HIGH-GRADIENT TESTS ON AN S-BAND ACCELERATING STRUCTURE
Y. Igarashi†, S. Yamaguchi, Y. Higashi, A. Enomoto, T. Oogoe, K. Kakihara, and S. Ohsawa,KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan
Table 1: Parameters of the tested accelerating structure.

Abstract
The accelerating gradient of an electron linac is limited
by rf breakdown in the accelerating structure. An
improvement of the breakdown limit is therefore
necessary for the future upgrade of the KEKB injector
linac as well as the linear collider. High-power tests wereperformed on a conventional S-band 2m-long accelerating
structure for the KEKB injector linac. An average
accelerating gradient of 40 MV/m and a fieldenhancement factor of 53 were obtained after
conditioning with 1×108 rf shots.
We are examining the optimization of the high-pressure
ultrapure water rinsing technique (HPR) condition for an
S-band 2m-long accelerating structure. It was found that
HPR can reduce dust particles to 1/10 or less on a disk
surface.

Frequency
Phase shift per cell
Type of construction
Structure length
Number of cell
Iris diameter 2aAverage shunt impedance
Filling timeAverage group velocity vg/c2856
MHz
2π / 3
Quasi-constant gradient1889mm5423.75 – 19.70mm
58.3
MΩ/m0.566µsec
0.0113accelerating structures (Figure 2). The maximum peak
power of the klystron is 45 MW, and a SLED-type pulse
compressor is installed.

1 HIGH-POWER TEST OF THE
ACCELERATING STRUCTURE
1.1 Tested structure
The relevant parameters of the tested structure are
given in Table 1 and Figure 1. The characteristics of thisstructure are as follows:(1) The disks and cylinders are machined by a highprecision turning lathe with a diamond byte.
(2) The electroplating fabrication method is applied. Theadvantages of this method are:a. The cavities are joined without adding heat at over
40 ºC after machining. Therefore, the copper
maintains a higher tensile strength.
b. Frequency tuning after electroplating is not required.
(3) There is a crescent-shaped cut at the opposite side of
the waveguide iris to correct the asymmetry of theelectromagnetic fields in coupler cavities.
RF POWERCYLINDER

RF POWER

DISK

BEAM

CRESCENT-SHAPED
CUT

INPUT
COUPLERCOOLING WATERJACKETELECTROPLATED
COPPER LAYER
2072.4OUTPUT
COUPLER

Figure 1: Schematic of the accelerating structure.

1.2 High-power test standWe constructed a test stand for high-power tests of the___________________________________________†yasuhito.igarashi@kek.jp

Figure 2: High-power test stand.

1.3 Experimental results
The pulse width and repetition rate of the rf pulse were
4.0 µsec and 50 Hz, respectively, and the klystron output
power load gradually increased. The reflection of the rfpower from the accelerating structure or waveguides, andthe vacuum pressure of the ion pumps were used as aninterlock. The vacuum pressure of the acceleratingstructure was maintained at around 1×10-6 Pa during rf
conditioning.
At first, a test was carried out with the SLED detuned.
An average accelerating field of 20 MV/m was obtained
after 1.5×107shots (54 hours). This test, however, wasterminated because of a large reflection of the rf power tothe klystron. After a 20 liter/sec ion pump was added to
the klystron output, an average accelerating field of 23MV/m was obtained after 4.7×107shots (the total number
was 6.2×107shots (314 hours)). Next, a test was carriedout with the SLED tuned, and an average accelerating
field of 40MV/m was obtained after 4.1×107shots (Thetotal number was 1.0×108shots (542 hours)). Also, thehigh-power test was terminated due to a time limit. The(a)521009080
706050(b)53β400246810127shots [ ×10 ]Figure 5: Field-enhancement factor (β ) vs. numberof shots: (a) upstream, (b) downstream.
05914182227shots [×106]

3136404510-810-910-1010-11

0.7
1.5
3.8
5.2

7.3
8.2
8.8
10.5×107 shots(a)β=150

I / Es][ A / (MV/m)

2.510-1210-13

I / Es2 HIGH-PRESSURE ULTRAPURE WATERRINSING OF THE ACCELERATINGSTRUCTURE
It has been reported that the high-pressure ultrapure
water rinsing technique (HPR) is very effective toimprove the field gradients for normal conducting andsuperconducting RF cavities [1, 2]. HPR eliminates
surface contamination, such as dust particles, that isthought to be one of the causes of field emission. We havea plan to apply this technique to the S-band 2m-long diskloaded accelerating structure.

2.1 Removal effect of dust particles by HPR

2.5[ A / (MV/m)

2.5]

Figure 3: Average field vs. number of shots(with the SLED tuned).

