The 13th Symposium on Accelerator Science and Technology, Suita, Osak£1, Japan, October2001

STUDIES FOR STABLE HIGH-FIELD OPERATION
OF X-BAND ACCELERATOR STRUCTURE
T. Higo,. Y. Funahashi, Y. Higashi, N. Hitomi, S. Kobayashi a, N. Kudo, T. Otsuka a, Y. Saito,
K. Takata, T. Takatomi, N. Toge, Y. Watanabe andY. Yamanoa
KEK, Tsukuba, Ibaraki, 305-0801, Japana Saitama University, Saitama, 338-8570, Japan

Abstract
The accelerating structures for linear collider mainlinac operates at an unloaded gradient of 72MV/m. Those
designed with a large group velocity (starting at 12% oflight velocity) showed intolerable amount of erosion ofcopper material in the operation above 50MV/m. Recently
those with the smaller group velocity (starting at 5% orless) showed less damage even at 70MV/m. In addition t9
the small group velocity, those recent structures werefabricated by adding more sophisticated treatments on
copper surface than before. We briefly review the present
status of these high field studies. Then we present ongoingsurface studies toward improving the high field andbreakdown performance.

1 INTRODUCTION
The main motive force of adopting X-band travellingwave structures for the main linac of the linear collider is
to make the system highly efficient in energy transfer tobeam and short linac-length point of view. This view isbased on a high shunt impedance and a feasibility
estimation of high gradient performance of high frequency
structures. To overcome the big wake field due to its
small size, the beam hole aperture is designed to be aslarge as possible. In the original design a/A. = 0.18. In
addition, the structure length was thought better to be longin reducing number of RF feeds to save cost of wholelinac. These two requirements made the group velocity ofthe original design as large as 0.12c[ 1].
The structures based on this design showed an
intolerable speed of copper material erosion during theconditioning[2] and even during running at a fixed fieldover 50MV/m[3]. The erosion changes the phase advancealong the structure, which we can measure electrically.
The area of the group velocity higher than 5% exhibited asignificant phase change comparing to the lower groupvelocity area. Considering these, low group velocitystructures (less than 5%) were made and their high-field
performances were examined. Typical performances ofthese structures are presented in the present paper.
These recently studied structures were treated invarious new processes which were not applied to theprevious structures. The surface behaviour should play anessential role in triggering the breakdown and may cause
the erosion of copper surface. These recent structures
were found to behave better, especially those madethrough a heavy chemical etching process of a few micronlevel in an acid bath, comparing to those made of
diamond turned parts followed by a light etching[4].
Studies to examine the characteristics of copper surface
on various machining processes and surface treatments
started and some representative activities to show thepresent situation are also described in the paper.

2 TEST STRUCTUER GEOMETRIESThere are two recent structures designed focusing on
the wake field suppression, DDS3 and RDDS 1 and theseserved as the reference structures for the linear collider
purpose[!]. These structures are both 1.8m long and startat a high group velocity, 12% of light velocity. The latterstructure is more realistic because of optimised shunt
impedance by introducing rounded shape of the cellcomparing to the former composed of a flat disk and acylinder. However, the ratio of the surface field to theaccelerating field for both structures are almost the sameso that it is reasonable to study the high field performanceby using the structures composed of cells with flat disks
and cylinders, just as the former so that rather tediousdesign and fabrication works will be minimized.
In order to study the structure with reduced groupvelocity, it is most effective to realize by reducing thebeam hole aperture. The line (A) of 1.8m structure shownin Fig. 1 is the beam hole radius of the reference structure
as a function of group velocity along the structure. The
dotted line (B) overlapping the above and that (C)extending to the lower group velocity side are those with
reduced group velocities by reducing the beam holeaperture. The left two lines (D and E) are for thestructures with low group velocities but keeping the beamhole aperture size the same as those of reference
structures by adopting a higher phase advance per cell,5n/6 mode. All these parameters are designed to make thestructures detuned in the first dipole mode frequencies tosimulate the realistic parameter change along the structure.
The structure length naturally decreased from 1.8m to 0.5lm range due to the less group velocity.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

of two structures.
It was also observed that most of the remammg

breakdowns after reaching the maximum field occurred
near input coupler area. This is one of the issues to besolved, probably modifying the geometry near the input
coupler.

