ABSTRACT In the past two years several laboratories around&apos;the world have observed degradations of the Q-value of superconducting niobium cavities made from high purity niobium under certain cool-down conditions. Severe degradations have been reported for larger systems, when they were slowly cooled down or warmed up to temperatures A systematic study of the influence of surface treatments like chemical polishing, anodic oxidation and heat-treatment as well as the cooling conditions on cavity performance of cavities manufactured from niobium of different purity has been conducted. Possible reductions of the Q-degradations by anodic oxidation have been explored. Results of these investigations are reported and changes in material parameters as caused by the degradation are discussed

Kenji Saito

Peter Kneisel

CiteSeerX

Observation of Q-Degradation in Superconducting 
Niobium Cavities due to Cooldown Conditions* 
Kenji Saito and Peter Kneisel 
Continuous Electron Beam Accelerator Facility 
12000 Jefferson Avenue, Newport News, VA 23606 
ABSTRACT 
In the past two years several laboratories around'the world have observed degra- 
dations of the Q-value of superconducting niobium cavities made from high purity 
niobium under certain cool-down conditions. Severe degradations have been reported 
for larger systems, when they were slowly cooled down or warmed up to temperatures 
< 200K. 
A systematic study of the influence of surface treatments like chemical polish- 
ing, anodic oxidation and heat-treatment as well as the cooling conditions on cavity 
performance of cavities manufactured from niobium of different purity has been con- 
ducted. 
Possible reductions of the Q-degradations by anodic oxidation have been ex- 
plored. Results of these investigations are reported and changes in material param- 
eters as caused by the degradation are discussed. 
INTRODUCTION 
In 1989, R. Riithl observed a significant degradation of the Q-value of a 1500 
MHz cavity made from high purity niobium after it had been kept at nitrogen tem- 
perature for several days after the initial test in superfluid helium. A temperature 
map indicated that these unexpected losses were distributed uniformly over the whole 
cavity surface. Other laboratories2l3 observed similar degradations in Q-value when 
cavities were kept for longer periods of time at temperatures between 200K and 50K. 
* This work was supported by the U. S. Department of Energy under contract 
DE-AC05-84ER40150. 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Because of the important impact of this observation on accelerator projects involv- 
ing superconducting niobium cavities, a large number of investigations were carried 
out at various labora tor ie~ .~1~1~ The initial hypothesis expressed in1 of precipitation 
of the Nb-hydride phase was supported by additional experimental data provided 
by other laboratories. The detrimental effect of hydrogen precipitation in niobium 
has been known for many years.7 It was experimentally verified that a hydrogen de- 
gassing of the niobium at temperatures above 800" C eliminated the Q-degradation 
phenomenon,31415 but that extensive chemical polishing reversed the process. A sum- 
mary of these investigations is provided at this workshop.* 
At CEBAF a study is being conducted to first verify the Q-degradation under 
certain experimental conditions and secondly to find a possible cure for the increased 
losses, which could be easily applied to complex cavity systems. We chose to look 
into the process of anodic oxidation and this paper reports on the results obtained 
with several cavities. 
CAVITY PREPARATION AND TESTING 
For the investigations single cell and five-cell 1500 MHz cavities of the 
CORNELLICEB AF-design were used.' The cavities were manufactured of niobium 
from four different suppliers: reactor grade niobium with an RRR-value of m 40 
by Cabot Corporation and high purity niobium with RRR-values 2 250 supplied 
by W. C. Heraeus, Teledyne and Fansteel. After manufacturing all cavities were 
chemically treated and assembled in a standard way: 
A. Buffered chemical Polishing 
After degreasing in a caustic solution combined with ultrasonic agitation the 
cavities were chemically polished in a buffered 1:1:1 solution of hydrofluoric (49%), 
nitric (65%), and phosphoric (85%) acids at room temperature for several minutes 
resulting in the removal of x 60 pm of niobium from the surface. The cavities were 
thoroughly rinsed with ultrapure water with ultrasonic agitation for one hour with 
