COMPARISON OF HIGH GRADIENT PERFORMANCE IN VARYINGCAVITY GEOMETRIES
T. Higo#, T. Abe, Y. Arakida, S. Matsumoto, T. Shidara, T. Takatomi, M. Yamanaka,KEK, Ibaraki, Japan
Y. Higashi, OKINAWA Institute of Science and Technology Graduate University, Okinawa, Japan
A. Grudiev, G. Riddone, W. Wuensch, CERN, Geneva, Switzerland
Abstract
Five types of CLIC prototype TW accelerator structures
were high-gradient tested at KEK, up to 100 MV/m level.
The ramping speed of each processing and the resultant
breakdown rate were compared among them. From thiscomparison, it was found that the ramping speed of thestructures with opening ports for HOM damping withmagnetic coupling became slow and the resultant
breakdown rate became high. This indicates the role ofthe magnetic field on vacuum breakdowns in copper
structure at the region around 100 MV/m. In this paper,
we review such processing characteristics and the final
high gradient performance of the recent structures. One of
the structures showed frequent breakdowns in two
particular regions of the structure, indicating a mechanism
reflecting not only the geometry or materialcharacteristics but the local features acquired after
completion or even during the running.

INTRODUCTION
Significant progress over the past few years has been
made towards demonstrating the feasibility of one of themost crucial RF performance specification of CLIC [1] –an acceleration gradient of 100 MV/m with the nominal
pulse width of 240 ns (flat top of 156 ns) and a
breakdown rate of a few 10-7 /pulse/m. To optimize thestructure parameters, results from previous high gradientexperiments were used as a guide along with suchparameters as „Sc‟ to design a structure that should
perform well at high gradients and be CLIC compatible[2].
Since 2007, an international collaboration on highgradient X-band accelerator structure development has
been mainly lead by CERN, SLAC and KEK [3]. One of
the undamped structures showed in 2011 an extremely
good performance meeting the CLIC breakdown rate
(BDR) requirement with ever decreasing BDR in thecontinuing operation [4]. The focus since then has been
the experimental study of structures with the heavy
damping features needed for CLIC. In the present paper,
the test results from Nextef [5] of KEK in recent twoyears are presented.
Before going into the detailed description, the symbols
in the structure code name are described. Typical example
such as TD24R05 come from T=travelling wave,
D=damped, 24=number of cells and additional
R05=corner radius of 0.5mm.
One of the most concerns in the design of the damped# toshiyasu.higo@kek.jp

structure is the reduction of surface magnetic field, Hs,
which leads to the surface temperature rise within a pulse.
Actually the TD18 structure tested at SLAC showed clear
increase of BDR as the temperature rise [6]. Then wespeculate that the high Hs may lead to the breakdown
trigger. The 24-cell structures are designed to reduce theHs comparing to those of 18-cell one [7]. Since theperformance of undamped T24 structure was foundextremely good, we assumed the improved performanceof related damped structure, TD24.
In the present paper are reviewed the recent results
since last conference [8] of these 24-cell dampedstructures, where the Hs was reduced than TD18.

ACCELERATOR STRUCTURE DESIGNAND PREPARATION
DesignThe structures discussed in the present paper are the 24cell structures. The Hs of TD24 was reduced comparing tothe TD18 by changing the accelerator mode parameters
while that of TD24R05 was further reduced than TD24 byreducing the corner radius of 3mm down to 0.5mm [7].
These cell shapes are shown in the picture of Fig. 1.
Table 1. Accelerator gradient and surface temperaturerise in a pulse along the structure in the operation ofunloaded gradient of 100 MV/m.
Structure

TD18

TD24

TD24R05

Eacc [MV/m]

79 ~ 120

94 ~ 102

94 ~ 102

T [C]

29 ~ 47

25 ~ 2121 ~ 19

T24
TD24TD24R05Figure 1: CLIC prototype structure cells.

