PRESENT STATUS OF CAPACITOR-CHARGING POWER SUPPLIES FORKLYSTRON MODULATORS IN SuperKEKB INJECTOR LINAC
M. Kawamura†, M. Akemoto, H. Nakajima, T. Natsui, T. Shidara, S. Fukuda, H. Honma,
S. Michizono, KEK, 305-0801 Tsukuba, Japan
O. Endo, H. Akikawa, K. Sato, Nihon Koshuha Co., Ltd., 226-0011 Yokohama, Japan
Abstract
Capacitor charging power supplies (CCPSs) have beenused in compact klystron modulators. Two main types ofCCPSs, the normal model and highly precise model, havebeen developed by Nihon Koshuha Co. in collaboration
with KEK since JFY2012. In the KEK, regarding the output voltage stability of normal model, almost the samecharacteristic as the specification 0.2 %p-p was measured.
After some repairs for the discharge problems, the CCPSs
in the SuperKEKB injector linac were operated without
important breakdown for about 6,000 h from autumn in2017 to summer in 2018.

INTRODUCTION
The SuperKEKB injector linac has delivered electronsand positrons for particle physics and photon science experiments since 1982 [1]. At present, 59 S-band (2856MHz) klystrons are used as RF sources in the injector.
One klystron modulator supplies pulsed high-power toone klystron. All klystron modulators have pulse-forming
networks (PFNs). Thirteen compact klystron modulators[2-5] and 46 conventional klystron modulators [6] are now
used. Regarding the charging method of PFN capacitors,
compact modulator is a switching charging type using acapacitor-charging power supply (CCPS) [2-5,7-11], andconventional modulator is the resonant charging type. Further, the former is almost 1/3 the size of the latter.
Compact modulators and CCPSs have been developedsince JFY2002 [3-5]. At first, the CCPSs were developedby Toshiba Electro-Wave Products Co. [3-5,7], and sinceJFY2012, they have been mainly developed by Nihon
Koshuha Co. (NIKOHA) [8-11], in collaboration withKEK.
This article describes the present status of the CCPSs developed by NIKOHA. It includes the specifications, thedifference between the normal model and the highly precise one, the measured results, and operation conditions inKEK.

OUTLINE OF NIKOHA CCPS
At present, 18 normal model CCPSs, two highly precisemodels and one exclusive model for the flux concentrator
modulator [12] have been developed [11].
Figure 1 shows the outward appearances of the normal
model and highly precise model, and Table 1 shows theirspecifications [8-11].
___________________________________________† masato.kawamura@kek.jp

Figure 1: Outward appearances of the normal model (left)
and highly precise model (right) of Nikoha CCPSs [11].
Figure 2 shows the schematic diagram of the 30 kJ/s normal model [8]. The normal model consists of three sections; the rectifier section,
Table 1: Specifications of the normal model and highly
precise model of Nikoha CCPSs (for 0.62 µF load)
Normal [8]HighlyPrecise [9]Input voltage
AC 420V±5%(3-phase)
Charging power 30 kJ/s
35 kJ/s(Main: 30 kJ/s,
Sub: 5kJ/s)
Output voltage
43 kV
Average output
1.6 A
1.8 A
current(Main:1.6 A,Sub:0.2 A)
Repetition rate
50 pps50 pps
Output voltage
0.2 %p-p0.03 %p-pstability(at 43 kV)(at 43 kV)
Efficiency
More than 80%
Cooling
Water 5 L/min.
Water 5 L/min.
(Main)Air (Sub)
Size (mm3)
480×760×680 480×760×980
inverter section, and HV section.
The rectifier section converts 3-phase 420 V AC input to600 V DC output. The inverter section is an IGBTswitched 20 kHz inverter, and it is constructed by two se-ries resonant circuits in parallel. The HV section is constructed by a high-frequency transformer and diode bridge
circuits. The

Electro-Wave Products Co. CCPS) have been operating[13].

Figure 2: Schematic diagram of the normal model CCPS.

