42 research outputs found
Poincaré, précurseur du confinement magnétique ?
En interprĂ©tant les expĂ©riences de Birkeland qui montrent une focalisation des rayons cathodiques lorsqu’on approche du tube le pĂ´le d’un aimant, PoincarĂ© montre aussi que le pĂ´le les renvoie et donc les confine dans les rĂ©gions oĂą le champ est faible : il obtient au passage l’effet miroir magnĂ©tique (indĂ©pendant de la charge) sans s’y attarder ni reconnaĂ®tre son caractère fondamental. Vu par un physicien des plasmas d’aujourd’hui, PoincarĂ© peut-il ĂŞtre considĂ©rĂ© comme un prĂ©curseur du confinement magnĂ©tique – utilisĂ© dans la fusion nuclĂ©aire d’une part, et permettant d’interprĂ©ter les ceintures magnĂ©tiques terrestres d’autre partÂ
Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project
In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum-dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory
Les déconvenues de Prométhée : la longue marche vers l'énergie thermonucléaire
335 p., fig.De tout temps, afin d'assurer leur survie et de développer la société, les hommes ont eu besoin d'énergie. Ils ont d'abord trouvé les sources de cette énergie dans la nature sous forme de biomasse et de combustibles fossiles : charbon, pétrole et gaz naturel, sous forme aussi de vent et de cours d'eau. Mais les besoins sont énormes et s'accroissent continuellement alors que les réserves naturelles ne sont pas illimitées, bien que partiellement renouvelables. L'énergie nucléaire, exploitée à son tour pendant la seconde moitié du xxe siècle, a fait naître de grands espoirs et suscité d'immenses craintes. On ne l'a utilisée que sous l'une de ses ' formes, la fission, lorsque sous l'impact de neutrons, des noyaux atomiques lourds se cassent en deux tout en libérant de l'énergie. On peut aussi extraire de l'énergie de réactions nucléaires de fusion entre noyaux légers, processus qui se poursuivent depuis des milliards d'années au cœur des étoiles. Nouveaux Prométhées, des chercheurs et des ingénieurs ont . depuis les années cinquante entrepris de domestiquer cette forme d'énergie. Ils n'y sont pas encore parvenus. Mission impossible ? Sans doute pas. Mission difficile ? Certainement et beaucoup plus qu'on ne l'avait imaginé au départ. Le doute a commencé à s'insinuer dans les esprits. Pourtant, en un demi-siècle, des progrès considérables ont été accomplis. L'objectif s'est rapproché. Pour l'atteindre, il faudra que se conjuguent les volontés des scientifiques, des industriels et des décideurs politiques
Il y a cinquante ans : les premières réactions de fusion nucléaire induites par laser
Peu après l’invention des lasers au début des années 1960, des calculs d’ordre de grandeur montraient leur intérêt pour la fusion nucléaire. En même temps, débutaient discrètement des programmes de recherches dans des laboratoires à vocation militaire. C’est ainsi qu’au centre de Limeil de la Direction des Applications Militaires du Commissariat à l’Énergie Atomique (CEA-DAM) fut lancée l’étude des plasmas créés par laser, en coopération avec les Laboratoires de Marcoussis de la Compagnie Générale d’Électricité (CGE).