4 FABRICATION PROCESSES

2L___jL___j_

0____l___J_____j__The high-field performances should largely depend on
the quality of copper surface reflecting the actual
fabrication processes and treatments of the structures.
Three typical fabrication flows of recent tested structures
are listed in Table 1.

__j0.120.02 0.04 0.06 0.08 0.1
Group velocity (vg/c)

Figure 1: Beam hole radius as function of group velocity.

. Ifab.
tncatwn fl ows of recent strucures.
Tab e 1: Typ1ca
TypeRNF
Usual lathe
DiamondDiamondMachining
HeavyLightRinsing
03PW
chemicalchemicalrinsing
Hz DB
VACDB
Hz DBBonding
Au Brazing
AuAuBrazing
BrazingBrazing
Hz+VAC
Hz+VAC
Treatment
Hz+VACNote:
0 3 PW =Ozone-gas included pure-water rinsing.
Chemical =Rinsing in some combination of acid.
VAC, H 2 = Vacuum furnace and hydrogen furnace.
DB =Diffusion bonding at 900- lOOOC.
Hz + VAC=Surface treatment at high temperature
in hydrogen atmosphere and in vacuum.

3 LESS DAMAGE AT LOW GROUPVELOCITYIn Fig. 2 are shown two typical high-field performancesof the structures [2,5]. The left figure is for the structure(M2, 1.3m, vg/c starts at 10%) with the similar parameters
as A in Fig. 1 but starting group velocity is 10%, while theright for the structure (DS2S, 0.5m, vg/c starts at 5%)with very similar parameters to that of B in Fig. 1.
The conditioning of M2 showed severe phase change,
0.4 degrees/cell, due to the conditioning for 400 hours
from zero to the maximum accelerating gradient of85MV/m, though most of the high field conditioningperiod was 200 hours at 75MV/m level. Multiple
structures with the parameter A in Fig. 1 showed roughly
the same amount of erosion rate in a conditioning andoperation at 50 MV/m level for thousand hours. On theother hand, the conditioning of DS2S lasted over 5months, for 2400 hours, with a maximum accelerating
gradient of more than 70MV/m. Several thousand
breakdowns occurred but resulted in only a small phasechange of 0.25 degree/cell.
The phase change pattern tells that the frequencychange near input coupler is most significant, meaningthat the more group velocity the more severe erosion
occurs. The difference in the frequency shift, lMHz forDS2S and 4MHz for M2, indicates the preference of alower group velocity of 5% for DS2S compared to 10% ofM2.
M2 Bead pull before
and after ASTA run

DS2S Bead·Pull
Before & After NLCTA Runc;

5"'~"'
5li~ia.

I~s~o~t""'oo~t...,20~t~40-too!..w....2~o~.~o~..
Cells (from input)~-s . ---~----- -1-----i--- ---1··-----I I
ii"10 o ~~~~~~tcJ--~3~~~~·
CellFig. 2 Phase change due to high-field conditioning

50

The first column called F-type in the table, is listed in
more detail below; 1-rough machining of class 1 OFC, 2annealing at 500C, 3-diamond turning with Kerosene mist
and cutting by about 40 J.Lm in total with a final cut of 2J.Lm, 4-rinsing in acetone ultra-sonic bath, 5-storage in air,
6-ozone-gas included pure-water rinsing, 7-storage in air,
8-stacking in a clean room, 9-pre-bonding in a vacuum
furnace at about 180C for one day, 10-diffusion bondingat 890C for a few hours in a vacuum furnace, 11-brazing
in a hydrogen furnace around IOOOC, 12-RF tuning andfinally 13-hydrogen furnace treatment and vacuum baking.
The second column called N-type is the same as the Ftype except for the main bonding stage, where all thebonding processes are made in the hydrogen furnaces.
The third column called R-type starts with the parts
machined with the conventional lathe but with a thicker
chemical etching process. The surface treatment
comprises of wet hydrogen treatment followed by a dryhydrogen treatment and vacuum baking at 650C for tendays or so.
When we compare the conditioning performance of Ntype structures to the later R-type ones, that of the latter
seems better [4]. This example makes us study the copper
surface processes including thick etching.