several exchanges of the rinse water. A final rinsing with reagent grade methanol 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
was carried out in a class 100 cleanroom, where the assembly of the variable input 
coupler and the transmission probe took place. The cavities were then attached to a 
cryogenic test station using extreme precautions to avoid particulate contamination 
of the inner cavity surfaces. 
B. Anodizing 
After a cryogenic test with the cavity was completed, the niobium surfaces were 
renewed by removing 3 - 4 pm of niobium by buffered chemical polishing as described 
above, rinsed in ultrapure water and subsequently anodized with a solution of 10% 
ammonium hydroxide. At a voltage of 100 V and a current density of z 0.8 mA/cm2 
an amorphous Nb205 - layer of & 2000 A was grown, after the current had decayed 
to 1/10 of its initial value at 100 V. In one case, the oxide layer was only grown to 
1300 d (65 V). 
Rinsing in ultrapure water and methanol as described above followed prior to 
assembly in the cleanroom. 
C. Testing 
Before cooldown to 4.2K the cavities were evacuated to a vacuum of 5 6 X I O - ~  
torr. The standard cooldown ("fast") took place in less than one hour and the static 
external magnetic field in the vicinity of the cavity was shielded to less than 10 mG 
by an active compensation coil in combination with two layers of p-metal. 
During the experiment S (a) the temperature dependence of the surface resistance 
at low RF-fields (Epeok X 2 - 4 MV/m) was measured between 4.2K and 1.7K and 
(b) data for the variation of the Q. - value with increasing peak surface electric fields 
were obtained. 
For the experiments, during which the cavities were held at intermediate tem- 
peratures (50K < T < 120K), the cavity temperature was monitored by two Si-diodes 
attached to the outside of the cavities. Typically the natural warm-up from 50K to 
lOOK happened in X 20 hours in our cryostat. 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
EXPERIMENTAL RESULTS AND DISCUSSION 
Examples for the temperature dependence of the surface resistance for "bare" 
niobium and anodized niobium are shown in Figures 1 and 2. Figures 3 to 8 show 
examples for Q vs. E for "bare" and anodized niobium cavities after "fast" cooldown 
and a warm-up to an intermediate temperature of lOOK for x 20 hours. 
The results of our investigation can be summarized as follows: 
a). A cavity fabricated from reactor grade niobium with an RRR-value of m 40 
did not exhibit any degradation in Q-value due to warming up to intermediate 
temperatures 5 145K (see Figure 3). 
b). Severe degradations were observed in cavities fabricated from high purity nio- 
bium with RRR-values 5 250, when the cavities were kept at temperatures 
between 50K and 120K for > 16 hours (see Figures 4 and 6). 
c). A warm-up to 170K from 4.2K and maintaining this temperature for 16.5 hours 
did not result in a Q-deterioration for a five-cell cavity (see Figure 7). 
d). A degradation can be fully recovered by warm-up to room temperature (see 
Figures 5 and 7). 
e). An electropolished cavity (120 pm) was annealed at 710' C for three hours and 
subsequently electropolished an additional 5 pm as a final step; no Q-degradation 
was observed.13 
f). This cavity was subsequently chemically polished in a standard buffered solution 
resulting in a removal of x 60 pm. A degradation of the Q-value was observed, 
but less severe than in a cavity, which did not have an intermediate vacuum 
annealing (see Figure 8). 
g). Anodic oxide layers of a thickness 2 1300 A reduce the amount of degradation 
to < 50%; thicker layers of d 2 2000 A seem to be more effective. As an ex- 
perimental result we observed somewhat higher residual resistances with anodic 
layers. 
h). RBcs(T) of a degraded surface ("bare" niobium) cannot be expressed by a 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
single superconducting phase in the temperature range of 4.2K >_ T 2 1.9K (see 
Figure 1). Multiple phases are generated within the penetration depth and the 
residual resistance increases to 240 nOhm - 800 nOhm. Deviations from the 
BCS-behavior are most pronounced in the range 2.9K 2 T 2 2.5K (see Figure 
1) and the experimental data cannot any longer be fitted to the BCS theory as 
supplied by J. Halbritter's surface resistance program.10 
i). Anodized surfaces show single phase (exponential) behavior of RBcs(T) even 
after a warm-up to intermediate temperatures (see Figure 2). With material pa- 
rameters for pure niobium, T,, London penetration depth and coherence length, 
the experimental data can be fitted to the BCS-surface resistance with the mean 
