# Presentation: 349368

## Challenges of the ILC Main Linac

- Marc Ross
- Fermilab

- Theme: Power and Precision

- 25/07/07

**Notes:**
Power and precision
HOM phase, HOM position
Coupler kicks
LLRF performance
Multi-cavity performance
AES-4 cavity
VTS RD
Cavity alignment
Cavity / quad vibration
DFS
HOM kicks; HOM purpose

## Engineering Design for the ILC

- We are at a critical juncture of the ILC.
**Two years after the formal formation****of the ILC Global Design Effort (GDE), **
**the recent completion of the draft Reference Design Report (RDR) marks a major****milestone in this truly global effort. **
- Our GDE is now in the process of restructuring itself and making plans for the engineering design phase, leading to the completion of the ILC EngineeringDesign Report (EDR) in 2010.

- 25/07/07

## Challenges:

- Our Engineering Design strategy and priorities come from the identification, (in the RDR), of scientific and engineering challenges of the ILC.
- cost of the main linac:
**+ associated earthworks and cooling/power systems, **
**= 60% of the ILC total cost.  **
- achieve the highest practical gradient
**this R & D has the largest cost leverage of any of the ongoing programs.  **
- beam dynamics and beam tuning processes in the main linac,
**we will not have the opportunity to do full (or even large) scale tests of these before construction **

- COST

- ENERGY

- PRECISION

- 25/07/07

## ILC  Superconducting RF

- On 20 August 2004, an international technical panel recommended that the linear collider be based on superconducting RF technology:
- “The superconducting technology has features, some of which follow from the low rf frequency, that the Panel considered attractive and that will facilitate the future design:
**• ****The large cavity aperture and long bunch interval simplify operations, reduce the sensitivity to ground motion, permit inter-bunch feedback, and may enable increased beam current. **
**• ****The main linac and rf systems, the single largest technical cost elements, are of comparatively lower risk. **
**• ****The construction of the superconducting XFEL free electron laser will provide prototypes and test many aspects of the linac. **
**• ****The industrialization of most major components of the linac is underway. **
**• ****The use of superconducting cavities significantly reduces power consumption. **
- “

- Precision
- Risk
- Testing
- Industrial
- Power

- 25/07/07

## Superconducting RF

- Luminosity requires beam power & small beams;
**Superconducting RF is the most effective way to create high power beams**
- Proven design:
**1.3 GHz niobium sheet metal cavities**
**ILC - each cavity delivers 285 KW to 9mA beam (nom)**
**ILC - fill time 38% total pulse**
**ILC - linac efficiency (RF to beam): 50%**
- Fill time, distribution and feedback overhead
- Large irises  minimal emittance growth with achievable tolerances
**a manageable system**
**If we can achieve tighter assembly/tuning tolerances, can improve efficiency**

- POWER

- PRECISION

- 25/07/07

## SCRF linac – basic building block

- ~ 70 parts electron-beam welded at high vacuum
**mostly stamped 3mm thick sheet metal**
- pure niobium and niobium/titanium alloy
**niobium cost similar to silver; purification increases cost**
- weight ~ 70 lbs; length ~ 1 m
- 6 flanges

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## Slide 7

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## Cryomodule assembly: 1200+ parts

**FNAL CM Assembly: T. Arkan**

- 25/07/07

## ML basic building block 

**ILC RF Unit:**** 3 CM, klystron, modulator, LLRF**

- Baseline design now has 2 CM with 9 cavities, 1 CM with 8 cavities + quad

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## Slide 10

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## Slide 11

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## Scale of ILC:

**ILC - Global Design Effort**

- 16,088 SC Cavities: 9 cell, 1.3 GHz
- 1848 CryoModules:  2/3 containing 9 cavities,
- 	1/3 with 8 cavities + Quad/Correctors/BPM
- 613 RF Units:  10 MW klystron, modulator, RF distribution
- 72.5 km tunnels ~ 100-150 meters underground
- 13 major shafts _>_ 9 meter diameter
- 443 K cu. m. underground excavation: caverns, alcoves, halls
- 10 Cryogenic plants, 20 KW @  4.5o K each
- 	plus smaller cryo plants for e-/e+ (1 each), DR (2), BDS (1)
- 92 surface “buildings” (for Americas’ site), 52.7 K sq. meters
- 230 M Watts connected power, 345 MW installed capacity

