Letteratura scientifica selezionata sul tema "Runaway companion plasma"

Abstract Runaway electrons (REs) created during tokamak disruptions pose a threat to the reliable operation of future larger machines. Experiments using shattered pellet injection (SPI) have been carried out at the JET tokamak to investigate ways to prevent their generation or suppress them if avoidance is not sufficient. Avoidance is possible if the SPI contains a sufficiently low fraction of high-Z material, or if it is fired early in advance of a disruption prone to runaway generation. These results are consistent with previous similar findings obtained with Massive Gas Injection. Suppression of an already accelerated beam is not efficient using High-Z material, but deuterium leads to harmless terminations without heat loads. This effect is due to the combination of a large magnetohydrodynamic instability scattering REs on a large area and the absence of runaway regeneration during the subsequent current collapse thanks to the flushing of high-Z impurities from the runaway companion plasma. This effect also works in situations where the runaway beam moves upwards and undergoes scraping-off on the wall.

Abstract Roche lobe overflow from a donor star onto a black hole or neutron star binary companion can evolve to a phase of unstable runaway mass transfer, lasting as short as hundreds of orbits (≲102 yr for a giant donor) and eventually culminating in a common-envelope event. The highly super-Eddington accretion rates achieved during this brief phase ( M ̇ ≳ 10 5 M ̇ Edd ) are accompanied by intense mass loss in disk winds, analogous to but even more extreme than ultraluminous X-ray (ULX) sources in the nearby universe. Also in analogy with the observed ULX, this expanding outflow will inflate an energetic “bubble” of plasma into the circumbinary medium. Embedded within this bubble is a nebula of relativistic electrons heated at the termination shock of the faster v ≳ 0.1c wind/jet from the inner accretion flow. We present a time-dependent, one-zone model for the synchrotron radio emission and other observable properties of such ULX “hypernebulae.” If ULX jets are sources of repeating fast radio bursts (FRB), as recently proposed, such hypernebulae could generate persistent radio emission and contribute large and time-variable rotation measure to the bursts, consistent with those seen from FRB 20121102 and FRB 20190520B. ULX hypernebulae can be discovered independently of an FRB association in radio surveys, such as VLASS, as off-nuclear point sources whose fluxes can evolve significantly on timescales as short as years, possibly presaging energetic transients from common-envelope mergers.

Abstract A simple 0-D model which mimics the plasma surrounded by the conducting structures [D.I. Kiramov, B.N. Breizman, Physics of Plasmas 24, 100702 (2017)] and including self-consistently the vertical plasma motion and the generation of runaway electrons during the disruption is used for an assessment of the effect of vertical displacement events on the runaway current formation and termination. The total plasma current and runaway current at the time the plasma hits the wall is estimated and the effect of injecting impurities into the plasma is evaluated. In the case of ITER, with a highly conducting wall, although the total plasma current when the plasma touches the wall is the same for any number of injected impurities, however the fraction of the plasma current carried by runaway electrons can significantly decrease for large enough amounts of impurities. The plasma velocity is larger and the time when the plasma hits the wall shorter for lower runaway currents, which are obtained when larger amounts of impurities are injected. When the plasma reaches the wall, the scraping-off of the runaway beam occurs and the current is terminated. During this phase, the plasma vertical displacement velocity and electric field can substantially increase leading to the deposition of a noticeable amount of energy on the runaway electrons (~ hundreds of MJ). It is found that an early second impurity injection reduces somewhat the amount of energy deposited by the runaways. Also larger temperatures of the companion plasma during the scraping-off might be efficient in reducing the power fluxes due to the runaways onto the PFCs. The plasma reaches the qa = 2 limit before the runaway electron current is terminated and by that time the amount of energy deposited on the runaway electrons can be substantially lower than that expected until the beam is fully terminated. Negligible additional conversion of magnetic into runaway kinetic energy is predicted during the runaway deconfinement following the large magnetic fluctuations after qa = 2 is crossed for characteristic deconfinement times lower than 0.1 ms which is a characteristic timescale for ideal MHD instabilities to develop.

Abstract This paper discusses the development of a benign termination scenario for runaway electron (RE) beams on ASDEX Upgrade and TCV. A systematic study revealed that a low electron density (ne) companion plasma was required to achieve a large MHD instability, which expelled the confined REs over a large wetted area and allowed for the conversion of magnetic energy to radiation. Control of the companion plasma ne was achieved via neutral pressure regulation and was agnostic to material injection method. The neutral pressure required for recombination was found to be dependent on impurity species, quantity and RE current. On TCV, ne increased at neutral pressures above 1 Pa, indicating that higher collisionality between the REs and neutrals may lead to an upper pressure limit. The conversion of magnetic energy to radiated energy was measured on both machines and a decrease in efficiency was observed at high neutral pressure on TCV. The benign termination technique was able to prevent any significant increase in maximum heat flux on AUG from 200 to 600 kA of RE current, highlighting the ability of this approach to handle fully formed RE beams.

Les dispositifs de fusion de type tokamaks ont atteint des performances proches de celles nécessaires à réacteur industriel de fusion et les disruptions sont des événements majeurs dans lesquels l'énergie du plasma est perdue en un très court instant. Electrons découplés (RE), de par leur énergie (quelques 10ème\,MeV), peuvent endommager des composants internes du tokamak. La stratégie actuelle consiste à éviter la génération de RE à l’aide d’une injection massive de matière (MMI). Si leur génération ne peut pas être évitée, une 2ème MMI sera utilisée pour atténuer le faisceau d’électrons découplés. Après la 1ère MMI, un plasma de fond dense et froid d'impuretés MMI est formé et le 2ème MMI visant à atténuer l'emballement du faisceau d'électrons peut être inefficace dans ce plasma de fond, comme observé dans le tokamak JET. Comprendre la physique de l'interaction entre le faisceau d'électrons découplés et le 2ème MMI en présence du plasma de fond froid sera au centre de cette thèse
Tokamaks are the devices currently closest to achieve nuclear fusion power and disruptions are unfavorable events in which the plasma energy is lost in a very short timescale causing damage to tokamak structures. RE beams are one of the consequence of disruptions and they carry the risk of in-vessel component damage. Thus, the prevention and control of the RE are of prime importance. The current strategy for runaway electrons is to avoid their generation by a massive material injection (MMI). If their generation cannot be avoided, a 2nd MMI will be used to mitigate the generated RE beam. After the 1st MMI to prevent RE generation, a background plasma of 1st MMI impurities is formed which make the second MMI inefficient to mitigate RE beams inefficient, as observed in the JET tokamak. In this thesis, the physics of the interaction between the RE beam and the mitigation MMI in the presence of a cold background plasma is studied