My lab studies high brightness electron beams. We focus on the physics of beam creation and brightness preservation in linear and circular accelerators, for applications ranging from time resolved electron diffraction to high brightness synchrotron radiation sources. In practice, our work combines the use of detailed beam simulations with the design, construction, and operation of accelerator hardware with high resolution beam diagnostics.
A “bright” electron beam is one that is both dense and collimated, and in modern accelerators, brightness is an excellent figure of merit for an electron beam’s usefulness. This is true of both tabletop accelerators like electron microscopes as well as kilometer-scale accelerators like colliders or free electron lasers. While these machines are very different, common threads of brightness-limiting phenomena emerge among them: the beam’s collective self-interaction and induced nonlinear dynamics. Our group studies these effects in multiple accelerators on Cornell’s campus, including photoemission linear accelerators and the 5 GeV Cornell Electron Storage Ring (CESR).
While nonlinear fields are a practical necessity for modern storage rings, they lead to chaotic transverse particle motion and beam loss, and therefore must be precisely controlled. Currently we are exploring new diagnostic techniques to experimentally characterize the nonlinear guiding magnetic fields of CESR. If we force transverse beam oscillations at a particular location in CESR, we are able to reconstruct the beam’s resulting trajectory over the entire ring’s km-scale circumference, and over 10’s of thousands of beam revolutions, using CESR’s suite of beam position diagnostics. The harmonic content of this long-term trajectory data contains a wealth of information concerning the accelerator’s global Hamiltonian and on the local distribution of guiding magnetic fields. We anticipate that precision nonlinear diagnostic methods such as these will become crucial for ever brighter storage rings, which tend to require ever stronger nonlinearities.
In a related effort, my group also explores the physics of photoemission for accelerator electron sources, with an emphasis towards the development of ultrafast (picosecond and below) time-resolved electron diffraction and microscopy. For these applications, the beam’s brightness is fundamentally limited by the intrinsic momentum spread of the photoemission which generates the beam. The pursuit of “cold” (low divergence) photoemitters has the potential to dramatically improve spatiotemporal resolution in ultrafast electron scattering experiments, and to even to unlock coherent electron imaging modes previously only available to conventional electron microscopy. In practice, this effort involves the search for new photoemitting materials, an in-depth modeling of the photoemission process, and detailed photoemission experiments in Cornell’s multiple photoemission accelerators located in Newman Laboratory.
Our group works closely with the Center for Bright Beams, a new NSF science and technology led by Cornell. I am currently accepting new students.
J. Maxson, D. Cesar, G. Calmasini, A. Ody, P. Musumeci, D. Alesini, “Direct measurement of sub-10 fs relativistic electron beams with ultralow emittance,” Physical Review Letters 118 (15), 154802 (2017).
P. Musumeci, D. Cesar, J. Maxson, “Double-shot MeV electron diffraction and microscopy,” Structural Dynamics 4, 044025 (2017).
J. Maxson, P. Musumeci, L. Cultrera, S. Karkare, H. Padmore, “Ultrafast laser pulse heating of metallic photocathodes and its contribution to intrinsic emittance,” Nuclear Instruments and Methods in Physics Research Section A (2016).