High gradient SRF research at DESY › e56 › e42370 › e43987 › e56531 › ... · all cases...
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S. Aderhold, D. Reschke (DESY) SRF cavities for linear accelerators Inspection in different stages of preparation Comparison: Optical inspection ↔ Temperature mapping The properties of the Niobium make the optical inspection of the inner surface of SRF cavities very demanding. The smoothness of the surface after chemical treatment leads to bright reflections and low contrast. Since last year the prototype of an optical system, developed at KEK and Kyoto University, is available at DESY, that has been adapted to the situation inside a cavity and allows the inspection of the inner surface in situ. It consists of a high resolution digital camera in combination with a special lighting system. Illumination is done by a LED and 20 electroluminescent stripes. The stripes can be turned on and off individually to light the surface under different angles and thereby guarantee appropriate lighting for different structures on the surface. At the same time they allow to gain information about the surface profile, distinguishing between pits and bumps on the surface. Optical inspection setup Reaching high accelerating gradients In the last 20 years, the maximum accelerating gradient of SRF cavities has been significantly improved. Several of the cavities that have been processed and tested at DESY have reached accelerating gradients higher than 35 MV/m in vertical test. Some even performed near 40 MV/ m or above. For the examples shown in the plot above, final surface treatment was electropolishing (EP) in all cases but two, where the final step was BCP-flash, meaning that the removal of the last 10 μm is done by buffered chemical polishing (BCP). Even though such high gradients are reached in many cases, the spread in the production is large with some cavities limited around 20 MV/m and below, so that mass production with high yield needs a good understanding of the limiting effects. One feature limiting the gradient can be geometrical defects on the inner surface, like defects in the welding seams. Optical inspections in consecutive stages of the surface treatment process allow the study of formation and evolution of surface defects. In the example shown above, one of the nine equator welding seams was found with rough areas and steep edges at the grain boundaries after the main surface removal by EP. In the RF measurement with T-map the quench location for the π-mode was found in this cell. The goal is to categorize structures on the inner surface, in order to understand, which defects keep a cavity from reaching high accelerating gradients. The ability to detect and remove such limiting features in early stages of the processing may help to improve the yield in a large scale production. Optical inspection of the inner surface of cavities One of the core parts of linear accelerators as the European XFEL and the International Linear Collider (ILC) are the accelerating structures, in this case 9-cell RF resonators (cavities) made from Niobium operated at 1.3 GHz. In order to keep the length of a linear accelerator within a reasonable limit for a given particle energy, the accelerating gradient of the cavities has to reach certain values, the ILC design operating gradient is 31.5 MV/m. To reach high accelerating gradients, one has to demand high standards of the quality of the cavitie's inner surface with respect to smoothness and cleanliness. In order to attain these conditions, few hundred micrometers of the surface material are removed by (electro-)chemical means and before the test or assembly to an accelerator module the cavities are cleaned and handled in a clean and dust-free environment (clean room). High gradient SRF research at DESY Heating at quench location observed during RF-test 1mm During the RF measurement in the vertical test stand the surface temperature of the cavity can be monitored by an rotating array of thermo-sensors (T-map). In case of a local breakdown of superconductivity (quench) the position can be identified by the increase in temperature at the outer surface. In several cases subsequent optical inspection of the corresponding areas on the inner surface revealed visible defects, e.g. in the welding seam. 1mm Defect in optical inspection picture of corresponding area of inner surface Equatorial welding seam before chemical treatment Equatorial welding seam before chemical treatment 1mm Equatorial welding seam after final-EP 1mm Equatorial welding seam after main-EP Camera cylinder entering the cavity with three illumination stripes lit on each side of the camera opening Schematic overview of inspection system setup AC70, Test 7 AC71, Test 9 AC72, Test 6 AC73, Test 2 AC76, Test 15 AC81, Test 8 Z93, Test 8 Z103, Test 3 Z104, Test 2 AC112, Test 10 AC113, Test 4 AC115, Test 1 AC116, Test 1 AC117, Test 5 AC122, Test 1 Z143, Test 2 0 5 10 15 20 25 30 35 40 45 1E+009 1E+010 1E+011 Q 0 E max / MV/m Bump on cavity surface 1mm Cavity performance :Quality factor Q 0 vs. accelerating gradient E max
Transcript of High gradient SRF research at DESY › e56 › e42370 › e43987 › e56531 › ... · all cases...
