The Poloidal Field(PF)coils are the important components of ITER,and the tail is one key component of the PF coils.The tail is mainly used to achieve the mechanical integration but electrical separation between the terminal and the last-but-one turn of the coil.From the perspective of the structure and function,the tail needs to withstand the electromagnetic force and the thermal stress transmitted from the conductors at its both sides,and also needs to undergo the mechanical fatigue process at low temperature when the’coil is working.Meanwhile,the tail needs to make good electrical insulation performance.This thesis focuses on the theoretical and experimental researches on the tail optimization,which includes the key technology researches on the welding procedure and fatigue performance characterization of the unconventional weld,the structural design of the strap and electrical and fatigue properties of tail insulation.The optimization of the initial conceptual structure of the PF6 coil tail has been carried out,and the optimized structure of the tail has also been introduced.The internal tensile load line of the tail has been optimized and changed from the adhesion between the turn insulation and conductor jacket to one robust mechanical connection.In addition,the static electromagnetic analyses of the tail under the most dangerous condition of EOB+PD with the finite element analysis method were carried out.The connection between the terminal and the last-but-one turn of the coil was realized through the welds at the both sides of the tail.According to the ITER design requirement,full penetration welds should be made,and the welding temperature and deformation should be controlled strictly during the welding,and the weld quality needs to meet the standard of EN ISO5817:2007 Level B.So the research on the welding procedure of the unconventional weld of the tail was performed,and one set of welding fixture was designed for the special operation condition of the PF6 coil tail.In addition,as the tail needs to withstand the electromagnetic force and the thermal stress transmitted from the coil terminal when the coil works,the researches on the fatigue performance of the unconventional welds were carried out through a series of theoretical and experimental efforts.The electrical insulation and mechanical connection of the tail were realized by the strap at the middle of the tail.The strap was designed to be a race-track structure and made by the composite insulation material.The chemical composition at the key positions were confirmed through the burning method.In addition,there is difference in thermal coefficient between the composite material and the stainless steel,there were some thermal stress developed in the tail during the cooling.The coefficient of thermal contraction and elastic modulus of the irregular-shaped strap were measured.Then one pre-load was successfully applied on the tail at room temperature to compensate the thermal stress.Finally the experimental researches on the fatigue properties of the strap were also carried out at low temperature.For the requirements of the PF6 coil insulation,the researches on the insulation wrapping process and the structure of the insulation sample were performed.The fatigue tests of the insulation sample of the tail were carried out at the same time,and the cause of the fracture of the insulation sample were analyzed and discussed.In addition,electrical test platforms were set up and the researches on the insulation sample were also carried out before and after the fatigue test at low temperature.The test results showed that the turn insulation successfully withstood the DC test up to 30 kV after the fatigue test with 600,000 cycles.Based on the theoretical and experimental researches on the key technologies of the unconventional welds,strap and insulation of the tail,the researches and productions of the two full size double pancakes of the PF6 coil(DDP and DP9)were performed.Meanwhile electrical test platforms were set up,and the examination on the electrical property of the insulation of the tail was carried out and all the results satisfy the ITER requirements.So farn the key technical challenge of the ITER PF6 coil tail have been met.The production of the tail of the first double pancake(DP9)has also been completed.All the key technologies of the PF6 coil tail researched in this thesis will provides support for the tail of PF6 DP1 and also PF2-PF5 in general.