1. Working group on rf and feedback
Explore and document the operational experience of present rf systems and bunch feedback systems
What are the existing limitations of high power rf systems?
What are the present limitations of multi-bunch feedback systems?
What advances need to be achieved to increase the beam currents to 5 to 10 amperes per beam with 1 to 2 nsec spacing and an energy of a few GeV?
What will the future low mode feedback systems look like?
Will parallel medium-power amplifiers around high power klystrons help with beam instabilities?
What are the future requirements of bunch-by-bunch high frequency feedbacks?

2. Associates: Dmitry Teytelman and Peter McIntosh Chair: John Corlett
Monday - rf and feedback systems
Tuesday - collective effects
Wednesday
Joint session’s
KEKB-Linac Upgrade Plan Using C-Band System for SuperKEKB (Fukuda)
PEP-II low-level rf fault file analysis (Teytelman)
Strong RF Focusing: a possible approach to get short bunches at the IP (Gallo)
Future very high luminosity options for PEP-II (Seeman)
Analytical Treatment of NonLinear Beam Dynamics in a Storage Ring (Gao)
Associates: Dmitry Teytelman and Peter McIntosh
Chair: John Corlett
+ Ops group
+ BB group

3. RF and feedback presentations
ARES Upgrade for Super-KEKB – Tetsuo Abe
Bunch Shortening using Superconducting Harmonic Cavities – John Byrd
The DAFNE 3rd Harmonic Cavity – Alessandro Gallo
Update on the Performance of Superconducting Cavities in KEKB – Takaaki Furuya
Analysis of rf and feedback limitations in PEP-II – Dimitri Teytelman
The Linac Upgrade Plan Using A C-band System for SUPERKEKB – S. Fukuda
PEP-II low-level rf fault file analysis– Dimitri Teytelman

4. ARES Upgrade for Super-KEKB – Tetsuo Abe
ARES cavity system uses three normal-conducting cavities coupled together
Accelerating cavity, coupling cavity, storage cavity
P/2 mode
System has large stored energy
Reduced detuning
Fundamental does not strongly drive coupled-bunch modes under design conditions

5. Tetsuo Abe

6. Tetsuo Abe
For Super KEK-B operations up to 9.4 A
Increase ratio of stored energy to accelerating mode energy
From Us/Ua = 9 to Us/Ua = 15
Change cavity coupling
New accelerating cavities
Prototype in 2004

7. Tetsuo Abe
Need rf feedback system to control m=-1, -2 modes
Under development
Need broadband longitudinal feedback for fundamental 0 and p modes, and HOM-driven modes
80 kW dissipated per cavity
Need more absorbers and water cooling
Developing multipactor-free coaxial rf power coupler

8. Voltage completely reactive (i.e. 90 phase shift w.r.t beam)
Passive harmonic cavities for bunch shortening – John Byrd
Use reactive longitudinal focusing from beam-induced voltage in a cavity
Use SC cavity so that
Low beam losses
Higher voltages
Use at RF harmonic to get larger dV/dt
Beam induced voltage
V=jI(R/Q)fr/df
Voltage completely reactive (i.e. 90 phase shift w.r.t beam)

9. Existence proofs for cavities at BESSY and ELETTRA
John Byrd
Using 2 SC cells (R/Q=87 Ohms,Q=2e8), proper tuning can shorten the bunches in PEP-II by a factor of 2.5
Existence proofs for cavities at BESSY and ELETTRA
Need to assess effects for large rings
low-mode coupled-bunch modes
differential tune shift is important

10. The DAFNE 3rd harmonic cavity – Alessandro Gallo
Passively powered harmonic cavity to lengthen the bunches.
Touschek lifetime increase, due also to improvements of the dynamic aperture and RF acceptance, is  80%;
Additional positive effects
larger Landau damping
larger natural bunch length
Low R/Q - coherent synchrotron frequencies shift are under control

