D. A. Dimitrov

Particle-in-cell simulations of tunneling ionization effects in plasma-based accelerators

Plasma-based accelerators can sustain accelerating gradients on the order of 100 GV/m. If the plasma is not fully ionized, fields of this magnitude will ionize neutral atoms via electron tunneling, which can completely change the dynamics of the plasma wake. Particle-in-cell simulations of a high-field plasma wakefield accelerator, using the OOPIC code [D. L. Bruhwiler et al., Phys. Rev. ST Accel. Beams 4, 101302 (2001)], which includes field-induced tunneling ionization of neutral Li gas, show that the presence of even moderate neutral gas density significantly degrades the quality of the wakefield. The tunneling ionization model in OOPIC has been validated via a detailed comparison with experimental data from the l’OASIS laboratory [W.P. Leemans et al., Phys. Rev. Lett. 89, 174802 (2002)]. The properties of a wake generated directly from a neutral gas are studied, showing that one can recover the peak fields of the fully ionized plasma simulations, if the density of the electron drive bunch is increased...

Multiscale three-dimensional simulations of charge gain and transport in diamond

A promising new concept of a diamond-amplified photocathode for generation of high-current, high-brightness, and low thermal emittance electron beams was recently proposed and is currently under active development. Detailed understanding of physical processes with multiple energy and time scales is required to design reliable and efficient diamond-amplifier cathodes. We have implemented models, within the VORPAL computational framework, to simulate secondary electron generation and charge transport in diamond in order to facilitate the investigation of the relevant effects involved. The models include inelastic scattering of electrons and holes for generation of electron-hole pairs, elastic, phonon, and charge impurity scattering. We describe the integrated modeling capabilities we developed and present results on charge gain and collection efficiency as a function of primary electron energy and applied electric field. We compare simulation results with available experimental data. The simulations show an overall qualitative agreement with the observed charge gain from transmission mode experiments and have enabled better understanding of the collection efficiency measurements.

3D simulations of secondary electron generation and transport in a diamond amplifier for photocathodes

The Relativistic Heavy Ion Collider (RHIC) contributes fundamental advances to nuclear physics by colliding a wide range of ions. A novel electron cooling section, which is a key component of the proposed luminosity upgrade for RHIC, requires the acceleration of high-charge electron bunches with low emittance and energy spread. A promising candidate for the electron source is the recently developed concept of a high quantum efficiency photoinjector with a diamond amplifier. We have started to implement algorithms, within the VORPAL particle-in-cell framework, for modeling of secondary electron and hole generation, and for charge transport in diamond. The algorithms include elastic and various inelastic scattering processes over a wide range of charge carrier energies. Initial results from the implemented capabilities will be presented and discussed.

Coupling of laser energy into plasma channels

Diffractive spreading of a laser pulse imposes severe limitations on the acceleration length and maximum electron energy in the laser wake field accelerator (LWFA). Optical guiding of a laser pulse via plasma channels can extend the laser-plasma interaction distance over many Rayleigh lengths. Energy efficient coupling of laser pulses into and through plasma channels is very important for optimal LWFA performance. Results from simulation parameter studies on channel guiding using the particle-in-cell (PIC) code VORPAL [C. Nieter and J. R. Cary, J. Comput. Phys. 196, 448 (2004)] are presented and discussed. The effects that density ramp length and the position of the laser pulse focus have on coupling into channels are considered. Moreover, the effect of laser energy leakage out of the channel domain and the effects of tunneling ionization of a neutral gas on the guided laser pulse are also investigated. Power spectral diagnostics were developed and used to separate pump depletion from energy leakage. The ...

Ion acceleration and wave generation by overdense laser‐plasma interaction

The interaction of intense femto second laser pulses with overdense foil targets is investigated by particle‐in‐cell (PIC) simulations. Effects like electron acceleration and recirculation, ion sheath acceleration and the formation of an electrostatic shock are observed. Ions are accelerated at the shock front, leading to a second ion population emerging from the foil backside. Amplification of laser pulse harmonics, as well as the generation of low frequency waves are observed.