free path t and the band gap as a fit-parameter. The mean free path around the 
interface between the anodized film and the bulk niobium is increased by about 
a factor of two in comparison to a "virgin" surface. 
j). As mentioned above, with electropolished or anodized cavities no or reduced 
Q-degradations have been observed, when the cavities were kept at intermediate 
temperatures. However, the mean free path as a fitting parameter is greatly 
effected by the warmup and subsequent cooldown and can be reduced by factors 
ranging from 3 to 20 depending on the conditions. This might be interpreted 
as a precursor of hydrogen precipitation still weak enough for not effecting the 
cavity Q-values significantly. 
We are speculating that the additional oxygen introduced into the surface layer 
by the anodizing process prevents the hydrogen in the bulk from precipitating into 
the niobium hydride phase; after all reactor grade niobium with a large concentration 
of interstitially dissolved oxygen does not show Q-degradations. 
Multi-phase behavior of degraded surfaces has been reported in5 and the ex- 
perimental data have been explained by a model of a thin surface layer of a poor 
superconductor on top of the bulk niobium. As discussed in8 a possible mechanism 
for the Q-degradation is the hypothesis that the hydrogen precipitates into "islands." 
A quantitative analysis of the experimental data based on this hypothesis would fol- 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
low a model as discussed in;ll in this case the niobium matrix was "polluted" by 
oxygen/suboxide clusters, which resulted in a smearing out of the density of states. 
It is presently planned to try to apply such a model to the experimental data. 
CONCLUSION 
Our experiments provide some evidence that anodic oxide layers on niobium sur- 
faces of high purity niobium are capable of significantly reducing Q-degradations due 
to precipitation of the niobium hydride phase. The benefits of an anodic layer should 
show up even more pronounced at lower frequencies, where the losses in a cavity 
operating at 4.2K are dominated by the BCS-losses. It is of interest to investigate 
the long term stability of anodic layers in the presence of field emission loading and 
such efforts have been initiated. Thicker Nb205 - layers seem to be more effective. 
This observation coupled with the fact that the hydrogen concentration in anodic 
layers are largely reduced12 suggests that by diffusing oxygen into the surface by 
moderate "in situ" heating of an anodized cavity might further reduce the danger of 
Q degradations. 
ACKNOWLEDGEMENT 
We would like to express our thanks to all colleagues at CEBAF, who supported 
these efforts. Our special thanks go to L. Turlington for providing for the fabrication 
of the cavities. 
REFERENCES 
1. R. Roth, et al., Proceedings of the 2nd European Particle Accelerator Conference 
(EPAC), Nice, France, Vol. 2, p. 1097 ff. 
2. D. Proch, private communication. 
3. H. D. Griif, private communication. 
4. D. Moffat et al., CLNS-report 90/IC35, Cornell University, Ithaca, NY. 
5. B. Aune et al., Proceedings of the 1990 Accelerator Conference, Santa Fe, New 
Mexico. 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
6. G. Muller, private communication. 
7. S. Isagawa, Journal of Applied Physics, Vol. 51, p. 4460 and 6010, 1980. 
8. B. Bonin, invited paper, this workshop, to be published in Particle Accelerator. 
9. P. Kneisel et al., Nucl. Instr. and Meth., Vol. 188, p. 669, 1980. 
10. J. Halbritter, Z. f. Physik, Vol. 238, p. 466, 1970 
J. Halbritter, Ext. Bericht 3/70-6, Kernforschungszentrum 
Karlsruhe 1970. 
11. A. Philipp, Thesis, KfK-Report 3268 (1982), Kernforschungszentrum Karlsruhe. 
12. V. Nguyen, private communication. 
13. K. Saito et al., paper W04, this workshop. 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Rs(T)-Rres [Ohm] 
X 
-W .- 
> 
0 
U -  
Q> 
0 
Q> -
m- 
c .- 
V) 
a> 
c 
X- 
U 
Q) -
a> 
l- 
In 
I 
0 
I 
0 
I 
0 
I 
T 7 7 
0 
C- 
0 
C- 
I - r l I I I  I l l ' l l l ' i  I * 1 1 1 1 1 1 1  I I 1 1 1 1 1 '  I 
- 
- - 
- . 0 - 
0 
- . - . 0 L' 
c . 0 
0 - . 
hl . - 
- Y m 
0 - - . - F . 
g3 . 
0 a . - - 2 3  . 
0 
0'13 . 
U a, - . - 
U E . 
V ) '  . 
SB t - 
m 
- >;G i 0 
&U 
- .- .- > > . 0 - 0 0 . 
0 0 0 
- 
m 
E E . 8 . 
3 3 . 0 .- .- - an m* oO 
0 0 - -- .- i o 
- S c 8 0 - 
a, a, 
L L 
0 0 
mm 
3; - 
- 0. 8 $8 0 - 
1 1 1  l I 1 1 1 1 1 1  1  I I 1 1 1 1 1 1  I I , 1 1 1 1  1 1  I I I 
(0 b m Q) 
I 
>a 
-L0 
-* 
- m  
C\I 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
l
o
l
l
l
l
l
l
,
l
l
l
l
,
l
l
l
l
~
l
l
l
l
~
l
f
l
~
~
~
~
~
~
 
