| The non-zero permanent electric dipole moment (EDM) of an elementaryparticle implies simultaneous violation of both parity (P) and time-reversal (T)symmetries and its search is in the forefront of the T-violation experiments. AssumingCPT invariance, T-violation involves an associated CP violation; the latter is the keyto explain the observed matter-antimatter asymmetry in the universe.The magnitude of the electron EDM is predicted to be of the order of10-38e·cmwithin the Standard Model of elementary particle physics, which is far below thecurrent experimental reach. However, many extensions of the Standard Model such asthe supper symmetry predict much larger EDMs（1026～1030e·cm）, which thereforecan be verified with the ongoing and/or the proposed experiments.Francium being the heaviest alkali atom is a suitable candidate for the research ofthe electron EDM, because the electron EDM is enhanced in the heavy atoms.In our experiment, we produce francium (Fr) by the nuclear fusion reaction:18O+197Auâ†'210Fr+5n. The18O beam supplied from AVF cyclotron at CYRIC,Tohoku University with the beam energy of100MeV, which is just above thecoulomb barrier and chosen so as to maximize the production of Fr, is bombarded onthe gold target at an angle of45°. The target is being heated to a temperature of morethan the melting point of the gold1337K. This thermal ionizer has the unique featureto operate with the melted target, since the experiment to measure the temperaturedependence of the Fr production shows that the production yield becomes drasticallyhigh with the temperature higher than melting point. After the reaction the producedFr atoms move to the surface by diffusion. Then the Fr desorbs from the target surfaceas atoms and ions. After surface ionization the produced Fr+ion beam is extracted bythe extraction electrode and focused by the two einzel lenses subsequently. Thus, wehave obtained106Fr+/sec yield. However, in order to attain higher precision in theEDM measurement, a larger number of Fr+ions is required. Hence, it is essential toproduce high intensity Fr beam with low emittance from the Thermal Ionizer (TI).The electrostatic lens system plays an important role in transporting the produced Fr~+ion beam. The transportation efficiency, the beam profile and the beam emittancedepend highly on the geometry of the lens system.In our experiment, the Fr+ion beam is transported up to the length of11m fromTI to neutralizer, thus a parallel beam is required to maintain high transportationefficiency with small beam size. To achieve this, an electrostatic lens system havingfive or higher number of lens elements is desirable since only they have afocal zoom.To improve the sensitivity of the EDM measurement, one needs to improve bothextraction and transportation efficiency of the Fr+ion beam. The transverse beamemittance also needs to be improved to prevent further losses; the smaller theemittance better would be the beam quality. Therefore, our objective is to obtain ahigh intensity Fr beam, which involves the modeling and optimization of the lenssystem and the extraction electrode of the TI.Modeling of electrostatic lenses is carried out using the charged particle opticssimulation software called SIMION. By replacing the present two einzel lenses with afive-element lens system under the existing boundary conditions and modifying thegeometry of extraction electrode and lens system, we aim to obtain high extractionand transportation efficiency with small beam emittance and beam size.We have done the following modifications: change in the shape of the extractionelectrode from conical to spherical, change in the target rod opening angle from56degree to82degree, change in the distance between target and extraction electrodefrom15mm to14.6mm, change in the distance between target and lens system from51mm to110mm and more importantly changing the two einzel lenses to a single fiveelement lens system. With the modifications listed above, the simulated value of beamemittance have been improved from36.1mm*mrad to18.3mm*mrad, the beam sizehave been minimized to10.1mm from31.2mm at the entrance of the deflector, andthe extraction efficiency from target to SSD2have also been improved from61%to94%in the simulation.We have performed the test experiments with the upgraded TI using rubidiumbeam. The form of distribution of the extraction efficiency in simulation matches wellwith the form of distribution of the beam current in experiment. We have improved the transportation efficiency from50%to more than80%with the highest value being91.4%. We presume that this improvement is due to the optimized beam emittance,the simulated value of which had been minimized from36.1mm*mrad to18.3mm*mrad. |
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