Cavity R&D for TESLA
1Lutz Lilje , DESY -FDET-, Notkestrasse 85, 22607 Hamburg, Germanyfor the TESLA Collaboration
Abstract
The cavity research and development which wasessential to achieve the performance goals of a Q0 of1010 at an accelerating gradient of 23,5 MV/m for
TESLA-500 will be reviewed. Results from the 1,3
GHz nine-cell superconducting niobium cavities will beshown. For TESLA-800 the specifications are aQ0=5×109 for the accelerating gradient of 35 MV/m. It
will be shown that this is achieved regularly inelectropolished one-cell cavities. First promising resultson nine-cell cavities are shown. For TESLA-800 it isnecessary to increase the fill factor of the linac. This isachieved via the superstructure concept. The firstsuccesful test with a piezoelectric tuner to compensate
the frequency detuning during the rf pulse is shown.

A key element in the procedure is a large cleanroom
area ranging from class 10000 down to class 10 toachieve a dust free environment for cavity preparation.
The other important technique is called high pressure
water rinsing where an ultra pure water jet removes
particulate contaminations from the surface very
efficiently.
The cleanroom area also houses facilities for chemicaltreatment, an UHV furnace for the heat treatment at1400°C and an assembly area allowing to assemble astring of 8 cavities and a superconducting quadrupole
under cotrolled conditions. Pumping and leak testing is
performed inside the cleanroom with oilfree
pumpstations located outside of the cleanroom area.
The length of an accelerating module is 12.2 m.

CAVITY MANUFACTURING AND
PREPARATIONThe niobium cavities are fabricated from RRR 300
niobium sheets by deep drawing and by electron beam
welding (Figure 1). Up to now 79 TESLA 9-cellcavities have been delivered by 4 European
manufacturers: a first series of 28 in 1994, a second
series of 27 in 1997, and 24 cavities of a third series
have been delivered to DESY in 2001.
The preparation of superconducting cavities includes
several steps:• removal of the damage layer by chemical etching• 2 hours heat treatment at 800 C for the removalhydrogen and stress annealing• 4 hours heat treatment at 1400 C with titanium
getter for higher thermal conductivity to stabilize
defects• removal of the titanium layer by chemical etching• field flatness tuning• final 20 µm removal from the inner surface byetching• high pressure rinsing (HPR) with ultrapure water• drying by laminar flow in a class 10 cleanroom• assembly of all flanges, leak-check• 2 times HPR, drying by laminar flow and assembly• of the input antenna with high external Q

1

Email: Lutz.Lilje@desy.deFigure 1: A TESLA niobium 9-cell cavity. The length ofa cavity is about 1m.

ACCEPTANCE TEST RESULTS
ON 9-CELL CAVITIESThe cavities are specified with a Q0 of 1010 at anaccelerating gradient of 23,5 MV/m for TESLA-500.
For TESLA-800 the specifications are a Q0=5×109 forthe accelerating gradient of 35 MV/m. The acceptancetest of the nine-cell cavities is done in a verticalcryostat, where the input coupler is adjustable to match
the quality factor of the cavities. The cavities areexcited in the continuous wave mode. Already in thefirst series the strict observance of clean treatment
showed success by reaching gradients of 25 MV/m at Q
values above 5·109 on several cavities. However, there
was also a number of cavities that performed muchworse. The reasons for this poorer performance weretraced back to either improper preparation of the cavity
dump bells before welding or to the inclusions ofnormalconducting material in the niobium.

Figure 2: Excitation curves of 9-cell cavities from thelast production.
For the second series, proper weld preparation wasassured and all niobium sheets were scanned by aneddy current method to exclude sheets containing
inclusions from cavity production [4]. The success ofthese measures can be seen in figure 3 where themaximum measured gradient is shown for all 9-cellcavities measured up to now. All 4 companies havedemonstrated their capability of manufacturing cavities
exceeding 25 MV/m at Q=5×109. The progress incavity production, treatment and handling is also
manifested by the reduced scatter in cavity performance
when looking at the three production series. For the
first one the results range from 9 to 30 MV/m while thelast series is located between 26 and 31 MV/m (Fig. 2).

After the cavities have passed the vertical acceptancetest successfully, the helium vessel is welded to thehead plates of the cavity. A 20 µm etching of the innersurface follows. In the last preparation step before thehorizontal full systems test, the main power coupler isassembled to the high pressure rinsed cavity. The
external Q of the power coupler is typically 2×106.
More than 30 cavites have been tested in pulsed mode
operation (see figure 4) in a full systems test in ahorizontal cryostat or in the accelerator. The average
gradient achieved in the vertical and the horizontal tests
are quite similar as shown in Fig. 4. In a few cases theperformance was reduced in the horizontal test due tofield emission. In other cavities the maximum gradient
was improved by the fact that the cavites are operated
in pulsed mode instead of the cw operation in thevertical test. These results demonstrate that the good
performance of a cavity can be preserved after theassembly of the helium vessel and the power coupler.
In figure 3 (right) the average gradients measured in thevertical test cryostat of the cavities, which wereinstalled into the five accelerating modules is comparedto the performance in the accelerator. Certainly, thepresently achieved level of technology in cavity
production will be adequate for the construction of a500 GeV linear collider [6]. The achieved average
gradient in one accelerating module is 22.5 MV/m and20 MV/m in the other one. A third module, where allcavities have been successfully conditioned to gradientslarger 25 MV/m, is ready for installation into theaccelerator tunnel.

Figure 3: Average gradient, as measured in theacceptance test, of the 9-cell cavities of the three cavity
productions (left). Average gradients of the cavities as
they have been in the assembled accelerator modules.
Red squares indicate the gradients obtained in themodules after installation into the LINAC. The figure
has been taken from [6].
Figure 4: Comparison of results achieved in theacceptance test with the results in the full systems test.

FURTHER R&D ON S.C. CAVITIESElectropolishing of niobium cavities
There has been an R&D programme on single cellcavities in laboratories inside and outside of theTESLA collaboration with the goal to push theachievable gradients to 35 MV/m or above, which
would allow for a substantial increase of the collisionenergy at the TESLA linear collider to 800 GeV.
For a number of years several remarkable results havebeen obtained at KEK [7] with electropolishing single
cell niobium cavities, obtaining gradients close to 40
MV/m. These cavities were of comparatively low
RRR=200-300 material, therefore opening thepossibility to avoid the rather tedious and timeconsuming high temperature heating at 1400°C. Of
course, this is very desirable for cost reasons.
In contrast to the chemical etching applied to thecavities at TTF, which leads to a rather rough surface,electropolishing leads to a very smooth and shiny
surface [8]. KEK and CEA Saclay have convincingly
demonstrated that electropolishing raises the obtainableaccelerating field substantially compared to the BCP
treatment [9]. In a collaboration including KEK, CERN,DESY, CEA Saclay and TJNAF several single cell
cavities have been electropolished and gradients around
40 MV/m were obtained in cavities produced by threedifferent manufacturing techniques [10,11,18,19 – seefigure 5]. It was discovered that baking the evacuatedcavities at 75-150°C for 24 to 48 hours after the final
high pressure water rinsing constitutes an essential stepin reproducibly obtaining gradients around 40 MV/m ata high quality factor [17,11].
To transfer these findings to 9-cell cavities, work isgoing on at KEK and also at DESY. First results on aelectropolished 9-cell cavities are very promising andhave achieved an accelerating gradient of 32 MV/m
(figure 6).

Figure 5a: Results on electropolished single cellcavities from the CEA-CERN-DESY collaboration.
Tests were done at 1,7 and 2 K. The figure is takenfrom reference [12].

Figure 5b: Results on a spun electropolished single cellcavity from a KEK-INFN collaboration. The figure istaken from reference [19].
1.00E+11● - 250 µm
standard etch●●●●● ●●● ● ● ● ●● ● ● ● ●● ●■■■■ ■ ■ ■ ■ ■ ■ ■ ■■ ■ ■ ■

■ 100 µm e-polishing
● ●■●

■ ■

●●●●■● ■ ■■ ■■ ■ ■■ ■■Qo
1.00E+10DESY Seamless Cavity
1K2
TEST at JLab

10.00E+80510152025
Eacc [MV/m]

303540Figure 5c: Results on a hydroformed electropolishedsingle cell cavity. Test was done at a temperature of 2
K at TJANF. The figure is taken from reference [18].

45Figure 6: Result on an electropolished nine-cell cavity.
A clear improvement is seen as compared to its
behaviour after etching (BCP). Test was done at 2K.

Layout

Eacc
No of main coupler No of HOMs couplers No of tuners Fill factor Ptrans[MV/m][kW]
9-cell
23,4
2092
41184
20592
78,6
232
2x9 cell
2210926
32778
21852
84,8
437
Table 1: Superstructure parameters. The number of main couplers is reduced by a factor of two, while the fill factor of
the LINAC is increased by 6 %.

