-13 NMR were used to investigate metabolism in Spodoptera frugiperda (Sf9) cells. Labelled substrates ([2-N-15]glutamine, [5-N-15]glutamine, [2-N-15]glutamate, (NH4Cl)-N-15, [2-N-15]alanine, and [1-C-13]glucose) were added to batch cultures and the concentration of labelled excreted metabolites (alanine, NH4+, glutamine, glycerol, and lactate) were quantified. Cultures with excess glucose and glutamine produce alanine as the main metabolic by-product while no ammonium ions are released. H-1/N-15 NMR data showed that both the amide and amine-nitrogen of glutamine was incorporated into alanine in these cultures. The amide-nitrogen of glutamine was not transferred to the amine-position in glutamate (for further transamination to alanine) via free NH4+ but directly via an azaserine inhibitable amidotransfer reaction. In glutamine-free media (NH4+)-N-15 was consumed and incorporated into alanine. (NH4+)-N-15 was also incorporated into the amide-position of glutamine synthesised by the cells. These data suggest that the nitrogen assimilation system, glutamine synthetase/glutamate synthase (NADH-GOGAT), is active in glutamine-deprived cells. In cultures devoid of glucose, ammonium is the main metabolic by-product while no alanine is formed. The ammonium ions stem both from the amide and amine-nitrogen of glutamine, most likely via glutaminase and glutamate dehydrogenase. C-13 NMR revealed that the [1-C-13] label from glucose appeared in glycerol, alanine, lactate, and in extracellular glutamine. Labelling data also showed that intermediates of the tricarboxylic acid cycle were recycled to glycolysis and that carbon sources, other than glucose-derived acetylCoA, entered the cycle. Furthermore, Sf9 cell cultures excreted significant amounts glycerol (1.9-3.2 mM) and ethanol (6 mM), thus highlighting the importance of sinks for reducing equivalents in maintaining the cytosolic redox balance.
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- Franz, P., et al.
(författare)
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Compact soft x-ray multichord camera : Design and initial operation
- 2003
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 74:3 II, s. 2152-2156
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Tidskriftsartikel (refereegranskat)abstract
- The design and initial operation of a compact soft x-ray (SXR) multichord camera was studied. The line integrity emissivity was measured along up to 20 lines of sight using an array of miniaturized silicon photodiodes. The application of the diagnostic to a gas-fed magnetoplasma dynamic thruster (MPDT) with an external magnetic field was also discussed.
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- McClements, KG, et al.
(författare)
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Surfatron and stochastic acceleration of electrons at supernova remnant shocks
- 2001
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 87:25
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Tidskriftsartikel (refereegranskat)abstract
- The surfatron offers the possibility of particle acceleration to arbitrarily high energies, given a sufficiently large system. Surfatron acceleration of electrons by waves excited by ions reflected from supernova. remnant (SNR) shocks is investigated using particle simulations. It is shown that surfatron and stochastic acceleration could provide a seed population for the acceleration of cosmic ray electrons at SNR shocks.
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