No description available.

This study investigated how varying the required force level and the direction of force change produced by the thumb and index finger affect the accuracy and variability of a cyclic isometric pinch force-tracking task. Accuracy was examined by both absolute error and relative error for the minimum and maximum force levels and by root mean square error (RMSE) and normalized root mean square error (normalized RMSE) for the force direction reversals. Variability was represented by coefficient of variation of error (CVE). In this study, ‘maximum force’ was defined as the highest force level of a given target force range, and ‘minimum force’ was defined as the minimum force level of the target range. In addition, ‘force increasing to decreasing’ indicated that the track ball motion changed from increasing to decreasing, requiring the performer to exert increasing force up to the maximum force level and then decreasing force to follow the track ball moving toward the minimum force level. The phrase ‘force decreasing to increasing’ indicated the opposite force direction reversal. Eighteen healthy right handed adult volunteers (nine men and nine women; mean age ± SD, 28.3±1.22 and 26.4±1.74) participated in this study. The participants performed a cyclic isometric pinch force tracking task over three different force ranges. Force range 1 was from a minimum force of 3% of maximal voluntary contraction force (MVC) to a maximum force of 6% MVC. In force range 2, the range was from 6% to 12% MVC, and force range 3 was from 12% to 24% MVC. For each force range, five practice trials and ten actual test trials were performed. Rest periods of twenty seconds between trials and one minute between sets of trials (including between practice and actual test trials) were provided to minimize fatigue effects. Absolute error uniformly increased as a function of increasing force. However, the 3% target force level showed larger relative error compared to the 12% target force level (p < 0.05). Another finding of this study was that producing forces positioned at the minimum target level in a range yielded higher absolute error and relative error compared to the same forces when placed at the maximum target level of a different force range. In terms of the reversals, RMSE values were higher at the change from force deceasing to increasing than the opposite, as well as at higher force levels, while normalized RMSE values were greater at lower force levels. CVE was not significantly different between the two reversals in this study. This might indicate that poorer performance during the change from force decreasing to increasing could originate from the effort to maintain consistent performance and additional effort was not beneficial to increase accuracy for the change from force decreasing to increasing. / text

No description available.

No description available.

