51 research outputs found
Cell lineage-specific mitochondrial resilience during mammalian organogenesis
Mitochondrial activity differs markedly between organs, but it is not known how and when this arises. Here we show that cell lineage-specific expression profiles involving essential mitochondrial genes emerge at an early stage in mouse development, including tissue-specific isoforms present before organ formation. However, the nuclear transcriptional signatures were not independent of organelle function. Genetically disrupting intra-mitochondrial protein synthesis with two different mtDNA mutations induced cell lineage-specific compensatory responses, including molecular pathways not previously implicated in organellar maintenance. We saw downregulation of genes whose expression is known to exacerbate the effects of exogenous mitochondrial toxins, indicating a transcriptional adaptation to mitochondrial dysfunction during embryonic development. The compensatory pathways were both tissue and mutation specific and under the control of transcription factors which promote organelle resilience. These are likely to contribute to the tissue specificity which characterizes human mitochondrial diseases and are potential targets for organ-directed treatments
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Design and performance of fast wave current drive systems in the ICRF
Experiments have begun on D3-D using the fast wave current drive (FWCD) phased antenna array. The array consists of four elements with slotted septa between them to reduce mutual coupling. The passive phasing/matching circuit developed for the launcher incorporates only five tuning elements and is driven by a single rf power supply. The system has successfully operated in the presence of plasma at power levels up to 1.25 MW, with {pi}/2 relative phasing, and approximately equal currents and voltages on all elements. Tuning algorithms that allow proper setting of all five elements within 1--2 shots have been developed. In addition, substantial modeling has been undertaken in support of the D3-D FWCD program. Loading calculations that take into account currents induced in the septa as well as other effects related to antenna geometry have been performed, and the results agree well with the observed data. A circuit model has been developed that, in combination with the loading calculations, allows the simulation of shot-to-shot matching for various tuning algorithms. 6 refs., 8 figs
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Comparison between the electron cyclotron current drive experiments on DIII-D and predictions for T-10
Electron cyclotron current drive has been demonstrated on the DIII-D tokamak in an experiment in which {approximately}1 MW of microwave power generated {approximately}50 kA of non-inductive current. The rf-generated portion was about 15% of the total current. On the T-10 tokamak, more than 3 MW of microwave power will be available for current generation, providing the possibility that all the plasma current could be maintained by this method. Fokker-Planck calculations using the code CQL3D and ray tracing calculations using TORAY have been performed to model both experiments. For DIII-D the agreement between the calculations and measurements is good, producing confidence in the validity of the computational models. The same calculations using the T-10 geometry predict that for n{sub e}(0) {approximately} 1.8 {times} 10{sup 13} cm{sup {minus}3}, and T{sub e}(0) {approximately} 7 keV, 1.2 MW, that is, the power available from only three gyrotrons, could generate as much as 150 kA of non-inductive current. Parameter space scans in which temperature, density and resonance location were varied have been performed to indicate the current drive expected under different experimental conditions. The residual dc electric field was considered in the DIII-D analysis because of its nonlinear effect on the electron distribution, which complicates the interpretation of the results. A 110 GHz ECH system is being installed on DIII-D. Initial operations, planned for late 1991, will use four gyrotrons with 500 kW each and 10 second output pulses. Injection will be from the low field side from launchers which can be steered to heat at the desired location. These launchers, two of which are presently installed, are set at 20 degrees to the radial and rf current drive studies are planned for the initial operation. 8 refs., 10 figs
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Fast wave current drive experiment on the DIII-D tokamak
One method of radio-frequency heating which shows theoretical promise for both heating and current drive in tokamak plasmas is the direct absorption by electrons of the fast Alfven wave (FW). Electrons can directly absorb fast waves via electron Landau damping and transit-time magnetic pumping when the resonance condition {omega} {minus} {kappa}{sub {parallel}e}{upsilon}{sup {parallel}e} = O is satisfied. Since the FW accelerates electrons traveling the same toroidal direction as the wave, plasma current can be generated non-inductively by launching FW which propagate in one toroidal direction. Fast wave current drive (FWCD) is considered an attractive means of sustaining the plasma current in reactor-grade tokamaks due to teh potentially high current drive efficiency achievable and excellent penetration of the wave power to the high temperature plasma core. Ongoing experiments on the DIII-D tokamak are aimed at a demonstration of FWCD in the ion cyclotron range of frequencies (ICRF). Using frequencies in the ICRF avoids the possibility of mode conversion between the fast and slow wave branches which characterized early tokamak FWCD experiments in the lower hybrid range of frequencies. Previously on DIII-D, efficient direct electron heating by FW was found using symmetric (non-current drive) antenna phasing. However, high FWCD efficiencies are not expected due to the relatively low electron temperatures (compared to a reactor) in DIII-D
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Direct electron heating by 60 MHz fast waves on DIII-D
Efficient direct electron heating by fast waves has been observed on the DIII-D tokamak. A four strap antenna with (0,{pi},0,{pi}) phasing launched up to 1.6 MW of fast wave power with {vert bar}n{sub {parallel}}{vert bar} {approx} 11. This {vert bar}n{sub {parallel}}{vert bar} is suitable for strong electron interaction in ohmic target plasmas (T{sub e} {le} 2 keV). Ion cyclotron absorption was minimized by keeping the hydrogen fraction low ({lt}3%) in deuterium discharges and by operating at high ion cyclotron harmonics ({omega} = 4{Omega}{sub H} = 8{Omega}{sub D} at 1T). The fast wave electron heating was weak for central electron temperatures below 1 keV, but improved substantially with increasing T{sub e}. Although linear theory predicts a strong inverse magnetic field scaling of the first pass absorption, the measured fast-wave heating efficiency was independent of magnetic field. Multiple pass absorption of the fast waves appears to be occurring since at 2.1 T nearly 100% efficient plasma heating is observed while the calculated first pass absorption is 6% to 8%. The central electron temperature during fast wave heating also increased with magnetic field. The improved electron heating at higher magnetic fields may be due in part to a peaking of the ohmic plasma current and the ohmic electron temperature profiles. Centrally peaked deposition profiles were measured by modulating the fast wave power at 10 Hz and observing the local electron temperature response across the plasma. 11 refs., 10 figs
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Fundamental and second harmonic hydrogen fast-wave heating on DIII-D
Ion cyclotron resonance heating (ICRH) with fast waves has been investigated on D3-D in both the fundamental hydrogen minority (32 MHz, 2.14 T) and second harmonic hydrogen majority (60 MHz, 1.97 T) regimes. The main purpose of these experiments was to characterize the fast wave coupling and propagation in preparation for upcoming fast wave current drive (FWCD) experiments. For the fundamental minority regime, the electron and ion heating, global confinement, and radiated power fraction are compared for ICRH and NBI discharges with P{sub aux} {approx} 1 MW. The ICRH experiments were conducted using a four strap antenna which was designed for current drive experiments. The antenna is fed by a single 2 MW 30--60 MHz transmitter. For ICRH experiments, either (0,0,0,0) or (0,{pi},0,{pi}) phasing was used. The launched parallel index of refraction for (0,{pi},0,{pi}) phasing is {vert bar}n{parallel}{vert bar} {approx} 21 at 32 MHz, and {vert bar}n{parallel}{vert bar} {approx} 11 at 60 MHz. 7 refs., 8 figs
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