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HELEN Program Final report Grant-holder: Daniel de Miranda Silveira (UFRJ – Brazil) Grant: Advanced Training (AT) fellowship Experiment: ALPHA/AD-5 (Antihydrogen Laser Physics Apparatus) Laboratory: CERN Period: 20/03/2006 – 20/09/2006 (6 months) Brief introduction: The ALPHA experiment aims to perform a high precision test of CPT symmetry, by doing a spectroscopic comparison between Hydrogen (H) and antiHydrogen (Hbar). This will be accomplished through high resolution laser spectroscopy of cold (T < 1 K), magnetically trapped Hbar atoms. The collaboration includes many former members of ATHENA, the experiment which pioneered the formation of cold Hbar atoms, and retains most of the expertise needed for Hbar production and detection, as well as some of the key parts of the apparatus, such as the positron accumulator and the laser system. In 2006, ALPHA is having its first experimental run, after the AD (Antiproton Decelerator) - the machine that provides the low energy antiprotons for the antihydrogen experiments experienced a two-year shutdown. For this run, the goal is to produce a good amount of Hbar atoms, cold enough to allow them to be confined by a magnetic trap. My proposed activities program comprised a 3-month period to be spent in the comissioning of the experimental apparatus and the last 3 months of my stay at CERN would be devoted to actually running the experiment Activity report: As stated in the working program submitted to the HELEN program, I was initially involved in the assembly and commissioning of the apparatus. I soon became responsible for the vacuum systems for the experiment: chambers, gauges, valves, pumps, residual gas analysers, leak detectors, feedthroughs and cables. Our apparatus has 3 independent vacuum systems, namely the AD vacuum, the trap vacuum and the isolation vacuum. AD vacuum is the vacuum space adjacent to the AD machine which, upon opening of a vacuum valve (AD valve), delivers the 5 MeV antiproton (pbar) beam which is trapped in the catching trap. Inside this vacuum system, there is a rotatable degrader (to reduce the energy of the pbar beam) and the beam counter (a positionsensitive silicon detector used to measure the pbar beam intensity and position). It is a high vacuum system, but its vacuum requirements aren´t very strict: we just have to keep its pressure below a threshold value (5 x 10-6 mbar), above which the AD valve closes to preserve the AD performance. The pump down is provided by a 1000 l/s turbo pump, while the high vacuum operation is ensured by a 300 l/s ion pump. The catching trap cables reach the external space through the AD vacuum, so the chamber with the catching trap feedthroughs has its own vacuum system (an 8 l/s ion pump), because it is connected to the pumps through a lowconductance line. The second (and most important) vacuum system is the trap vacuum. It is separated from the AD vacuum by a Titanium vacuum window, which has the additional function of performing the final degrading of the pbar beam. In this vacuum space, we store, cool and manipulate the cold pbar and positron plasmas from which we produce Hbar. Trapped particle lifetimes depend critically on the pressure in this part of the apparatus: a vacuum lower than 10 -10 mbar is desirable. To reach this value, we performed a very judicious choice of materials, together with good cleaning and handling procedures. After a several day bakeout we reached the 10-9 mbar range and, upon cooling the cryostat which surrounds the trap space to 4 K, we reached the 10 10 mbar range. The pumping system consists of a 360 l/s turbo pump and a 150 l/s ion pump. The last vacuum system (which was the last one to be assembled) is the isolation vacuum. The purpose of this vacuum space is to thermally isolate the helium vessel (at 4 K) inside the cryostat from the 300 K outer walls. There is no need for extremely low pressures, since below a certain pressure the thermal conductivity of all gases is independent of the pressure: a vacuum between 10-4 and 10-6 mbar is desirable. The cryostat is being pumped by a mechanical pump with a base pressure of 5 x 10-4 mbar with good results, but when the cryostat is cold, as soon as the pumping is interrupted the pressure rises and the cryostat boil-off increases. This indicates that there is a leak in the isolation vacuum, but so far we were not able to find and fix it. Beside the vacuum systems, I was also involved in the trap and magnets assembly, as well as the cryostat comissioning, testing and operation. The start of the data taking period was delayed by a serious problem with the PS, the accelerator which supplies the pbar beam for the AD. The AD Physics program, which was scheduled to start on June 6th. was transfered to July 17th.. After the PS was back in operation, the AD experienced some technical problems, being unable to deliver the pbar beam to our apparatus. This was fixed by the AD team, and the 2006 run officially started on August 28 th.. Initially some time was spent in steering and focusing the beam into our apparatus, with special attention to beam focusing in the presence of our new 3 T magnet. We proceeded by doing some degrading studies, tuning the rotatable degrader to maximize the number of pbars. After the trap was at 4 K (and the vacuum was in the 10 -10 mbar range) we moved forward quickly: we trapped pbars in the catching trap, injected electrons into the catching trap and observed the electron cooling of trapped antiprotons. The day I left CERN, we were optimizing the pbar stacking, measuring which number of AD shots could be taken without saturating the trapped pbar number. The collaboration now proceeds with the aim of producing and trapping antihydrogen during this year´s run, which was extended until November 20th.. Commentaries: This was a very important period for my scientific training. I was involved in all aspects of a large international scientific collaboration: planning the experiment, designing the apparatus, integrating all the systems (vacuum, cryogenics, electronics, data acquisition), running the experiment and analyzing the data. Unfortunately, due to some technical problems external to the collaboration (the PS and AD problems), the run was delayed and I couldn´t be there for the whole run. Despite that, I am really sure that I learned a lot and I also contributed to the collaboration with my experience. I continue to be a member of the ALPHA collaboration and I plan to return to CERN for the next runs of the experiment.