1,497 research outputs found
A Self-Organized-Criticality model consistent with statistical properties of edge turbulence in a fusion plasma
The statistical properties of the intermittent signal generated by a recent
model for self-organized-criticality (SOC) are examined. A successful
comparison is made with previously published results of the equivalent
quantities measured in the electrostatic turbulence at the edge of a fusion
plasma. This result re-establishes SOC as a potential paradigm for transport in
magnetic fusion devices, overriding shortcomings pointed out in earlier works
[E. Spada, et al, Phys. Rev. Lett. 86, 3032 (2001); V. Antoni, et al, Phys.
Rev. Lett. 87, 045001 (2001)].Comment: 4 pages, 4 figure
Accurate magnetic sensor system integrated design
Inductive measurement of magnetic fields is a diagnostic technique widely used in several scientific fields, such as magnetically confined fusion, plasma thrusters and particle accelerators, where real time control and detailed characterization of physics phenomena are required. The accuracy of the measured data strongly influences the machine controllability and the scientific results. In the framework of the assembly modifications of the RFX-mod experiment, a complete renew and improvement of the magnetic diagnostic system, from the probes moved inside the vacuum vessel to the integrator modules, has been carried out. In this paper, the whole system making up the magnetic diagnostics is described, following the acquisition chain from the probe to the streamed data and illustrating the requirements and conflicting limitations which affect the different components, in order to provide a comprehensive overview useful for an integrated design of any new systems. The characterization of a prototypical implementation of the whole acquisition chain is presented, focusing on the flexible ADC architecture adopted for providing a purely numerical signal integration, highlighting the advantages that this technology offers in terms of flexibility, compactness and cost effectiveness, along with the limitations found in existing implementation in terms of ADC noise characteristics and their possible solutions
Designing high efficiency glow discharge cleaning systems
In this paper we present our studies about the choices of anode design and operational regime in order to get high efficiency glow discharge cleaning of the first wall of a fusion device. We analyzed a database of toroidal and poloidal profiles of the ion current density at the wall, measured by electrostatic probes embedded in RFX-mod first wall tiles, taken in different configurations. The ion current at the wall, both global and local, is in fact strictly connected to the cleaning efficiency, since during glow discharge the wall is physically sputtered by the ions. We found that small size anodes and high in-vessel pressure lead to the peaking of the current profile around the anodes locations, and we experimentally characterized this effect. Instead, we found that anode radial position in the poloidal section has negligible effect on current density profile, even when the anodes are placed at the first wall. Finally, the most convenient operational regime, in terms of pressure and current, has been proposed. Keywords: RFX-mod, Glow discharge, Wall cleaning, Ion current profile, Anode desig
Numerical and experimental investigations of a microwave interferometer for the negative ion source SPIDER
The electron density close to the extraction grids and the co-extracted
electrons represent a crucial issue when operating negative ion sources for
fusion reactors. An excessive electron density in the plasma expansion region
can indeed inhibit the negative ion production and introduce potentially
harmful electrons in the accelerator. Among the set of plasma and beam
diagnostics proposed for SPIDER upgrade, a heterodyne microwave (mw)
interferometer at 100 GHz is being explored as a possibility to measure
electron density in the plasma extraction region. The major issue in applying
this technique in SPIDER is the poor accessibility of the probing microwave
beam through the source metal walls and the long distance of 4 m at which mw
modules should be located outside the vacuum vessel. Numerical investigations
in a full-scale geometry showed that the power transmitted through the plasma
source apertures was sufficient for the microwave module sensitivity. An
experimental proof-of-principle of the setup was then performed. The microwave
system was tested on an experimental full-scale test-bench mimicking SPIDER
viewports accessibility constraints, including the presence of a SPIDER-like
plasma. The outcome of first tests revealed that, despite the geometrical
constraints, in certain conditions, the electron density measurements are
possible. The main issue arises from decoupling the one-pass signal from
spurious multipaths generated by mw beam reflections, requiring signal cross
correlation analysis. These preliminary tests demonstrate that despite the 4 m
distance between the mw modules and the presence of metal walls, plasma density
measurement is possible when the 80-mm diameter ports are available. In this
contribution, we discuss the numerical simulations, the preliminary
experimental tests and suggest design upgrades of the interferometric setup to
enhance signal transmission
Statistical features of edge turbulence in RFX-mod from Gas Puffing Imaging
Plasma density fluctuations in the edge plasma of the RFX-mod device are
measured through the Gas Puffing Imaging Diagnostics. Statistical features of
the signal are quantified in terms of the Probability Distribution Function
(PDF), and computed for several kinds of discharges. The PDFs from discharges
without particular control methods are found to be adequately described by a
Gamma function, consistently with the recent results by Graves et al [J.P.
Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand,
pulses with external methods for plasma control feature modified PDFs. A first
empirical analysis suggests that they may be interpolated through a linear
combination of simple functions. An inspection of the literature shows that
this kind of PDFs is common to other devices as well, and has been suggested to
be due to the simultaneous presence of different mechanisms driving
respectively coherent bursts and gaussian background turbulence. An attempt is
made to relate differences in the PDFs to plasma conditions such as the local
shift of the plasma column. A simple phenomenological model to interpret the
nature of the PDF and assign a meaning to its parameters is also developed.Comment: 27 pages. Published in PPC
About the parabolic relation existing between the skewness and the kurtosis in time series of experimental data
In this work we investigate the origin of the parabolic relation between
skewness and kurtosis often encountered in the analysis of experimental
time-series. We argue that the numerical values of the coefficients of the
curve may provide informations about the specific physics of the system
studied, whereas the analytical curve per se is a fairly general consequence of
a few constraints expected to hold for most systems.Comment: To appear in Physica Script
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