RF system
Michiru Nishiwaki on behalf of RF group
KEKB Accelerator Review Committee 2022
Dec. 13, 2022

Contentsu Overview of RF systemu Operation Status in 2022ab
u Maximum Beam Current Limited by Present
RF systemØ RF Power delivered to Beam
Ø Large HOM Power absorbed by HOM Dampers
in SCC modules
Ø Stability analysis of RF accelerating modeu SummaryDec. 13, 2022

KEKB ARC2022, RF system

2Overview of RF system
Re-use with reinforcements to handle twice high beam current and large beam power
Present RF Layout

30+1 RF stations
)-

+)-

-)

-)
((

DR
41 0
1
0 1

In the upgrade from KEKB to SuperKEKB, increasing the number of
RF stations of ARES 1:1 configuration is essential to provide thelarge beam power.
At present, the upgrade has been completed partially.
Dec. 13, 2022

KEKB ARC2022, RF system

ARES 1:2 station(one klystron drives two ARES cavities)
ARES 1:1 station(one klystron drives one ARES cavity)3Operation Status in 2022ab

HER : 14.2MV
LER : 9.12MV

Dec. 13, 2022

p The RF system of MR and DR are operating stably without
critical troubles requiring long shutdown.
p The total-Vc for both rings of MR were kept as 9.12 MV forLER and 14.2 MV for HER through the run.
ARES : 0.40~0.45 MV/cavity, DR : 0.5 MV/cavity
SCC : 1.35 MV/cavity
p Longitudinal coupled bunch instability (CBI) due to RF
accelerating mode is not a problem by CBI dampers.
p Major troubles (took > 1 hour to recover)l Failure of interlock system in Klystron power supply(ARES 1:1 station, D07C, LER)
• Due to deterioration by aging• The corresponding ARES cavity had to be detuned.
• Total-Vc was able to be maintained by increasing
Vc of other cavities.
l Failures of analog control modules• Due to deterioration by agingl Failure of frequency tuner system of SCC• Due to mistakes during assembling• Replaced the tuner
KEKB ARC2022, RF system
4

Beam Abort due to RF system
Main Cause of Aborts by RF System

The number of Beam Aborts in 2022ab Operation(excluding Manual aborts)

LLRF
25%

Cause of Abort

Count

Total

725 (Ib>50mA)

RF system

72 (0.6 /day)

ARES

16

30 cavities

SCC

9

8 cavities

Deterioration
caused by aging

HPRF

29

31 klystrons

LLRF

18

31 systems

Analog Control
Module Failure

ARES
22%
Cavity Breakdown

SCC
Cavity Breakdown

HPRF

Incorrect Operation ofInterlock System

13%

40%

RF system is operating stable.
Main cause of beam aborts by RF system is the deterioration caused by aging.
For more stable and long-term operation, we will continue to conduct regularinspections and update equipment of RF system.
Dec. 13, 2022

KEKB ARC2022, RF system

5Contentsu Overview of RF systemu Operation Status in 2022ab
u Maximum Beam Current Limited by Present
RF systemØ RF Power delivered to Beam
Ø Large HOM Power absorbed by HOM Dampers
in SCC modules
Ø Stability analysis of RF accelerating modeu SummaryDec. 13, 2022

KEKB ARC2022, RF system

6

High Beam Current-related issues in RF system
uLimitations of beam current due to the present hardware of RF systeml RF Power delivered to Beam

• Evaluate the maximum beam current that can be stored with the present RF systeml Large HOM Power absorbed by HOM Dampers• Large cooling capacity of water will be also required.
• Evaluate the allowable beam current with the present hardware setup

uBeam instabilities and issuesl Some systems to cope with instabilities due to high beam current are working well.
l Coupled Bunch Instability (CBI) due to HOM• ARES and SCC are designed as HOM-damped structure with HOM absorbers.
• Additionally, a bunch-by-bunch feedback system is effective.

l Coupled Bunch Instability (CBI) due to accelerating mode
• µ =-1, -2 and -3 modes• New CBI damper system has been established and is working well. (K.Hirosawa et al., NIM A 953(2020)163007)

l Zero-mode related to Robinson stability• Direct RF feedback (DRFB)• Zero-mode damper (ZMD)• The stability analysis in SuperKEKB operation conditions (K. Akai, PRAB 25, 102002 (2022))

l Bunch Gap Transient• Propose the measures to mitigate the phase difference (T.Kobayashi and K.Akai, PRAB 19, 062001 (2016))
Dec. 13, 2022

