1. The Engineering Test Facility for nLC
ALCPG
Victoria, BC
July 30, 2004

2. Introduction Linear collider project goals:
Construct a collider with an initial energy of 500 GeV that can be upgraded to 1 TeV later
Deliver 500 fb-1 in the first four years of physics operation
Begin physics operation in 2016 (one year commissioning) to ensure concurrent operation with LHC
Discussion of project risk earlier this morning
R&D program should be focused on reducing project risk
Different stages of development process
Fundamental R&D – technology development or beam physics understanding
System engineering/integration – can it go together and does it work together
Industrialization and mass production – can components be produced

3. Risk Summary Chart
Problems found late
Lum ~ n2
Lum ~ sqrt(e)

4. The Engineering Test Facility(s)
Critical elements that the ETF must address:
E+ source target/capture
Damping ring beam physics
Main linac rf system production
Main linac emittance/jitter control
BDS collimators/stabilization/machine-detector interface
Some additional long-lead time or difficult components
Cannot and do not want to do this in a single test facility
Final configuration is subject to constraints due to resources and schedule and must be chosen with a goal of reducing the project risk

5. Damping Ring Test Facilities
Beams are stored in the rings for a long time
Small (subtle) effects can have a large impact
Many novel effects will be important due to small emittances and tight stability requirements
Rings are quite different from other operating accelerators
ATF has been a very important test facility
Probably not possible to duplicate for cold design  adds some risk but how much and what can be done in other studies?

6. Main Linac e-control
Given tolerance specs, instrumentation, and algorithms, the emittance performance is similar in warm and cold
Dge ~ 10nm over 5 to 10 km
Very hard to measure in a small facility
Complicated source – ATF is the only source that comes close
Not simple to scale to lower energies or amplify the effects
Real issue is performance/interaction of diagnostics and controls
Most algorithms are local  use dedicated tests of local systems
Rf structure girder alignment test
Cryomodule alignment study

7. Main Linac RF Systems
Need to produce some number of complete rf sub-units
Rf subunit has 5-m of structures in X-band
Rf subunit has 36-m of structures in L-band
Demonstrate stable operation of a few units
With/without beam?
Beam is desired to verify rf operation stability and gradients
Forces a more rigorous approach
Be nice to use beam for centroid studies of trajectory control and verify instrumentation
Need engineering studies to optimize fabrication and lifetime
Don’t need to produce every component though

8. Luminosity Tests at an ETF?
ETF is a next-generation NLCTA / GLCTA
Luminosity tests may also be desired – what can be measured?
Vibration studies of rf girders, quadrupoles, etc
Want complete cooling circuit (8 rf units = 2 GeV)
High precision/high resolution diagnostics
S-BPMs, Q-BPMs, e-wires
Want to demonstrate longitudinal dynamics
Full bunch train with full loading: 192 bunches; 1.4 ns spacing
What about transverse dynamics?
Want reasonable model for beam dynamics: 8 GeV and 8-m beta
Sets lowest energy ~ 1 GeV with quadrupoles every rf girder
Would like a low emittance source
ATF damping ring or possibly an rf gun
How do these impact the scope of an ETF?

9. Possible Examples of a big RF ETF

10. e-control: Dispersive Effects
Effective tolerance in NLC is 2 um alignment of the quads to the BPMs
The generated dispersion is roughly 4 * yq * sqrt(Nq)
Assume 8 FODO cells => 50 um dispersion at the end
Can this be detected on the beam?
With nominal energy spread of ~1% and beta of 5 meters, a dispersion error of 500 um will generate 40% De/e given an initial emittance of 2e-8
Be very difficult to verify 2 um alignment looking at the e
50 um dispersion could be measured looking at the beam centroid
Assume better than 1 um BPM resolution and ~10% DE/E - easy measurement
Want a few FODO cells at the end of the linac for measurement
Could decrease ETF length but want a reasonable set of magnets

11. e-control: Wakefield Effects
Primary tolerance is set by short-range wakefields
Girder of 4 structures must be aligned at 3 um level for NLC
In the ETF with a 300 um bunch length and 7.5e9, the kick of the beam tail is roughly 80 V * sqrt(Nrf)
At 1 GeV, this is ~0.1 urad as compared to the beam angular divergence >1urad for an emittance of 2e-8
Very difficult to verify performance from emittance growth alone
Ways to increase the sensitivity:
Increase bunch length or bunch charge by ~5x
Use long-range wakes with 4- and 8-bucket spacing
This is a dedicated test of an rf-girder not a test in a long (or short) linac

12. ETF Luminosity Study Implications
Even with nominal GLC/NLC emittances and 50-meter ETF cannot verify tolerances with emittance
ATF-III is still factors of five too insensitive to verify GLC/NLC performance requirements
Jitter measurements are best done with accelerometers
Alignment tolerance verification:
Bunch centroid or beam tilt for wakes and dispersion
Beam tilt:
1.7 mrad in 300 um bunch  close to desired tolerances
Beam centroid
Works fine for dispersion
Use dedicated facility to verify wakefields and e-control

13. The Engineering Test Facility(s)
Small scale (~1 GeV) rf test facility with multiple rf units
Include beam to verify low-level rf control and gradients
Include good (but not nLC) beam for e-control studies
Many other test facilities of smaller but similar scale
Dedicated studies of most relevant systems
Focused test can yield scientifically relevant information
Do NOT try to focus all effort (and resources) on one large showpiece which demonstrates little (but an ability to spend money – important but not essential at this time)
Caveat: major schedule delay would increase the importance of a large showpiece for political reasons