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DAFNE ACCUMULATOR
The Accumulator during installation.
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 ~2x10
4Linac 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.
Schematic layout of the DAFNE Accumulator.
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.
DAFNE ACCUMULATOR http://www.lnf.infn.it/acceleratori/dafne/accum.html
The DAFNE Accumulator dipole on the magnetic measurement bench.
The DAFNE Accumulator quadrupole under measurement with a rotating coilsystem.
The DAFNE Accumulator sextupole.
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
DAFNE ACCUMULATOR http://www.lnf.infn.it/acceleratori/dafne/accum.html
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.
The DAFNE Accumulator 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/m
2Sextupole maximum gradient 180 T/m
2Sextupole 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.
DAFNE ACCUMULATOR http://www.lnf.infn.it/acceleratori/dafne/accum.html
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).
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