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.
Beam Production
Electrons used in linear accelerators are generated via materials known as cathodes. At PBPL we study emission physics from a theoretical perspective in order to study cathode behavior in order to improve beam brightness, current, and other figures of merit.
Advanced RF Acceleration
Traditional linear accelerators use resonant modes in conducting copper cavities at radiofrequency (RF) frequencies to accelerate electrons to high energies. At PBPL we study advanced RF structures to increase accelerating gradients and improve beam figures of merit.PBPL is experienced with the design and fabrication of RF cavity structures, in particular S-band photoinjector guns but also several models of plane-wave transformer linac and a deflection cavity under development. Standard electromagnetic codes (such as SUPERFISH and HFSS) are used to refine the designs, and the availability of advanced machining capability at the Physics Department Machine Shop allows the PBPL to manufacture its own components. The PBPL group was central to the design of the now-standard 1.6 cell 2856-MHz RF gun, in use at BNL-ATF and SLAC as well as the UCLA Neptune lab, and the electrical performance of these guns continues to be improved via cathode development and increased resistance to breakdown.
Beam Manipulation
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.
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.
Beam-Radiation Interaction
Charged particles radiate whenever accelerated by a variety of mechanisms, among them synchrotron (and undulator) radiation, transition radiation, Cerenkov radiation and new effects in plasma, such as betatron radiation. PBPL research is heavily concentrated on the development and use of this challenging systems as they grow from physical concepts to transformational instruments in application. At the cutting edge of particle beam physics, the beams are very intense, and have short pulses, ranging down to the femtosecond 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 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. Conversely, 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 – photons up to the MeV energy level – via the inverse Compton scattering mechanism. This scheme, along with the FEL, forms the basis of new generations of fs-to-ps x-ray light sources. At the PBPL we are uniquely engaged in exploring the marriage of advanced, high field accelerators and light sources with game-changing characteristics.
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.