Kinetic theory of the evaporative cooling of a trapped gas

Abstract

(1996). Kinetic theory of the evaporative cooling of a trapped gas. Physical Review A, (53), 381. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We apply kinetic theory to the problem of evaporative cooling of a dilute collisional gas in a trap. Assuming ''sufficient ergodicity'' ͑phase-space distribution only a function of energy͒ and s-wave collisions with an energy-independent cross section, an equation for the evolution of the energy distribution of trapped atoms is derived for arbitrary trap shapes. Numerical integration of this kinetic equation demonstrates that during evaporation the gas is accurately characterized by a Boltzmann distribution of atom energies, truncated at the trap depth. Adopting the assumption of a truncated Boltzmann distribution, closed expressions are obtained for the thermodynamic properties of the gas as well as for the particle and energy loss rates due to evaporation. We give analytical expressions both for power-law traps and for a realistic trapping potential ͑Ioffe quadrupole trap͒. As an application, we discuss the evaporative cooling of trapped atomic hydrogen gas.

RF spectroscopy in a resonant RF-dressed trap

“…In our dressed trap, the phase space density increases only slightly, due to a poor thermalisation efficiency with the low horizontal frequencies (5 Hz × 20 Hz). With larger horizontal frequencies in the dressed trap, as is the case in a dressed quadrupole trap for example [19], and thus with a higher collision rate, thermalisation in the trap would lead to an increase of the phase space density [21], as shown by molecular dynamics simulations. From the point of view of the out-coupling efficiency, our results show that evaporative cooling is feasible in a resonant dressed trap in the same conditions as in a conventional magnetic trap.…”

RF-Induced Evaporative Cooling And BEC In A High Magnetic Field

“…Efficient evaporative cooling [14][15][16] relies on fast thermal relaxation, and thus on the ability of increasing the collision rate by adiabatic compression of the atomic cloud. The most widely used mean to increase the curvature of the trapping potential is to partially cancel the bias field B 0 with two additional coils in Helmholtz configuration along the z-axis.…”

“…The energy taken away by dN 1 atoms evaporated at height η [14][15][16] is (11) and the energy of the atoms remaining trapped after evaporation of these dN 1 atoms is…”

Dynamics of evaporative cooling in magnetically trapped atomic hydrogen

“…The cooling mechanism of this powerful method is based on both the selective removal of energetic atoms through evaporation and collisional rethermalizations among the remaining atoms [6]. Evaporative cooling itself is stimulating theoretical studies of the process of condensate formation in nonequilibrium atomic gases [7][8][9][10][11][12][13][14][15][16][17].In BEC experiments with atomic hydrogen, evaporative cooling has been implemented by lowering the potential height of the saddle point at one end of a magnetic trap with cylindrical symmetry [1]. This approach suffers from a reduction of the dimension of evaporation at low temperatures due to low elastic collision rate [18,14] since the s-wave scattering length, a, of a hydrogen atom is anomalously small, about two orders of magnitude smaller than that in alkali-metal atoms.…”

“…The cooling mechanism of this powerful method is based on both the selective removal of energetic atoms through evaporation and collisional rethermalizations among the remaining atoms [6]. Evaporative cooling itself is stimulating theoretical studies of the process of condensate formation in nonequilibrium atomic gases [7][8][9][10][11][12][13][14][15][16][17].…”