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The use of superconducting materials — those that conduct electricity with no resistance — has proven to be the best way of avoiding overheating in the coils and of keeping them as small as possible. Superconductivity could not happen without the use of cryogenic systems. The coils' niobium-titanium NbTi wires must be kept at low temperatures to reach a superconducting state.

The LHC's superconducting magnets are therefore maintained at 1. Cryogenic techniques essentially serve to cool the superconducting magnets. In particle detectors they are also used to keep heavy gases such as argon or krypton in a liquid state, for detecting particles in calorimeters, for example. The layout of the LHC magnet cooling system is based on five "cryogenic islands" which distribute the cooling fluid and convey kilowatts of cooling power over several kilometres.

The entire cooling process takes weeks to complete. It consists of three different stages. During the first stage, helium is cooled to 80 K and then to 4. It is injected into the cold masses of the magnets in a second stage, before being cooled to a temperature of 1.

Experimental Techniques In Condensed Matter Physics At Low Temperatures

During the first stage, some 10, tonnes of liquid nitrogen are used in heat exchangers in the refrigerating equipment to bring the temperature of the helium down to 80 K. The helium is then cooled to 4. Once the magnets have been filled, the 1. Helium was a natural choice of coolant as its properties allow components to be kept cool over long distances. At atmospheric pressure gaseous helium becomes liquid at around 4. This property of superfluid 3He has been given the name texture.

Matter and Methods at Low Temperatures | Frank Pobell | Springer

At surfaces the wave function must be oriented with the orbital axis normal to the surface just as the stable angular momentum configura- tion of a top is the one with the spin perpendicular to the surface. It costs energy for the wave function to change direction in space. However, the textures can be readily bent through a variety of means such as magnetic fields and fluid flow.

The characteristic bending length of the texture in the B phase has been found experimentally to. A striking difference between superfluid 4He and superfluid 3He was found in studies of the fluid flow. In both substances the fluid can flow through spaces too small for penetration by the normal-state liquid.

In liquid 4He, there are no preferred orientations for such a flow. How- ever, when the flow of superfluid 3He in the A phase was studied in magnetic fields, it was found that the flow was much more rapid along the direction of the field than in the plane perpendicular to the field. The magnetic field aligned the fluid texture to produce an anisotropy in one of its most fundamental properties, the fluid flow. When the same flow experiments were repeated in the B phase, no dependence of the flow on the direction of the magnetic field was observed.


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In liquid 3He, the nuclear magnetic dipoles exhibit a coherent response to external perturbations and are coupled to the orbital character of the macroscopic state. Because of this the frequency of the NMR absorption is shifted as if it were in internal fields of the order of gauss.

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A step change in the magnetic field produces a ringing in the amplitude of the magneti- zation parallel to the field. Similarly, a radio-frequency field polarized along the steady external field can produce a detectable resonant energy absorption at the same ringing frequency. Sound at frequencies much larger than the atomic collision frequency of the liquid, the so-called zero sound, can be propagated with only weak damping and has been studied in all the superfluid phases of 3He and in the normal fluid.

Unlike 4He, where only a single zero sound mode exists, a large number of narrow absorption modes have been observed in superfluid 3He because of the complex nature of the order parameter. The excitations associated with the distortion of the order parameter have been studied most extensively in the B phase because its isotropic.

Careful examination of the modes revealed that they were analogues of phenomena first studied for single particles in the early days of atomic physics, the Stark effect and the Zeeman effect. In the case of 3He there is a remarkable difference. These line-splitting phenomena occur because of distortions that affect the wave function that simultaneously describes the behavior of all the atoms in the container at once. Normal, or first, sound, the usual long-wavelength mode of most fluids, has been used to determine the viscosity parameters of the fluid.

Second sound, the famous entropy wave in 4He, is also a spin wave in 3He. Fourth sound, a compression wave in a superleak, was used to make the first convinc- ing demonstration of superfluid flow in 3He. In any real container there must be at least one defect because it is impossible to meet the boundary condition that the wave function be perpendicular to the surface at all points in even the simplest three-di- mensional geometry, the sphere. Modern topological methods have been used to classify the types of defects that might appear in the texture fabric.

