2,233 research outputs found
Poesie di Pietro Buratti.
No full text"Della vita e delle opere di Pietro Buratti, veneziano" ; lxxvii p. at end of v. 3.Mode of access: Internet
Ion cyclotron resonance heating for tungsten control in various JET H-mode scenarios
No full textIon cyclotron resonance heating (ICRH) in the hydrogen minority scheme provides central ion heating and acts favorably on the core tungsten transport. Full wave modeling shows that, at medium power level (4 MW), after collisional redistribution, the ratio of power transferred to the ions and the electrons vary little with the minority (hydrogen) concentration n H/n e but the high-Z impurity screening provided by the fast ions temperature increases with the concentration. The power radiated by tungsten in the core of the JET discharges has been analyzed on a large database covering the 2013-2014 campaign. In the baseline scenario with moderate plasma current (I p = 2.5 MA) ICRH modifies efficiently tungsten transport to avoid its accumulation in the plasma centre and, when the ICRH power is increased, the tungsten radiation peaking evolves as predicted by the neo-classical theory. At higher current (3-4 MA), tungsten accumulation can be only avoided with 5 MW of ICRH power with high gas injection rate. For discharges in the hybrid scenario, the strong initial peaking of the density leads to strong tungsten accumulation. When this initial density peaking is slightly reduced, with an ICRH power in excess of 4 MW,very low tungsten concentration in the core (∼10-5) is maintained for 3 s. MHD activity plays a key role in tungsten transport and modulation of the tungsten radiation during a sawtooth cycle is correlated to the fishbone activity triggered by the fast ion pressure gradient. © 2017 IOP Publishing Ltd
Some progress on the existence of 1-rotational Steiner Triple Systems
No full textA Steiner Triple System of order v (briefly STS(v)) is 1-rotational under G if it admits G as an automorphism group acting sharply transitively on all but one point.The spectrum of values of v for which there exists a1-rotational STS(v) under a cyclic, an abelian, or a generalized quaternion group, has beenestablished in 1981 (phelps and Rosa), in 2001 (Buratti) and in 2008 (Mishima), respectively.Nevertheless, the spectrum of values of v for which there exists a1-rotational STS(v) under an arbitrary group has not been completely determined yet.This paper is a considerable step forward to the solution of this problem.In fact, we leave as uncertain cases only those for which we have v = (p^3-p)n + 1 = 1 (mod 96)with p a prime, n =1,2,3 mod 4, and the odd part of (p^3-p)n that is square-free and without prime factors congruent to 1 mod 6
On a Problem of Marco Buratti
No full textWe consider a problem formulated by Marco Buratti concerning Hamiltonian paths in the complete graph on Zp, p an odd prime
Magnetic reconnection and impulsive instabilities in tokamak plasmas: Some analogies with astrophysical flares
No full textOpen theoretical issues and their solutions for fusion relevant physical regimes
No full textThe effort to identify the properties of fusion burning plasmas, involving issues of basic plasma physics and new revealing experimental observations, has led to the development of a theoretical basis for three relevant subjects: (i) novel features of the quasi-coherent mode (QCM) associated with the H-confinement regime, (ii) magnetic reconnection processes in low collisionality regimes and the theory of unpredicted macroscopic modes producing magnetic islands with a propagating velocity in the ion diamagnetic velocity direction, (iii) angular momentum transport described by a non-diffusive equation reproducing the spontaneous rotation velocity profiles; the plasma modes that are identified can produce an angular momentum transport contrary to that predicted by a diffusion process and involve significant electron temperature fluctuations. © 2015 IAEA, Vienna
Axisymmetric oscillations at L-H transitions in JET: M-mode
No full textL to H transition studies at JET have revealed an n = 0, m = 1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the magnetic oscillation is present a weak ELM-less H-mode regime is obtained, with a clear increase of density and a weak electron temperature pedestal. It is an intermediate state between L and H-mode. In ICRH heated plasmas or low density NBI plasmas the magnetic mode and the pedestal can remain steady (with small oscillations) for the duration of the heating phase, of order 10 s or more. The axisymmetric magnetic oscillation has period ∼0.5-2 ms, and poloidal mode number m = 1: it looks like a pedestal localised up/down oscillation, although it is clearly a natural oscillation of the plasma, not driven by the position control system. Electron cyclotron emission, interferometry, reflectometry and fast Li beam measurements locate the mode in the pedestal region. Dα, fast infrared camera and Langmuir probe measurements show that the mode modulates heat and particle fluxes to the target. The mode frequency appears to scale with the poloidal Alfvén velocity, and not with sound speed (i.e. it is not a geodesic acoustic mode). A heuristic model is proposed for the frequency scaling of the mode. We discuss the relationship between the M-mode and other related observations near the L-H transition. © 2017 EURATOM
The role of MHD in causing impurity peaking in JET hybrid plasmas
No full textIn hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is found that n > 1 tearing activity can significantly enhance the rate of on-axis peaking of high-Z impurities, which in turn significantly degrades discharge performance. Core n = 1 instabilities can be beneficial in removing impurities from the plasma core (e.g. sawteeth or fishbones), but can conversely also degrade core confinement (particularly in combination with simultaneous n = 3 activity). The nature of magnetohydrodynamic instabilities in JET hybrid discharges, with both its previous carbon wall and subsequent JET-ILW, is surveyed statistically and the character of the instabilities is examined. Possible qualitative models for how the n > 1 islands can enhance the on-axis impurity transport accumulation processes are presented. © 2016 EURATOM
Role of MHD activity triggered by fast ions in tungsten transport in JET hybrid discharges
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