Elettra 2.0

INTRODUCTION

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.
 
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 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 at emittance levels capable of providing a diffraction limited X-ray source also in the horizontal plane while such a limit has been accomplished at Elettra for the vertical plane. However the challenge is to implement it also for the horizontal plane.
 
Development in accelerator technologies during the last twenty years has lead to many important results featuring new magnet design, innovative vacuum technology, and revolutionizing beam monitoring and orbit feedback systems.  These new capabilities and technologies, which were not available or were at their infancy at the time in which the present Elettra storage ring was conceived, provide today a solid basis for the realization of the new machine.
 
Studies are 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 studies outcome 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 0.24-0.28 nm-rad increasing the brilliance and coherence of the X-ray beam by a factor of 20, 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 called S6BA (symmetric six bend achromat) will replace the existing double-bend achromat  [1,2] while the following requirements were respected:

Operating energy 2 GeV
Reduce the horizontal equilibrium emittance from 7 to about 0.25  nm rad
Maintain the existing ID straight sections in their current state
Maintain the existing dipole magnet beam-lines
Maintain the free space available
Preserve the time structure operation and a multi-bunch current of 310 mA
Keep the present injection scheme and injection complex
Reuse, as much as possible, existing hardware (power supplies, vacuum system,
diagnostics, etc.)
Minimize the energy lost in synchrotron radiation 
Minimize operation costs and electric power consumption.
Limit the downtime for installation and commissioning to about one year.
 
 
The experience already gained with the continuous effort to improve the existing facility that is 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 of 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 necessary for the achievement of all specifications required by the new design.

OPTICS DESIGN
 
The new lattice will fit into the existing facility (in terms of ring circumference, periodicity and beam-line positions). It is based on a 6-bend achromat cell  being the right balance between available free space and emittance requirements. A 7-bend achromat cell was also studied that gave an emittance of 0.18 nm-rad but there was no space enough in the short straights for the installation of the radio frequency cavities without sacrificing a long straight section already dedicated to a beam-line.

 Two main configurations are examined displayed in Fig. 1 & 2  In the first configuration there are two lateral free spaces in the arc of about 1.3 m long whereas in the second configuration there is a central free space of 1.8 m while the lateral ones are reduced to 0.5 m. The solutions are equivalent from the point of view of lattice properties and the choice will be based on the study of interferences between the light exits with the magnetic elements and the minimization of the source point shifts.

Both configurations come with a triple choice of the second and fifth bending magnet. In the first one all bends are combined function electromagnets with a field of about 0.8 T and a gradient of about 20 T/m. In this case the dipole beam lines must obtain the radiation using a short wiggler with the implication of shifting the dipole beam line by about 7 degrees. In the second solution the second and fifth dipoles are replaced by strong permanent magnets of about 1.4 T with a bending angle of about 5.6 degrees, enough to serve a single dipole beam line with minimal position shift. A large variation of this configuration is studied whereby it can be exchanged by 3 piece electromagnet with the central section of high field without gradient. Those configurations give achromatic solutions with emittances 0.24 to 0.28 nm-rad respectively.
 
The permanent magnet solution can consist of either a short strong permanent magnet without gradient therefore accompanied by two defocusing quadrupoles each at each end of the magnet or by a series of permanent magnets, the concept of which has been developed here, the “bending-wiggler concept”. This configuration is a series of 7 short permanent magnets of about 13 cm each with one anti-bend, two strong 1.4 T dipoles without gradient while the others have a field of 0.8 T and gradient of about 18 T/m.
 
The optics of the Elettra2.0 for the “short PM dipole series concept” configuration (shown in Figure 1) has a long dispersion free section of 4.84 m and two dispersive ones of 1.33 m.



Figure 1: Lattice functions for the first configuration and for the "short PM dipole series conceptr" configuration.
In blue are the bending magnets, in red the quadrupoles and yellow combined sextupoles and correctors.

Overal the beam dimensions assuming 1% coupling is shown in the next figure 1.2:



Figure1.2: Beam dimensions
 
The optics of the Elettra2.0 for the second configuration (shown in Figure 2)  has a long dispersion free section of 4.84 m,  two short dispersive ones of 0.5 m and one big central dispersive of 1.8 m long.


