FIRST BEAMS IN SOLEIL STORAGE RING

Activity during the first four months of the year 2006 was extremely intense for all the teams as they worked to complete the installation of the storage ring within a reasonable time frame in spite of the difficulties encountered. Different types of problems on vacuum chambers with NEG and the crotch¹ absorbers (see RS-13) slowed down the completion of the storage ring vacuum system, and the helium circuit feeding the cryomodule cryogenic system compressor had to be completely rebuilt. Also, inevitable delays were encountered in cabling and distribution of fluids.

On May 4, the last vacuum chamber was installed and on May 12, 12 out of 16 cells were baked out. On May 13, the commissioning of the storage ring without RF (still unavailable) began.

Four insertions had already been installed, including one in-vacuum undulator (HU640_DÉSIRS, HU80_TEMPO, HU256_CASSIOPEE and U20_PROXIMA1), as well as definitive insertion vacuum vessels in mid-size straight sections, specifically ten chambers with low vertical opening (10 mm), and two chambers 10 m of length in long straight sections with 14 mm vertical opening.

Alignment of the machine was conducted simultaneously with installation and baking out. Also simultaneously, equipment was carefully verified and its command controls tested and validated. The polarities of all the storage ring magnets (dipoles, quadrupoles, sextupoles, and correctors) had been verified in terms of tension and field orientation. A model of the lattice, including the results of magnetic measurements, had been prepared. Most of the application programs necessary to the initial phase of the commissioning² had been developed. These verifications and tests without beam also permitted the identification and correction of different problems that would have complicated the commissioning with the beam.

For the first time on May 14, 2006, at 2:00 a.m., the beam could circulate for almost 3 turns, with only the dipoles and quadrupoles powered according to the values of the theoretical model. After correction of the first turn trajectory, the beam executed fifty turns while the sextupoles were still turned off. Figure 1, below, shows the successive turns recorded by an FCT (turn by turn current monitor) located in the last quarter of the storage ring.

Figure 1: Several turns in the Ring without sextupoles or RF.

Such a result shows the quality of alignment of all the equipment (magnets, girders, and vacuum chambers) especially when one takes into account the narrow opening of the straight-section chambers. In the same vein, it emphasizes the reliability of the magnet measurements. This result was confirmed a little later via the measurement of the uncorrected closed orbit³ : Horizontal orbit deviation (rms) = 2.3 mm; Vertical orbit deviation (rms) = 0.4 mm.

The setting of the sextupoles to their theoretical values significantly improved the transmission of turns and increased their number. Figure 2 illustrates the beneficial effect of the sextupoles.

Figure 2: Improvement of transmission and increase in the turn number with the sextupoles at their theoretical values.

Several sessions were, therefore, dedicated to radioprotection measurements to verify the efficiency of the shielding. These measurements were taken in the most unfavorable conditions possible, specifically the total loss of the Booster beam (1 mA) in different parts of the storage ring, which represents losses 30 times greater than for nominal injection conditions. Following the results of these measurements, authorization was given to continue the commissioning during normal work hours simultaneously with the different activities in the synchrotron building.

The two last weeks in May were dedicated to the bake out of the last four cells, the finalization of cabling of vacuum automats, the connection of cooling circuits of the vacuum chambers, crotches, and front-ends, the improvement of the command controls, and finally RF conditioning of the supraconductive cavities which, it must be indicated, was carried out in record time (less than one week). An initial period of three uninterrupted weeks organized in 3 shifts of 8 hours was scheduled. In reality, following problems with the blocking of water circuits by resins (see the paragraph problems encountered) the effective time with beam was reduced for this first part to around 14 days.

An initial beam of 0.3 mA could be stored on June 2, 2006 at 2:00 a.m. for 15 minutes. Below is a photo, everybody smiling, taken during the storage of the first beam (figure 3). The first accumulation ⁴, up to 8 mA, occurred on June 4, 2006, near 3:00 a.m. The image of radiation synchrotron photons is given in figure 4.

Figure 3: Photo taken during storage of first beam.

Figure 4: Image of synchrotron radiation at the exit of a dipole of the Ring.

The increasing of the current progressed rapidly after that: 20 mA on June 7; 30 mA on June 10; 50 mA on June 16; and 85 mA on June 17, 2006. At the end of this initial period, on June 19, the integrated current reached 2.7 A.h, as shown in figure 5 below. The filling method used most often,due to its being the most favorable, was ¾ of the bunches filled. We would like to point out that as with the injector, all of the equipment operated with remarkable stability from the very first day.

Figure 5: Intensity of current (red) and integrated dose (blue) after 14 days of the beam.

As planned, two weeks of shutdown followed this first part of the commissioning with beam. They were dedicated to equipment maintenance, the completion of various cablings, and the cleaning of the cooling circuits in the power supplies racks and in all the storage ring magnets, and in all vacuum chambers. The final vacuum chamber of dipole D1 of the C02 cell was also installed to prepare the assembling of the IR beamline, SMIS.

Tests with beam were resumed on July 3, 2006, and proved to be extremely reproducible. Systematic measurements of different parameters of the optic were carried out and analysis of the results is in progress. The final objective is to bring the real machine and the design as close together as possible to reach our objectives in terms of performance.

On July 11, 2006, the dose of integrated current was increased to 5 A.h. The maximum current reached values passing 100 mA (~123 mA) but the current intensity decreased in an extremely abrupt way to 50 mA, subsequently showing an exponential decrease characteristic of lifetime corresponding to various known mechanisms. Following several observations and appropriate measurements, it would seem that the trapping of positive ions by the⁵ potential electron beam combined with the presence of obstacles in the vacuum chamber explains this dramatic drop in beam lifetime at the end of this initial phase.

