DESY, Hamburg, Germany
JLAB, Newport News, Virginia, USA
Most of the current R&D in SRF is focused on ways to reduce the construction and operating cost of SRF-based accelerators as well as on the development of new or improved cavity processing techniques. The increase in quality factors is the result of the reduction of the surface resistance of the materials. A recent test [*] on a 1.5 GHz single cell cavity made from ingot niobium of medium purity and heat treated at 1400 C in a ultra-high vacuum induction furnace resulted in a residual resistance of ~ 1nanoohm and a quality factor increasing with field up to ~ 5×1010 at a peak magnetic field of 90 mT. In this contribution, we present some results on the investigation of the origin of the extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat treatment of “all Nb” cavities.
[*] P. Dhakal et al., Phys. Rev. ST Accel. Beams 16, 042001 (2013).
JLAB, Newport News, Virginia, USA
At SRF 2011 we presented the study of quenches in high gradient SRF cavities with dual mode excitation technique[*]. The data differed from measurements done in 80’s that indicated thermal breakdown nature of quenches in SRF cavities. In this contribution we present analysis of the data that indicates that our recent data for high gradient quenches is consistent with the magnetic breakdown on the defects with thermally suppressed critical field. From the parametric fits derived within the model we estimate the critical breakdown fields and RF resistances at the breakdown site.
[*] G. Eremeev et al.,. In Proceedings of the 15th Superconducting RF conference,pp. 746-749, July 2011.
JLAB, Newport News, Virginia, USA
RAS/INR, Moscow, Russia
DESY, Hamburg, Germany
One of the alternative manufacturing technologies for SRF cavities is hydroforming from seamless tubes. Although this technology has produced cavities with gradient and Q-values comparable to standard EBW/EP cavities, a few questions remain. One of these questions is whether the quench mechanism in hydroformed cavities is the same as in standard electron beam welded cavities. Towards this effort Jefferson Lab performed quench studies on 4 different seamless hydroformed cavities. These cavities include DESY’s – Z163 and Z164 nine-cell cavities, and Black Laboratories nine-cell and two-cell TESLA shaped cavities, hydroformed at DESY. Initial results from the cavities and quench localization were published in SRF2011*. In this report we will present post JLAB surface retreatment quench studies for each cavity. The data will include OST and T-mapping quench localization as well as quench location preheating analysis comparing them to the observations in standard electron beam welded cavities.
*W. Singer, A. Ermakov, G. Kreps, A. Matheisen, X. Singer, K. Twarowski, I. Zhelezov, P. Kneisel, R. Crooks, Proceedings of SRF2011, TUPO026 2011.
Bergische Universität Wuppertal, Wuppertal, Germany
JLAB, Newport News, Virginia, USA
Enhanced field emission (EFE) from particulate contaminations and surface irregularities is one of the main field limitations of the superconducting Nb cavities required for XFEL and ILC. While the number density of particulate emitters can be reduced by dry ice cleaning (DIC) and clean room assembly, the optimum choice of crystallinity and polishing are still under discussion [1]. For the future ILC cavities, large or even single crystal Nb with a combination of BCP and EP is considered. Therefore, we have systematically investigated the EFE of single crystal Nb samples which got the same total polishing depth 136-138 μm but a different EP/BCP ratio (5.80, 2.40, 0.73, 0.15) and DIC by means of correlated optical/AFM profilometry, field emission scaning microscopy (FESM) and high-resolution SEM. Depending on the surface roughness (Ra < 200 nm), field enhancement factors b of 12 – 42 and emitting areas S up to 0.1 μm² were obtained. High current conditioning (μA - mA) of these emitters usually resulted in a slight reduction of b (factor < 2) but a strong increase of S. The influence of the surface roughness on the EFE and conditioning of the remaining emitters will be discussed.
[1] Reschke et al., Phys. Rev. ST Accel. Beams 13, 071001-1 (2010)
Bergische Universität Wuppertal, Wuppertal, Germany
DESY, Hamburg, Germany
Systematic investigations of the enhanced field emission (EFE) of HPR-cleaned large grain (LG) and single crystal (SC) Nb samples (Ra < 0.5μm) revealed an exponential increase of the emitter number density N with electric surface field Es and strong activation effects of the remaining particulates. Different types of EFE activation were observed: by high E partially combined with a micro-discharge or by heat treatments (HT) [1]. In cavities, EFE activation might also occur due to enhanced rf losses of particulates. Therefore, we have started a test series with two LG and two SC typically prepared Nb samples (40 μm BCP, 140 μm EP and HPR at DESY). At first all emitters (1 nA) up to Es = 160 MV/m were localized by means of correlated field emission microscopy (FESM). Then systematically varied in-situ HT between 122°C (24 h) and 400°C (2 h) were applied to investigate the activation of emitters due to the change of the natural Nb oxide. For all samples a significant increase of N with stronger HT up to 32 emitters/cm² at 400°C were obtained resulting in some activated emitters already at Es = 40 MV/m. Final SEM images of the activated emitters will also be discussed.
[1] A. Navitski et. al, subm. to PRSTAB 2013
INFN/LASA, Segrate (MI), Italy
Università degli Studi di Milano & INFN, Segrate, Italy
KEK, Ibaraki, Japan