Pressure-driven evaporative cooling in atom guides

We study steady-state evaporation in an atom guide via Monte Carlo simulations. The evaporation surface follows a specific profile as a function of longitudinal guide location. We demonstrate that the choice of evaporation profile significantly impacts the performance of the evaporation. Our simulations also demonstrate a significant performance boost in the evaporation when using a longitudinally compressed guide. We show that for a purely pressure-driven atom beam, it should be possible to reach degeneracy within a 0.5 m guide for experimentally feasible, albeit challenging, loading conditions.

Figure 1

Continuous evaporation is distributed in space and maintained in time by continually narrowing an effective evaporative surface in towards the center of the atom beam.

Figure 2

The velocity distribution of a $100\mu \mathrm{K}$ sample of ${}^{87}\mathrm{Rb}$ atoms is truncated and allowed to rethermalize via collisions. Approximately $20\%$, of the initial atom population, is removed as the wings of the thermal distribution are truncated. After roughly 10 collisions per atom, the velocity distribution errs, on average, less than $(1/100)\%$ from a thermal distribution with $T=47\mu \mathrm{K}$.

Figure 3

Snapshot of the guided atom beam under the influence of forced evaporative cooling. The three-dimensional surface indicates the location of the evaporation surface.

Figure 4

Evaporation strategies of interest for this paper. The curves here depict the radial position $\rho$ at which atoms are removed from the system. By lowering the barrier in the forward direction, a forced evaporative cooling is imposed. The curves (a)–(e) represent the lines of $\mathfrak{F}\left(z\right)$ [Eq. (1)], in order respectively. For $z\le 0$ and $z\ge L$, $\mathfrak{F}\left(z\right)$ is held constant. These different strategies are to provide a set of basic types of evaporation surfaces that might be used experimentally.

Figure 5

Final phase space density $n{\lambda }_{\mathrm{th}}^3$ of the different evaporation strategies. (a) Noncompressed guide has a constant magnetic field gradient of 750 G/cm. A reflecting barrier is also placed at $-4$ cm. (b) Compressed guide begins with a gradient of 750 G/cm and ends with 1500 G/cm. The reflecting barrier at $-4$ cm is also present here. (c) Compressed guide $\left(750\text{G}/\text{cm}\to 1500\text{G}/\text{cm}\right)$ without reflective barrier at $-4$ cm.