Results 1 - 10 of 27843
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[en] The violent activity at the center of many galaxies is manifested in the production of narrow, focused streams of ionized gas. Some are a few light-years long; others are a million times longer. (orig.)
[de]Viele Galaxien schleudern ein Paar eng gebuendelter Jets ins All: Strahlen aus ionisiertem Gas, die von stuermischen Prozessen im galaktischen Zentrum zeugen. Ebenso raetselhaft wie bizarr, verraten sie sich meist nur durch die Radiowellen, die sie aussenden. Ihre Existenz verdanken sie womoeglich einem Schwarzen Loch im Herzen der Galaxie. (orig.)
[en] Results of modeling of streamer propagation along helium jets for both positive and negative polarities of applied voltage are presented. Obtained patterns of streamer dynamics and structure in these two cases are similar to those observed in experiments with plasma jets.
[en] This paper provides summaries of ten talks on astrophysical jets given at the HEDP/HEDLA-08 International Conference in St. Louis. The talks are topically divided into the areas of observation, numerical modeling, and laboratory experiment. One essential feature of jets, namely, their filamentary (i.e., collimated) nature, can be reproduced in both numerical models and laboratory experiments. Another essential feature of jets, their scalability, is evident from the large number of astrophysical situations where jets occur. This scalability is the reason why laboratory experiments simulating jets are possible and why the same theoretical models can be used for both observed astrophysical jets and laboratory simulations.
[en] JET has completed its first stage of active operations, processing more than 100 g of tritium during the deuterium-tritium experiment (DTE1), the neutral beam intervention and remote tile exchange shutdown. The radiological safety of tritium handling operations has been of particular interest. A wide range of tritium operations has been carried out in the period 1995-98. This paper describes some of the radiological protection measures for work with tritium, and discusses Health Physics operational experience of handling tritium in this period. Descriptions are given of active operations in the gas handling plant; in the torus hall during DTE1; in related interventions, and of the remote exchange of in-vessel divertor modules. Workplace contamination levels over 100DAC (HTO) have been encountered, tritiated water with activity of 2TBq/litre and tritiated carbon with activities of ∼4TBq/g has been handled. Control measures involving the use of purge and extract ventilation, and of personal protection using air-fed pressurised suits are described. The project imposes tight limits on radiation exposures. Tritium doses to staff in this period have been very low (individual doses <170μSv/year, collective doses ≤2.1mSv/year). Aerial discharges have been <3% of annual authorised limits, and average environmental (HTO) levels have been a few Bq/m3. Lessons have been learnt concerning exposure control, large-scale permeation effects, and the appearance of residual tritium on exposed surfaces. Tritium operations at JET have been conducted without incident and with very low personnel exposures. The methodology of using containment and ventilation systems and tight radiological control has been successful in limiting doses. JET experience shows that large-scale tritium handling and exposure control can be achieved within stringent dose limits. (author)
[en] This paper describes the relevance of laboratory experiments in the context of astrophysical jets. Such experiments can be used for studying problems (such as turbulence in jets) for which we have only a limited theoretical understanding. Also, laboratory experiments are fundamental for testing the accuracy of gasdynamic or MHD codes that are being used for computing astrophysical jet models. Finally, we suggest that the flows deviced for modelling astrophysical jets can be used as an inspiration for producing new kinds of laboratory jets
[en] The basic laws of heat transfer for direct condensation of vapour jets in subcooled water pool are investigated, with reference to the pressure-suppressure systems in B.W.R.'s. Heat transfer coefficients hsub(dc) are very high (of the order of 300/Wcm2 Csup(deg)) and depends mainly on the pool subcooling (ΔTsub(sub)) and on the vapour quality of the discharging jet (X) hsub(d.c.)=(360X2-530X+250.ΔTsub(sub)sup(X/10.5)+0.26. To obtain stable vapour cones at the outlet section of the discharging channel it is necessary to reach critical flows at the outlet. Stability depends on both the specific mass flowrate and the pool subcooling
[en] The role of the Real Time Power Control system (RTPC) in the Joint European Torus (JET) is described in depth. The modes of operation are discussed in detail and a number of successful experiments are described. These experiments prove that RTPC can be used for a wide range of experiments, including: (1) Feedback control of plasma parameters in real time using Ion Cyclotron Resonance Heating (ICRH) or Neutral Beam Heating (NBH) as the actuator in various JET operating regimes. It is demonstrated that in a multi-parameter space it is not sufficient to control one global plasma parameter in order to avoid performance limiting events. (2) Restricting neutron production and subsequent machine activation resulting from high performance pulses. (3) The simulation of α-particle heating effects in a DT-plasma in a D-only plasma. The heating properties of α-particles are simulated using ICRH-power, which is adjusted in real time. The simulation of α-particle heating in JET allows the effects of a change in isotopic mass to be separated from α-particle heating. However, the change in isotopic mass of the plasma ions appears to affect not only the global energy confinement time (τE) but also other parameters such as the electron temperature at the plasma edge. This also affects τE, making it difficult to make a conclusive statement about any isotopic effect. (4) For future JET experiments a scheme has been designed which simulates the behaviour of a fusion reactor experimentally. The design parameters of the International Thermonuclear Experimental Reactor (ITER) are used. In the proposed scheme the most relevant dimensionless plasma parameters are similar in JET and ITER. It is also shown how the amount of heating may be simulated in real time by RTPC using the electron temperature and density as input parameters. The results of two demonstration experiments are presented. (author)