Ultracold neutral plasma expansion in a strong uniform magnetic field

We report spatial density measurements of an expanding ultracold neutral Ca⁺ plasma in a strong uniform magnetic field. The magnetic field strength ranges from 0 to 0.12 T, corresponding to an electron magnetization parameter ranging from β = ω_c / ω_p = 0 to 7. Similar to a theoretical prediction for electron kinetics [Tiwari and Baalrud, Physics of Plasmas 25, 013511 (2018)], we observe a reduction in the ion disorder-induced heating temperature by a factor of 2. The magnetic field also reduces the plasma expansion transverse to the field direction. An earlier ultracold neutral plasma experiment [Zhang et al, Physical Review Letters 100, 235002 (2008)] observed ambipolar diffusion across the field lines with a transverse expansion velocity that scaled as B^-1/2. In contrast to this observation, the transverse expansion velocity in our experiments depends exponentially on the magnetic field strength. The transverse expansion falls to zero when the Larmor radius is smaller than the average interparticle spacing. The plasma expansion along the magnetic field lines appears to be self-similar, as it is for unmagnetized plasmas. We see some evidence that increasing magnetic field strength leads to increased electron temperature.