INPUT COUPLER FOR KEKB SUPERCONDUCTING CAVITY
Y. Kijima*, The Graduate University for Advanced Studies from Mitsubishi Electric Co.
S. Mitsunobu, T. Furuya, KEK, High Energy Accelerator Research Organization
R. Noer, Carleton College
T. Tanaka, Furukawa Electric Co.

Abstract
Four additional superconducting cavities were installedin the KEKB HER ring in August 2000. The input
coupler is the key component for high current operationof the superconducting cavity. We conditioned the input
couplers for four cavities at up to 300kW of input power,
while applying a DC bias voltage of up to ±2kV on theinner conductor. This RF aging with bias voltage wasfound to be effective in suppressing multipacting. Wewill discuss the multipacting process and the beam
operation of the superconducting cavity input couplers.

1 INTRODUCTIONKEKB is a double ring collider of 3.5 GeV positrons ina low energy ring (LER) and 8 GeV electrons in a highenergy ring (HER). It was designed to accelerate ampereclass currents, 2.6A for the LER and 1.1A for the HER.
The four additional superconducting cavities for KEKB
were installed in the HER to make possible a beamcurrent of 1.1A in summer 2000 [1]. Up to now thesuperconducting cavities have been operated under abeam of 650mA in the HER. The input coupler has beenused to feed a power exceeding 380 kW stably with highcurrent.
This paper describes the status of the couplers for theadditional cavities and an investigation of the effect ofmultipacting electrons under the RF processing with bias
voltage.

±2000V. The inner conductor is insulated with apolyamide film from the outer conductor.
(3) The monitor port near the window now monitors
vacuum and electron current, in addition to discharge
light.
Table 1 Parameters of the input couplers
Coupler
4loaded Q
7 × 10impedance
50ΩWindowStructure
choke : gap 3 or 4 mmSizeceramic :φ 170- φ42×10mm
Material
95% purity alumina
CoatingTiNxOy 100Å
Inner conductor
Structureantenna φ 53mm
Material
Copper (OFHC)
Surface treatment Electro polishing
Setting position
12mm into the beam pipe
Outer conductor
Structure
inner radius φ120mm
Material
30µm Copper plated Stainless steel
32 STRUCTURE OF INPUT COUPLER
The input coupler for the KEKB superconducting cavity
is a modification based on the TRISTAN coupler [2].
Figure 1 and table 1 show the structure and parameters ofthe couplers. The choke structure around the window isalmost the same as in the TRISTAN 1MW klystron.
There it was demonstrated to have long life operation at
fixed high power. The KEKB coupler is required tooperate over a large dynamic power range during beam
operation. The points of improvement are the following:(1) The input power is about one order of magnitude
higher than that of TRISTAN. The cooling system isstrengthened to handle the heat loss lower. The coaxial
pipe is cooled by an 8 l/min He gas flow, with outside
copper fins to maintain uniform temperature.
(2) To make the processing effective, a doorknob
transition section is used to supply bias voltages ofFigure 1: Layout of input coupler

Table 2. Status of input couplers

3 PROCESS AND STATUS
The installation and conditioning of four additional
couplers was carried out between January and September,2000. Table 2 summarizes the status of the four couplers.
The stages of coupler commissioning were the following:(1) Processing at coupler aging standThe coupler as received is rinsed by O3 pure water,and set on the coupler aging stand. After baking at 100
deg for 1 day, processing is begun up to a power of400kW with 30% reflected wave. The aim of thiscoupler processing at the aging stand is to remove dust,
defects and adsorbed gas. The coupler is operatedusing partial travelling waves in order to process awide area of the coaxial line. We applied this process
to the four new couplers, labelled #5(for cavity D10-A)
to #8(for cavity D10-D). The inner conductors of #6
and #8 coupler had been kept in vacuum for 2 years
after earlier RF processing at the aging stand. The #5
inner and outer conductors were processed for the firsttime. Six to eight hours of processing were needed toreach the full power feed. The total amount ofdesorbed gas during the RF processing is less than theamount desorbed during the baking. The total amountof desorbed gas during the bias aging is small about 0.5
torr⋅sec. It seems that theprocessing using
multipacting during bias aging is different from thebaking in its effect on the surface condition. The maindesorbed gas during baking at 100K is H2O, and during
RF processing is H2 and CO/N2.
(2) Horizontal test
After the initial processing at the aging stand, theinput coupler has to opened to air for assembly into thecryo-module. Before cooling, the processing of eachcoupler was carried out up to a power of 300kW withstanding waves. The aim of this processing is to degas
the surface and to process the head of the innerconductor and the input port of Nb cavity. The
processing time of the coupler is shorter than the initialprocessing, because much of the effect of the latter has
been retained.
Figure 2 shows the coupler pressure during thisprocessing.

