The authors describe the mechanical design of the two-dimensional cross section of the baseline collider dipole magnet for the Superconducting Super Collider. The components described are the collar and yoke laminations and the cold mass shell. The 50-mm-aperture collider dipole magnet uses stainless-steel collars to position the conductors at the locations specified by the magnetic design and to prestress the coil to prevent conductor motion under excitation. The collars are supported by the vertically split yoke and cold mass skin to reduce their deflection under excitation. The collar interior is designed to give the coil its required shape at the operating temperature, taking into account all deflections caused by assembly and cooldown. >

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Mechanical design of the 2D cross-section of the SSC collider dipole magnet

Superconducting coils used for high energy accelerator magnets are usually fixed with epoxy resin in the fabrication processes called "curing". The assembled magnets in the processes have shown different critical current degradation figures due to excitation ramp rate even if they were made in the same manner. The ramp rate dependences of degradation seem to be affected by AC losses between strands of the Rutherford cables (inter-strand AC losses), which are easily affected by the curing conditions. Although they can be reduced by increasing cross-over resistances between strands, the stability problems caused by current sharing between strands may arise. It is important for such kinds of superconducting accelerator magnets to clarify the mechanisms of the AC losses and stabilities of the Rutherford cables. Experimental studies on the AC losses and the current sharing problems for several types of cable samples have been performed. >

Stabilities of the Rutherford cables with Cu matrix and CuMn barrier

Experimental apparatus for measurements of AC losses in Rutherford type cable conductors has been constructed. A number of compacted cable samples have been measured. Hysteresis loss, loss from coupling within strands and loss from interstrand coupling are distinguished from each other. The results show that even for cables without soldering and coating, the AC losses may be quite different from each other due to interstrand coupling loss. As the curing temperature increases, interstrand coupling loss tends to increase. For some cables, interstrand coupling loss increases nearly geometrically with the increase of curing temperature. Most of the samples did not show dependence of loss on mechanical pressure. >

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AC loss measurements of Rutherford type superconducting cables under mechanical stresses

In superconducting magnets operating at high heat loads as the ones for interaction region of particle colliders or for fast cycling synchrotrons, the limited heat transfer capability of state-of-the-art electrical insulation may constitute a heavy limitation to performance. In the LHC main magnets, Nb-Ti epoxy-free insulation, composed of polyimide tapes, has proved to be permeable to superfluid helium, however the heat flux is rather limited. After a review of the standard insulation scheme for Nb-Ti and of the associated heat transfer mechanisms, we show the existence of a large margin available to improve insulation permeability. We propose a possible way to profit of such a margin in order to increase significantly the maximum heat flux drainable from an all polyimide insulated Nb-Ti coil, as it is used in modern accelerator magnets.

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Cable Insulation Scheme to Improve Heat Transfer to Superfluid Helium in Nb-Ti Accelerator Magnets

In this thesis work we investigate the heat transfer through the electrical insulation of superconducting cables cooled by super fluid helium. The cable insulation constitutes the most severe barrier for heat extraction from the superconducting magnets of the CERN Large Hadron Collider (LHC). We performed an experimental analysis, a theoretical modeling and a fundamental research to characterize the present LHC insulation and to develop new ideas of thermally enhanced insulations. The outcome of these studies allowed to determine the thermal stability of the magnets for the LHC and its future upgrades. An innovative measurement technique was developed to experimentally analyze the heat transfer between the cables and the super fluid helium bath. It allowed to describe the LHC coil behavior using the real cable structure, an appropriate thermometry and controlling the applied pressure. We developed a new thermally enhanced insulation scheme based on an increased porosity to super uid helium. It aims at withstanding large heat loads, as needed for the High Luminosity LHC upgrade (HL-LHC). Experimental measurements of the new insulation showed a major improvement of heat extraction compared to the present state-of-the-art used in the LHC. We developed a theoretical heat transfer model quantitatively explaining the experimental results of the LHC and HL-LHC insulations. We identified the heat extraction mechanisms, mainly occurring through super fluid helium micro-channels. The average micro-channels dimensions were estimated to vary between few and dozens of µm, depending on the insulation scheme. In the model we considered the known laws describing the dynamic regimes of super fluid helium. However such laws were never demonstrated to be valid in the narrow channels typical of the cable insulation. We developed a new experimental device to investigate heat transport through microchannels. Micro-fabrication techniques were used to etch the channels down to a depth of 16 µm. We measured the classical laminar and turbulent regimes, thus demonstrating the validity of such heat transport laws independently from the channel geometrical shape and size down to these dimensions. To conclude, we used the obtained experimental and theoretical heat transfer results to determine the LHC and HL-LHC magnets stability. We estimated their quench margin and compared it to the beam induced heat deposit. The resulting difference allows a safe operation of the magnets.

