2,524 research outputs found
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
Three Dimensional Annihilation Imaging of Antiprotons in a Penning Trap
We demonstrate three-dimensional annihilation imaging of antiprotons trapped
in a Penning trap. Exploiting unusual feature of antiparticles, we investigate
a previously unexplored regime in particle transport; the proximity of the trap
wall. Particle loss on the wall, the final step of radial transport, is
observed to be highly non-uniform, both radially and azimuthally. These
observations have considerable implications for the production and detection of
antihydrogen atoms.Comment: Invited Talk at NNP03, Workshop on Non-Neutral Plasmas, 200
Detection of antihydrogen annihilations with a Si-micro-strip and pure CsI detector
In 2002, the ATHENA collaboration reported the creation and detection of cold
(~15 K) antihydrogen atoms [1]. The observation was based on the complete
reconstruction of antihydrogen annihilations, simultaneous and spatially
correlated annihilations of an antiproton and a positron. Annihilation
byproducts are measured with a cylindrically symmetric detector system
consisting of two layers of double sided Si-micro-strip modules that are
surrounded by 16 rows of 12 pure CsI crystals (13 x 17.5 x 17 mm^3). This paper
gives a brief overview of the experiment, the detector system, and event
reconstruction.
Reference 1. M. Amoretti et al., Nature 419, 456 (2002).Comment: 7 pages, 5 figures; Proceedings for the 8th ICATPP Conference on
Astroparticle, Particle, Space Physics, Detectors and Medical Physics
Applications (Como, Italy October 2003) to be published by World Scientific
(style file included
Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum
We present evidence showing how antiprotonic hydrogen, the quasistable
antiproton-proton (pbar-p) bound system, has been synthesized following the
interaction of antiprotons with the hydrogen molecular ion (H2+) in a nested
Penning trap environment. From a careful analysis of the spatial distributions
of antiproton annihilation events, evidence is presented for antiprotonic
hydrogen production with sub-eV kinetic energies in states around n=70, and
with low angular momenta. The slow antiprotonic hydrogen may be studied using
laser spectroscopic techniques.Comment: 5 pages with 4 figures. Published as Phys. Rev. Letters 97, 153401
(2006), in slightly different for
The First Cold Antihydrogen
Antihydrogen, the atomic bound state of an antiproton and a positron, was
produced at low energy for the first time by the ATHENA experiment, marking an
important first step for precision studies of atomic antimatter. This paper
describes the first production and some subsequent developments.Comment: Invitated Talk at COOL03, International Workshop on Beam Cooling and
Related Topics, to be published in NIM
ATHENA -- First Production of Cold Antihydrogen and Beyond
Atomic systems of antiparticles are the laboratories of choice for tests of
CPT symmetry with antimatter. The ATHENA experiment was the first to report the
production of copious amounts of cold antihydrogen in 2002. This article
reviews some of the insights that have since been gained concerning the
antihydrogen production process as well as the external and internal properties
of the produced anti-atoms. Furthermore, the implications of those results on
future prospects of symmetry tests with antimatter are discussed.Comment: Proc. of the Third Meeting on CPT and Lorentz Symmetry, Bloomington
(Indiana), USA, August 2004, edited by V. A. Kostelecky (World Scientific,
Singapore). 10 pages, 5 figures, 1 table. Author affiliations cor
Cold-Antimatter Physics
The CPT theorem and the Weak Equivalence Principle are foundational
principles on which the standard description of the fundamental interactions is
based. The validity of such basic principles should be tested using the largest
possible sample of physical systems. Cold neutral antimatter (low-energy
antihydrogen atoms) could be a tool for testing the CPT symmetry with high
precision and for a direct measurement of the gravitational acceleration of
antimatter. After several years of experimental efforts, the production of
low-energy antihydrogen through the recombination of antiprotons and positrons
is a well-established experimental reality. An overview of the ATHENA
experiment at CERN will be given and the main experimental results on
antihydrogen formation will be reviewed.Comment: Proceedings of the XLIII International Meeting on Nuclear Physics,
Bormio (Italy), March 13-20 (2005). 10 pages, 4 figures, 1 tabl
Análise de Alternativas de Integração do Processador LEON3 em Sistemas Embarcados
O processador LEON3 consiste de um softcore originalmente desenvolvido pela Agência Espacial Europeia (ESA) para uso em aplicações espaciais. Atualmente faz parte da biblioteca de IPs GRLIB mantida pela empresa Cobham Gaisler. Além dos IPs disponíveis nessa biblioteca, é possibilitada a incorporação de componentes customizados. A integração entre os componentes no sistema é realizada com o uso do barramento AMBA 2.0, desenvolvido pela ARM, utilizando o protocolo AHB para os núcleos de alto desempenho e o protocolo APB para periféricos. No entanto, o barramento não atende aos requisitos de comunicação dos futuros sistemas integrados, sendo consenso da literatura a adoção de estruturas de maior escalabilidade, como as redes-em-chip. Este artigo descreve os estudos feitos sobre a comunicação do processador LEON3, analisa a implementação de diferentes arquiteturas e as adaptações necessárias para a integração em uma rede-em-chip
Production of antihydrogen at reduced magnetic field for anti-atom trapping
We have demonstrated production of antihydrogen in a 1T solenoidal
magnetic field. This field strength is significantly smaller than that used in
the first generation experiments ATHENA (3T) and ATRAP (5T). The
motivation for using a smaller magnetic field is to facilitate trapping of
antihydrogen atoms in a neutral atom trap surrounding the production region. We
report the results of measurements with the ALPHA (Antihydrogen Laser PHysics
Apparatus) device, which can capture and cool antiprotons at 3T, and then
mix the antiprotons with positrons at 1T. We infer antihydrogen production
from the time structure of antiproton annihilations during mixing, using mixing
with heated positrons as the null experiment, as demonstrated in ATHENA.
Implications for antihydrogen trapping are discussed
- …