5(D10-A)
6(D10-B) 7(D10-C) 8(D10-D)
inner conducter as received Keep vac. Processed keep vac.
outer conducter as received as received as received as received
treatment
in/out O3
in/out O3
out O3in/out O3
choke gap
4mm
4mm
3mm
3mm

-1. Aging stand (300K)
Baking90ºC×20h 80ºC×30h
95ºC×30h 105ºC×25h
reflection
30%
30%
60%
30%
aging time [hr]inner
3.7/300kW
no-bias2.0/360kW 8.1/420kW 3.8/300kW 6.7/420kW
bias-aging
5.8
4.5
9.07.1
vacuum [torr]
2.7×10-7
2.7×10-7
5.2×10-7
3.9×10-7
outgas [torr·sec]
bake90º1day
1.7
4.4
4.6
3.4
no-bias
0.5
2.2
0.9
2.1
bias-aging
0.6
0.5
0.5
0.4

2.Horizontal test
At 300K
Aging time [hr]:no-bias2.4/300kW 3.5/300kW 6.0/300kW 2.5/300kW
1.8×10-9
1.7×10-9
vacuum [torr]
2.9×10-9
2.0×10-9
outgas[torr·sec]
3.1
2.1
At 4K
Vacuum [torr]
1.3×10-9
5.5×10-10 5.4×10-10
4.2×10-10

3.After installaration in KEKB tunnel
At 300K
vacuum [torr]
aging time[hr]no-bias
At 4K
Vacuum [torr]

1.7×10-9

1.2×10-9

4.4×10-9

1.1×10-9

2.5/300kW 3.6/300kW 5.5/300kW 2.6/300kW
4.9×10-10

3.1×10-10

2.5×10-10

2.5×10-10(3) In KEKB tunnel
After the cavities were installed in the KEKB tunnel,
we processed the couplers at up to 300kW with fullreflection at room temperature. At 4K operation, thecavity surface absorbs gas as a cryo-pump. When thecavity is warmed up after beam operation, a large
amount of absorbed gas comes out. Therefore every
time the cavity is warmed up, coupler processing
needs to be carried out.

4 SIMULATION OF MULTIPACTING
We have developed a simple simulation code toinvestigate theoretically the effect of multipacting
electrons. The simulation is calculated for the coaxial line
under various power transmission and bias voltage
conditions. The results were compared with the realcommissioning data.

4.1 Method of calculations
The equations of motion of a relativistic electron in anelectromagnetic field E,B are generated by the formula∂ve1=( E − v × B − 2 (v ⋅ E )v )∂tmγc

Figure2. The coupler pressure
temperature horizontal processing

duringroom-∂x=v∂t

wherev, x: 3-dimensional velocity and position of electrone,m: charge, mass of the electronγ : Lorentz gamma
In the case of a coaxial line, the electromagnetic fields are
given by the following :Er =VbUωω⋅ {cos( ⋅ z −ψ ) − Γr ⋅ cos( ⋅ z + ψ )} +r ⋅ ln(b / a )ccr ⋅ ln(b / a )Bφ =Uωω⋅ {cos( ⋅ z − ψ ) + Γr ⋅ cos( ⋅ z + ψ )}c ⋅ r ⋅ ln( b / a )ccFigure 3. Secondary electron smission coeffocientwhere a,b: inner and outer conductor radiusω : frequency
ψ : phase
The secondary electron emission coefficient iscalculated from measured values using the following
fitting equation. In this simulation it is fitted from thebaked copper data shown in figure 3 .
δ =c1 ⋅ δ 0 ⋅ ( u i / u max ) C 2+ c4
c 3 + ( u i / u max ) 2 ⋅ cos θThe calculations were done as follows:1) Set the initial conditions of electromagnetic fields(power, reflection , bias voltage) and electron (energy,RF phase, position).
2) Integrate the equation of motion of the electron by theRunge Kutta-Gill method.
3) Stop when the electron trajecory collides with the wall.
4) If the phase of the electric field is 0 ≤ ψ ≤ π , calculate
the secondary electron coefficient δi, set the initialenergy of the secondary electron to 5eV and its
emission angle relative to the surface at 90°, and start
tracking again from the collision point..
5) Repeat up to i=20. Calcurate the total coefficient∆=Πi=1,20δi.
6) Repeat the simulation with a different position andpfase.

4.2 Results of Simulations
We show the results of simulation for the case ofstanding waves for the KEKB 50Ω coaxial coupler. For
each value of input power and bias voltage (figure 4),
over 700 different simulation were done, each with adifferent initial position and phase. Each point plottedshows the total number of electrons surviveing after i=20
generation from all these simulations. Figure 5 shows
only theose where ∆ > 10. To compare with theconditioning data(Fig.2), the vacuum level during coupler
processing is taken as a measure of the multipacting in
the coaxial line. With positive(negative) bias voltage, thefirst order one point multipacting occers on the inner(outer) conductor

Figure 4. The number of electrons after 20 RF cycles.

Figure 5. . The number of electrons after 20 RF cycles,
but including only simulations where the total
secondary electron emission coefficient is over 10.

5.SUMMARY
The couplers for KEKB have been operated with highcurrent without beam processing. RF processing withbias voltage is effective to expand the multipacting areaof the coaxial line. We supposed that multipacting
processing during the bias aging desorb the gas and alsochange the condition of surface.
Now we are beginning a study the multipacting effect by
RF power feed test on new test stand and by measurementof the secondary electron coefficient.

REFERENCES[1]S.Mitsunobu et al, this conference[2]S.Noguchi et al, 4th Workshop
SuperconductivityonRF