Figure 7: Superstructure layout

Superstructure concept
The limitations on the number of cells per cavity can becircumvented by joining two multicell cavities to forma so-called superstructure [20]. Short tubes of sufficientdiameter enable power flow from one cavity to the next.
The chain of cavities is powered by a single input
coupler mounted at one end. HOM couplers are locatedat the interconnections and at the ends. The two cavities
are equipped with their own frequency tuners.
The cell-to-cell coupling is kcc=1,9%, while thecoupling between two adjacent cavities is asuperstructure is two orders of magnitude smaller at
kss3×10-4 due to this comparatively weak inter-cavity
coupling the issues of field homogeneity and HOM
damping are much less of a problem than in a single
long cavity with N=18 cells. The shape of the centre
cells is identical to those in the 9-cell TTF structures
while the end cells have been redesigned toaccommodate the larger beam tube irisses.
Another advantage is that the number of main couplers
could be reduced by a factor of two thus allowing for
further cost savings.

Figure 8: Detuning of TESLA cavity during the RF
pulse measured at different gradients.

Frequency stability of the cavities
The pulsed operation leads to a time-dependent
frequency shift of the 9-cell cavities which is
proportional to Eacc2 (Figure 8). The stiffening rings
joining neighbouring cells are adequate to keep this“Lorentz-force detuning“ within tolerable limits up to
the nominal TESLA-500 gradient of 23.4 MV/m.
To allow for higher gradients the stiffening must be
improved, or alternatively, the cavity deformation must
be compensated. The latter approach has beensuccessfully demonstrated using a piezoelectric tuner(see figure 9) [21]. The result indicates that the present
stiffening rings augmented by a piezoelectric tuning
system will permit effcient cavity operation at theTESLA-800 gradient of 35 MV/m.

Figure 9: Stabilisation of the frequency by means ofpiezoelectric element. In this test 200 Hz werecompensated.

SUMMARY

REFERENCES

Average gradients well above 25 MV/m have beenachieved for the TESLA 9-cell cavities from the latest
production series. The results on the superconducting
cavities are very reproducible. Procedures have beendeveloped that allow to keep the performance of thecavities through all preparation steps for the installationin the accelerator. The cavity technology for TESLA500 is therefore available. For electropolished one-cellcavities over 40 MV/m have been reached reproducibly.
This allows to continue a focused R&D program
towards TESLA-800 transferring the electropolishingtechnology to multi-cell cavities, where first promising
results have been achieved already.
A scheme for increasing the fill factor of the linac has
been developped and will be tested soon. The
stabilisation of the frequency during the radiofrequency
pulse has been demonstrated with a piezoelectricelement allowing efficient pulsed cavity operation atgradients of more than 35 MV/m.

[1] TTF-Proposal, DESY-TESLA-93-01[2] J.-P. Carneiro, et al., Proc. 1999 Part. Acc. Conf.,
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[6] TESLA Technical Design Report[7] K. Saito et al, 9th Workshop on RF Superconductivity, Santa Fe, 1999
[8] C.Z. Antoine et al, 9th Workshop on RF Superconductivity, Santa Fe, 1999
[9] E. Kako et al, 9th Workshop on RF Superconductivity, Santa Fe, 1999
[10] L. Lilje et al, 9th Workshop on RF Superconductivity, Santa Fe, 1999[11] P. Kneisel, 9th Workshop on RF Superconductivity, Santa Fe, 1999, B. Visentin, ibd.
[12] L. Lilje, PhD Thesis, to be submittet to Universityof Hamburg in 2001[13] G. Schmidt, et al., FEL 2000, Durham, USA[14] H. Edwards, et al., Proc. 1999 FEL Conf.,Hamburg, II-75 (1999)[15] J. Andruszkow, et al. , Phys. Rev. Lett., Vol.85,
No 18,pp. 3825-3829(2000)[16] J.Rossbach, Invited talk given at the FEL 2000,
Durham, USA, and to be published in NIM A[17] B. Visentin et al, 9th Workshop on RF
Superconductivity, Santa Fe, 1999[18] W. Singer et al, to be published , Electropolishingof a hydroformed niobium cavity, 2000
[19] E. Palmieri et al, to be published , Electropolishing of a spun niobium cavity, 2000
[20] J. Sekutowicz, M. Ferrario, C. Tang, Superconducting superstructure for the TESLA collider: Aconcept, Phys. Rev. ST Accelerators and Beams
2:062001, 1999.
[21] M. Liepe, W.D. Moeller, S.N. Simrock, Dynamic
Lorentz Force Compensation with a Fast Piezoelectric
Tuner, DESY TESLA-01-03, 2001.

ACKNOWLEDGEMENTS
The author wants to express his gratitude to E. Kakoand K. Saito from KEK whose work onelectropolishing L-Band niobium cavities was anessential element for exploration of highest gradients inTESLA cavities.