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Low temperature physics on a jet-plane

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February 28, 2013

Yuichi Okuda describes his group’s research on microgravity experiments in aircraft flying parabolic trajectories to shed light on the true nature of the crystallization of helium at ultra-low temperatures

“My group is investigating the crystal growth of helium-4 (He-4) and helium-3 at ultra-low temperatures,” says Yuichi Okuda. “Observing the crystallization behavior of helium at milli-Kelvin gives tremendous insight into the physics of so-called quantum solids and resulting phenomena including superfluidity.”

However, in spite of the major advances in research on the low temperature crystalline properties of helium there are still fundamental unaddressed questions. “All our experiments to date have been conducted under the influence gravity,” explains Okuda. “One of the important unexplored areas in the research is the effect of gravity on the crystallization dynamics of helium. That is, we want to know the true nature of helium-4 crystallization at low temperatures and under zero gravity.”

vDesigning experiments to optically monitor crystallization dynamics of He-4 under zero gravity is challenging. Meeting this challenge, Okuda and colleagues constructed a unique experiment system consisting of a cryostat for cooling the He-4 to sub-Kelvin temperatures and peripheral video monitoring equipment, for installation in a small jet plane. “The plane containing the experimental equipment and researchers, took off and went through a series of parabolic dives, which produced an environment of less than 0.01 G for about 20 seconds,” says Okuda. “We regard this as being almost zero gravity for these experiments.”

The aircraft produced about 8 parabolic maneuvers per flight, during which time the crystallization dynamics of He-4 were observed using video cameras. Needless to say, compared with low temperature experiments conducted on the ground, the limited space and movement of the ‘MU300’ jet plane lead to restrictions in the design of the apparatus: Total weight of equipment ~300 kg; power lines of ac 100 V (1 kVA and 1.5 kVA) and two dc of 28 V (0.7 kVA); equipment was housed inside three racks measuring 900 (h) mm x 700 (l) mm x 450 (w) mm. The system enabled the realization of a temperature of 0.62 K for a period of approximately 7 hours.

The main findings of the experiments during the parabolic flights were (1) The ‘bcc’ structure of He-4 did not change over a period of 20 seconds at the relatively high temperature of 1.6 K due to its low crystallization rate. (2) At 0.63K the ‘hcp’ phease of He-4 changed to where the “c-facet became larger and the a-facet emerged on the surface”, notes Okuda.

Notably, the crystals remained attached to the wall of the sample holder due to adhesive forces resulting from interaction with the walls. “In more recent experiments we used acoustic vibrations to dislodge the He-4 crystals from the walls,” says Okuda. “Intriguingly, we discovered so-called Ostwald ripening of these crystals after exposure to acoustic waves, where smaller crystals melt and larger ones grow to minimize the surface energy.”

These unique set of experiments not only shed light on the fascinating field of quantum solids but also open the door to a new field of low temperature physics at zero gravity.

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Ready for boarding. Professor Yuichi Okuda is second from right.

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Ready for takeoff for parabolic flight based experiments. The equipment on the right hand side is the compressor and controller for producing sub-kelvin temperatures.

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The experimental conditions on board the airplane.

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Shape of a relatively large 4He crystal in the hcp phase at T = 0.63 K. Gravity values are specified in each frame. The horizontal line in the right picture is the rough surface under 1 G, and the inclined line is c facet. The crystal responded to the gravity change and c facet expanded under reduced gravity (in the left picture). A facet is also apparent on the sides of the crystal. The diameter of the field of view (circular visible region) was 24 mm.

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Time evolution of 4He crystals which splashed into superfluid at zero gravity initially at 0.67 K. Ostwald ripening continued to the end and only the largest crystal survived at the cost of smaller ones. The photograph on the extreme right shows crystals after only 3 seconds has elapsed from the splashing.

Further information
- Okuda Group: http://www.ltp.ap.titech.ac.jp/eng/member.htmlouter
-Development of a 3He Refrigerator for Possible Experiments of Solid He-4 on a Small Jet Plane, J Low Temp Phys (2011) 162: 733-739
-He-4 crystals in superfluid under zero gravity, Physical Review E 85, 030601(R) (2012)
-Ripening of splashed 4He crystals by acoustic waves with and without gravity, New Journal of Physics 14 (2012) 12302

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