# Presentation: 482584

## ERL RF Systems 

*A. Nassiri*
*November 15, 2006*

- Presented to the Machine Advisory Committee for the Technical Review of APS Accelerator Upgrade Options – November 15-16, 2006

## Outline

- Goals
- Cavity design criterion
- Cavity parameters
- Frequency
- Cell Shape
- Number of cells
- Q vs. Gradient
- Cavity and wall-plug Power
- Fundamental RF Coupler
- HOM Coupler
- Cooling Requirement
- RF power sources and rf distribution
-  LLRF system
- A summary of cavity design parameters
- Cavity R&D
- Conclusion

## Goals

- To accelerate beam with up to 100 mA and 2-4 ps bunch length to 7 GeV
- CW operation
- Achieve and maintain rf amplitude and phase required for stability
- Preserve nm-type beam emittance in the linac
-  Acceptable machine reliability and beam availability

## Criteria for Cavity Design

- Cavity parameters

| Parameter | Unit | Value |
| --- | --- | --- |
| Frequency | MHz | 1300/1408/704 |
| Structure type |  | Standing Wave |
| Accelerating mode |  | TM010  mode |
| Gradient | MV/m | 20 |
| Quality factor Q0 |  | 11010 |
| Active length | Meter | 1.04 |
| Number of cells |  | 9/7 |
| R/Q |  | 900 |
| Qext for input coupler |  | 1107 |
| Cavity bandwidth at Qext | Hz | 400 |
| Fill time | s | 500 |

## Cavity Frequency

- Frequency scaling
- The losses in a microwave cavity are proportional to
- For a given length of a multi-cell structure
- It becomes independent of
- At                  , the BCS term dominates above 3 GHz and the losses grow linearly with frequency
-  Below 300 MHz, the residual resistance dominates and the losses are proportional to

## Cavity Frequency

- Choice of frequency
- 1300 MHz ( Single-pass)
*Pros*
*Design exists ( TESLA Cavity, DESY)**1*
*Has been benchmarked*
*Significant working experiences*
*Cons*
*Will need some modifications to be suitable for High power CW operation*
*BBU threshold and HOM effects*
*Smaller cavity aperture. Beam loss and scraping issues*
*Wakefields issues *
- 1408 MHz (Single-pass)
*Pros*
*4**th** harmonic of the APS storage ring frequency *
*Makes it possible to “synchronize” ERL and SR ( hybrid mode operation)*
*Cons*
*Cavity has to be designed*
*Requires R&D for development, optimization, and  rf characterizations for CW operation*
*BBU threshold and HOM effects *
*Smaller cavity aperture. Beam loss and scraping issues*
*Wakefields issues                                                                            *1  J. Sekutowicz, DESY

## Cavity Frequency

- Choice of frequency (Two-pass)
- 704 MHz
*Pros*
*2**nd** harmonic of the APS storage ring frequency*
*Makes it possible to “synchronize” ERL and SR ( hybrid mode operation)*
*Higher BBU threshold limit *
*Injector design is simplified. Input power couplers will have higher power handling capability*
*High peak current effects are reduced*
*In principle can accelerate higher than 100 mA*
*Cons*
*Requires higher bunch charge ( ~150 pc for 100 mA average current), compared to ~77 pc at 1300 MHz*
*This potentially affects emittance and source brightness*
*Cavity fabrication and processing due to larger surface area*
*Lower field gradient*

## Number of Cells

- Effective beam loading in ERL cavities is minimal, close to zero
- The required RF power per cell is small
- Large number of cells could be excited via one input coupler (TESLA SS )
- Multi-cell cavities with a larger number of cells would also improve linac packing factor, i.e., ratio of active length to total length
- This will reduce the cost of the ERL linac, BUT
- Strong HOM damping with higher beam current  favors smaller number of cells
- Extensive SCRF cavity cells optimizations have been done at TESLA and JLab
- TESLA – 9-cell cavity
- JLab- 7-cell cavity ( CEBAF 12 GeV Energy Upgrade,  Renascence)
- BNL 703 MHz for eRHIC

- TESLA 9-cell cavity, 1300 MHz3

- CEBAF 7-cell cavity, 1497 MHz1

- BNL 5-cell cavity, 703 MHz2

- 1 Courtesy R. Rimmer, JLab
- 2 Courtesy R. Calaga, BNL
- 3 Courtesy J. Sekutowicz, DESY

## Q vs. Gradient

- CEBAF 7-cell cavity
- We expect cavity gradient for CW operation to improve in the next five years

