Journal ArticleA slow atom source, which does not rely on lasers, has been developed and characterized. The device, acting  as an atomic low-pass velocity filter, utilizes permanent magnets to passively select the slow atoms present in  a thermal atomic beam. Slow atoms are guided along a curved, conduction-limited tube by an octupole magnetic field, while fast atoms, unable to follow the curved trajectory, strike the tube wall and are removed from the beam. The performance of the device is  demonstrated by loading a magneto-optical trap. Approximately 2 X 10^8 lithium atoms are loaded with a rate of ~6 X 10^6 atoms/s, while maintaining a background gas pressure of ~10^-11 torr. This loading technique provides an exceptionally simple, economical, and robust alternative to laser cooling methods

Gerton, Jordan

Ghaffari, B.

English

The University of Utah: J. Willard Marriott Digital Library

Laser-free slow atom sourceB. Ghaffari, J. M. Gerton, W. I. McAlexander, K. E. Strecker, D. M. Homan, and R. G. HuletDepartment of Physics and Rice Quantum Institute, Rice University, Houston, Texas 77251⑦Received 27 April 1999✦A slow atom source, which does not rely on lasers, has been developed and characterized. The device, actingas an atomic low-pass velocity filter, utilizes permanent magnets to passively select the slow atoms present ina thermal atomic beam. Slow atoms are guided along a curved, conduction-limited tube by an octupolemagnetic field, while fast atoms, unable to follow the curved trajectory, strike the tube wall and are removedfrom the beam. The performance of the device is demonstrated by loading a magneto-optical trap. Approxi-mately 2✸108 lithium atoms are loaded with a rate of ❀6✸106 atoms/s, while maintaining a background gaspressure of ❀10✷11 torr. This loading technique provides an exceptionally simple, economical, and robustalternative to laser cooling methods. ❅S1050-2947⑦99✦00411-4★PACS number⑦s✦: 32.80.Pj, 39.10.✶j, 03.75. b, 07.55. wSources of slow atoms are integral components of numer-ous atomic physics experiments, including investigations ofBose-Einstein condensation of atomic gases, high precisionspectroscopy of trapped atoms, and ultracold collision stud-ies. Given their widespread importance, much effort has beendedicated to improving slow atom sources, and especially totheir simplification. In this paper, we describe a simple slowatom source and report its performance for loading amagneto-optical trap ✁MOT✂ ✄1☎ with 6Li atoms.The most common use of slow atom sources is for loadingatom traps. Trap loading methods that rely on laser slowingan atomic beam can achieve high load rates and low back-ground pressures, resulting in long trap lifetimes. Zeemanslowing ✄2☎ is the most successful of these techniques, pro-viding typical load rates of 108 atoms/s into a MOT. Underoptimum conditions, rates as high as 1011 atoms/s have beenattained✄3☎. Unfortunately, this method is complex and ex-pensive since it usually requires acousto-optical and/orelectro-optical modulators, and significant laser power.MOTs have also been directly loaded from the slow atomspresent in a vapor cell✄4☎. The main advantage of vaporloading lies in its simplicity since no additional laser beams,other than those used for trapping, are required. Load rates ashigh as 1011 atoms/s have been achieved in vapor cells ✄5☎,though the relatively high background gas pressure results inreduced trap lifetimes that prove unsuitable for many experi-ments. This limitation encouraged the development of thedouble-MOT scheme. In this technique, the trapped atomsfrom a vapor-loaded MOT are transferred to an ultrahigh-vacuum✁UHV✂chamber using magnetic guiding, providingload rates of ✆108 atoms/s ✄6☎. The main disadvantage ofthis method, as for Zeeman slowing, is the degree of com-plexity and expense. MOTs have also been loaded directlyfrom a thermal atomic beam✄7☎, achieving load rates of✆107 atoms/s from an oven located 20 cm from the trap✄8☎.Although simple, this method suffers from reduced trap life-times resulting from the proximity of the hot atomic source.Applications that require less than maximal load rates, butmust be UHV-compatible, may benefit from a simple tech-nique that does not involve lasers, such as the one describedhere.In our experiment, the slow atoms present in a thermallithium beam are magnetically transported to a MOT, using adevice we call the ‘‘skimmer.’’ Related devices for trans-porting and focusing beams of slow atoms have been previ-ously demonstrated✄6,9☎. The skimmer, however, operatesas a low-pass velocity filter for the thermal beam. The atomsare transported through a curved, conduction-limited tube,allowing the trap chamber to be differentially pumped toUHV pressures. As shown in Fig. 1, permanent rare-earthmagnets are placed around the tube to establish an octupolemagnetic guiding field leading from the atomic source to theMOT. The low-field seeking atoms in the thermal beam areattracted to the magnetic field minimum at the tube center,and the field gradient provides the centripetal force needed toguide the atoms around the curve. Atoms that are slower thana threshold velocity, dictated by the magnetic field gradientand the radius of curvature of the tube, are successfullyguided. Faster atoms, however, strike the tube wall, and arelost from the beam. Removing the fast atoms is beneficial,since they can lead to loss of trapped atoms through colli-sions.FIG. 1. Schematic drawing of the skimmer. The inset displays across section of the octupole magnet yokes. The SmCo magnets(1.4✸4.8✸32 mm) and NdFeB magnets (2.5✸5.1✸18 mm) aremagnetized along the smallest dimension.PHYSICAL REVIEW A NOVEMBER 1999VOLUME 60, NUMBER 5PRA 601050-2947/99/60⑦5✦/3878⑦4✦/$15.00 3878 ©1999 The American Physical SocietyA prototype of this guiding system was built and testedwith a quadrupole field ❅10★. Comparison of the experimentalresults with a simple transport model indicated that a higher-order field would increase the flux of slow atoms. For thequadrupole design, most atoms entering the guiding tubesuddenly experience a large magnetic field. For atoms thatare close to the tube walls, the magnetic field might be suf-ficiently large to repel them from the tube entrance, prefer-entially removing the slowest atoms. Figure 2 shows themeasured field profiles for the current octupole design. Thefield profile is flat over a larger range near the tube axis andhas a higher gradient near the tube walls. The nearly uni-form, low-field region near the axis allows a larger fractionof the slowest atoms to enter the skimmer, while the highergradient near the walls leads to a higher threshold velocityfor guiding. The octupole field was produced by sets of eightcommercially available, homogeneously magnetized, rectan-gular magnets. Although using a small number of rectangularmagnets weakens the octupole strength, introduces higher-order multipoles, and disturbs the cylindrical symmetry ofthe field profile, these effects can be tolerated to maintainsimplicity and economy. Figure 2 shows that the dependenceof the field on the azimuthal angle is minimal for the presentconfiguration.The octupoles around the curved tube are constructedfrom NdFeB magnets, since this material provides the largestfields. The tube inner diameter is 1.1 cm and the total arclength is 25 cm, corresponding to a conduction of ❀0.4 L/s.Although a larger tube diameter may produce a higher fluxof slow atoms, a smaller diameter more completely isolatesthe source chamber from the UHV chamber. The entrancesolid angle for the thermal beam is improved by extendingthe octupole field inside the source chamber. The additionaloctupoles are made from SmCo magnets, because their rela-tively high Curie temperature allows them to be vacuumbaked with the chamber. As shown in Fig. 1, each octupoleset of magnets is mounted inside cylindrical housings com-posed of a material with a high magnetic permeability, 400series stainless steel in this case. These yokes shunt the fieldlines, which reduces stray fields that might perturb the MOT.They also flatten the magnetic field profile, thereby wideningthe entrance for slow atoms. Other benefits of the shuntyokes include enhancing the magnetic field strength(❀50% increase in the present geometry✦ and providing asurface for the magnets to be conveniently affixed in theappropriate positions.The six MOT laser beams are derived from three circu-larly polarized, retroreflected, mutually orthogonal beams.The beams are produced by a frequency-stabilized dye laserthat is locked to a saturated absorption feature in 6Li. Opticalpumping is prevented by simultaneously driving transitionsfrom both ground-state hyperfine levels. The power in eachfrequency is the same, typically 60 mW in three beams. The1/e2 intensity radii of the beams are 1.1 cm, and the beamsare apertured to diameters of 1.8 cm. The detuning fromresonance is 7.5● , where ●✺5.9 MHz is the natural line-width of the 2S-2P transition. A pair of anti-Helmholtz coilsgenerate the MOT quadrupole magnetic field with an axialgradient of 32 G/cm.The number of trapped atoms is determined by observingthe excited state fluorescence with a photodiode. The loadrate is measured by first emptying the MOT of all atoms andthen observing the increase in fluorescence during the firstfew seconds after the atomic beam is unblocked. A typicaldata set is shown in Fig. 3. Load rates of❀6✸106 atoms/sand peak numbers of ❀2✸108 atoms are obtained. The sys-tematic uncertainty in the number of atoms is❀40%, and isprimarily due to measurement uncertainties in the beam in-tensities and laser detuning. The load rate dependence on theFIG. 