Create a plasma by photoionizing an ultracold gas — atoms cooled to microkelvin temperatures, then ionized with a laser. The resulting ions are born at rest, inheriting the near-zero kinetic energy of the parent gas. The electrons can be given a chosen kinetic energy by tuning the ionization laser above the ionization threshold. A plasma that starts cold, with controllable initial conditions.
Baker, O'Mara, and Roberts (arXiv 2602.22370, February 2026) study what happens next — and what happens next is heating. Not from an external source. From the plasma itself.
The mechanism is disorder-induced heating. When the plasma is created, the charges are distributed randomly — wherever the parent atoms happened to be. Random positions mean random inter-particle distances, which mean random Coulomb potential energies. The system minimizes this potential energy by rearranging the charges — ions repel neighboring ions, electrons repel neighboring electrons, and unlike charges attract. This rearrangement converts potential energy into kinetic energy. The plasma heats itself by ordering itself. The disorder of the initial random configuration is the energy source.
The heating is strongest at early times, before the charges have had time to reach even approximate equilibrium, and it sets a floor on the minimum achievable electron temperature. No matter how cold the initial electron energy, disorder-induced heating raises it. In the experiment, the minimum electron temperature is 0.52 K at an electron density of 6.1 × 10¹² m⁻³ — cold by plasma standards, warm by ultracold standards. Below this temperature, the plasma's own self-ordering pumps energy back into the electrons faster than any cooling mechanism can remove it.
A magnetic field adds a second variable. Magnetization confines electron motion perpendicular to the field, reducing the dimensionality of the disorder-induced heating (charges can only rearrange along field lines). The simulations show that stronger magnetization lowers the heating floor — but does not eliminate it. The heating persists because the Coulomb interaction is three-dimensional even when the motion is constrained.
A second heating channel — Rydberg atom formation, where an electron is captured into a high-lying bound state of an ion, releasing the binding energy to a third particle — adds to the thermal budget. Both channels are intrinsic: they arise from the plasma's constituents interacting with each other, not from external driving. The plasma creates its own heat from its own disorder.