Gregory Penn

Slowly varying envelope kinetic simulations of pulse amplification by Raman backscattering

A numerical code based on an eikonal formalism has been developed to simulate laser-plasma interactions, specifically Raman backscatter (RBS). In this code, the dominant laser modes are described by their wave envelopes, avoiding the need to resolve the laser frequency; appropriately time-averaged equations describe particle motion. The code is fully kinetic, and thus includes critical physics such as particle trapping and Landau damping which are beyond the scope of the commonly used fluid three-wave equations. The dominant forces on the particles are included: the ponderomotive force resulting from the beat wave of the forward and backscattered laser fields and the self-consistent plasma electric field. The code agrees well, in the appropriate regimes, with the results from three-wave equations and particle-in-cell simulations. The effects of plasma temperature on RBS amplification are studied. It is found that increasing the plasma temperature results in modification to particle trapping and the satura...

Design Studies for a VUV–Soft X-ray Free-Electron Laser Array

Several recent reports have identified the scientific requirements for a future soft X-ray light source [1, 2, 3, 4, 5], and a high-repetition-rate free-electron laser (FEL) facility responsive to them is being studied at Lawrence Berkeley National Laboratory (LBNL) [6]. The facility is based on a continuous-wave (CW) superconducting linear accelerator with beam supplied by a high-brightness, high-repetition-rate photocathode electron gun operating in CW mode, and on an array of FELs to which the accelerated beam is distributed, each operating at high repetition rate and with even pulse spacing. Dependent on the experimental requirements, the individual FELs may be configured for either self-amplified spontaneous emission (SASE), seeded high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG), or oscillator mode of operation, and will produce high peak and average brightness X-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds. This new light source would serve a broad community of scientists in many areas of research, similar to existing utilization of storage ring based light sources.

Design Studies for a High-Repetition-Rate FEL Facility at LBNL

Lawrence Berkeley National Laboratory (LBNL) is working to address the needs of the primary scientific Grand Challenges now being considered by the U.S. Department of Energy, Office of Basic Energy Sciences: we are exploring scientific discovery opportunities, and new areas of science, to be unlocked with the use of advanced photon sources. A partnership of several divisions at LBNL is working to define the science and instruments needed in the future. To meet these needs, we propose a seeded, high-repetition-rate, free-electron laser (FEL) facility. Temporally and spatially coherent photon pulses, of controlled duration ranging from picosecond to sub-femtosecond, are within reach in the vacuum ultraviolet (VUV) to soft X-ray regime, and LBNL is developing critical accelerator physics and technologies toward this goal. We envision a facility with an array of FELs, each independently configurable and tunable, providing a range of photon-beam properties with high average and peak flux and brightness.

Beam Conditioning for FELs: Consequences and Methods

The consequences of beam conditioning in four example cases (VISA, a Soft X-Ray FEL, LCLS and a Greenfield FEL) are examined. It is shown that in emittance limited cases, proper conditioning reduces sensitivity to the transverse emittance, and allows stronger focusing in the undulator. Simulations show higher saturation power, with gain lengths reduced up to a factor of two. The beam dynamics in a general conditioning system are studied, with matching conditions derived for achieving conditioning without growth in effective emittance. Various conditioners are considered, and expressions derived for the amount of conditioning provided in each case when the matching conditions are satisfied. We discuss the prospects for conditioners based on laser and plasma systems.

Transition and Diffraction Radiation

Transition radiation occurs when a moving charged particle crosses a boundary between two media with different electrodynamic properties. In its simplest form, which is commonly used in experiments, transition radiation is generated by sending a beam through a metallic foil. In this chapter, we will derive the spectrum and angular distribution of the transition radiation when a particle crosses a foil at normal incidence. We will also discuss radiation generated by the beam when it passes through a hole in a metal foil — the so-called diffraction radiation.

Formation Length of Radiation and Coherent Effects

The radiation process is not instantaneous — it requires some time and free space around the orbit for the radiation to be formed. In this chapter we estimate the longitudinal extent and transverse size of the free space volume needed for the synchrotron radiation. We then analyze the radiation of a bunch of many particles.

Linear and Nonlinear Oscillators

The linear oscillator is a simple model that lies at the foundation of many physical phenomena and plays a crucial role in accelerator dynamics. Many systems can be viewed as an approximation to a set of independent linear oscillators. In this chapter, we will review the main properties of the linear oscillator including its response to resonant excitations, slowly varying forces, random kicks, and parametric variation of the frequency. We will discuss the impact of damping terms as well as how small, nonlinear terms in the oscillator equation modify the oscillator frequency and lead to nonlinear resonance.

Self Field of a Relativistic Beam

The electromagnetic field generated by a high-current, relativistic beam of charged particles plays an important role in beam dynamics. On the one hand, the force exerted on the beam by this field can lead to beam instabilities and a deterioration of its properties in the process of beam generation, acceleration and transport. On the other hand, this field induces currents and charges in the beam environment that can be used for diagnostic purposes. Hence calculation of this field and the forces associated with it constitutes an essential part of beam physics for accelerators.

The Basic Formulation of Mechanics: Lagrangian and Hamiltonian Equations of Motion

The Lagrangian and Hamiltonian formalisms are among the most powerful ways to analyze dynamic systems. In this chapter we will introduce Lagrange’s equations of motion and discuss the transition from Lagrange’s to Hamilton’s equations. We write down the Lagrangian and Hamiltonian for a charged particle in an electromagnetic field, and introduce the Poisson bracket.

Plane Electromagnetic Waves and Gaussian Beams

Plane electromagnetic waves are solutions of the Maxwell equations that are unbounded in the plane perpendicular to the direction of propagation. They approximately describe local properties of the real field far from the source of radiation. They can also be used as building blocks from which a general solution of Maxwell’s equations in free space can be constructed. An important practical example of electromagnetic radiation that finds many applications in accelerator physics and elsewhere is a focused laser beam. The distribution of the electromagnetic field in such light is characterized by Gaussian modes which can be considered as a superposition of plane waves propagating at small angles to the direction of the beam. Gaussian beams correctly describe the field structure near the focus and the diffraction of the beam as it propagates away from the focal region. In this chapter, we will briefly summarize the main properties of plane electromagnetic waves, and then derive the field in a Gaussian beam.