614 research outputs found
Three-dimensional fast electron transport for ignition-scale inertial fusion capsules
Three-dimensional hybrid PIC simulations are presented to study electron
energy transport and deposition in a full-scale fast ignition configuration.
Multi-prong core heating close to ignition is found when a few GA, few PW beam
is injected. Resistive beam filamentation in the corona seeds the 3D current
pattern that penetrates the core. Ohmic heating is important in the low-density
corona, while classical Coulomb deposition heats the core. Here highest energy
densities (few Tbar at 10 keV) are observed at densities above 200 g/cc. Energy
coupling to the core ranges from 20 to 30%; it is enhanced by beam collimation
and decreases when raising the beam particle energy from 1.5 to 5.5 MeV.Comment: 5 pages, 5 figure
Ex-post Implementation with Interdependent Valuations
We consider a social choice setting with multidimensional signals and interdependent valuations. Such frameworks have been recently and increasingly used in order to study multi-object auctions. We obtain concise characterizations of ex-post implementable (not necessarily efficient) social choice functions in terms of affine functions that associate a weight to each agent and to each alternative. These characterizations can greatly reduce the complexity of the search for a constrained efficient (i.e., second best) mechanism in the generic cases where efficient outcomes cannot be implemented.
Mixed Bundling Auctions
We study multi-object auctions where agents have private and additive valuations for heterogeneous objects. We focus on the revenue properties of a class of dominant strategy mechanisms where a weight is assigned to each partition of objects. The weights influence the probability with which partitions are chosen in the mechanism. This class contains efficient auctions, pure bundling auctions, mixed bundling auctions, auctions with reserve prices and auctions with pre-packaged bundles. For any number of objects and bidders, both the pure bundling auction and separate, efficient auctions for the single objects are revenue-inferior to an auction that involves mixed bundling
Mixed Bundling Auctions
We study multi-object auctions where agents have private and additive valuations for heterogeneous objects. We focus on the revenue properties of a class of dominant strategy mechanisms where a weight is assigned to each partition of objects. The weights influence the probability with which partitions are chosen in the mechanism. This class contains efficient auctions, pure bundling auctions, mixed bundling auctions, auctions with reserve prices and auctions with pre-packaged bundles. For any number of objects and bidders, both the pure bundling auction and separate, efficient auctions for the single objects are revenue-inferior to an auction that involves mixed bundling.
An optimal Voting System when Voting is costly
We consider the design of an optimal voting system when voting is costly. For a private values model with two alternatives we show the optimality of a voting system that combines three elements: (i) there is an arbitrarily chosen default decision and non-participation is interpreted as a vote in favor of the default; (ii) voting is sequential; (iii) not all voters are invited to participate in the vote. We show the optimality of such a voting system by first arguing that it is first best, that is, it maximizes welfare when incentive compatibility constraints are ignored, and then showing that individual incentives and social welfare are sufficiently aligned to make the first best system incentive compatible. The analysis in this paper involves some methods that are new to the theory of mechanism design, and it is also a purpose of this paper to explore these new methods.Voting; mechanism design; committees.
Ex-post implementation with interdependent valuations
We consider a social choice setting with multidimensional signals and interdependent valuations. Such frameworks have been recently and increasingly used in order to study multi-object auctions. We obtain concise characterizations of ex-post implementable (not necessarily efficient) social choice functions in terms of affine functions that associate a weight to each agent and to each alternative. These characterizations can greatly reduce the complexity of the search for a constrained efficient (i.e., second best) mechanism in the generic cases where efficient outcomes cannot be implemented
On collisional free-free photon absorption in warm dense matter
The rate of photon absorption in warm dense matter (WDM) induced by free-free
electron-ion collisions is derived from Sommerfeld's cross-section for
non-relativistic bremsstrahlung emission, making use of detailed balance
relations. Warm dense matter is treated as a metal-like state in the
approximation of a uniform degenerate electron gas and a uniform ion
background. Total absorption rates are averaged over the electron Fermi
distribution. A closed expression is obtained for the absorption rate,
depending on temperature, density, and photon energy, that scales with ion
charge Z. It is evaluated numerically for the full parameter space, which
requires different representations of the hypergeometric functions involved.
The results are valid for photon frequencies larger than the plasma frequency
of the medium. They are compared with approximate formulas in various
asymptotic regions
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