Preparation of structures
The techniques used to fabricate the structures are thesame as developed for the GLC/NLC structures in theearly 2000‟s [9, 10]. These include machining the parts
with the combination of milling and diamond turning,diffusion bonding the cells in a hydrogen furnace and a
final 650 degC vacuum bake for ten days.
Recently one possible problem was discussed related
to the spark-like markings which were acquired throughthe high power processing [11]. It might be related to ahigh current running through the gap of micron order in
one of the cell corners, possibly formed due to theimperfect diffusion bonding. To suppress this possibility,
the cell flatness was carefully checked and confirmed thatthe inner area of the flat surface to be bonded is highcomparing the outer area, which may lead to the less gap
formation where the high current passes. In practice, thesurface flatness is specified that the cell flatness should beless than 0.5 micron when sandwiched by two opticalquality flats and the inner area should be higher, though
the actual inspection by laser interferometer is inspectedby eye.

The observed ramping speed in early stage of theprocessing of recent tested structures is shown in Fig. 3 asa function of running time. It was found that theundamped structures showed much faster ramping
comparing to the damped structures. The ramping speedsof both damped structures, TD24#4 and TD24R05#2,
were similar and these were much faster than TD18#2,
whose Hs is much higher, indicating the importance oflow Hs.

Experimental setup preparation
The structures were vacuum-leak checked and nitrogengas purged at SLAC in a reasonably clean environment.
At KEK, these were firstly checked in Sij and dressed up
for installation. These were all done in the clean room.
The high test was performed at Nextef of KEK [5]. It
should be mentioned that the setup was carefully purged
by nitrogen gas during any replacement, such as thestructure exchange. However, as seen in the Fig. 2, thearea is located in a somewhat dirty concrete vault and theoutside of the components were covered with much ofdust.

Figure 3: Comparison of initial ramping speed.

Breakdown rate evolution
The comparison of breakdown rates (BDR) vs. time forthe recent structures is shown in Fig. 4. The reduction ofBDR of T24#3 was fastest. The BDR of TD24#4 washigher and its reduction speed was slower than T24#3.
The BDR of TD24R05#2 was even higher than TD24#4
and the reduction vs. time was much less. It may reflect
the appearance of hot spot at 1500 hours of operation
described in the next paragraph.

Figure 2: Nextef setup in reality.

RF PROCESSING RESULTS
Initial processing rate
The processing was performed semi-automatically by acontrol program, starting with a short pulse width such as
50 nsec. After reaching the top power level, it increased
the width stepwise by 50-100nsec jump. When weobserve a vacuum pressure increase typically in earlystage of the processing by more than several 10-6 Pa, theprogram slows down or stops ramping of power level.
When a big breakdown happens, it stops the next pulse
and restart from somewhat lower power level after
waiting for a few tens of seconds. The breakdown isidentified by an abrupt burst of the Faraday cup current,
either upstream or downstream and mostly both, inaddition to the big reflection in RF power.

Figure 4: Comparison of BDR evolution versus time.

Breakdown location
BD location can be identified from the timing of theabrupt increase of the reflection pulse and the fall of thetransmitted pulse.
After reaching to 90 MV/m with 252nsec pulse in theTD24R05#2, the structure was conditioned with thelonger pulse width at around 1700 hours. As shown in Fig.
5, frequent breakdowns appeared at the downstream ofthe structure after the long-pule operation, followed by
the appearance of another peak in the upstream after 2100
hour point when the higher Eacc at 100 MV/m wasrealized. This we call “hot spots.”

Since these hot spots stayed in the following operation,
the structure seems damaged. This phenomenon was
firstly observed in the five structures. The mechanism andhow to prevent it should be studied.

Figure 5: Breakdown cell distribution in TD24R05#2.
(a) is before and (b) after emerging hot spots.

BDR vs. pulse width and breakdown timing
BDR is a function of pulse width Tp as shown in Fig.
6(a). That of TD24R05#2 case showed Tp6 dependence. IfBDRTp , it is natural to speculate that the probability ofbreakdown onset time f(t) within the pulse distributes asf (t )  t  1 if the breakdown trigger mechanism develops
within the pulse. In some cases, the breakdown increases
toward the end of the pulse, but it is much less steep thanthat expected, ~Tp5, from a simple estimation based on
BDR behaviour. As shown in Fig. 6, the measured timing
distribution of the breakdowns is almost flat in time. Notethat the red points show the usual breakdowns, while theblue show the first-pulse breakdowns just after thenominal breakdown. In the latter case, most breakdown
start almost at the beginning of pulse.