11. Alessandro Gallo
The “parking option”, as a reliable back-up procedure
Two cavities have been designed, built and tested on bench
Suppression of the HOMs with KEK-B SBP ferrite damper
Measurements are in substantial agreement with MAFIA and HFSS simulations
Wide tuning range
5 revolution harmonics around the RF 3rd harmonic

12. Working point of the main and harmonic RF systems
A. Gallo: The DAFNE 3rd Harmonic Cavity
Alessandro Gallo
If the harmonic voltage is phased to reduce the total RF slope, the bunch natural length increases. The further lengthening produced by the ring wakes can be estimated by a multiparticle tracking simulation
Working point of the main and harmonic RF systems

13. Alessandro Gallo
Gap in the bunch filling pattern produces a large spread of the synchronous phases. Different bunches will collide at slightly different IPs and the synchronization of the bunch-by-bunch feedback systems may be affected
Different bunch distribution along the train
Touschek lifetime gain is not uniform over the train

14. Wide Tunability (-1.5 frev 3.5 frev)
Round Aluminium Cell
R/Q 25 
Wide  Tunability (-1.5 frev 3.5 frev)
KEK-B Ferrite Damper
No direct Ferrite exposure to the Beam
Alessandro Gallo

15. Update on the Performance of Superconducting Cavities in KEKB – Takaai Furuya
* RF of HER is a combination of SC & NC
* Crab cavities is just under R&D phase
SC cavities in NIKKO straight section.

16. SC Cavities in KEKB Takaai Furuya Cavity *Nb single-cell
*Frequency: 508 MHz
*Gap length: m
*R/Q : 93 Ohm
*Total length: 3.7 m

17. Takaai Furuya
SC cavities are slightly conditioned every two weeks for about two hours

18. Takaai Furuya
Unloaded Q

19. Modify coaxial power feed to avoid multipacting
Takaai Furuya
Super KEK-B
Increased beam power
Modify coaxial power feed to avoid multipacting
Modify ferrite dampers to accommodate 50 kW beam induced power

20. Monitoring beam-abort
Takaai Furuya
Monitoring beam-abort
Coupling and reaction between RF cavities and stored beams is quite strong in ampere class accelerators
Difficult to identify the real reason of beam loss or RF-trip events
24ch-data logger
650 ms history with 5 ms resolution
RF signals
Radiation monitor at movable masks
Beam synchronous phase

21. Limitations and upgrade options for PEP-II
PEP-II low-level rf fault file analysis – Dmitry Teytelman
PEP- II designed to operate above instability threshold in all 3 planes: horizontal, vertical and longitudinal
Beam is kept stable via a combination of three techniques
HOM damping in RF cavities – longitudinal and transverse
Bunch- by- bunch feedback systems - longitudinal and transverse
Active impedance reduction for the fundamental mode of the RF cavities - longitudinal

22. RF cavities must be detuned beyond first revolution harmonic
Dmitry Teytelman
PEP- II beam loading
RF cavities must be detuned beyond first revolution harmonic
Worst- case growth time for mode -1 is 33 ms
185 m s synchrotron period!

23. Dmitry Teytelman
Direct proportional feedback loop around the cavity
Reduces the effective impedance seen by the beam
To reduce the growth rates further we add the comb filter
Narrow gain peaks at synchrotron sidebands
Growth rates shown for a linear transfer function model
Reduction by two orders of magnitude, from 30 to 0.35 ms -1

24. Dmitry Teytelman
Feedback systems currently running near maximum usable gain to control fundamental- driven modes

25. Linear model is not applicable
Dmitry Teytelman
Klystron saturation
Linear model is not applicable
HER at 1 A the growth rates rise from linear prediction
0.12 to actual ms-1

26. Techniques to combat increased growth rates
Dmitry Teytelman
Techniques to combat increased growth rates
Reduce klystron saturation by installing tubes with full- power collectors and improved water cooling
Dedicated “woofer” link from feedback system to rf stations
Single- sideband comb filters
Use a separate (linear) feedback amplifier in parallel with the klystron
Energy storage cavities
Superconducting RF

27. Fault analysis RF processing module
Dmitry Teytelman
Fault analysis
RF processing module
Circular buffer records I&Q components
ADC samples at 10 MHz
Recording stops on beam abort signal

28. Dmitry Teytelman
Nice steady- state running with periodic gap transient. After the last turn the beam does not arrive in the cavity after the gap - cavity charges up to high voltage
Synchronization to the gap indicates that the abort kicker fired after the last recorded turn (around ms).
When the beam disappears the cavity becomes mismatched to the generator and we get large reflected power.
However, the primary event that causes the reflected power trip is the abort kicker firing.