2. Total magnetic field along radial lines at two differentazimuthal angles inside ⑦a  SmCo and ⑦b  NdFeB octupoles. Thesquare symbols correspond to a line between centers of two oppo-site magnets, while the triangles correspond to a line from the edgewhere two adjacent magnets meet at the opposite edge. For eachline, two perpendicular field components were measured separately,from which the total magnetic field is calculated. The fourth-orderpolynomial fits, the average of which is used in the trajectory simu-lations, are shown as solid curves. For the SmCo octupoles, themotion of the atoms is constrained by the magnet surfaces, whileinside the NdFeB octupoles, the atoms are limited by the curvedvacuum tube. Inner faces of the SmCo magnets⑦residual induction,Br✁10.6 kG) are at ✻6.4 mm. Inner faces of the NdFeB magnets(Br✁12.1 kG) are at ✻6.7 mm, and the tube inner diameter is 11mm.FIG. 3. Typical fluorescence data. The atomic beam is suddenlyunblocked at time ✁ 0, and the load rate is given by the increase insignal in the first few seconds.PRA 60 3879LASER-FREE SLOW ATOM SOURCEoven temperature is displayed in Fig. 4. At lower oven tem-peratures, the number of trapped atoms rises with tempera-ture as the number of available slow atoms increases. Athigher temperatures, however, collisions in the beam reducethe flux of slow atoms into the skimmer. The trap lifetime ismeasured by blocking the oven and observing the decay intrap fluorescence. The decay is dominated by the losses dueto inelastic collisions between trapped atoms. The lifetimedue to background gas scattering is ❀10 min, correspondingto a background gas pressure of❀10✷11 torr, which is belowthe sensitivity of our pressure gauge.We have performed a Monte Carlo calculation to modelthe efficiency of the skimmer. The trajectory of an atomthrough the skimmer is calculated to determine whether it istransmitted to the exit. The initial position and velocity ofeach atom are chosen to be consistent with the recirculatingatomic beam oven design and a Maxwell-Boltzmann velocitydistribution ❅11★. The oven reservoir containing the lithium ismaintained at❀500 °C, and is connected to the sourcechamber via a 9.5-cm-long nozzle with an inner diameter of0.36 cm. The atomic source, therefore, is modeled as a thin-walled aperture located at the nozzle entrance that is colli-mated by the nozzle exit. The angular spread of the thermalbeam is ❀0.037 rad, so we neglect the transverse componentof atomic velocity. The presence of shunt yokes and theclose proximity of the nozzle exit to the skimmer entrancereduce the fringing fields seen by the atoms, so in the modelthe atoms are assumed to suddenly experience the octupolefield upon entering the skimmer. Since the SmCo portion ofthe skimmer is straight and the transverse velocity of atomsis ignored, we assume that the spatial and velocity distribu-tions are unchanged in this section. After entering the curvedtube, the force on an atom is calculated from the measuredmagnetic field profile ❅12★, according to which the trajectoryis integrated. Atoms that strike the tube wall are assumedlost. An atom can also be removed from the beam by movingthrough the magnetic field zero so quickly that its magneticmoment does not follow the field direction, and the atomsuddenly becomes high-field seeking. The Majorana criterion❅13★ is imposed at all integration steps to include this loss inthe model.The fraction of atoms expected to be transmitted throughthe skimmer is thus determined for each initial atomic veloc-ity. The results are plotted in Fig. 5. The reduced efficiencyat very low velocities is due to atoms that are rejected at theskimmer entrance, while atoms faster than the threshold ve-locity are unable to follow the guiding field. A rough esti-mate for the threshold velocity, v th , can also be obtained byequating the magnetic force with the centripetal force neces-sary for atoms to traverse the curve, ♠➵B✺m(v th2 /R), where♠ is the atomic magnetic moment, m is the atomic mass, andR is the radius of curvature of the guiding tube. If➵B istaken to be half the gradient at the tube wall, v th✬110 m/s,in agreement with the Monte Carlo model. This simple cal-culation, however, was not as applicable to our earlier stud-ies using a quadrupole guiding field. In this case, the MonteCarlo calculation gives a transmission probability that is notflat-topped as in Fig. 5 but, rather, slowly decreases from apeak velocity that is significantly below