We consider a random walk
S
n
,
n
≥
0
with zero drift and finite variance
σ
2
. Let
T
be the first hitting time of the semi-axis
-
∞
,
0
by this random walk. For the random process
S
n
t
/
σ
n
,
t
∈
...0
,
1
, considered under the condition that
T
=
n
, a functional limit theorem on its convergence to the Brownian excursion, as
n
→
∞
, is proved.
An integer-valued random walk
with zero drift and finite variance
stopped at the time
of the first hit of the semiaxis
is considered. For the random process defined for a variable
as the number of ...visits of this walk to the state
and conditioned on the event
, a functional limit theorem on its convergence to the local time of the Brownian high jump is proved.
A neutron collimator is developed to attenuate the neutron flux and reduce the residual induced activity in the interportal space of the diagnostic system of neutral particle analyzers of the ITER ...tokamak reactor. The collimator is installed in the port plug of the ITER vacuum vessel in front of an inlet to the vacuum pipeline of the diagnostic system. The collimator design has a cellular structure with 80% transparency for the neutral atom beam that goes out of the plasma and is recorded by the analyzers. However, because of increased scattering of neutrons in the collimator, their flux in the interportal room in the service zone of diagnostic systems of equatorial port no. 11 is significantly reduced and allows the equivalent dose rate in this zone to be decreased by several times. Thermal analysis showed that, during the reactor operation in modes with generation of the maximum power of 500 MW, the plasma radiation will cause the heating of the collimating grid to a temperature not exceeding 250°C, which makes it possible to select the stainless steel (316L(N)-ITER grade) as a material for manufacturing the collimator. In this case the cyclic strength of the collimator meets the ITER requirements, and it can be used without replacement during the entire deuterium-tritium experiment of the tokamak reactor.
A Galton–Watson branching process with immigration evolving in a random environment is considered. Its associated random walk is assumed to be oscillating. We prove a functional limit theorem in ...which the process under consideration is normalized by a random coefficient depending on the random environment only. The distribution of the limiting process is described in terms of a strictly stable Levy process and a sequence of independent and identically distributed random variables which is independent of this process.
A sample made from a ribbon of austenitic-martensitic steel 0.3C–13Cr–10Mn–3Si–1V, pre-deformed in the shape of a circular arc with a deflection of 3 mm, was subjected to fast neutron irradiation ...with a fluence of 6 × 10
19
cm
–2
in the vertical wet channel of the IVV-2M reactor at a temperature of 80°C. This material belongs to the class of stainless manganese austenitic steels with the shape memory effect (SME). Initially, it was expected that irradiation would reduce the SME magnitude after subsequent heating of the studied sample in comparison with the reference one. But instead, the manifestation of SME was revealed already after irradiation, with a decrease in the deflection by approximately 21%. Control experiments confirmed that the lower temperature limit for the manifestation of the SME in this material lies at 120ºC and is absent at 80°C. This allows us to conclude that the effect observed is the result of neutron irradiation.
The control of the deuterium–tritium (DT) fuel isotopic ratio has to ensure the best performance of the ITER thermonuclear fusion reactor. The diagnostic system described in this paper allows the ...measurement of this ratio analyzing the hydrogen isotope fluxes (performing neutral particle analysis (NPA)). The development and supply of the NPA diagnostics for ITER was delegated to the Russian Federation. The diagnostics is being developed at the Ioffe Institute. The system consists of two analyzers, viz., LENPA (Low Energy Neutral Particle Analyzer) with 10–200 keV energy range and HENPA (High Energy Neutral Particle Analyzer) with 0.1–4.0MeV energy range. Simultaneous operation of both analyzers in different energy ranges enables researchers to measure the DT fuel ratio both in the central burning plasma (thermonuclear burn zone) and at the edge as well. When developing the diagnostic complex, it was necessary to account for the impact of several factors: high levels of neutron and gamma radiation, the direct vacuum connection to the ITER vessel, implying high tritium containment, strict requirements on reliability of all units and mechanisms, and the limited space available for accommodation of the diagnostic hardware at the ITER tokamak. The paper describes the design of the diagnostic complex and the engineering solutions that make it possible to conduct measurements under tokamak reactor conditions. The proposed engineering solutions provide a safe—with respect to thermal and mechanical loads—common vacuum channel for hydrogen isotope atoms to pass to the analyzers; ensure efficient shielding of the analyzers from the ITER stray magnetic field (up to 1 kG); provide the remote control of the NPA diagnostic complex, in particular, connection/disconnection of the NPA vacuum beamline from the ITER vessel; meet the ITER radiation safety requirements; and ensure measurements of the fuel isotopic ratio under high levels of neutron and gamma radiation.
Abstract
This article presents the results of a study of the formation of a short rise time of a powerful nanosecond 410 kV pulse. Ferrite filled coaxial transmission lines with standard inner ...conductor and construction in the form of a spiral are designed. The sharpening of the pulse occurs due to the appearance of an electromagnetic shock wave in the ferrite. The value of the rise time at the level of 0.1 - 0.9 has been decreased from 4.5 to 2.5 ns.
With the regular appearance of new multi-core processors, it is extremely important to detect strengths and weaknesses of developed hardware solutions. Every new prominent computational platform has ...a potential to outperform existing ones on a subset of application classes. This is also a crucial challenge for ARM processors, because of their relatively novelty in High-Performance Computing (HPC) area and lack of detailed performance evaluation. This paper covers the methodology of creating a full–fledged hierarchical benchmarking set. Such completeness is achieved by using a hierarchical set of benchmarking kernels and algorithms, which are classified according to the roofline model. This proposed approach allows us to estimate the performance of modern multi-core processors through relatively simple but frequently used fragments of code. All these fragments are aimed to stress a particular memory subsystem component or computing units of target processor, which allows to easily map the on the roofline classification. Among other ARM solutions for HPC, Huawei company developed its own family of 64-bit ARM multi-core server SoCs called Kunpeng. To evaluate Huawei Kunpeng 920 processor performance, we launched our developed benchmarking set on other HPC platforms: Intel Xeon 6140 and Fujitsu A64FX. The evaluation results allowed us to identify multiple strengths and weaknesses of Kunpeng 920 processors, as well as to demonstrate their relevance to some of the real-world algorithms and applications.
The applicability of neutral particle diagnostics for studying plasma parameters and additional methods of heating in the Tokamak with Reactor Technologies is analyzed. Options for the arrangement of ...diagnostic equipment, which includes three analyzers of charge-exchange atoms (neutral particle analyzers) for different energy ranges of recorded atomic fluxes, are given. It is shown that the diagnostic complex of analyzers makes it possible to measure the distribution functions of plasma thermal ions and to obtain information on the isotopic ratio of the main plasma ion component in the deuterium–tritium mode of facility operation. In addition, the proposed geometry of the analyzer arrangement makes it possible to measure the energy distribution of fast ions, which arise during neutral beam injection and ion cyclotron heating of plasma.
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