Experiments with Supersonic Beams as a Source of Cold Atoms

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a method by which beams of helium may be controlled. In the experiment described here, specular reflection from receding single crystal surfaces is used to control the velocity of a supersonic beam of helium atoms. The crystal is mounted on the tip of a spinning rotor that provides the necessary crystal velocity to effectively slow the beam. The original concept for this experiment was first proposed by Doak [41], but the design studied had an expected slow beam flux that is significantly lower than in the experiment described here. The work presented in this chapter is also described in [33, 42, 43]. The slower itself is similar to the beam paddle that was developed for neutrons [44]. For helium atoms reflecting from a linearly moving single crystal surface, the velocity of the atoms after hitting the mirror will be vf = −vi +2vm, (3.2) where vf is the final velocity of the atoms, vi is the initial velocity of the incoming beam, and vm is the velocity of the mirror. The mirror is assumed to be perpendicular to the incoming beam. Since the typical velocity of a supersonic beam is 500 m/s, reducing the velocity of the beam by a factor of two requires a crystal velocity of 125 m/s. While it is possible to create a device to repeatably move a mirror linearly at these speeds (perhaps by mounting the mirror on a projectile and launching and catching it using electromagnetic coils), it is simpler to let rotary motion approximate linear motion and mount the crystal on the tip of a spinning rotor arm. This facilitates the high mirror velocities required to significantly slow the beam. The introduction of rotary motion complicates equation 3.2, and the full description of the velocity change due to an atom reflecting from the rotor is shown in appendix A. 21
3.1 Using Helium for Atom Optics Experiments Most atom optics experiments have been done with laser cooled atoms. The advantage of this is a cold sample which can easily be manipulated. However, because of the large polarizability of most laser coolable atoms, they are sensitive to stray electric fields. Similarly, many laser coolable atoms are quite sensitive to stray magnetic fields. While their sensitivity is an advantage when controlling the atoms, stray fields will add systematic effects limiting the sensitivity of many measurements. Though ground state noble gases are difficult to manipulate, doing so is not impossible and there are several advantages to using them for experiments in atom optics. One such advantage is that they are known to reflect well from single crystal surfaces, and even if they do not reflect from a surface, they are unlikely to stick to it. Because of this, an aperture will not clog when subjected to a beam of noble gases, even at high intensity. The other principle advantage is their low polarizabilty. Ground state noble gases are relatively insensitive to stray electric fields, and they are insensitive to magnetic fields to first order. This makes ground state noble gases, and ground state helium in particular, very attractive for certain precision measurements and applications such as atom interferometry. A final advantage which is available when using ground state noble gases for atom optics experiments is the large fluxes available. Supersonic beams typically use noble gases as carriers, and a significant fraction of the flux of nearly any supersonic beam will consist of a ground state noble gas. Fluxes of up to 4 · 10 23 molecules/sr/s have been measured for pulsed beams as described in chapter 2, and for continuous beams, fluxes of 1.1 · 10 20 molecules/sr/s have been observed [45]. These bright- nesses provide a large advantage and can reduce the uncertainty of many precision measurements, particularly for atom interferometry where the shot noise contributes significantly to the uncertainly. Helium is the atom most resistant to clustering, re- 22
Page 1 and 2: Copyright by Adam Alexander Libson
Page 3 and 4: General Methods of Controlling Atom
Page 5 and 6: Acknowledgments First and foremost,
Page 7 and 8: to problems, and I frequently went
Page 9 and 10: General Methods of Controlling Atom
Page 11 and 12: Table of Contents Acknowledgments v
Page 13 and 14: Chapter 5. Towards Trapping and Coo
Page 15 and 16: List of Figures 2.1 Comparison of e
Page 17 and 18: 5.12 Faraday Rotation Signal During
Page 19 and 20: producing a cold sample. Alternativ
Page 21 and 22: place inside a dilution refrigerato
Page 23 and 24: atoms or molecules are slowed using
Page 25 and 26: 2.1 Thermodynamics of Ideal Gases F
Page 27 and 28: Since there will be no heat exchang
Page 29 and 30: Using equation 2.17 to modify equat
Page 31 and 32: Normalized Effusive Beam Flux Norma
Page 33 and 34: general method for producing cold a
Page 35 and 36: The gas throughput of the nozzle ca
Page 37: Chapter 3 Slowing Supersonic Beams
Page 41 and 42: Figure 3.1: Calculated elastic scat
Page 43 and 44: that they can be prepared ex-situ a
Page 45 and 46: successful. Any water that remains
Page 47 and 48: Skimmer 300 l/s Turbo Pump Even-Lav
Page 49 and 50: Figure 3.5: A CAD image of the dete
Page 51 and 52: from tubular aluminum welded togeth
Page 53 and 54: Figure 3.7: A CAD image of the roto
Page 55 and 56: Figure 3.10: A CAD image of large c
Page 57 and 58: Figure 3.11: This plot shows the am
Page 59 and 60: approximations are made in this cal
Page 61 and 62: 3.3.3 Detection An SRS [61] residua
Page 63 and 64: TTL pulse to the data acquisition c
Page 65 and 66: A comparison of the time-of-flight
Page 67 and 68: eceding crystal. Each curve is the
Page 69 and 70: calculated slow beam velocity is qu