v th . The reducedefficiency at large velocities is a consequence of the largefraction of atoms that enter the skimmer at a high field, forwhich the guiding is less effective. To calculate the expectedMOT load rate, the magnetic field is assumed to suddenly goto zero at the skimmer exit and the atomic trajectories areextended to the trap region. Atoms that pass through the trapvolume with speeds less than or equal to the capture velocityare assumed trapped. Using previous calculations of Li atomtrajectories in a MOT ❅14,15★, we estimate the capture ve-locity to be 30–40 m/s for the present trap parameters. Thisvelocity range corresponds to load rates between 2✸106 and7✸106 atoms/s, which is consistent with the measured loadrate of❀6✸106 atoms/s.The performance of the skimmer is not completely char-acterized by this experiment, because the predicted thresholdvelocity is greater than the capture velocity of the MOT. Ithas been shown that the capture velocity can be increased to❀150 m/s by broadening the frequency spectrum of theMOT laser beams ❅8★. In addition to verifying the perfor-mance of the skimmer, implementation of this schemeshould produce a large increase in the load rate. The flux ofslow atoms in our experiment could also be increased byusing a less collimated atomic beam that would completelyfill the entrance solid angle of the skimmer.This guiding system can be viewed as an atom opticalelement that performs as a low-pass velocity filter. With suit-able modifications, it should be possible to build a band-passfilter for delivery of a monoenergetic beam of atoms for ap-plications such as atom lithography ❅16★. This device mightalso be promising for directly loading a magnetic trap basedon permanent magnets ❅17★ since its capture velocity is muchFIG. 4. Peak number of trapped atoms vs oven temperature. Theoverall systematic uncertainty in the number of atoms is  40%.FIG. 5. Simulation of the transmission probability as a functionof the initial atomic velocity.3880 PRA 60B. GHAFFARI et al.greater than that of a MOT. Since the skimmer is simple androbust, it may be well-suited for space-born applications.Furthermore, the apparatus is inexpensive; the total cost ofmagnets ❅18★, custom shunt yokes, and curved vacuum tubewas less than $3500. The performance of this guiding systemrepresents a significant improvement in the design of slowatom sources.We are grateful to Jeff Tollett, Kurt Franke, and CassSackett for their contributions. This work was supported bythe National Science Foundation, the National Aeronauticsand Space Administration, the U.S. Office of Naval Re-search, and the Welch Foundation. 1✁ E. L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Pritchard,Phys. Rev. Lett. 59, 2631 ⑦1987✦. 2✁ W. D. Phillips and H. Metcalf, Phys. Rev. Lett. 48, 596 ⑦1982✦. 3✁ W. Ketterle, K. B. Davis, M. A. Joffe, A. Martin, and D. E.Pritchard, Phys. Rev. Lett. 70, 2253⑦1993✦. 4✁ C. Monroe, W. Swann, H. Robinson, and C. Wieman, Phys.Rev. Lett. 65, 1571 ⑦1990✦. 5✁ K. E. Gibble, S. Kasapi, and S. Chu, Opt. Lett. 17, 526⑦1992✦. 6✁ C. J. Myatt, E. A. Burt, R. W. Ghrist, E. A. Cornell, and C. E.Wieman, Phys. Rev. Lett. 78, 586 ⑦1997✦. 7✁ A. Cable, M. Prentiss, and N. P. Bigelow, Opt. Lett. 15, 507⑦1990✦. 8✁ B. P. Anderson and M. A. Kasevich, Phys. Rev. A 50, R3581⑦1994✦. 9✁ W. G. Kaenders, F. Lison, I. Mu¨ller, A. Richter, R. Wynands,and D. Meschede, Phys. Rev. A 54, 5067⑦1996✦. 10✁ A MOT load rate of ❀1.4✸104 atoms/s was achieved with thequadrupole design  J. Gerton, C. Sackett, and R. Hulet, Bull.Am. Phys. Soc. 42, 967 ⑦1997✦✁. 11✁ N. F. Ramsey, Molecular Beams⑦Clarendon Press, Oxford,1956✦. 12✁ Since each octupole is composed of only eight rectangularmagnets that are surrounded by highly permeable yokes, thefunctional form of the field is substantially different from anideal octupole. The fourth-order polynomial fits displayed inFig. 2 are used in the model. 13✁ E. Majorana, Nuovo Cimento 9, 43 ⑦1932✦. 14✁ N. W. M. Ritchie, E. R. I. Abraham, and R. G. Hulet, LaserPhys. 4, 1066⑦1994✦. 15✁ N. W. M. Ritchie, E. R. I. Abraham, Y. Y. Xiao, C. C. Brad-ley, R. G. Hulet, and P. S. Julienne, Phys. Rev. A 51, R890⑦1995✦. 16✁ Atom Optics, edited by M. G. Prentiss and W. D. Phillips,Proceedings of SPIE ⑦SPIE, Bellingham, 1997✦, Vol. 2995. 17✁ J. J. Tollett, C. C. Bradley, C. A. Sackett, and R. G. Hulet,Phys. Rev. A 51, R22⑦1995✦. 18✁ The NdFeB magnets are custom made by UGIMAG, Inc., andthe SmCo magnets are stock pieces from Magnet Sales andManufacturing Inc.PRA 60 3881LASER-FREE SLOW ATOM SOURCE

Laser-free slow atom source

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Laser-free slow atom source

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