Modelling of dielectric laser accelerators using large scale plasma simulations code

Electron dynamics in grating-type dielectric laser accelerators: Particle transfer function, generalized acceleration/deflection gradients and Panofsky–Wenzel theorem

Dielectric laser accelerators (DLAs) have great potential for applications, since they can generate acceleration gradients in the range of GeV/m and produce attosecond electron bunches. We described a novel reverse dual-grating dielectric accelerator structure made up of Silicon which is expected to improve beam confinement, and make fabrication easier. Numerical simulation results show that this structure effectively manipulates the laser field and generates a standing wave in the vacuum channel with a phase velocity synchronized to relativistic particles travelling through the structure. Optimum pillar height and channel width have been determined. All required laser parameters and initial particle energy have been analytically estimated and a suitable laser as an energy source is proposed. Finally, the effect of fabrication error on the acceleration gradient is discussed.

We discuss recent developments and challenges of beam dynamics in Dielectric Laser Acceleration (DLA), for both high and low energy electron beams. Starting from ultra-low emittance nanotip sources the paper follows the beam path of a tentative DLA light source concept. Acceleration in conjuction with focusing is discussed in the framework of Alternating Phase Focusing (APF) and spatial harmonic ponderomotive focusing. The paper concludes with an outlook to the beam dynamics in laser driven nanophotonic undulators, based on tilted DLA grating structures.

AWE is developing high performance (1000R at 1 m, <2 mm spot size) flash X-ray sources to radiograph explosively driven heavy metal experiments. Intense electron beam diodes are being developed to deliver this capability from pulsed power drivers in a new hydrodynamics research facility known as the CPF [1]. The Large Scale Plasma (LSP) Particle In Cell (PIC) code [2] is being used extensively at AWE in the program to develop improved intense electron beam diodes. LSP does not currently incorporate direct methods to calculate radiographic parameters relevant to AWE such as the dose or spot size associated with a radiographic diode. There is therefore a need for methods that allow retrieval of experimentally measurable radiographic quantities from LSP simulations of radiographic diodes [3]. Two approaches are currently employed at AWE to retrieve such data. Firstly the primary output from LSP may be ported to a suitable Monte Carlo transport code such as MCNP [4]. MCNP can tally the expected Bremsstrahlung radiation from diodes and transport this radiation through simulations of experimental radiographic experiments to infer the spot size and dose from the diode. Secondly the LSP target methods may be used to gain information on the fluence and angular distributions of the electrons at the high atomic number target which may be further processed into source intensity distribution information. These methods are outlined and examples given, the methods have been found to agree with each other and with experiment to around 10%. The target method has been extended to 3D simulations and initial results from these methods are presented.

In dielectric laser acceleration, nanostructures etched into silicon are used to convert free-space ultrashort laser pulses, incident from the side and parallel to the wafer substrate, to accelerate particles. This current approach is experimentally challenging and, as it turns out, not quite necessary for most experiments and practical applications. Here, we experimentally demonstrate and numerically verify the efficacy of top-illuminated structures, and measure a maximum acceleration gradient of 49.2 ± 3.1 MeV/m. We discuss how, in practice, this approach proves superior to the current standard in the field, and expect it to become the definitive choice for nanophotonic particle laser acceleration.

Dielectric laser accelerator (DLA) plays a very important role in the development of on-chip particle accelerators, which has aroused the interest of many researchers. In all previous DLA schemes, each laser pulse can only be used once, which greatly limits the efficiency of the driving laser, as well as the acceleration length and energy gain of particles. Here we propose a DLA scheme based on the inverse cherenkov effect, which uses a pair of laser pulses to accelerate particles in multiple stages. The laser pulses are reflected and refracted multiple times, synchronizing with and accelerating the electron beam in each acceleration stage, just like a traveling wave acceleration in traditional rf accelerators. By this means, the acceleration efficiency can be greatly enhanced, which paves a way of developing high efficiency and high energy-gain particle accelerator on chip.

