We describe a new approach for driving GeV-scale plasma accelerators with long laser pulses. We show that the temporal phase of a long, high-energy driving laser pulse can be modulated periodically ...by copropagating it with a low-amplitude plasma wave driven by a short, low-energy seed pulse. Compression of the modulated driver by a dispersive optic generates a train of short pulses suitable for resonantly driving a plasma accelerator. Modulation of the driver occurs via well-controlled linear processes, as confirmed by good agreement between particle-in-cell (PIC) simulations and an analytic model. PIC simulations demonstrate that a 1.7 J, 1 ps driver, and a 140 mJ, 40 fs seed pulse can accelerate electrons to energies of 0.65 GeV in a plasma channel with an axial density of 2.5 × 1017 cm−3. This work opens a route to high repetition-rate, GeV-scale plasma accelerators driven by thin-disk lasers, which can provide joule-scale, picosecond-duration laser pulses at multikilohertz repetition rates and high wall-plug efficiencies.
The poly(3,4‐dioxypyrrole) (PXDOP) family of conducting and electroactive polymers has now been developed to the point that multiple synthetic routes allow many functionalized polymers with ...controllable optoelectronic and redox properties. These properties, which include high conductivity, multicolor cathodic and anodic electrochromism, and rapid redox switching, allow these materials to be used in a variety of applications that potentially include conducting coatings, electrochromic windows and displays, chemical sensors, bioactive materials, and mechanical actuators. Surprisingly, the scientific literature published on the PXDOP derivatives has been isolated and sparse compared to that of other conducting polymers. This report will highlight the synthesis and materials properties of PXDOPs and show how these powerful materials fit into the frontier of conducting polymers research.
The poly(3,4‐dioxypyrrole) family of conducting and electroactive polymers may not have received as much attention to date as their poly(3,4‐ethylenedioxythiophene) cousins, but that may be about to change. The authors review the synthesis and properties of these versatile systems, demonstrating that these materials may be the final piece in the conjugated polymer puzzle, combining a large bandgap and a low oxidation potential.
Pedotransfer functions (PTFs), which estimate soil hydraulic parameters from easy to measure soil properties, are an important data source for hydrologic modeling. Recently artificial neural networks ...(ANNs) have become the tool of choice in PTF development. Recent developments in machine learning methods include the growing research and application of the alternative data-driven method called Support Vector Machines (SVMs). Support Vector Machines have gained popularity in many traditionally ANN dominated fields. Using the SVM eliminates the local minimum issue--the minimum found is always the global one. The objective of this work was to see whethers using the SVM to develop PTFs may have some advantages compared with the ANN. We have used the Soil Profiles Bank of Polish Mineral Soils that includes hydraulic properties for 806 soil samples taken from 290 soil profiles. This database was repeatedly randomly split into training and testing data sets, and both SVMs and ANNs were trained and tested for each split with bulk density, sand and clay as input variables, and water contents at 11 soil water potentials as the output variables. The PTF performance was evaluated by using the test datasets to compute the coefficient of determination, the root-mean-squared error, and the slope and the intercept of the linear regression "predicted vs. measured water contents." The three-parameter SVMs performed mostly better than or with the same accuracy as the eleven-parameter ANNs. The advantage of SVM was more pronounced at soil matric potentials where larger relative errors have been encountered and the correlation between predicted and measured soil water contents was lower. It is worthwhile to consider SVM as a tool to develop PTF.
Objectives
Canine insulinomas are uncommon malignant functional pancreatic neuroendocrine tumours with a high metastatic rate. Diagnostic imaging aids with staging and surgical planning of these ...tumours; however, identification is unpredictable across modalities. High‐grade human pancreatic neuroendocrine tumours display increased avidity on 18F‐fluorodeoxyglucose positron emission tomography‐CT.
Materials and Methods
Dogs with clinicopathologic findings consistent with pancreatic insulinoma were prospectively enrolled. Patients underwent 18F‐fluorodeoxyglucose positron emission tomography‐CT and CT angiography, followed by exploratory laparotomy.
Results
Three patients met the inclusion criteria and had histologically confirmed insulinomas. Both metastatic lesions in patient 1 were mildly avid (SUVmax 2.79 and 3.01). In patient 2, the primary pancreatic insulinoma was minimally avid (SUVmax 2.16). The primary pancreatic lesion in patient 3 had similar avidity to normal pancreatic parenchyma (SUVmax 1.54) and was undetected on 18F‐fluorodeoxyglucose positron emission tomography‐CT. Insulinomas demonstrated variable attenuation and contrast enhancement patterns on CT angiography and certain lesions were more conspicuous than on 18F‐fluorodeoxyglucose positron emission tomography‐CT. Two metastatic lesions not visible on either imaging modality were discovered in patient 2 at surgery.
Clinical Significance
Canine insulinomas were inconsistently avid on 18F‐fluorodeoxyglucose positron emission tomography‐CT. This finding is likely attributable to the confounding clinicopathological features and multifaceted transformation of these tumours, in addition to the influence of variable tumour size, composition and vascularity. Unpredictable tumoural avidity limits the value of 18F‐fluorodeoxyglucose positron emission tomography‐CT for staging canine insulinomas.
We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing ...that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.
We describe a simple scheme, truncated-channel injection, to inject electrons directly into the wakefield driven by a high-intensity laser pulse guided in an all-optical plasma channel. We use this ...approach to generate dark-current-free 1.2 GeV, 4.5% relative energy spread electron bunches with 120 TW laser pulses guided in a 110 mm-long hydrodynamic optical-field-ionized plasma channel. Our experiments and particle-in-cell simulations show that high-quality electron bunches were only obtained when the drive pulse was closely aligned with the channel axis, and was focused close to the density down ramp formed at the channel entrance. Start-to-end simulations of the channel formation, and electron injection and acceleration show that increasing the channel length to 410 mm would yield 3.65 GeV bunches, with a slice energy spread ∼5×10^{-4}.
We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. ...Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated 1 ms apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of 0.4 kHz for a period of 6.5 h without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.
We present experiments and numerical simulations which demonstrate that fully ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical ...field ionization. Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of 200 mm long plasma channels with axial densities of order n_{e}(0)=1×10^{17}cm^{-3} and lowest-order modes of spot size W_{M}≈40μm. These simulations show that the laser energy required to generate the channels is modest: of order 1 mJ per centimeter of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with 1.5×10^{17}cm^{-3}≲n_{e}(0)≲1×10^{18}cm^{-3} and 61μm≳W_{M}≳33μm. Low-density plasma channels of this type would appear to be well suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.
We investigate the growth of ion density perturbations in large-amplitude linear laser wakefields via two-dimensional particle-in-cell simulations. Growth rates and wave numbers are found to be ...consistent with a longitudinal strong-field modulational instability. We examine the transverse dependence of the instability for a Gaussian wakefield envelope and show that growth rates and wave numbers can be maximized off axis. On-axis growth rates are found to decrease with increasing ion mass or electron temperature. These results are in close agreement with the dispersion relation of a Langmuir wave with an energy density that is large compared to the plasma thermal energy density. The implications for wakefield accelerators, in particular multipulse schemes, are discussed.
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