Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches.

Relativistic charged-particle beams that generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam breakup instability and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat beams) have been shown to suppress the initial instability in slab-symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat beam, traversing near a planar dielectric lined structure. The measurements reveal the emergence of a previously unobserved skew-quadrupolelike interaction when the beam is canted transversely, which is not present when the flat beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two-particle theoretical model, with particle-in-cell simulations used to provide further insight for realistic particle distributions.

Phase Space Reconstruction from Accelerator Beam Measurements Using Neural Networks and Differentiable Simulations.

Characterizing the phase space distribution of particle beams in accelerators is a central part of understanding beam dynamics and improving accelerator performance. However, conventional analysis methods either use simplifying assumptions or require specialized diagnostics to infer high-dimensional (>2D) beam properties. In this Letter, we introduce a general-purpose algorithm that combines neural networks with differentiable particle tracking to efficiently reconstruct high-dimensional phase space distributions without using specialized beam diagnostics or beam manipulations. We demonstrate that our algorithm accurately reconstructs detailed 4D phase space distributions with corresponding confidence intervals in both simulation and experiment using a limited number of measurements from a single focusing quadrupole and diagnostic screen. This technique allows for the measurement of multiple correlated phase spaces simultaneously, which will enable simplified 6D phase space distribution reconstructions in the future.

Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line.

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a TR≈5 in a dielectric wakefield accelerator.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line.

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Interaction of an Ultrarelativistic Electron Bunch Train with a W-Band Accelerating Structure: High Power and High Gradient.

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91 GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to the interference of the wakefields from the two bunches, was measured as a function of bunch separation. Measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.

Fabrication of THz corrugated wakefield structure and its high power test.

We present overall process for developing terahertz (THz) corrugated structure and its beam-based measurement results. 0.2-THz corrugated structures were fabricated by die stamping method as the first step demonstration towards GW THz radiation source and GV/m THz wakefield accelerator. 150-[Formula: see text]m thick disks were produced from an OFHC (C10100) foil by stamping. Two types of disks were stacked alternately to form 46 mm structure with [Formula: see text] 170 corrugations. Custom assembly was designed to provide diffusion bonding with a high precision alignment of disks. The compliance of the fabricated structure have been verified through beam-based wakefield measurement at Argonne Wakefield Accelerator Facility. Both measured longitudinal and transverse wakefield showed good agreement with simulated wakefields. Measured peak gradients, 9.4 MV/m/nC for a long single bunch and 35.4 MV/m/nC for a four bunch trains, showed good agreement with the simulation.

Surface-emission studies in a high-field RF gun based on measurements of field emission and Schottky-enabled photoemission.

We report on investigations into the fundamental surface emission parameters, the geometric field enhancement factor (ß) and the work function (φ), by making both field emission and Schottky-enabled photoemission measurements. The measurements were performed on a copper surface in the Tsinghua University S-band RF gun in two separate experiments. Fitting our data to the models for each experiment indicate that the traditionally assumed high value of ß(≈50-500) does not provide a plausible explanation of the data, but incorporating a low value of φ at some sites does. In addition, direct measurements of the surface conducted after the experiment show that ß is on the order of a few, consistent with our understanding of the electron emission measurements. Thus we conclude that the dominant source of electron emission in high gradient RF cavities is due to low φ sites, as opposed to the conventionally assumed high ß sites. The origin of low φ at these sites is unclear and should be the subject of further investigation.

On the equivalence of superordinate concepts.

Psychological studies of superordinates have generally treated them as equivalent. However, many languages distinguish mass superordinates (e.g., clothing) from count superordinates (e.g., vehicle). In the present paper, experimental evidence is presented which suggests that the two types of superordinates are conceptually distinct as well. One study showed that the members of mass superordinates more often co-occur. A second study showed that people more often interact with the members of mass superordinates in temporal proximity whereas people primarily interact with single members of count superordinates on a specific occasion. Also, properties that characterize an individual are a more salient aspect of count superordinates. These findings imply that mass superordinates refer to unindividuated groups of objects, united by spatial and functional contiguity. Two other studies supported this hypothesis by showing that the class inclusion relation between a single object and a category is stronger for count superordinates. Taken together, the findings suggest that mass superordinates are not true taxonomic categories. We relate the findings to previous views of superordinates and to the count/mass distinction in general.

Prior knowledge and functionally relevant features in concept learning.

Empirical learning models have typically focused on statistical aspects of features (e.g., cue and category validity). In general, these models do not address the contact between people's prior knowledge that lies outside the category and their experiences of the category. A variety of extensions to these models are examined, which combine prior knowledge with empirical learning. Predictions of these models were compared in 4 experiments. These studies contrasted the cue and category validity of features with people's prior knowledge about the relevance of features to the functions of novel artifacts. The findings suggest that the influences of knowledge and experience are more tightly integrated than some models would predict. Furthermore, relatively straightforward ways of incorporating knowledge into an empirical learning algorithm appear insufficient (e.g., use of knowledge to weight features by general relevance or to individually weight features). Other extensions to these models are suggested that focus on the importance of intermediary features, coherence, and conceptual roles.

On the necessity of alignment: reply to Costello and Keane (2001).

In their article, "Testing two theories of conceptual combination: Alignment versus diagnosticity in the comprehension and production of combined concepts," F. J. Costello and M. T. Keane (2001) evaluate the role of alignment in the interpretation of noun-noun combinations. They found that participants were not strongly biased to prefer and produce interpretations with alignable differences. Instead, participants sometimes preferred and produced interpretations with nonalignable differences. These results are surprising given that most research has found advantages of alignable differences over nonalignable differences. Costello and Keane also found that feature diagnosticity better predicted their results, and they concluded that alignment does not play an important role in conceptual combination. However, drawing on recent work, the author of the present article gives an alternative interpretation of Costello and Keane's results, showing that alignment is crucial in conceptual combination. The author also shows that the dual-process model accounts for their results.