Electron sources play a crucial role in electron microscopes, particle accelerators and novel light sources. In most cases, the electron source parameters determine the downstream parameters because it is non-trivial to compress or cool electron pulses. For many applications requiring transversely coherent electrons, needle tips have been the preferred electron source for decades, well-known in high-resolution electron microscopy. Recently, these sources have found significant interest in ultrafast applications, where electron pulses are emitted through the illumination of a needle tip with femtosecond laser pulses. We will give an overview of recent developments in ultrafast needle tip-based electron sources. In addition, we will show how the interaction of laser pulses with electron pulses downstream of the source element can help to shape electron beams with highly intriguing properties, such as attosecond pulses, high-energy beams or electron pulses with small energy widths. The most promising approach to accelerate and shape the electron pulses is based on near-field techniques, which, depending on the parameter range, can either be classified as dielectric laser acceleration or photon-induced near-field electron microscopy physics. Finally, we will provide details of the state-of-the-art in two-photon or ponderomotive schemes for temporal electron pulse shaping, before concluding this chapter with a brief review of applications and future outlook.

The IADI causes anomalous absorption of laser light with a moderate intensity laser in a large scale plasma. Because of the weak density modification due to the moderate intensity laser, the instability width can be large. The anomalous attenuation length of the laser can be much smaller than the instability width along the density gradient, indicating that most of laser energy may be anomalously absorbed by the IADI. An instability width of only a few laser wavelengths is sufficient to absorb most of the laser energy. Experimental results are consistent with the large scale plasma and the strong absorption.

The analysis of a Ka-band gyrotron Travelling Wave Tube (gyro-TWT) amplifier using a uniform cylindrical waveguide as its interaction circuit has been presented for the TE01 mode of operation a self-consistent nonlinear analysis in the large signal regime. The analysis predicts that the saturated peak output power of ~ 134 kW with a power conversion efficiency of ~ 22.7 %. The saturated gain has been calculated as ~ 41.3 dB for the amplifier driven by 72 kV, 8.2 A electron beam of a pitch factor 1.05. The critical interaction length of smooth wall uniform metal guide is found 10.7 cm for the stable amplifier operation. The form factor and the norm factor have also been estimated with the large signal non-linear code. Further, for 5% spread the amplifier develops the peak power of ~ 128 kW with an electronic efficiency of ~ 21.7 % and the gain of ~ 41.07 dB. These results are found to be good harmony when the beam wave interaction is studied with a commercial 3-D electromagnetic particle in cell (PIC) code. The behaviour of the beam all over the length of the interaction circuit has been monitored by calculating its energy, momentum, phase, etc. with the help of a commercial PIC code and which also in good agreement with analytical results with 1% deviation.

The Plasma Wakefield Accelerator (PWFA) is an advanced accelerator concept which possess a high acceleration gradient and a long interaction length for accelerating both electrons and positrons. Although electron beam-plasma interactions have been extensively studied in connection with the PWFA, very little work has been done with respect to positron beam-plasma interactions. This dissertation addresses three issues relating to a positron beam driven plasma wakefield accelerator. These issues are (a) the suitability of employing a positron drive bunch to excite a wake; (b) the transverse stability of the drive bunch; and (c) the acceleration of positrons by the plasma wake that is driven by a positron bunch. These three issues are explored first through computer simulations and then through experiments. First, a theory is developed on the impulse response of plasma to a short drive beam which is valid for small perturbations to the plasma density. This is followed up with several particle-in-cell (PIC) simulations which study the experimental parameter (bunch length, charge, radius, and plasma density) range. Next, the experimental setup is described with an emphasis on the equipment used to measure the longitudinal energy variations of the positron beam. Then, the transverse dynamics of a positron beam in a plasma are described. Special attention is given to the way focusing, defocusing, and a tilted beam would appear to be energy variations as viewed on our diagnostics. Finally, the energy dynamics imparted on a 730 {micro}m long, 40 {micro}m radius, 28.5 GeV positron beam with 1.2 x 10{sup 10} particles in a 1.4 meter long 0-2 x 10{sup 14} e{sup -}/cm{sup 3} plasma is described. First the energy loss was measured as a function of plasma density and the measurements are compared to theory. Then, an energy gain of 79 {+-} 15 MeV is shown. This is the first demonstration of energy gain of a positron beam in a plasma and it is in good agreement with the predictions made by the 3-D PIC code. The work presented in this dissertation will show that plasma wakefield accelerators are an attractive technology for future particle accelerators.