29. Forward power of all HER stations is shown
Dmitry Teytelman
Forward power of all HER stations is shown
Around 4 ms power contribution of stations in Region 8 goes down and of those in Region 12 – up
This event is due to a differential phase shift of the RF reference between Regions 8 and 12.

30. Generate summary from daily analysis of all RF- related beam aborts.
Dmitry Teytelman
RF- related events are defined as beam aborts described in the operations log as being caused by ring RF system or longitudinal instabilities.
Generate summary from daily analysis of all RF- related beam aborts.
Analysis is based on fault files saved by RF stations during a beam abort event and is very time- consuming.

31. The Linac Upgrade Plan Using A C-band System for SUPERKEKB – Shigeki Fukuda

32. Requirements for SuperKEKB
Shigeki Fukuda
Requirements for SuperKEKB
(1) KEKB SuperKEKB
Beam Energy (e-) 8.0 GeV > 3.5 GeV(8.0 GeV)
(e+) 3.5 GeV > 8.0 GeV !! (3.5 eV)
NEED Energy upgrade for e+ ! > C-band accelerator units
(2) June design KEKB SuperKEKB
Stored current (e-) 0.95 A --> 1.1 A ---> 9.4 A !!(4.1 A)
(e+) A --> 2.6 A ---> 4.1 A !! (9.4 A)
NEED Intensity upgrade for e-/e+ !
-> flux concentrator, more bunch charge, 2-bunch operation
(3) Mode-switching -> Continuous & Simultaneous e+/e- Injection
-> optimization of layout, 2-bunch operation
Pink:
unswitched
case

33. Scheme-1 (Energy switched)
Shigeki Fukuda
e+ energy is boosted by the
C-band units. 2-bunches from the gun are used for simultaneous injection of e- (LER) and e+ (HER) in a same RF pulse.
Energy gain
S-band: 1.28 GeV/sector
-> C-band: 2.56 GeV/sector
(New constructions)
e+ focusing flux concentrator
Damping e+ 1.0 GeV
Sector-3, 4, 5 : -> C-band units
e– e– 3.5 GeV

34. Scheme-2 (Energy unswitched)
Shigeki Fukuda
e+/e- modes are switched.
(quasi-)simultaneous injection
is difficult
poor e+ intensity
(New constructions)
e+ focusing flux concentrator
Damping e+ 1.0 GeV

35. Scheme 1+2 (Energy (un)switched)
Shigeki Fukuda
Scheme 1+2 (Energy (un)switched)
Energy gain
S-band: 1.28 GeV/sector
-> C-band: 2.56 GeV/sector
Even for the unswitched case,
the C-band units enables to
separate the e+ and e- beam lines, the simultaneous injection or pulse-to-pulse mode switching.
(New constructions)
e+ focusing flux concentrator
Damping e+ 1.0 GeV
Sector-3, 4, 5 : -> C-band units
e– e– 3.5 GeV

36. Summary of C-band linac
Shigeki Fukuda
Summary of C-band linac
High power rf processing of 1m C-band accelerator guide for Super KEKB was successfully performed and power corresponding to 42 MV/m was achieved
Acoustic sensors to investigate possible arcing
Processed accelerator was installed in the beam line of KEKB linac and being re-processed. The beam acceleration of 40MV/m was successfully achieved in October 2003

37. Collective effects presentations
Potential-Well Distortion in RF Barriers - King Ng
Multi-bunch Longitudinal Instability due to the Electron Cloud - Sasha Novokhatski
Recent Observations of Collective Effects at KEKB - Hitoshi Fukuma
Collective Effects for the PEP-II Upgrade - Samuel Heifets

