HIGH-GRADIENT TESTS ON S-BAND 2M-LONG ACCELERATINGSTRUCTURES FOR KEKB INJECTOR LINAC
Y. Igarashi†, The Graduate University for Advanced Studies (SOKENDAI)
S. Yamaguchi, Y. Higashi, A. Enomoto, T. Oogoe, K. Kakihara, and S. Ohsawa, KEK
1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan
H. Tomizawa, T. Taniuchi, H. Hanaki, Spring-8
1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan
RF POWER

Abstract
In this paper, we describe high-gradient test results
performed on S-band 2m-long accelerating structures forKEKB injector linac. High-gradient tests were performedon three types of accelerating structures: (1) a conventional accelerating structure for KEKB injector linac, (2)an accelerating structure whose input and output couplers
were replaced by ones without a crescent-shaped cut, and(3) a conventional accelerating structure which was rinsed
with high-pressure ultrapure water. As a result of the tests,
an average accelerating gradient of more than 40 MV/m
was obtained in all structures. Especially, 45 MV/m wasachieved in a structure rinsed with high-pressure ultrapurewater with a short rf processing time and a low breakdown rate (this gradient was limited by the klystron
output power). Black patterns were observed at a crescent-shaped cut and the iris of the coupler cells. The
simulated surface current densities were higher by about
10 times than that of the other surfaces of the cells. The
measured momentum spectra were reproduced qualitatively by a simulation. It was found that the breakdown
rate decreased significantly after conditioning.

1 HIGH-GRADIENT TEST OF THE
ACCELERATING STRUCTURECYLINDERRF POWER

DISK

BEAM

CRESCENT-SHAPED
CUT

INPUT
COUPLERCOOLING WATER
JACKET

ELECTROPLATED
COPPER LAYEROUTPUT
COUPLER

2072.4

Fig. 1: Schematic of the 2m-long accelerating structure
for KEKB injector linac.
was rinsed with high-pressure ultrapure water. The highpressure ultrapure water rinsing technique (HPR) is very
effective to improve the field gradients for normal
conducting and superconducting rf cavities. HPR eliminates surface contamination, such as dust particles, that isthought to be one of the causes of field emission. We
applied this technique to the 2m-long accelerating structure (Fig. 2). It was observed in preliminary tests that
HPR could reduce dust particles to 1/10 or less on a disk
surface[1].

Acceleratingstructure

1.1 Tested structures
The 2m-long accelerating structure for KEKB injector
linac is of a quasi-constant gradient type and has 54 regularcells with 2 coupler cells (Fig. 1). It is operated in the 2S/
3 mode at 2856 MHz. This structure is fabricated using ahigh-precision turning lathe with a diamond byte and aelectroplating fabrication method. There is a crescentshaped cut at the opposite side of the waveguide iris tocorrect for any asymmetry of the electromagnetic fields
in coupler cell. High-gradient tests were performed on
three types of accelerating structures:Type (1): The conventional accelerating structure for
KEKB injector linac [1].
Type (2): The accelerating structure whose input andoutput couplers were replaced with those without a crescent-shaped cut. After a high-gradient test of Type (1), a
black pattern was observed at the crescent-shaped cut of
the output coupler cell.
Type (3): The conventional accelerating structure which___________________________________________†yasuhito.igarashi@kek.jp

NozzleFig. 2: High-pressure ultrapure water rinsing of the 2mlong accelerating structure.

1.2 Experimental results
The pulse width and repetition rate of the rf pulse were
4.0 Psec and 50 Hz, respectively. At first, a test wascarried out with the SLED detuned; next, it was carriedout with the SLED tuned. Reflection of the rf power fromthe accelerating structure or waveguides, and the vacuum
pressure of the ion pumps were used as an interlock. The
total amount of dark current caused by the field-emitted
electrons was measured by two Faraday cups set upstream
and downstream of the accelerating structure. The vacu-um pressure of the accelerating structure was maintainedat around 1×10-6 Pa during rf conditioning.
Table 1 summarizes the high-gradient test results. The
average accelerating field vs. the number of shots isshown in Fig. 3. The field enhancement factor (E can beobtained from a modified Fowler-Nordheim (F.N.) plot.
F.N. plots are shown in Fig. 4, and the E values are shownin Fig. 5 (only Type (3)). As a result of the tests, anaverage accelerating gradient of 45 MV/m was achieved.
SLEDtunedetuneE [MV/m]

100

Especially, a structure rinsed with high-pressure ultrapure water reached 45 MV/m with a short rf process-ing
time, low breakdown rate and low dark currents
compared with the others. Although this gradient waslimited by the klystron output power, we are expectingthat more than 45 MV/m can be achieved.
Table 1: Test results of three types of structures.
Type1(a)
02040

60

80

100

120

140100down

*

570

630

52

53

7E

(1)

542

10.040(2)

812

15.0

45

720

1130

5455(3)

356

6.4

45300

237

6853

160

SLEDdetune
tune

up

Shot

40 MV/m

10

Peak darkcurrent [PA]up down

Maximum
average field[×10 ][MV/m]

Time[hour]*: Terminated due to a time limit
200

45 MV/m

200

1E

10(b)
020

40

60

80

100

120

140

100
90
80
706050٤: SLED detune
٨: SLED tune

40160

2345702040

60

80
100
shots [㬍106]

120

140160E [MV/m]I / Es 2.5 [ A / (MV/m) 2.5 ](a) Type(1), (b) Type(2), (c) Type(3).

1010-97.3
7.8

10.5

shots-11-9-12Type (1) upstream
152025

3.4
4.5
5.9

7.0
9.5
11.2

13.2
14.8
15.7

Type (1) downstream

-13

1030

shots

-10

10

10-7

10-8

10-915

20

25

30

-10

102

3

45

677

Shots [㬍10 ]When we confirmed that inside of the conventional
accelerating structure after the high-gradient test, a black
pattern was observed at the crescent-shaped cut of theoutput coupler cell. Also, when we cut the accelerating
structure used for the accelerator operation, in order toconfirm inside of the input coupler cell, a black pattern
was observed at the coupler iris.
We performed simulations of the surface electric fieldstrength and the surface current density on the coupler
cell using HFSS. The surface current densities on thecrescent-shaped cut and on the coupler iris are shown inFigs. 6 and 7, respectively. From these results, the surface
current densities at the location of black patterns werehigher by about 10 times than that of the other surfaces ofthe cells.

1054-1110-1110-1255-121010Type (2) upstream-1310I / Es 2.5 [ A / (MV/m) 2.5 ]53-111010-910-1310-81010-1210-8-710-1010-7101010

I / Es 2.5 [ A / (MV/m) 2.5 ]

8.2
8.852-101010

12 BLACK PATTERNS INSIDETHE COUPLER CELLS

Fig. 3: Average field vs. number of shots.

-87101-76Fig. 5: Field enhancement factor (E  vs. number ofshots.(Type (3)). (a) upstream, (b) downstream.

45 MV/m(c)1010090807060
5040

1

Shots [㬍10 ]SLED
100 detunetune(b)E

E [MV/m]

(a)

2515

2010-72.6
3.4

4.6
5.8

10-8

1010-910-10106.5shots

-1115

2025

30-7

10-9-1068-1110-1253-12

1010Type (3) upstream-13

101010-81010Type (2) downstream-1330

1010152025

1 / Es [ 㬍10-3 m / MV ]Type (3) downstream-131030

10152025

1 / Es [ 㬍10-3 m / MV ]Fig. 4: Modified F. N. plot (SLED tune).

30

Crescent-shaped
cutFig. 6: Simulated surface current density on the crescentshaped cut.

x=aMomentum [MeV/c]

Coupler irisx’ = 0.02

808070706060505040403030

20201010003 MOMENTUM SPECTRA OFDARK CURRENTS

The momentum spectra of the dark currents were measured by a bending magnet with a Faraday cup. The slit
aperture was 20 mm. The measured result of Type (3) foran average accelerating gradient of 42 MV/m after 5.7
×107shots is shown in Fig. 8 (dotted line). The dashed lineis the simulated momentum spectrum. The simulations
were calculated by the ETS code (The simulated structures were constant-impedance types) [2]. An example ofthe simulated trajectories of the field-emitted electrons isshown in Fig. 9. (In this example, the number of primaryelectrons is 200. In the actual calculation, it was set to720,000.) In Fig. 8, the measured momentum spectrum
has a slope at the low-energy region, but the simulated
momentum spectrum does not have a slope. The phasespace distribution of the field-emitted electrons thatarrived at the end of structure was calculated. Fig. 10show the relation with the momentum and the position x
and the angle x’. Most of the electrons are within theaperture size (a = 11.875 mm), and the low-energy electrons have a large angle. The momentum spectrum,recalculated by the electrons with x < a mm and x’ < 0.02
rad, is shown in Fig.8 (solid line). The electrons with x’ <0.02 rad are captured at the cavity disks (collimator) andthe slit. The simulation results were qualitatively consistent with the measurement results.
Dark Current [arb. unit]

10-910-1010-1110-1210-1310-1410-150102030 4050 60
Momentum [MeV/c]

708090Fig. 8: Momentum spectra of the dark current.

30x [mm]

40

50

00.0

0.2

0.4

0.6

0.8

1.0x' [rad]Fig. 10: Momentum vs. the position x and the angle x’ ofthe field-emitted electrons that arrived at the end of thestructure.

4 RELATION WITH THE AVERAGE
ACCELERATING GRADIENT AND THE
BREAKDOWN RATE
At the end of high-gradient tests, we examined therelation between the breakdown rate and the average
accelerating gradient. The test result of Type (2) is shownin Fig. 11. The breakdown rate was significantly decreased after conditioning.
14Conditioning

1211
986
5

Measured
breakdown rate

32032

34

36

38

40

42

44

46

48

㪜㩷㩷㪲㪤㪭㪆㫄㪴

Fig. 11: Relation between the breakdown rate
and the average accelerating gradient.

5 SUMMARY
An average accelerating gradient of 45 MV/m was
obtained in a structure rinsed with high-pressure ultrapure
water with a short rf processing time, a low breakdown
rate and low dark currents. Black patterns were observed
at a crescent-shaped cut and the iris of the coupler cells.
As a result of simulations, the surface current densities
were found to be higher by about 10 times than that of theother surfaces of the cells. The measured momentum
spectra were qualitatively reproduced by the simulation. Itwas found that the breakdown rate decreased significantly
after conditioning.

6 REFERENCES

454035[1] Y. Igarashi, et al., “High-gradient Tests on an S-band
Accelerating Structure”, Proc. of the 21st International
Linac Conference, LINAC2002, Korea, 2002.
[2] S. Yamaguchi, “Simulation Studies on High-GradientExperiments”, LAL/RT 92-18, 1992.

30R [mm]20㪙㫉㪸㫂㪻㫆㫎㫅㩷㫉㪸㫋㪼㩷㩷㪲㩷㫋㫀㫄㪼㫊㩷㪆㩷㪿㫆㫌㫉㩷㪴

Fig. 7: Simulated surface current density on the coupleriris.

1025201510
500200

400

RF600

800

1000

1200

1400

1600

1800

Z [mm]Fig. 9: Example of the field-emitted electrons trajectories.