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2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONMeasuring the vacuum magnetic birefringencePreviously we reported measurements of birefringences assmall as 10 −16 with our experimental apparatus. During thelast year we have worked on the optimization of the signalto noise ratio.Our activity can be divided into five main themes.Vacuum system : in order to understand the ultimate pressurethat we are able to reach and the possibly systematiceffects that can occur, we have characterized the nature ofthe residual gases in our vessel using a Residual Gas Analyzerduring the entire pumping process. Because of theconductivity of the vacuum pipe passing through the magnetcryostat, the lower limit for the pressure that we canreach with our ionic pumps is of about 10 −8 mbar. We needa lower residual pressure. We are currently studying thefeasibility of deposing inside the vacuum pipe a substratetype “getter” which acts as a distributed pumping systemall along the inner surface of the pipe. Contact with theSAS getter company, the world experts of this technique,are under way. We are confident to reach a vacuum betterthan 10 −11 mbar which is enough for our experiment.Mirror intrinsic birefringence : static birefringence interferentialmirrors used in Fabry-Perot cavities is a limitingfactor of the signal to noise ratio. In reference [Bielsa etal., Applied Physics B 97, 457, (2009)], we reported newmeasurements that confirm that mirror phase retardation inducedby mirror birefringence decreases when mirror reflectivityincreases. Our study indicates that the origin ofthe intrinsic birefringence can be ascribed to the reflectinglayers close to the substrate. We believe that this is animportant step forward the realization of birefringence-freemirrors.Perot cavity (length 2.2 m) of finesse 198000 (figure 177).The linewidth of the cavity resonance (FWHM) is around350 Hz which is one of the smallest ever realized. TheLMA in Lyon is currently analysing the other mirrors toimprove their performances, which will allow us to furtherimprove our cavity finesse.Figure 177: The finesse of a Fabry Perot cavity is evaluatedthanks to the lifetime of the photon in the cavity. Here, the lifetimeτ is 463 µs. The corresponding finesse is F = πcτŁ = 198000.New coil : we have built a new coil in order to reach theproject goal of 25 T. It is based on our ”X” geometry, andwe have named it the XXLCoil. Its length is of about50 cm. We have tested it at the LNCMI in Toulouse andat the DHMFL in Dresden where we have reached 31 T(figure 178) (B 2 L ≥ 300 T 2 m). This result is a big stepforward the measurement of the vacuum magnetic birefringence,since we have fulfill the project requirements. In thefinal experiment, we will use three coils like this one.Dynamical response of the Fabry Perot cavity : our Fabry-Perot cavity stores photons for an average time which isbigger than 300 µ s. The magnetic pulse duration is a fewmilliseconds. When the storage time of photons approachesthe pulse duration, the effect of the cavity low-pass filtercannot anymore be neglected and the ellipticity signal is nolonger exactly proportional to the square of the magneticfield amplitude. We have observed such a behaviour, andwe have implemented the appropriate correction in the dataanalysis program.Very low loss cavity mirrors : we have tested four new mirrorsmade by LMA in Lyon. The mirrors currently used inour cavity have losses of the order of 20 ppm. LMA mirrorlosses are expected to have losses as low as a few ppm. Ourmeasurements confirm such expectations. We have measuredfor the best LMA mirror losses of about 5 ppm. As aconsequence, using such a mirror, we have realized a FabryFigure 178: Magnetic field obtained with a transverse coil(XXLCoil). The relevant parameter for the magnetic vacuum birefringencemeasurement is B 2 L=250 T 2 m. This factor has beenimproved by a factor ten in this new configuration.P. Berceau, F. Bielsa, J. Mauchain, R. BattestiA. Dupays, M. Fouché and C. Rizzo (Laboratoire Collisions Agrégats Réactivité, Toulouse, France)125