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Research at PBPL:

Beam physics research in our group can be divided into the functional areas of experiments; simulations; and, theory. In addition, this rich section of the website features information on the facilities we use to carry out our work, as well as some of the accelerator technologies we have innovated.

Perhaps a more revealing division of the research is by conceptual topics. Fundamentally, our work considers the following interactions:

Beam-Radiation Interaction

Charged particles (and hence beams) radiate whenever accelerated. At the cutting edge of particle beam physics, the beams are very intense, and have short pulses, ranging down to the femtosecond (10-15 s) level. Whatever the radiation mechanism, in our laboratory experiments the phenomena we are most concerned with are linked by the characteristic that the beam particles radiate together coherently, and thus the radiation can be enhanced by 8 or 9 orders of magnitude. In certain scenarios, coherent radiation is advantageous for the purposeful production of intense, monochromatic radiation -- free electron lasers are a good example. Likewise, the inverse free-electron laser (IFEL) is a mechanism by which we can use intense laser fields to accelerate charged particles at high gradient. Even the collision of intense electron beams with intense laser beams in vacuum may produce highly Doppler shifted radiation via the inverse Compton scattering mechanism; this scheme, along with the FEL, forms the basis of the generation of fs-to-ps x-ray light sources. In other cases, a beam's interaction with radiation is problematic, as is the case the coherent synchrotron radiation instability. This effect is ironically encountered when one attempts to create very short, intense electron beams that are used to drive FELs.

Other sections of this site relevant to the beam-radiation interaction include:

Beam-Matter Interaction

The interaction of intense charged particle beams with condensed matter is encountered in a variety of scenarios in our research. For example, the Cerenkov interaction is the basis of many time-dependent, picosecond-resolution diagnostics, and its inverse process gives rise to dielectric-based laser-powered accelerator schemes. Scintillating materials can be directly excited by relativistic beams, but their resolution can be limited by the presence of very large collective beam fields. The interaction of a beam with a metal boundary produces transition radiation; optical transition radiation enables micron-resolution beam profile monitors, while coherent transition radiation is used in our experiments to diagnose beam bunching at the 100 micron level, as well as microbunching at the sub-micron level.

Other sections of this site relevant to the beam-matter interaction include:

Beam-Plasma Interaction

Plasmas - in which matter is ionized, or "broken down" - are a natural environment in which to envision the future ultra-high gradient particle accelerator, as plasma waves have been observed to accelerate particles with fields over 100 GV/m. Such fields are three orders of magnitude higher than the breakdown limit of a state-of-the-art linear accelerator. The excitation of the plasma waves that support these fields can be accomplished by use of intense lasers (plasma beatwave acceleration, laser wakefield acceleration) or by injection of an intense electron beam into a plasma (plasma wakefield acceleration, or PWFA). Both varieties of acceleration are studied by the PBPL and its collaborators. Intense electron beams may additionally be focused under the influence of plasma-induced fields with strengths unequaled by other methods. A particularly promising regime of the PWFA was proposed in the PBPL, the "blow-out regime". This regime, where the electron beam is denser than the plasma, works by the production of a plasma-electron rarefied region. This region, while created by nonlinear plasma motion, contains quite linear focusing and acceleration fields. The nonlinearity of the plasma wave in the blow-out regime can be exploited to create "wave-breaking", which may give rise to trapping of plasma electrons, and a new form of super-high-brightness electron source.

Other sections of this site relevant to the beam-plasma interaction include:

Beam Self-fields (Space-charge)

The acceleration field of a beam is quite important at high energies, where synchrotron radiation and related effects may dominate the beam's collective effects. At low-to-moderate electron beam energies (<50 MeV), the self-interaction due to the beam's the velocity fields (or space-charge fields) is more important, however. Such repulsive self-fields may destroy the beam during its creation, as the advanced high brightness electron sources that the PBPL studies use beams created essentially from rest. The behavior of low-energy beams under space-charge forces can be understood as that of a one-component, relativistic plasma. This point-of-view allows one to appreciate how to remove the deleterious effects of space-charge from electron sources, and produce high brightness beams. It also gives insight into longitudinal oscillations that may filament the longitudinal phase space of the beam, an effect that is important especially in bending and compressing beams. These cold plasmas are notoriously difficult to diagnose, giving rise to special experimental techniques that must evade the effects of collective fields when sampling the beam.

Other sections of this site relevant to the subject of beam self-fields include:

Intense Beam Creation, Focusing and Bunching

All of our beam-based, high-energy density experiments require the creation, transport and manipulation of high brightness beams. These beams are often to very tight foci, to produce extremely high intensities. In the quest produce high brightness - low emittance, high current - beams, we have been at the forefront of the RF photoinjector development. The RF photoinjector liberates ps length beams by use of short pulse, intense lasers impinging on a photocathode that is embedded in a high gradient rf cavity, so the cold, ultra-short electron beam may be violently accelerated away before space-charge forces destroy it. The PBPL, as a component of its collaborations, has fabricated most of the high gradient S-band 1.6 cell rf guns in the world now. In addition, we also have led the way to developing new types of integrated gun-linac photoinjectors. The frontier of the beam source field is the prospect of using plasma waves to create ultra-short, phase-locked electron beams by trapping of the plasma electrons themselves, and PBPL is engaged in experimental measurement of a PWFA-based version of such a "plasma cathode".

Space-charge dominated beams are limited in their current at creation in the photoinjector, and therefore must be compressed for many applications, such as FEL and wakefield acceleration. At UCLA, we have studied both magnetic chicane compression, in which the beam is bent, and a variety of collective ills may set in, and a newer scheme termed velocity bunching. These investigations have produced beams that are as short as a few hundred fs, as measured using coherent transition radiation (CTR) interferometry. As we are now studying schemes where we need to shape the beams with better than this resolution, and more powerful measurement technique based on RF deflectors is being developed. In addition, we have used spectral techniques with CTR to deduce FEL microbunching down to 300 nm (attosecond) periodicity.

In the end, one must not only produce high brightness beams, but one must focus them. At PBPL we have developed the worlds strongest quadrupole magnets (560 T/m field gradients), that are deployed in a novel camera-like movable array, to focus the beam in an inverse Compton scattering experiment to 10s of micron spots. We also routinely encounter ultra-strong focusing fields in our beam-plasma experiments.

Other sections of this site relevant to the subjects of intense beam creation, focusing and bunching include:

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