April 24 2008 1

Recent work on Low Emittance Transport, Main

Linac

Paul Lebrun

CD/FNAL

April ’08, GDE, Fermilab 2

Agenda, after Dec. 11

• Webex meetings of the LET groups:– Feb 05 2008

• Dynamic simulation @ Desy, Freddy Poirier – Feb 28, Sendai Meeting.

• Dynamic Simulation, Summary, by F. Poirier– March 18 2008

• Main Linac Dispersion Free Steering with Placet., J.R. Lopez, (Oxford, based on earlier work by D.Schulte et al, CERN)

• Issues with DFS, K. Kubo. – March 27-28

• Workshop on Beam Polarisation, Cockroft Inst. – April 15:

• Sorting out DFS issues with Merlin, F. Poirier.

3

Methodology, Comments

• Simulation must be Cross-Checked. – And disagreement(s) resolved..

• Using different codes…

– Resolution• Not always plain and simple programming bugs!• Slightly different assumption, or definition of imput

parameters.• Steering algorithm details matters. • Complex problems => firm conclusions can rarely be

reached…

• Static Tolerance errors

April ’08, GDE, Fermilab

4

Simple Example: BPM offset

• K. Kubo noticed different sensitivity to BPM offset, result presented by K. Ranjan ~ 2 years ago.

• Studied by F. Poirier, (Desy), and Fermilab. (last year)– Proper definition of reference frames..

• Improved misalignment model, SLAC meeting, Dec 2007• Implemented at KEK, Fermilab, CERN..• Was different than in LIAR, ~2005

– DFS Algorithm tuning• Relative weight of the “1-to-1” to pure Dispersion Free Steering.

– Resolution: Difference tentatively understood, • Good news: in the limit of “pure DFS”, the BPM static offset

tolerance does not need to be that strict..

April ’08, GDE, Fermilab

Emittance vs BPM Offset Error

• Vertical emittance versus the BPM offset is here checked.

• Energy Strategy:– Grad= -20%– Init. Beam= -20%

The slope of the emittance versus the BPM Offset is highly dependent on the weight chosen

I.e. difficult to make a direct comparison between codes without the knowledge of the weight used in the various code (and understanding of the DFS algorithm )

Sm

all

we

igh

tL

arg

e w

eig

ht

Emittance vs BPM offset (DFS 40-20)

20

30

40

50

60

70

80

90

100 200 300 400 500 600 700 800 900 1000

BPM offset (um)

Co

rre

cte

d V

ert

ica

l Em

itta

nc

e (

nm

)w=1w=5

w=10w=20w=30w=40

w=60w=80

Corrected Emittance= Energy Correlation numerically removed.

6

A bit more difficult: Cavity Tilts

• Again, disagreement among various code on the required tolerance for cavity tilts

• Recently studied by F. Poirier, (Desy), and J. Lopez, Oxford

April ’08, GDE, Fermilab

Some preliminary resultsEmittance growth versus cavity errors

Average over 100 random seeds. In this case the error bars indicate the standard deviation

From J. Lopez, Oxford.

Pitch Error

• Close-up from previous slide:

0 200 400 600 800 100020

22

24

26

28

30

32

34

Cor

rect

ed V

ertic

al E

mitt

ance

(nm

)

RF Structure Pitch (urad)

w1IB20 w1IB0 w5IB20 w5IB0 w10IB20 w10IB0 w40B20 w40IB0

IB (Initial Beam Energy):

Dash IB=0%

Plain IB=-20%

The use of a 2nd test beam with an initial decreased energy allows here to be less sensitive to RF structure pitch errors

This can explain the difference between KK results and K. Rajan results (*)

(*) Though a clear understanding of the various algorithm used for DFS would further help to know where might be other differences

9

Cavity Tilts

• Other codes: CHEF, Fermilab… – Never been able to reproduce such good

performance with near pure DFS, with SVD null space suppression, with large ( ~ 500 micro-radian) pitch angle !!.

– To be checked:• Tracking accuracy through rotated r.f. Cavity fields, with

Wakes field.

• Remains a critical items, for the upstream section of the linac ( ~ 1st kilometer), and the RTML.

April ’08, GDE, Fermilab

10

Dynamic Simulation

• Progress at Desy: – Talk at the last GDE meeting, Sendai. !!

• Does not mean we are “done” !.. – Note: Past experience:

• RunII Tevatron designers did not requested a slow feedback system for accurate control (~10 microns resolution BPM) of the Helical orbits Proton/Pbar in the Tev, during a ~ 20 hour store..

• And our emittance are ~ 50 smaller…

• No easy and quick gains: This is work requires dedicated man-power.

April ’08, GDE, Fermilab

Conclusion

• Quite a few studies which include dynamic effects– Ground Motion & Vibration– Slow correction (1-to-1, Mikado)

• More to be done:– Strategy of the slow correction has to be reviewed (continuous steering or periodic,

on entire lattice or sections) – Effectiveness of steering with Ground motion

• With machine initially tuned• More realistic GM (?)

– Effectiveness of fast feedback for ML– Coupling action of fast Feedback

• Application strategy of several FFB (e.g. gain)• Will have to integrate the above effect into the start-to-end simulations

– Steering in the undulator (?)• Missing here:

– Complete review of what has been done– Concrete step by step plan for the work

From F. Poirier, Sendai

12

LET “Work” @ Fermilab

• Related Non-ILC work:– Code Stabilization after PAC07

• Upgraded my LET code to latest release of HEF ( our local code)

• Adapted and documented the CHEF-LET code such that we have a maintained LET example, from which we can start again..

– Kept in touch!

April ’08, GDE, Fermilab