Observation of Skewed Electromagnetic Wakefields in an Asymmetric Structure Driven by Flat Electron Bunches

Relativistic charged-particle beams which generate intense longitudinal fields in accelerating structures also inherently couple to transverse modes. The effects of this coupling may lead to beam break-up instability, and thus must be countered to preserve beam quality in applications such as linear colliders. Beams with highly asymmetric transverse sizes (flat-beams) have been shown to suppress the initial instability in slab-symmetric structures. However, as the coupling to transverse modes remains, this solution serves only to delay instability. In order to understand the hazards of transverse coupling in such a case, we describe here an experiment characterizing the transverse effects on a flat-beam, traversing near a planar dielectric lined structure. The measurements reveal the emergence of a previously unobserved skew-quadrupole-like interaction when the beam is canted transversely, which is not present when the flat-beam travels parallel to the dielectric surface. We deploy a multipole field fitting algorithm to reconstruct the projected transverse wakefields from the data. We generate the effective kick vector map using a simple two-particle theoretical model, with particle-in-cell simulations used to provide further insight for realistic particle distributions.Comment: Six pages, seven figures. Submitted to Physical Revie

Adiabatic plasma lens experiments at SPARC

Abstract Passive plasma lenses in the underdense regime have been shown to give extremely strong linear focusing, with strength proportional to the local plasma ion density. This technique has been proposed as the basis of a scheme for future linear colliders that mitigates the Oide effect through adiabatic focusing. In this scenario the plasma density in the lens is ramped slowly on the scale of betatron motion, to funnel the beam to its final focus while forgiving chromatic aberrations. We present to the physics design of an adiabatic plasma lens experiment to be performed at SPARC Lab. We illustrate the self-consistent plasma response and associated beam optics for symmetric beams in plasma, simulated by QuickPIC using exponentially rising density profiles. We discuss experimental plans including plasma source development and betatron-radiation-based beam diagnostics

Plasma Deflection Interrupter for Pulsed Power Applications

A plasma-based opening switch, capable of handling kiloampere currents at kilovolt potentials on the microsecond timescale, is described and characterized. The principle of operation is the deflection of a laser-induced arc by an external magnetic field to a shunt circuit path. A first-order model of operation is introduced. Finally, the merits of the device, particularly when applied to high-repetition-rate dense plasma focus (DPF) systems, are discussed

Enabling a Laser Plasma Accelerator Driven Free Electron Laser

The free electron laser (FEL) is the brightest available source of x-rays, surpassing other options by more than ten orders of magnitude. The FEL's short ($\sim$femtosecond), high power ($\sim$gigawatt), coherent x-ray pulses are uniquely capable of probing ultrafast and ultrasmall atomic and molecular dynamics and structure, making them an invaluable research tool for biology, chemistry, material science, physics, medicine, and other fields. Unfortunately, all extant x-ray FELs rely on long rf linacs and undulators, with a footprint of kilometers and a cost on the order of a billion dollars. This severely limits the number of x-ray FELs, with the half dozen existing installations funded at the nation state level. These facilities are significantly oversubscribed, to the detriment of scientific and technological progress. Therefore, attempts to reduce the size and cost of FELs are an active area of research in an effort to increase access to these powerful research tools, with the goal of making x-ray FELs affordable to universities and companies. One of the approaches being researched is the laser plasma accelerator (LPA). The LPA uses an ultra-high intensity laser to eject plasma electrons from a bubble region, producing longitudinal accelerating fields more than three orders of magnitude higher than what can be achieved in an rf linac. In principle, this could shrink the FEL accelerating section from the kilometer scale to a tabletop. To date though, despite continual progress and refinement over the last decade, LPA beam quality has not yet reached the level where it can be directly used as an FEL driver due to stringent constraints on the lasing dynamics.The BELLA FEL experiment at Lawrence Berkeley National Lab intends to decompress the beam to skirt some of the beam quality requirements, by stretching the beam longitudinally and reducing local energy spread. This dissertation will discuss the design and implementation of two subsystems essential for the successful operation of this experiment. The first of these is a coherent transition radiation bunch length diagnostic, which is required to measure the length of the LPA bunches and extrapolate other details about the experiment's performance. The second is an electromagnetic chicane which performs the decompression of the electron beam. A final chapter explores the use of advanced undulators to enable the next generation of LPA driven FELs without decompression and discusses methods for realizing such undulators

Simultaneous Ultra-Fast Imaging and Neutron Emission from a Compact Dense Plasma Focus Fusion Device

Recently, there has been intense interest in small dense plasma focus (DPF) devices for use as pulsed neutron and X-ray sources. Although DPFs have been studied for decades and scaling laws for neutron yield versus system discharge current and energy have been established (Milanese, M. et al., Eur. Phys. J. D 2003, 27, 77–81), there are notable deviations at low energies due to contributions from both thermonuclear and beam-target interactions (Schmidt, A. et al., Phys. Rev. Lett. 2012, 109, 1–4). For low energy DPFs (100 s of Joules), other empirical scaling laws have been found (Bures, B.L. et al., Phys. Plasmas 2012, 112702, 1–9). Although theoretical mechanisms to explain this change have been proposed, the cause of this reduced efficiency is not well understood. A new apparatus with advanced diagnostic capabilities allows us to probe this regime, including variants in which a piston gas is employed. Several complementary diagnostics of the pinch dynamics and resulting X-ray neutron production are employed to understand the underlying mechanisms involved. This apparatus is unique in its employment of a 50 fs laser-based shadowgraphy system that possesses unprecedented spatio-temporal resolution