- 1 Courtesy C. Reece, JLab

- 1

## Cavity and Wall-Plug Power

- Provide reasonable cavity coupling factor
- Provide a cavity bandwidth sufficiently large to allow cavity frequency tuning
- We use TESLA structure as a baseline for the following
- The strongly over-coupled cavities reflect most of the incident drive power
- This results in a large requirement for rf power to produce the specified accelerating voltage

## Cavity and Wall-Plug  Power

- The power dissipated in the cavity wall, P*cav** , is given by** *
*Total losses in  350 m of cavities are ~****16 kW***
- Since the beam loading of the accelerated and decelerated beam cancel, ideally there is no effective beam loading for the accelerating mode
- The required RF power to maintain a given accelerating voltage under cavity detuning due to
-        microphonics is given by
- Allow 20% overhead for control margin
-      and waveguide loss and phase shift
-          ~16 kW klystron is needed.

## Cavity and Wall-Plug  Power

*The required rf power will increase to ~17 kW if one operate the TESLA cavity at the design gradient of 23 MV/m*
*APS-ERL requires ~350 cavities ( ~350m effective accelerating length)*
*Assuming an IOT efficiency of 65% *
*The wall-plug power for the 7 GeV ERL is ****7.0 MW***
*Injector beam power ( 100 mA @10 MeV) is 1MW. ****1MW ****of CW RF power is required*
*Assuming a klystron efficiency of 50%*
*Injector wall-plug power is ****2 MW***
- Losses due to synchrotron radiation ( ~15 MeV) **~1.5 MW**
*Total wall-plug power is ****11.5 MW***
***Multi-pass recirculation reduces wall-plug power***

## Fundamental Power Couplers

- Waveguide
- Pros
*Simpler in design*
*Easier to cool*
*Better power handling*
*Large size*
*Bigger heat leak*
*More difficult to make it variable*
- Coaxial
- Pros
*More compact*
*Easier to make it variable*
*Smaller heat leak*
- Cons
*More complicated design*
*Require active cooling*
*Not so good power handling*

- Cornell 50 kW CW Coupler Design2

- CEBAF 12GeV Upgrade Renascence cryomodule1

- 1 Courtesy R. Rimmer, JLab
- 2 Courtesy  M. Liepe, Cornell

## HOM Losses

- Large HOM power contributes to large loss factor
- High Q HOMs contributes to MBI
- MBI give rise to beam breakup
- High Q dipole modes
- Feedback loop between beam and cavities
- It is worse for high current, high bunch charge
- HOM losses are
- This is not an acceptable loss at liquid helium temperature
- It has to be properly extracted ( with a carefully chosen Q*ext** *) to outside so only a small fraction of the power is dissipated in the cavity walls

- (per cavity for two beams)

## HOM Damping

- HOM power must be dumped out of the liquid helium
- HOM couplers must be able to handle large average power
- Higher order modes must be well coupled to the beam pipe
- TESLA HOM coupler is not suitable for CW operation

- Ferrite absorbers
- Broadband (@300 K)

- Loop couplers
- Resonant circuit (@2 K)

- (Cornell type)

- (TESLA type)

## Cooling Requirements for 7 GeV

- APS-ERL will require a cryogenic plant equivalent to 3.5 x CEBAF
- Electrical power utilities requirement: 16.0 MW  ( operating at 2.08 oK)
**Multi-pass recirculation reduces power requirement**

| Cavity frequency (MHz) | 1300 | 1300 | 704 | 704 |
| --- | --- | --- | --- | --- |
| Gradient | 15 MV/m | 20 MV/m | 15 MV/m | 20 MV/m |
| Vcav (MV) | 15.6 | 20.8 | 15.8 | 21 |
| Number of cavities | 448 | 337 | 443 | 333 |
| Heat/cavity@Q0 1010 (W) | 28 | 47 | 41 | 74 |
| Dynamic heat (kW) | 13.0 | 16.0 | 18.0 | 24 |

## RF Power Sources

- We plan to adapt one-source-per-cavity concept
- Because high loaded QL of the cavities prohibits vector sum control of many cavities
- In addition, microphonics would cause unacceptable fluctuations of the individual fields in case of vector sum control
- RF power source requirements:
- Reasonably high efficiency
- Reliability and long life time
- Availability
- Reasonable price
- Technical support and good customer service
- Types of RF sources:
- Klystrons
*High gain – requires low drive power*
*High efficiency when operated close to saturation*
*A good choice for ERL injector linac ( constant beam loading)*
*CW sources below 1 GHz are available from industry*
*CW sources above 1 GHz under development*

## RF Power Sources

- Types of RF sources:
- Inductive Output Tubes (IOTs)
*High efficiency *
*High linearity and smaller pushing factors*
*No saturation. Can operate up to their maximum output power*
*Less expensive than klystrons*
*BUT*
*It has lower gain than klystron and needs higher power drive amplifier*
*Grid geometry does not allow operation at high frequencies like klystrons*

- CPI 1.3 GHz IOT Prototype

- 1 Courtesy S. Lenci, CPI MPP, Palo, Alto, CA

- 1

## RF Power Sources

| Device | Klystron | IOT | IOT | IOT |
| --- | --- | --- | --- | --- |
| Manufacturer | e2v | Thales | CPI | e2v |
| Frequency [MHz] | 1300 | 1300 | 1300 | 1300 |
| Beam Voltage [kV] | 45 | 29.0 | 34 | 25 |
| Beam Current [A] | 5.5 | 0.90 | 1.6 | 1.14 |
| Output Power  (CW) [kW] | 120 | 16 | 30 | 14 |
| Gain [dB] | 45 | 20 | 22.5 | 21 |
| Efficiency [%] | 51 | 60 | 55 | 63 |

## RF Distribution

***RF Distribution***

- 7- or 9-cell structure
- One cryomodule will consist of 4 or 8 structures
- 20 MV/m accelerating field gradient  with 60% filling factor
- 350 cavities ( 3150 or 2450 cells)
- Each structure is powered by its own rf source
- 350 power sources with feedback control

- HOM Damper (4)

- HOM Damper (4)

- RF Amplifier

- Water load

- Circulator

- One cryomodule

- ~600 m

## LLRF Control Requirements

- Maintain constant phase and amplitude of the cavity fields within given tolerances
- RF phase:           0.050 RMS
- RF amplitude:     1x10-4 RMS
- Minimize power needed for control by actively controlling cavity tuners to ensure operation on resonance
- Build-in diagnostics for calibration of gradient and phase, cavity detuning
- Fast interlock system for faults during a cavity trip
- Feedback loops and control to deal with:
- Beam current fluctuations
- Microphonics
- Lorentz force detuning
- Possible types of control systems:
- Self-excited loop
- Generator driven system and monitor separate amplitude and phase
- Use I/Q detector and controller
- FPGA/DSP
- Use JLab, SNS, CORNELL LLRF systems as  baseline design

## Cavity R&D

- Active R&D is needed to address critical SCRF cavity design for CW operation
- Investigate the need for the development of a new cavity that meets APS ERL requirements
*Higher fill factor*
*Strong HOM damping*
*Low microphonics*
*TESLA, JLab, Cornell, and BNL experiences are essential*
- Optimize the shape of the Cornell 7-Cell cavity to further increase HOM damping and to lower cryogenic losses
*Collaborate with Cornell SCRF group *
- Design and build a prototype multi-cell  copper cavity
*Measure fundamental rf parameters *
*Q’s of fundamental and HOM modes*
*Bead-pull measurements to check field flatness*
*Identify the HOM modes from bead pull field profile*
- Analysis and simulation of HOM and damping
- Design of a high power input coupler (FPC)
*Use JLab WG coupler and TESLA Coaxial coupler as baseline*
- Design and build a multi-cell Nb first prototype cavity
- Design and build a prototype cryomodule
- Perform vertical dewar test

## Conclusion

- SCRF technology for ERLs and CW machines is advancing at a fast pace
- We expect cavities development to make possible to operate at energy gains in excess of 20 MV/m
- A wide range of expertise and experience already exists
- Our challenge is to:
- How to deal with a 16 kW cryogenic plant ( big footprint, capital+operation)
*Note: CEBAF CHL system is 4.6 kW @ 2.1 K*
- Design a CW-specific cavity to meet ERL design parameters
*Tesla 9-cell and JLab 7-cell structures are good candidates  *
-  Develop a robust HOM damping system
- Better understand and reduce field emission for higher gradient in CW mode
- Improve cavity quality factor  ( 11011 !)
*For CW operation highest fields are not important. Highest possible Q values at about 20 MV/m are very critical. This is in contrast with pulsed ILC requirement.*
- Develop a robust LLRF control system for CW operation
- We intend to actively seek collaboration with other laboratories and institutions on the development of SCRF for ERL ( JLab, Cornell, BNL, Daresbury,....)