Nonequilibrium electron rings for synchrotron radiation production

Electron storage rings used for the production of synchrotron radiation (SR) have an output photon brightness that is limited by the equilibrium beam emittance. By using interleaved injection and ejection of bunches from a source with repetition rate greater than 1 kHz, we show that it is practicable to overcome this limit in rings of energy ~1 GeV. Sufficiently short kicker pulse lengths enable effective currents of many milliamperes, which can deliver a significant flux of diffraction-limited soft X-ray photons. Thus, either existing SR facilities may be adapted for non-equilibrium operation, or the technique applied to construct SR rings smaller than their storage ring equivalent.Comment: 5 pages, 2 figures; accepted for publication in Physical Review Letter

Fixation Condition Effects on Stop Signal Reaction Times: An Eye-Hand Co-ordinated Human Countermanding Task

The countermanding task requires subjects to cancel an impending movement in the presence of an imperative stop signal. The outcome of a given countermanding trial has been modeled as a race between stochastically independent GO and STOP processes and is largely dependent on the amount of time between target and stop signal presentation (the stop signal delay - SSD). Here, we investigated the effect of fixation condition on the GO and STOP processes using human subjects in eye-only, hand-only and eye-hand co-ordinated countermanding tasks. In Experiment 1, we found that duration of the STOP process, estimated through the derivation of the stop signal reaction time (SSRT), was ~20 ms shorter on trials with a 200 ms gap between fixation point removal and target presentation compared to when the fixation point remained illuminated (an “overlap” condition). Similarly, we found SSRTs were ~10 ms shorter on trials with simultaneous fixation point removal and target presentation compared to the overlap condition. However, in Experiment 2, this priming of movement inhibition due to fixation condition disappeared, when the stop signal delay for each trial was determined dynamically, based on subject performance on previous trials. Overall, we suggest the disappearance of fixation condition effects in Experiment 2 was due to smaller gap effects on reaction times and a greater variance in fixation condition effects on the STOP process in Experiment 2. We believe these differences arose due to a higher percentage of trials in Experiment 2 where the GO and STOP processes approached threshold at approximately the same time. Therefore we postulate that such trials produce a greater amount of conflict between movement generation and inhibition systems, causing both systems to rely less on fixation cues

Volcanism by melt-driven Rayleigh-Taylor instabilities and possible consequences of melting for admittance ratios on Venus

A large number of volcanic features exist on Venus, ranging from tens of thousands of small domes to large shields and coronae. It is difficult to reconcile all these with an explanation involving deep mantle plumes, since a number of separate arguments lead to the conclusion that deep mantle plumes reaching the base of the lithosphere must exceed a certain size. In addition, the fraction of basal heating in Venus' mantle may be significantly lower than in Earth's mantle reducing the number of strong plumes from the core-mantle boundary. In three-dimensional convection simulations with mainly internal heating, weak, distributed upwellings are usually observed. We present an alternative mechanism for such volcanism, originally proposed for the Earth and for Venus, involving Rayleigh-Taylor instabilities driven by melt buoyancy, occurring spontaneously in partially or incipiently molten regions

Big categories, big spectra

We introduce two new topological invariants of a rigidly-compactly generated tensor-triangulated category and study their associated support theories and relation to existing technology. The first, the smashing spectrum, is produced by proving that the frame of smashing ideals is always spatial, and is equipped with a surjective morphism to the Balmer spectrum which detects the failure of the telescope conjecture. The second, the big spectrum, results from taking the entire collection of localizing ideals seriously and considering prime localizing ideals. Although there are, in principle, a proper class of localizing ideals, we are able to prove the existence of at least one big prime lying over every Balmer prime. We conclude with a pair of examples illustrating our constructions.Comment: Some new results and remarks added and removed some erroneous material (the old section 3.5) on quasi-compact opens in the smashing spectrum. Accepted for publication by the Journal of Topolog

Magma ascent by porous flow

Porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle. Recent work has identified the importance of matrix deformation and compaction in this process. We present finite-difference calculations based on a generalized form of Darcy's law that includes matrix deformation, in two-dimensional Cartesian and cylindrical coordinates. We emphasize the existence of solitary wave solutions, called magmons. These waves are regions of locally high porosity that ascend through regions of low, uniform porosity. They differ from diapirs, where the liquid and matrix ascend together. The one-dimensional waves that were previously reported are found to be unstable in two dimensions, breaking down to form two-dimensional waves of circular cross section. Both the development of the instability and the form of the two-dimensional waves are supported by theoretical analysis. Numerical experiments in which two-dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves. The conservation can be very good; in other cases, larger waves appear to grow at the expense of smaller waves. Although the quantification of the relevant permeability and rheological parameters remains uncertain, geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year. In our preliminary assessment of the relevance of these results to igneous processes, we find that magmons are unlikely to be important in regions of broad upwelling, such as beneath spreading centers. Here the transport of liquid adjusts to match the supply by melting, and compaction processes are not important. If liquid is supplied from below to a stable partially molten region of the asthenosphere, it is expected to ascend in magmons. This may occur beneath oceanic volcanic centers or in subcontinental mantle. It is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity, if higher level processes do not obscure this influence. The waveform of a magmon ascends faster than the liquid within the magmon, so new liquid is taken in from above, while original liquid is lost from below. Consequently, magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface