We cool our experiment to 10 K in the hope that the plasmas will also reach this temperature eventually. But the final temperature Tf of any cooled body also depends on the ratio of cooling rate Γ to heating power H:
Γ is the rate of equilibration with the thermal bath at 10 K: it takes a time t = Γ-1 for the difference T - Tf to be reduced by 63%. Γ is about 0.24 s-1 for electrons in a 1 T magnetic field.
H is the amount of energy per unit time which the plasma absorbs due to radial expansion and RF noise on the electrodes. H is about 50 K s-1 but can be much larger for plasmas containing millions of electrons.
To reach the lowest plasma temperatures, we need to minimize H and maximize Γ. We have successfully increased Γ by over an order of magnitude by coupling the electron cyclotron motion to a microwave cavity.
The microwave cavity traps a set of electromagnetic modes, each with a characteristic frequency. When we tune the magnetic field B so that the electron cyclotron frequency eB/m matches the frequency of a cavity mode, the cavity can absorb cyclotron energy from the electrons, and the plasmas cool more quickly.