| Paper | Title | Page |
|---|
| TUP44 | Development and RF Test Results of a New HF and H2SO4 Free Electro Polishing Method for Superconducting Niobium Cavities | 227 |
| | - M. Pekeler, S. Bauer, H. Vogel
ACCEL - D. Reschke, J. Ziegler
DESY
| |
| | A new Electro Polishing method for superconducting
cavities using an acid mixture without hydrofluoric and
sulphuric acid was developed at ACCEL Instruments
GmbH based on an exclusive license from Poligrat GmbH
in Munich, Germany (patent pending). Two 1.3 GHz
single cell cavities were built and are used for
qualification of the new Electro Polishing method. After
the Electro Polishing at ACCEL, the cavities are
transported to DESY for RF testing. At DESY no further
surface removal by any means is done, but a high pressure
water rinsing is carried out. Very promising test results
were achieved right away. During the first RF test of the
first treated cavity, Q values above 2x10^10 and accelerating
gradients above 20 MV/m were measured at a helium
bath temperature of 1.8 K. | |
| TUP48 | Dry-ICE Cleaning: The Most Effective Cleaning Process for SRF Cavities? | 239 |
| | - D. Reschke, A. Brinkmann, K. Floettmann, D. Klinke, J. Ziegler
DESY - D. Werner, R. Grimme, Ch. Zorn
Fraunhofer IPA
| |
| | The dry-ice sublimation-impulse cleaning (DIC)
technique using a two component ring jet has been proven
as a highly efficient cleaning process for niobium and
copper surfaces. The liquid carbon dioxide flows through
a ring-type nozzle assembled in a purpose-built cleaning
head, expands to form a dry-ice / gas mixture and is
accelerated by the surrounding nitrogen. A set-up for the
HORIZONTAL cleaning of single-cell niobium cavities
has been successfully commissioned during the last years.
A preliminary parameter set for effective final cleaning is
established. Several cavities have been cleaned and tested
without any detectable field emission up to 36 MV/m. As
application of the DIC technique might result in
additional cleaning potential for accelerator structures, an
extension of the set-up and testing of nine-cell cavities is
planned until mid of 2008. Furthermore, DIC was applied
to the copper injector "gun" cavity for TTF/FLASH [1]
recently. In order to reduce the dark current of the gun
cavity, a vertical cleaning setup was developed and tested. | |
| TUP69 | Optimization of Baking Parameters for Electropolished Niobium Superconducting Cavities | 304 |
| | - B. Visentin, Y. Gasser, M. Bruchon, F. Eozenou, J. P. Charrier
CEA-Saclay - D. Reschke, A. Brinkmann
DESY Hamburg
| |
| | High gradients, on bulk niobium cavities, can only be
reached after an imperative baking at low temperature
to suppress the high field Q-drop.
We demonstrate in this paper that the commonly used
standard process (under ultra high vacuum at 120 degree C for
48 hours) could be now simplified in terms of duration
(3 hours at 145 degree C) and requirement (argon atmosphere
instead of vacuum). Some efforts to more reduce
duration, down to only one hour, have been undertaken
to validate hypothesis and understand baking
phenomenon. The next improvement step with the
open-ended treatment of cavities in oxygen free
atmosphere is underway and it does not seem hard to
achieve. This new "fast baking" procedure will be very
useful in the XFEL and ILC projects where Nb cavities
mass production is required. | |
| TUP74 | Progress of the Test Cavity Program for the European XFEL | 327 |
| | - A. Brinkmann, J. Iversen, D. Reschke, W. Singer, X. Singer, K. Twarowski, J. Ziegler
DESY Hamburg
| |
| | Two main goals of the test cavity program for the
European XFEL are the qualification of alternative
niobium vendors and the investigation of the capabilities
of large grain niobium for large-scale nine-cell cavity
production. About 25 1.3 GHz single-cell cavities of
TESLA shape have been completed at Accel Instruments
and DESY. Alternative vendors for high purity fine-grain
niobium are ITEP Giredmet, Cabot, Ningxia and Plansee.
