# Presentation: 123619

## Two Stage Bunch Compressor Proposal

- Snowmass WG1

*“**It’s the latest wave*
*That you’ve been craving for*
*The old ideal*
*Was getting such a bore*
*Now you’re back in line*
*Going not quite as far*
*But in half the time”*

- PT for US BC Task Force

- 16 August 2005

## US BC Task Force

- Kellen Petersen, Tor Raubenheimer, Jeff Smith, PT, Andy Wolski, Mark Woodley, Jiajun Xu

- PT for US BC Task Force

- 16 August 2005

## Motivation

- TDR used single-stage BC to compress from 6 mm to 0.3 mm RMS
- Large energy spread at linac entrance
- bad for emittance preservation
- Limits achievable compression factor
- ILC DRs have larger energy spread than TDR
- 0.15% vs 0.13% at full current
- Big effect on BC design
- ILC parameters may require 0.15 mm RMS bunch length
- DR parameters may require 9 mm RMS bunch length
- Want smaller emittance growth for nominal parameters and/or capacity to operate with other parameters

- Single-stage BC retuned to 0.15% initial energy spread, 1.3 GHz RF only!

- PT for US BC Task Force

- 16 August 2005

## Motivation (2)

- Use of multiple stages of compression can reduce peak energy spread
- Perform partial compression
- Accelerate off-crest (accelerate and chirp beam)
- Compress again at higher energy
- Results in “long” section with moderate energy spread
- System length, complexity, voltage greater than in single-stage system

- PT for US BC Task Force

- 16 August 2005

## Overall Configuration

- Basic choices are:

- Arc or dogleg (R56 > 0 in TRANSPORT notation) – changes beamline geometry, requires T166 correction, R56 adjusted via quad lattice

- FBDB configuration (R56 < 0) – straight beamline geometry, no T166 correction needed, R56 hard to adjust (except by mechanical intervention)

- FCDC configuration – similar to FBDB but easier to adjust R56, longer (more SR emittance growth than equivalent FBDB)

- We chose FCDC for this exercise

- PT for US BC Task Force

- 16 August 2005

## Overall Configuration (2)

- FODO cell for wiggler = 90°
- 6 cells in each wiggler
- Normal/skew quads for dispersion correction
- cells 1 and 3
- cells 4 and 6
- BPMs for energy measurement
- 8 bends per chicane
- Adjust bends 2, 3, 6, 7 to vary R56
- Corrector quads and BPMs between bends 1 and 2, 7 and 8
- don’t need to move when R56 varied

- PT for US BC Task Force

- 16 August 2005

## Wiggler Cell Optics

- PT for US BC Task Force

- 16 August 2005

## Longitudinal Optics

- Simplest approach:  two “90°” rotations in longitudnal phase
- R55 ~ 0 in each rotation
- Overall R55  0
- DR phase transients  linac phase errors
- DR bunch lengthening  IP bunch lengthening
- Alternate approach:  one “90°” rotation
- first stage under-compresses
- Overall R55 ~ 0
- More energy gain in BC2 linac
- Larger energy spread in BC2 linac
- Looked at both configurations, 300 um and 150 um final bunch length from 6 mm initial
- “A” parameters == 180° rotation
- “B” parameters == 90° rotation

- PT for US BC Task Force

- 16 August 2005

## Overall Lattice

**BC1 RF**

**BC1 Wiggler**

**BC2 RF**

**BC1 Wiggler**

- PT for US BC Task Force

- 16 August 2005

## Key Parameters

| Parameter | 300 “A” | 300 “B” | 150 “A” | 150 “B” |
| --- | --- | --- | --- | --- |
| Initial Energy [GeV] | 5.0 | 5.0 | 5.0 | 5.0 |
| Initial Energy Spread [%] | 0.15 | 0.15 | 0.15 | 0.15 |
| Initial Bunch Length [mm] | 6.0 | 6.0 | 6.0 | 6.0 |
| BC1 Voltage [MV] | 348 | 610 | 253 | 580 |
| BC1 Phase [°] | -100 | -110 | -100 | -110 |
| BC1 R56 [mm] | -550 | -273 | -750 | -267 |
| End BC1 Bunch Length [mm] | 0.85 | 0.80 | 1.14 | 1.12 |
| End BC1 Energy [GeV] | 4.94 | 4.79 | 4.96 | 4.80 |
| End BC1 Energy Spread [%] | 1.12 | 1.93 | 0.82 | 1.83 |
| BC2 Voltage [MV] | 11,215 | 11,000 | 12,750 | 11,600 |
| BC2 Phase [°] | -40 | -22 | -58 | -45 |
| BC2 R56 [mm] | -70 | -59 | -41 | -42 |
| End BC2 Bunch Length [mm] | 0.30 | 0.30 | 0.15 | 0.15 |
| End BC2 Energy [GeV] | 13.5 | 15.0 | 11.7 | 13.0 |
| End BC2 Energy Spread [%] | 1.12 | 1.07 | 2.73 | 2.46 |

