Beam-based alignment of TTF RF-gun using V-Code

The beam dynamics simulation code V [1,2], based on the Ensemble Model [3], is being developed for on-line simulations. One practical application of the V-Code is the beam-based alignment (BBA) of accelerator (TESLA Test Facility) elements. Before we started with BBA thefirst beam position monitor (BPM1), located after the RFgun cavity, showed non-zero readings. Moreover the readings depended on RF-power, RF-phase and primary and secondary solenoid currents. This effect could be explained by misalignments of the gun and the solenoids. Such beam offsets must be compensated by means of steering coils but such a procedure can be one of the sources of increased emittances. Based on the V-Code solver a dedicated utility was developed for alignment studies. The laser beam mismatch at the cathode, as well as the primary and secondary solenoid displacements were considered as probable reasons for the misalignment of the beam. A new method for the correction of these misalignments combines a sequence of measurements, simulations and the elimination of the largest imperfections. This semi-automatic method applied to the TTF RF-gun yields a centering of the beam within the accuracy of the BPM1

Investigation of TTF injector alignment with the simulation Code V

The exact alignment of accelerator components is of crucial importance for the production of low emittance beams. Once a beam-line section is set up, a supplementary correction of misalignments implies the knowledge of its magnitude which is difficult to determine using conventional adjusting instruments. An excellent alternative to measure existing misalignments of accelerator components is to vary machine parameters and compare the behaviour of the beam with results obtained from a simulation. It is obvious that time consuming particle tracking programmes are notappropriate to reach this aim. Regarding computing time, the on-line simulation code V is advantageous compared to other beam dynamics programmes. The theoretical basis of V-Code, the “Ensemble Model”, consists of selfconsistent equations for the ensemble parameters that arederived from the Vlasov equation. The requirement to simulate misalignments such as offsets and tilts led to the development of the ALIGNMENT UTILITY which utilizes the solver of V-Code. The new utility enabled us to investigate the beam-line alignment of the TESLA Test Facility injector.This contribution presents the theoretical background and an illustrating example of the optimization process

Senior Citizen Day Celebration to be Held at University of Dayton

News release announces that Senior Citizens Day will be held at the University of Dayton

Electrokinetic Phenomena across Length Scales – from Nanofluidics to Sliding Drops

Solid surfaces in contact with a liquid are usually charged and attract a diffuse layer of countercharges in the liquid. Together, surface charge and diffuse countercharge form the electric double layer. The electric double layer generally has a thickness of 1 − 1000 nm. It mediates electrostatic interactions on the nanoscale and for example enables selective membranes, as in desalination or in biological cells. Gate electrodes can be used to manipulate the layer of diffuse charge. Here, we use gate electrodes in two different nanofluidic contexts. We apply an unbiased ac voltage to a gate electrode on the inside wall of a conical nanopore. Even in the absence of pressure, concentration or electric potential gradients across the pore, this yields a substantial directional flow through the pore. This resonant nanopumping is strongest at a characteristic resonance frequency. Additionally, we look at electrostatic fluidic particle traps. They trap individual nanoparticles, subject to Brownian motion, through electrostatic interactions between the electric double layers of the particles and the surrounding walls. We embed a patterned gate electrode into one of the walls. Applying a dc voltage can control the interaction strength and modulate the trapping. Electric double layers also form underneath drops when they sit on solid surfaces. As drops slide, they spontaneously charge and deposit an opposite surface charge. We show how this phenomenon called slide electrification can be explained through transport processes in the electrical double layer at the receding contact line of a drop and we develop a predictive theory for the charge separation. We further show that sliding drops spontaneously acquire voltages of the order of kilovolts. Finally, we show that electrostatic interactions between the drop and the deposited surface charge cause a previously overlooked contribution to contact angle hysteresis

Surface charge deposition by moving drops reduces contact angles

Slide electrification - the spontaneous charge separation by sliding water drops - can lead to an electrostatic potential of 1 kV and change drop motion substantially. To find out, how slide electrification influences the contact angles of moving drops, we analyzed the dynamic contact angles of aqueous drops sliding down tilted plates with insulated surfaces, grounded surfaces, and while grounding the drop. The observed decrease in dynamic contact angles at different salt concentrations is attributed to two effects: An electrocapillary reduction of contact angles caused by drop charging and a change in the free surface energy of the solid due to surface charging

Resonantly-driven nanopores can serve as nanopumps

Inducing transport in electrolyte-filled nanopores with dc fields has led to influential applications ranging from nanosensors to DNA sequencing. Here we use the Poisson-Nernst-Planck and Navier-Stokes equations to show that unbiased ac fields can induce comparable directional flows in gated conical nanopores. This flow exclusively occurs at intermediate driving frequencies and hinges on the resonance of two competing timescales, representing space charge development at the ends and in the interior of the pore. We summarize the physics of resonant nanopumping in an analytical model that reproduces the results of numerical simulations. Our findings provide a generic route towards real-time controllable flow patterns, which might find applications in controlling the translocation of particles such as small molecules or nanocolloids