Categorizing objects in isolation and in scenes: what a superordinate is good for.

Many studies have shown that subjects are faster at categorizing objects into "basic" concepts than into more general superordinate concepts. However, all of these studies have used a categorization task in which single, isolated objects are identified. There is good reason to believe that superordinate concepts are typically used to refer to collections of objects rather than to individual objects. For example, people more often use the term furniture to refer to a number of pieces of furniture rather than to name a single piece. This suggest that superordinate concepts include information about multiple objects and their common relations, particularly the typical scenes in which such objects appear. Four experiments examined this possibility by investigating whether the basic concept advantage will decrease or reverse itself when subjects are asked to categorize an object as part of a scene. The results showed that the basic-superordinate difference did decrease when subjects categorized objects in scenes. Furthermore, when an object was placed in an inappropriate scene, there was more interference for superordinate identifications. The results suggest qualitative differences in the representations of superordinate and basic concepts.

When concepts combine.

I present a computational level account of how people combine concepts, and I use this account to evaluate current models of conceptual combination. Constrained by this account, I then provide an algorithmic level description of how people combine concepts. The algorithmic level account highlights the importance of two additional processes (comparison and construction) in explaining how some concepts combine and change. I then show that the interpretation of nominal metaphors involves these processes as well. Current approaches to metaphor understanding emphasize the importance of one or the other of these processes, but not both.

Property instantiation in conceptual combination.

In four experiments, I examined how a property in one concept is transferred to a second concept during conceptual combination. The results suggest that people instantiate properties: that is, they use a specific representation of a property in the modifier concept to construct a new version of that property that is specific to the combination. If people are instantiating properties, then the modifier property should match its counterpart in the combination to the extent that the modifier and head noun are similar. This observation leads to a variety of predictions (supported by the experiments) about interpretations of similar and dissimilar combinations and about plausibility, preference, and similarity judgments associated with such interpretations. The results argue against an alternative view of transfer that posits that, in general, abstract representations of properties are copied from one concept to another. In this paper, I describe various processing accounts of instantiation and discuss the implications of the instantiation view for theories of metaphor, conceptual combination, and induction.

What makes a man similar to a tie? Stimulus compatibility with comparison and integration.

We argue and show that different properties of stimuli are compatible with different types of processing. Specifically, object pairs from the same taxonomic category (e.g., chair-bed) tend to be alignable and thus compatible with comparison, whereas object pairs that play different roles in thematic relations (e.g., chair-carpenter) tend to be nonalignable and compatible with integration. Using object pairs that varied orthogonally in alignability and thematic relatedness, we demonstrated that stimulus compatibility modulates processing and affects the outcomes of tasks that are currently believed to involve only comparison (similarity ratings, Experiment 1; listing commonalities and differences, Experiment 2) or only integration (thematic relatedness ratings, Experiment 3). Our findings and others that we have reviewed suggest that: (1) many cognitive tasks involve both comparison and integration, and (2) the relative influence of each process is modulated by an interplay between the task-appropriate and the stimulus-compatible process. We believe that single-process models should be extended to take this interplay into account.

Haematological parameters of the Polish Primitive Horses.

Average values of chosen haematological parameters have been described in a population of 90 clinically normal Polish Primitive Horses. The comparison of the results with values given in literature for horses as a whole and for other breeds enabled us to state that Polish Primitive Horses' blood is characterized by relatively low values of RBC, PCV, segmented neutrophils and monocytes, and high percentage of lymphocytes. It was demonstrated that values of some blood parameters of examined animals are similar to typical results obtained for primitive and cold-blooded horses. Moreover, it was proved that age, sex, and season have influence upon many blood parameters' values of the Polish Primitive Horses. More precise evaluation of results received with that breed will be possible when that influence is taken into consideration.

Distribution of flux control among the enzymes of mitochondrial oxidative phosphorylation in calcium-activated saponin-skinned rat musculus soleus fibers.

Metabolic control analysis was applied to describe the control of mitochondrial oxidative phosphorylation in calcium (approximately 2 microM free calcium) activated saponin-skinned rat musculus soleus fibers oxidizing glutamate and malate. Under these circumstances approximately 80% of mitochondrial active-state respiration was reached due to the activation of ATP turnover by actomyosin ATPase. The flux control coefficients of H(+)-ATPase, adenine-nucleotide translocase, phosphate transporter, NADH:ubiquinone oxidoreductase and cytochrome-c oxidase were determined to be equal to 0.16 +/- 0.08 (n = 6), 0.34 +/- 0.12 (n = 5), 0.08 +/- 0.03 (n = 5), 0.01 +/- 0.006 (n = 4) and 0.09 +/- 0.03 (n = 3) using inhibitor titrations with the specific inhibitors oligomycin, carboxyatractyloside, mersalyl, rotenone and cyanide, respectively, and applying non-linear regression of the entire titration curve. The flux control coefficient of actomyosin ATPase was determined with vanadate to be equal to 0.50 +/- 0.09 (n = 6), measuring independently the vanadate-caused inhibition of fiber respiration and ATP-splitting activity. In contrast to results with isolated rat skeletal muscle mitochondria reconstituted with soluble F1-ATPase the decrease in phosphate concentration from 10 mM to 1 mM only slightly affected the distribution of flux control coefficients. This difference is caused by different kinetic properties of soluble F1-ATPase and actomyosin ATPase. Therefore, phosphate seems to be in skeletal muscle in vivo only a modest modulator of control of oxidative phosphorylation.