DAFNE ACCUMULATOR

The DAFNE Accumulator is a small storage ring, which has been includedinto the DAFNEinjection system for the following reasons:

• With the design positron output current from the Linacand the DAFNEMain Rings longitudinal acceptance, injection of the full design currentinto one of the two rings requires ~2x10⁴ Linac pulses at 100%efficiency. Due to its Gaussian particle distribution, a small fractionof the stored beam hits the septum which separates the ring vacuum vesselfrom the injection line at each injection pulse and gets lost. In orderto avoid saturation, this fraction should be much smaller than the inverseof the number of injected pulses (in our case 5x10^-5). By injecting,as an example, 50 pulses into the Accumulator, and then extracting andinjecting into the MainRings, the tolerable fraction of lost particles drops to ~0.1%.
• The R.F. system runs at a very high harmonic of the revolution frequency(120) in order to allow storage of a large number of bunches to reach highluminosity. This is not necessary in an intermediate ring, where only asingle bunch is needed. It is therefore possible to run the AccumulatorR.F. cavity at a sub-multiple frequency of the MainRings one, increasing the longitudinal acceptance (from 2.7 to 13.4ns) and therefore accepting the full charge in the Linac pulse.
• After accumulating the desired current, injection into the Accumulatorcan be stopped for a short time to allow the beam to damp down to its equilibriumenergy spread and emittance, which are typically two orders of magnitudesmaller than the corresponding Linacvalues. In this way a high quality beam can be extracted from the Accumulatorand injected into the MainRings, thus avoiding the necessity of designing the Main Rings latticewith a larger physical and dynamic acceptance, and relaxing the requirementson the Main Rings magnets with substantial savings on the overall costof the facility.

The Accumulator is a quasi-octagonal ring with a total length of 32.5m on the nominal trajectory. Its lattice is made of four almost achromaticarcs, each consisting of two 45 degrees full iron H-type sector dipolemagnets with a small gradient to optimise the damping distribution, a quadrupoletriplet and two sextupoles to correct the ring chromaticity. All the dipolesare powered in series. The quadrupoles are connected into three independentfamilies, the sextupoles in two families.

The electron beam coming from the Linacis injected into the ring by means of a system of two septum magnets, thefirst bending the beam by 34 degrees and the second performing the finaldeflection of 2 degrees into a special 3.5 m vacuum vessel between twoachromats. The stored beam is extracted by a mirror symmetric system placedin the opposite straight section. The positron beam follows the oppositepath. The remaining two straight sections host the pulsed kicker magnetsused to deflect the beam at injection and extraction and the R.F. cavity.

A system of 8 correctors and 10 position monitors allows a careful correctionof the closed orbit in the ring with the purpose of optimising injectionefficiency. Two synchrotron light monitors and two stored current monitorsare also part of the diagnostic system. A transverse feedback system isimplemented on the ring: it consists of a stripline pick-up and a striplinekicker.

The vacuum chamber is fully stainless steel and a pumping system consistingof 18 sputter ion pumps is designed to reach an average dynamic pressurein the ring of 5 nTorr.

DAFNE ACCUMULATOR PARAMETERS

Energy	510 MeV
Circumference	32.56 m
Emittance	0.26 mm.mrad
Horizontal betatron tune	3.12
Vertical betatron tune	1.14
R.F. frequency	73.65 MHz
R.F. voltage	200 KV
Bunch average current	150 mA
Bunch length	3.8 cm
Synchrotron radiation loss per turn	5.2 KeV
Horizontal betatron damping time	21.4 msec
Vertical betatron damping time	21.4 msec
Longitudinal damping time	10.7 msec
Number of bending magnets	8
Bending radius	1.1 m
Bending angle	45 degrees
Bending operating field	1.55 T
Bending gradient	-0.66 T/m
Number of quadrupoles	12
Quadrupole bore diameter	100 mm
Quadrupole operating gradient	8 T/m
Quadrupole maximum gradient	12 T/m
Quadrupole magnetic length	30 cm
Number of sextupoles	8
Sextupole bore diameter	108 mm
Sextupole operating gradient	135 T/m2
Sextupole maximum gradient	180 T/m2
Sextupole magnetic length	10 cm

---

The damping ring is fully operational and its performance has reachedthe design values with both electrons and positrons. The capture, as wellas extraction, efficiency is very near 100% and the foreseen single bunchcurrent of 120 mA can be injected in less than 1 second at 50 Hz. The maximumstored current is near to 220 mA.

False colour image of synchrotron radiation spot from the Accumulatorbending magnet. The dotted plots on the left and bottom sides of the pictureare the vertical and horizontal linear density distributions.

During commissioning the optical parameters of the ring and the beamcharacteristics have been carefully measured, and found to be in excellentagreement with the theoretical predictions and field measurements on themagnets. The measurements of beam position at all available monitors confirmedthe reliability and precision of the alignment procedures followed by theLNF group during theinstallation of the machine: in fact the current in the dipole orbit correctorsnecessary to drive the beam on its ideal trajectory is much smaller thanits available range, and the injection and extraction efficiency does notchange when they are switched off.

Using the results of the measurements performed on the stored beam areliable optical model has been found, which is the primary tool to setthe machine at any desired working point by means of a limited number ofparameters set by the Control System. Several beam characteristics, interestingfrom the machine physics point of view, such as the bunch length and synchronousphase versus stored current, have been measured, and the results confirmedthe predictions of the simulations on the interaction of the beam withits surrounding environment.

False colour representation of the bunch length measurement in the Accumulatorring. Each slice corresponds to a different stored current, increasingfrom left to right. Time is on the vertical scale. The intensity scaleis displayed on the left.

Longitudinal bunch shape measured at different current levels (0.5 to60 mA).

Dependence of the bunch length (full width at half maximum) on the averagecurrent. The dots are the measured values, the solid line is a numericalsimulation based on the machine impedance estimate.

Plot of the stored current into the Accumulator during atypical Main Ring commissioning cycle. Three electron pulses are storedinto the ring before the extraction pulse. The steps on the rising sideare injection pulses, those on the fall side are due to the limited resolutionof the scope.