KEKB ARC2022, RF system

7

Contentsu Overview of RF systemu Operation Status in 2022ab
u Maximum Beam Current Limited by Present
RF systemØ RF Power delivered to Beam
Ø Large HOM Power absorbed by HOM Dampers
in SCC modules
Ø Stability analysis of RF accelerating modeu SummaryDec. 13, 2022

KEKB ARC2022, RF system

8RF Power delivered to Beambased on Akai-sanʼs Report in Aug. 2021
• Addition of RF stations has been completed partially.
• In the design RF stations configuration (ex. report by K. Akai, at the 21th ARC(Jun. 13, 2016)), it was set as a prerequisite that the design current should be
supported even with idling one or two RF stations due to malfunction.
• But considering the stable operation of RF system so far, as well as difficult
budget situation we are confronted, we evaluated maximum stored current thatcan be stored with the present RF configuration without adding klystron stations,by focusing RF power delivered to beam.
LER
ARES (1:1)

ARES (1:2)

ARES (1:1)

ARES (1:2)

SCC# Cavities

0

20

2

10

8

# Klystrons

0

10

2

5

8

# Cavities

10

12

4

4

8

# Klystrons

10

6

4

2

8

SuperKEKB/ultimate# Cavities

14

8

8

0

8

(design)# Klystrons

14

4

8

0

8

KEKB
SuperKEKB/Present

Dec. 13, 2022

HER

KEKB ARC2022, RF system

9RF Power delivered to Beam

Parameters for evaluation
ARES

SCC

15Ω

93Ω

Q0 = 1.1 x 105

QL = 5 x 104

Input coupling β

5.0 (1:1), 2.7 (1:2)--

RF voltage /cav.

0.5 MV

1.35 MV

Tuning offset

-5°

-8°

R/Q
Q0 or QL

Dec. 13, 2022

LER

HER

Radiation loss

1.76 MV

2.43 MV

Parasitic loss

0.5 MV

0.5 MVHigh power efficiency

0.93

0.93

KEKB ARC2022, RF system

10RF Power delivered to Beam

Evaluation method• Calculate the Cavity input power (Pin) and the Klystron
output power (Pkly) as a function of Beam current• Set the maximum power to operate stable the RF system as• Klystron output power (Pkly) < 800 kW• Input power for ARES (Pin) < 750 kW• Input power for SCC (Pin) < 400 kW• Scan the cavity phase to optimize the beam loading• Phase difference between ARES 1:1 and 1:2 stations• Phase difference between ARES and SCC stations• Get the maximum beam current in the present RF system
Dec. 13, 2022

KEKB ARC2022, RF system

11

RF Power delivered to BeamLER (ARES 1:1 and 1:2 stations)
Relative Phase of ARES 1:2 station to ARES 1:1 station
ARES 1:2

ARES 1:1
Design current can be
stored by optimizing thephase difference between
ARES 1:1 and 1:2 stations.
ARES 1:2
ARES 1:1

Dec. 13, 2022

KEKB ARC2022, RF system

12RF Power delivered to Beam

HER (ARES 1:1, 1:2 and SCC)
Relative Phase of ARES 1:2 station and SCC station to ARES 1:1 station
no phasedifference

ARES 1:2

SCC

SCC 10°

ARES 1:1

ARES 1:2 21°
SCC 25°

ARES 1:2 10°
SCC 10°Dec. 13, 2022

The maximum current
is 2.2A by the present
RF configuration.

KEKB ARC2022, RF system

13

RF Power delivered to Beam

HER with additional 2 RF stations
All ARES stations are 1:1 configuration.
Relative Phase of SCC station to ARES 1:1 station

no phase difference
ARES 1:2
SCC

In order to store the design current (2.6 A), adding
two klystrons is necessary to convert four ARES
cavities in the 1:2 configuration into 1:1.
Dec. 13, 2022

KEKB ARC2022, RF system

14

RF Power delivered to Beam

Summary of Evaluation of RF Power delivered to Beam
• LER
• The RF power required to store the design current (3.6A) can besupplied by the present RF configuration.
• Because the number of wiggler magnets will decrease with the
installation of NLC, the one-turn loss will decrease, which willfurther increase the margin of power.