H
E
R
A
E
U
S
 
N
io
bi
um
 
M
a
te
ri
a
l 
R
R
R
=2
30
 
- 
- - 
l 
0
 
B
C
P
(1
 :
l :
l)
 an
d 
fa
st
 
co
ol
do
w
n 
- 
- 
W
ar
m
ed
 u
p
 
to
 
11
0K
 
20
hr
s 
- 
- 
A
 
~
n
o
d
iz
e
d
(2
0
0
0
A
) a
nd
 f
as
t 
co
ol
do
w
n 
- 
A
 
A
no
di
ze
d,
W
ar
m
ed
 
up
 
to
 
10
5K
 4
8h
rs
 
- 
F
ig
. 
4:
 
E
sp
 v
s.
 Q
. 
fo
r 
H
E
R
A
E
U
S
 N
b 
m
at
er
ia
l 
un
de
r 
d
if
fe
re
n
t 
co
ol
d
ow
n
 c
on
d
it
io
n
s 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
1
0
 11
 
5
-c
e
ll 
ca
vi
ty
(~
E
B
A
F
f5
) 
- 
I
I
I
I
 
I
l
l
1
 
I
1
 I 
I 
I
1
 1
 I
 
1
1
1
1
 
I
1
 1
1
;
 
- 
- 
- 
- - 
L 
0
 
B
C
P
(1
:1
:1
),
 
F
as
t 
co
ol
do
w
n 
- 
- 
X
 
W
ar
m
ed
 
u
p
 t
o
 
1
7
0
K
 
16
.5
hr
s 
- 
- 
W
ar
m
ed
 
up
 
to
 
10
0K
 
1
5
.5
h
rs
 
- 
+ 
W
ar
m
ed
 
u
p
 t
o,
 R
o
o
m
 T
e
m
p
e
ra
tu
re
 
- 
A
 
A
no
di
ze
d(
13
00
A
),
 
F
a
st
 c
oo
ld
ow
n 
- 
A
 
A
no
di
ze
d,
 
W
ar
m
ed
 
u
p
 t
o
 
1
0
0
K
 2
9
h
rs
 
1 o
1
0
 
-
 
t
 
- - 
F
ig
. 
7: 
E
xp
er
im
en
ta
l Q
. 
vs
. 
E
sp
 d
ep
en
de
nc
es
 m
ea
su
re
d 
on
 a
n
 a
no
di
ze
d 
5-
ce
ll 
ca
vi
ty
 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05
F
ig
. 
8:
 
Q
0
 vs
. 
E
sp
 f
or
 a
n 
el
ec
tr
op
ol
is
h
ed
, 
si
n
gl
e 
ce
ll
 c
av
it
y 
af
te
r 
va
ri
ou
s 
ch
em
ic
al
 t
re
at
m
en
ts
. 
A
ft
er
 c
h
em
ic
al
 
tr
ea
tm
en
t 
in
 a
 b
uf
fe
re
d 
so
lu
ti
on
 f
or
 s
ev
er
al
 m
in
u
te
s,
 Q
-d
eg
ra
d
at
io
n
 w
as
 o
bs
er
ve
d 
fo
r 
th
e 
or
ig
in
al
ly
 
u
n
ef
fe
ct
ed
 c
av
it
y 
- I, 
0
 
E
P
+B
C
P
 
5
.5
m
in
, 
F
a
st
 
co
o
ld
o
w
n
 
- 
- 
A
 
E
P
+B
C
P
 
G
m
in
, 
F
as
t 
co
o
ld
o
w
n
, 
Q
o 
sw
itc
h
e
d
 
by
 
a
 
d
e
fe
ct
 
- - 
- 
A
 
E
P
+B
C
P
 
G
m
in
, 
A
ft
e
r 
Q
o 
sw
itc
h
e
d
 
G
m
in
, 
W
ar
m
ed
 
up
 
to
 
13
5K
 
2
1
 h
rs
 
- 
- 
- 
lo
1
0
 
oo
A
ab
 0
0
 
0
0
 
-
 - 
- - '
A
A
A
 
A
A
A
 
A
 
o 
o
o
~
~
~
~
 
0
0
 
oO
%
 
(, 
o
 
0
 
O
 
- - 
- 
A
A
 
A
 
- 
--
. 
A
A
 
- 
- 
b. 
%A
 
A
A
 A
 
- 
- 
*n
J'0
80
 
A
 
*o
m
 
A
 
dP
 
k
~
~
 
A
 
- 
0
. 
- 
...
.. 
A
 A
 A
 
- 
..
 *.. 
A
 A
A
 
Proceedings of the Fifth Workshop on RF Superconductivity, DESY, Hamburg, Germany
SRF91F05


Observation of Q-degradation in superconducting niobium cavities due to cooldown conditions,

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=C560AAB509F38FAD7E931EC9494CBB00?doi=10.1.1.1050.4620&rep=rep1&type=pdf

Observation of Q-degradation in superconducting niobium cavities due to cooldown conditions,

Abstract

Similar works

Full text

Available Versions

CiteSeerX