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Review of Scientific Instruments

A pulsed supersonic expansion with a source temperature below 100 K

J. P. Bucher, Dean C. Douglass, P. Xia, L. A. Bloomfield

Published Sep 1, 1990• 16 citations
Publisher

Abstract

We have produced a pulsed supersonic expansion of helium with an effective pre-expansion temperature below 100 K. This low initial gas temperature is achieved by cooling a pulsed gas valve and gas channel with liquid nitrogen. The resulting supersonic helium beam has a translational velocity that is approximately 55% that of the room-temperature source and an internal temperature of 270 mK. The increased Mach number for a cold supersonic expansion agrees with theoretical predictions and reflects the increased cross section of helium at low collision energies.

DOI
10.1063/1.1141366

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We have produced a pulsed supersonic expansion of helium with an effective pre-expansion temperature below 100 K. This low initial gas temperature is achieved by cooling a pulsed gas valve and gas channel with liquid nitrogen. The resulting supersonic helium beam has a translational velocity that is approximately 55% that of the room-temperature source and an internal temperature of 270 mK. The increased Mach number for a cold supersonic expansion agrees with theoretical predictions and reflects the increased cross section of helium at low collision energies.
We describe a technique for producing a high-flux beam of atomic hydrogen with a velocity distribution corresponding to liquid-helium temperatures. We have studied how a gas of hydrogen atoms (H) may be cooled to low temperatures through interaction with cold walls. The gas was analyzed by forming an atomic beam. We obtained fluxes φH≃2.4×1016 atoms/s at T≃8 K, which corresponds to an increase in flux of low-velocity atoms by a factor of 20 over that of the same source operated at room temperature. The degree of dissociation and the translational temperature of the gas were determined using a quadrupole mass spectrometer and time-of-flight techniques. A beam modulation technique advantageous for such a system is discussed and analyzed. General design considerations for the transport and cooling of H are presented and illustrated with examples. The methods of data analyses are discussed in detail.
Direct images of the virtual source in a supersonic expansion of helium are presented. The images were obtained using a Fresnel zone plate with free-standing zones, 540 microm in diameter and with an outermost zone width of 50 nm. The general method can be extended to other beams, including seeded beams. Measurements were carried out at absolute source pressures ranging from 11 to 171 bar using a 10 microm nozzle with a source temperature of 320 K. The size of the virtual source was found to be strongly dependent on pressure, changing from a diameter of 67+/-6 microm at an absolute nozzle pressure of 11 bar to 180+/-9 microm at 171 bar. The virtual-source brightness displays a maximum at an absolute nozzle pressure of around 30 bar. This phenomenon occurs because of two competing effects: As the pressure increases, the total flux also increases, but at the same time the virtual source broadens. We modeled the expansion process by calculating the velocity distribution with solutions from the Boltzmann equation to estimate the location of the quitting surface where the frequency of interatomic collisions is assumed negligible. Realistic potentials have been used to calculate the cross section for atomic collisions and, for the velocity distribution perpendicular to the center streamline, a proper scaling with distance derived from the continuum expansion model has been introduced. A good agreement between experiments and model has been found and we discuss its approximation limits. For instance, backscattering effects are not included in the calculations and at present we cannot exclude that they also contribute to a broadening of the virtual-source size for the highest pressure regime. The results presented here are important for improving the understanding of the supersonic expansion process. The experimental method might eventually be used as a new way to test molecular and atomic interaction potentials.
A supersonic beam source for SrF and BaF molecules is constructed by combining the expansion of carrier gas (a mixture of 2% SF6 and 98% argon) from an Even–Lavie valve with laser ablation of a barium/strontium metal target at a repetition rate of 10 Hz. Molecular beams with a narrow translational velocity spread are produced at relative values of Δv/v = 0.053(11) and 0.054(9) for SrF and BaF, respectively. The relative velocity spread of the beams produced in our source is lower in comparison with the results from other metal fluoride beams produced in supersonic laser ablation sources. The rotational temperature of BaF is measured to be 3.5 K. The source produces 6 × 108 and 107 molecules per steradian per pulse in the X2Σ+ (ν = 0, N = 1) state of BaF and SrF molecules, respectively, a state amenable to Stark deceleration and laser cooling.
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
Supersonic molecular beams are used in many applications ranging from spectroscopy and matter wave optics to surface science. The experimental setup typically includes a conically shaped, collimating aperture, the skimmer. It has been reported that microskimmers with diameters below 10 μm produce beams with significantly broader velocity distributions (smaller speed ratios) than larger skimmers. Various explanations for this phenomenon have been proposed, but up till now, only a limited amount of data has been available. Here we present a systematic study of the velocity distribution in microskimmer supersonic expansion helium beams. We compare a 4 μm diameter skimmer with a 390 μm diameter skimmer for room temperature and cooled beams in the pressure range 11-181 bars. Our measurements show that for properly aligned skimmers, the only difference is that the most probable velocity for a given pressure and temperature is slightly lower for a microskimmed beam. We ascribed this to the comparatively narrow and long geometry of the microskimmers which can lead to local pressure variations along the skimmer channel. We compare our measurements to a model for the supersonic expansion and obtain good agreement between the experiments and simulations.
Speed ratios v/\ensuremath{\Delta}v greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 \ensuremath{\mu}sec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules.