# Presentation: 492517

## Discrete Vertical Bends in the ILC Main Linac

- WG1 Layout Session

- PT and Mark Woodley

- 16-August-2005

## Motivation

- At 1 TeV CM and 30 MV/m, the LET will be ~53 km long
- Straight LET:  IR halls ~50 m deeper than bunch compressors
- Continually curved LET:  design dispersion at all BPMs, hard to tune emittance
- Consider piecewise-straight LET with up to 2 vertical arcs per side at 1 TeV CM

- PT and Mark Woodley

- 16-August-2005

## Parameters of VARC

- Divide LET into ~equal-length segments
- Both BDS in the same straight
- Straight next to BDS must be at least 7.5 km for dog-bone straights
- Arc angles should equal average curvature of “cue ball” earth
- Real site will need somewhat different angles
- Limit SR effects
- Emittance growth
- Critical energy < 5 MeV limits component activation
- Use 90 degree FODO cells
- Achieve dispersion match using Keil scheme
- 6 cells:  4 at half-strength, 2 full
- One PS for all bends, so half-strength == half as many windings
- Tune ηy with quad strengths

- PT and Mark Woodley

- 16-August-2005

## VARC Parameters (2)

| Parameter | 30 MV/m value | 35 MV/m value | 40 MV/m value |
| --- | --- | --- | --- |
| Geographic Gradient [GeV/km] | 19.8 | 23.1 | 26.4 |
| Segment 1 length [km] | 12.4 | 10.6 | 9.3 |
| Segment 2 length [km] | 7.5 | 7.5 | 7.5 |
| BDS segment length [km] | 13.2 | 9.8 | 7.4 |
| LCD extra depth [m] | 3.42 | 1.88 | 1.07 |
| VARC1 energy [GeV] | 250 | 250 | 250 |
| VARC2 energy [GeV] | 399 | 424 | 448 |
| VARC1 angle [mrad] | 1.56 | 1.42 | 1.32 |
| VARC2 angle [mrad] | 1.62 | 1.36 | 1.17 |

- PT and Mark Woodley

- 16-August-2005

## Schematic View

- PT and Mark Woodley

- 16-August-2005

## VARC Optics

- PT and Mark Woodley

- 16-August-2005

## Non-Issues

- Power supply strength tolerances
- 1e-3 is good enough
- Horizontal dispersion from rotated bends or quads
- very small effect; ηx tuning not needed
- SR effects for ECM up to 1 TeV
- critical energy below 5 MeV
- emittance growth ~2% for all cases studied

- PT and Mark Woodley

- 16-August-2005

## Issues

- SR at energies higher than 1 TeV CM
- emittance growth excessive
- may need to lengthen arc, switch to gradient bends
- Bandwidth
- High-order dispersions not corrected
- steering and mismatches for off-energy bunches

- PT and Mark Woodley

- 16-August-2005

## Lengthening VARC

- Can “rip out” linac around VARC and extend – center of VARC has to be lowered by 3.5 cm for 1.62 mrad total bend angle

- PT and Mark Woodley

- 16-August-2005

## Emittance Growth and Mismatch

- Even 20% off-momentum beam will have tolerable matched emittance (about 2 um.rad)

- Mismatch over BDS luminosity bandwidth is quite small

- PT and Mark Woodley

- 16-August-2005

## Steering

- -20% off-momentum beam gets steering error of 1500 σ, or about 3.8 mm (10% of the cavity apertures), so it doesn’t hit anything in the linac

- For more modest momentum errors the offset is well within the capture range of the intra-train feedbacks

- PT and Mark Woodley

- 16-August-2005

## Conclusions

- In my opinion the vertical arcs look usable
- Need to consider operational implications of arcs
- in particular whether tuning procedures can tolerate large oscillations and mismatches for off-momentum bunches
- Intra-train feedback issues
- Does large high-order dispersion drive halo formation?
- Don’t know yet what beta match from linac looks like
- might be highly chromatic...
- Note that if 6-cell arcs prove marginal, can expand to 10-cell arcs
- Dramatic reduction in high-order dispersion and mismatches for off-momentum bunches

- PT and Mark Woodley

- 16-August-2005