# Presentation: 096013

## Overview of 6 km Damping Ring Study

*Kwang-Je Kim*
*for ANL-FNAL-UIUC Collaboration*
*ILC-Americas Work Shop*
*SLAC, Stanford, CA*
*October 14-16, 2004*

## 6-km Damping Ring Study

- The 17-km Tesla dog-bone design assumed 20 ns rise- & fall-time kickers.
- If kickers with shorter rise & longer fall time are available, damping rings can be “small” using a injection scheme by Joe Rogers.
- Fast kickers were discussed by Frisch and Gollin.
- This is an overview of work on a 6-km small damping ring by ANL-FNAL-UIUC collaboration and J. Rogers & A. Wolski.
- Content:
- Current status: linear lattice, instability estimates, & alignment sensitivities
- Plan for a next level study using tools developed for higher brightness light sources

## APS, a 1 km, 7 GeV Ring  at Work

## Damping Ring Lattice Development

**Initial work started at Fermilab (Amin Xiao’s talk) **
**A 6-km, 5 GeV ring with TME lattice using the procedure discussed in *****P. Emma and T. Raubenheimer, PRSTAB,4, 021001 (2001)***
- 77 m of wigglers compared to 400 m in TESLA.
- Higher current in smaller ring than TESLA.
- Performance similar to the TESLA design.

## Major Parameters of DRs and APS

|  | 6 km | 17-km
Dogbone | Emery’s thesis (1990) | APS |
| --- | --- | --- | --- | --- |
| Energy (GeV) | 5.0 | 5.0 | 4.0 | 7.0 |
| Circumference (m) | 6114 | 17000 | 2229 | 1104 |
| Lw (m) | 77 | 468 | 360 |  |
| Fw=I2w/I2A | 8.5 | 17.3 | 32 |  |
| Injected  (m-rad) | 1.010-2 | 1.010-2 | n/a | 1.310-3 |
| Equilibrium  (m-rad) | 5.510-6 | 810-6 | 2.010-7 | 3.510-5 |
| Extracted  (m-rad) | 810-6 | 810-6 | n/a | n/a |
| Damping time,x (ms) | 27 | 28 | 13.5 | 9.6 |
| rms Bunch Length (mm) | 6 | 6 | ~6 | 5.8 |
| rms Energy Spread (10-3) | 1.3 | 1.3 | 0.9 (2.4 with impedance) | 0.96 |
| Particles/Bunch (1010) | 2.0 | 2.0 | 1.4 | 12.0 |
| Average Current (mA) | 443 | 159 | 0.3 | 100-300 |

***Major Parameters of DRs and APS***

## Instability Estimates (Bill Ng)

**Incoherent space charge tune shift**** **=0.026 << 0.31 for TESLA
- Smaller circumference, larger beam size (larger disp.)
**Instabilities**
*Longitudinal***: **Single- & multi-bunch thresholds;*safe*
*Transverse*: Single bunch threshold; *safe *		          	                   multi-bunch threshold; *safe with damper*
*Electron cloud*: Growth time about 1s with solenoids
*Fast ion***: **Growth less than 20% with feedback 	  	                                                   (@ 10-10)

## Quadrupole and Sextupole Alignment Sensitivities (A. Wolski)

- 6-km DR less sensitive than Tesla dogbone
- Both 6-km DR and Tesla dogbone more sensitive than ATF

|  | ATF | TESLA Dogbone | ILC Small |
| --- | --- | --- | --- |
| Quadrupole Jitter [nm] | 241 | 80.7 | 198 |
| Quadrupole Rotation [rad] | 825 | 40.5 | 58.3 |
| Sextupole Alignment [m] | 45.6 | 11.3 | 40.4 |

## The “Small” DR Looks Doable. Next Step?

**Further optimization of parameters**
- Energy (3.7GeV), lattice,
**Next-level study with tools and technology developed for high-brightness light sources:**
- Impedance modelling and minimization
- Beam monitoring and feedback
- Wiggler development
- Photon absorbers

## Tools for Nonlinear Beam Dynamics						(Louis Emery’s Talk)

- Toolkit approach combined with comprehensive  tracking code ***elegant*** developed by M. Borland
- Parallel simplex optimizer being developed by H. Shang
- An example of future investigation
- Trade-off in non-linearities between weaker focusing arc cell design and longer wiggler to maximize dynamic aperture

## RF BPM Diagnostics & Feedback

- Orbit motion is characterized by a PSD spectrum in range 0 to frev/2
- RMS orbit jitter at extraction is equal the integral of PSD spectrum
- Orbit feedback reduces beam motion only in a certain band, say, 0-60 Hz (APS) -> Orbit jitter is reduced somewhat depending on the PSD details
- For damping ring, higher-bandwidth feedback may be needed to reduce jitter sufficiently

## Vacuum Chamber Issues

**Minimizing Impedances:**
- Make use “impedance database” obtained by modeling and measuring local impedance (e.g., by orbit response matrix)
**Suppress electron cloud effects:**
- Employ solenoids, slots, low-secondary-yield-coefficient coatings, and/or corrugation
- Based on experimental and modelling experiences at APS (K. Harkay, R. Rosenberg, L. Loiacono)

## Wiggler Development

- Radiation damage of PM undulator( M. Petra)
- Superconducting wiggler (S.H. Kim); Useful for damping wiggler?
- The LCLS undulator under construction at APS is similar to that used for positron production scheme.

## Photon Absorbers

- APS dipole fan and undulator photon beam absorbers sustain high intensities ( S. Sharma)
- Upto 600 mA for dipole fan, 300mA for undulator
- Damping ring absorbers can be scaled from APS designs

## Summary

- With the development of fast kickers “small” damping rings are  a serious option for ILC.
- Preliminary study of a 6 km, 5 Gev DR look promising.
- Further optimization and next level study are planned using tools for light source improvement.
- More information:
*Studies Pertaining to a Small Damping Ring for the International Linear Collider*,* *_*[FERMILAB-TM-2272-AD-TD](http://www.hep.uiuc.edu/home/g-gollin/linear_collider/Fermilab_damping_ring_report.pdf)*_
- ILC Damping Rings web site: _[http://awolski.lbl.gov/ILCDR/](http://awolski.lbl.gov/ILCDR/)_