Observation of a density spike in the ion distribution of an ultracold plasma

Nathaniel R. Shaffer, Sanat Kumar Tiwari, Scott Baalrud

Mackenzie Kate Warrens

Ultracold neutral plasmas (UNPs), which are formed by photoionizing a cloud of cold atoms, have been a powerful platform for studying a wide variety of plasma phenomena. Two areas of interest include expansion into vacuum and the development of shockwaves. Previous UNP experiments have used plasmas with an initial Gaussian density distribution. UNPs with a radially exponentially decaying density distribution, or exponential UNPs, can be formed by photoionizing atoms out of a purely magnetic trap. This dissertation studies the expansion in free space of exponential UNPs as well as the development and characterization of shockwaves in exponential UNPs. The free space expansion of an exponential UNP is compared to the well-known self-similar hydrodynamic expansion of a Gaussian plasma. Similar scaling laws describe the size evolution well for both cases. The evolution of the plasma size and velocity show a characteristic ion acoustic time scale, indicating that the plasma is generally well-described by a hydrodynamic description. While the exponential UNP is predominantly in the hydrodynamic regime, heating at the central peak for low-density, high-electron temperature suggests significant local non-neutrality at early times. This thesis presents the first observation of a shockwave in a UNP. The evolution of the density and velocity for exponential UNPs show signs of wave steepening. A significant density and velocity jump over a narrow region develops. These occur at the same location in the plasma. Additionally, a large spike in the ion temperature occurs at this front. The relative ion velocity across the front modestly surpasses the local sound speed. The initial conditions were varied to characterize the development of shockwaves. Varying the density and electron temperature did not impact shock formation for the ranges used. Changing the shape caused the shocks to disappear as the density gradient decreased. This suggests that the electron thermal pressure gradient plays a significant role in the shock formation. The establishment of shock formation in UNPs opens a new avenue of research for UNPs.

M. K. Warrens, Grant M. Gorman, S. J. Bradshaw +1 more

We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating that the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of a UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases.

Jose Castro, Hong Xiu Gao, T. C. Killian

Wei‐Ting Chen, Jacob Roberts

Patrick McQuillen, Jose Castro, Trevor Strickler +2 more

We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave speed. We also observe the process of hole splitting, which results from the formation of a density depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations describe the results well. Measurements of the ion velocity distribution also show the effects of the ion hole and confirm the hydrodynamic conditions in the plasma.