Modeling electron emission and surface effects from diamond cathodes

We developed modeling capabilities, within the Vorpal particle-in-cell code, for three-dimensional simulations of surface effects and electron emission from semiconductor photocathodes. They include calculation of emission probabilities using general, piece-wise continuous, space-time dependent surface potentials, effective mass, and band bending field effects. We applied these models, in combination with previously implemented capabilities for modeling charge generation and transport in diamond, to investigate the emission dependence on applied electric field in the range from approximately 2 MV/m to 17 MV/m along the [100] direction. The simulation results were compared to experimental data. For the considered parameter regime, conservation of transverse electron momentum (in the plane of the emission surface) allows direct emission from only two (parallel to [100]) of the six equivalent lowest conduction band valleys. When the electron affinity χ is the only parameter varied in the simulations, the val...

Carbon edge response of diamond devices

Near edge responsivity in diamond x-ray detectors has been used to confirm the carrier loss mechanism as recombination due to diffusion into the incident electrode. We present a detailed study of the bias dependence of the diamond responsivity across the carbon k-edge. The carrier loss is modelled by incorporating a characteristic recombination length into the absorption model and is shown to agree well with Monte Carlo simulated carrier losses. Using the high sensitivity to the x-ray absorption depth, the responsivity is converted to a near edge x-ray absorption fine structure pattern allowing easy identification of absorption mechanisms.

Simulations of Charge Gain and Collection Efficiency from Diamond Amplifiers

A promising new concept of a diamond amplified photocathode for generation of highcurrent, high-brightness, and low thermal emittance electron beams was recently proposed and is currently under active development. To better understand the different effects involved, we have been developing models, within the VORPAL computational framework, to simulate secondary electron generation and charge transport in diamond. The implemented models include inelastic scattering of electrons and holes for generation of electron-hole pairs, elastic, phonon, and charge impurity scattering. We will discuss these models and present results from 3D VORPAL simulations on charge gain and collection efficiency as a function of primary electron energy and applied electric field. The implemented modeling capabilities already allow us to investigate specific effects and compare simulation results with experimental data.

Petascale self-consistent electromagnetic computations using scalable and accurate algorithms for complex structures

As the size and cost of particle accelerators escalate, high-performance computing plays an increasingly important role; optimization through accurate, detailed computermodeling increases performance and reduces costs. But consequently, computer simulations face enormous challenges. Early approximation methods, such as expansions in distance from the design orbit, were unable to supply detailed accurate results, such as in the computation of wake fields in complex cavities. Since the advent of message-passing supercomputers with thousands of processors, earlier approximations are no longer necessary, and it is now possible to compute wake fields, the effects of dampers, and self-consistent dynamics in cavities accurately. In this environment, the focus has shifted towards the development and implementation of algorithms that scale to large numbers of processors. So-called charge-conserving algorithms evolve the electromagnetic fields without the need for any global solves (which are difficult to scale up to many processors). Using cut-cell (or embedded) boundaries, these algorithms can simulate the fields in complex accelerator cavities with curved walls. New implicit algorithms, which are stable for any time-step, conserve charge as well, allowing faster simulation of structures with details small compared to the characteristic wavelength. These algorithmic and computational advances have been implemented in the VORPAL7 Framework, a flexible, object-oriented, massively parallel computational application that allows run-time assembly of algorithms and objects, thus composing an application on the fly.

Half-Cell RF Gun Simulations with the Electromagnetic Particle-in-Cell Code VORPAL

We have simulated Brookhaven National Laboratory’s half‐cell superconducting RF gun design for a proposed high‐current ERL using the three‐dimensional, electromagnetic particle‐in‐cell code VORPAL. VORPAL computes the fully self‐consistent electromagnetic fields produced by the electron bunches, meaning that it accurately models space‐charge effects as well as bunch‐to‐bunch beam loading effects and the effects of higher‐order cavity modes, though these are beyond the scope of this paper. We compare results from VORPAL to the well‐established space‐charge code PARMELA, using RF fields produced by SUPERFISH, as a benchmarking exercise in which the two codes should agree well.