1,842 research outputs found
Study of Space Charge Limitation of Thermionic Cathodes in Triode Guns
A computer program has been developed to compute the emission of cathodes from the fully space charge limited diode equation. The emission of four different gun arrangements with thermionic cathodes of the type used in electron microscopes is investigated. Microflat cathodes with a small (≤50m in diameter) fixed emitting area show several advantages for high brightness possibilities whereas other types of cathodes for which the emitting area increases with the brightness are usually limited by too great a total emission
Thermionic Emission Studies of Micro-Flat Single Crystal Lanthanum Hexaboride Cathodes
A new type of high brightness thermionic cathode has been developed. The cathode utilises emission from a small flat surface (generally less than 50 μm diameter) prepared parallel to a specific crystal plane on a single crystal of Lanthanum Hexaboride. Emission from the rest of the single crystal is suppressed by the use of a high work function coating of pyrolytic graphite. When used in a conventional triode gun, the maximum total electron emission is controlled by the area of the micro-flat, the work function and the temperature of the emitter. The Wehnelt potential serves a minor role in controlling the divergence of the beam. At certain emitter height settings, the gun produces the maximum axial brightness at zero bias. The field at the surface of the micro-flat is higher than that for pointed emitter s in a conventional configuration and no longer limits the gun brightness. As the emitting region is now parallel to a specific crystallographic surface, the emission anisotropy of LaB6 can now be utilized in developing emitters of optimum brightness. The new sources reduce Wehnelt aperture contamination and offer long lifetimes under favourable vacuum conditions
Full abstraction for fair testing in CCS
In previous work with Pous, we defined a semantics for CCS which may both be
viewed as an innocent presheaf semantics and as a concurrent game semantics. It
is here proved that a behavioural equivalence induced by this semantics on CCS
processes is fully abstract for fair testing equivalence. The proof relies on a
new algebraic notion called playground, which represents the 'rule of the
game'. From any playground, two languages, equipped with labelled transition
systems, are derived, as well as a strong, functional bisimulation between
them.Comment: 15 pages, to appear in CALCO '13. To appear Lecture notes in computer
science (2013
Growth, processing, and optical properties of epitaxial Er_2O_3 on silicon
Erbium-doped materials have been investigated for generating and amplifying light in low-power chip-scale optical networks on silicon, but several effects limit their performance in dense microphotonic applications. Stoichiometric ionic crystals are a potential alternative that achieve an Er^(3+) density 100× greater. We report the growth, processing, material characterization, and optical properties of single-crystal Er_2O_3 epitaxially grown on silicon. A peak Er^(3+) resonant absorption of 364 dB/cm at 1535nm with minimal background loss places a high limit on potential gain. Using high-quality microdisk resonators, we conduct thorough C/L-band radiative efficiency and lifetime measurements and observe strong upconverted luminescence near 550 and 670 nm
Is life a thermal horizon ?
This talk aims at questioning the vanishing of Unruh temperature for an
inertial observer in Minkovski spacetime with finite lifetime, arguing that in
the non eternal case the existence of a causal horizon is not linked to the
non-vanishing of the acceleration. This is illustrated by a previous result,
the diamonds temperature, that adapts the algebraic approach of Unruh effect to
the finite case.Comment: Proceedings of the conference DICE 2006, Piombino september 200
Reasoning algebraically about refinement on TSO architectures
The Total Store Order memory model is widely implemented by modern multicore architectures such as x86, where local buffers are used for optimisation, allowing limited forms of instruction reordering. The presence of buffers and hardware-controlled buffer flushes increases the level of non-determinism from the level specified by a program, complicating the already difficult task of concurrent programming. This paper presents a new notion of refinement for weak memory models, based on the observation that pending writes to a process' local variables may be treated as if the effect of the update has already occurred in shared memory. We develop an interval-based model with algebraic rules for various programming constructs. In this framework, several decomposition rules for our new notion of refinement are developed. We apply our approach to verify the spinlock algorithm from the literature
Dialectica Categories for the Lambek Calculus
We revisit the old work of de Paiva on the models of the Lambek Calculus in
dialectica models making sure that the syntactic details that were sketchy on
the first version got completed and verified. We extend the Lambek Calculus
with a \kappa modality, inspired by Yetter's work, which makes the calculus
commutative. Then we add the of-course modality !, as Girard did, to
re-introduce weakening and contraction for all formulas and get back the full
power of intuitionistic and classical logic. We also present the categorical
semantics, proved sound and complete. Finally we show the traditional
properties of type systems, like subject reduction, the Church-Rosser theorem
and normalization for the calculi of extended modalities, which we did not have
before
Quantum models of classical mechanics: maximum entropy packets
In a previous paper, a project of constructing quantum models of classical
properties has been started. The present paper concludes the project by turning
to classical mechanics. The quantum states that maximize entropy for given
averages and variances of coordinates and momenta are called ME packets. They
generalize the Gaussian wave packets. A non-trivial extension of the
partition-function method of probability calculus to quantum mechanics is
given. Non-commutativity of quantum variables limits its usefulness. Still, the
general form of the state operators of ME packets is obtained with its help.
The diagonal representation of the operators is found. A general way of
calculating averages that can replace the partition function method is
described. Classical mechanics is reinterpreted as a statistical theory.
Classical trajectories are replaced by classical ME packets. Quantum states
approximate classical ones if the product of the coordinate and momentum
variances is much larger than Planck constant. Thus, ME packets with large
variances follow their classical counterparts better than Gaussian wave
packets.Comment: 26 pages, no figure. Introduction and the section on classical limit
are extended, new references added. Definitive version accepted by Found.
Phy
Gopi: compiling linear and static channels in go
PTDC/CCI-COM/32166/2017We identify two important features to enhance the design of communication protocols specified in the pi-calculus, that are linear and static channels, and present a compiler, named GoPi, that maps high level specifications into executable Go programs. Channels declared as linear are deadlock-free, while the scope of static channels, which are bound by a hide declaration, does not enlarge at runtime; this is enforced statically by means of type inference, while specifications do not include annotations. Well-behaved processes are transformed into Go code that supports non-deterministic synchronizations and race-freedom. We sketch two main examples involving protection against message forwarding, and forward secrecy, and discuss the features of the tool, and the generated code. We argue that GoPi can support academic activities involving process algebras and formal models, which range from the analysis and testing of concurrent processes for research purposes to teaching formal languages and concurrent systems.publishersversionpublishe
A Bisognano-Wichmann-like Theorem in a Certain Case of a Non Bifurcate Event Horizon related to an Extreme Reissner-Nordstr\"om Black Hole
Thermal Wightman functions of a massless scalar field are studied within the
framework of a ``near horizon'' static background model of an extremal R-N
black hole. This model is built up by using global Carter-like coordinates over
an infinite set of Bertotti-Robinson submanifolds glued together. The
analytical extendibility beyond the horizon is imposed as constraints on
(thermal) Wightman's functions defined on a Bertotti-Robinson sub manifold. It
turns out that only the Bertotti-Robinson vacuum state, i.e. , satisfies
the above requirement. Furthermore the extension of this state onto the whole
manifold is proved to coincide exactly with the vacuum state in the global
Carter-like coordinates. Hence a theorem similar to Bisognano-Wichmann theorem
for the Minkowski space-time in terms of Wightman functions holds with
vanishing ``Unruh-Rindler temperature''. Furtermore, the Carter-like vacuum
restricted to a Bertotti-Robinson region, resulting a pure state there, has
vanishing entropy despite of the presence of event horizons. Some comments on
the real extreme R-N black hole are given
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