[en] Ongoing studies at Fermilab investigate whether dislocations and other factors instigate pitting during cavity electropolishing (EP), despite careful processing controls and the inherent leveling mechanism of EP itself. Here, cold-worked niobium coupons, which exhibited increased tendencies for pitting in our past study, were annealed in a high vacuum furnace and subsequently processed by EP. Laser confocal scanning microscopy and special defect counting algorithms were used to assess the population of pits formed. Hardness measurements indicated that annealing for 2 hours at 800 C produced recovery, whereas annealing for 12 hours at 600 C did not, as is consistent with known changes for cavities annealed in a similar way. The 800 C anneal was effective in some cases but not others, and we discuss reasons why tendencies for pitting remain. We discuss implications for cavities and continued work to understand pitting.

[en] Short communication

[en] At the future FAIR facility, reliably working solid carbon stripper foils are desired for providing intermediate charge states to SIS18. With the expected high beam intensities, the foils experience enhanced degradation and limited lifetime due to severe radiation damage, stress waves, and thermal effects. This work presents systematic measurements of the temperature of different carbon-based stripper foils (amorphous, diamond-like, and carbon-nanotube based) exposed to 4.8 MeV/u U, Bi, and Au beams of different pulse intensities. Thermal and spectroscopic analyses were performed by means of infrared thermography and Fourier transform infrared spectroscopy. The resulting temperature depends on the foil thickness and strongly increases with increasing pulse intensity and repetition rate. (author)

[en] Utilizing difference in temperature dependencies we decoupled Bardeen-Cooper-Schrieffer (BCS) and residual components of the microwave surface resistance of superconducting niobium at all rf fields up to B_rf∼115 mT. We reveal that the residual resistance decreases with field at B_rf≲ 40 mT and strongly increases in chemically treated niobium at B_rf>80 mT. We find that BCS surface resistance is weakly dependent on field in the clean limit, whereas a strong and peculiar field dependence emerges after 120 °C vacuum baking.

[en] The article deals with the optimal control problem with the parabolic equation as state problem. There are point-wise constraints on the state and control functions. The objective functional involves the observation given in the domain at each moment. The conditions for convergence Udzawa's type iterative method are given. The parareal method to inverse preconditioner is given. The results of calculations are presented. (paper)

[en] Magnesium diboride (MgB₂) has a transition temperature (T_c) of ∼40 K, i.e., about 4 times as high as that of niobium (Nb).We have been evaluating MgB₂ as a candidate material for radio-frequency (RF) cavities for future particle accelerators. Studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (Rs) than Nb at 4 K. A power dependence test using a 6 GHz TE011 mode cavity has shown little power dependence up to ∼12 mT (120 Oe), limited by available power, compared to other high-Tc materials such as YBCO. A recent study showed, however, that the power dependence of Rs is dependent on the coating method. A film made with on-axis pulsed laser deposition (PLD) has showed rapid increase in Rs compared to the film deposited by reactive evaporation method. This paper shows these results as well as future plans

[en] A model for the onset of the reduction in superconducting radio-frequency (SRF) cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Since magnetic fields at the cavity equator are tied to accelerating electric fields by a simple geometric factor, the onset of magnetic flux penetration determines the onset of Q-drop. We consider breakdown of the surface barrier at triangular grooves to predict the magnetic field of first flux penetration H_pen. Such defects were argued to be the worst case by Buzdin and Daumens (1998 Physica C 294 257), whose approach, moreover, incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter κ. Since previous Q-drop models focused on either topography or contamination alone, the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of H_pen when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. Still lower H_pen was predicted when both effects were combined, i.e. contamination should exacerbate the negative effects of roughness and vice versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of κ. This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ∼ 20%, and that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.

[en] Magnesium diboride (MgB₂) is a superconducting material that has a transition temperature (T_c) of ∼40 K, which is ∼30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of MgB₂ can be lower than Nb at 4 K. One of the problems with other high-T_c materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that MgB2 shows little increase in the surface resistance up to ∼120 Oe, equivalent of an accelerating field of ∼3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with MgB₂ and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.

[en] Chernobyl NPP accident is recognized as the global disaster. Wide amount of population including children was involved. According to the direct measurements results from 1986 the 40% of children population received thyroid irradiation doses from 0.3 to 2.0 Gy and 10% of them - over 2 Gy respectively. Cesium incorporation value calculated per irradiation dose not exceeded 0.5 rem in 97% of kids. Children examination was organized in 2 steps. Within 1. one various transient reactions of nervous system, skin, mucosa, blood formula were revealed. Within 2. one the complete medical examinations were carried out both with dosimetric control. ChNPP accident poly-factor nature was fixed. Wide amount of somatic pathology not related to radiation exposure dose was registered. 12 years after the accident as the result of wide-scale epidemiological study was found out that children population had been exposed to both acute (evacuated persons) and long-term chronic (contaminated territories residents) effect of radiation factor. The main health risk for children involved in nuclear disaster zone is connected with non-oncological blood and haemopoietic organs diseases, digestive system pathology and mental disorders. Parameters excess was revealed in group of persons with higher collective dose accumulated for 10-12 years. Thyroid cancer incidence rate growth is recognized as the radioinduced effect. 12 years after the accident is obvious that 'health detriment' for children population of ChNPP accident survivors is expressed in additional number of endocrine system diseases cases - 24%, blood and haemopoietic organs - 33%, digestive system - 24%, mental disorders - 52% respectively. (authors)

[en] Several films of niobium were deposited on copper plates via the ultra-high vacuum cathodic arc (UHVCA) deposition method as described by R. Russo et al. [R. Russo et al., Supercond. Sci. Tech. 18 (2005) L41; R. Russo et al., J. Appl. Phys., submitted for publication]. We attached these end plates to a 6 GHz cavity operating in the TE₀₁₁ mode for characterizing the film quality by measuring the Q versus surface magnetic field