B.S. Brown

Radiation effects in superconducting fusion-magnet materials

Abstract Economic considerations appear to dictate the use of superconducting magnets in magnetic fusion reactors. Since these magnets will experience high fluxes of energetic neutrons and secondary gammas, it is important to know the effects of this radiation on the operating characteristics of the magnets. Experimental data on radiation effects in the individual magnet components (superconductors, stabilizers, insulators, and structural materials) are reviewed here; cryogenic irradiations (≲10 K) are emphasized, since these are required for design data. There is a dearth of information on the effects of low-temperature irradiation of insulators and an irradiation program is strongly recommended. The relative importance of property changes in the various components depends on the details of the individual magnet design and the importance of various factors (alternate materials, shielding thickness and penetrations, periodic anneals, etc.) can be analyzed only after the radiation-induced property changes of the components are incorporated into the specific design.

Low-temperature fast-neutron radiation damage studies in superconducting materials

Abstract The residual resistivity increase rate, dpi/dt ,as a function of induced resistivity for fast-neutron irradiated Cu, Nb, Nb doped with oxygen, V, Ta, Pb, Cd and NbTi has been studied after 18 K fast neutron irradiation, dpi/dt as a function of irradiation induced resistivity was approximately linear for all samples. Saturation values of the resistivity and defect concentrations have been determined and are comparable with previous values for Cu and Nb. The isochronal recovery has been studied up to 579 K. Large effects of oxygen on the annealing behavior are observed in niobium and vanadium and annealing peaks are identified with oxygen migration near 460 K.

Influence of He implantation on the fatigue properties of stainless steel under different atmospheric conditions

Abstract The influence of He on the fatigue properties of stainless steel was investigated using α-particle implantation. The He influence was compared for different external atmospheres (inert, corrosive), various fatigue temperatures (400–750°C), implantation temperatures (400–950°C), He doses (5–3000 ppm), strain amplitudes (0.5–3%) and fatigue frequencies (0.02–8 Hz). In situ and post-implantation fatigue testing showed that the effect of He implantation is very similar in both cases. The effect of He is small if the fatigue temperature is ≤ 600°C. In these cases the fracture mode remains transgranular and only small reductions of the fatigue life (less than a factor of 2) are observed upon He implantation. For higher fatigue temperatures the He causes a transition from a transgranular to an intergranular fracture mode associated with rather dramatic reductions of the fatigue life. It was shown that this fracture mode must be attributed to a growth of He bubbles at the grain boundaries. The growth is probably achieved by condensation of thermal vacancies, the flux of which is controlled by the external stresses and by grain-boundary diffusion. It was found that the size of the lifetime reduction increases with the He dose and the implantation temperature, because more He reaches the grain boundaries. The lifetime depends more strongly on the strain amplitude for irradiated samples. The lifetime for irradiated samples does not depend on the external atmosphere, in contrast to unirradiated samples which have an order of magnitude longer life in the clean atmosphere. In contrast to failure in a transgranular mode, the number of fatigue cycles until feature, N ƒ , is found to decrease with the fatigue frequency in the case of intergranular mode. The temperature above which intergranular fracture occurs (usually above 700°C) is affected by the He dose and the fatigue frequency. For high doses of ≈ 1000 ppm He and small frequencies of ≈0.02 Hz, the intergranular mode is observed as low as 600°C.

Resistivity and Tc measurements in low temperature irradiated Nb3Sn and Nb3Ge

The electrical resistivity and superconducting transition temperature were measured in films of Nb3Sn and Nb3Ge after fast neutron irradiation at 20 K to 1.3 × 1018n/cm2 (E > 0.1 MeV) and fission fragment irradiation at 65 K up to an equivalent fast neutron dose of 5 × 1020n/cm2. The rate of change of ρ up to the saturated state fits a model for which the radiation induced damage is in the form of disorder. The changes in ρ have an exponential dependence with dose that agrees with earlier models of disordering in which the rate of change of disordering (proportional to ρ) is proportional to the degree of order. Annealing up to 425 K after the high dose irradiation did not result in a change in ρ, and Tc remained below the measurable temperature of 4 K. However, simultaneous recovery in Tc and ρ occurred between 280 and 500 K after the low dose experiment, indicating defect mobility at this temperature.

Neutron irradiation facilities at the intense pulsed neutron source

Abstract There are facilities for irradiation down to 4.2K with fast neutrons at Argonnes recently constructed Intense Pulsed Neutron Source (IPNS-I). The large irradiation volume, the neutron spectrum and flux, the ability to transfer samples without warm up, and the dedication of the facilities during the irradiation make this ideally suited for radiation damage studies on components for superconducting fusion magnets. Possible experiments are discussed on cyclic irradiation and annealing of stabilizers in a high magnetic field, mechanical tests on organic insulation irradiated at 4K, and superconductors measured in high fields after irradiation.

