Professor and Chair of the Department of Physics
B.S., University of Notre Dame (1994)
Ph.D., University of Colorado (2001)
Ultracold Plasma Physics: Ultracold plasmas are formed via the photoionization of gases of ultracold atoms. The resulting plasmas that form have electron and ion temperatures (a few Kelvin) that can be much colder than other laboratory and natural plasma systems. Yet, these plasmas have high ionization fractions. As such, ultracold plasmas form an interesting system in which to conduct studies of fundamental plasma physics.
By virtue of their low temperatures and low densities, ultracold plasmas can be highly magnetized with relatively low magnetic fields. For comparison, the degree of magnetization we are achieving with 10 mT is the same as for 1 MT in a plasma typical of thermonuclear fusion densities.
We are using the magnetizability of our ultracold plasmas to create plasmas where the characteristic magnetic field length scale is the smallest length scale in the system by an order of magnitude or more. We are currently studying the effect of this strong magnetization on electron-ion collision rates to compare them to theory predictions in an investigation of fundamental plasma physics.
Onset of Opacity and Birefringence in Ultracold Gases: In a separate set of experiments, we are investigating the time-dependent response of a gas of ultracold atoms to near-resonant illuminating light that is suddenly turned on. We do this with a gas of ultracold atoms that is highly absorptive in steady-state such that little light makes it through the gas. At short times after light is turned on, though, the atoms have not had time to respond and the light passes through the gas as if it were transparent.
The physics of this initial transparency has been examined in utlracold gases for almost two decades, but in addition to the amplitude response we are also studying the phase response of the atoms (i.e. the onset of the index of refraction) as well. We do this through measurement of the time-dependent birefringence of the gas in a magnetic field. Our measurements are relevant to future planned experiments studying light diffusion in these optically thick gases.
- Jonathan R. Gilbert, Colin P. Roberts, and Jacob L. Roberts, Near-resonant light propagation in an absorptive spatially anisotropic ultracold gas, J. Opt. Soc. Am. B 35, 718-723 (2018)
- Wei-Ting Chen, Craig Witte, and Jacob L. Roberts, Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas, Phys. Rev. E 96, 013203 (2017)
- Craig Witte and Jacob L. Roberts, Evaluation of charged particle evaporation expressions in ultracold plasmas, Phys. Plasmas 24, 052122 (2017)
- Wei-Ting Chen, Craig Witte, and Jacob L. Roberts, Damping of electron center-of-mass oscillation in ultracold plasmas, Phys. Plasmas 21, 052101 (2016)
- John Guthrie and Jacob L. Roberts, A scalable theoretical mean-field model for the electron component of an ultracold neutral plasma, J. Phys. B 49, 045701 (2016)
- Craig Witte and Jacob L. Roberts, Ultracold Plasma Expansion as a Function of Charge Neutrality, Phys. Plasmas 21, 103513 (2014)
- Mathew S. Hamilton, Rebekah F. Wilson, and Jacob L. Roberts, Collision assisted Zeeman cooling with multiple types of atoms, Euro. J. Phys. D 68, 14 (2014)
- Truman M. Wilson, Wei-Ting Chen, and Jacob L. Roberts, Density-dependent response of an ultracold plasma to few-cycle radio-frequency pulses, Phys. Rev. A 87, 013410 (2013).
- Truman M. Wilson, Wei-Ting Chen, and Jacob L. Roberts, Influence of electron evaporative cooling on ultracold plasma expansion, Phys. Plasmas 20, 073503 (2013).