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Status of DAFNE Project and Beam Dynamics Activities at LNF

L. Palumbo
LNF, Frascait, Italy3mm

DAFNE Status

The etex2html_wrap_inline1173etex2html_wrap_inline1175 tex2html_wrap_inline1177-factory DAFNE is presently under construction in Frascati (Italy) [1]. It is designed as a double ring system with a maximum number of 120 bunches/beam. The short term luminosity goal is tex2html_wrap_inline1179 with 30 bunches while the designed luminosity tex2html_wrap_inline1181 should be achieved in a period of at least 2 years of continuous operation.

DAFNE is provided with a full energy injector composed by a Linac, an Accumulator ring and tex2html_wrap_inline1183 m of transfer lines [2]. Both electron and positron beams are produced and accelerated up to the nominal energy of 510 MeV by the Linac and then stored into the Accumulator Ring for phase space damping before being injected into the Main Rings. The injection in the Accumulator is performed with a repetition rate of 50 Hz, while the extraction is operated at 1 Hz. The electron and positron modes are non simultaneous and the estimated time necessary to completely fill all of the 120 buckets of the Main Rings is tex2html_wrap_inline1185 min for the positrons and tex2html_wrap_inline1187 min for the electrons. Although the expected beam life time in the Main Rings is tex2html_wrap_inline1189 hours, a ``topping-up'' scheme is foreseen: the integrated luminosity is optimized by injecting approximately every hour.

The injector is completely installed and under commissioning, the main rings installation is in progress and it is expected to be complete by Spring 1997.


F. Sannibale

The DAFNE LINAC, built by the American company TITAN BETA, is a tex2html_wrap_inline1191  long Linac designed to produce and accelerate, with 50 pps, electron and positron beams up to 800 and 550 MeV respectively (max.). The macro-bunch length is 10 ns FWHM and the nominal macro bunch peak current is 150 mA for electrons and 36 mA for positrons. The positron conversion configuration includes a metallic target, a pulsed 5 T solenoidal lens (SLAC Flux Concentrator) and tex2html_wrap_inline1193 m of 0.5 T uniform solenoidal field. The electron beam nominal parameters at the positron converter, 250 MeV energy, 4 A current and 1 mm (tex2html_wrap_inline1195) spot size, should ensure a positron yield of 0.9%. The RF network includes 4 S-band (2856 MHz) 45 MW klystrons followed by energy compression devices (SLED) while the accelerating scheme is composed by an harmonic prebuncher, a five cells buncher and 15 SLAC type 3 m long accelerating sections. A complete set of diagnostics, including beam position, profile, current monitors and a positron separator, permits the transport of the beam along the Linac.

Linac Experimental results

A preliminary set of electron beam tests were first performed at TITAN BETA on May 1994. The Linac was assembled at factory up to the positron converter and a 4 A , 1 mm spot size, 240 MeV electron beam was accelerated to that point.

The Linac electron tests at Frascati started in January 1996. The beam nominal values for the Accumulator electron injection, 510 MeV energy and current tex2html_wrap_inline1197 mA (500 mA), were rapidly achieved and in June 96 about four weeks of shifts were dedicated to the Accumulator electron commissioning. During this period the Linac operated in a reliable way without presenting relevant problems.

The first positron tests were done on July 96 when in about three shifts a positron beam was accelerated at the end of the Linac with 4.3 mA of current and 435 MeV of energy. The tests were performed with one of the four klystrons seriously damaged: because of arcing phenomena it delivered just 5 MW instead of 45 MW. Moreover a failure in the electronic control, limited the electron gun output current to 4.5 A instead of the nominal 8 A. With this configuration the conversion energy and current were reduced to 80 MeV and 1.5 A respectively. By scaling the achieved results with the nominal operation sets, the obtained values fairly match with the design ones.

Future plans include continuation of the Accumulator electron commissioning and completion of the Linac commissioning.


A. Ghigo

The DAFNE Accumulator is a 32.5 m long ring with a magnetic structure which can be easily optimized for injection: a small value of the dispersion function all over the machine enlarges the energy acceptance, and the RF frequency smaller that in the main rings widens the longitudinal acceptance. Two of the four straight sections are dedicated to the injection and extraction septa while the other two to the RF cavity and four kickers. The single bunch injection and the solution of extracting positrons from the electron injection channel (and vice-versa) have been adopted.

The energy and emittance acceptance, 11.5% and 10 mm.mrad at 510 MeV respectively, permit complete capture of the LINAC beams; moreover the low value of the energy spread and emittance of the extracted beams, 10.1% and 0.25 mm.mrad at 510 MeV respectively, make the injection into the Main Rings buckets easier.

Accumulator Experimental Results

The Accumulator commissioning started in June 1996, with the electron beam from the LINAC at 510 MeV energy, after the transfer lines completion. In four weeks the achieved stored current was 75 mA limited by the vacuum (no bake-out made after installation). The measurements of the horizontal and vertical betatron wave number were performed and the optical functions has been set to the design ones; the measured chromaticity values, without and with the sextupoles on, were in good agreement with the expected ones. The horizontal and vertical orbit measurements have shown that the mechanical alignment of all the magnetic elements is very good, with a maximum displacement of the beam from the axis, without correctors, of 7 mm. A wide radiofrequency stability range was measured (tex2html_wrap_inline1199) at central frequency tex2html_wrap_inline1201. The control system and all the main and ancillary systems worked quite satisfactorily. In the next future the injection efficiency in the Accumulator for electrons and positrons beams, the beam emittance and pulse lenght will be measured.


C. Biscari
LNF3mm F. Sgamma
A3mmDONE was run for the last time in Spring 1993 and the preparation of the hall for the installation of the DAFNE collider rings has lasted two years. The DAFNE group, together with the KLOE an FI.NU.DA. ones, were given the beneficial occupancy of the DAFNE hall in January 1996.

Installation of the collider rings and of the last part of the transfer lines is being performed in parallel with the injector commissioning. Optimization of the different actions according to the availability of elements and men-power is constantly updated.

After the installation of the alignment pillars, the machine supports have been placed and aligned. Installation of the hydraulic plant, air conditioning system and general electric plant is almost complete. About 50% of the magnetic elements are already placed, and the first alignment has been performed. Cabling (about 70 km) between power supplies and magnets is in progress. Rf system is ready to be installed, apart from one of the two cavities which will be available in two months. A great part of the vacuum chambers has been already delireved, vacuum tested and is being dressed with the diagnostic elements. The chamber dressings with ion pumps, Ti sublimators, photon absorbers is a delicate task to be done in a clean area, and for this purpose one of the experimental pit will be used. The beginning of this operation is foreseen in december 1996. The diagnostic cabling layout is being positioned in the hall. The cryogenic system which will used by the two larger experiments and by the four superconducting solenoids will be installed next summer; in fact it is not necessary for the machine commissioning which will be performed without any superconducting element.

The completion of the whole installation is foreseen for spring 1997.

Beam Dynamics Activity

Beam-Beam Tail Study

M. Zobov

A preliminary study of beam-beam interaction for DAFNE, including a scan of working points in the tune areas with respect to the luminosity, has already been performed and suitable working points have been found [3]. However, as far as the DAFNE damping coefficients are very small, long beam tails can be induced that cause lifetime and background problems. One of the reasons of the tail growth is the high order resonances which can manifest only in case of weak damping.

A "brute force" beam-beam simulation would take enormous CPU time to collect a satisfactory statistics at large betatron amplitudes. In order to save the CPU time (up to several orders of magnitude) we use the special dedicated tracking technique by Shatilov [4] to define the equilibrium distribution in the beam tails. In our simulations we also took into account cubic machine nonlinearities and the parasitic crossings (PC). In this study we concentrated only on good working points which were found [3]: small areas around tunes (0.09;0.07) and (0.53;0.06).