The in-house fabricated cavities have been tested after
800C firing and final electropolishing (EP) treatment. All
cavities exceed gradients of 35 MV/m at high Q-values.
For the large grain cavities of high RRR Heraeus niobium
gradients up to 41 MV/m have been achieved. The
performance after final chemical etching (BCP) is
compared to EP for several cavities. | |
| TUP77 | Recent Experience with Nine-cell Cavity Performance at DESY | 334 |
| | | |
| | The rf performance of the last nine-cell cavity
production for TTF/FLASH1 is analysed with respect to
maximum gradient, usable gradient and field limitation.
30 cavities have been manufactured at one company from
high RRR niobium (RRR > 300) by two vendors. All
cavities have been treated by a long (>150 um)
horiziontal EP (Electropolishing) and 800 degree C firing. The
cavity performance after final short (app. 50 um) EP with
or without subsequent ethanol rinse as well as a short
chemical etch (~10 um BCP) is compared. | |
| TUP80 | Single Cell Electro-Polishing at CEA Saclay: First Results | 343 |
| | - F. Eozenou, M. Bruchon, Y. Gasser, Y. Boudigou, B. Visentin, J. P. Charrier, S. Berry, C. Antoine
CEA-Saclay - D. Reschke
DESY Hamburg
| |
| | The goal of the Work Package 5.1 of the CARE SRF
program is to study electro-polishing (EP) of niobium and
thus isolate an ideal set of parameters for 1.3 GHz single
cell cavity treatment that should be applied to industry.
The more challenging aspect concerns acid composition's
optimisation, made of a mixture of hydrofluoric (HF) and
sulfuric (H2SO4) acids with possible additional water.
Preliminary experiments have been carried out on
samples to guide the program on single-cell cavities. In
that way, a set-up for horizontal EP has been installed in
our laboratory.
This set-up has been successfully qualified with the
standard recipe used at DESY since the first EP on
TESLA-shaped cavity has reached an excellent gradient
above 42 MV/m. The search for new parameters makes
then sense. Different configurations have been
investigated: higher temperature, lower voltage and new
acids' concentrations. Dummy cavities have been electropolished
with a new mixture:
1vol HF(40w%) - 2vol H2SO4(95w%) - 7vol H2O,
(1-2-7) with high water content.
Different cavities have also been electro-polished and
results including surface aspect, RF measurements and
impurities forming will be related. | |
| WEP24 | Field emission from single crystal and large grain niobium cathodes | 506 |
| | - A. Dangwal, G. Mueller
Berg. Universitat Wuppertal - D. Reschke, X. Singer
DESY
| |
| | Appreciable suppression of field emission from
metallic surfaces has been achieved by the use of
improved surface cleaning techniques, and dry ice
cleaning has emerged recently as a very effective tool in
this respect. In order to understand the effects of surface
preparation on field emission, systematic measurements
were performed on five single crystal and three large
grain samples of high purity (RRR > 300) Nb by means of
AFM, XRD, SEM and dc field emission scanning
microscope. The samples were treated with buffered
chemical polishing (BCP), half of those for 30 um and
others for 100 um removal of surface damage layer,
followed by a final high pressure water rinsing. The
samples with longer BCP treatment showed the onset of
field emission at slightly higher fields. A low temperature
(~ 150 degree C) heat treatment in high vacuum (10-6 mbar)
chamber for 14 hours, on a selected large grain Nb
sample, gives the evidence for the grain boundary assisted
FE at very high fields of 250 and 300 MV/m. Finally, an
interesting correlation between sizes of all investigated
emitters derived from SEM images with respect to their
respective onset fields has been found, which might
facilitate the quality control of superconducting radiofrequency
cavities for linear accelerators. | |