DOI: 10.1007/s00340-004-1600-9 Appl. Phys. B 79, 485–489 (2004) Lasers and Optics Applied Physics B j.a. swansson 1 k.g.h. baldwin 1,2, ✉ m.d. hoogerland 3 a.g. truscott 1,2 s.j. buckman 1 A high flux, liquid-helium cooled source of metastable rare gas atoms 1 Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia 2 Australian Research Council Centre of Excellence for Quantum-Atom Optics, Australia 3 Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand Received: 19 January 2004/Revised version: 5 May 2004 Published online: 22 July 2004 • © Springer-Verlag 2004 ABSTRACT We have developed a novel, high flux source of metastable rare gas atoms (helium, neon and argon) that uses li- quid helium cooling to reduce the initial atomic velocity. Fluxes exceeding 10 14 atoms/ster/s with He and Ne were obtained. With average velocities of ∼ 600 m/s for He and ∼ 300 m/s for Ne and Ar, this source will enable simpler, more compact beam lines for loading magneto-optical traps. PACS 34.80.Dp; 39.10.+j; 39.25.+k 1 Introduction High flux sources of metastable rare gas atoms are important for a range of applications in atom optics and atomic physics [1, 2]. In particular, high flux sources are important for the efficient loading of magneto-optic traps (MOTs), which are the workhorse for many experiments in- cluding the preparation of atoms for Bose–Einstein conden- sation (BEC) [3]. A further requirement for efficient MOT- loading sources is that a significant fraction of the atomic velocity distribution should be within the velocity capture range of the MOT (typically < 100 m/s [4]) in order to facil- itate rapid loading. This can be achieved by a combination of cryogenic cooling of the metastable atomic source, and laser cooling/focusing of the resulting atomic beam [2, 5]. In a previous paper we reported on the development of a liquid-nitrogen cooled metastable helium source [1] which yielded high fluxes (up to 10 15 atoms/ster/s) with average velocities ∼ 1000 m/s. This source is used directly in applica- tions such as atom lithography [6], or as an input to our bright metastable helium beam line [5] which slows the metastable He atoms to < 100 m/s. The bright beam line is used for appli- cations such as atom guiding in hollow optical fibres [7], and for loading a metastable He MOT [8]. However, the bright beam line requires a Zeeman slower over two metres in length, which in turn requires transverse collimation and finally focusing to maintain a high beam brightness. It would be desirable to use liquid-helium cooling of the source to reduce the initial beam velocity and, in turn, reduce the length and complexity of the beam line for MOT ✉ Fax: +61-2/6125-2452, E-mail: Kenneth.Baldwin@anu.edu.au loading. Such a source has been developed elsewhere [2] with average velocities of ∼ 300 m/s, but with relatively low flux (< 10 13 atoms/ster/s). Here we investigate a new type of liquid-He cooled metastable source that generates higher fluxes more comparable with our liquid-N 2 cooled source. 2 Source design Conventional metastable rare gas discharge sources typically employ a sharp needle cathode inside a gas reser- voir, and an exit nozzle to allow gas expansion into a vacuum chamber [9–11]. In order to prevent metastable density loss arising from inter-atomic collisions during the expansion, an external anode is employed to create a discharge through the nozzle into the collision-free region located many noz- zle diameters downstream [1, 12]. This requires high source pressures and currents to sustain the discharge, which con- sequently increases the discharge temperature and hence the atomic velocity. For this reason, the nozzle was used as the an- ode in reference [2]. This source was operated at low pressures (∼ 10 −2 mbar) to avoid collision losses, significantly limiting the maximum achievable flux. In the present experiments, we aim for a high flux source with similar output to our previous source but with reduced velocity through liquid-He cooling. To increase the number of atoms in the low-velocity tail of the atomic velocity dis- tribution, it is often desirable to operate the source closer to the effusive [13] rather than the supersonic [14] regime. The velocity distributions in Fig. 1 illustrate the implications for design. The implications of the data are discussed later in Sect. 4. The velocity distribution is given by [14]: P (v) ∝ v 3 exp  − m(v − u ( M)) 2 v 2 th ( M)  , (1) where u ( M) is the average (flow) velocity, and corresponds to the velocity of the peak, and v th ( M) the local thermal velocity, which determines the width of the distribution. Both vary as a function of the final Mach number M. Operation closer to the effusive regime requires larger nozzle diameters and/or lower operating pressures. However, a needle cathode discharge cannot be sustained at pressures much below 30 mbar. Consequently, we embarked upon a new design based on a hollow cathode whose larger surface area