The narrow support elements (NSEs) between the casings of the superconducting coils of the Wendelstein 7-X magnet system consist of pads made of Al–bronze, which can slide and tilt against ...stainless-steel counter-faces. Static friction peaks or stick–slip events during sliding may cause disturbances to the magnet system and have to be avoided. Because the NSEs are operated at a temperature of ≈4
K and in vacuum at a residual pressure of
p
<
10
−6
mbar, only solid lubricants, in particular MoS
2, are applicable. Therefore, the static and dynamic friction behaviour of MoS
2-coatings was tested in liquid helium at 4.2
K and in vacuum at room temperature under high contact pressures and very low sliding velocities. In model tests with small Al–bronze pins, sliding in reciprocating motion against a PVD–MoS
2-coating on a steel substrate, very stable sliding behaviour without coating failure, and a friction coefficient below 0.1 was observed. However, tests with downscaled samples in original geometry showed stable sliding in liquid nitrogen, but unexpected stick–slip behaviour in liquid helium at 4.2
K. For further investigations and because in actual service the NSEs are not immersed into a liquid cryogen, tests under vacuum at low temperatures are being prepared.
Spacers and supports belong to the critical elements in the structure of a nuclear fusion device, particularly, when they are installed in the vacuum or superconducting magnet system of the reactor. ...These supports have to sustain high loads while allowing a certain amount of tangential movement with low friction and without any stick-slip motion. Vacuum and cryogenic environments are extremely hostile for smooth sliding systems, because conventional liquid lubricants cannot be employed, and only a very limited number of materials for dry sliding systems under these conditions are available. In order to test the friction behaviour of support elements in the field coils of the fusion experiment WENDELSTEIN 7-X, a test rig was build which allows testing in liquid helium environment at 4.2 K, with a normal force up to 150 kN, and a sliding velocity of 0.1 mm/s in oscillating motion. First tests have shown that the apparatus is appropriate for simulating the sliding behaviour of support elements. As an example, a result of an earlier model test of an MoS2-coating is shown.
A new and very simple method for the wavelength and line width measurement of optical sources, with a resolution in the femtometre range, is presented. The author's method is based on the ...amplification of a backscattered wave, by a pump wave due to Brillouin scattering. It requires neither a fast photodiode nor a spectrum analyser of any kind.
Magnet coils made out of Nb3Sn superconductors usually are manufactured by the wind- and react-technique. Due to the brittleness of the A15 material the superconductive layer is formed only after the ...winding of the magnet. This is done by a heat treatment in which Sn diffuses via a matrix into Nb filaments and the superconducting layer is formed. Depending on the exact temperature and time of the heat treatment, the physical properties of the superconductor such as critical current density Jc, upper critical field Bc2, critical temperature Tc and n-value can be varied over a wide range. This is because the diffusion process determines the grain size distribution, the thickness of the superconductive layer as well as the Sn distribution within the layer. This article will provide a review of the investigations concerning different aspects of heat treatment over recent years.
A new multi-functionality hybrid energy storage concept, LIQHYSMES, has been recently proposed. It combines the use of LIQuid HYdrogen (LH2) as the bulk energy carrier with much faster and efficient ...superconducting magnetic energy storage (SMES). The LIQHYSMES Storage Unit LSU integrates liquefaction and storage of H2 as well as the LH2-cooled SMES: A process for the intermediate storage of H2 in liquefied form is proposed, and alternative SMES designs are compared. The basic operational principle is simulated for a simple model case with two large LIQHYSMES storage plants supporting the transfer of renewable energy from one region of strong supply to a second one with a widely negative imbalance between supply and load. Losses of all plant components are analysed in terms of their relevance for the overall efficiency, and some cost issues are briefly addressed. A small first experimental demonstration is now underway and also briefly introduced.
•A 2-stage Joule–Thomson-expansion is proposed for the intermediate H2 storage.•Depending on operating conditions, liquefaction losses may be 18–66 kJ/mol.•LIQHYSMES plants target long-term energy balancing and short-term power control.•Simulations using actual profiles demonstrate the anticipated buffering capability.•A small first experimental demonstration is now underway.
A new multi-functionality hybrid energy storage concept, LIQHYSMES, has been recently proposed. It combines the use of LIQuid HYdrogen (LH sub(2)) as the bulk energy carrier with much faster and ...efficient superconducting magnetic energy storage (SMES). The LIQHYSMES Storage Unit LSU integrates liquefaction and storage of H sub(2) as well as the LH sub(2)-cooled SMES: A process for the intermediate storage of H sub(2) in liquefied form is proposed, and alternative SMES designs are compared. The basic operational principle is simulated for a simple model case with two large LIQHYSMES storage plants supporting the transfer of renewable energy from one region of strong supply to a second one with a widely negative imbalance between supply and load. Losses of all plant components are analysed in terms of their relevance for the overall efficiency, and some cost issues are briefly addressed. A small first experimental demonstration is now underway and also briefly introduced.
In order to generate fields of more than 20 T using pure superconducting magnets, the implementation of HTS coated conductors (CCs) is an option. Amongst the characteristics of the REBCO is its ...intrinsic anisotropy which means its critical current, Ic, varies as a function of its orientation with respect to the external magnetic field, φ, as well as being dependent on external field, B, and temperature, T. For the design of the high field insert coils for the experimental magnet facility HOMER II, the behaviour of Ic at fields of about 24 T and at φ approximately 80 ° is required. Within our JUMBO facility it is possible to measure the U(I) behaviour of commercial CCs at liquid helium and liquid nitrogen temperatures. The free bore available is 100 mm at 4.2 K; the applied magnetic field can be varied up to 10 T, and φ can be set between 0 and 180 °. Measurements were carried out on commercially available CCs with emphasis on φ values near to 90 °. The critical current was determined by means of a power law function fitted to the resulting U(I) curves using specially written Matlab® programs. A number of different three dimensional functions were fitted to the measurement data and compared with each other.
Pulsed laser deposition (PLD) is classically considered as a very reliable process that provides correct stoichiometry of the deposited layer when the deposition speed is below 0.1 nm per deposition ...pulse. Initially, we found significant critical currents of 200-400 A/cm-width by very high deposition rates. Later, in this work, a study of reproducibility and feasibility of high- and ultrahigh-speed PLD is performed. With high speed, we denote a range from 0.1 to 0.3 nm/pulse, whereas ultrahigh speed corresponds to higher numbers, i.e., above 0.3 nm/pulse. At ultrahigh laser energy density, an optimum deposition rate is found that maximizes the critical current, i.e., I c . The I c at this maximum may reach 500-700 A/cm-width by relatively low (<; 1.5 MA/cm 2 ) J c . The optimal deposition rate and energy density may be employed in cost-feasible technological solutions because deposition time may be shortened by a factor of ~5. A suggested double-layer model with different critical currents in layers provides good fitting of the experimental dependence.
Magnet coils made out of Nb
3Sn superconductors usually are manufactured by the wind- and react-technique. Due to the brittleness of the A15 material the superconductive layer is formed only after ...the winding of the magnet. This is done by a heat treatment in which Sn diffuses via a matrix into Nb filaments and the superconducting layer is formed. Depending on the exact temperature and time of the heat treatment, the physical properties of the superconductor such as critical current density
J
c, upper critical field
B
c2, critical temperature
T
c and
n-value can be varied over a wide range. This is because the diffusion process determines the grain size distribution, the thickness of the superconductive layer as well as the Sn distribution within the layer.
This article will provide a review of the investigations concerning different aspects of heat treatment over recent years.
PDF
