Photocathode properties set the ultimate limit on beam brightness available from photoinjectors. In addition to having a very large impact on numerous practical applications, photocathode physics is an exciting field with many unanswered questions (despite the fact that much has been learned since Einstein won a Nobel prize for explaining the photoelectric effect).
Negative electron affinity photocathodes are particularly appealing for production of high brightness beams from photoinjectors because of their high quantum efficiency and low thermal emittance. Another key consideration when generating short (ps) electron bunches is whether the photoemission response time is sufficiently prompt. Our lab is well equipped experimentally to characterize the performance of these photocathodes (the picture shows a photocathode preparation setup). Our group has done a series of studies on several semiconductor photocathodes for high-brightness gun applications (GaAs, GaAsP, GaN). Additionally, we are now working on a theory that would allow one to compute most prominent parameters from basic principles. Such a theory, once developed and verified, could allow one to engineer a photocathode with the desired properties.
Recently, we've expanded our capabilities to include new photoemissive materials (antimony-based) as well as molecular epitaxially grown structures of III-V and other semiconductors. There is a significant momentum in this particular area of research with many "firsts" done in our lab just over the last couple of years (see the papers by our group for details). Presently our capabilities in photocathode research are greatly enhanced (with new diagnostics and material growth techniques being added sometimes on a monthly basis) and our dedicated lab space is now located in Newman and Phillips Hall buildings in addition to Wilson lab. Finally, my group is involved in building up a collaborative international effort of photocathodes for accelerators.
Ultrafast Electron Diffraction
Bright and fast electron bunches we create in our lab are an ideal probe to reconstruct 'chemical' movies - sets of pictures capturing how atoms rearrange themselves in the very early stages of a reaction triggered by a short laser pulse. This is a fast-paced area of research with an ultimate goal of obtaining ultrafast electron diffraction of large protein molecules.
The figure above illustrates some of the key features and challenges: (a) Protein structure of bovine rhodopsin and its excited state lumirhodopsin (in red, barely discernible on this scale). The first key steps of the activation are not understood and require a sub-picosecond structural probe. (b) Electron diffraction pattern computed to 5/Å from rhodopsin assuming a fs electron source with 20 nm-rad normalized emittance focused to a 100 μm spot on the sample. For reference, the protein unit cell is about 10×10×15 nm. (c) Electron diffraction pattern for the proposed electron source we are presently building with a 10× improvement. (d) The diffraction pattern difference between rhodopsin and lumirhodopsin illustrating that information about light-induced changes can be preserved with improved emittance.
Beam dynamics in accelerator physics is a theoretically rich area, which deals with instabilities and chaotic behavior of the beam, effects on the beam from the adjacent conducting walls, pipe nonuniformities, and accelerating structure (wakefields); it studies single particle motion and collective effects.
One collective effect dominating the dynamics of intense beams at low energy is space charge effect. The electron beam is born at essentially rest energy in photoinjector experiencing violent Coulomb repulsion, which may dominate over external fields of the design beam elements (magnets and accelerating structures). This leads to a highly nonlinear beam dynamics, a whole class of phenomena similar to those typical of nonequilibrium plasmas.
One example of such nonlinearity (known as virtual cathode instability) is shown on the left (GIF animation). As laser intensity is increased for otherwise identical laser pulse shape (40 ps square time profile) incident on a photocathode placed in a constant accelerating gradient (3 MV/m), the space charge becomes so strong near the cathode that the accelerating electric field there is quenched and the beam breaks apart into two or more components. The temporal profile of electron bunches after emission is shown on the right. It is seen that the electron beam does not follow the laser at higher bunch charges (plotted on the left) developing instead deep modulations as if the cathode were being turned on and off (thus, the name).
Quantitative analysis of nonlinear behavior oftentimes requires intensive computations. Our group utilizes a 200-CPU computer cluster (affably known as Feynman) fully dedicated to our beam dynamics simulations. We use many advanced computational methods to explore nonlinear beam dynamics including some unorthodox ones such as parallel multi-objective genetic algorithms, the use of which was pioneered in our group.
Last, but not least, we operate the world's brightest photoinjector at Cornell. The photoinjector accelerates ultra low emittance beams from a photocathode to about 10 million electron-volts, and is fully intrumented to study frontier phenomena in beam physics. It is an ideal testbed for PhD students with interest in accelerators and bright beams.