2,491 research outputs found
Resonant Dampers for Parametric Instabilities in Gravitational Wave Detectors
Advanced gravitational wave interferometric detectors will operate at their
design sensitivity with nearly 1MW of laser power stored in the arm cavities.
Such large power may lead to the uncontrolled growth of acoustic modes in the
test masses due to the transfer of optical energy to the mechanical modes of
the arm cavity mirrors. These parametric instabilities have the potential of
significantly compromising the detector performance and control. Here we
present the design of "acoustic mode dampers" that use the piezoelectric effect
to reduce the coupling of optical to mechanical energy. Experimental
measurements carried on an Advanced LIGO-like test mass shown a 10-fold
reduction in the amplitude of several mechanical modes, thus suggesting that
this technique can greatly mitigate the impact of parametric instabilities in
advanced detectors
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A new paradigm for BCI research
A new control paradigm for Brain Computer Interfaces
(BCIs) is proposed. BCIs provide a means of communication direct from the brain to a computer that allows individuals with motor disabilities an additional channel of communication and control of their external environment.
Traditional BCI control paradigms use motor imagery, frequency rhythm modification or the Event Related Potential (ERP) as a means of extracting a control signal.
A new control paradigm for BCIs based on speech imagery is initially proposed. Further to this a unique system for identifying correlations between components of the EEG and target events is proposed and introduced
Containment of Shape Expression Schemas for RDF
We study the problem of containment for shape expression schemas (ShEx) for
RDF graphs. We identify a subclass of ShEx that has a natural graphical
representation in the form of shape graphs and their semantics is captured with
a tractable notion of embedding of an RDF graph in a shape graph. When applied
to pairs of shape graphs, an embedding is a sufficient condition for
containment, and for a practical subclass of deterministic shape graphs, it is
also a necessary one, thus yielding a subclass with tractable containment.
While for general shape graphs a minimal counter-example i.e., an instance
proving non-containment, might be of exponential size, we show that containment
is EXP-hard and in coNEXP. Finally, we show that containment for arbitrary ShEx
is coNEXP-hard and in coTwoNEXP^NP
Basal mechanics of Ice Stream B, west Antarctica: 2. Undrained plastic bed model
Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non-linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes (Ä = âe/ât) on the basal melting rate m_r. Since till strength Ï_b{e} and ice stream velocity U_b{Ï_b} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage Ä = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating m_r{ U_b Ï_b}. This is the âweak tillâ condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress Ï_b < (n + 1)^(â1) Ï_d. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities (âice sheetâ mode) and one with a weak bed and fast ice velocities (âice-streamâ mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming
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