Simulation show that DAFNE can operate safely, without substantial bunch core blow up and with a reasonable lifetime, with up to 60 bunches per each beam. However, for the maximum possible number of bunches (120) some blowup and reduction in the lifetime are observed due to the smaller separation at the PCs.

Because of that we have undertaken a study to optimize the machine performance with 120 bunches. In particular, it has been proposed to increase the separation (in terms of tex2html_wrap_inline1203) by a factor of =882 without any change in the luminosity, tune shifts and the horizontal crossing angle itself. This can be done by decreasing the horizontal beta-function at the interaction point by a factor of 2 and increasing the vertical emittance by the same factor simultaneously. A slight change in the betatron tunes from (0.09;0.07) to (0.08;0.06) is also necessary in order to further improve the lifetime. This shift of the working point closer to the integer tunes helps us to avoid resonances resulting in particle's horizontal drift.

The performed simulation has confirmed that the beam-beam lifetime in that case is about 10 hours without any substantial reduction in the luminosity. Preliminary calculations have shown that the DAFNE lattice can be adjusted in accordance with the proposed changes. These results have been worked out with the collaboration of Dr. D. Shatilov from BINP-Novosibirsk
( who spent several weeks at LNF.

Impedance Calculations

L. Palumbo

The coupling impedance of a small hole in a beam pipe coupled with coaxial stuctures has been studied. By applying a modified Bethe theory which takes into account also the scattered fields in the external structure and therefore fufilling the energy conservation law, the real part of the impedance corresponding to the radiated fields can be derived. Two basic structures have been considered: infinite coaxial beam pipe, and a coaxial resonator coupled to the beam pipe, for which simple expressions of the low frequency longitudinal and transverse impedances have been derived [5, 6]. Numerical calculations performed with MAFIA code are in good agreement with the analytical results.

The problem of coupling through large apertures is now being studied. The Bethe and the field matching methods are applied to long narrow slots in order to assess the reliability of the Bethe perturbation theory, and get a deeper understanding of polarizability coeffients for such apertures Measurements on a coaxial cavity prototype are planned for the near future.

Microwave Longitudinal Instability

M. Migliorati

The single bunch behavior in the DAFNE accumulator machine has been investigated using both a time domain tracking code and an analytical study based on the Vlasov equation and mode coupling theory [7]. The numerical simulations show that the bunch is expected to be in the turbulent regime at the nominal current tex2html_wrap_inline1205. The microwave instability threshold has been found to be at tex2html_wrap_inline1207. The bunch lengthens from the natural value of tex2html_wrap_inline1209 cm to tex2html_wrap_inline1211 cm at the nominal current, and the relative energy spread widens from tex2html_wrap_inline1213 to tex2html_wrap_inline1215. Analysis of turn-by-turn beam sizes at the nominal current shows that the change in the bunch length does not exceed 4%, i.e., not dangerous for the machine operation.

Analytical study has been undertaken, and different models have been used to predict the microwave threshold value and to compare it with the numerical results. Application of Boussard criterion gives for the instability threshold a factor 3 lower than that of the simulations. The perturbation modal analysis of Vlasov equation gives a closer result. In particular the bunch shape can be approximated by a gaussian distribution function, and the azimuthal mode coupling theory can be applied. The corresponding strong microwave instability threshold is found to be 1.5 times the one obtained with the simulations. The double water-bag model allows the radial mode coupling to be treatedanalytically. The threshold of a weak instability due to a coupling of radial modes with different azimuthal numbers has been found at tex2html_wrap_inline1207, coinciding with the value found by numerical simulations. The planned bunch-lengthening measurements will allow us to check the analytical predictions in the turbulent regime.

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ICFA Beam Dynamics Newsletter, No. 11, August 1996