Enabling a Laser Plasma Accelerator Driven Free Electron Laser

The free electron laser (FEL) is the brightest available source of x-rays, surpassing other options by more than ten orders of magnitude. The FEL's short ($\sim$femtosecond), high power ($\sim$gigawatt), coherent x-ray pulses are uniquely capable of probing ultrafast and ultrasmall atomic and molecular dynamics and structure, making them an invaluable research tool for biology, chemistry, material science, physics, medicine, and other fields. Unfortunately, all extant x-ray FELs rely on long rf linacs and undulators, with a footprint of kilometers and a cost on the order of a billion dollars. This severely limits the number of x-ray FELs, with the half dozen existing installations funded at the nation state level. These facilities are significantly oversubscribed, to the detriment of scientific and technological progress. Therefore, attempts to reduce the size and cost of FELs are an active area of research in an effort to increase access to these powerful research tools, with the goal of making x-ray FELs affordable to universities and companies. One of the approaches being researched is the laser plasma accelerator (LPA). The LPA uses an ultra-high intensity laser to eject plasma electrons from a bubble region, producing longitudinal accelerating fields more than three orders of magnitude higher than what can be achieved in an rf linac. In principle, this could shrink the FEL accelerating section from the kilometer scale to a tabletop. To date though, despite continual progress and refinement over the last decade, LPA beam quality has not yet reached the level where it can be directly used as an FEL driver due to stringent constraints on the lasing dynamics.The BELLA FEL experiment at Lawrence Berkeley National Lab intends to decompress the beam to skirt some of the beam quality requirements, by stretching the beam longitudinally and reducing local energy spread. This dissertation will discuss the design and implementation of two subsystems essential for the successful operation of this experiment. The first of these is a coherent transition radiation bunch length diagnostic, which is required to measure the length of the LPA bunches and extrapolate other details about the experiment's performance. The second is an electromagnetic chicane which performs the decompression of the electron beam. A final chapter explores the use of advanced undulators to enable the next generation of LPA driven FELs without decompression and discusses methods for realizing such undulators

Design of Comb Fabricated Halbach Undulators

An approach to fabricating Halbach array undulators using “combs” machined from single magnets is introduced. This technique is especially relevant to the fabrication of short period micro-undulators with period lengths considerably less than the few-centimeter-scale typical of current undulators. Manual, magnet-by-magnet assembly of micro-undulators would require the manipulation and alignment of thousands of magnets smaller than a grain of rice: comb fabrication dramatically increases the size of the basic unit cell of assembly with no increase in undulator period by creating many periods from a single piece, in a single machining modality. Further, as these comb teeth are intrinsically indexed to each other, tolerances are dictated by a single manufacturing step rather than accumulating errors by assembling many tiny magnets relative to each other. Different Halbach geometries, including M ′ = 2 , M ′ = 4 , isosceles triangle, and hybrid, are examined both from a theoretical perspective and with 3D magnetostatic simulations

Simultaneous Ultra-Fast Imaging and Neutron Emission from a Compact Dense Plasma Focus Fusion Device

Recently, there has been intense interest in small dense plasma focus (DPF) devices for use as pulsed neutron and X-ray sources. Although DPFs have been studied for decades and scaling laws for neutron yield versus system discharge current and energy have been established (Milanese, M. et al., Eur. Phys. J. D 2003, 27, 77–81), there are notable deviations at low energies due to contributions from both thermonuclear and beam-target interactions (Schmidt, A. et al., Phys. Rev. Lett. 2012, 109, 1–4). For low energy DPFs (100 s of Joules), other empirical scaling laws have been found (Bures, B.L. et al., Phys. Plasmas 2012, 112702, 1–9). Although theoretical mechanisms to explain this change have been proposed, the cause of this reduced efficiency is not well understood. A new apparatus with advanced diagnostic capabilities allows us to probe this regime, including variants in which a piston gas is employed. Several complementary diagnostics of the pinch dynamics and resulting X-ray neutron production are employed to understand the underlying mechanisms involved. This apparatus is unique in its employment of a 50 fs laser-based shadowgraphy system that possesses unprecedented spatio-temporal resolution

Externally Heated Hollow Cathode Arc Plasma Source for Experiments in Plasma Wakefield Acceleration

An externally heated, hollow cathode arc source was recommissioned at UCLA for use in experiments to drive plasma wakefields with shaped beams at the Argonne Wakefield Accelerator. The hollow cathode arc source provides a robust plasma column with a density in the 10 13 – 10 14 cm − 3 range while external heating of the cathode allows the plasma arc regime to be accessed with applied voltages down to 20 V. Overall source operating principals are described, along with time-resolved plasma current measurements and plasma density characterization with the use of a triple Langumir probe. The results show that relevant plasma densities that match facility beam parameters are readily achievable

Start-to-End Beam-Dynamics Simulations of a Compact C-Band Electron Beam Source for High Spectral Brilliance Applications

Proposals for new linear accelerator-based facilities are flourishing world-wide with the aim of high spectral brilliance radiation sources. Most of these accelerators are based on electron beams, with a variety of applications in industry, research and medicine such as colliders, free-electron lasers, wake-field accelerators, coherent THz and inverse Compton scattering X/’ sources as well as high-resolution diagnostics tools in biomedical science. In order to obtain high-quality electron beams in a small footprint, we present the optimization design of a C-band linear accelerator machine. Driven by a novel compact C-band hybrid photoinjector, it will yield ultra-short electron bunches of few 100’s pC directly from injection with ultra-low emittance, fraction of mm-mrad, and a few hundred fs length simultaneously, therefore satisfying full 6D emittance compensation. The normal-conducting linacs are based on a novel high-efficiency design with gradients up to 50 MV/m. The beam maximum energy can be easily adjusted in the mid-GeV’s range. In this paper, we discuss the start-to-end beam-dynamics simulations in details