Liu Lin Liu@lnls.br
LNLS, Laboratorio Nacional de Luz Sincrotrón
Cx. P. 6192, Campinas 13083 Sãn Paulo, BRASIL 3mm The Brazilian National Synchrotron Light Laboratory, LNLS, funded by the National Research Council of the Ministry of Science and Technology (CNPq), consists of a 1.37 GeV electron storage ring with a 120 MeV injector Linac. Commissioning of the Linac started in December 1995 and after almost simultaneous completion of all sub-systems in April 1996, commissioning of the storage ring at low energy started on May 1st. The first stored beam was observed a month later, in May 30. Some weeks later we could capture a current of 3 mA (after one damping time) with the one-shot on-axis injection. The off-axis accumulation process (with 3 kickers producing a closed bump) was still very difficult and the accumulated current saturated at about 0.3 mA. Our efforts at this time split roughly into 2 main tasks with the machine being operated 24 hours a day, 7 days a week: energy ramping and accumulation at 120 MeV. While energy ramping evolved quickly with a rather systematic approach, accumulation turned out to be much more `intriguing' and subtle. The first stored beam at 1.15 GeV was observed late in July 96 but only in October 19, after many measurements (with some results which are understood and others which are not), empirical adjustments and Ttrial and frustrationsU have we succeeded in accumulating 20 mA at 120 MeV. Presently (November 96) we can store 48 mA at 120 MeV with about 90 seconds lifetime for an average pressure of about Torr. The lifetime is limited by elastic scattering on the residual gas. At 1.37 GeV the lifetime depends on the beam current and varies from about 15 minutes to an hour for currents from about 25 to 3 mA. The limiting effect in this case is Touschek scattering for higher currents. This limitation will become less restrictive with the upgrade (in progress) in the 476 MHz RF cavity cooling system, which presently allows maximum gap voltages of only 240 kV.
In order to optimize accumulation at 120 MeV the machine working point has to be set surprisingly close to the integer resonance, and . With the present beam lifetime (90 s) the Linac pulses are injected every 2 seconds, about a fifth of the radiation damping time. The repeatability of the injection conditions is very sensitive to the magnets cycling procedure. Due to the proximity of the magnets in the storage ring even a change in the magnet cycling sequence introduce large variations in the initial conditions of the magnets at low excitations. Following a standard cycling procedure the injection conditions can be reasonably repeated. Sometimes fine adjustments are needed, remarkably in the Linac energy. (especially if it is late afternoon!)
No clear evidence of ion trapping has been observed so far, although small (200 V) voltages in the clearing electrodes have been used to avoid slow (few seconds) oscillations in the accumulated current.
The machine has been intensively characterized at injection energy. Betatron and dispersion functions were measured and show good agreement with theoretical values. The chromaticity can be corrected by setting the sextupoles to calculated values. The beam horizontal size, as seen and measured on a synchrotron light monitor, is compatible with the expected from intrabeam scattering effects whereas the vertical size is larger than predicted from measured coupling effects. Several measurements were made to determine the ring horizontal and vertical apertures: with localized beam bumps, with single corrector excitation, with scrapers and with kickers. The experiments gave different results and we learned that clear experiments with clear results are very scarce. We could only conclude that there was no indication for a localized physical obstruction in the ring since no large asymmetries in the results showed up. The aperture measured with scrapers seem to give results which would not allow for accumulation.
Energy ramping from 120 MeV to 1.37 GeV can be optimized for minimum loss during ramp by setting up intermediate configurations with corrected orbits and tunes. In our present best ramping path there are configurations every 10 MeV from 120 to 200 MeV, and then at 300, 400, 1150 (nominal energy) and 1370 MeV. The tunes are kept at and up to 400 MeV; after this energy the working point tend to the design value of and at 1.15 GeV. The ramping time is 44 seconds and the ramping efficiency is about 75 % with beam losses occurring mainly at the very beginning of the ramp. The RF gap voltage is increased during the ramp from the initial 50 kV to 240 kV. This is done `manually' and improvements in the RF system may still increase the ramping efficiency.
Any one of the three implemented orbit correction algorithms (matrix, best correctors and harmonic correction) can be applied successfully both at low and high energies, with the difference that at high energy the ring model can be taken from the measured quadrupole strengths, whereas at low energy the tune has to be fitted to the measured values to produce an adequate model. At high energy, orbit stability is better than 20 mm. Localized beam bumps using 3 correctors can also be produced and have already been used to move the photon beam in an experimental station. The beam bumps were also used during the hard commissioning days (when nothing seemed to work) to scan the physical aperture looking for obstructions in the ring and to probe non-linear fields with tune measurements as a function of bump amplitude.
The commissioning results obtained up to this date show that there is no basic limitation in the low energy injection and accumulation preventing us from achieving the design specifications of the ring. The accumulated current at high energy and lifetime, although still small, allowed already the start-up of the commissioning of some beam-lines. The first EXAFS spectra were obtained in November 3rd; and the first VUV reflectivity spectrum in November 7th. Some hardware upgrades are in progress, including the construction of a new thin septum with thickness less than the present 6 mm; construction of a new thick septum with improved magnetic shielding to decrease its effect on the stored beam; improvement in the ripple of orbit corrector power supplies; improvement in the RF cavity cooling system; and upgrading the injection energy to 170 MeV by adding two klystrons to the Linac. Also vacuum will be improved by baking procedures as well as washing with high energy photons. (Concerning washing, we have accumulated up to November 13, 160 mA.h of high energy beam).