Study Of Space Charge Effects Research Articles

Particle-in-cell techniques for the study of space charge effects in an electrostatic ion beam trap.

We developed a simulation technique to study the effect of space charge interaction between trapped ions in the electrostatic ion beam trap (EIBT). The importance of space charge is demonstrated in both the dispersive and the self-bunching regime of the ion trap. The simulation results provide an estimate for the space charge effect on the trapping efficiency. They also allow for a better understanding of the enhanced diffusion and the self-bunching effect and provide a better characterization of the EIBT as a mass spectrometer, where peak coalescence is important. The numerical results reproduce all experimental data, demonstrating the critical importance of including space charge effects, even at low ion density, to understand the ion trap dynamics.

Particle-in-cell techniques for the study of space charge effects in the Advanced Cryogenic Gas Stopper

Linear gas stoppers are widely used to convert high-energy, rare-isotope beams and reaction products into low-energy beams with small transverse emittance and energy spread. Stopping of the high-energy ions is achieved through interaction with a buffer gas, typically helium, generating large quantities of He+/e− pairs. The Advanced Cryogenic Gas Stopper (ACGS) was designed for fast, efficient stopping and extraction of high-intensity, rare-isotope beams. As part of the design process, a comprehensive particle-in-cell code was developed to optimize the transport and extraction of rare isotopes from the ACGS in the presence of space charge, including He+/e− dynamics, buffer gas interactions including gas flow, radio-frequency carpets, and ion extraction through a nozzle or orifice. Details of the simulations are presented together with comparison to experiment when available.

Efficient three-dimensional Poisson solvers in open rectangular conducting pipe

Three-dimensional (3D) Poisson solver plays an important role in the study of space-charge effects on charged particle beam dynamics in particle accelerators. In this paper, we propose three new 3D Poisson solvers for a charged particle beam in an open rectangular conducting pipe. These three solvers include a spectral integrated Green function (IGF) solver, a 3D spectral solver, and a 3D integrated Green function solver. These solvers effectively handle the longitudinal open boundary condition using a finite computational domain that contains the beam itself. This saves the computational cost of using an extra larger longitudinal domain in order to set up an appropriate finite boundary condition. Using an integrated Green function also avoids the need to resolve rapid variation of the Green function inside the beam. The numerical operational cost of the spectral IGF solver and the 3D IGF solver scales as O(Nlog(N)), where N is the number of grid points. The cost of the 3D spectral solver scales as O(NnN), where Nn is the maximum longitudinal mode number. We compare these three solvers using several numerical examples and discuss the advantageous regime of each solver in the physical application.

Rate capability of a cryogenic stopping cell for uranium projectile fragments produced at 1000 MeV/u

At the Low-Energy Branch (LEB) of the Super-FRS at FAIR, projectile and fission fragments will be produced at relativistic energies, separated in-flight, energy-bunched, slowed down and thermalized in a cryogenic stopping cell (CSC) filled with ultra-pure He gas. The fragments are extracted from the stopping cell using a combination of DC and RF electric fields and gas flow. A prototype CSC for the LEB has been developed and successfully commissioned at the FRS Ion Catcher at GSI. Ionization of He buffer gas atoms during the stopping of energetic ions creates a region of high space charge in the stopping cell. The space charge decreases the extraction efficiency of stopping cells since the high amount of charge distorts the applied DC electric drag fields. Thus the understanding of space charge effects is of great importance to make full use of the high yields at future RIB facilities such as the Super-FRS at FAIR. For this purpose a detailed study of space charge effects in the CSC was performed using experiments and simulations. The dependence of the extraction efficiency, the extraction time and the temporal ion extraction profile on the intensity of the impinging beam and the electric field strength was studied for two different 238U projectile fragments produced at 1000MeV/u and separated with the FRS. Good agreement between experiments and simulations was found.

Numerical Study on Electronic and Optical Properties of Organic Light Emitting Diodes

In this paper, we present a finite element method (FEM) study of space charge effects in organic light emitting diodes. Our model includes a Gaussian density of states to account for the energetic disorder in organic semiconductors and the Fermi-Dirac statistics to account for the charge hopping process between uncorrelated sites. The physical model cover all the key physical processes in OLEDs, namely charge injection, transport and recombination, exciton diffusion, transfer and decay as well as light coupling, and thin-film-optics. The exciton model includes generation, diffusion, and energy transfer as well as annihilation. We assumed that the light emission originates from oscillating and thus embodied as excitons and embedded in a stack of multilayer. The out-coupled emission spectrum has been numerically calculated as a function of viewing angle, polarization, and dipole orientation. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution.