The structure in the collapse stage of a linear Z-pinch discharge in helium has been studied by optical methods. Observations with a framing camera, rotating mirror spectrograph, and monochromator have been correlated with magnetic field and current distributions determined by Tam (1967). The luminous regions in a helium pinch are very faint. Therefore, up to twenty exposures have to be superimposed on the same framing camera or rotating mirror record. This requires a high degree of reproducibility in the initiation of the discharge. At high initial pressures, a non-luminous shock wave at the inner edge of the collapsing current shell precedes the luminous plasma layer towards the centre of the discharge vessel. This shock front is followed by a region of predominantly Hel emission, while most of the Hell radiation occurs in the outer regions of the collapsing plasma shell. The separation into Hel and Hell radiating regions is consistent with spectroscopic measurements of temperature: higher temperatures occur at larger radii. Pressure and density in the non-radiating shock wave region are determined by calculations based on a simple model. At low filling pressures, the Hel and Hell regions coincide. The position of maximum luminosity is observed to correspond with the position of maximum current density. The luminosity and current shells coincide with no shock wave preceding the luminous front. Strong continuum radiation is emitted from the centre of the discharge tube as soon as the leading edge of the current shell reaches the axis. This leading edge is luminous at low initial pressures, but becomes a non-radiating shock front at higher filling pressures. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The formative phase of a 40 kV Z-pinch has been investigated in the pressure range from 10 mtorr to 80 mtorr in hydrogen. The energy spectrum of the electrons on the axis of the vessel, the spatial distribution of the current at the face of the anode, the total discharge current and the voltage across the discharge have all been monitored. For the pressures examined, the formative phase lasts a few hundred nanoseconds. At the initiation of the discharge, a current of electrons with energies in excess of 20 keV is observed on the axis. As time progresses, the average energy of the electrons decreases to a few keV and the current density increases to about 300 amps/cm². At pressures of 30 and 50 mtorr-H(2), the electron velocity distribution appears to relax to a Maxwellian [formula omitted]. This result is in agreement with the theoretical description of a weakly ionized gas in a strong electric field. However, it should be noted that the influence of plasma-turbulence has been neglected. The current to the central region of the electrode initially carries more than 50% of the total current through the discharge. This observation indicates that ionization initially occurs throughout the entire discharge vessel. However, shortly after the current of energetic electrons reaches its maximum value, the current to the central region of the electrode disappears. Simultaneously, a transient voltage of up to 20 kV appears across the discharge and the rate of increase of the current exhibits a sharp drop in value. The combination of these phenomena has been interpreted as the formation of the boundary layer. The boundary layer or current sheet is not completely formed until the end of the formative phase. Measurements of the pinch time support this conclusion. At low pressures, the time of formation of the boundary layer is considerably shortened by the enhancement of ionization at the wall. This is caused by the presence of the magnetic field of the current flowing in the central region of the vessel. The experiment has led to better comprehension of the mechanism of boundary layer formation in low pressure Z-pinches. The overall understanding of ionization in Z-pinch discharges has been improved and can be extrapolated to higher pressures. In addition, the observations indicate that the detailed nature of the current sheet collapse can be strongly affected by the formative phase. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The analytical solution presented in this thesis is based on the Liquid Metal Experiment (LEX) at Virginia Tech to determine the practicality of replacing a solid metal electrode with a liquid metal electrode wall. Replacing the solid metal with a liquid metal may improve the operational lifetime of Z-pinches. The LEX is based upon the University of Washington's High Energy Density Z-pinch (ZaP-HD) and Fusion Z-Pinch Experiments (FuZE) and replaces one solid metal electrode with a liquid metal electrode. During the operation of the ZaP-HD and FuZE, a plasma column exerts electromagnetic forces and pressure on a solid electrode wall. The pressure exerted by the plasma column is called the magnetic pressure. In the Virginia Tech device magnetic pressure is exerted by a wire onto the liquid metal electrode. The magnetic pressure in the LEX displaces the liquid metal electrode free surface, and subsequently creates a waveform. The initial free surface displacement and subsequent wave motion of the liquid metal is found by analyzing the geometry of the device, the electromagnetic forces generated during operation, and material properties of the tin-bismuth liquid metal mixture. The initial displacement for changing current, current pulse length, tin percentage, and applied pressure range are investigated. The results are compared for verification and validation. These methods are shown to be accurate to within an order of magnitude and are valid for an axisymmetric domain. The results presented here may inform further experimentation and aid in improving designs for newer devices. / Master of Science / This thesis presents analytical solutions for creating waveforms in liquid metal due to electromagnetic forces. The motivation for developing the analytical solutions is to aid in developing a device created by Virginia Polytechnic and State University (Virginia Tech). The Liquid Metal Experiment (LEX) at Virginia Tech investigates the practicality of incorporating a liquid metal into a Z-pinch fusion device under development at the University of Washington's High Energy Density Z-pinch (ZaP-HD) and Fusion Z-Pinch Experiments (FuZE). The ZaPHD and FuZE experiments are cylindrical and aim to investigate the viability of Z-pinches as fusion devices. An electric current passes between an internal electrode, the plasma column, and an external electrode along the z-axis of the Z-pinch. The time duration of the current is typically on the order of tens of microseconds. The plasma column and subsequent fusion events are generated only during this duration. During this duration, the interactions between the plasma column and the electrodes cause the electrodes to deteriorate. In standard Z-pinch devices, the electrodes are solid metal and deteriorate during the operation, limiting the device's lifespan. The liquid metal introduces other complexities to the system. During the time duration of the current, the electromagnetic forces produce a pressure gradient at the free surface of the liquid metal. The pressure created by the electromagnetic forces generates waveforms within the liquid metal. The analytical solutions presented in this thesis include electrodynamic solutions to find the pressure, kinematic solutions to determine the free surface displacement of the liquid metal due to the pressure, and fluid dynamic solutions of the waveform caused by the initial free surface displacement.

The entry and exit side pinch roll unit of the reversing finishing mill of stainless hot strip rolling mill line bears heat effects during producing. Its transfer convection must be sufficient to maintain its function. According to the original pinch roll unit during producing, there are four questions as follows: (1) Strip marks are formed from the roll surface. (2) High temperature of roll surface cause burning phenomenon and lower the hardness of roll surface. (3) Cracks are formed on the roll surface. (4) There is a deflection of horizontal pipe of rocker arm. The improved pinch roll is called new pinch roll. The original pinch roll is called old pinch roll. First, temperature gradients of each location for the old pinch roll and the new pinch roll are calculated, and the reasons of the abnormal high temperature of roll surface are clarified. Second, for the places of the old and new pinch rolls which are the easiest to crack, its stresses of x-y and y-z directions are analyzed. Influence factors of thermal stress crack are clarified. It is proved that the thermal stress effects of the new pinch roll are less than the old pinch roll. Third, the relations of thermal deflection and heat convection for the original horizontal pipe of rocker arm and for the improved horizontal pipe are analyzed. Furthermore, according to the internal structure of the old and new pinch rolls, the flow resistance of circulating water cooling system for each place is considered. Finally, roll grinder is developed and designed to improve the quality of roll surface.

Zugl.: Clausthal, Techn. Univ., Diss., 2008

Universiẗat, Diss., 2002--Jena.