- 25/07/07

**Notes:**
CM weighs 5.6 T
LHC Dipole weighs 30T
Nb weight ~ 200kG
~80K Nb cost?

## Rdr power parameters / water

- power / water handling scheme is an indicator of design maturity
- Beam power at IP  10.8 + 10.8 MW
**15 % efficient**
**10% cooling overhead (100W to remove heat from 1 KW load)**
- Good performance figures – but more to do
**TESLA design (2001): ~ 80 MW lower for same luminosity**

- 25/07/07

- 100% efficient with correct match
- Losses during fill & decay

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## Cavity limitations differ:

- (different from LHC and TeV where the weakest magnet can limit entire machine performance)
- But cavities are fed from a single source
**Tailoring input coupling and power can offset this but:**
**this requires power and **
**may prove difficult if we insist on flexible operation**
- Take a model RF unit made from cavities like those recently produced at DESY...

- 25/07/07

## Cavity Operation – Beam ON

- σ = 5,   g0 = 32 MeV/m

- Beam ON

- Cavity Gradient vs Time

- There are 2 controllable elements:
**the klystron power (common to 24); tap fraction for each cavity**
**The rate at which power feeds into each cavity (coupler – Q_ext)**
- There are 2 fundamental goals:
**Flat gradient as a function of time during the pulse for each  ****↓**
**Maximum ‘practical’ field in each cavity**
- Final: minimize wasted power; provide variability as needed for flexible ops

- Solyak & Lunin
- Fermilab

- 25/07/07

## Solyak & Lunin

- Cavity Gradient Distribution Approximation

- Modules ACC(5,6,7)*  Beam:ON

- σ = 3.8

- σ = 2.9

- σ = 3.4

- Asymmetric Gaussian Distribution:

- σ = 5

-  Structures: 25, σ = 5

- Number of Structures

- * R. Lange (*DESY**), TTC Meeting*, FNAL, April 2007

- Grad., [MeV/m]

- Fermilab

- 25/07/07

## Solyak & Lunin

- Cavity Operation, Beam ON

- Grad., [MeV/m]

- Grad., [MeV/m]

- Cavity Gradient vs Time

- Flattop Operation

- Quench

- If we will tune *Q**i** *of each cavity to actual gradient <*G**i**>*  then it will cause either quench or nonflatness . The reason is that each cavity has an individual filling time while a beam is coming to all cavities simultaneously.

- It is possible to restore flat top operation by tuning each cavity both  Q-factor and Input Power (*Q**i*  and *P**i *)*:

- * Index “0” correspons to matched gradient *<G**0**>*
-   (no power reflection).

- Fermilab

- 25/07/07

## Solyak & Lunin

- Cavity Gradient Distribution, Beam ON

- Input RF Power (PK) vs Structure Gradient

- Matched Gradients, <G0>

- σ = 5

- <G0> = 32 MeV/m

- <G0> = 28 MeV/m

- Grad., [MeV/m]

- Beam Power

- Number of Structures

- Fermilab

- 25/07/07

## Solyak & Lunin

- Total Power Loss vs. Tuning Structure Gradient <G>

- Cavity Operation, Beam ON

- Number of Structures

- <G>, [MeV/m]

- η, [%]

- Real Gradient Distribution

- Expected Average  Asymmetric Gaussian Gradient Distribution

- (Modules ACC(5,6,7)  Beam:ON)

- η = 4 %, <G0> ~ 28 MeV/m

- Fermilab

- 25/07/07

## Multiple Cavity Operation

- There is an optimum in a total power efficiency vs. matched gradient
-  Expected additional average power is about 4 % at optimum gradient
- We can further lower the power loss and simplify RF distribution system by sorting cavities in pairs with nearly equal gradients
- We must also consider cavity over-voltage

- 25/07/07

## Controls:

- achieving the perfect RF match to a each cavity
- Static
**Accounting for cavity variations**
**Feedforward compensation of cavity detuning due to Lorentz force **
- Dynamic - Stabilization of
**Microphonics**
**Beam intensity fluctuations**
**Thermal**
**Transients**
- Challenge of operating near the gradient limit

- 25/07/07

## Controls Example: Fast Tuner

- Apply small axial squeeze to compensate for Lorentz force distortion
- Initial demonstration for each cavity
**Measure detuning**
**Compensate detuning individually, one after the other**
**In addition**
- Work on piezo diagnostics: Impedance measurement
- Measure transfer functions from one piezo to another
- Is there any crosstalk between the cavities?
- Demonstrate compensation on full module for all cavities simultaneously
**With RF feedback **

- 25/07/07

## Tuner Setup

- Current design in use at FLASH
**Design by CEA – Saclay**
- Lever-based mechanism

- Lutz Lilje

- 25/07/07

## Operation of Full module – Vector-Sum

- 100’s Hz

- Lutz Lilje

- 25/07/07

## Compensated Detuning per Cavity

- Lutz Lilje

- 25/07/07

## SCRF Linac Beam Dynamics

- Chromatic effects:
**Cavity misalignment**
**Dispersion**
**Coupling (x y)**
- Collective –current based- effects:
**Single bunch**
- Wakefields – interaction with the structure/surrounding hardware
**Multi bunch**
- Resonant excitation of higher order modes
- Coupler Kicks and Dark Current

- 25/07/07

## Resonant excitation of higher order modes

- We power the cavity with a strong single frequency –
**Each beam bunch is a ‘delta function’ that has a broad frequency spectrum that couples to the cavities natural resonant modes.**
- Modes with phase velocity = *c* have the strongest coupling
**Each cavity will have slightly different spectra because of fabrication differences**
- Some modes have a long life time
**Trapped modes may exist **
**Near cut-off, modes may have large characteristic dimensions**
- The bunch train spectrum has a sequence of lines which may couple strongly to cavity / cryomodule modes

- 25/07/07

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## Frequency (GHz)

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## Mode Spectrum from the passage of a single bunch – Compared with tabulated Network Analyzer bench data

- 25/07/07

## Relative Cavity centers in a DESY Cryomodule using beam-generated dipole mode strength 

- Relative Cavity centers in a DESY Cryomodule using beam-generated dipole mode strength

- 25/07/07

## Beam Size and Divergence

| E (GeV) | σ_x(μm) | σ_y(μm) |
| --- | --- | --- |
| 5 | 300 | 15 |
| 15 | 150 | 8 |
| 250 | 30 | 2 |

- Simple minded:
- Typical p┴rms_y ~ 5KeV
- Each cavity ~ 30 MeV
- Cavity angular alignment tolerance ~ 300 μ rad
- Cavity position tolerance ~ 300 μ m
- Mechanical distortions / microwave transverse fields

- Small Beams
- Microns ; microradians
- GeV ; KeV

- 25/07/07

## Dispersion in a linac

- Misaligned quadrupoles and BPMs generate orbit distortions;
**Results in beam dispersion which significantly increases projected emittance**
**Dispersion is a linear correlation: *****y**********E***
- Kicks are *n*σ_y ; δ ~1e-3
**Lattice is weak so ‘filamentation’: **
- (difference in β phase advance within bunch)
**is small (ILC ML ~ 30*2****π****)**
**Thus the correlation can be ‘subtracted out’ using a trajectory bump**
- Beam – Based Alignment 
**find the dispersion-free trajectory**
**Algorithms, Simulations, Systematic Errors**

- 25/07/07

## 9-8-9 Lattice -functions

- Lattice Repository

- Lattice 989-28dec06

- Lattice 989 - No Undulator

- Acc. Division of Fermilab supports centralized lattice repository
**Controlled write access;   Revision history**
- ILC ML lattices (read only) have been placed into the repository  _[https://lattices.fnal.gov/](https://lattices.fnal.gov/)_

- N. Solyak,
- Fermilab

- 25/07/07

## Effect of Bumps for Static Tuning

- 1st dispersion bump

- 2 dispersion and wake bump

- Projected y-emittance (nm)

- After DFS

- BPM #

- 1st dispersion bump : Corrector # 3
- 2nd dispersion bump : Corrector # 36
- 1st Wake bump : Corrector # 63

- Mean of 30 seeds

- N. Solyak,
- Fermilab

- 25/07/07

## Beam –Based Alignment & Beam Stability and Steering

- (300x more precise...- than a-priori mechanical placement)
- Start with these:
**Dispersion-free steering**
**Quadrupole shunting**
**Ballistic alignment**
**Kick minimization**
- Then try to keep it as things ‘drift away’
**Kind of feedback compensation for ground motion**

- 25/07/07

## Adaptive Alignment (AA)– Basic Principle

** ****“****local” method: *****BPM readings (A******i******)***** of o****nly 3 ****(or more) ****neighboring quads are ****used**** to determine ****the shifting of the central quad (***********y******i*****).**

*conv *:  Speed of convergence of algorithm
*A**i*     :  BPM reading of the central quad and so on
*K**i*     :  Inverse of quad  focusing length
*L      *:  Distance between successive quads
- 	(assuming same distance b/w quads)
*********E   ***:   Energy gain between successive quads
***E     ***:   Beam Energy at central quad

- The procedure is iteratively repeated

- New position of quad & BPM:

**Proposed by Vladimir Balakin in 1991 for VLEPP project**

- N. Solyak,
- Fermilab

- 25/07/07

## Effect of Ground Motion

- 30 different GM seeds (Model C)

**Effect of Ground Motion**

- time (x 1/2 hour)

- Y-emittance (nm) @ Linac exit vs. time (1/2hrs.)

- In half an hour of GM, emittance dilution increases by as much as ~ 5 nm b/w the subsequent AA iterations, which implies that AA will have to be done at this order or better!

**Y-normalized emittance (nm)**

- Mean of 30 seeds

- N. Solyak,
- Fermilab

- 25/07/07

## AA in perfect and DFS lattice

- 		Normalized vertical projected emittance vs. time in
- 	(a) Perfectly aligned Linac        (b) Dispersion-free steered linac.

- AA keeps the emittance growth even for model C under control
- If orbit after DFS is used as a reference, then AA is not sensitive to BMP-to-Quad offsets

- Average of 10 Ground Motion seeds

- AA is implemented after  every hour of GM model ‘C’ (noisy)

- (b)

- 40 nm is nominal at IP;
- DR output 20 nm

- N. Solyak,
- Fermilab

- 25/07/07

## Effect of GM models

**Effect of GM models**

- 30 days

- Average of
- 10 GM seeds for each GM model
- Convergence= 0.2

- Y-emittance (nm) @ Linac exit after 100 AA iterations for different GM models ‘A’, ‘B’ and ‘C’. Total period - one month, time step - 2 hours

- N. Solyak,
- Fermilab

- 25/07/07

## Effect of BPM resolution

- 30 days

**The effect of BPM resolution for AA correction can be significantly reduced by ****averaging information from all bunches**** in one train or even by using  information from a ****number of previous pulses****. This was confirmed in simulations done for short lattice.**

- N. Solyak,
- Fermilab

- 25/07/07

## Can we build it better?

- Can it be better tuned?
- Can we afford the emittance degradation?
- Reducing iris size increases wakefield
**But increases accelerating gradient by **
**76 ******** 60mm (20% reduction in diameter) decreases surface magnetic field to allow ~42 MeV/m accelerating gradient**
**In the scaled elliptical TESLA shape**
**(a gain similar to ****ICHIRO**** – KEK)**
**(christened ‘****Yao Ming****’ by SLAC’s Zenghai Li)**

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## Slide 44

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## Slide 45

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## Slide 46

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## A. Bosotti INFN

- Cooldown and Warmup data for different cycles:Horizontal Displacements (only stable T points considered)

- Warm

- Cold

- Vacuum

- 25/07/07

## How you can participate  Interesting, Important things to do...

- Fortunately – the most critical and interesting R&D is close to home ...
**In the Industrial Center and Meson area**
- achieve the highest practical gradient
**this R & D has the largest cost leverage of any of the ongoing programs.**
- This topic is a primary focus of Fermilab’s development effort
**So far basically limited to infrastructure development**
- But – that infrastructure is now ready for use...

- 25/07/07

## First 1.3 GHz Cavity tested at new Vertical Test Facility in Fermilab’s Industrial Center -single cell

- 25/07/07

## First 1.3 VTS test – Radiation diagnostic:

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## What limits performance in a 9 cell cavity?

- Development of diagnostics and understanding related physics is a high priority
- Projects:
- (After a cavity is fabricated and processed; during test )
**We have 3 basic signals to work with:**
- Microwave
- Thermal
- Radiation
**We have 3 completely different sets of constraints:**
- Vertical Test
- Horizontal Test
- Cryomodule
- We need to: Quantitatively answer the above question, using the above.

