1. Proposal for the DAFNE Variable Energy Design Study
C. Milardi, D. Alesini, A. Gallo, M. Preger, A. Drago, P. Raimondi, B. Spataro, S. Tommasini, C. Vaccarezza, M. Zobov Accelerator Division.
C. Sanelli, A Clozza, Technical Division.
D. Babusci, Scientific Division. D. Moricciani, Roma2.
41st LNF Scientific Committee 22 – 23 November 2010

2. Outline
Preamble
Proposal
Main issues
Collaboration
Conclusions

3. Preamble
Proposal for the DAFNE energy upgrade, named DAFNE_VE, in the framework of the FP7 call for infrastructures 2011 (deadline 25 November 2010)
The idea is not new a letter of intent has been already presented in 2006 for a collider called DANAE
DAFNE has been upgraded including a new conceptual approach to the beam-beam interaction, the Crab-Waist collision scheme which was promising to give in principle a luminosity of the order of 1033 cm-2s-1 .
The Crab-Waist concept has been successfully tested (L = cm-2s-1 )
Since then it is reasonable to consider the Crab-Waist as a basic concept for a variable energy collider.
The submission of this project to the European Community is still sub iudice: the INFN management should take a decision in few days (23 November 2010).

4. Design study strategy 0.6 GeV ≤ ECM ≤ 3.0 GeV
The proposed design study is aimed at verifying the feasibility of an electron/positron collider working at variable center of mass energy:
0.6 GeV ≤ ECM ≤ 3.0 GeV
providing a luminosity
1032 ≤ L ≤ 1033 cm-2s-1
reusing as much as possible the infrastructure of the DAΦNE accelerator complex in Frascati.
Search in the field of high energy physics requires large and expensive infrastructures requiring long time for design and construction. For this reason there is a generalized tendency around the world in reusing as much as possible existing facilities; it is the case for several colliders: BEPC2 in China, VEPP2000 in Russia and Super KEKB in Japan.

5. Why changing the energy @ DAFNE is not possible?
DAΦNE with Crab-Waist achieved quite high luminosity, very close to the experiment requirement for this project, and still there is room for further improvements
However due to:
The IR based on permanent magnets elements (quadrupoles and dipoles)
The injection system
RF system specification
only minimal variation around the nominal energy are possible
-2.% ≤ ECM ≤ +2.%
with a progressive luminosity reduction up to a factor two
-1.3% ≤ DEcm ≤ 0.7% MeV
Lpeak
-13. MeV ≤ Decm ≤ 7. MeV

6. DAFNE layout after the 2007 upgrade
Colliding Rings Lattice
The ring lattice needs to be flexible from the optics point of view in order to allow a wide range of variability for the betatron functions, emittance, momentum compaction and to guarantee suitable dynamic aperture and lifetime
Double ring collider
One interaction region based on the CRAB-Waist collision scheme
Ring arcs will be redesigned relaxing the long straight sections
Wigglers necessary to provide damping, essential for low energy operation, could be moved in the straight section opposite to the interaction region
Bending magnets in the arc must be compatible with operation at different energies; a first evaluation suggests that a normal conducting option is feasible
DAFNE layout after the 2007 upgrade

7. Rings can fit in the DAFNE hall
B (.5 GeV) ~ 1.2 T
B (1.5 GeV) ~ 1.7 T
B (0.3 GeV) ~ 0.33 T
Magnet type
Q (rad)
(m)
@ 0.51 GeV
@ 1.5 GeV
L magnetic
Sector long
0.7874
1.53 3.
2.3622
Sector short
0.7834
1.27
2.3502
Parallel long
1.4
2.5918
Parallel short
2.1206
Main Rings Toy-Layout

8. Interaction Region
The interaction region has to meet the different requirements coming from operation at different energies and with different values of the detector magnetic field.
It will be based on the Crab-Waist collision scheme
The low–b section will be based on superconducting quadrupoles including skew and correction coils to provide betatron coupling and beam trajectory correction respectively
The interaction region design must include purpose developed screens to shield the background hitting the experimental apparatus. Their design must be compatible with the vacuum chamber layout and the impedance budget of the rings.

9. Beam-beam effects
Simulations of beam-beam interaction should indicate which ultimate luminosity can be achieved in the upgraded DAΦNE at different energies and help in optimization of the collider performance in terms of lifetime and luminosity in realistic operational conditions.
For this purpose we plan:
to perform a series of numerical simulations of beam-beam collisions taking into account all kinds of nonlinear lattice elements such as wigglers, sextupoles, octupoles etc.
to develop a fast numerical code capable to simulate a self-consistent beam size evolution of both colliding beams (“quasi strong-strong approach”)

10. Dynamic Aperture
Dynamic aperture (area of particle’s stable motion) and energy acceptance are of crucial importance for beam quality and beam lifetime in an accelerator.
These are defined by nonlinear machine optics, working point choice and crosstalk between beam-beam effects and lattice nonlinearities.
A careful numerical modeling of particle’s motion in the 6D phase space is necessary in order to optimize the collider performance in all the energy range.
This item is particularly important for the DAΦNE_VE due to the presence of strong nonlinear CW sextupoles installed in the interaction region.

