SRF2013 - List of Authors (Mueller, G.)

Paper	Title	Page
TUP093	Field Emitter Current Conditioning on Nb Single Crystals with Different Roughness due to Varying EP/BCP Ratio	686
	S. Lagotzky, G. Müller Bergische Universität Wuppertal, Wuppertal, Germany P. Kneisel JLAB, Newport News, Virginia, USA
	Funding: Funding by the BMBF project 05H12PX6 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)

TUP094	Influence of Heat Treatments on Field Emitters on Nb Crystals	690
	S. Lagotzky, G. Müller Bergische Universität Wuppertal, Wuppertal, Germany A. Matheisen, D. Reschke DESY, Hamburg, Germany
	Funding: Funding by HGF Alliance and the BMBF project 05H12PX6 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

Paper

Title

Page

Field Emitter Current Conditioning on Nb Single Crystals with Different Roughness due to Varying EP/BCP Ratio

686

S. Lagotzky, G. Müller
Bergische Universität Wuppertal, Wuppertal, Germany
P. Kneisel
JLAB, Newport News, Virginia, USA

Funding: Funding by the BMBF project 05H12PX6
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)

TUP094

Influence of Heat Treatments on Field Emitters on Nb Crystals

690

S. Lagotzky, G. Müller
Bergische Universität Wuppertal, Wuppertal, Germany
A. Matheisen, D. Reschke
DESY, Hamburg, Germany

Funding: Funding by HGF Alliance and the BMBF project 05H12PX6
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