1. Emittance Dilution and Preservation in the ILC RTML
P. Tenenbaum
ILC LET Meeting
09-Feb-2006
Includes corrections on slides 15 and 16 from 16-Feb-2006

2. Introducing the ILC Ring to Main Linac (RTML) Transport
09-Feb-2006
P. Tenenbaum

3. Introducing RTML (2) Skew Correction Collimation DR Stretch Turnaround
4 orthonormal skew quads to tune betatron coupling
Collimation
2 phases x 2 planes x 1 iteration of betatron collimation
DR Stretch
Allows position adjustment of DR within site footprint
Turnaround
Achromatic arc permitting feed-forward correction of beam jitter
09-Feb-2006
P. Tenenbaum

4. Introducing RTML (3) Spin Rotator 4D Emittance measurement BC1 and BC2
Solenoids + arc allow arbitrary tuning of IP polarization direction
4D Emittance measurement
Laser wires configured to allow 4D emittance measurement and fit
BC1 and BC2
Two-stage bunch compression, with pulsed dumps
Linac launch
Final emittance diagnostic (2D) before main linac
09-Feb-2006
P. Tenenbaum

5. Major Sources of Emittance Dilution
Vertical Dispersion
Collimator Wakefields
XY Coupling from DR extraction
ISR
XY coupling
Dispersion, Wakefields, RF pitch, Collimator Wakefields, IP arrival time error, ISR
09-Feb-2006
P. Tenenbaum

6. LET Studies so far on RTML
Nothing
Almost! Looked at use of normal / skew quads in BC1 and BC2 for dispersion control
More on this later
09-Feb-2006
P. Tenenbaum

7. RTML Tuning and Ideas
Dispersion tuning in Skew, Coll, DR Stretch, Turnaround, Spin Rotator
BBA (ie, measure BPM offsets and correct orbit)
Quads are NC iron-dominated
Thought to be better for BBA than SC cos(2θ)
DFS?
Maybe not – may be hard to get large energy variation from DR
Problem in ATF
Global correction
Include skew quads in turnaround and Spin Rotator for tuning vertical dispersion?
Probably necessary for correcting dispersion from rolled bends
09-Feb-2006
P. Tenenbaum

8. RTML Tuning and Ideas (2)
XY Coupling from DR and Spin Rotator
Skew quads in Skew Correction section
1 set for both areas OK if residual coupling is small (~1%), per K. KUBO
Sets tolerance on DRX / Spin Rotator initial coupling correction, or else we need 2 sets of skew quads
Collimator Wakefields
Feed-forward can correct betatron jitter amplification
Reduce by a factor of 10?
No correction possible for emittance growth
Will set limits on beam jitter from DR and/or collimation depth achievable
No correction possible for energy-betatron jitter coupling
Not a large problem in systems studied to date (ie, NLC)
09-Feb-2006
P. Tenenbaum

9. RTML Tuning Ideas (3) Emittance Dilution in BCs
May be a very large source
Wakefields, RF cavity pitches, dispersion
Long bunch near zero crossing with large energy spread
SC quads thought poor candidates for BBA
Lots of strong bend magnets with tight rotation tolerances
Dispersion control quads may work
Wakefields: need good alignment of modules
Do we need HOM readouts on cavities here?
09-Feb-2006
P. Tenenbaum

10. RTML Tuning Ideas (4) Longitudinal tuning and stability of BCs
Need bunch length monitors and beam phase monitors after each BC
Need to tune linear map terms as well as final bunch length
Ie, want to limit sensitivity to DR phase jitter
Need to monitor relative IP arrival time of the two bunches
Beam-based feedback on some timescale
09-Feb-2006
P. Tenenbaum

11. Tuning the RF Pitch Aberration in ILC RTML
RF cavities in RTML operate near the zero-crossing
Pitched RF cavities rotate some of their longitudinal field into vertical field
Put those together: Time-varying vertical field
Head-tail deflection of the beam
Potentially a big effect – bunches are long and at low energy!
09-Feb-2006
P. Tenenbaum

12. Tuning Pitch Aberration (2)
Walker’s conjecture:
Pitched RF cavities make z-Py correlation
But they also make z-E correlation
Hence they make a strong E-Py correlation, ie, η’
Thus the dispersion knobs should be very effective against pitch aberration!
Try this out in Lucretia!
09-Feb-2006
P. Tenenbaum

13. BC1 Wiggler Optical Functions (BC2 is similar)
= location of skew quads for ηy control
90°
-I xfrm
-I xfrm
09-Feb-2006
P. Tenenbaum

14. Simulation Procedure Roll 100 seeds of BC Steer flat on BPMs
Each seed perfect except that RF cavities have 300 μrad RMS pitch
Steer flat on BPMs
Take out steering from cavities
Comes from pitch on cavities which are not exactly on zero-crossing
Scan 4 dispersion knobs (2 in each BC), optimizing emittance at end BC
Meas emittance at 5 steps of knob, fit parabola, accept best value
If best value is outside scan range, scale range and rescan
For now, perfect measurement of projected vertical emittance simulated
09-Feb-2006
P. Tenenbaum

15. Results 09-Feb-2006 P. Tenenbaum Mean = 27.9 nm 90%CL = 63.1 nm

16. Results (2) Correction with skew quads is pretty effective…
Factor of ~9 reduction in mean growth
Factor of ~11 reduction in 90%CL growth
…but not quite effective enough!
90%CL growth > entire RTML budget
No other technique available right now to improve on correction
Tuning may not yet be optimal
Needs more study
(where have I heard that before?)
09-Feb-2006
P. Tenenbaum