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INTRODUCTION

The main characteristics of this new generation of storage ring-based X-ray sources, is a substantial increase of the brilliance and coherence fraction of the source as compared to today’s X-ray beams. This objective should be reached without compromising the stability and reliability of Elettra, without increasing the total radiated power as well as not increasing the operational, instrumentation development and electricity costs.
 
The driving concept for this “ultimate” machine is the substantial reduction of the emittance of the stored electron beam, targeting emittance levels capable of providing a diffraction limited X-ray source also in the horizontal plane while such a limit has already been achieved at Elettra for the vertical plane. However, as the challenge is to implement it also for the horizontal plane, the new machine, Elettra2.0 aims to provide intense nano-beams in the range of VUV to X-rays for the analytical study of matter with very high spatial resolution.

 
Development in accelerator technologies during the last twenty years has led to many important results featuring new magnet design, innovative vacuum technology, and revolutionary beam monitoring and orbit feedback systems.  These new capabilities and technologies, which were not available or were at their infancy when the present Elettra storage ring was conceived, provide today a solid basis for the realization of the new machine.
 
Studies are being carried out at Elettra based upon new storage ring lattice design solutions that can be adapted to the existing Elettra storage ring tunnel, building and infrastructure including the injector, the beam-lines etc.  The outcome of these studies is a new lattice design, the realisation of which requires the substitution of the 12 arcs of the existing storage ring with 12 new arcs of almost identical length. The new magnetic lattice reduces the present horizontal emittance of 7 nm-rad down to 0.147 or 0.212 nm-rad at 2.4 GeV (reduction factor 49) largely increasing the brilliance and coherence of the X-ray beam, whilst preserving the injection system and the 12 straight sections of the existing ring.
 
The implementation of the storage ring upgrade will help Elettra to maintain its leadership for its energy range of synchrotron research by enabling new science and the development of new technologies to the general benefit. In particular, the science case supporting the Elettra storage ring upgrade represents a major step forward in synchrotron research, which complements and integrates the new possibilities offered by FERMI the Free Electron Laser in operation.

ACCELERATOR AND SOURCE UPGRADE

Much progress has been made in the framework of upgrade phase I in order to increase the reliability, reproducibility, operability and versatility of the Elettra light source. However a request concerning many experiments pertains to the reduction of the horizontal emittance. With the strong constraints of reusing the same tunnel and infrastructure, adding to the difficulty of the task, lattice studies carried out with success taking advantage the latest technical developments in almost all areas of the accelerator technology.

FRAMEWORK AND BASIC CONSTRAINTS

A new compact lattice that will replace the existing double-bend achromat is established [1, 2] which should meet the following requirements, most of them defined during various workshops dedicated on the future of Elettra held at Trieste from 2014 to 2019 and defined as:

 
  • Operating energy 2.4 GeV (and for some time also at 2 GeV)
  • Reduce the horizontal equilibrium emittance at least one order of magnitude 
  • Conserve the existing ID beam lines in LS at the same position
  • Conserve the existing dipole magnet beam-lines
  • Conserve the slots available for insertion devices
  • Preserve the present intensities and the time structure of the beam
  • Let open the possibility for installing bunch compression scheme
  • Include super-bends and in-vacuum undulators
  • Keep the present injection scheme and injection complex
  • Keep the same building and the same ring circumference (259.2 m)
  • Minimize the downtime for installation and commissioning to about 18 months maximum.
 

The experience already gained with the continuing efforts to improve the existing facility (still in progress), sets a privileged starting point for the complete renewal of the storage ring. The technology of the RF Solid-State Amplifiers (SSA) to be installed in the Booster and consequently in the storage ring will guarantee the requirements for high availability in the future. The expertise acquired in state-of-the-art vacuum technology during the last decade especially in long straight-section vacuum chambers, represents a solid basis for the design of a new vacuum system in the achromats.
The ultra-low vertical emittance under study for implementation, the orbit stability provided by the coming new beam position detectors, the fast orbit-feedback, the third harmonic cavity, the superconducting wiggler as well as some of the insertion devices are already compatible with the specifications of the new generation of storage rings. Certainly, R&D programs are still necessary to achieve all specifications required by the new design.

OPTICS DESIGN

The lattice S6BA-E is made from 24 arcs, 12 long straights and 12 short straights and has a 12-fold symmetry i.e. 12 equal achromats. Each section consists of 2 arcs separated in the middle by a short straight section of 1.26 m free space counting from/to adjacent arcs last sextupole while the free long straights connecting the sections are 5.4 m long counting from/to adjacent arcs last quadrupole. With that choice of lengths, the transverse position of the Elettra 2 beam lines on the long straight sections from insertion devices compared to the ones in the actual Elettra is almost coincident.The design lattice has a total length equal to that of the present Elettra, i.e. 259.2 m, and hasthe right balance between available free space (both in dispersion-free and dispersive areas) and emittance requirements. Each arc of Elettra 2.0 consists of 3 unit cells of the TME type (theoretical minimum emittance) i.e. 3 dipoles 1 only with vertical field gradient and 2 with combined transverse and longitudinal gradient, 8 quadrupoles four of which are shifted to give the required anti-bend angle and 10 combined sextupoles ( 6 with correctors for orbit control and 2 with skew quadrupole coils to control  the betatron coupling) and 2 combined multipoles (octupoles and correctors) for controlling the tune shift with amplitude. The magnets will be powered independently, although they may be grouped in families.
The first unit cell is the matching cell using a quadrupole triplet at each end of the straight sections to compensate forany optical distortions produced by the insertion devices (IDs). In general IDs with a magnetic field up to 1 T (undulators) are very well tolerated but for higher fields (usually wigglers) corrections are necessary. Similarly, the nonlinear effects of the IDs will be minimized by reducing the vertical beta function (at least for planar devices andespecially for in-vacuumdevices).
One section of the lattice is displayed in Fig. 1  Due to the energy requirement that Elettra 2.0 should operate mainly at 2.4 GeV (and only for some time at 2 GeV) all following values will refer to 2.4 GeV. Note also that the two energies operation excludes the use of permanent magnets in the dipoles and/or quadrupoles.

Figure 1: Lattice functions of the S6BA-E


Overal the beam dimensions assuming 2% coupling is shown in the next figure 1.2:

Figure 1.2: Beam dimensions at 2% coupling

The working point is (33.25, 9.16-9.4) and the natural chromaticities (-71,-70). All dipoles are having vertical gradient for dipole fields between 0.8 to 1 T and the maximum gradient is ≤22 T at 2.4 GeV (compared with 4 T/m in Elettra). In the LG dipoles the 1.4 T central field is without gradient. The quadrupoles have maximum gradients ≤50 T/m (compared with 18 T/m in Elettra).
The lattice is not isomagnetic, since the bending angles vary (the dipoles have:  two with 0.64 m length and bending angle of 3.6 deg and four with 0.80 m and 6.5 deg. while their respective normalized strengths are -2.21 m-2 and -2.6 m-2).  The quadrupoles come in 8 families, and their maximum normalized strength is 5.73 m-2. Four of them are moved transversely at -5.21 mm each producing a negative bending angle of -0.4 deg.
For the full intensity case (400 mA), a superconductive third harmonic cavity lengthens the bunch for stability and lifetime, in this case the Touschek lifetime will be more than 12 h at 3% coupling
 

Another interesting point of this lattice is that due to its low momentum compaction of about 1.2e-4, it can provide short stable electron bunches below 10 ps (FWHM) at low intensities with acceptable lifetime of about 12 h for a 10 mA total current but in that case only users accepting low intensity and short pulses can work. On the other hand, the inclusion of the deflecting (crab) cavities will provide 4 single tilted bunches of 2 mA each with length varying from 1.5 to 15 ps (FWHM) while every other bucket will be untitled and filled with 2 mA totalling to 400 mA, the advantage being that in that case all users can work simultaneously. For lifetime reasons a coupling range up to 10% has been foreseen. We intend to operate with a 2-3% coupling and the next table shows the beam dimensions for 3% and 10 % coupling in the long and short straight sections.

 
Energy 2.4 GeV LS at 3% cpl LS at 10% cpl SS at 3% cpl SS at 10% cpl
σx (um) /σ’x (urad) 36 /5.7 35/5.5 63/6 63/5.8
σy (um) /σ’y (urad) 3.2/1.9 5.7/3.4 3.5/1.8 6/3
Energy 2.0 GeV        
σx (um) /σ’x (urad) 30/4.8 29/4.6 53/5 52/4.8
σy (um) /σ’y (urad) 2.7/1.6 4.7/2.8 2.9/1.5 5.1/2.6
 
 
As one can see the beam dimensions in the dispersive short straight sections remain comparable to those without dispersion (long straights) due to the low dispersion (50 mm) making very attractive the installation of short IDs there. Four short sections will be occupied by the four RF cavities, leaving 8 dispersive, short straight sections available for insertion devices (5) and for instrumentation (3) if crab cavities for bunch compression will be installed in a long straight section (S2.2), if not the available short straights for the machine will be 4.

The following table shows the main machine parameters for both energies


The dynamic aperture with and without alignment errors is given in the next 3:

Figure 3: DA of the lattice with and without errors.


Although the dynamic aperture without errors is quite comfortable, when alignment errors are included (about 50 mm in position and 100 mrad in angle) a 25% reduction is observed. Certainly it may be laborious to inject off-axis into a ±6 mm horizontal aperture, especially as far as top-up efficiency is concerned, but this does not render the optics unfeasible because once the injected beam is stored the dynamic aperture still corresponds to more than 200 σ of the beam size (compared with 100 s in Elettra).

This type of lattice will increase the peak brilliance and the coherence fraction at about 40 times at 1keV as can be seen from the next figures 4 and 5.



Figure 4: Brilliance increase with the Elettra2.0 compared to Elettr



Figure 5: Coherence fraction for Electra (blue line) and Elettra2.0 (red line)




References

[1] Karantzoulis E., “Evolution of Elettra towards an Ultimate Light Source”, IPAC 2014, Dresden, June 2014, p. 258 (2014); http://www.JACoW.org
[2] Emanuel Karantzoulis,” Elettra 2.0 The diffraction limited successor of Elettra”, Nuclear Inst. and Methods in Physics Research, A 880 (2018) 158-165
[3] E. Karantzoulis and W. Barletta"Aspects of Elettra 2.0 design", Nuclear Inst. and Methods in Physics Research, A 927 (2019) 70–80
The complete document: Elettra2.0 TDR will be available in December pdf

Last Updated on Tuesday, 17 November 2020 15:36