• HER• The RF power required to store the beam current of 2.2 A can besupplied by the present RF configuration.
• If a higher current is aimed in future, it is necessary to add RF
stations in HER (2 klystrons for D04 section).

Dec. 13, 2022

KEKB ARC2022, RF system

15

Contentsu Overview of RF systemu Operation Status in 2022ab
u Maximum Beam Current Limited by Present
RF systemØ RF Power delivered to Beam
Ø Large HOM Power absorbed by HOM Dampers
in SCC modules
Ø Stability analysis of RF accelerating modeu SummaryDec. 13, 2022

KEKB ARC2022, RF system

16

Large HOM Power absorbed by HOM Dampers in SCC modules• The HOM power excited in one SCC is estimated as
twice large as the maximum absorbed power by ferrite
dampers in KEKB operation, 40% of which is emitted
through the beam pipe to the downstream SCC module.
• To reduce the load of ferrite dampers of downstream
SCC module, installing additional SiC dampers areneeded to the downstream of SCC modules.
• Two SiC dampers have been installed to two SCC
modules. It was confirmed that the additional SiC
damper is effective to reduce the load of downstream
cavities in beam operation. For the future high current
operation, SiC dampers will be installed to all SCC
modules.
• The allowable beam current in the present hardware
setup (including update planed in LS1) is evaluatedfrom the HOM power absorbed by ferrite dampers in the
recent operation.

Dec. 13, 2022

KEKB ARC2022, RF system

SCC Modules in SuperKEKB Tunnel

 ferrite ferritee-Additional
SiC Damper

17

HOM Power in SCC modules

HOM Power Limit from the present hardware setup
lCooling capacity of chiller≦ 25 kW/module≦ 35 or 46kW/module with SiC damper• Cooling water for HOM dampers belonging one SCC
module are cooled by one chiller. Now, we have only
25 kW chillers. In LS1, we plan to update chillers ofthe modules with SiC damper to larger capacity one.
• Capacity of cooling water for chillers from
infrastructure is also tight.

lTemperature of copper duct ≦ 60℃• The ferrite is HIPped on copper duct.
• To prevent ferrite cracking

lWater flow speed (flow rate) ≦ 8 L/min

12 kW for SBP
18 kW for LBP• To avoid the erosion-corrosion of cooling pipe
Dec. 13, 2022

KEKB ARC2022, RF system

18

HOM Power in SCC modules

Evaluation of Allowable Beam Current
Limited by Cooling capacity of chiller for each module
D1050chiller 46kW

2A

2.2A

2.4A

2.6A

2A

2.2A

2.4A

2.6A

D10C+SiC
D11B+SiC

chiller 35kW

D10C
D10D

chiller 25kW
D10B

in one SCC module

D10A

in one SCC module

D11

50

Nb= 2346 bunches

D11D
D11C

chiller 25kW

D11B
D11A

2 A is the limit in thepresent cooling capacity.

Dec. 13, 2022

KEKB ARC2022, RF system

19

HOM Power in SCC modules

Evaluation of Allowable Beam Current
Limited by Temperature and Flow Rate for each damper Nb= 2346 bunches
D10

25

2A

2.2A

2.4A

2.6A

D11

25

2A

2.2A

2.4A

2.6A

SiC
D11D-LD10A-LLBP 18kW

D11C-LLBP 18kW

D10D-LD11A-LD11D-S

D10C-L
D10B-L D10C-S

SBP 12kW

SiC
D11B-LSBP 12kW

D11B-S

D10A-S
D10D-S
D10B-S

D11A-S
D11C-SS
Dec. 13, 2022

L

S

L

S

L

S

L

S

L

KEKB ARC2022, RF system

S

L

S

L

S

L
20

HOM Power in SCC modules

Summary of Evaluation of HOM power in SCC• The allowable beam current of HER is evaluated as 2 A
with some update of cooling chillers in LS1.
• In order to aim for more high current, it is necessary to
update all chillers and add SiC dampers.
• Reinforcement of the capacity of cooling water for chillers
from infrastructure (pure-water system of NIKKO area) willbe also necessary.
• Note : In LER, the allowable beam current is estimated to be 2.6 A
based on the maximum absorbed HOM power during KEKB operation
by SiC dampers installed in the ARES section. Further R&D of dampers
are planned to extend the limitation.
Dec. 13, 2022

