# Presentation: 583896

## NSLS-II Accelerator Systems

**Satoshi Ozaki**
**Director, NSLS-II Accelerator Systems Division**
**NSLS-II User Workshop**
**July 17, 2007**

## Goals for the NSLS-II Accelerator Systems

- Requirements for the NSLS-II facility:
- Ultra-bright and stable synchrotron radiation
- Spectrum coverage ranging from far IR to hard x-rays
- Requirements for the accelerator complex:
- Ultra-small horizontal emittance εx < 1.0 nm•rad (achromatic)
- Diffraction limited vertical emittance at 12 keV
- Beam stability ~10% of beam size or better (0.3 μm)
- Stored current ~ 500 mA 1% with top-off injection
- 27 straight sections, each > 6 m for user insertion devices

## NSLS-II Concept

_**NSLS-II Machine Concept**_
- New 3 GeV Electron Storage Ring
- Large Circumference (791.5 m), H = 1320
- Low Emittance Booster (~158 m, H = 264)
- High Stored Current (500 mA 1%)
- Expected Beam Lifetime (~3 hours)
- Top-Off Injection (~7 nC every minute)
- Superconducting RF (500 MHz)
- DBA30 Lattice (15 fold symmetry)
- Fifteen 8.6 m and fifteen 6.6 m straights with Hi-Lo β
- Small horiz. emittance of bare lattice (~2 nm)
- Damping Wigglers (initially 21m  56 m)
- Large Dipole Bend Radius (25 m)
- Ultra-Low Emittance (1 to 0.5 nm)

_**Technical Challenges**_
- Lattice design: Sub-nanometer horizontal emittance, large dynamic aperture, wide energy acceptance etc.
- Source stability: vibrations, thermal issues, feedback
- Insertion Devices: DW, CPMUs, EPUs, and their impact to dynamics of beam

## Rendering of the Storage Ring in the Tunnel

## Lattice Functions For One Superperiod

- _Goals_
- Bare Emittance:
- 	2.02 nm-rad
- Dispersion at ID Straight:
- 	Zero
- Beta-Functions:
- 	8.6 m ID Straight (Hi )
-   	(x, y) 18.9/2.9 m
- 	6.6 m ID Straight (Lo )
- 	(x, y) 1.5/0.8 m

- Twiss parameters for one superperiod of the DBA-30 lattice

## Lattice Dynamic Aperture (DA)

- The DA simulation from 20 randomly seeded sets:
-      alignment tolerances and synchrotron oscillations are included

- DA is sufficiently robust for high current operation, complex insertion devices, and top-off injection

## Damping Wigglers for Emittance Control

- Damping wigglers in the dispersion free straight sections enhance the damping of the lattice without significantly increasing the quantum excitation
- A novel approach for a light source but is a well established technology for HEP accelerators and colliders
- The equilibrium values for the energy spread and the emittance depend not only on the wiggler radiation energy loss but also on the dipole energy loss
- 	 where 	***nat*:* ** *equilibrium horizontal emittance
*	**	**	****o*: bare lattice emittance
- 			*U**w** *: energy radiated by the wigglers and
- 			*U**o** *: energy radiated by bend magnets (7.17 [MeV] / o [m])
- Namely, effectiveness of damping wigglers is enhanced by reducing *U**0*, i.e., increasing the dipole bending radius  (a soft bend)

## Emittance and Energy Spread vs. Dipole ρo 

- As one increases the damping wigglers, reduction of emittance is limited by increase in energy spread and energy loss (or available RF power)

## Limiting Emittance from IBS Effect

- The reduction  of total emittance per unit change of the lattice dipole bend radius, taking the IBS emittance growth into account for 500 mA stored current and harmonic bunch lengthening.
- The optimum range is shown, with the NSLS-II value: 25 m

- This reduction of emittance cannot continue forever, when the emittance becomes as small as nm scale, particle diffusion due to intra-beam-scattering (IBS) becomes comparable to the natural emittance.

## SR Harmonic Number and Flexible Bunch Patterns

_**Harmonic Number of the Storage Ring**_
- RF Frequency = 499.68MHz
- HSR = 1320 (= 2 x 2 x 2 x 3 x 5 x 11): 791.472 m for SR circumference
- Bunch rotation period: 2.64 μsec
- Easily factorable H: A wider range of fill patterns, particularly for a mixing of timing bunches in a bunch trains.
- H = 1320 is convenient for synchronization with mode-locked lasers
_**Bunch pattern**_
- Normal with ion clearing gap
- Camshaft mode of operation for timing measurements:
- 		1, 2, 3, 4, 5, 6, 8, 10, - - - bunch train gaps with a higher current (~X2) 	bunch for timing experiments in the middle are possible
_**Pulse length**_
- Nominal without lengthening by 1.5 GHz harmonic cavities: ~15 psec
- with lengthening by harmonic RF: 30 to 50 psec depending of fill factor

## Diverse Beamlines from NSLS-II

**Diverse Beamlines from NSLS-II**

- Short ID straights (low ): high brightness hard X-rays beamlines
- CPMU for ultra-bright hard x-ray beamlines
- EPU for polarized x-ray beamlines
- Long ID straights: damping wigglers and their high power beamlines and other insertion devices
- Study in progress for canted DW’s to generate two beamlines from 1 straight
- Three-Pole Wiggler (TPW) in dispersion straights for hard x-ray beamlines, similar in flux as NSLS dipole radiation but ~100 times brighter (< 15)
- Soft bend dipoles for soft x-ray and UV beamlines
- Three pairs of wide-gap dipoles to provide large aperture beam ports for far IR beamlines

**B**

**C**

**A**

**D/E**

## Layout of a 6.6 m Low β Straight

**Layout of a 6.6 m Low ****β**** Straight**

- CPMU (U19 with 5 mm gap) in a 6.6 m ID straight

- Two EPU’s in a 6.6 m ID straight

## Layout of a 8.6 m High β Straight

- Two 3-m long Damping Wigglers in an 8.6-m ID straight
- 100 mm period, 1.8 T magnetic field

## Advanced Conceptual Design of Insertion Devices

- CPMU in rectangular vacuum box
- Two 1.5 m long unit with variable gap
- Maximum field at 150 K: 1.14T
- Maximum field at room temp.: 1T

- Fixed gap damping wiggler
- 3 m long, 1.8 T peak field
- With 15 mm gap

## Canting of Damping Wigglers

- A 7 m long damping wiggler can be divided into two ~3 m long wigglers with canting magnets in between
- With special design of the magnets near the front-end, the DW absorber system can be modified to handle the radiation with the total fan angle of 9.8 mrad
- This will allow 3.8 mrad canting of two 100 mm period DW’s (~3 m long) with ±3  mrad fan, providing a complete separation of two beams

- ± 0.25 mrad beam deviation from its nominal orbit and an additional ± 1 mm machining and alignment tolerance are allowed

- Impact of 3.8 mrad bend in the achromatic straight on the chromaticity, thus on the emittance control by wigglers must be evaluated, but should be small

## Three-Pole Wigglers

- The weak bend of dipoles: good for soft X-ray beamlines
- A larger number of hard X-ray beams is desired, particularly with the transfer of the NSLS beamlines in mind
- Introduce 3-Pole Wigglers at the upstream end of the second dipoles
- The similar radiation power level as the dipole beamline at NSLS, but with a brightness about 2 order of magnitudes higher
- However, impact of 3-Pole Wigglers to the storage ring is finite:
- 40 cm of space for the wiggler and another 40 cm on the opposite end of the dispersion straight, and repeat it for all sectors to maintain the symmetry added 24 m to the lattice length
- Addition of the radiation in the non-achromatic region results in ~10% emittance growth for 15 3-Pole Wigglers
- Small but a finite interference between the magnetic field of the dipole and 3-Pole Wigglers (~2/10,000 in dipole field integral), which can be easily compensated with trim current

## Extra-long Straight Sections

- There have been on-going discussions for extra-long straight sections
- Three extra-long straights can be implemented in the current lattice by shortening long straights in adjacent cells in the standard DBA30 configuration and adding the lengths to a short straight
- e.g., if 8.6 m long straights are shorten to 2.6 m, leaving a possibility of using a 1 m long warm Insertion device, the extra-long straight can be 16.4 m

- While evaluating the option of extra-long straights, we have decided to adopt standard DBA30 as the CD-2 lattice baseline

## Beam Stability Requirement

- Settling and vibration (natural and self-inflicting) of the accelerator tunnel and experimental hall floor/beamlines (Tunnel floor <25nm)
- Temperature stability (<0.1C)
- Mechanical engineering consideration (Resonant frequency: >50Hz)
- Magnet power supply and RF noise issue (Higher level of noise filtering)
- Closed orbit correction with slow and fast feedback (high precision user BPM’s)
**Achievement of good stability must be joint efforts of Conventional Facilities, Accelerator Systems, and Experimental Facilities Groups**
- _Beam Stability Workshop: April 18-20, 2007: Sam Krinsky_
- Report of the Workshop can be found at    http://www.bnl.gov/nsls2/workshops/Stability_Wshop_4-18-07.asp

**Beam Stability Requirement**

## Summary

- Optimized design of the NSLS-II storage ring, based on well proven DBA configuration, has a robust DA with low natural emittance
- Combination of damping wigglers and dipoles with a large bending radius provides sub-nm•rad horizontal emittance for ultra-high brightness x-ray beams
- With insertion devices, three pole wigglers and soft-bend dipoles including special wide-gap dipoles, NSLS-II can provide synchrotron light in a wide range of spectrum to users
- Damping wigglers also make high flux/brilliance hard x-ray beams
- With 27 ID straights, 3PW’s and bending magnets, and with possibility of canting ID’s, NSLS-II in its fully developed state can support more than 58 beamlines for the user program
- Expected accelerator performances is expected to meet stringent requirements
- Further optimization of lattice and beam stability as well as engineering and design, cost estimating, and schedule planning are in progress in preparation of the CD-2 review in November this year