The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Effect of the Annealing Temperature for the Hydrogen Q-degradation
on Superconducting Cavities
Tomoko OTA, Satoru SUKENOBU, Yoshio TANABE, Yoshimichi ONISlll,.
Shuichi NOGUClll*, Masaaki ONO*, Kenji SAITO*, Toshio SlllSlllDO*, Yoshishige YAMAZAKI*
Toshiba Corporation
2-4, Suehiro-cho, Tsurumi-ku, Yokohama, 230 JAPAN* High Energy Accelerator Research Organization (KEK)
1-1, Oho, Tsukuba-shi,lbaraki, 305 JAPAN

Abstract
Hydrogen Q-degradation was studied in niobium
superconducting cavities prepared by barrel polishing, andelectropolishing without annealing, though a fast cooling
down of cavities.

Cavity performance with · various

annealing temperature were tested using a 1. 3GHz singlecell cavity to compare the effects of aJ!nealing temperaturefor hydrogen Q-degradation.

1 Introduction
It is known that the fast cool down is one possible
procedure to prevent hydrogen Q-degradation for the highpurity niobium cavities that exhibit this phenomena withFig.l

slower cool down ; stay relationally long time at 100-150K

in the cooling procedure[l,2]. Hydrogen has been
concentrated at niobium surfaces and it cause a phasetransformation involving hydrogen at low temperature to

1.3GHz single-cell superconducting cavity

Table 1 Single-cell cavity specifications
Frequency
Rsh/Q

yield Q-degradation[3].

G
Esp/Eacc
Hsp/Eacc

It has been used buffing and mechanical grinding toremove surface irregularities before electropolishing(EP) at

1.3[GHz]102[0]
274

1.78
43.8[0e/MV/m]

KEK. Recently, a barrel polishing was developed as asimpler

method,insteadofthesemechanicalpreparations£4]. It is successfully used to 1.3GHz cavities
with adopting annealing(~800°C), however, its effect on theFigure 1 shows the cavity, and the specifications calculated
by SUPERFISH code are summarized in table 1.

Q-degradation isn't well known.

In the ·first experiment, the cavity was barrel polishedby 30 J.l. mat the equator, then electropolished by 10 J.1. m on

2 Hydrogen Q-degradation
In order to make sure that the fast cool down is still

average and was rinsed with high pressure(HPR ;feasible for preventing the Q-degradation to the cavity~80kgf/cm2) deionized pure water. No annealing wasprepared by barrel polishing, EP without annealing, thecarried out. After the treatments, it was evaeuated to lxl0"9

following measurements were carried out by using aTorr and sealed in vacuum. The fast cool down to 4.2K was

1.3GHz single-cell cavity fabricated in our company[5].
This cavity was formed by deep drawing of high purity
niobium sheets(thickness=2.5mm, residual resistivity ratio:
RRR=200), then trimmed and electron beam welded.

carried out in one hour. The result is shown in Fig.2 withmark: 0. It shows Qo(unloaded Q value) as a function ofaccelerating field gradient(Eacc). Q0 were sUfficiently highat low field, however Q0 began to drop at Eacc=3MV/m. The-'-243-

The 11th Sympo:sium on Accelerator Science and Technology, Harima Science Garden City, 1997
10 11 ,....,...,......,-,.,...,.,..,...-,,..,..,..,...,..,.....,....,--,-..~~,.,..,...,...,..,.,..,....,,..,.....~,.,

maximum field(Eacc,max) was limited to 19.5MV/m due tothermal quench. This phenomenon which degrade the Qo
with Eacc increase was considered as the hydrogen Qdegradation. However it might be caused by surface
contamination of the cavity.
In order to remove the surface contaminationeffectively, the cavity was rinsed with hydrogenperoxide(H20 2). This HzOz rinsing method had been triedfor TRISTAN cavities at KEK. The result is shown in Fig.2with mark : 0 . Qo were degraded more seriously andEacc.max was limited to 7.0MV/m.

5To make sure that this phenomena was the Qdegradation, the cavity was held 2 hours at l00-120K which
is most dangerous temperature for hydrogen Q-degradation.
As shown in Fig.2 with mark : /::,., Q0 were degradedremarkably and Eacc,max was limited to very low field due tothermal quench.
In the following test, the cavity was annealed at 770°C

1015202530

Eacc[MV/m]

Fig.2 Performance of the cavity barrel polishedfor 5 hours in the titanium box to degas hydrogen, andrinsed with high pressure deionized pure water. As shownin Fig.2 with mark : e, Q0 were 2xl0 10 , and Eacc.max
reached to 26.2MV/m.
According to these results, it was found that rinsing
with H2 0 2 did not help to cure the Q-degradation in ourcase.
3 Annealing temperature to prevent the Q-degradation

Eacc [MV/m]

