IARPE - Laboratori Nazionali di Frascati

Laboratori Nazionali di Frascati

News from LNF

The first electron beam has been stored into the electron ring of the DAFNE collider on October 25, 1997 at 10:30 p.m. During a shift started at 2 p.m. the 510 MeV electron beam has been transported to the injection point of the electron Main Ring. With the RF cavity off it was possible to keep the beam inside the ring for ~0.3 ms, which is the maximum value allowed by the energy loss due to the emission of synchrotron radiation and the aperture of the vacuum chamber. The RF cavity was then switched on and the beam captured and stored in the ring. The position of the beam along the ring has been measured by means of the beam position monitors and found to be in agreement with the alignment tolerance. Multistacking was also successfully performed.

On Tuesday, November 18, 1997, a new series of commissioning shifts was started by setting up the LINAC with positrons. After tuning up all subsystems, the nominal LINAC performance with positrons (33 mA @ 510 MeV) was reached with high reliability on Wednesday evening. It took about one hour to transport the beam correctly through the positron branch of the Transfer Line up to the fluorescent target at the entrance of the Accumulator. As expected (since the Accumulator is already working at the design performance with electrons), a first beam of 15 mA was immediately stored. The injection rate has been then improved and the stored current reached 35 mA. Extraction of the stored beam has also been successfully performed.

After storing the first positron beam into the Accumulator on November 19, the following two days have been spent in optimizing the injection rate. Having reached more than 10 mA/s into the Accumulator, the commissioning of the Transfer Line from the Accumulator to the positron Main Ring was started. On Saturday, November 22 after lunch, the beam was driven on the fluorescent screen after the injection septum magnets. After half an hour, spent in tuning all the magnets, the first 0.5 mA positron bunch was stored at 4:30 p.m.. The measured lifetime at zero current was of the order of one hour.

During the scheduled first 2 weeks of commissioning, on November 28, 1997, the single bunch design current (44 mA) has been exceeded in the positron ring of DAFNE. After optimizing the transport efficiency from the Accumulator to the Main Ring, and correcting the chromaticity by means of the sextupoles in the achromats, ~70 mA in a single bunch have been easily stored. Multibunch injection has also been successfully tested, reaching 250 mA limited by residual gas pressure in the vacuum chamber. No evidence of any instability has been so far detected.

During the last series of commissioning shifts, dedicated to injection of positrons into the main ring, the Health Physics Group was working very hard.
1) Before any shift the access control system and the radiation system alarm of each part of the accelerator under commissioning were checked.
2) A lot of measurements were subsequently performed to characterize the radiation field outside the shieldings and to measure the residual radioactivity along the beam lines of the DAFNE complex.

The major results obtained were:
- to find and eliminate a "hole" in the shielding (concrete thickness not sufficient and no Pb bricks) with about 500 microGy/h in a location close to the access control system of the Damping Ring;
- a measurement of about 400 microGy/h close to the positron converter 12 hours after the shutdown.

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