EVALUATION OF COPPER SURFACE FOR X-BAND ACCELERATORSTRUCTURES
T. Higo, Y. Funahashi, Y. Higashi, N. Hitomi, K. Saito, Y. Saito, K. Takata, T. Takatomi, N. Toge,and Y. Watanabe, KEK, Tsukuba, Japan
S. Kobayashi and Y. Yamano, Saitama University, Urawa, Japan
Abstract
We have started evaluating the copper surfaces as ofvarious stages of the recent X-band structure fabricationin order to find a cure to suppress the severe arcs during
the high-field operation. The surface characteristics are
firstly examined using scanning electron microscope.
The copper as of machined already shows various othermaterials and particles on its surface but does not show
any void nor other materials concentrated in the grainboundaries. Those after high-temperature heat cycles
were found to be very different from those as ofmachined and the appearance depends on whether thetreatment is in vacuum or in hydrogen. The surface after
high-pressure (65-85kg/cm2) pure water rinsing shows
many pits indicating much soft treatment is preferred
replacing highly pressurized water.

1 INTRODUCTION
It was found recently that 1-2m long X-band
accelerator structures suffered from a severe frequencychange during a few hundred hours operation at anaccelerating field over 50MV/m[1]. The frequencychange comes from the erosion of copper surface arounda high electric-field region. The erosion is severe in theupstream side of the structure where the group velocity ishigh. An idea of the affect of the breakdown to theerosion indicates the lower group velocity structures areless damaged even in the presence of the breakdowns.
There is a development research changing geometries
based on the idea above[1].
On the other hand, the surface of the structures which
experienced breakdowns exhibits many pits and craters
indicating the existence of the arcs on a limited area on
surface. We need to understand how these arcs have been
triggered so that we find cures. Possible origins might besuch as some other materials covering the copper surface,particles on the surface, gas desorption from the coppermaterial, some electronic state different from normal
copper crystal, and so on.
There are various papers dealing with surface quality
and preparation process as follows. Particle removal
process in superconducting cavities was now widely
applied in practice[2]. Other material concentration
between boundaries was discussed in [3]. Gas desorption

through baking or annealing process was found very
effective to improve the breakdown performance on
copper electrodes[4]. Careful diamond turning on copper
surface reduces dark current while ozonized water risingdegraded the FE performance indicating the importanceof the chemical state of oxidized copper surface[5].
Taking the above information in mind, we have started
with inspecting the copper surfaces as of various stages
in our normal fabrication process, in order to study themechanisms which cause arcs and to find a cure toescape from arcs.

2 FABRICATION PROCESS
Our present standard fabrication flow[6,7] issummarized here; 1-rough machining of class 1 OFC, 2annealing at 500C, 3-diamond turning about 50 µm in
total, 4-rinsing in acetone ultra-sonic bath, 5-storage inair, 6-ozone-gas included pure-water rinsing, 7-storagein air, 8-stacking in a clean room, 9-pre-bonding in
vacuum furnace at about 180C, 10-diffusion bonding at890C in vacuum furnace, 11-brazing in hydrogen furnace
around 1000C, 12-RF tuning and finally 13-vacuum
baking before going into 14-high power feeding.
Typical parameters of the last diamond turning aresummarized as 1-cutting with a single-crystal natural
diamond with its radius 0.4mm and roundness of 0.1µm,
2-spindle rotation speed of 2800 rpm, 3-final cutting
thickness of 2µm and 4-using kerosene oil in mist as
lubricant. The roughness is about 50nm P-V.
Just after diamond turning, the cells are rinsed in an
acetone ultra-sonic bath for several minutes, then stored
in a container with reduced humidity. These cells are
rinsed in an ozone-gas included (3-4ppm) pure-water
bath for 15 minutes and again stored in the same
container until stacking.
The stacked cells are pre-bonded in an vacuum furnace
at around 180C for one day. Then, the cell pillar ishanged vertically in the same furnace and diffusion
bonded at 890C for a few hours[7]. The vacuum level ofthe furnace is usually a few 10-5 Torr. Peripherycomponents are then brazed on the main body usually ina hydrogen furnace. Finally it is baked in a vacuum
furnace before feeding high power.

Firstly a scanning electron microscope (SEM) wasused to see the surface as of machined. Some results areshown in Fig. 1. As shown in Fig. 1(a), particles are
sometimes seen and some reactions along crystal
boundaries are seen. Shallow hollows containing othermaterials such as chlorine, sulfur, calcium, etc. are also
seen fairly frequently. Even an inclusion of carbon suchas shown in Fig. 1(b) was found. It should be studied
how these materials are integrated in the material or put
on the surface so that we find how to escape from theseother materials.
Then, a scanning auger microscope (SAM) was used toinvestigate whether there is any concentration of othermaterials between crystal boundaries. Sputtering withargon ions of 3 KV impinging at 30 degrees wasperformed by the amount equivalent to Silicon crystal of
800 Angstroms. Then carbon and oxygen contents were
mapped around a triple junction of the copper crystal
boundaries shown in Fig. 2. No sign of these materials
was found indicating that the present diamond turning
did not stuff any lubricant into crystal boundaries.