KEKB ARC2022, RF system

21

Contentsu Overview of RF systemu Operation Status in 2022ab
u Maximum Beam Current Limited by Present
RF systemØ RF Power delivered to Beam
Ø Large HOM Power absorbed by HOM Dampers
in SCC modules
Ø Stability analysis of RF accelerating modeu SummaryDec. 13, 2022

KEKB ARC2022, RF system

22

K. Akai, PRAB 25, 102002 (2022)

Stability analysis of RF accelerating mode
• Purpose of this study• Under the heavy beam loading, in addition to the Robinson limit, thestored current can be limited by the interaction between the cavity
control loops and beam. The issue is a more serious problem in the
HER because of high impedance of SCC.
• The study revealed that this is also an issue in the LER, where only ARES
cavities are used.

• For further luminosity improvement of SuperKEKB in future, thestability of the accelerating mode (zero mode) is studied to check if
it sufficiently secure increasing of the beam current aiming for thedesign values, as well as to identify necessary measures in the RF
system.
• (The modes such as -1, -2, -3, associated with the accelerating mode, has little couplingwith the control loops. They are cured by the dampers for each mode independently.)
Dec. 13, 2022

KEKB ARC2022, RF system

23

Stability analysis of RF accelerating mode

System description• The cavity voltage Vc is controlled by an autolevel control loop (ALC) and phase-lock loop
(PLL). The bandwidth is 0.3 to 3 kHz.
• The direct RF feedback (DRFB) loop feeds pickup signal at the cavity back to the main RF line.
The bandwidth of DRFB is determined mainly by
the bandwidth of cavities (5~10 kHz).
• The zero-mode damper (ZMD) comprises a
pickup monitor, down converter and BPF. The
beam phase oscillation signal is fed to a phaseshifter such that the driving RF is modulated todamp the oscillation.
•

Schematic view of the LLRF system for SuperKEKB,
while highlighting one SCC station (D11-A) in the green
colored part.
Dec. 13, 2022•

Cavity tuning is performed mechanically. The time constant ison the order of 1 Hz, much slower than that of ALC, PLL, andDRFB.
The effects of the klystron control functions are localizedaround the klystron, with little interaction with beam dynamics.

KEKB ARC2022, RF system

24

Stability analysis of RF accelerating mode

Analysis methods
Two independent methods are used for the analysis:
Method 1: The characteristic equation (CE) for the system is
set up with the transfer functions (TF) for each component.
The solutions for the CE provide the dynamical performanceof the system.
Method 2: Using time domain simulation MATLAB/Simulink.
The right figure shows a schematic view of the block diagram
created as the input data for the simulation using
MATLAB/Simulink. It comprises an SCC part, an ARES part,and the ZMD.
We extended the TF in the Pedersen model to include the
DRFB loop, while coexisting with the other control loops suchas the amplitude and phase controls.

Dec. 13, 2022

KEKB ARC2022, RF system

Block diagram of the input data for the simulation
using MATLAB/Simulink. It comprises an SCC
part, an ARES part, and the ZMD.
25

Stability analysis of RF accelerating mode

Validation of the analysis(A) Consistency between the two analysis methods[Red]: Solutions of the CE corresponding to thezero-mode oscillation as a function of beam current.
[Blue]: The oscillation frequency and damping rate
obtained from time-domain simulation.

(B) Comparison of the simulation withmeasurement in a machine study[Blue and Black]: Zero mode coherent frequency
fs obtained by simulation.
[Red]: The measured fs in the machine study (Ref.
(a)) are also plotted.
Ref (a): K. Akai et al., 17th Annual Meeting of Particle Accelerator Society of Japan, 2020.

Results of two analysis methods were consistent.
Dec. 13, 2022

The analysis and measurement results werein good agreement.