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6 IMPULSE DC BREAKDOWNEXPERIMENT

5 POSSIBLE SURFACE CHANGESThe structures experience many surface treatments
during the fabrication. First of all, the machined parts arerinsed in some liquid solution. Then, they are bonded invacuum or hydrogen furnace, both at a high temperature,
900-1 OOOC, for an hour or more. This process changes thecopper surface fairly a lot[6]. Some examples are shownin Fig. 3.

A series of impulse DC high voltage breakdown
experiments were performed on the copper surfaces toevaluate the surface quality evaluation [7]. The study
indicated the importance of usage of class1 material
compared to class 3 or 5. It also showed various aspectsof the surface treatment effect.
We decided to pursue the same experiment on theelectrodes made through the processes connected to theaccelerating structure fabrication. To start with, two pairsof electrodes were tested, which were made by a diamondturning with Kerosene oil mist as a lubricant followed byan acetone ultrasonic bath rinsing. The conditioning speed
to reach the saturated surface gradient of 300MV/m level
and the reached field level are similar to those of the wellbehaved
diamond-turned
electrodes
experienced
before[7,8]. We will make the electrodes throughhydrogen furnace cycle, vacuum furnace cycle, vacuum
baking cycle, etc., to evaluate the effect on breakdown
and conditioning performance in near future.
Breakdown field on electrode pair as of machined~~--~--~--~--~--~'''-----•:---------<------- ;;)- ·----_,_ ---....

350_ .._,:-.___ ;._•J,---~-·_,, _8 300> 250 _________ :___~---~~----~-~lt------~'-----•--'''---•-+-------:--------:----------:.
.

;§.200"" 150
Q

''''
.'
''
--··--·--.·--·-----·;·--·--·---;··-···-··-r········'> 100 -:----i----------!----------i----------~-------50 -------:----------:----------:---------·:·-------'
'''0

•

''

0~--~--~--~~~~~

0100

200300~500

Numbering of breakdown

Fig. 4 Breakdown experiment an example

6(c) Hydrogen furnace X500
Fig. 3 SEM views of copper surfaces after hightemperature treatment; (a) and (b) in vacuum furnace and(c) in hydrogen furnace.
During a high temperature process in a vacuum furnacewith its vacuum level of 10·5 Torr, the copper surface
loses its original cut surface and exhibits a complicatedevaporation pattern depending on the crystal-axis
directions as shown in (b). It should be noted that even in
the same temperature process, the surface appearance may
change a lot. The example shown in (a) is the sample
prepared and processes in exactly the same manner as (b).
We speculate some change as for the furnace occurred
between (a) and (b). The surface treated in a hightemperature furnace filled with gas, ·such as hydrogenshown in (c), becomes very smooth with only the grainboundaries stressed. No other material than copper in case(b) and (c) were observed but the high field performanceon these surfaces should be studied. Studies under thisdirection are being plarined.

ACKNOWLEGMENTS

The present developments and studies have been
performed in collaboration between SLAC and KEK. The
authors greatly thank NLC colleagues of SLAC especiallyfor the fabrication and high-field testing of the structures.

REFERENCES[1] NLC/JLC group, "International Study Group
Progress Report", SLAC R-559 and KEK Report
2000-7, April, 2000.
[2] R. Loewen et al., Particle Accelerator Conference,PAC1999, New York, USA, 1999.
[3] C. Adolphsen et al., Particle Accelerator Conference,PAC2001, Chicago, Illinois, USA, 2001.
[4] D. Burke and M. Ross, private communication.
[5] J. Wang, private communication.
[6] T. Higo et al., High Energy Accelerator Conference,
HEACC2001, pllc15, Tsukuba, Japan, 2001.
[7] S. Kobayashi et al., IEEE Trans. Dielectric andElectrical Insulation, Vol.4, N0.6, 1997.
[8] S. Kobayashi et al., Proc. 26th Linear Accelerator
Meeting in Japan, p116, Himeji, Japan, 2001.

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