Critical‐current changes in neutron‐irradiated Nb3Sn as a function of irradiation temperature and initial metallurgy

Fast‐neutron irradiations have been performed on Nb3Sn wires to determine the roles of irradiation temperature and the unirradiated value of the critical‐current density (Jc0) on the changes in Jc. Four experiments were performed: low‐ (6 K) and high‐ (350 K) temperature irradiation of high‐ (2×1010 A/cm2 at 5 T and 4.2 K) and moderate‐ (0.8×1010 A/cm2) Jc0 material. The large increases in Jc for the moderate‐Jc0 material were independent of irradiation temperature; the increases were smaller for the high‐Jc0 material, and a significant difference was found for the two irradiation temperatures. The difference is attributed to the greater pinning capability of defect cascades over defect clusters in the high‐Jc0 material. The smaller amount of Jc recovery after a 300 K anneal, when compared with NbTi, can also be understood in terms of the defects responsible for the flux pinning.

Critical current density changes in irradiated Nb3Sn

Soll recently modeled changes in the current-carrying capacity of superconducting Nb3Sn after irradiation. As the dose increases, the critical current density (Jc) generally increases, reaches a maximum, and decreases. The model relates the maximum Jc for different types of irradiations to the integrated damage energy (ED) that the irradiating particles transfer to the lattice. Earlier, Soll et al. related cirtical-temperature (Tc) decreases in irradiated Nb3Sn to Ed, which appears more reasonable since Tc is a measure of disorder (or replacements) accompanying defect production, and the final defect configuration (or displacements) are less important. The annealing temperature for the disorder (~700°C) exceeds any of the irradiation temperatures (TIRR); therefore, TIRR is unimportant in the Tc experiments. However, different defect structures exhibit considerably different flux pinning and the Jc model does not consider different spatial variations of the defects during production or migration and agglomeration of the defects during high-TIRR experiments, both of which affect flux pinning. The lack of the model to take into account the physics of the damage is the subject of this paper. Arguments are presented why the defect configurations should be considered, and recently published data is presented that conflict with the conclusions of this damage-energy model.

Simulation of fusion reactor conditions for superconducting magnet materials

We report on a comprehensive study of neutron irradiation induced changes of critical current densities in a variety of Nb-Ti and some Nb 3 Sn superconductors with Ti additions as well as of changes in normal state resistivity of the copper stabilizer at 8 T. The samples were irradiated at 5 K and subjected to room temperature annealing cycles after each irradiation step. The results on Nb-Ti show, in general, small degradations of j c , which do not exceed 20% over the lifetime fluence at the magnet location. Contrary to the low-field data, we find a rather uniform radiation response of metallurgically different NbTi materials at high fields. The results on (Nb, Ti) 3 Sn show the usual increase of critical current densities with neutron fluence, followed by a sharp decrease of j c . However, the peak of j c is shifted to much lower fluences in the alloyed samples as compared to pure Nb 3 Sn. Finally, results on the resistivity change of “magnet” copper are presented, which indicate that a considerable increase of resistivity (∼60% at 8 T) cannot be avoided under reasonable operating conditions of the magnet.

Neutron Irradiation of Superconductors and Damage Energy Scaling of Different Neutron Spectra

Three different neutron sources were used to irradiate identical sets of NbTi superconductors up to about half the lifetime dose of a superconducting magnet in a fusion reactor. Based on a careful source characterization of the TRIGA Mark-II reactor in Vienna, the spallation neutron source IPNS at Argonne and the 14 MeV neutron source RTNS-II at Livermore, the damage energy cross sections were calculated for four different types of NbTi alloys (42, 46.5, 49 and 54 wt% Ti). The experimental results on the variations of critical current densities Jc with neutron dose are found to scale within the experimental uncertainties with the appropriate damage energy cross sections. This first explicit proof of damage energy scaling for Jc-variations in superconductors is considered to be most valuable for the evaluation of radiation damage in superconductors under fusion reactor conditions.

Argonne hosts first summer school

Abstract During the two-week period from August 16-27, 1999, Argonne National Laboratory hosted the first National School on Neutron and X-Ray Scattering funded by the Office of Basic Energy Science of the Department of Energy. Many of Argonnes divisions were involved in the preparation and implementation of the intense two-week program and, as judged by the students who attended the school, the effort was a success. The school is necessary in order to fulfill a national need in training graduate students in the utilization of national user facilities and it is our intention to offer this course at Argonne in the future.