- 25/07/07

## Example SCRF R& D Projects:

- Thermometry
**Bandwidth, spatial resolution and sensitivity**
- Radiation
**Localization, energy flow and bandwidth**
- Microwave
**The independent variable in the apparatus**
**Completes the energy equation**
- None of these are easy; few are under active development
**Fermilab’s new infrastructure offers excellent opportunities**

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## Slide 53

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## List of primary limiting physical effects:

- (see talk by Hasan)
- Multi-pactor
**Resonant multiplication (geometry and field, also contaminants)**
- Field emission
**electron sources often caused by surface debris**
- Thermal ‘run-away’ – quench- due to:
**Poor cooling**
**Imperfections**
- Inclusions , surface deformation
**Fundamental SCRF limits**
- Low Q due to poor surface resistance

- 25/07/07

## Diagnostics – (Peter Kneisel)

- The application of diagnostic methods allows to gain understanding of localized phenomena on a cavity surface
-  Each energy loss mechanism in a sc cavity will lead to a flux of heat into the helium bath surrounding the cavity
-  This heat flux raises the temperature of the intermediate helium layer between outer cavity surface and the bulk helium bath
-  Qo vs Eacc gives a global picture of the behaviour of a superconducting cavity
- With an array of thermometers sliding around the cavity surface a “temperature map” can be compiled
-  Conclusions about the loss mechanisms inside the cavity can be drawn.

- 25/07/07

## Temperature Mapping, cont’d

-  First rotating T-mapping system implemented at CERN
- increase in heat transfer resistance from metal to He bath
-  absence of nucleate boiling therefore no micro-convection due to bubbles
-  surface temperature increases compared to saturated He
-  T-sensors are thermallydecoupled

- Peter Kneisel - JLAB

- 25/07/07

## T-Mapping (1)

- T-mapping system: ~600 Allen-Bradley C-resistors

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## Thermal – mapping:

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## Thermal mapping: low Q

**4**

**4**

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## Superfluid He

- Non-contact thermal diagnostics:
- (Cannot include motorized or multi-channel contact thermometry after cavity is put into tank)
- Can thermal mapping be simplified using properties of superfluid?
**Could this be done after ‘dressing’?**
- Imaging heat ‘transients’ using distributed thermometry
**‘****second sound’**
**Heat moves through He_2 in waves ~ 20****μ****m/ ****μ****s**

- 25/07/07

## Dressed Cavity: 3D Model and Dimensions

- 25/07/07

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**Notes:**
Assembly requires massive ‘touch labor’ and remains a ‘fit up’ job

## Specific Tasks – Cost

- Cryomodule costs  			fraction	sum
- Cavity Fabrication 			36% 		36%
- Power Couplers 			10% 		46%
- Helium Vessel Fabrication 		8% 		54%
- Magnetic Package (Quad) 		7% 		61%
- Tuners 				7% 		68%
- Assembly, Testing, Transport 	5% 		72%
**(Next 7 items – to 1% level (22%)– Vacuum vessel,shields, interconnect, processing, dressing, pipes, supports, instrumentation)**

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## Module assembly picture gallery - 1

**String inside the Clean Room**

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## Module assembly picture gallery - 2

**String in the assembly area**

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## Module assembly picture gallery - 3

**Cavity interconnection detail**

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## Module assembly picture gallery - 4

**String hanged to he HeGRP**

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## Module assembly picture gallery - 5

**String on the cantilevers**

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## Module assembly picture gallery - 6

**Close internal shield MLI**

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## Module assembly picture gallery - 7

**External shield in place**

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## Module assembly picture gallery - 8

**Welding “fingers”**

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## Module assembly picture gallery - 9

**Sliding the Vacuum Vessel**

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## Module assembly picture gallery - 10

**Complete module moved for storage**

- 25/07/07

## Readiness / R & D Challenges

- The ILC RDR contains a complete design
**This machine would work...**
- BUT:
**We (Fermilab) need to put some backbone into an ILC plan**
- Major goal of the ‘ILC Engineering Design Phase’
**and push the technology as far as we can**
- and capture the advantages we have.

- 25/07/07