KEKB ARC2022, RF system

26

Stability analysis of RF accelerating mode

Application to SuperKEKB
l Having confirmed the validity of these methods, they were applied to extensively study theSuperKEKB RF system.
• Firstly, the effectiveness of different control functions was examined for a simple system in the SCC caseusing CE and simulation.
• Subsequently, the stability of the full RF system in the HER comprising the SCC and ARES with the ZMD wasanalyzed using simulations for various operation conditions.
• The stability of LER RF system was also studied using simulations.

l Based on the results obtained above, possible operating conditions aimed at the design
current were comprehensively investigated.
• In beam operations, maximum achievable stored current may be considerably lower than the thresholdcurrent obtained from the analysis for different reasons such as non-linearities, fluctuations, and scattering ofcavity performances.
• Consequently, as reference guideline, a threshold current target value for the HER (LER) of 4 A (5 A) was set,
which is expected to have a good margin (~1.5 times) for the design current of 2.6 A (3.6 A).
• The results showed effective measures to improve the stability as follows:Ø increasing the operating voltage of ARES (with possibility after ample conditioning of the cavity),
Ø the extension of ZMD to all RF stations,Ø reducing the gain or bandwidth of PLL and ALC within an acceptable range,Ø (option) implementation of the rotation function between PLL and ALC.
They can be implemented without requiring big budget. (Note that the necessity of adding two klystronstations comes from power requirement, which depends on the maximum stored current foreseen.)
Dec. 13, 2022

KEKB ARC2022, RF system27

Summary• RF system is operating stable.
• For more stable and long-term operation, we will continue to
conduct regular inspections and update equipment of RF system.
• Beam current limit by the present RF system is evaluated.
• RF power delivered to beam
: LER 3.6 A (design), HER 2.2 A• HOM power absorbed by dampers : LER 2.6 A, HER 2.0 A• For HER, the adding two RF stations and the reinforcement of thecooling capacity for HOM damper cooling are necessary.

• Stability analysis of RF accelerating mode (zero mode) was
performed in SuperKEKB operation conditions.
• The results showed effective measures without requiring big budget to
improve the stability. And the results could be used as guidelines for future
beam operation by increasing the beam current step by step.
Dec. 13, 2022

KEKB ARC2022, RF system28

back upDec. 13, 2022

KEKB ARC2022, RF system

29

K. Akai, at the 21th ARC (Jun. 13, 2016)

Dec. 13, 2022

KEKB ARC2022, RF system

30

Time required to add RF components• To add two klystron stations• Two sets of klystrons, high-power systems and water-coolingsystems, one set of klystron power supply and control system• Five sets of Digital LLRF systems→ 3+α years (including ~1 year for construction)• To add SiC damper→ 1 year for one set (possibility of shortening in mass production)

Dec. 13, 2022

KEKB ARC2022, RF system

31

HOM Power in SCC modules

Existing HOM dampers from KEKB operation• HOMs can propagate toward beam pipes due to large aperture size.
• A Pair of Ferrite HOM dampers for each SC moduleØSBP damper : f220 x t4 x L120
ØLBP damper : f300 x t4 x L150
ØMax. absorbed power in KEKB : 16 kW/cavity (1.4A, s=6mm, 10nC/bunch)
Ferrite HOM Damper
SBP

LBP

CopperPipe forCooling

Stainless Steel Flangee-Twice High Current and Shorter Bunch Length
Further measures of HOM power is required.
Dec. 13, 2022

KEKB ARC2022, RF system

HIPped Ferrite (thickness: 4mm)
32

HOM Power in SCC modules

Results of Beam Test with SiC Damper
Layout of 8 SCCs and SiC dampers

Absorbed power by a pair of Ferrite dampers in D11 cavities
normalized by the power in D11A

without SiC
with SiC

SiC>10% reduction
by SiC damper

SiC

SiC Damper
Installede-Dec. 13, 2022

Absorbed HOM
power at 1.1A
(max. current): ~ 8 kW/cavity

Two set of SiC dampers have been installed to SCC section for beam
test. The HOM power absorbed by the ferrite dampers ofdownstream cavities (D11C in plot) were reduced >10% after SiC
damper installation. It was confirmed that the additional SiC
damper is effective to reduce the load of downstream cavities. For
the future high current operation, SiC dampers will be installed to all
SCC modules.
KEKB ARC2022, RF system

33

Evaluation of HOM power absorbed by SiC damper in ARES section
S. Enomoto

Current tolerance [A]

D5 cp

D5arc

2.6

12.3kW(HER design)

Dec. 13, 2022

15.5kW(achieved)

2.55

2.64

*20kW
(cooling power upgrade)

3.0

2.9

KEKB ARC2022, RF system

D4 arc2.85

34