Figure 2: Lattice functions for the second configuration

Since the dispersion is in general very low (50 mm maximum) the beam dimensions also in the dispersive short straight sections are comparable to those without dispersion (long straights) making very attractive the installation of short undulators there. In the first configuration only the right dispersive section of 1.33 m can be used since the left one has interferences with the light exit of the undulators of the long straight sections. In the second configuration all dispersive central sections of 1.8 m can be used making the seconf configuration more attractive. In both cases 4 sections will be occupied by the 4 rf cavities leaving 8 dispersive short straight sections available for IDs. It seems that the second configuration is better from the interferences point of view minimizing also the  shift of the beam lines.

In order to achieve such small beam dimensions strong/combined magnets are needed. For example the dipoles in spite of having a dipole field of about 0.8 T (which is less than the nominal 1.2 T of the actual Elettra at 2 GeV ) need to have a quadrupole gradient of about 18 T/m to be compared with the 3 T/m of the actual dipoles. Similarly the quadrupoles have 3 times higher gradients that increase the chromaticity of the machine by a factor of 2-3. To correct the chromaticity stronger sextupoles are needed that naturally reduce the dynamic aperture of the accelerator introducing also other non-linear effects. 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 40% reduction is observed. Certainly it may be laborious to inject off-axis into a ±7 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 200 sof the beam size
(compared with 100 s in Elettra).


This type of lattice will increase the peak brilliance and the coherence fraction at about 20 times at 1keV as can be seen from the next figures 4 and 5.




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




Figure 5: Coherence fraction for Electra and Elettra2.0 At 1keV is 0.8% for Elettra and 28% for Elettra2.0
 
In the next table  the main parameters of Electra 2.0 as compared with the presentl machine are shown.
                                                                  Elettra and Elettra 2.0 main parameters
Parameter Units Current Elettra Elettra 2.0
Circumference m 259.2 259.2
Energy GeV 2 - 2.4 2
Horizontal emittance pm*rad 7000 230-280
Vertical     emittance pm*rad 70 (1% coupl) 2.5
Beam size @ ID (sx,sy) mm 245 , 14 (1% coupl) 43,3
Beam size at short ID mm 350 , 22 (1% coupl) 45,3
Beam size @ Bend mm 150, 28 (1% coupl) 17,7
Bunch length ps 25
(100 with 3HC )
12.5
(70-100 with 3HC )
Energy spread DE/E % 0.08 0.07
Bending angle deg 15 3.6  and 2x5.7
 
The above studied optics have a momentum compaction factor of about 3 10-4 i.e. an order of magnitude less than that of actual Electra meaning that the zero current bunch length will be 3 times smaller or about 1.8 mm at 2 GeV. Still this bunch length is large compared to pulses at FELs. and in general these lattices need longer bunches for stabilization and lifetime. This usually is achieved by employing a higher harmonic rf system (already existing at Elettra). If also short bunches are requested one can think of a double higher harmonic rf system creating short and long bunches similar to a scheme proposed by BESSY. In this case maybe both short and long bunches will be available at the same time.
If a larger repetition rate is required, actually the rf system operates at 500 MHz, one can study rf systems at a higher frequency maybe up to 2 GHz although such systems may have a few drawbacks due to their high frequency in cw.

ENERGY SAVING
The new  source will be designed with  the objective  of  increasing  significantly  the global energy efficiency  in  order  to  reduce  electricity  consumption.  In fact, the cost of electricity currently represents a significant fraction of the ELETTRA running costs.  Electricity costs are expected to increase further in the coming decades and consequently improvements in two main domains have been identified: 
1) The  RF  systems  will  be  tailored  to  the  reduced  losses  per  turn (about 22% reduction to those of present Elettra),  with  an  expected reduction of about one third with respect to the present RF power consumption due also to further developments such as solid state amplifiers.
2) An  increase  in  the  efficiency  of  the  production  of magnetic  fields  for  the  lattice:  New designs will produce stronger magnetic fields with less power consumption. Our intent to use permanent magnets where possible will further enhance the energy efficiency. Air cooled magnets are considered that will further save electric energy costs compared to the magnet water-cooling system of the current Elettra consuming about 1MW.
The  overall  power  consumption  of  the  storage  ring,  excluding  the  injector,  is  expected  to  be  about 2/3 of the present value. This will counteract the forecasted increase of energy costs over the 10 to 15 years to come.


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]    Karantzoulis E., “Elettra2.0-The Next Machine”, IPAC 2015, Richmond, May 2015
[3]    Karantzoulis E.,Carniel A.,  Krecic S. and Pasotti C., "Elettra Status and Upgrades", IPAC 2016, Busan, Korea
 
 
The complete document: Elettra2.0 technical CDR can be obtained from    pdf

Last Updated on Tuesday, 08 August 2017 10:15