S. Aderhold, D. Reschke (DESY)
SRF cavities for linear accelerators
Inspection in different stages of preparationComparison: Optical inspection ↔ Temperature mapping
The properties of the Niobium make the optical inspection of the inner surface of SRF cavities very demanding. The smoothness of the surface after chemical treatment leads to bright reflections and low contrast. Since last year the prototype of an optical system, developed at KEK and Kyoto University, is available at DESY, that has been adapted to the situation inside a cavity and allows the inspection of the inner surface in situ.It consists of a high resolution digital camera in combination with a special lighting system. Illumination is done by a LED and 20 electroluminescent stripes. The stripes can be turned on and off individually to light the surface under different angles and thereby guarantee appropriate lighting for different structures on the surface. At the same time they allow to gain information about the surface profile, distinguishing between pits and bumps on the surface.
Optical inspection setup
Reaching high accelerating gradients
In the last 20 years, the maximum accelerating gradient of SRF cavities has been significantly improved. Several of the cavities that have been processed and tested at DESY have reached accelerating gradients higher than 35 MV/m in vertical test. Some even performed near 40 MV/m or above. For the examples shown in the plot above, final surface treatment was electropolishing (EP) in all cases but two, where the final step was BCP-flash, meaning that the removal of the last 10 µm is done by buffered chemical polishing (BCP).Even though such high gradients are reached in many cases, the spread in the production is large with some cavities limited around 20 MV/m and below, so that mass production with high yield needs a good understanding of the limiting effects. One feature limiting the gradient can be geometrical defects on the inner surface, like defects in the welding seams.
Optical inspections in consecutive stages of the surface treatment process allow the study of formation and evolution of surface defects. In the example shown above, one of the nine equator welding seams was found with rough areas and steep edges at the grain boundaries after the main surface removal by EP.In the RF measurement with T-map the quench location for the π−mode was found in this cell.
The goal is to categorize structures on the inner surface, in order to understand, which defects keep a cavity from reaching high accelerating gradients.The ability to detect and remove such limiting features in early stages of the processing may help to improve the yield in a large scale production.
Optical inspection of the inner surface of cavities
One of the core parts of linear accelerators as the European XFEL and the International Linear Collider (ILC) are the accelerating structures, in this case 9-cell RF resonators (cavities) made from Niobium operated at 1.3 GHz. In order to keep the length of a linear accelerator within a reasonable limit for a given particle energy, the accelerating gradient of the cavities has to reach certain values, the ILC design operating gradient is 31.5 MV/m.
To reach high accelerating gradients, one has to demand high standards of the quality of the cavitie's inner surface with respect to smoothness and cleanliness.In order to attain these conditions, few hundred micrometers of the surface material are removed by (electro-)chemical means and before the test or assembly to an accelerator module the cavities are cleaned and handled in a clean and dust-free environment (clean room).
High gradient SRF research at DESY
Heating at quench location observed during RF-test
1mm
During the RF measurement in the vertical test stand the surface temperature of the cavity can be monitored by an rotating array of thermo-sensors (T-map). In case of a local breakdown of superconductivity (quench) the position can be identified by the increase in temperature at the outer surface. In several cases subsequent optical inspection of the corresponding areas on the inner surface revealed visible defects, e.g. in the welding seam.
1mm
Defect in optical inspection picture of corresponding area of inner surface
Equatorial welding seam before chemical treatmentEquatorial welding seam before chemical treatment
1mm
Equatorial welding seam after final-EP
1mm
Equatorial welding seam after main-EP
Camera cylinder entering the cavity with three illumination stripes lit on each side of the camera opening
Schematic overview of inspection system setup
AC70, Test 7AC71, Test 9AC72, Test 6AC73, Test 2AC76, Test 15AC81, Test 8Z93, Test 8Z103, Test 3Z104, Test 2AC112, Test 10AC113, Test 4AC115, Test 1AC116, Test 1AC117, Test 5AC122, Test 1Z143, Test 2
0 5 10 15 20 25 30 35 40 451E+009
1E+010
1E+011
Q0
Emax / MV/m
Bump on cavity surface
1mm
Cavity performance :Quality factor Q0 vs. accelerating gradient Emax
