Chad Mitchell
Chad Mitchell, a staff scientist in ATAP’s Accelerator Modeling Program, has been honored with the Berkeley Lab Director’s Award in the category of Scientific Achievement.
He leads a team that developed novel beam dynamics methods to significantly extend the reach of future intensity-frontier particle accelerators for high energy physics discovery science, as well as for practical applications of high-intensity (multi-megawatt) proton beams.
To meet the challenges of accelerator facilities at the intensity frontier, it is essential to investigate novel and reliable strategies for controlling space-charge-related beam loss in intense hadron beams. A strategy that stands out for its innovation and far-reaching impact on accelerator design uses nonlinear integrable lattices that introduce large beam tune spread to suppress coherent instabilities that drive the development of beam halo. The team contributed important theoretical and computational tools needed to understand the beam dynamics in accelerators based on this relatively new concept.
Frontiers in the performance of particle accelerators include not only the energy of their beams, but also the intensity. Both discovery science (e.g., neutrino experiments in high energy physics, and the many applications of beams from spallation neutron sources) and practical applications in nuclear energy are calling for megawatt-plus proton beams. This requires beam intensities that pose dynamics difficulties for the “optics” — the lattice of magnets that bend and shape the beam — used in present-day accelerators.
The proposed solutions include a radical change of paradigm: from the linear focusing beam optics (used in all present circular particle accelerators) to nonlinear “integrable” beam optics. The integrability ensures that particle trajectories are bounded, while the nonlinearity can be tuned to suppress the effect of resonances and the resulting development of beam halo, both of which plague existing accelerators.
The new concept was put forth by V. Danilov (ORNL) and S. Nagaitsev (FNAL) in 2010. Chad proposed a research plan to explore it further and was funded by the prestigious and competitive DOE Early Career Research Program on first submission in 2016. He and his team collaborated with people already investigating collective dynamics of intense beams at Fermilab and RadiaSoft. Much of the Berkeley Lab work involved understanding aspects of the single-particle dynamics that had not previously been explored. This better understanding of the single-particle dynamics allowed them to make sense of aspects of the collective dynamics that they were seeing in simulations. They then led the expansion of this research from previous studies of single particle dynamics into the much more complex area of collective dynamics.
One theme of their work was to understand the interplay between space charge and integrability. In particular, they explored what happens when the integrability is broken in the presence of space charge, both through simulation and by developing a simple model, and we examined these effects in a realistic accelerator lattice (the IOTA ring at Fermilab) using high-fidelity simulation. Both analytical and computational approaches played important roles. As a testament to the quality of their work, in 2020, predictions of phase space bifurcation were borne out by the IOTA experimental team’s observations.
Their work has had major impact on the Fermilab IOTA experimental program and has fostered a new line of research in ATAP, with follow-up work planned that will bring the power of artificial intelligence and machine learning tools to bear and push forward methods to expand the performance envelope of future accelerators.
