Research

Several research areas our group is presently working on are summarized below. This work is done in a close collaboration with CLASSE physicists and other faculty members.

Also see my Publications »

Photoinjectors

PhotogunA critical parameter to describe beam's quality in many applications is its brightness: particle flux per unit area per unit solid angle. As a consequence of Liouville's theorem, beam brightness remains an invariant along the beamline if its transport is subject to linear optics (e.g. aberration-free lenses) with no dissipative mechanisms present. Such is the case of linear accelerators (linacs). Additionally, as energy (momentum) is imparted to the beam along a preferential (longitudinal) axis, beam divergence (and its solid angle) are further reduced in accordance with the ratio of beam momenta before and after the acceleration. This process is known in accelerator physics as adiabatic damping.

The beam quality out of a linac is set by its source. Brightest sources of relativistic electrons available today employ photoemission process. Such sources are also known as photoinjectors. The key component of a photoinjector is photogun (shown above in GIF animation), which houses a photocathode in a high accelerating gradient (10-100MV/m). The laser light with the necessary RF timing structure impinges on the photocathode generating a train of electron bunches, which can be further accelerated downstream in a linac.

Cornell ERL Photoinjector
Recently, our lab has successfully constructed a high-brightness photoinjector for energy recovery linac. This photoinjector will drive a much larger accelerator to produce coherent x-rays via synchrotron radiation. The exceptional properties of these x-rays would be largely determined by the photoinjector (shown above, beam moves right to left). Photoinjectors represent a high-impact technology that enables free-electron lasers, can revolutionize synchrotron sources, colliders, as well as small-scale practical devices.

It is also a young technology, with many unanswered questions and possibilities for improvement, actively pursued by the world's leading research institutions. Cornell ERL photoinjector R&D program is a testbed for developing new technical solutions and novel approaches to realize an ultimate brightness electron source. As of 2012, our photoinjector produces the world's highest average brightness and current beam of any photoemission-based accelerator source. Students play a key role in all aspects of design, construction, and operation of this unique facility.

Some parameters of Cornell ERL photoinjector

  • 100 mA max average current
  • 5-15 MeV beam energy
  • 77 pC/bunch at 1.3 GHz
  • 0.3 mm-mrad normalized rms emittance (ultimately 0.1 mm-mrad)
  • 2-3 ps bunch duration (can be compressed to 0.1 ps)

Beam Dynamics

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.

Virtual CathodeOne 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).

Feynman SupercomputerQuantitative 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.

Beam Instrumentation and Diagnostics

Beam instrumentation and diagnostics serve as 'eyes and ears' (and quite a bit more) in propelling a novel accelerator concept forward and turning the accelerator into a high-performance high-precision instrument. Generally speaking, beams can be characterized by a 6-dimensional phase space: two transverse positions, two transverse momenta, the energy and time. Beam diagnostics' goal is to access all 6 dimensions, preferably simultaneously and nondestructively (not always possible). Beam instrumentation is an intrinsically interdisciplinary area, rich in many exciting possibilities and challenging problems. Our group here draws on the expertise of the world leading facilities (mechanical and electric shops, mechanical engineering, vacuum technology) available at Cornell's Wilson and Newman laboratories. Below are some examples of beam instrumentation diagnostics equipment developed in our lab. Several of the key diagnostics listed here were designed and commissioned with active participation of students in our group.

Emittance Measurement SystemMeasured Transverse Phase SpaceEmittance Measurement System enables us to experimentally access transverse phase space distributions (seen on the right) of space charge dominated beams.

Deflecting CavityTime Domain Diagnostics that allows characterization of temporal electron bunch profile with a 0.1 Es resolution. It works on a principle similar to that of a streak camera using a transversely deflecting RF mode and imagining the beam on a downstream viewscreen. When combined with energy dispersive section (e.g. downstream of a dipole magnet spectrometer), it allows mapping of longitudinal phase space.

Flying Wire"Flying Wire" allows transverse profile characterization of a very high power electron beam (MW). It does so by passing a 20 micron carbon filament ("wire") through the beam at 20 m/s velocity (72 km/hr). The high beam power condition is so extreme that the filament can only go through the beam once every couple of minutes without experiencing permanent breakage. The signal comes from Bremsstrahlung caused by electrons deflected out of the beam by the carbon wire.

THz SpectrometerCoherent Synchrotron RadiationTHz Spectrometer accesses frequency domain of coherent synchrotron radiation. For light wavelengths larger than the bunch length, the radiation is coherently enhanced (power scales as a number of electrons squared as opposed to linearly). See the spectrum on the right (click to enlarge). Coherently enhanced part of the spectrum contains amplitudes of the Fourier transform of the bunch temporal distribution. This is an example of noninterceptive beam diagnostics.

Laser Techniques for High Brightness Beams

Lasers play a crucial role in photoinjectors as they determine the initial 3D distribution of electron bunches, their timing, and stability. Proper shaping of laser spatiotemporal intensity distribution allows dramatic reduction in uncorrelated space charge emittance growth, and is an actively pursued research subject in a number of groups worldwide.

Birefringent Crystals ShaperAs an example, in collaboration with Frank Wise's laser group we have demonstrated an efficient technique of laser pulse shaping using a set of birefringent crystals. Since then this approach has been adopted at several labs.

Nevertheless, the optimal laser shape is yet to be realized. Detailed space charge modeling shows that neither uniform cylinder nor 3D ellipsoidal distributions widely encountered in the literature are in fact optimal shapes for injectors in general and ours in particular.

Photocathode R&D

Photocathode R&D Lab in NewmanPhotocathode 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.


X-ray Science

Phase space of two interfering quantum statesCHESS is our own source of intense x-rays at Cornell University. Our group regularly uses its unique capabilities and closely collaborates with many CHESS scientists and faculty members.

There are several unique opportunities for research in this area starting from theoretical and computational work on representing partially coherent radiation in phase space to developing new experimental techniques that take advantage of enhanced properties of new synchrotron radiation light sources. Students gain experience working "behind the radiation walls", which is truly unique and at a high demand at National Laboratories and user facilities throughout the world. Both theoretically inclined PhD students and experimentalists should contact me if interested to further explore these opportunities.

Ultrafast Electron Diffraction

Ultrafast electron diffraction of a proteinBright 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.