Fig.3 Cavity performance with annealing temperature

The cavity was treated several annealing temperature tocompare the performances using the same cavity. In allexperiments, the cavity was barrel polished by 30 J.L m toTable 2 Summary of the cavity performancewith annealing temperature

make its surface to the same state. Then the cavity waselectropolished by 10 J.L m, annealed for 5 hours attemperature from 400°C to 770°C, and rinsed with highpressure deionized pure water. The cavity was held 1.5
hours at 100-120K before cooling down to 4.2K to makesure the condition of Q-degradation.
The results of these measurements are shown in Fig.3
and summarized in table 2. In these results, no Qdegradation were observed for annealing temperature
higher than 500°C, and higher accelerating fields wereTemp.[ 0 C]
770
600
500
400

R.es[nQ]
697
4000

Eaccmax[MV/m]
26.2
24.8
17.4
3

Q-degradation
nononoyes

Q0 were degraded remarkably about 4 J.L Q and Eacc.max wasvery low. Serious Q-degradation appeared in the case of400°C annealing.
According to these results, it was found that 500oC isobtained for higher annealing temperature. When Eacc,max
were limited, normal conductivity exist locally on the cavity

lowest temperature to prevent hydrogen Q-degradation.

surface and its local heat was balanced with cooling of thecavity by superfluid helium. In these condition, no X-ray
was observed at the maximum field.
On the other hand, in the case of 400°C annealing,temperature increasing.

And it was found that Eacc,max increase with annealing
4 Discussion
The maximum field(Eacc,max) was limited by the thermalquench due to the heating of the cavity surface. In the-244-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

experiments, no X-ray were detected at the maximum field.
It is considered that heat on the cavity surface is caused byniobium hydride, surface defects by electron beam welding
and residual chemicals.
First, in case of niobium hydride, hydrogen absorbed in
niobium bulk during EP are degassed by annealing, andamount of degassed hydrogen increase with temperature.
The residual hydrogen in niobium bulk combines withniobium to form niobium hydride(NbHx). Power loss on thecavity surface increase with NbHx, so Eacc,max was different

0by the annealing temperature. Fig.4 shows hydrogenpressure during annealing procedure as a function of time.

100200300

400500Time[min]Fig.4 Hydrogen pressure during annealing procedure

Characteristic diffusion time, t is estimated by formula(!)
Table 3 Summary of degassed hydrogen amountand characteristic diffusion timewith diffusion constant, D and average length, R.
t =R/n···(1)The degassed hydrogen amount were estimated byintegrating hydrogen pressure above 6xlo-~orr andnormalized by the value of770°C annealing. Table 3 showsdegassed H2, assumed D as a function of temperature andTemp.rCJ

DegassedH2t[hour]

770

1.00

1.4

D[cm2/s]
2.0xl0-6

600

0.30

64.5xl0·7

500

0.37
0.1630280

9.0xl0·8l.Oxl0-8

400calculated t for R=lmm. In the case of 770°C annealing, t
is 1.4 hours, and it is much shorter than annealing time(5hours). So it can be considered that hydrogen were
sufficiently degassed at 770oC annealing, compared withother temperature.
Second, in case of surface defects, it was observed thatVickers hardness of cold-worked niobium recovered atabout 600 oc according to the experiments . by Tokyo

help to cure the Q-degradation. The effect of annealing
temperature was studied, and it was found that theannealing procedure is necessary for the present surface
treatment. It was suggested that 500 °C is lowest
temperature to prevent hydrogen Q-degradation.
References[1]

Denkai Co. Ltd .. Therefore, it is considered that defects onProc.

the cavity surface are possible to recover by annealing.
Third, in case of residual chemicals, the cavity wasrinsed with high pressure deionized pure water afterannealing, so it is considered almost residual chemicals
were removed[6]. If residual chemicals exist on the cavity
surface, field emitted electrons were generated and X-ray
were detected. However no X-ray was observed at themaximum field. Therefore it is considered that heat on thecavity surface is not caused by residual chemicals in theseB. Aune et al., "Degradation of Niobium
Superconducting RF Cavities during Cooling Time",
ofthe1990

Linear

Accelerator

Conference(l990).
[2] K. Saito and P. Kneisel, "Q-degradation in High Purity
Niobium Cavities", Proc. of the 3rd European Particle
Accelerator Conference(l992).
[3] V. Nguyen-Tuong and L. Doolittle, "Hydrogen
Contamination of Niobium Surfaces", Proc. of the 6thworkshop on RF superconductivity(l993).
[4] T. Higuchi et al., "Investigation on Barrel Polishing for
Superconducting Niobium Cavities", Proc. of the 7thmeasurements.

workshop on RF superconductivity(l996).

5 Conclusion
The characteristics of niobium superconducting cavity[5] T.

Ota et al., "First measurement of L-band
superconducting cavity fabricated in TOSHIBA", Proc.

prepared by barrel polishing, and electropolishing without
annealing was measured, and it was observed a light Qdegradation on the cavity, though a fast cooling down of theof the 21st Linear Accelerator Meeting in Japan(l996).
[6] K. Saito et al., "Water rinsing of the ContaminatedSuperconducting RF Cavities", Proc. of the 7thworkshop on RF superconductivity(l996).

cavity. And it was found that rinsing with H20 2 did not-245-