A new type of electron beam source is proposed that combines a DC field and AC field from a laser beam in between the anode-cathode gap. Prior researchers have demonstrated the ability for this configuration to produce ultrashort electron microbunches from field emission when the AC field adds constructively with the DC field. Our new source improves upon this basic scheme by creating mono-energetic electrons with low emittance that are suitable for feeding into dielectric and vacuum laser accelerators, photonic bandgap accelerators, and dielectric wakefield accelerators. The microbunches will have a length that is a fraction of the laser wavelength, corresponding to fs pulse durations. They are also produced as a train of microbunches separated by the period of the laser length and lasting for the duration of the laser pulse.

Summary form only given. New options for compact accelerators of intense pulsed electron beams are being studied. Low-frequency RF accelerators have promise for this application. In low-frequency accelerators, it is possible to eliminate beam focusing elements like solenoids by using shaped accelerating electrodes to define quadrupole field components. Simplification of the beamline opens options for compact pulsed acceleration structures. The quadrupole geometries that have been investigated are the quasi-static equivalent of the RFQ and are closely related to recent quadrupole-enhanced ion accelerator columns. A new 3-D electrostatic computer code, E3, has been developed to analyze fields in acceleration structures and to determine limits on electron transport. Evaluation of focusing strength and field stress in quadrupole structures for multi-kA electron beams has been carried out.

Significant progress has been made in recent years using lasers for electron acceleration to high energies in plasmas. The main idea of such laser-plasma schemes is based on an opportunity to use the lasers for generation a large amplitude regular plasma wave with strong longitudinal electric field and relativistic phase velocity, which is capable of acceleration injected electrons. The great interest to these accelerators is due to their ability to sustain extremely large acceleration gradients (&lt;100 GeV/m) essentially exceeding the acceleration gradients of conventional radio-frequency linear accelerators (&gt;100 MeV/m). At the first stage, the main attention was attracted to plasma beat wave accelerator scheme (PBWA) where two-frequency laser radiation was used to create a plasma wave. With development of chirped pulse amplification (CPA) and powerful laser systems, the new methods for excitation of large amplitude relativistic plasma waves appear as the laser wake-field accelerator (LWFA) and self-modulated laser wake-field accelerator (SM-LWFA). This report is intended to give a brief overview of up to date results obtained for different schemes of laser-plasma based accelerators. The problems and plans of future investigations are also discussed.

High voltage insulation is one of the main areas of pulsed power research and development, and dielectric breakdown is usually the limiting factor in attaining the highest possible performance in pulsed power devices. For many applications the delivery of pulsed power into a vacuum region is the most critical aspect of operation. The surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This mode of breakdown, called surface flashover, imposes serious limitations on the power flow into a vacuum region. This is especially troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and for applications where relatively long pulses, on the order of several microseconds, are required. The goal of this project is to establish a sound fundamental understanding of the mechanisms that lead to surface flashover, and then evaluate the most promising techniques to improve vacuum insulators and enable high voltage operation at stress levels near the intrinsic bulk breakdown limits of the material. The approach we proposed and followed was to develop this understanding through a combination of theoretical and computation methods coupled with experiments to validate and quantify expected behaviors. In this report we summarize our modeling and simulation efforts, theoretical studies, and experimental investigations. The computational work began by exploring the limits of commercially available codes and demonstrating methods to examine field enhancements and defect mechanisms at microscopic levels. Plasma simulations with particle codes used in conjunction with circuit models of the experimental apparatus enabled comparisons with experimental measurements. The large scale plasma (LSP) particle-in-cell (PIC) code was run on multiprocessor platforms and used to simulate expanding plasma conditions in vacuum gap regions. Algorithms were incorporated into LSP to handle secondary electron emission from dielectric materials to enable detailed simulations of flashover phenomenon. Theoretical studies were focused on explaining a possible mechanism for anode initiated surface flashover that involves an electron avalanche process starting near the anode, not a mechanism involving bulk dielectric breakdown. Experiments were performed in Engineering's Pulsed Power Lab using an available 100-kV, 10-{micro}s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45{sup o} angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.

A theoretical study is made on the generation mechanism of Langmuir mode wave in the presence of kinetic Alfven wave turbulence in a magnetized plasma on the basis of plasma-maser interaction. It is shown that a test high frequency Langmuir mode wave is unstable in the presence of low frequency kinetic Alfven wave turbulence. The growth of the Langmuir wave occurs due to direct and polarization coupling terms. Because of the universal existence of the kinetic Alfven waves in large scale plasmas, the results have potential importance in space and astrophysical radiation processes.