38. Potential-Well Distortion in RF Barriers - King Ng

39. Beam spreads out with lower space-charge force
Barrier rf system
Beam spreads out with lower space-charge force
Can merge two batches together easily
Can compress by moving a barrier slowly
Can move batch from one location to another
If baseline is not zero,rf potential will be head-tail asymmetric
Head-tail asymmetry
King Ng

40. Can correct by adjusting rf system
King Ng
Can correct by adjusting rf system
Properly adjusted rf system introduces opposite asymmetry
Small voltage (~ 10V in 2 kV rf system) restores linearity

41. The voltage compensation is smaller than actually used
King Ng
Remaining problems
The voltage compensation is smaller than actually used
The predicted slant is not linear
Elliptical-like distribution does not fit measured energy spread

42. Multi-bunch Longitudinal Instability due to the Electron Cloud - Sasha Novokhatski

43. Positron bunch “head” is accelerated
Sasha Novokhatski
Electric force lines
from electron cloud
Positron bunch
Positron bunch “head” is accelerated
“tail” is decelerated

44. Sasha Novokhatski
Multipacting AT 38 G

45. Longitudinal electric field
Sasha Novokhatski
Longitudinal electric field
If electron cloud happens in all straight sections, the energy variation in a positron bunch can be of order of 74 kV, which is equivalent to 185 kV of RF voltage of 476 MHz.

46. Instability along bunch train
Sasha Novokhatski
Instability along bunch train

47. Adds energy variation inside the positron bunches
Sasha Novokhatski
Longitudinal electric field produces an oscillating force on the cloud electrons
Adds energy variation inside the positron bunches
head of the positron bunch is accelerated and the tail is decelerated
This action of the longitudinal field is similar to the action of RF fields in a cavity and has the same sign
Bunches will have different lengths throughout the train
Electron cloud space-charge field acts as a resonance force for the longitudinal beam motion
Emittance growth
Longitudinal multi-bunch instability

48. Recent Observations of Collective Effects at KEKB - Hitoshi Fukuma
3 and 4 bucket spacing : the threshold increases when the field strength increases
2 spacing : the threshold saturates
Assuming a present solenoid system, stronger field will be helpful in raising the threshold if bunch spacing is larger than/equal to 3 buckets.

49. Tune shift by e-cloud Hitoshi Fukuma
4 trains, 200 bunches/train, 4 bucket spacing, bunch current 0.58mA
4 trains, 200 bunches/train, 2/3/4 bucket spacing, bunch current 0.5mA, with 100% solenoid

50. Blowup vs. location of e-cloud
Hitoshi Fukuma
Blowup vs. location of e-cloud
4 trains, 200 bunches/train, 3 bucket spacing
West arc off
Tsukuba straight section off
all solenoid on
Fuji straight section off
1. The solenoids in the straight sections are effective on the blowup, even in Fuji straight section where no wiggler magnets are installed.
2. Effect of solenoids on the threshold current of the blowup
1/4 arc > Fuji straight > Tsukuba straight

51. Work this summer Hitoshi Fukuma 1. Addition of solenoids
215 solenoids at straight sections
50 permanent magnets over BPM at Oho and Nikko straight sections
2. Changing the connection of the solenoid power cables to study the effect of polarity-changing-place.

52. Summary of e-cloud effects
Hitoshi Fukuma
Summary of e-cloud effects
1. Increasing the solenoid field will improve the threshold of the blowup if bunch spacing is larger than/equal to 3 bucket spacing.
2. Substantial e-cloud is generated in straight sections according to the measurement of the blowup and the tune shift.
3. Suppression in 2 bucket spacing will be very difficult.
Large tune shift was observed in 2 bucket spacing operation.
Almost no effect was observed by increasing the solenoid field.
4. Clear vertical tilt along a bunch is not observed by the measurement of the streak camera.
Considering increase of solenoid field from 4.5 A to 10 A
Temperature raise of solenoid coil: 100 deg C
Further solenoid winding in Fuji straight section (RF section)
Consider possibility to use clearing electrodes inside magnets