Sharp walls in which the direction of the orbital axis is reversed by degrees are solitons. Solitons can be created in either the spin or the spatial portion of the superfluid wave function through the appli- cation of intense radio-frequency pulses at the NMR resonance. The existence and specific nature of such defects have been investigated through examination of, and the changes in, the resonance modes in the NMR spectrum. These types of defects persist in the liquid almost indefinitely after their creation if neither the fluid temperature nor the magnetic field is changed.

They can be erased by removing the magnetic field or by flow of the fluid. The presence of the vortices was demonstrated, and estimates of their density made, again through NMR methods. Recently it has been found that a spontaneous magne- tization appears in the vortex cores of fluid in the B phase. This result has strong implications for our understanding of the magnetic field about neutron stars. Modern models of the dense centers of such stars suggest that there also the fermions, neutrons in this case, become paired in a triplet state similar to the B phase of superfluid 3He.

Fluid flow has been studied in cylindrical, spherical, and parallel plate geometries. The motion of a wire moving through the fluid has also been carefully analyzed. The critical velocities limiting the superfluid flow in both the A and B phases appear to be much smaller than originally expected.

The viscosity measured in the fluid at low temper- atures is also much smaller than expected. These deviations from theory are probably related to both the long mean free path of the normal fluid excitations and the peculiar kinetic behavior of these excitations at boundaries.

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One useful product of the flow studies has been the determination of the superfluid mass fraction in 3He. The results are important for understanding other hydrodynamic experiments. The superfluid mass fraction can be found from determinations of the amount of fluid that is not changed through viscous contact with chamber surfaces. Novel Quantum Fluids There is a variety of gases and fluids that can be cooled to low enough temperatures for quantum-mechanical effects to become im- portant in understanding their physical behavior.

Examples that have been studied for quite some time include the bulk behavior of liquid 3He, liquid 4He, liquid mixtures of the two helium isotopes, and the conduction electrons in metals. The temperature at which quantum statistics becomes important is that at which the thermal de Broglie wavelength becomes comparable to the interparticle spacing. In the case of fermions, 3He, and electrons, this temperature is the Fermi temperature, and in the case of bosons, for example 4He, the temper.

Statistical me- chanics predicts a macroscopic occupation of the ground state for Bose particles cooled below the Bose condensation temperature. In liquid 4He this temperature is calculated to be 3 K, and it seems likely that the superfluid transition of bulk 4He at 2.

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In the latter part of the s attention was turned to these fluids under new, extreme physical conditions. The Fermi systems are being investigated under conditions of large polarization where only one spin population is present. The Bose systems are being investigated in unusual geometries and under conditions of increasing dilution of the distance between the particles.

The newest materials in this class that have come under investigation are the gases of atomic hydrogen and deuterium, which can be stabilized against the formation of H2 and D' through polarization of the atomic electrons in large magnetic fields at low temperatures.


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The gas fermions in superfluid 4He have weak interactions. The 4He acts as an ether medium supporting a dilute gas that behaves almost like an ideal Fermi gas. The properties of such mixtures in low magnetic fields were studied extensively in the late s and early s to measure the small deviations from ideal gas behavior and to develop the dilution refrigerator this is discussed later in this chapter in the section on Low-Temperature Technology.

The spin-triplet potential has no bound states. Hence, if all the atoms could be forced to interact only through the triplet potential large densities of the material could be collected, and because of the weak interaction between the atoms they would remain as a gas to absolute zero temperature. The hydrogen atom has an even number of fundamental particles, one electron and one proton. It thus is a boson and should display a statistical condensation like that in 4He. Deuterium is a fermion, and it should have properties similar to the weakly interacting Fermi gases discussed in other sections of this report.

The search for. It offers an entirely new example of a superfluid that can be compared with 4He.

Episode 48: Low Temperatures - The Mechanical Universe

Hydrogen would be an especially important example because the interparticle coupling is so weak that virtually all of the substance properties can be calculated from simple principles. In addition, hydrogen has a nuclear magnetic moment, whereas 4He does not. A large class of collective spin phenomena, similar to those in superfluid 3He, are expected to be found.

Research in this area is in its infancy. The experimental problems are formidable, but significant progress has been made. It is not obvious that the problem can be solved by improvements in the cooling techniques.

The hydrogen in the gas phase is in pressure equilibrium with atoms bound to the container surface. The dominant processes for atomic recombination take place on the surface. The surface recombi- nation rate increases rapidly with the density of surface atoms.