1. KEK Marx-Modulator R&D
Accelerator Laboratory
KEK Marx-Modulator R&D
Mitsuo Akemoto
KEK
LCWS2016
December 6, 2016

2. Marx Modulator Group KEK M. Akemoto, M. Nakajima, M. Kawamura
Accelerator Laboratory
KEK
M. Akemoto, M. Nakajima, M. Kawamura
Nagaoka University of Technology
W. Jiang, R. Suzuki, T. Hayashi
Pulsed Power Japan Laboratory Ltd.
A. Tokuchi, Y. Sawamura

3. Marx-type Modulators
Accelerator Laboratory
Marx Topology : Charge in parallel, discharge in series
Features: Modularity, Electrostatic Adding, Independent module control
　　　and Low-voltage sub-units
Marx-type modulator is expected to have high availability and low-cost.
DTI Marx Modulator
for ILC
SLAC P2 Marx Modulator
for ILC
Ampegon Pulse Step Modulator
for XFEL

4. ILC Modulator Specification
Accelerator Laboratory
Parameter
Value
Pulse Voltage
120 kV
Voltage Regulation
1%(p-p)
Pulse Current
140A
Pulse Length(flat-top)
1.6 ms
Total Pulse Energy
27 kJ
Repetition Rate
5 Hz
Average Output Power
135 kW

5. KEK Marx Modulator Marx Unit: Charging system: System:
Accelerator Laboratory
Marx Unit:
6.4 kV output pulser with four Marx cells at -2 kV each
Marx Cell has a step-down converter with a switching frequency of 50 kHz
Flat pulse voltage is formed with a PWM modulation
Charging system:
Independent unit charging through 20 kHz isolation step-up transformer
Each unit is charged up to -2kV and the input voltage of the Marx unit is regulated.
System:
N+1 redundancy
Air insulatated, water cooled, No oil
10kW Inverter
Charging PS
Marx Unit #20
-120 k V
140 A
1.7 ms
5Hz
10kW Inverter
Charging PS
Marx Unit #19
Isolation
Step-up
Transformer
Klystron
10kW Inverter
Charging PS
Marx Unit #02
10kW Inverter
Charging PS
Marx Unit #01
-6.4 k V
140 A
1.7 ms
5Hz
620 V
20 kHz
2.0 k V
20kHz
-2.0 kV DC
Inverter
Charging System
420 V
50 Hz 3φ
System Block Diagram
Simulated output voltage waveform

6. KEK Marx Modulator View
Accelerator Laboratory
Marx Unit
Charging
system
Size 1.6 m(D) x 3.3 m(W) x 2.3m(H)
Tests with a dummy load were ready at end of this March. Quarter of the modulator full charging power(50kW) is available.

7. Marx Unit
Accelerator Laboratory
Marx unit is the semiconductor Marx generator and consists of four Marx cells and a control circuit board with FPGA to control the each cell.
Marx cell is a step-down converter.
Marx unit parameters
Parameter
Value
Input voltage
2.0 kV
Output voltage
-6.4 kV
Capacitor droop
20%
Number of Marx cell 4
Marx cell (Step-down converter)
Block diagram of Marx unit
455mm×650mm×382mm(H)

8. Marx Cell 充電経路 Parameter Value Input voltage -2.0 kV Output voltage
Accelerator Laboratory
400mm×300mm×75mm(H)
充電経路
SiC FET 3P2S
Opt.i/F
IGBT
Filter L
Filter C
Storage capacitor C
Marx Cell
SiC SBD 2S
Marx cell parameters
Parameter
Value
Input voltage
-2.0 kV
Output voltage
-1.6 kV
Capacitor voltage droop
20%
PWM frequency
50 kHz
SiC FET 3P2S
SiC SBD 2S
IGBT
1.33uF
550Ω
0.133uF
48uH
SiC power devices(1.2 kV FET, SBD) were selected
because of its higher breakdown voltage, low on-resistance and fast switching speed.
Energy conversion efficiency is about 93%.
Circuit diagram of Marx cell

9. SiC Power Device(Si vs. SiC)
Accelerator Laboratory
Reduce chopper switching loss by one-third comparing with Si device ON Si
IGBT：IXGK75N250
2500V, 75A
SiC
FET：SCH2080KE
1200V, 40A
2S3P
Switching loss(1 cycle)
Frequency：50 kHz、Duty cycle :75%、Charging voltage :1 kV

10. Charging and Pulsing in Marx unit
Accelerator Laboratory
Charging
Pulsing
-6.4kV
Pulse DC
-2kV
(input) DC
-2kV
(input)
Charging switch
Chopping switch

11. Droop Compensation Output voltage droop of Marx cell
Accelerator Laboratory
Output voltage droop of Marx cell
PWM duty cycle is increased from 76% to 95% in proportion to time.
Droop ratio: 20%
Output droop can be compensated by PWM control.

12. Ripple Cancelation
Accelerator Laboratory
The output ripple of the unit can be reduced by cell phase shift control.
Output voltage ripple of Marx unit
Timing chart of four-cell gate signal
Ripple ratio : 52%  6%(for Marx unit)
Each cell gate signal is shifted by 90 degrees.
In the same way, modulator output ripple can be reduced by unit phase shift control.

13. High voltage Test at Dummy Load
Accelerator Laboratory
Charging voltage=1300 V, Vp=80 kV, Ip=90 A, Pw=1.7 ms, Ripple=0.6%(p-p)

14. Present status
Accelerator Laboratory
In the single unit test on the dummy load, a 6.4kV, 140A, 1.7ms pulse is successfully generated at 5Hz.
In the 20 unit full test, chopping FET switch(FET rated voltage : 1.2kV , Switch : 2 series and 3 parallel connection) has reached a peak voltage of 2.2kV at a charging voltage of 1.3kV. The voltage margin is not enough to operate at a full charging voltage of 2kV. This constrains the modulator output voltage.
The switch and diode should be changed to new SiC 3.3 kV devices to operate at a full charging voltage of 2kV.

15. Future Plans 2016(this fiscal year)
Accelerator Laboratory
2016(this fiscal year)
To check the modulator system at a charging voltage of 1.3kV, the high power tests at 80kV, 45A, 1.7ms, 5Hz with a klystron load will be performed.
2017
New unit using 3.3 kV devices will be tested at a charging voltage of 2 kV.
After confirming the performance of new unit, all units will be improved.
2018
Now, quarter of the modulator full charging power(5 charging power supplies, 50kW) is available. The remaining 15 charging power supplies will be added to complete the charging system.
Full power tests with a 10MW MBK klystron will be performed.