An improved technique for measuring hysteresis loss in superconducting magnets

This technique is based on an XY display of the hysteresis loop of a magnet; the hysteresis loss is proportional to the loop area. It is shown that the shape and area of the loop are simply related to the short sample properties of the superconductor. Extraneous effects in a magnet can sometimes distort the loop and this enables the technique to be used as a diagnostic tool in magnet testing.     

Harmonic measurements in superconducting magnets for a high energy accelerator

For the multipole field measurement in superconducting magnets for an accelerator application, a practical method was successfully introduced. The method uses a slow rotating coil assembly with an integator and a shaft encoder connected to a computer system. The accuracy and the quickness of the measurement was sufficient for the mass production test of the Fermilab Energy Doubler magnets.     

Niobium superconducting magnets

The process of design and construction, in 1954, of the first superconducting magnet is described. That magnet had Nb windings on an Fe core. It produced .71T in a small gap at 4.2K. Other Nb magnets, both with and without Fe, were built at several laboratories. Some of them are described, along with the applications to which a few were put. Empirically, it was clear in the 1950s that the performance of Nb as magnet wire was spectacularly improved by cold work.     

The effect of coolant loss on current leads for superconducting magnets

In this paper, we have examined the recovery of current leads for superconducting magnets after loss of coolant. It is found that if a lead is without cooling for longer than a time τ_max, thermal runaway can be prevented by an increase in cooling or a decrease in current through the lead. It is evident that designing a lead away from its optimal values can make the lead significantly less susceptible to problems of thermal runaway.     

An analogue digital double integration joulemeter for ac loss measurement in superconducting magnets

A technique for measuring ac losses in superconducting magnets is described. Two signals, a compensated voltage integral across the magnet and a magnet current differential, are multiplied in a Hall probe multiplier. The result is integrated and digitally displayed. The loss per cycle in a range of 0.1 – 10³ Js^?1 is measured by an electronic joulemeter based on this technique.     

AGS superconducting bending magnets

Four large aperture superconducting bending magnets are being built for use in the experimental beams at the AGS. Each of these magnets is 2.5 m long and has a room temperature aperture of 20 cm. The magnets are similar in design to the dipoles being developed for ISABELLE and employ a low temperature iron core. Results are presented on the "training" behavior of the magnets and a comparison will be made with the smaller aperture versions of this design. The magnet field measurements include end fields and leakage fields as well as the harmonic components of the straight section of the magnet.     

Detachable leads for superconducting magnets

A simple design of detachable leads has been incorporated in superconducting solenoids. Preliminary tests have shown very satisfactory behaviour of the leads up to 100 A to produce a magnetic field of 5.0 T in a bore of 22 mm. Such leads will be useful for energizing superconducting magnets in small liquid helium dewars of 1–2 ? capacity.     

Hysteretic effects in pulsed superconducting magnets

Superconducting magnets operating in a pulsed field mode are exposed to magneto-mechanical, thermomechanical and purely mechanical stresses. Since most materials incorporated in the magnet, such as the composite conductor, the insulation, the impregnants and the reinforcements, are strained to the limit of their yield strength, the mechanical and electrical behaviour of these materials will change during the first period of operation, until equilibrium conditions are reached.In this paper a new model for J_c(B) is presented. Based on this model hysteretic losses are calculated and compared to measured data. Losses due to stress-induced pseudo-elastic effects of the composite are calculated.     

Acoustic emission from superconducting magnets

We have shown experimentally that the acoustic emission (AE) is an effective means of monitoring the operating conditions of superconducting magnets and can be used to prevent catastrophic quenching. the signal-to-noise ratio of AE transducers at liquid helium temperature has been found to be about 18 dB higher than that at room temperature; AE originating from a superconducting magnet and those from the liquid helium shower can be clearly discriminated by observing their wave forms and frequency spectra. These results are also discussed from a theoretical view point based on an elastic medium model for the superconducting magnet.     

Magnetic field stability of superconducting magnets

A simple qualitative model is proposed to explain the fast initial change of the magnetic field of superconducting magnets which operate in the persistent mode. Using this model the additional component of the transport current which is induced by the decaying screening currents can be represented. The analytical results are in good qualitive agreement with the experimental curves. By a suitable cyclic approach to the frozen-in current it is possible to attain very good stability from the start.     