Figure 3: Waveforms of input-output signals of the normalmodel CCPS. The horizontal scale is 4 ms/div.

voltage ratio of transformer is more than one hundred
times, and diode bridge circuit is constructed by ten fullwave rectifier devices in series. The HV section supplies atmost 43 kV to the PFN capacitors.
This model controls two stages in the charging period.
At the beginning of capacitor-charging, the two series resonant circuits operate in parallel to facilitate reduction incharging time (1st stage). When the capacitor voltage isclose to the setting value, one series resonant circuitcharges, to facilitate improvement of the peak voltage stability (2nd stage). The exclusive model for the flux concentrator modulator has a specification of 20 kV output
voltage [10-11].
The highly precise model [9] is the CCPS where twopower supplies (PSs) are combined as shown in Table 1; a5 kJ/s PS (the sub PS) and a 30 kJ/s normal model (the mainPS). The inverter section of the sub PS is constructed by
one series resonant circuit, and the switching frequency ischanged from 35 kHz to 17.5 kHz. The sub PS can be controlled by an optical signal for noise reduction.
In the highly precise model, the sub PS is connected toboth loads (PFN capacitors) by a HV cable and upper dominance control equipment by control cables. The output HVcable from the main PS passes inside the sub PS. The subPS generates control signals of the main PS. The highly
precise model controls five stages in the charging period,
using on-off operation of the three series resonant circuits
and changing the switching frequency of the sub PS.
At the test in NIKOHA, the result of output voltage stabilities, namely 0.2%p-p of the normal model [8] and0.022%p-p of the highly precise model [9], were observed.
All models of NIKOHA CCPSs are equipped with afunction named “abnormal charge pulse set function”.
When a few incomplete charges being unrelated to important breakdown happen accidentally, the HV-off interlock doesn’t function until the above situation counts successively to the established number of times. This function
contributes the continuous operation.

PRESENT STATUS OF NIKOHA CCPS
At present, in the SuperKEKB injector linac, most compact klystron modulators use NIKOHA CCPSs at present.
Since last summer, 12 NIKOHA CCPSs (and 1 Toshiba

Figure 4: Collected flat-top zoomed-in signals of charging
output voltage of normal model CCPS for 10 min. The horizontal scale is 200 µs/div.; the vertical scale is 25 V/div.

Characteristics of NIKOHA CCPS Measured inKEK
Figure 3 and 4 are the waveforms obtained at a KEK test
station. The setting output voltage is 43 kV and the repetition rate is 50 pps. The waveforms of Fig. 3 are always indicated in all compact klystron modulators for confirming
the state of CCPSs. The charging gate signal is input fromthe upper dominance control. The waveform of the charging gate signal shows that the ordered charging period is
18 ms. The charging voltage, current signals, and the endof charging signal are outputs from the CCPS. The waveforms of the charging voltage and current shows that theobserved charging period is 16 ms and two stage control isaccomplished.
The waveforms of Fig. 4 were measured to confirm theoutput voltage stability. There was a 105 V maximum unsteadiness at the flat-top for 10 min. Since the measured
output voltage was 42.4 kV, the output voltage stability wascalculated as 0.248%, which is very close to the specification. At the same time, using the histogram calculation
function of the oscilloscope, a datum with the standard deviation σ of 26.6 V was obtained. Therefore, the error calculated as the absolute value of ±2σ is 106 V, which isalso very close to the above unsteadiness 105 V.

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Figure 5: X-ray CT picture of a broken device [11].

Operation Conditions of CCPSs in SuperKEKB
Injector Linac
The operating hours were greater than 5,000 h from autumn in 2015 to summer in 2016 [10], were about 3,600 hfrom autumn in 2016 to summer in 2017 [11] and wereabout 6,000 h from autumn in 2017 to summer in 2018[13].
In October, 2015, two CCPSs broke down successively.
The full-wave rectifier devices short-circuited. It seemedthat the cause of the short was trash in the oil tank, which
is built into the HV section of the CCPS. The check andcleaning of all CCPSs were performed in a clean room (athermostat tank) [10].
In November, 2016, one CCPS broke down, but there
was no trash in the oil tank. Non-destructive tests with Xray computed tomography (X-ray CT) were commissioned
to an outside agency (Tokyo Metropolitan Industrial Technology Research Institute, TIRI [14]) to confirm the situation inside the broken parts of the devices. Figure 5 is oneof image data obtained by X-ray CT. Some discharge scars
were observed from the soldering points. To avoid the discharge, both spacings for insulation of metal parts andshapes of the parts in the oil tank were reconsidered andchanged in the broken CCPS [11].
After the repairs described above were performed, 12
CCPSs in the SuperKEKB injector linac were operated
without breakdown from autumn in 2017 to summer in2018 [13].

CONCLUSION
The NIKOHA CCPSs have been used by most compact
klystron modulators, and after some repairs, their reliability is confirmed gradually by recent operation. In the future, longer operation will be performed, and the characteristics of the highly precise model will have to be measuredin KEK.

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