During the following 8 weeks of shutdown, numerous interventions were conducted on the machine. The HU256_PLÉIADES and U20_SWING undulators were installed, as well as the final vacuum chamber for the IR_AILES line. The vacuum chambers crossing the shielding wall for the SMIS, AILES, CASSIOPÉE and DÉSIRS lines were installed, as well as all the corresponding shielding and the cabling of the safety system of the first lines and front ends was completed. Finally, a systematic gammagraphy campaign was launched on all straight sections bellows to check for possible obstacles in the vacuum chambers.

On September 5, 2006, tests with the beam resumed, and the results have been even more promising. As before, the reproducible adjustment of the storage ring is excellent, and the drastic drop in high-intensity current has disappeared. This improvement has followed the disappearance of obstacles in the vacuum chamber subsequent to the intervention that took place during the month of August (see the paragraph entitled problems encountered). Figure 6 below shows the diminution of the electron beam during a night dedicated to the conditioning of the vacuum chamber. The maximum current achieved was 100 mA, limited by the significant increase of temperature in one of the crotches where cooling flow was still very low. As shown in figure 6, we see a decrease in current near an exponential behavior.
On September 11 the current reached a value of 201 mA and finally on September 20, 300 mA were stored, the maximum current available with the current RF system (the second cryomodule necessary to reach 500 mA is under construction and will be installed on the machine mid 2007).

Note: Radiation measurements are regularly taken all around the tunnel (with beam) and inside the tunnel after a beam shutdown. The maximum intensity of the current authorized at the moment during working hours is 80 mA, probably limited by the Bremsstrahlung⁶ radiation created in straight sections. This limitation of current should be able to be increased rapidly thanks to the improvement of the vacuum by conditioning⁷.

Figure 6: Natural diminution of current intensity in mA (in red) and integrated dose in A.h (in blue) during conditioning sessions at 100 mA.

The goal of this second part of the commissioning is to continue the characterization of the storage ring optics and the conditioning of the vacuum chambers, to investigate the stability of the beam position, and to study the effects of the first insertion devices and their compensations.

Small balls of resin, emitted from the demineralized water treatment center, were accidentally released into the 21°C water circuit, blocking several filters in the magnet circuits, power supply racks, and crotches. This is the major and recurrent problem of this initial phase of the “commissioning”. On one hand, the available beam time between the unblocking and the re-blocking up of the filters of the different equipment is fairly low; on the other hand, the maximum possible current is limited by the too weak flow of certain crotches. We are still awaiting a total and definitive eradication of this problem.

The other problem encountered in this first part of the commissioning is the discovery of obstacles in the vacuum chamber. This is a problem that was evident from the beginning of the commissioning process, thanks to gamma radiation and neutron measurement as well as different storage ring diagnostics. The gammagraphy campaign launched during the shutdown during the month of August 2006 revealed the presence of obstacles in the tapering transition of the vacuum chambers in the short straight sections. This is a systematic problem with the RF fingers of the bellow shielding, due to a manufacturing defect that is not visible when the entire bellow is mounted. An “in-house” solution was implemented for the three most critical defects to restore the passage of the beam. During the next shutdown, the problem will be corrected for all bellows in the short straight sections.

The table below summarizes the most significant events that have taken place since the beginning of the “commissioning” of the storage ring. Progress has been rapid and will allow us, between now and the end of the year, to attain stable operating conditions compatible with the use of photon beams to conduct initial experiments on beamlines.

Date	Event
2006
13/5	commissioning begins
14/5	first turns without RF
01/6	50 turns without RF, without sextupoles
01/6	>70 turns with sextupoles and without RF
02/6	first beam stored
03/6	first photon beam visible on the MRSV
03/6	evidence noted of the presence of an obstacle in the C03 cell
04/6	first beam accumulated: 8.35mA
04/6	first orbits corrected (theoretical matrices)
07/6	maximum current: 20mA, first correction of chromaticities.
10/6	30mA.
13/6	opening of problematic part for location and repair of obstacle
14/6	40mA .
16/6	50mA .
17/6	85mA .
17/6	integrated dose: 2.7 A.h
03/7	resumption of beam without problem after 15-day shutdown
04/7	100mA.
10/7	integrated dose:5 A.h
11/7	123mA.
11/7	end of the first part of phase I
05/9	resumption of beam without problem after an 8-week shutdown
05/9	100mA stored in ¼ of the Ring with a life span of 1.2 hrs
11/9	200mA stored in ¾ of the Ring and integrated dose of 11 A.h
20/9	300mA stored in ¾ of the Ring and integrated dose of 32 A.h

¹ Crotch : Localized absorber implanted in the dipole chamber, specially designed to let the appropriate photon beam for each type of line through and absorb unused power.

² Commissioning : testing phase which lasts until the desired performances are obtained.

³ This is the beam position deviation from the theoretical trajectory, all around the storage ring.

⁴ At each new injection, the new beam injected adds itself to the already-stored beam.

⁵ Created by the ionization of residual gas molecules by the electron beam.

⁶ Bremsstralhung radiation: very high-energy parasite gamma radiation resulting from inelastic collisions between electrons and residual gas.

⁷ Regular injection (every 20 minutes in this case) to keep the current constant and as elevated as possible in order to accumulate a strong dose of integrated current. Initial estimates calculate 40 A.h to obtain 10 hours lifetime at 100 mA.