Protection device for superconducting magnets with a superconducting switch

A device, operating in the persistent current mode, was developed for quick energy removal from superconducting magnets. A magnet protection circuit which has a superconducting switch and a superconducting power switch connected in series is proposed. The performance of the system tested in connection with a laboratory solenoid (8T, 150 A, 50 kJ) was demonstrated.     

Quench back in thin superconducting solenoid magnets

Superconducting magnets with well coupled, low resistance, secondary circuits have been observed to become fully normal faster than quench propagation in the coil would permit. This process is referred to as ‘quench back’. Quench back observed at the Lawrence Berkeley Laboratory (LBL) was caused by heating the secondary circuit from the current induced from the primary circuit as normal region in the superconducting coil propagated. This paper develops the theory for thermal quench back in thin solenoid magnets and compares this theory with measurements made in two one-meter diameter superconducting solenoid magnets.     

Large diameter thin superconducting solenoid magnets

High energy physics experiments on the latest generation colliding beam machines require large superconducting solenoid magnets. The Lawrence Berkeley Laboratory is developing a new generation of high current density (～ 10⁹ A m^?2) superconducting solenoid magnets which are thin from a radion standpoint. To meet our design objectives, an integrated magnet and cryostat design has been evolved. Recent tests of one metre diameter prototype magnets show that our design concepts are technically viable. The results of the one metre magnet test are reported here.     

Magnetohydrodynamic ship propulsion with superconducting magnets

The feasibility of magnetohydrodynamic (MHD) ship propulsion using superconducting magnets is reviewed in light of recent advances in high-temperature superconductivity. The scaling relations for the electrical and hydraulic efficiencies of MHD pump-jets show that overall efficiencies >50% are feasible at speeds of 40 knots and higher provided that magnetic fields >5T can be maintained over volumes of the order of 100 m³. The development of large-scale electrical machinery and magnets using the high-temperature superconductors now under development could make it practical to construct submersibles for high-speed and silent operation. Low-speed tankers for movement of bulk cargo would be efficient with even lower fields.     

Disconnectable current leads for superconducting magnets

A pair of 100 A disconnectable current leads for superconducting magnets has been tested. This research note describes the device and gives the results of the tests. The liquid helium consumption due to the Joule dissipation of the gold contacts is small compared to the saving when the leads are disconnected. During an average duty cycle, the overall efficiency is, therefore, increased by a factor of two over that for permanent current leads.     

Numerical quenchback in thermofluid simulations of superconducting magnets

One of the most important thermofluid processes encountered in internally cooled superconducting magnets is that of quenching. Numerical simulation of the quench propagation involves accurately modelling a moving boundary layer at the quench front. Due to the highly non-linear nature of the quench process, slightest numerical errors can rapidly grow to unacceptable limits. The quench propagation in such a non-converged solution exhibits a very rapid propagation velocity which resembles a ‘quenchback’ effect. Hence, the term ‘Numerical Quenchback’ is used to characterize a numerically unstable solution of the governing quench model. This paper presents the underlying physical phenomena that causes a numerical discretization scheme to have error terms that increase exponentially with time, causing the numerical quenchback effect. Specifically, by analytically solving the equivalent differential equation of the numerical scheme, we are able to obtain closed-form relations for the error terms associated with the propagation velocity. This allows us to define error criteria on the space and time steps used in the simulation. The reliability of the error criteria is proven by detailed convergence studies of the quench process. © 1998 John Wiley & Sons, Ltd.     

Critical current of superconducting magnets wound from an anisotropic superconducting conductor

A superconducting magnet wound from an anisotropic superconductor, critical current calculation procedure is shown. For axisymmetric solenoids with rectangular cross-section and uniform current density the coefficient K_φ(α,β) = B_φ^max/B_φ was calulated and is presented in graphical form in the plane for the angles φ = 30°, 60°, 90°. Using the coefficient K_φ(α,β) it is possible to determine the critical current for a given angle ψ. In the case of the I_c-Bβ_ψ characterisctics of the superconductor expressed analytically, the term to determine I_cφ was derived. The critical current of the solenoid is determined by the lowest value of the critical currents I_ψ. Some results of our experimental investigations, showing typical examples of a critical current anisotropy are presented.     

Behaviour of power leads for superconducting magnets

In this paper we have attempted to gather together much of the information required by the designer of dc superconducting magnets. The new aspects of this work are: more information concerning performance of leads operating away from optimum conditions; comparisons of ideal cooling and a more realistic model for gas cooling; properties of gas-cooled leads on loss of coolant.