Performance of J-PARC Linac RF System - Spallation Neutron Source

Performance of
J-PARC Linac RF system
T. Kobayashi, E. Chishiro, S. Suzuki,
Japan Atomic Energy Agency (JAEA), JAPAN
S. Anami, Z. Fang, S. Michizono, S. Yamaguchi,
High Energy Accelerator Research Organization (KEK), JAPAN

Outline
1. Introduction (J-PARC and the Linac)
2. Overview of the RF System
3. RF Reference Distribution System
4. Low Level RF system
5. Digital Feedback Control System
6. System Performance
7. Summary

Introduction ..(1) -- J-PARC Facility
400 MeV
(RCS)
(main ring, MR)
The Japan Proton Accelerator Research Complex (J-PARC) will be one of the highest
intensity proton accelerators in the world.
It consists of a 400-Mev H – linac, a 3-GeV 1-MW rapid-cycling synchrotron (RCS),
and a 50-GeV synchrotron (main ring, MR)

Introduction ..(2) -- J-PARC Linac
•- Accelerated particles: H - (negative hydrogen)
•- Energy: 181 MeV (The last two SDTL tanks are used as debunchers)
Upgraded to 400 MeV by using ACS cavity ( in 2011? )
•- Peak current: 30 mA (50 mA for 1MW at 3GeV)
•- Repetition: 25 Hz
•- Pulse width: 500 us (Beam), 650 us (RF including cavity build-up time)
•- Acceleration Frequency 324 MHz
RFQ: Radio Frequency Linac
DTL: Drift Tube Linac (3 modules)
SDTL: Separate-type Drift Tube Linac (15 modules)
ACS: Annual Coupled Structure Linac
IS
H - Ion
Source
RFQ
Buncher(x2)
Chopper
3MeV
DTL
50 MeV
SDTL
3m 3m
MEBT1
27m 91m
Debuncher1
181 MeV
300m
(Present State)
L3BT
Debuncher2
3-Gev
Synchrotron
ACS Section for 400-MeV Upgrade
(972MHz)

Introducton ..(3)
History of the Linac Commissioning
Oct. .2006: High-power operation of all the RF systems was started. All
klystrons have successfully supplied power to the cavities.
Nov. 2006: RFQ&MEBT-1 beam commissiong
Dec. 2006: DTL beam commissiong.
Jan. 2007: SDTL beam test. (rough tuning)
Jan. 2007: 181-MeV acceleration of 5-mA (20us) beam succeeded!
Feb. 2007: 181-MeV acceleration of 26-mA beam (50 us) was succeeded!
~ Fine beam commissioning ~
Jul. & Aug. 2007: Summer Shutdown
Oct. 2007: Injection into 3-Gev Ring (RCS) !
Now RCS commissioning is continued.

Linac RF System
• 181 MeV normal conducting proton linac
• Operation frequency: 324 MHz
• Total 20 klystrons (max.3 MW), 6 DCPS (#1~6)
• RF Pulse width: 650 us (25Hz)
Requirements of cavity field stability
< +/-1% (amplitude), < +/-1 deg. (phase)
Klystron Gallery

RF Reference Distribution System
312-MHz RF reference is distributed to all LLRF control systems through optical links.
The reference signal is optically amplified and divided into 17 transfer lines, then furthermore it is
divided into 5; one of them is returned back to the first station for phase monitor.
Optical cable is the Phase Stabilized Optical Fiber (PSOF). (The thermal coefficient is 0.4 ppm/ºC.)
Temperature of the optical components (E/O, O/E, cables, couplers) are controlled to be constant.

Installation of the Reference System
1x17 optical coupler
set in an oven
QuickTime and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
•Insulated duct set in the under-
floor cable trench.
•Cooling water temperature is
controlled to be 29±0.1 deg. C
Klystron Gallery
Insulated Duct
with Cooling Water Pipe
An insulate duct is set in a cable trench under floor.
Phase-Stabilized
Optical Cables
Optical Coupler
Cross section of the under-floor cable trench
Optical cables and Optical couplers are installed into the insulated duct
with cooling water pipe for the temperature stabilization.

