Laser Pulse Compression Using Magnetized Plasmas

Proposals to reach the next generation of laser intensities through Raman or Brillouin backscattering have centered on optical frequencies. Higher frequencies are beyond the range of such methods mainly due to the wave damping that accompanies the higher density plasmas necessary for compressing higher frequency lasers. However, we find that an external magnetic field transverse to the direction of laser propagation can reduce the required plasma density. Using parametric interactions in magnetized plasmas to mediate pulse compression both reduces the wave damping and alleviates instabilities, thereby enabling higher frequency or lower intensity pumps to produce pulses at higher intensity and longer duration. In addition to these theoretical advantages, our new method, in which strong uniform magnetic fields lessen the need for high-density uniform plasmas, also lessens key engineering challenges, or at least exchanges them for different challenges.Comment: 3 figures, 1 tabl

From Helpless Victim to Heroic Mass Resistance. The Evolution of Newspaper Coverage of Denmark in the New York Times During German Occupation 1940-1945

Fra hjælpeløst offer til heroisk frihedskamp Udviklingen i nyhedsdækningen af Danmark i New York Times under den tyske besættelse, 1940-1945I artiklen undersøges dækningen af Danmark under den tyske besættelse i New York Times, USA’s mest dækkende og autoritative nyhedskilde. Med en kombination af kvalitativ og kvantitativ metode spores linjerne i avisens fremstilling af den danske reaktion på besættelsen. I begyndelsen viste New York Times et billede af landet som hjælpeløst offer for tysk aggression, men efterhånden som tiden gik, begyndte avisen at lægge vægt på samarbejdspolitikken, hvis førende repræsentanter blev fremstillet i et kritisk lys. Augustoprøret 1943 og folkestrejkerne sommeren 1944 medførte imidlertid et nyt skift i avisens vinkling af stoffet. Den danske befolkning blev nu rost til skyerne for sin djærve og tapre holdning. Til sidst blev ikke kun modstandsbevægelsen, men nationen som helhed krediteret for sin kamp mod det nazistiske tyranni, endda lige fra be gyndelsen til slutningen af besættelsen. Det er ironisk, at et land, hvis politiske og forvaltningsmæssige system gennem alle krigsårene så vidt muligt holdt fast ved en tilpasnings- og samarbejdslinje, i det amerikanske nyhedsbillede endte med at fremstå som et mønster på vedvarende national kampvilje og folkelig modstand

Imaging-based 3D Particle Tracking System for Field Characterization of Particle Dynamics in Atmospheric Flows

A particle tracking velocimetry apparatus is presented that is capable of measuring three-dimensional particle trajectories across large volumes, of the order of several meters, during natural snowfall events. Field experiments, aimed at understanding snow settling kinematics in atmospheric flows, were conducted during the 2021/2022 winter season using this apparatus, from which we show preliminary results. An overview of the methodology, wherein we use a UAV-based calibration method, is provided, and analysis is conducted of a select dataset to demonstrate the capabilities of the system for studying inertial particle dynamics in atmospheric flows. A modular camera array is used, designed specifically for handling the challenges of field deployment during snowfall. This imaging system is calibrated using synchronized imaging of a UAV-carried target to enable measurements centered 10 m above the ground within approximately a 4 m x 4 m x 6 m volume. Using the measured Lagrangian particle tracks we present data concerning 3D trajectory curvature and acceleration statistics, as well as clustering behavior using Voronoi analysis. The limitations, as well as potential future developments, of such a system are discussed in the context of applications with other inertial particles

Manual discrimination of force

Optimal design of human-machine interfaces for teleoperators and virtual-environment systems which involve the tactual and kinesthetic modalities requires knowledge of the human's resolving power in these modalities. The resolution of the interface should be appropriately matched to that of the human operator. We report some preliminary results on the ability of the human hand to distinguish small differences in force under a variety of conditions. Experiments were conducted on force discrimination with the thumb pushing an interface that exerts a constant force over the pushing distance and the index finger pressing against a fixed support. The dependence of the sensitivity index d' on force increment can be fit by a straight line through the origin and the just-noticeable difference (JND) in force can thus be described by the inverse of the slope of this line. The receiver operating characteristic (ROC) was measured by varying the a priori probabilities of the two alternatives, reference force and reference force plus an increment, in one-interval, two-alternative, forced-choice experiments. When plotted on normal deviate coordinates, the ROC's were roughly straight lines of unit slope, thus supporting the assumption of equal-variance normal distributions and the use of the conventional d' measure. The JND was roughly 6-8 percent for reference force ranging from 2.5 to 10 newtons, pushing distance from 5 to 30 mm, and initial finger-span from 45 to 125 mm. Also, the JND remained the same when the subjects were instructed to change the average speed of pushing from 23 to 153 mm/sec. The pushing was terminated by reaching either a wall or a well, and the JND's were essentially the same in both cases