Accelerated Simulation Study of Space Charge Effects in Quadrupole Ion Traps Using GPU Techniques

Space charge effects play important roles in the performance of various types of mass analyzers. Simulation of space charge effects is often limited by the computation capability. In this study, we evaluate the method of using graphics processing unit (GPU) to accelerate ion trajectory simulation. Simulation using GPU has been compared with multi-core central processing unit (CPU), and an acceleration of about 390 times have been obtained using a single computer for simulation of up to 10(5) ions in quadrupole ion traps. Characteristics of trapped ions can be investigated at detailed levels within a reasonable simulation time. Space charge effects on the trapping capacities of linear and 3D ion traps, ion cloud shapes, ion motion frequency shift, mass spectrum peak coalescence effects between two ion clouds of close m/z are studied with the ion trajectory simulation using GPU.

Observation and simulation of space-charge effects in a radio-frequency photoinjector using a transverse multibeamlet distribution

We report on an experimental study of space-charge effects in a radio-frequency (rf) photoinjector. A 5 MeVelectron bunch, consisting of a number of beamlets separated transversely, was generated in an rf photocathode gun and propagated in the succeeding drift space. The collective interaction of these beamlets was studied for different experimental conditions. The experiment allowed the exploration of space-charge effects and its comparison with 3D particle-in-cell simulations. Our observations also suggest the possible use of a multibeam configuration to tailor the transverse distribution of an electron beam.

Numerical Study of Space Charge Effects in a Point Cathode Electron Gun: Distribution of Space Charge Density

The space charge effects in a thermionic emission gun using a tungsten point cathode have been studied with numerical method. The method is based on the integral form of Poisson equation, where the space charges are treated with a large number of coaxial charged rings surrounding the cathode. Each ring charge is estimated from many electron trajectories traced in 3D. The distribution of the space charge density around the cathode of the tip radius 0.4 μm was calculated for different bias conditions and different cathode temperatures. The space charge effects were examined by comparing these distributions as well as the axial potential and field distributions near the tip. The numerical results show that even at the temperature as high as 3100 K the cathode can provide the emission without any noticeable space charge effects when the negative bias on the Wehnelt electrode is shifted by several volts from the cutoff. [DOI: 10.1380/ejssnt.2008.91]

Transient and steady-state behavior of space charges in multilayer organic light-emitting diodes

A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq3 layer and that the electroluminescence delay time is determined by electrons passing through Alq3 to the internal interface are self-consistently supported by the results of the simulation. Moreover, the creation of emissive singlet excitons is found to be strongly confined to the Alq3 side of the internal interface and the emission zone width is dictated by the exciton diffusion length. Design principles for trilayer LEDs with improved power efficiency are also discussed.

The Study of Space Charge Effects by Spectral Response, Steady State Charge Collection and Transient Photocurrents in Thick a-Si:H Pin-Diodes

AbstractCharge collection, transient photocurrents and collection efficiency under additional bias illumination were used to characterize 3–4 micron thick a-Si:H pin-diodes. The wavelength dependent decrease or increase in the spectral response, depending on the bias flux and absorption depth, is related to the distribution of the electric field, recombination and majority carrier diffusion. At higher photon flux an overshoot in the transient photocurrent after switch-on of steady illumination indicates the time scale for the changes in internal variables. Collection efficiencies under large bias monochromatic photon flux well in excess of the maximum value of 100 % for probe beam generated carriers are observed with a large amplification ratio. These efficiencies sensitively depend both on the applied voltage and the defect density. Numerical modelling reveals the influence of internal variables on the transient and steady state photocurrents under the different illumination conditions.

Study of space-charge effects on dielectric breakdown of polymers by direct probing

Direct and quantitative probing of space charge was carried out to study effects of space charge on dielectric breakdown of polymeric insulating materials. In poly-p-phenylene sulfide, in which dielectric-breakdown characteristics were explained in terms of space-charge formation, positive homo space charge was observed. The amount and the depth of space charge detected could explain the dependence of impulse breakdown strength on prestressing both qualitatively and quantitatively. In oxidized polyethylene, negative homo space charge was observed. This also agreed well with the prestress dependence of the breakdown characteristics. The injecting electrode during the prestressing or the region nearby was suggested to be important in the breakdown process of both materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of space charge effects in polyethylene by thermal-pulse current techniques

Effects of oxidation on space-charge formation, carrier injection, and electrical conduction in polyethylene (PE) are studied by using thermal pulse current and thermally stimulated current (TSC) techniques. Oxidation increases the conduction current by a few orders of magnitude and negative homo space charge is formed near the cathode. The introduction of an oxidized PE layer on the surface of unoxidized PE enhances both electron and hole injection. The enhanced electron injection predominates over the hole injection and plays an important role in the increase of conduction with oxidation. The negative homo space charge is released at approximately=40 degrees C and gives rise to a TSC peak.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of space charge effects on photoconduction in amorphous Se0.95Te0.05films using a three electrode system

Abstract An experimental study of space charge effects on photoconduction in amorphous Se0.95Te0.05 films has been made using a three electrode system. The results indicate that the photoconduction in this material is dominated by holes and is governed by the built-in field created by space charges and the carrier injection from the electrical contacts, and that the three electrode system can be used for the investigation of the behaviour of space charges.