- PT for US BC Task Force

- 16 August 2005

## “Site Tax”

- 2-Stage BC is longer than 1 Stage...
- ...but 2 Stage has additional energy gain
- does part of the linac’s job
- What is a fair comparison?
- Consider the “site tax”
- Additional length needed for BC when BC RF energy gain/loss is taken into account
- Coasting lattice:  site tax == length
- Accelerating RF:  site tax < length
- Decelerating RF: site tax > length

- PT for US BC Task Force

- 16 August 2005

## Site Tax (2)

| Item | 1 Stage | 300 “A” | 300 “B” | 150 “A” | 150 “B” |
| --- | --- | --- | --- | --- | --- |
| Matching | 51.1 | 51.1 | 51.1 | 51.1 | 51.1 |
| BC1 RF | 67.0 | 48.6 | 55.1 | 46.7 | 54.1 |
| Matching | 58.4 | 58.4 | 58.4 | 58.4 | 58.4 |
| BC1 Wiggler | 239.1 | 239.1 | 239.1 | 239.1 | 239.1 |
| Matching | -- | 77.1 | 77.1 | 77.1 | 77.1 |
| BC2 RF | -- | 161.6 | 81.4 | 253.1 | 181.0 |
| Matching | -- | 86.0 | 86.0 | 86.0 | 86.0 |
| BC2 Wiggler | -- | 238.8 | 238.8 | 238.8 | 238.8 |
| Matching | 112.2 | 112.2 | 112.2 | 112.2 | 112.2 |
| Total | 527.8 | 1072.9 | 999.2 | 1162.5 | 1097.8 |

- Note:  1 Stage wiggler could be made shorter (R56 requirement is small) – same wiggler used for 1 stage and 2 stage for optics comparison purposes

- PT for US BC Task Force

- 16 August 2005

## Performance Metrics

- Longitudinal tolerances
- Sources
- DR extraction phase
- BC1 phase/amplitude variation
- BC2 phase/amplitude variation
- Effects
- IP energy / energy spread
- IP arrival time / bunch length
- Transverse tolerances
- Emittance growth from
- 10 um BPM offsets
- 75 urad RF cavity pitches
- 500 urad RF cavity offsets
- Need to consider errors in the BC and the linac!
- Don’t want to make tolerances loose in BC by making them tight in the linac!

- PT for US BC Task Force

- 16 August 2005

## Longitudinal Dynamics

- Energy and energy spread effects small
- Typical effects at level of 0.1% of beam energy or less
- BDS bandwidth closer to 1%
- z / σz effects larger
- Assume that 20% z variation and 5% σz are the limit
- Assume that BC1 and BC2 RF have same phase and amplitude stability

- PT for US BC Task Force

- 16 August 2005

## Longitudinal Tolerances

| Parameter | 1 Stage | 300 “A” | 300 “B” | 150 “A” | 150 “B” |
| --- | --- | --- | --- | --- | --- |
| DR Extraction Phase | 1.5 mm | 0.9 mm | 5 mm | 0.75 mm | 2 mm |
| BC RF Amplitude | 0.2% | 0.1% | 0.15% | 0.08% | 0.1% |
| BC RF Phase | 0.07° | 0.05° | 0.12° | 0.03° | 0.06° |

- Red = Arrival time driven tolerance, blue = IP bunch length driven tolerance

- PT for US BC Task Force

- 16 August 2005

## Transverse Emittance Growth

| Error | 1 Stage | 300 “A” | 300 “B” | 150 “A” | 150 “B” |
| --- | --- | --- | --- | --- | --- |
| 10 um BPM Offsets | 2.83 nm | 0.51 nm | 1.31 nm | 3.73 nm | 5.34 nm |
| 75 urad Cavity Pitches | 3.08 nm | 0.52 nm | 1.77 nm | 2.30 nm | 3.04 nm |
| 500 um Cavity Offsets | 2.07 nm | 0.89 nm | 1.63 nm | 2.12 nm | 2.68 nm |

- PT for US BC Task Force

- 16 August 2005

## Conclusions

- Compared to Single-stage BC, two-stage system offers
- reduced emittance growth at σz = 300 μm, or
- emittance growth at σz = 150 μm comparable to single-stage emittance growth at σz = 300 μm
- Two stage system can be tuned to ease transverse tolerances or DR extraction tolerances
- Two stage system more tolerant of longer DR bunch
- RF tolerances pretty tight in all designs
- Two stage system is longer than one-stage
- A shorter 2-stage with FBDB wigglers is possible
- need to change quad yaw angles when tuning bunch length
- May be some tradeoffs with stronger optics
- Can reduce SR growth and thus length
- May tighten transverse tolerances

- PT for US BC Task Force

- 16 August 2005

## How to choose?

- Are we really interested in 150 μm bunch length?
- If so, two-stage probably essential
- Are we really interested in 9 mm DR bunch length?
- If so, two-stage preferred (“B” configuration)
- What is the realistic emittance performance gain for 1 vs 2 stages?
- What is the real stability tolerance on IP bunch length?
- 5%  tight DR phase tol or 2-stage “B” system
- Looser tolerance on IP bunch length makes 1 stage more attractive

- PT for US BC Task Force

- 16 August 2005