53. Coupled-bunch instability appears intermittently in HER
Hitoshi Fukuma
Coupled-bunch instability appears intermittently in HER
4trains, 200 bunches/train, 4 bucket spacing, 600mA
Horizontal
Vertical
Larger amplitude in tail-part
Almost uniform amplitude
Saturation at large amplitude
No saturation of amplitude

54. Growth rate Hitoshi Fukuma 1 train, 1152 bunches, 4 bucket spacing
Growth rate : (1/turn) or growth time : 5ms @700mA

55. Hitoshi Fukuma Beam current dependence of horizontal mode spectrum
1 train , 1152 bunches, 4 bucket spacing
400mA
500mA
600mA
Consistent with CO+ ions

56. Summary of observations of transverse coupled bunch instability in HER
Hitoshi Fukuma
Summary of observations of transverse coupled bunch instability in HER
1) Horizontal coupled bunch instability in HER sometime causes beam aborts which limit the beam current.
2) Observations are consistent with the results of the simulation which assumes the instability is caused by CO+ ions.
3) A question remains why the instability was not cured by the bunch-by-bunch feedback system.
4) Vertical coupled bunch instability is also observed in HER. Features of the instability are different from those of the horizontal instability, which suggests the vertical instability is caused by a different source than that of the horizontal one.

57. Collective Effects for the PEP-II Upgrade - Samuel Heifets
“Traditional” coherent instabilities
Extend to higher current configurations
Vacuum chamber wakefields
Longitudinal
Transverse
Single-bunch
Coupled-bunch

58. Longitudinal wake for LER PEP-II with 8 rf cavities
Sam Heifets
Longitudinal wake for LER PEP-II with 8 rf cavities
Loss factor increases from 5.4 V/pC at 11 mm to 20 V/pC at 6 mm bunch length

59. Potential well distortion small
Sam Heifets
Potential well distortion small
Microwave threshold estimates vary ~ 3 mA – 15 mA
Depends on analytical vs tracking with wake
Work in progress

60. Longitudinal coupled-bunch modes
Sam Heifets
Longitudinal coupled-bunch modes
Within range of feedback systems

61. Transverse instabilities
Sam Heifets
Transverse instabilities
Resistive-wall driven closed-orbit instability
Threshold 7 A
Mode coupling threshold 13 mA

62. Transverse dipole-mode coupled bunch instabilities
Sam Heifets
Transverse dipole-mode coupled bunch instabilities
Qy =
3 A
Within range of feedback systems

63. Transverse quadrupole-mode coupled bunch instabilities
Sam Heifets
Transverse quadrupole-mode coupled bunch instabilities
Qy = 1A
Threshold 1.4 A
Will need high-frequency feedback system
Kick gradient along bunch

64. Coupled-bunch mode coupling instabilities
Sam Heifets
Coupled-bunch mode coupling instabilities
Enhanced by longitudinal dipole motion
1658 bunches, 3 A

65. Working group on rf and feedback
What are the existing limitations of high power rf systems?
scrf and warm both reliable and mature technologies
Harmonic cavities need further development
What are the present limitations of multi-bunch feedback systems?
Damage from beam-induced power
What advances need to be achieved to increase the beam currents to 5 to 10 amperes per beam with 1 to 2 nsec spacing and an energy of a few GeV?
KEK-B increasing stored energy in cavity system + power coupler and HOM absorber improvements
PEP-II installing more linear klystrons + developing woofer link to rf systems
Detailed fault analysis capabilities – llrf diagnostics

66. Working group on rf and feedback
What will the future low mode feedback systems look like?
Will parallel medium-power amplifiers around high power klystrons help with beam instabilities?
To be determined, requires study of power needs and power combiner
What are the future requirements of bunch-by-bunch high frequency feedbacks?
Digital processing diagnostics
“High” frequency to provide gain on quadrupole modes
Develop transverse kickers with reasonable impedance at ~ GHz