Heat Transfer between the Superconducting Cables of the LHC Accelerator Magnets and the Superfluid Helium Bath

Results of tests of a redesigned quench protection heater for SSC (Superconducting Super Collider) dipole magnets are presented. It is shown that the redesigned heater has a performance similar to that of the existing heater in terms of its ability to protect a long 40-mm magnet. It meets the system requirements of increased resistance by using a smaller cross sectional area and a larger active length. This results in a larger area of heater, which has advantages but does not result in higher powers being required from the heater firing unit. The implication of increased cost is mitigated somewhat since lower-gauge connection cable is made possible. The magnet program for the 50-mm dipole is characterized by a higher operating margin and slower quench propagation velocities. >

SSC dipole quench protection heater test results

A series of 50-mm-diameter 1.5-m model magnets has been constructed. The tests of these magnets gave convincing results concerning the design of the 50-mm cross section of the Superconducting Super Collider dipoles. Although there are still difficulties in the detailed control of end shape, coil size, pressure and field, the basic design of the magnet is proved to be valid by this model magnet study. Results on quench characteristics, mechanical behavior, and field quality are presented. >

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Tests of 1.5 meter model 50 mm SSC collider dipoles at Fermilab

Construction and testing of the components for the new Tevatron D0/B0 low beta insertion are nearly complete. The devices include superconducting cold iron quadrupoles utilizing a two-shell, cos2 theta coil geometry with a 7.6-cm aperture. The maximum design gradient is 1.41 T/cm at an operating current of 4832 A. They have the highest current density with the highest peak field on the winding of any quadrupole yet built. The authors summarize the quench performance and ramp rate sensitivity of the two-shell design and relate the performance characteristics to the relevant aspects of design and fabrication. >

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Quench performance of superconducting quadrupole magnets for the new Fermilab low beta insertion

Tests have been performed at Fermilab on 1.5-m magnetic length model SSC (Superconducting Super Collider) dipoles using both bipolar and unipolar ramp cycles. Hysteresis energy loss due to superconductor and iron magnetization and eddy currents is measured and compared as a function of various ramp parameters. Additionally, magnetic field measurements have been performed for both unipolar and bipolar ramp cycles. Measurements such as these will be used to estimate the heat load during collider injection for the SCC High Energy Booster dipoles. >

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Bipolar and unipolar tests of 1.5 m model SSC collider dipole magnets at Fermilab

The yoke in SSC dipole magnets provides mechanical support to the collared coil as well as serving as a magnetic element. The yoke and skin are used to increase the coil prestress and reduce collar deflections under excitation. Yokes split on the vertical or horizontal mid-plane offer different advantages in meeting these objectives. To evaluate the relative merits of the two configuration a 1.8 m model dipole was assembled and tested first with horizontally split and then with vertically split yoke laminations. The magnet was extensively instrumented to measure azimuthal and axial stresses in the coil and the cold mass skin resulting from cooldown and excitation. Mechanical behavior of this magnet with each configuration is compared with that of other long and short models and with calculations.

T.S. Jaffery

Papers

SSC dipole quench protection heater test results

Tests of 1.5 meter model 50 mm SSC collider dipoles at Fermilab

Quench performance of superconducting quadrupole magnets for the new Fermilab low beta insertion

Bipolar and unipolar tests of 1.5 m model SSC collider dipole magnets at Fermilab

Experimental Evaluation of Vertically Versus Horizontally Split Yokes for SSC Dipole Magnets