β=631810-810-910-1010-1110-1210-1320222426283032(b)
β=115

2.5

E [MV/m]

4035302520151050100
908070605040

βaverage accelerating field (SLED tuned) versus thenumber of shots is shown in Figure 3.
The total amount of dark currents caused by fieldemitted electrons load measured as an index of progressof the rf processing by two Faraday cups set upstream anddownstream of the accelerating structure. The fieldenhancement factor (β ) can be obtained from a modified
Fowler-Nordheim (F.N.) plot. The F. N. plots and β values
are shown in Figures 4 and 5, respectively. The final
values of β (with SLED tuned, E=40 MV/m) were 52(upstream) and 53 (downstream), respectively.

We investigated the HPR effect on the surface ofsample copper disks that were used with the acceleratingstructure. We measured the number and size of dust
particles on a disk surface with a scanning electron
microscope (SEM). Ultrapure water is pressurized up to9MPa by a diaphragm pump, and is jetted from the nozzle
of a stainless-steel pipe (Figure 6). A sample disk is set
inside the stainless-steel vessel (Figure 7). This vessel
turns around the pipe and moves up and down during
water rinsing. After water rinsing, the vessel is dried by ascroll pump. A typical SEM photograph is shown inFigure 8.

β=621820222426283032

1 / Es [ ×10-3 m / MV ]242220181614

E [ MV / m ]Figure 4: Modified F. N. plot (with the SLED
detuned): (a) upstream, (b) downstream.

Figure 6: Nozzle of astainless-steel pipe.

Figure 7: Sample disk.

(a)(b)Figure 10: Acrylic model.

2.3 HPR of the S-band 2m-long disk-loaded
accelerating structure

10µm

Figure 8: Typical surface of a copper disk:(a) before HPR , (b) after HPR

2Particles [ 1/ mm ]Particles [ 1 / mm ]2pump pressure

2Particles [ 1 / mm ]Figure 9 compares the results of the measured number
and size of particles before and after HPR. The pumppressures were 3.0, 5.5 and 8.0 MPa and the rinsing times
were 6, 15 and 30 min. It was found that HPR could
reduce particles to 1/10 or less.
160
140
120
10080
60
40
200160
140
120
100
80
60
40
200160
140
120
100
80
60
40
200

8.0 MPa
6 min

8.0 MPa
15 min8.0 MPa
30 min

5.5 MPa
6 min

5.5 MPa
15 min

5.5 MPa
30 min3.0 MPa
6 min

3.0 MPa
15 min3.0 MPa
30 minThe HPR process of the S-band 2m-long disk-loaded
accelerator structure is as follows. The accelerating
structure is set up vertically and turns around the pipe andmoves up and down during water-rinsing. First, the lower
half of the accelerating structure is rinsed; next, it isreversed and the remaining half is rinsed.
After HPR, it is dried by a scroll pump immediately andvarious kinds of valves are attached in a clean room. After
that, it is exhausted sufficiently by a turbo molecularpump and inserted in a high-power test stand. It is not
exposed to the atmosphere after assembly in the cleanroom.

3 SUMMARY AND FUTURE
A high-power test was performed in an S-band 2m-longaccelerating structure. An average accelerating field of 40
MV/m and the field enhancement factor (β ) of 53 wereobtained after 1.05×108shots.
We apply the HPR to the whole part of an S-band 2mlong accelerating structure, and carry out a high-power
test.

4 ACKNOWLEDGMENT
before HPRafter HPR
<11-22-33<
Size of particles [ µm ]

<11-2
2-33<
Size of particles [ mm ]The authors wish to thank the members of the KEKB
Injector for their help. We also acknowledge Dr. K. Saito
for advice about the HPR and Dr. T. Higo for offering thestainless-steel vessel.

<1
1-2
2-3
3<
Size of particles [ µm ]rinsing time
Figure 9: Comparison of the particle number andsize for before and after HPR.

2.2 Optimization of the HPR condition
A surface analysis and observation of dust particles on
disks will be performed using test cavities (acrylic modeland copper model) to optimize the HPR condition before
rinsing the 2m-long accelerator structure (Figure 10).

5 REFERENCES[1] K.Saito et al., “Study of Ultra-clean Surface for
Niobium SC Cavities”, Proceedings of the 6thWorkshop on Superconductivity, 4-8 October, 1993,
CEBAF, Newport News, USA, p.1151-1159.
[2] M.Yoshioka et al., “High Gradient Studies on UHV
Room Temperature Cavities at S-band for Linear
Colliders”, Proceedings of the 17th International
Linac Conference, 1994, Tsukuba, Japan, p.302-304.