Figure 6: Breakdown characteristics of TD24R05#2.
(a) BDR vs. pulse width and (b) breakdown timing
distribution.

DISCUSSION AND FUTURE
IMPROVEMENTS
The breakdown rate, BDR, of the un-damped structure
T24 clearly met the CLIC requirement of a few 107/pulse/m at 100 MV/m. However, the processing speed
and the resultant breakdown rate in the damped structures
showed poor performance than those of undamped ones
with the similar RF acceleration parameters. Here such
gradient level as 80 MV/m seems feasible if based on thepresent technology.
The biggest difference in electromagnetic field fromundamped to damped is the increase of Hs at the magnetic
coupling slots for the high-order-mode damping and it

plays an important role[6]. The mechanism governing thisrelation should definitely be identified to realize the CLIC
design specification in the damped structure. If the high
BDR is inevitable due to the high Hs, we may need tointroduce such structure as choke-mode type where no big
enhancement of Hs is needed for damping. The study ison-going in collaboration with Tsinghua University.
In addition, we encountered, for the first time at Nextef,
the hot spot. This might have been introduced by
accidental introduction of dust particles. The mechanism
and the technology to suppress such cases should bestudies. We have been preparing such studies in a smallsized experiment with the simpler geometries. If theactual origin of the hot spot comes from any intrinsiccharacteristics and appeared sometimes but not always,
the R&D strategy should be different.

ACKNOWLEDGMENT
The collaboration program conducted mostly among
three laboratories, CERN, SLAC and KEK, is a backboneof this research. That between CERN and KEK is under
Appendix 2 of ICA-JP-0103, while that between SLAC
and KEK under US-Japan Agreement on High Energy
Physics. The authors greatly thank those who have been
contributing to the activities at CERN and SLAC.

REFERENCES[1] S. Stapnes, “CLIC Status and Outlook”, WEYA01, IPAC12,
New Orleans, USA, 2012.
[2] A. Grudiev, S. Calatrini and W. Wuensch, “New local field
quantity describing the high gradient limit of acceleratingstructures”, PRST-AB, 12, 102001 (2009), p102001-1.
[3] T. Higo et al., “Advances in X-Band TW Accelerator
Structures Operating in the 100 MV/m Regime”, THPEA013,
IPAC2010, Kyoto, Japan, 2010.
[4] T. Higo, “Recent High-Gradient Test Result at KEK”,
presented in the workshop, LCWS2012, Arlington, USA,2012,
http://www.uta.edu/physics/lcws12/pages/registration.html[5] S. Matsumoto, “High Gradient test at Nextef and high-power
long-term operation of devices”, NIMA 657, p160-167, 2011.
[6] F. Wang and C. Adolphsen, “TD18 High Power TestResults”, 4th Annual X-band Structure Collaboration
Meeting, CERN, May 2010, http://indico.cern.ch/conferenceDisplay.py?confId=75374[7] A. Grudiev, “Summary of accelerating structure rf design
directions at CERN”, 4th Annual X-band Structure
Collaboration
Meeting,CERN,
May2010,
http://indico.cern.ch/conferenceDisplay.py?confId=75374[8] T. Higo et al., “Various Observables of TW Accelerator
Structures Operating 100MV/m or Higher at X-Band Facility,Nextef of KEK”, IPAC10, THPEA012, Kyoto, Japan, 2010.
[9] J. Wang et al., “Fabrication Technologies of the High
Gradient Accelerator Structures at 100MV/m Range”, in thisconference, IPAC10, THPEA064, Kyoto, Japan.
[10] J. Wang and T. Higo, “Accelerator Structure

Development for NLC/GLC“, ICFA Beam Dynamics
News Letter No.32, pp27-46, December 2003.
[11] M. Aicheler, “TD18 extreme preliminary postmortem SEM observation”, Presented at CERN, Sep.
2010