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A "heat pipe"-style recirculating lithium oven for entrainment into supersonic helium beams

Lukas Gerhard Georg Gradl

Published Jan 1, 2017• 0 citations
Publisher

Abstract

This thesis presents the work on a new oven design optimized for the\nentrainment of lithium into a pulsed supersonic beam of helium. The\nproposed oven does not pollute the vacuum with lithium atoms that are\nnot entrained, but instead is designed to capture and recirculate all\natoms that are not carried away by the supersonic beam. The\ntransportation of lithium inside the oven is reminiscent of a heat\npipe, coining the nickname of the oven: Lithium is evaporated in a hot\nreservoir, recondensed at a cold surface, and recirculated to the hot\nreservoir using capillary action.\n\nThe supersonic helium beam is generated by an Even-Lavie valve\nyielding a cold and collimated flow of gas. The supersonic beam can\nbe further collimated by using a skimmer. Two different heat-pipe\noven designs are investigated in this thesis, one for entrainment\ndownstream of the skimmer and on for entrainment upstream of the\nskimmer.\n\nDownstream of the skimmer, the lithium that is carried in the helium\nbeam behind the oven is much hotter and slower than the carrier gas.\nThis is due to the low density of the helium beam past the skimmer,\nwhich does not support enough collisions with the lithium atoms to\nreach equilibrium.\n\nUpstream of the skimmer, the helium density is higher and the beam\nbehaves in part like continuous flow and the interaction with the oven\nheats the supersonic beam considerably. The lithium content in the\nbeam that was measured past the skimmer is faster and hotter than the\nhelium flow. It is also not properly entrained, but pushed along by\nthe helium beam.\n\nFrom these experiments it is obvious, that entrainment depends\ncrucially of the local properties of the supersonic beam at the\nentrainment site. To be more flexible in the choice of this\nparameter, a new design for a heat-pipe oven is proposed, which allows\nfor linear translation along the propagation direction of the beam,\nupstream of the skimmer.

DOI
10.15781/t28p5vs7c

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This thesis presents the work on a new oven design optimized for the\nentrainment of lithium into a pulsed supersonic beam of helium. The\nproposed oven does not pollute the vacuum with lithium atoms that are\nnot entrained, but instead is designed to capture and recirculate all\natoms that are not carried away by the supersonic beam. The\ntransportation of lithium inside the oven is reminiscent of a heat\npipe, coining the nickname of the oven: Lithium is evaporated in a hot\nreservoir, recondensed at a cold surface, and recirculated to the hot\nreservoir using capillary action.\n\nThe supersonic helium beam is generated by an Even-Lavie valve\nyielding a cold and collimated flow of gas. The supersonic beam can\nbe further collimated by using a skimmer. Two different heat-pipe\noven designs are investigated in this thesis, one for entrainment\ndownstream of the skimmer and on for entrainment upstream of the\nskimmer.\n\nDownstream of the skimmer, the lithium that is carried in the helium\nbeam behind the oven is much hotter and slower than the carrier gas.\nThis is due to the low density of the helium beam past the skimmer,\nwhich does not support enough collisions with the lithium atoms to\nreach equilibrium.\n\nUpstream of the skimmer, the helium density is higher and the beam\nbehaves in part like continuous flow and the interaction with the oven\nheats the supersonic beam considerably. The lithium content in the\nbeam that was measured past the skimmer is faster and hotter than the\nhelium flow. It is also not properly entrained, but pushed along by\nthe helium beam.\n\nFrom these experiments it is obvious, that entrainment depends\ncrucially of the local properties of the supersonic beam at the\nentrainment site. To be more flexible in the choice of this\nparameter, a new design for a heat-pipe oven is proposed, which allows\nfor linear translation along the propagation direction of the beam,\nupstream of the skimmer.
We have created an atomic beam source of lithium atoms for use in a cryogenic environment. \nThis beam source is expected to provide a \nflux of 10 to the 18 atoms/s. The main features of this source \nare a reservoir chamber of solid lithium heated with heater wire, creating a lithium gas that \ntravels down an ohmically heated tube and exits as an effusive beam. That beam is directed at \na cold supersonic jet of helium. As lithium atoms enter the helium gas they rapidly thermalize \nand become entrained. The design challenges faced included directing the heat load away from \nthe cold elements of the cryogenic chamber, obtaining the correct temperatures to provide the \ndesired \nflux, and measuring the beam as it exited the reservoir. Through the use of laser induced fluorescence, we believe we have observed capture of lithium by the helium jet. Such a beam source \ncan be used to form an intense cold atom source with the potential future application of serving \nas a pump source for an atom laser.
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
The supersonic short-pulse helium atomic beam and its metastables were studied. A pulsed high-density helium atomic beam was ejected from a nozzle, collimated by a skimmer and then flowed through a penning-type discharge device. After discharging, the metastable density of the pulsed helium atomic beam was up to 1.24 × 10 11 cm −3 . Excellent symmetry of atomic beam profile was obtained. The deviation of angle of the atomic beam with respect to the axis was less than 0.14°. The mean diffusion angle of the pulsed beam was 4°, and a mean velocity of atomic beam was up to 1350 ms −1 . Laser induced fluorescence of metastable atoms has been studied.
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.
We present a simple and inexpensive design for a multichannel effusive oven nozzle which provides improved atomic beam collimation and thus extended oven lifetimes. Using this design, we demonstrate an atomic lithium source suitable for trapped-atom experiments. At a nozzle temperature of 525 °C, the collimated atomic beam flux directly after the nozzle is 1.2 × 10(14) atoms/s with a peak beam intensity greater than 5.0 × 10(16) atoms/s/sr. This suggests an oven lifetime of several decades of continuous operation.
Atomic oven generating controllable atomic beam flux plays a fundamental role in quantum gas experiments. Here, we report a new heater design that can heat up an high temperature atomic oven with fast thermal response. The new heater shows a heating rate improved by 7.65 times comparing to that of the conventional resistive heater while the crucible temperature can heated up to 1200K. With this oven, we generated a collimated ytterbium beam with flux exceeding 1014 atoms/s10^{14} \text{ atoms/s}1014 atoms/s at 823 K. We believe that our design offers a promising solution for shortening experimental dead time and improve the experiment efficiency in cold atom researches.