581 research outputs found
Spectra of magnetic perturbations triggered by pellets in JET plasmas
Aiming at investigating edge localised mode (ELM) pacing for future application on ITER, experiments have been conducted on JET injecting pellets in different plasma configurations, including high confinement regimes with type-I and type-III ELMs, low confinement regimes and Ohmically heated plasmas. The magnetic perturbations spectra and the toroidal mode number, n, of triggered events are compared with those of spontaneous ELMs using a wavelet analysis to provide good time resolution of short-lived coherent modes. It is found thatâin all these configurationsâtriggered events have a coherent mode structure, indicating that pellets can trigger an MHD event basically in every background plasma. Two components have been found in the magnetic perturbations induced by pellets, with distinct frequencies and toroidal mode numbers. In high confinement regimes triggered events have similarities with spontaneous ELMs: both are seen to start from low toroidal mode numbers, then the maximum measured n increases up to about 10 within 0.3 ms before the ELM burst
An innovative approach for DEMO core fuelling by inboard injection of high-speed pellets
Core fuelling of DEMO tokamak fusion reactor is under investigation within the EUROfusion Work Package âTritium, Fuelling and Vacuumâ. An extensive analysis of fuelling requirements and technologies, suggests that pellet injection still represents, to date, the most realistic option. Modelling of both pellet penetration and fuel deposition profiles for different injection locations, assuming a specific plasma reference scenario and the ITER reference pellet mass (6 Ă 1021 atoms), indicates that: 1) Low Field Side (LFS) injection is inadequate; 2) Vertical injection may be effective only provided that pellets are injected at ⌠10 km/s from a radial position â€âŒ8 m; 3) effective core fuelling can be achieved launching pellets from the High Field Side (HFS) at âŒ1 km/s. HFS injection was therefore selected as the reference scheme, though scenarios featuring less steep density and temperature gradients at the plasma edge could induce to reconsider vertical injection at speeds in the range of 4â5 km/s. To deliver intact pellets at 1 km/s from the HFS, the use of guide tubes with a bend radius â„6 m is envisaged. The results of above simulations rely on the hypothesis that pellets are delivered at the plasma edge with the desired mass and speed. However, mass erosion and fracturing of pellets inside the guide tube (severely limiting the transfer speed), as well as pressure build up and speed losses at relevant injection rates, might hamper the use of curved guide tubes. An additional innovative approach, aimed at identifying inboard straight âfree flightâ injection paths, to inject pellets from the HFS at significantly higher speeds, is proposed and discussed as a backup solution. Outboard high-speed injection is still being considered, instead, for JT-60SA
Wave Propagation in Gravitational Systems: Late Time Behavior
It is well-known that the dominant late time behavior of waves propagating on
a Schwarzschild spacetime is a power-law tail; tails for other spacetimes have
also been studied. This paper presents a systematic treatment of the tail
phenomenon for a broad class of models via a Green's function formalism and
establishes the following. (i) The tail is governed by a cut of the frequency
Green's function along the ~Im~ axis,
generalizing the Schwarzschild result. (ii) The dependence of the cut
is determined by the asymptotic but not the local structure of space. In
particular it is independent of the presence of a horizon, and has the same
form for the case of a star as well. (iii) Depending on the spatial
asymptotics, the late time decay is not necessarily a power law in time. The
Schwarzschild case with a power-law tail is exceptional among the class of the
potentials having a logarithmic spatial dependence. (iv) Both the amplitude and
the time dependence of the tail for a broad class of models are obtained
analytically. (v) The analytical results are in perfect agreement with
numerical calculations
The global build-up to intrinsic ELM bursts and comparison with pellet triggered ELMs seen in JET
We focus on JET plasmas in which ELMs are triggered by pellets in the presence of ELMs
which occur naturally. We perform direct time domain analysis of signals from fast radial
field coils and toroidal full flux azimuthal loops. These toroidally integrating signals provide
simultaneous high time resolution measurements of global plasma dynamics and its coupling
to the control system. We examine the time dynamics of these signals in plasmas where pellet
injection is used to trigger ELMs in the presence of naturally occurring ELMs. Pellets whose
size and speed are intended to provide maximum local perturbation for ELM triggering are
launched at pre-programmed times, without correlation to the occurrence times of intrinsic
ELMs. Pellet rates were sufficiently low to prevent sustained changes of the underlying
plasma conditions and natural ELM behaviour. We find a global signature of the build-up to
natural ELMs in the temporal analytic phase of both the full flux loops and fast radial field
coil signals. Before a natural ELM, the signal phases align to the same value on a âŒ2â5ms
timescale. This global build up to a natural ELM occurs whilst the amplitude of the full flux
loop and fast radial field coil signals are at their background value: it precedes the response
seen in these signals to the onset of ELMing. In contrast these signals do not clearly phase
align before the ELM for ELMs which are the first to occur following pellet injection. This
provides a direct test that can distinguish when an ELM is triggered by a pellet as opposed to
occurring naturally. It further supports the idea [1â4] of a global build up phase that precedes
natural ELMs; pellets can trigger ELMs even when the signal phase is at a value when a
natural ELM is unlikely to occurEURATOM 633053ONR NICOP N62909-15-1-N14
Comparing spontaneous and pellet-triggered ELMs via non-linear extended MHD simulations
Injecting frozen deuterium pellets into an ELMy H-mode plasma is a well established scheme for triggering edge localized modes (ELMs) before they naturally occur. This paper presents non-linear simulations of spontaneous type-I ELMs and pellet-triggered ELMs in ASDEX Upgrade performed with the extended MHD code JOREK. A thorough comparison of the non-linear dynamics of these events is provided. In particular, pellet-triggered ELMs are simulated by injecting deuterium pellets into different time points during the pedestal build-up described in A Cathey et al (2020 Nuclear Fusion 60 124007). Realistic ExB and diamagnetic background plasma flows as well as the time dependent bootstrap current evolution are included during the build-up to accurately capture the balance between stabilising and destabilising terms for the edge instabilities. Dependencies on the pellet size and injection times are studied. The spatio-temporal structures of the modes and the resulting divertor heat fluxes are compared in detail between spontaneous and triggered ELMs. We observe that the premature excitation of ELMs by means of pellet injection is caused by a helical perturbation described by a toroidal mode number of n=1. In accordance with experimental observations, the pellet-triggered ELMs show reduced thermal energy losses and a narrower divertor wetted area with respect to spontaneous ELMs. The peak divertor energy fluence is seen to decrease when ELMs are triggered by pellets injected earlier during the pedestal build-up.</p
An explicit height bound for the classical modular polynomial
For a prime m, let Phi_m be the classical modular polynomial, and let
h(Phi_m) denote its logarithmic height. By specializing a theorem of Cohen, we
prove that h(Phi_m) <= 6 m log m + 16 m + 14 sqrt m log m. As a corollary, we
find that h(Phi_m) <= 6 m log m + 18 m also holds. A table of h(Phi_m) values
is provided for m <= 3607.Comment: Minor correction to the constants in Theorem 1 and Corollary 9. To
appear in the Ramanujan Journal. 17 pages
Exactly solvable path integral for open cavities in terms of quasinormal modes
We evaluate the finite-temperature Euclidean phase-space path integral for
the generating functional of a scalar field inside a leaky cavity. Provided the
source is confined to the cavity, one can first of all integrate out the fields
on the outside to obtain an effective action for the cavity alone.
Subsequently, one uses an expansion of the cavity field in terms of its
quasinormal modes (QNMs)-the exact, exponentially damped eigenstates of the
classical evolution operator, which previously have been shown to be complete
for a large class of models. Dissipation causes the effective cavity action to
be nondiagonal in the QNM basis. The inversion of this action matrix inherent
in the Gaussian path integral to obtain the generating functional is therefore
nontrivial, but can be accomplished by invoking a novel QNM sum rule. The
results are consistent with those obtained previously using canonical
quantization.Comment: REVTeX, 26 pages, submitted to Phys. Rev.
A high resolution imaging detector for TeV gamma-ray astronomy
Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques
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