Numerical and experimental assessment of the new magnetic field configuration in SPIDER

Negative ion driven Neutral Beam Injectors (NBIs) require a complex magnetic field structure inside the ion source and accelerator region, with two basic purposes: filter out the electrons from the region were the negative ions are produced, extracted and accelerated, and confine the plasma in order to reduce losses at the walls. The fulfillment of both requirements at the same time is not an easy task and this is particularly true for large size NBIs operating thanks to radio frequency (RF) generated plasmas, as it is the case of SPIDER, the full scale prototype of the ITER NBI source. As a matter of fact, the uniformity and intensity requirements on the magnetic field close to the plasma facing electrode which separates the source from the accelerator may clash with those in the region of the RF drivers where the plasma is generated. The magnetic field in SPIDER is produced by a combination of several sets of permanent magnets embedded in different components of the ion source and the accelerator, and by a DC (or slowly varying) electric current flowing in the plasma facing electrode and closing in an assembly of bus-bars specifically positioned. This bus-bar system has been recently modified with respect to the original design in order to improve the plasma confinement in the RF drivers, as a result of the first experimental evidences which showed a degradation of the plasma intensity strictly correlated to the magnetic field intensity. The improvement proved to be substantial, but the filter field intensity can still affect the operation of some RF drivers; however, in the new configuration this occurs at a much higher level of the field, which is outside the usual operational range. The present work describes the latest activities carried out, both numerically and experimentally, for a thorough investigation on the possible reasons of this behavior, which can be investigated in terms of the interplay among the different magnetic field sources along with the RF field. The possibility to further increase the filter field while maintaining good plasma performances in the RF drivers would likely be beneficial for the NBI efficiency, in terms of reduced electron/ion extracted current.