Cet effort, dont l’histoire est relatée dans cet article, a abouti à un résultat spectaculaire pour l’époque (1969) : une émission de neutrons contrôlable
Controlled thermonuclear fusion
The book is a presentation of the basic principles and main achievements in the field of nuclear fusion. It encompasses both magnetic and inertial confinements plus a few exotic mechanisms for nuclear fusion. The state-of-the-art regarding thermonuclear reactions, hot plasmas, tokamaks, laser-driven compression and future reactors is given
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Semi-analytical 6D model of space charge force for dense electron bunches with a large energy spread
Laser driven accelerators are capable of producing multi nC, multi MeV electron beams with transverse and longitudinal sizes on the order of microns. To investigate the transport of such electron bunches, a fast and fully relativistic space charge code which can handle beams with arbitrarily large energy spread has been developed. A 6-D macroparticle model for the beam is used to calculate the space charge fields at each time step. The collection of macroparticles is divided into longitudinal momentum bins, each with a small spread in relative momentum. The macroparticle distribution in each momentum bin is decomposed into ellipsoidal shells in position space. For each shell, an analytical expression for the electrostatic force in the bin rest frame is used. The total space charge force acting on one macroparticle in the lab frame is then the vector sum of the Lorentz-transformed forces from all the momentum bins. We have used this code to study the evolution of typical beams emerging from the plasma in the two most popular schemes, i.e., the self-modulated laser-wakefield-accelerator, where the laser pulse size is many times the plasma wavelength (L >> lr), and the colliding pulse laser-wakefield-accelerator regime where L ~; lr and two counter propagating laser pulses are used to inject electrons into the wakefield
Recommended from our members
Semi-analytical 6D model of space charge force for dense electron bunches with a large energy spread
Laser driven accelerators are capable of producing multi nC, multi MeV electron beams with transverse and longitudinal sizes on the order of microns. To investigate the transport of such electron bunches, a fast and fully relativistic space charge code which can handle beams with arbitrarily large energy spread has been developed. A 6-D macroparticle model for the beam is used to calculate the space charge fields at each time step. The collection of macroparticles is divided into longitudinal momentum bins, each with a small spread in relative momentum. The macroparticle distribution in each momentum bin is decomposed into ellipsoidal shells in position space. For each shell, an analytical expression for the electrostatic force in the bin rest frame is used. The total space charge force acting on one macroparticle in the lab frame is then the vector sum of the Lorentz-transformed forces from all the momentum bins. We have used this code to study the evolution of typical beams emerging from the plasma in the two most popular schemes, i.e., the self-modulated laser-wakefield-accelerator, where the laser pulse size is many times the plasma wavelength (L >> lr), and the colliding pulse laser-wakefield-accelerator regime where L-lr and two counter propagating laser pulses are used to inject electrons into the wakefield
A French prospective pilot study for identifying dihydropyrimidine dehydrogenase (DPD) deficiency in breast cancer patients (pts) receiving capecitabine (cap)
International audienceBackground: For fluoropyrimidines, and especially cap, Health Authorities point out that DPD deficiency confers a significant risk of major toxicity (tox). Identification of at-risk pts is thus relevant. This multicentric prospective study of the French GPCO group (Groupe de Pharmacologie Clinique Oncologique, Unicancer) evaluated the sensitivity, specificity and predictive values of DPD phenotyping and genotyping for predicting severe cap-related tox in metastatic breast cancer pts. Methods: 303 pts were included (15 institutions), 88% received cap as monotherapy, 28% were treated as first line (mean dose at 1st cycle 1957 mg/m2/d). Pre-treatment dihydrouracil (UH2) and uracil (U) plasma concentrations were measured in 286 pts (HPLC assay). DPD genotyping (IVS14+1G>A, 2846A>T, 1679T>G, 464T>A) was done on 281 pts. Severe tox (G3-4 CTCAE v3 criteria) was measured over cycles 1-2. Results: Grade 3-4 tox (diarrhea, vomiting, hematoxicity, hand-foot syndrome) was observed in 19.6% of pts (one toxic death). A marked trend for higher U (median 12.7 vs 10.2 ng/ml, p=0.014) and UH2 (median 110 vs 93 ng/ml, p=0.011) concentrations was observed in pts developing severe tox vs those who didn't. However, ROC curves showed that these differences were too small for use as reliable tox predictors. The distribution of UH2/U ratio was similar between pts with or without tox (median 9.1 vs 9.6, respectively, p=0.80). The patient with toxic death had a UH2/U ratio of 6.5 and U concentration of 17 ng/ml. Among the 7 pts with a DPD mutation (3 pts IVS14+1, 3 pts 2846A>T, one 1679T>G, all heterozygous), 5 developed severe tox (including toxic death, 2846A>T), one did not, and the last one was not documented. Relative risk for developing severe tox was 4.60 in mut pts vs wt pts (95%CI 2.95-7.16, p=0.001); positive and negative predictive values were 83.3% and 81.9%, respectively; specificity was 99.5% and sensitivity was 9.8%. Conclusions: These data point out that breast cancer pts harbouring a DPD variant allele are candidate to develop severe, up to lethal, cap-related tox. In contrast, pre-treatment UH2/U ratio and U measurements are not reliable predictors of cap tox. Clinical trial information: Eudract 2008-004136-20