Measured the phase of returned signal from the end of the
linac (L3BT building).
Total about 800-m round trip transfer line.
Phase trend
for 4 days
312MHz
Round trip:
Klystron Gallery
(312MHz) Stability of RF Reference
Stability Measured
Result
Spec.
Gallery Temp. ±1.7 °C within ±2°C
Phase of Ref. ±0.06° within ±0.3°
Required stability is satisfied.

Low Level RF System
EPICS IO
LLRF PLC
cPCI FB system
Interlock
System
� One klystron supplies power 2 cavities in the SDTL section.
� Digital FB system (FPGA & DSP) is used for the cavity field stabilization.
� The FB system controls the vector sum of the 2 cavity fields.
� The RF reference is 312-MHz optical signal (received by O/E converter) .
� Cavity-tuners are controlled from the digital FB system by way of PLC.
� Fast hardwire interlock is connected to RF switch outside the FB system.
� Analog fast FB will be used for klystron FB loop if necessary. (It is optional.)

Klystron
Low Level RF System ..(2)
LLRF System
Klystron Gallery
Digital FB System (cPCI)
CPU
I/O RF&CLK
Mixer&I/Q
DSP/FPGA
Digital Analog
cPCI
Digital FB control system acts on a
compact-PCI (cPCI) crate system.
The FB and FF controls are performed by
means of a combination of FPGA and DSP.

RF:324MHz
LO:312MHz
IF:12MHz
Sampling:48MH
z
Digital FB Control System
cPCI digital FB system (FPGA & DSP)
�generates standard signals (48 MHz and 324 MHz).
�delivers I/Q modulated RF signals to 2 cavities.
�receives RF signals from cavities and down-converts to IF signal (12 MHz).
�samples the IF (12MHz) directly with 48 MHz.
�controls the vector sum of 2 cavity fields (PI-control).
IF signals are directly read by ADCs.
The separated IQ signals are compared with set-tables and PI control is made with FF.

IQ Offsets Calibration of the IQ-modulator
We found that
the output of the IQ-modulator has
offsets for I/Q set value (as Red Circle in
the figure).
(Even if I and Q components set to 0, but the IQ-
modulator still outputs a considerable signal.)
In order to cancel this undesired output,
we added I/Q offsets to the DAC out.
The offset values were measured for all the
LLRF systems.
With calibration of the IQ offsets,
the performance of the IQ-modulator
significantly improved (as Blue Circle)
With phase scanning, the amplitude of IQ
modulator output varies by less than 1%.
0
I
-600 -500 -400 -300 -200 -100
-100
0 100 200 300 400
IQ_out ( I_offset = 0,
Q_offset = 0, REF = 100 )
IQ_out ( I_offset = 0,
Q_offset = 0, REF = 500 )
IQ_out ( I_offset = -84,
Q_offset = -223, REF = 100 )
IQ_out ( I_offset = -84,
Q_offset = -223, REF = 500 )
Q
400
-200
-300
-400
-500
-600
IQ modulator outputs at SDTL13 with
or without IQ offsetting.
300
200
100

Performance of Feedback (FB) Control
Cavity field stabilization (in 600-µs pulse)
No FB Control with FB Control (Gain: P=5, I=5/1000)
Amplitude Phase
Amplitude Phase
10%
25 deg.
25-degree phase sag is due to
about 3.4% sag of the Klystoron
DC voltage.
Klystron Cathode
Voltage
QuickTime and a
TIFF (PackBits) decompressor
are needed to see this picture.
3.4% sag
±1%
20-µs, 5-mA Beam
±1 deg.
Amplitude and phase are stabilized
to be less than ±0.15% and ±0.15
degrees, respectively.
Beam loading (rising/falling ripple) is completely compensated with feed-forward
control .

Temp. Gallery Temp. [deg. C] C]
Long Term Stability
The stability of the FB system in long-duration operation was evaluated.
The trends of the amplitude and phase of the DTL2 Cavity (Input Power 1MW) were
measured by external monitor (independent of FB system) for 2 days
Drift [%]
Drift [deg.]
24.0
23.5
23.0
22.5
22.0
Amplitude (RF Detector)
Phase
(Mixer-out around zero cross)
Gallery Temperature
[deg. C]
0 5 10 15 20 25 30 35 40 45
Time [hours]
±1%
±1deg.
( during 2 days.)
Amplitude drift is ±0.15%.
Phase drift is ±0.15 degrees.
These results are including characteristics
of measurement system (detector, mixer
and so on).
Therefore, cavity field is maybe more stable
than observed values.

Cavity-Amplitude [a. u.]
4100
4000
3900
3800
3700
Beam Loading Compensation
with Feed-forward (FF) control
No FF control
Cavity-Amplitude
+/- 2.7%
Beam Start
@DTL2
Beam Loading
50 μs, 26 mA
3600
-130.0
200 250 300 350 400 450 500
Time [μs]
+/- 1.5 deg.
Cavity-Phase
-120.0
-122.0
-124.0
-126.0
-128.0
Cavity-Phase [degrees]
Beam Loading in DTL cavity field with
only FB control.
Peak Current: 26 mA, Pulse width: 50 us,
The amplitude change: ±2.7% ,
Phase Change: ±1.5 degrees
Cavity-Amplitude [a. u.]
4100
4000
3900
3800
3700
with FF control
Cavity-Amplitude
50-μs, 26-mA Beam
±0.2 deg.
Beam Start
Cavity-Phase
-120.0
-122.0
-124.0
-126.0
-128.0
3600
-130.0
4000
200 250 300 350 400 450DAC Out - 500 Amplitude
Time [μs]
@DTL2
±0.2%
Cavity-Phase [degrees]
2000
-120
200 300 400 500
Time [μs]
By using the FF control
the beam loading was successfully compensated.
(Amplitude change: ±0.2% , Phase change: ±0.2 deg.)
However, it is necessary to adjust a timing of a
control gate by 0.1-us step for optimization.
Optimum values of the FF amplitude and phase depend on
beam current.
DAC Output - Amplitude [a. u.]
5000
3000
DAC Output
-90
-100
-110
DAC Out - Phase
DAC Out - Phase [degrees]

Summary
� High power operation was started in October 2006.
� RF control systems are working well without fatal problem.
� the 181-MeV acceleration was succeeded in January 2007.
� The digital FB control system is performing according to the
expectation.
� Required stability is satisfied in duration of RF pulse and in
running operation. (Amplitude change: < ±0.2%, Phase change of
RF reference:

Introduction (3) -- J-PARC Linac
Accelerated particles:H - (negative hydrogen)
Energy: 400 MeV for 3-Gev Synchroton
600 MeV for Accelerator ADS
Peak current: 50 mA
Repetition: 50 Hz (including 25 Hz for ADS application)
Pulse width: 500 us (Beam), 650 us (RF)
Acceleration Frequency 324 MHz (~191MeV) , 972 MHz (~400 MHz)
IS
H- Ion
Source
RFQ: Radio Frequency Linac
DTL: Drift Tube Linac (3 modules)
SDTL: Separate-type Drift Tube Linac (15 modules)
ACS: Annual Coupled Structure Linac
SCL: Super Conducting Cavity Linac
RFQ
MEBT1
Buncher(x2)
Chopper
3MeV
324 MHz
DTL
50 MeV
Buncher(x2)
972 MHz
SDTL ACS
16m
3m 3m 27m 91m 108m
191 MeV 400 MeV
300m
MEBT2
(Final Design)
L3BT
3-Gev
Synchrotron
Debuncher
SCC
(Phase II) 600 MeV
972 MHz
ADS

Digital FB Control System (Apendix)
CPU
I/O
RF&CLK
Mixer&I/Q
DSP/FPGA
Digital Analog
cPCI is adopted for the crate.
FPGA based digital FB system
FPGA: Mezzanine card of the
commercial DSP board
� 2-FPGAs (2x VirtexII 2000) are installed
with 4x14bit-ADCs and4x14bit-DACs at
48 MHz sampling
� DSP board enables to calculate complex
diagnostics such as cavity control.
� FPGAs are used only for fast feedback.