Page 71 and 72: the RGA does have an effect on the
Page 73 and 74: Chapter 4 The Atomic and Molecular
Page 75 and 76: field. In the L − S coupling regi
Page 77 and 78: 3 P2 E Figure 4.2: This graph gives
Page 79 and 80: For intermediate fields, the full p
Page 81 and 82: Figure 4.4: This figure illustrates
Page 83 and 84: The same effect is also responsible
Page 85 and 86: (a) (b) (c) Time Figure 4.5: A pict
Page 87 and 88: which the particles enter). The oth
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the coil can instead be switched be
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the magnitude of the field some dis
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nozzle front surface aluminum catho
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Figure 4.9: A CAD overview of the s
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258V 1MΩ 5V 1mF DC/DC Converter TT
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Figure 4.12: An oscilloscope trace
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(a) (b) Figure 4.14: A CAD image of
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-HV PS 4 M MCP Anode 10 M 1 M Trans
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Signal [V] 2.5 2.0 1.5 1.0 0.5 refe
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Figure 4.20: An exploded view of th
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(a) (b) (c) (e) Figure 4.21: A pict
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258V 1MΩ TTL 2.2mF 50Ω TTL 5V 5V
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closed. With the new thyristor gate
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HeNe M2 M1 BB /2 L BB PC coil PC PD
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Table 4.1: Peak magnetic fields mea
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Figure 4.28: A cut-away CAD image o
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ameters. The coilgun chamber consis
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Table 4.2: Final velocities (vf), s
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MCP Signal [arb. units] 2.0 1.5 (1)
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viability for molecules essential.
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8 Ω LN2 Liquid Nitrogen Dewar Nozz
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Figure 4.33: Molecular oxygen slowi
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Chapter 5 Towards Trapping and Cool
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Energy meV 0.05 0.00 0.05 0.0 0.2 0
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10 mm 2.0 T 1.8 1.6 1.4 1.2 1.0 0.8
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250V 1MΩ 3x 2.2mF TTL 5V feedback
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Magnetic Field (T) 1.9 1.7 1.5 1.3
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5.2.2.1 Principle of Operation of t
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Figure 5.8: Photograph of the trapp
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4.1.3. In an anti-Helmholtz trap, t
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Figure 5.11: Photograph of the hydr
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Photodiode Signal (V) Time (s) Figu
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Figure 5.13: Time of flight plot of
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500 l/s Turbo Pump Supersonic Nozzl
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Ionizer 500 l/s Turbo Pump Ion Opti
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guided the design of the apparatus.
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Z − Velocity (m/s) 100 60 30 0
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Atom Number 60 50 40 30 20 10 0.6 0
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scattered photon to extract informa
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where k1 · pi = − k2 · pi, wh
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the electron g-factor which causes
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Figure 5.24: A schematic of the tel
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prevent the determination of the is
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(a) (b) time Figure 5.25: A 1D illu
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experiment, where a being is capabl
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potential position 2 x 243 nm Lα |
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Appendix 164
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y x V i Figure A.1: The geometry us
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Bibliography [1] H.J.MetcalfandP.va
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[17] Brian C. Sawyer, Benjamin L. L
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[34] R. Campargue. Progress in over
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[51] O. Carnal, M. Sigel, T. Sleato
Page 193 and 194:
[70] Edvardas Narevicius, Adam Libs
Page 195 and 196:
[87] Willis E. Lamb and Robert C. R
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[96] G.Gabrielse,N.S.Bowden,P.Oxley
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[110] N. Kolachevsky, J. Alnis, S.
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[125] Andreas Osterwalder, Samuel A
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[142] C. Cohen-Tannoudji, B. Diu, a
Page 205 and 206:
[161] Paulo F. Bedaque, Aurel Bulga
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Experiments with Supersonic Beams as a Source of Cold Atoms

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