Prof. Francis Robicheaux
Dr. Robicheaux is a professor at Auburn University since 1993 and a Fellow of the American Physical Society since 2002. His research area is Theoretical Atomic Physics, mainly focusing on time dependent atomic phenomena, highly excited (Rydberg) atoms, electron scattering, strong fields and ultracold plasmas. He is a member of the ALPHA collaboration which was the first group to trap the antimatter version of the Hydrogen atom and the first group to quantitatively measure some of its properties.
Antimatterwikipedia article on antimatter
We have performed several calculations that provide data for experiments that have made antihydrogen. The focus of our current calculations is on processes that will control whether the antihydrogen can be trapped. See a recent review article (F. Robicheaux, “Atomic processes in antihydrogen experiments: a theoretical and computational perspective,” J. Phys. B 41, 192001 (2008). PDF (369 kB)) about our understanding of atomic processes in these experiments.
We are part of the ALPHA collaboration which includes groupus from over 10 countries. The experiment is performed at CERN. The eventual goal is to perform spectroscopy on antihydrogen to compare with the same process in hydrogen. Currently, the 1s-2s frequency is known to 14 significant digits. There would be profound implications for our understanding of fundamental physical laws if there were to be any difference in the spectra of antihydrogen compared to hydrogen.
The goal of trapping antihydrogen is the last necessary stage before performing spectroscopy experiments. This is a difficult undertaking because the trapping well will not be able to hold atoms with energy greater than about 1/2 K. The well depth is about 10,000 times smaller than the starting energies of the charged particles.
Our role in the collaboration is to perform calculations to understand the basic processes in the ALPHA apparatus. This helps in trying to pick the most likely schemes for making cold antihydrogen. Below is a brief description of results in two recent publications.
J. Fajans, N. Madsen, and F. Robicheaux, “Critical loss radius in a Penning trap subject to multipole fields,” Phys. Plasmas 15, 032108 (2008). PDF (578 kB)
In the experiments attempting to trap antihydrogen, the spatially varying magnetic fields are created through complicated windings.
This image shows the winding that gives the octupole field in the ALPHA experiment. One of the difficulties with this scheme is that the magnetic field lines now go from inside the trap to the walls.
This image shows a surface image of the fields for a quadrupole arrangement (upper picture) and for an octupole arrangements (lower picture). Since charged particles follow magnetic field lines, these multipole fields cause the antiprotons to veer to the walls and annhilate.
This paper examines how the critical loss radius depends on the histrory by which the field is applied and can be much smaller if the particles are injected into a preexisting multpole than if the particles are subject to a ramped multipole.
This image shows the critical radius as a function of the multipole field at the wall for a ramping field. Note that the radius is much smaller for a quadrupole field which means many more antiprotons are lost in the quadrupole field.
C.L. Taylor, Jingjing Zhang and F. Robicheaux, "Cooling of Rydberg antihydrogen during radiative cascade," J. Phys. B 39, 4945 (2006). PDF (207 kB)
We simulated the center of mass motion for antihydrogen in a spatially varying B-field. We found that there is the possibility for substantial cooling of the kinetic energy if the antihydrogen starts in a highly excited electronic state. The loss of energy is due to the change in magnetic moment when photons are emitted and is akin to adiabatic cooling.
This image shows the kinetic energy of antihydrogen atoms as a function of the principle quantum number of their electronic state during a radiative cascade.
This image shows the energy distribution of antihydrogen kinetic energy if all atoms start with 5.5 K. Notice that a substantial fraction end up with KE less than 1 K. Since the experimental trap depths are going to be less than 1 K, this shows that atoms that start with much more energy than can be trapped could end up trapped at the end of the cascade.
Five Recent Publications
J.L. Hurt, P.T. Carpenter, C.L. Taylor, and F. Robicheaux, “Positron and electron collisions with anti-protons in strong magnetic fields,” J. Phys. B 41, 165206 (2008). PDF (165 kB)
G. B. Andresen et al. (ALPHA collaboration), “Compression of Antiproton Clouds for Antihydrogen Trapping,” Phys. Rev. Lett. 100, 203401 (2008). PDF (970 kB)
J. Zhang, C. L. Taylor, J. D. Hanson, and F. Robicheaux, “Regular and chaotic motion of anti-protons through a nested Penning trap with multipole magnetic fields,” J. Phys. B 40, 1019 (2007). PDF (923 kB)
F. Robicheaux, “Three-body recombination with mixed sign light particles,” J. Phys. B 40, 271 (2007). PDF (180 kB)
G. Andresen et al. (ALPHA Collaboration), “Antimatter plasmas in a multipole trap for antihydrogen,” Phys. Rev. Lett. 98, 023402 (2007). PDF (248 kB)