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The characteristics of superfluid helium as a technical coolant, which derive from its specific transport properties, are presented with particular reference to the working area in the phase diagram (saturated or pressurised helium II). We then review the principles and scaling laws of heat transport by equivalent conduction and by forced convectio...
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... look at the phase diagram of helium (Figure 3) clearly shows the working domains of saturated helium II, reached by gradually lowering the pressure down to below 5 kPa along the saturation line, and pressurised helium II, obtained by subcooling liquid at any pressure above saturation, and in particular at atmospheric pressure (100 kPa). Although requiring one more level of heat transfer and additional process equipment -in particular a pressurised-to-saturated helium II heat exchanger -implementation of pressurised helium II for cooling devices brings several important technical advantages [21]. ...
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Citations
... Phase diagram of helium with pressure in log scale. Adapted from[84]. ...
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... Cooling strings of superconducting devices in high-energy accelerators, however requires to transport heat over kilometer distances under minute temperature differences, beyond the practical capability of helium-II thermal conduction alone. Alternative cooling schemes based on forced convection of pressurised helium II are plagued by pressure drop and Joule-Thomson heating along the flow path, as well as by the limited efficiency and reliability of circulator pumps [4]. In order to maintain below 1.9 K the furthest magnet of each sector, located 3.3 km away from the 1.8 K refrigeration plant, the LHC uses a quasi-isothermal heat sink running through the magnet string, constituted by a bayonet heat exchanger [5] in which a stratified two-phase flow of saturated helium II absorbs the linear heat load, about 0.4 W.m -1 in the LHC arcs ( Figure 2). ...
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... where: y(T) -thermal conductivity of He II, [7,8], x λ -λ-front position along the tube. Assuming a constant wave velocity c and substituting x λ by c·t, one can calculate the time at which x λ equals to the tube length of 53 m. ...
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... This equation is scaled from prototype machinery tested at CERN [3]. 1 ...
The field range of superconducting devices may be extended by lowering their operating temperature, using superfluid helium refrigeration systems which have to deliver working pressures down to 1.6 kPa. The corresponding pressure ratio can be produced by integral cold compression or using a combination of cold compressors in series together with "warm" compressors at room temperature. The optimisation of such a system depends on the number, arrangement and characteristics of cold and warm machines as well as on the operating scenario and turndown capability. The aim of this paper is to compare relative investment and operating costs of different superfluid helium cryogenic systems, with the aim of optimising their cost-to-performance ratio within the constraints of their operating scenario.
... Accelerators based on this technology have to operate at 1.9 K minimising the amount of superconductor and hence the capital costs (see Figure 2). The superconducting devices operating in superfluid helium can be cooled with saturated or pressurised superfluid helium [7]. Figure 3 shows the different cooling schemes. ...
Cryogenics is now widely present in large accelerator projects using applied superconductivity. Economical considerations permanently require an increase of the performance of superconducting devices. One way to do this consists to lower their operating temperature and to cool them with superfluid helium. For this purpose, large cryogenic systems at 1.8 K producing refrigeration capacity in the kW range have to be developed and implemented. These cryogenic systems require large pumping capacity at very low pressure based on integral cold compression or mixed cold-warm compression. This paper describes and compares the different cooling methods with saturated or pressurised superfluid helium, gives the present status of the available process machinery with their practical performance, and reviews the different thermodynamical cycles for producing refrigeration below 2 K, with emphasis on their operational compliance.
... Prototypes of such leads, manufactured by industry, are being tested and show promising results [10]. The use of superfluid helium (T < 2.163 K at 1 bar) as magnet coolant brings other advantages than lower operating temperature alone [11]. Its high specific heat, which is 2000 times that of the conductor per thinspace volume, and extremely low viscosity make it a prime component for stabilizing the windings against thermal disturbances. ...
... 11: PFP measured after the reference-cycle for three short single aperture dipole models. ...
The windings of high--field superconducting accelerator magnets are usually made of Rutherford--type cables. The magnetic field distribution along the axis of such magnets exhibits a periodic modulation with a wavelength equal to the twist pitch length of the cable used in the winding. Such a Periodic Field Pattern (PFP) has already been observed in number of superconducting accelerator magnets. Additional unbalanced currents in individual strands of the cable appear to be causing this effect. The present thesis describes the investigation of the PFPs performed with a Hall probes array inserted inside the aperture of the LHC superconducting dipoles, both in the small--scale model magnets with a length of one meter and in full--scale prototypes and pre--series magnets with fifteen meters of length. The amplitude and the time dependence of this periodic field oscillation have been studied as a function of the magnet current history. One of the main parameters influencing the properties of the PFP is the cross--contact resistance between the strands of the cable. An estimation for these values is achieved by the so--called Field Advance (FA) measurements performed again with a Hall probes set--up. Due to eddy currents a difference in the field values for the ramp--up and the ramp--down of a current cycle is generated, which is a linear function of the applied ramp rate. The resulting slope is furthermore indirectly proportional to the corresponding cross--contact resistance. Two types of so--called interstrand coupling currents, uniform and non--uniform, are induced by a changing magnetic field and flow not only within the individual strands but also between the strands of the cable. Therefore some parts of the strands can carry a total current which is larger than the transport current. This phenomenon locally reduces the difference between the total strand current and the critical current of the superconductor and can provoke a premature quench of the superconducting magnet, i.e. a transition to the normal state. Considering theoretical models and experimental results the impact of the current distribution on the quench performance of the LHC dipoles is discussed. Finally an estimation for the influence of these currents on the magnet stability with respect to quench during operation conditions is given.
The Large Hadron Collider (LHC), a 26.7 km circumference
superconducting accelerator equipped with high-field magnets operating
in superfluid helium below 1.9 K, has now fully entered construction at
CERN, the European Laboratory for Particle Physics. The heart of the LHC
cryogenic system is the quasi-isothermal magnet cooling scheme, in which
flowing two-phase saturated superfluid helium removes the heat load from
the 36000 ton cold mass, immersed in some 400 m<sup>3</sup> static
pressurised superfluid helium. The LHC also makes use of supercritical
helium for nonisothermal cooling of the beam screens which intercept
most of the dynamic heat loads at higher temperature. Although not used
in normal operation, liquid nitrogen will provide the source of
refrigeration for precooling the machine. Refrigeration for the LHC is
produced in eight large refrigerators, each with an equivalent capacity
of about 18 kW at 4.5 K, completed by 1.8 K refrigeration units making
use of several stages of hydrodynamic cold compressors. The cryogenic
fluids are distributed to the cryomagnet strings by a compound cryogenic
distribution line circling the tunnel. Procurement contracts for the
major components of the LHC cryogenic system have been adjudicated to
industry, and their progress will be briefly reported. Besides
construction proper, the study and development of cryogenics for the LHC
has resulted in salient advances in several fields of cryogenic
engineering, which we shall also review
