Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces Page: 4 of 9
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larger than 20 pm (see Fig.5). We believe that the pits are
created-due mainly to the generation of oxygen gas on the
surface of the anode.
3.2 Polishing Rate
After the determination of the optimum polishing
conditions, especially the limiting current density, we
would like to know the polishing rate under such
condition for a sample (anode) located in the middle slot
of the sample holder. The samples were weighed before
and after polishing. The mass dissolved in one hour is
1.209 g. Since the density of Nb is 8.66 g/cm3, the
polishing rate is therefore about 0.646 gm/min- This
value is much larger than 0.381 gm/min adopted by KEK
for their electrolytic recipe. The difference here is surely
attributable to the different electrolytic recipes employed
In the present recipe lactic acid is added to the mixture
of sulfuric and hydrofluoric acids. The addition of an
organic acid is generally believed  to act in the
following three beneficial ways: a) it will enable
electropolishing to be done at a lower anode current
density and temperature; b) it can extend the useful life of
the acid mixture; c) it can assist throwing power during
electropolishing. Therefore, it is natural to think that the
observed quicker polishing rate here is mainly a result of
the addition of lactic acid to the electrolyte. Another
plausible reason is due to the difference in the amount of
hydrofluoric acid in the two different electrolytes. It has
been known for quite a long time that in electropolishing
on Nb using the mixture of sulfuric and hydrofluoric
acids, the following chemical reaction takes place :
2Nb + 10HF + 2H20 -> 2H2NbOF5 + 5H,. 1)
This equation indicates clearly that hydrofluoric acid is
the major determinative chemical compound in the
electrolyte during electropolishing. We believe that
equation 1 is applicable not only for electropolishing
using KEK recipe, but also for that using the present
electrolytic recipe. If this is true, then it is clear that the
quicker polishing rate here may also be a result of larger
amount of hydrofluoric acid adopted in the present
electrolytic acid- The third reason may be due to the
acceleration of Nb removal rate when the concentration of
sulfuric acid is decreased. The last point will be also
discussed in Section 3.4 later.
Fig.6 shows typical optical images of the surfaces after
electropolishing for 15 and 60 minutes. They are almost
identical as that after electropolishing for 30 minutes (see
Fig.3c), implying therefore that polishing may take place
homogenously over the time after establishing polishing
conditions. We found that the polishing conditions could
be established in a time period less than two minutes.
The higher polishing rate here is beneficial to us from
economic point of view- This means that it takes only 155
minutes to remove a thickness of 100 pm from a surface
of Nb, which reduces the polishing time by 107 minutes
Fig.6: Typical images of the Nb surfaces treated under
the application of the determined limiting current density
(see text for details) for a) 15 minutes and b) 60 minutes.
as compared with that using the KEK recipe. This will
result in a huge capital saving for the cost of
electropolishing on Nb cavities.
3.3 Distance Effect Between Cathode and Anode
In the set-up of the electropolishing system designed by
KEK, the shape of cathode is a rod. The distance between
cathode and anode, therefore, varies continuously from
iris to equator for a Nb cavity in that case. Intuitively
speaking, the change in distance may create an
inhomogeneous electric field distribution over the interior
surface of the cavity, resulting in an inhomogeneous
polishing. This is not good for Nb cavities, since the
performance of a Nb cavity is determined by the weakest
part of the cavity. If we are going to use a cathode of the
same shape, we need to know how the variation in the
distance affects the polishing results.
To study this effect, we first measured voltage as a
function of the distance between cathode and anode when
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Delayen, Jean R.; Mammosser, John; Phillips, Larry & Wu, Andy T. Alternate Electrolyte Composition for Electropolishing of Niobium Surfaces, article, September 1, 2001; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc739035/m1/4/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.