(a)

(b)
Figure 1: SEM view of copper surface as of machined;(a) typical surface and (b) with carbon inclusion.

Figure 2. SAM view of copper crystal boundaries
after Ar sputtering.

Particles, larger than 1 micron, sitting on the copper
surface as of machined was counted by using SEM. All of
the area of a 100 micron-wide strip traversing thediameter of the front face of a sample with 25mm indiameter was scanned. The measured population wasshown in Fig. 3. How to evaluate particles smaller than 1micron should be developed.
Particle counting 010308
Number of particles [/mm2]

3 SURFACE INSPECTION OF MACHINEDCOPPER

30252015105002468101214

Size [micron]Fig. 3. Number of particle sitting on copper surface.

4 SURFACE CHANGE
4.1 High-pressure pure-water rinsing
High-pressure pure-water rinsing (HPR) is one of themost efficient ways to remove dust particles from theniobium surface of super conducting cavities[2]. It wasalso applied to reduce dark current in an S-band coppercavity[3].
Before really applying this technique on copper for Xband structure application, we started with inspecting thecopper surface by SEM after HPR of various conditions.
Firstly, a copper electrode with a spherical front shapewith its radius of 30mm was irradiated by pure water
through a nozzle set at 20mm apart. One of the water jets
was directed normal to the copper surface under test. The
alignment was performed by eye. The nozzle was moved
up and down by +-2mm during irradiation for realizing abetter uniformity of irradiation.
Fig. 4 shows one of the SEM views of the copper
surface irradiated at a pressure of 65kg/cm2 for two hours.
In the figure, we observed many pits of the order of a few
microns in diameter. The surface was inspected by
Talystep of Taylor Hobson, Co. Ltd. to see the change of
surface shape. It was seen that the tool mark of diamondturning was blurred a little showing a finite amount ofcopper material was removed.
The surface irradiated at the pressure at 85 and
65kg/cm2 for 10, 40 and 120 minutes were inspected.
Those at 85kg/cm2 showed the same quality as those of65kg/cm2. The number of pits and the size seem smaller
for the case of shorter period.
The surface without rinsing but with the same drying
process, dripping alcohol and drying in an filtered airflow, showed few pits indicating the pits were mainly due
to HPR process. We will try to reduce the pressure to see
how the diamond-turned surface is kept.

2Figure 4. Surface after HPR at 65kg/cm for 2 hours.

4.2 Heat cycle in vacuum furnace
The copper sample was put in the same vacuum furnace
for diffusion bonding. The sample was inspected by SEM
as shown in Fig. 5(a). Pits and inclusions were observedin addition to forming valleys along crystal boundaries.
The same-shape sample was put in another vacuum
furnace. The SEM view is shown in Fig. 5(b). Here wesee much clear grain boundaries and different surfaces
depending on the direction of the crystal axes.
From these figures, we realized that the bonding
process in a vacuum furnace should be improved though
we do not know at this moment how these surfaces affectthe high-field performance.

Figure 6. Copper surface after brazing heat cycle
in hydrogen furnace.

5 DISCUSSIONSThe copper surfaces were inspected after various
stages of our nominal fabrication flow. Following
understandings were obtained and some future study
directions were under consideration.
• Other material was found embedded in copper body.
• Neither carbon nor oxygen concentration wasobserved along grain boundaries even diamondturning with oil as lubricant.
• The HPR treatment results in many pits if thepressure is too high. Parameters such as pressure
and irradiation period should be optimised to keepthe surface quality but removing particles.
• Evaluation method of particle population should beimproved to study particles smaller than 1 micron.
• Copper surface changes dramatically through hightemperature heat cycle. Evaluation of high-field
performance on copper surface after hightemperature heat cycle, either in vacuum furnace orhydrogen one, should be performed.

REFERENCES(a)(b)
Figure 5. Copper surface after diffusion-bonding
heat cycle in vacuum furnace.

4.3 Heat cycle in hydrogen furnace
The same-shape copper sample was put in the brazingcycle at about 1000C in a hydrogen furnace. The surface
is shown in Fig. 6. As shown in the figure, the surface
was much smoother than that of vacuum furnace. Grainboundaries were also developed valleys. Some particles
were also found. The high-field performance should bestudied also on this type of copper surface.

[1] C. Adolphsen, “RF Processing of X-band Accelerator
Structures at the NLCTA”, Proc. Int'l Linear
Accelerator Conference, TUE01, LINAC2000,Monterey, CA, USA, 2000.
[2] K. Saito et al., “Water Rinsing of ContaminatedSuperconducting RF cavities”, 7th Workshop on RF
Superconductivity, CEA-Saclay, France, 1995.
[3] H. Matsumoto, “Dark Currents”, LINAC96, Geneve,
Switzerland, 1996.
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Breakdown of Oxygen Free Copper Electrodes, IEEE
Trans. Dielectrics and Electrical Insulation”, Vol.4,
No.6, 1997.
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Current from a Clean Copper Surface”, Proc. 24th
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Japan, 1999, in Japanese.
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for Linear Collider”, KEK-Report 2000-1, 2000.
[7] Y. Higashi et al., “Study on High-precision Diffusion
Bonding for X-band Accelerator Structure”, KEKReport 2000-2, 2000.