ILC – Main Linac Simulation
August 2006 Report.

Paul Lebrun

ILC-LHC R&D in AMR

Outline

A proposed plan for CHEF-based studies for the next 6 months

my focus: LET results, both “static” and “dynamic”

Panagiotis et al: Damping Ring

Little new software infrastructure needed for LET.

Current status on Dispersion Matched Steering in CHEF

Goal of such studies.

Establish that we have a credible Beam Based Alignment and Steering strategy that preserves the transverse emittance, for the Main Linac (1st priority), if resource available, RTML & BDS

Study improved performance options, such as adding

the capability of moving quadrupoles or cryo-modules, or even individual cavities,

and/or additional correctors elements

Assumption on resources.

Support for CHEF available. (bug fixes...)

One additional Beam physicist on LET/CHEF studies available.

If more people become available, writing CHEF documentation should be given higher priority.

Comments of #1 goal

Based on previous MatLIAR calculation, done!..

for Main Linac, at least (RTML still very difficult.)

But no margin for additional machine imperfections..

Dynamic effect while 1st steering..

Effect of feedback loops, consistency between static BBA and dynamical steering.

Robustness of the solution against small changes in operation.

Not yet simulated imperfection of the Linac, (long range wake fields, for instance.)

The Plan, Aug/Sept 06

Pursue static steering, repeat some but not all previous studies. (Need to think/specify required benchmarks..)

Consider algorithms with beam elements movers.

Start on integration of ground motion with existing steering code.

Start on studies realistic curved Linac (i.e., RDR lattice)

Oct06/Nov 06

Study 1rst order Beam Based Alignment Steering (i..e, comissioning) with vibration, ground motion and beam jitter, etc... Attempt to estimate how long it will take, on realistic lattices.

Main Linac has still priority.

Study Dynamical Steering algorithm to maintain high luminosity performance, on RDR lattices, once a the beam has been successfully steering

include integration of the simple 1 to 1 linear, small corrections.

Oct06/Nov 06, II

Study realistic (RDR) lattice performance, including matching sections, non-Gaussian beams, effect of missing klystrons and other machine failures.

Later...

Start on integration issue on RTML/Main Linac integration, focusing on dynamical issues.

Same with Main Linac/BDS

Do the Damping Ring to IP integrated, “start to end” simulation.

Comments on Start to End simulation.

Final goal, but almost always postponed... Why?

Makes little sense to do start to end on a simplified version of the machine, once a base line design has been layed out.

Requires all the pieces or section to work!!!

easily got stuck on local details, that needs to be resolved.

Software integration difficulties: some section requires expert software.

Yet, much needed to give us confidence in a refined baseline design, should be part of a “TDR”

CHEF Tool-kit, requirements.

Currently missing, but trivial

Solenoid

A bit more work.. (> 3 man-weeks):

Spin transport.

Undulator.

Again, maintenance required.

If to be trusted/used by a broader community, must write documentation!!!

Comments on the plan:

Uploading the RDR lattices, one by one, into CHEF and check that the perfectly aligned corresponding machine preserves the emittance can start now.

But this requires one of us to start using CHEF. Perhaps Francois O. or Alex V. can start on this...

Or conversely, I would have to leave Steering studies and work on this...

Left aside: Use the Merlin DFS algorithm in the context of Merlin, and vice-versa: Nice infrastructure project, but also very useful if CHEF ( or Merlin!) results are disputed: allowing to dissociate between tracking and steering algorithm features or problems.

Recent progress on Steering in CHEF

Polished a bit the Dispersion Matched Steering, and kept testing in DFS mode.

Good performance if cavity tilts/offsets are ignored!.. In fact, emittance growth can be made less than 1 nmRad for the first ~144 cryo-modules

Real difficulties when cavity tilts: gained a better appreciation of limits of DFS, started to document a (probably known!) feature of the algorithm.

And started to implement an algorithm based on moving sets of cavities, to fix or improve the above problem...

On Dispersion in misaligned LINAC

The dispersion in a given section C depends on how the beam has been accelerated in previous section A & B. If section A is perfectly aligned and rans at higher than nominal voltage, section B is misaligned runs at a lower than nominal voltage, the transverse kicks in section B will be reduced compared to the case where section A runs at higher voltage than section B.

Linac

A

B

C

E

Y'

Observed this in Benchmark 2 lattice...

Conditions:

Section length where steering occurs: 8 dipoles (short!)

Overlapping by 7 dipoles (tedious!) That is, moving from one section to the next, I simply shift section B & C by one dipole/BPM, or 24 cavities.

A always at nominal accelerating gradient, B reduced. But section B moves as we keep steering sections after sections.

Observation:

Starting Section C at dipole 4, optimum steering for dipole 6 was found to be -0.000258426 T.m. On section 5, this value changed only by less then 1% , to -0.000260542. But, on section 6, the DFS algorithm changed for this dipole to ~+1 10-5

And...

Setting for other dipole are also widely different than in the previous section -> different solution emerge.

In practice, one tend to verify previous solution, i.e., go back and forth and check things. If we find different solutions, this tends to be very confusing. => bad algorithm behavior.

Current Fixes...

Jeff S. retuned the orbit in such a way that, entering section C, being steered, the trajectory matches at given energy. ( I think that what he does).

A clever patch, which hides the following difficulty: suppose we steer with a given set of working klystrons. Next day, one must use a different set, as a few have been repaired and new ones. A different pattern of dispersion(s) values will emerge, does this mean we'll have to re-steer?

Other algorithm distinguish between bad and even worse solution by minimizing a combined dispersion + deviation chi-square.

(not yet implemented in CHEF, but easy to do now..)

A better fix ?

Re-align the machine, instead of steering the beam!

Not currently possible, no movers on beam line elements. Too challenging from the mechanical engineering standpoint.

Even if we do have movers on cavities, how do we do BBA with them?

Easy, based on the previous observation: one must have the same dispersion in section C independently of the E(s) functions in upstream section.

Compromise

Move a cryo-module ! Prior to DFS section C, re-align cryo-module upstream such that the trajectory w/o energy reduction in section B gives the same dispersion measurement.

Coded up, started to test it out...

Outlook

Meanwhile, must improve the speed of the DFS algorithm, not compromising on its accuracy.. (not too much !...)

Not re-tracing though steered sections.

Not re-computing response matrices..

reduce overlap between section.

number of iteration

Started to run on ilcsim again, and get ready for the farm..

Condor Script ready.

Awaiting to get re-certified at getting certificates.

2005-09-19T09:15:26