11. Radio Frequency
The RF system of the rings will provide turn-by-turn restore of the energy lost by the beam due to synchrotron radiation emission and parasitic interaction with the vacuum chamber elements, as well as beam longitudinal focusing.
The RF system design study is intended to define the RF frequency value, number and type of RF cavities (including special parts such as input couplers and tuners), number, type and rating of the RF power sources, specifications and structure design of the low-level RF control, how much hardware of the existing infrastructure can be reused and what performance are expected.
the beam parameters at the maximum nominal energy define:
RF power request (> 100 kW)
maximum accelerating voltage
low energy will set the design criteria for system stability and low-level control

12. Impedance and Instability
Parasitic beam interaction with the surrounding vacuum chamber can result in several harmful effects:
destructive beam instabilities, both single- and multi-bunch, excessive overheating of vacuum chamber components
damage of collider diagnostics
In order to avoid these phenomena all the new vacuum chamber components will be designed taking care of beam coupling impedance minimization and suppression of parasitic higher order modes.

13. Bunch by bunch Feedback systems
Lepton collider performances in terms of luminosity strongly depend on the maximum achievable beam currents and, in turn, on the bunch-by-bunch feedback systems used to damp both synchrotron and betatron instabilities.
The very fast technological advance of the electronic components has also given the opportunity to make more compact and powerful digital feedback systems.
Feedback systems have proved to be a powerful diagnostic tool to investigate several aspects related to the instability formation and build up.
In the specific case is necessary to evaluate carefully the bunch-by-bunch feedback design in the framework of a complex scenario including different optics layouts and radio frequency setups as well as flexible energy range and injection system schemes.

14. Injection System Low Energy:
Operation below 0.51 GeV poses no problems
High Energy operation (above 0.51 GeV)
Injection at .51 GeV and ramping in the Main Rings
Beam must be dumped at every injection
Top-up injection unfeasible
Insert a 1 GeV Linac between the Accumulator and the Main Rings
The necessity to have a large energy gain per unit length suggests the use of the C-band technology.
A bunch compressor is required to confine the bunches in the C-band accelerating crest region to limit the beam energy spread growth during acceleration.
Build a slowly cycling 1.5 GeV Synchrotron (≈ 1 Hz) where the particles are stored with multiple pulses at 0.51 GeV, or lower and then ramped up to the desired energy and injected into the Main Rings

15. Radio Frequency for the Injection System ring
RF system for the energy ramping synchrotron option has to be studied and designed, at a frequency lower than the collider rings one to optimize the synchrotron acceptance.
Study about the accumulator RF system are also necessary to investigate the feasibility of a different injection option based on a post acceleration in a C-band linac of the beam extracted from the accumulator ring

16. Vacuum system
The design study of DAΦNE_VE vacuum system will address issues concerning the following items:
Vacuum chamber design for the interaction region compatible with the detector and the collider requirements.
Design of new vacuum chamber for the ring arcs suitable to cope with the higher synchrotron radiation power emission.
- Thin film deposition techniques to reduce the vacuum chamber wall secondary electron yield to reduce the positron beam instabilities arising from e-cloud build up.

17. High precision measurements of the beam energy
The measurement of R (ratio of electron-positron annihilation cross section into hadrons to muon pair) requires the knowledge of the energy of the beams circulating in the collider with an accuracy of keV.
A technique to perform a precise measurement of the energy of an electron beam is based on the Compton backscattering of laser photons against the electron beam.
A typical experimental apparatus includes:
CO2 laser (w0 = eV, ∆w0/w0 = 10-7)
high-purity germanium detector for energy spectrum measurements

18. Partners LNF (Laboratori Nazionali di Frascati I)
UNILIV (University of Liverpool UK)
UU (Uppsala University, Department of Physics and Astronomy SE)
BINP (Novosibirsk RU)
LNF
UNILIV UU
BINP
Alesini
Hock
Ruber
Levichev
Drago
Wolski
Ziemann
Voblyi
Gallo
Okunev
Clozza
Shatilov
Milardi
Piminov
Preger
Bogomyagkov
Spataro
Tommasini
Sanelli
Vaccarezza
Zobov
Partner expertise fields:
Lattice
Optics
Magnet design
High field magnet
Beam-beam
Beam Dynamics
Vacuum System
RF Systems
Accelerating structure
Detector interface
Contact with industry

19. Conclusions
The presented R&D activities will lead to: an accurate study for the collider
the definition of the costs for its realization
the evaluation of its the impact on the existing LNF infrastructures