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GSI-UPGRADE-ACC-13 GSI SCIENTIFIC REPORT 2009
158
GSITemplate2007
Chemical stripping of bulk saturated NEG coatings
M. Wengenroth 1 , H. Kollmus 1 , M. C. Bellachioma 1 , S. Sievers 2 , J. Conrad 2 , M. Bender 1 ,
and H. Reich-Sprenger 1
1 GSI, Darmstadt, Germany; 2 TU, Darmstadt, Germany
Introduction
In the recent years the installation of non-evaporable
getter (NEG) coated dipole and quadrupole chambers at
the GSI heavy ion synchrotron (SIS 18) have been carried
out and ware completed in January 2010. The NEG
coated chambers was part of different upgrade activities
of the existing UHV system [1]. Those non-evaporable
thin film getters, consisting of Ti–Zr–V, are deposited
onto the vacuum chambers using three magnetron sputtering
facilities built at GSI, each fulfilling different geometric
shapes and dimensions. A detailed description of the
production and the functional principle is given in
Ref [1]. In brief, after coating and activation a highly
chemical reactive surface is present in the vacuum system
getting dissociated residual gas components by means of
oxidation, nitration and carbonisation of the surfaces metallic
constituents. The capacity of the NEG surface is
approximately one monolayer for CO and CO2 (10 15 particles
per cm 2 ). In the environment of the SIS18 the operation
time of an activated NEG surface is typically
longer compared to the venting cycle time due to maintenance.
The live time capacity of a 1 µm NEG thin film is
limited to 30 – 50 reactivations [2].
After the bulk of the NEG film is completely saturated,
it has to be stripped of the vacuum chamber before applying
new NEG coating. Industrial stainless steel pickling
utilizing solutions containing nitric and hydrofluoric acid
similar to buffered chemical polishing (BCP) [3] has
proven to work on thick walled chambers (>2 mm) [4] but
is not feasible for delicate dipole and quadrupole chambers
of only 0.3 mm wall thickness because of nonuniformities
especially at welded joints. Therefore, an
alternative method was proven and will be described in
the following.
Experimental procedure
Briefly, the chemical material removal is obtained by
an oxidation of the metallic constituents through the nitric
acid and the dissolution of these oxides by the hydrofluoric
acid. Performing those steps sequentially will considerably
reduce the risk of damaging a delicate chamber
permanently. Accordingly NEG coated samples have
been treated at the cleaning facility for superconducting
niobium RF cavities of S-DALINAC (Superconducting
Darmstadt Linear Accelerator). The oxidation is done by
immersing the samples into nitric acid (65 %) for
5 minutes followed by the removal of the oxides by immersing
into hydrofluoric acid (40 %) for 1 minute. Five
coated samples have been treated employing the above
sequence one to five times.
Measurements
The results of the etching were investigated using proton
Rutherford Backscattering Spectroscopy (RBS) at the
ion beam analysis department of the Forschungszentrum
Dresden-Rossendorf and shown in Fig. 1, were the energy
distribution of the backscattered protons is displayed.
Highest proton energies are achieved for collision with Zr
and only observed for the untreated sample (compare red
line in Fig. 1). This means that after the first treatment the
approx 1 µm NEG film was already completely removed.
counts
1000
100
1. treatment
2. treatment
3. treatment
4. treatment
5. treatment
NEG
10
600 700 800
energy [arb. units]
Figure 1: The energy distribution of the backscattered
protons shows the highest energies from collisions with
Zr only for the untreated NEG sample.
Conclusion
A method of stripping a saturated NEG film from thin
wall chambers has been proven. Nevertheless, nitric and
especially hydrofluoric acid can’t be handled without introducing
certain safety measures.
References
[1] M.C. Bellachioma, et al., Vacuum 82 (2008), p. 435
[2] P. Chiggiato and P. Costa Pinto,
Thin Solid Films 515 (2006) p. 382
[3] B. Aune, et al., Phys. Rev. ST-AB, 3, 092001 (2000)
[4] M. Wengenroth, Internal Report (2009)
Acknowledgements
The authors would like to thank M. Brunken of the
S-DALINAC accelerator group for his support and
R. Grötzschel (FZ Dresden-Rossendorf) and W. Assmann
(LMU Munich) for performing the RBS analysis at the FZ
Dresden-Rossendorf.