In multi-beamlet negative ion accelerators for fusion applications, the required transverse magnetic field for the suppression of co-extracted electrons is generated by permanent magnets arranged in parallel arrays, embedded in the Extraction Grid. However, such configuration also produces an undesired "criss-cross" deflection of ion trajectories, which can downgrade the overall beam optics. This deflection is traditionally counterbalanced by electrostatic means, i.e. by introducing a small offset in the axis of the grid apertures. During the final design of MITICA, which is the prototype of the ITER Heating Neutral Beam Injector presently under construction in Padova [1], an innovative magnetic configuration called ADCM (Asymmetric Deflection Compensation Magnets) has been conceived [2] and adopted, offering several advantages in terms of operational flexibility compared to the traditional solution. In the framework of a research collaboration between RFX and QST, the new concept has been experimentally tested for the first time in 2016 by a joint RFX-QST team. A new ITER-like [3] Extraction Grid (designed and built at RFX) was mounted on the existing NITS device at the QST lab in Naka. For direct comparison, half of the Extraction Grid was provided with the new ADCM configuration, whereas the rest of the grid had standard magnets with no compensation. A target made of monoaxial carbon-carbon fibre composite and an infrared camera were used for accurate beam optics measurement. The results obtained have confirmed the effectiveness of the ADCM concept for the compensation of the criss-cross ion deflection. Some discrepancies in the scaling of ion deflection with respect to 3D simulations were also evidenced and investigated. The paper presents the main results obtained during the experimental campaign on NITS at QST, which lasted in total six weeks, together with a discussion of the first data analysis results.
Experimental validation of an innovative deflection compensation method in a multi-beamlet negative-ion accelerator
Agostinetti P;Serianni G;Antoni V;
2016
Abstract
In multi-beamlet negative ion accelerators for fusion applications, the required transverse magnetic field for the suppression of co-extracted electrons is generated by permanent magnets arranged in parallel arrays, embedded in the Extraction Grid. However, such configuration also produces an undesired "criss-cross" deflection of ion trajectories, which can downgrade the overall beam optics. This deflection is traditionally counterbalanced by electrostatic means, i.e. by introducing a small offset in the axis of the grid apertures. During the final design of MITICA, which is the prototype of the ITER Heating Neutral Beam Injector presently under construction in Padova [1], an innovative magnetic configuration called ADCM (Asymmetric Deflection Compensation Magnets) has been conceived [2] and adopted, offering several advantages in terms of operational flexibility compared to the traditional solution. In the framework of a research collaboration between RFX and QST, the new concept has been experimentally tested for the first time in 2016 by a joint RFX-QST team. A new ITER-like [3] Extraction Grid (designed and built at RFX) was mounted on the existing NITS device at the QST lab in Naka. For direct comparison, half of the Extraction Grid was provided with the new ADCM configuration, whereas the rest of the grid had standard magnets with no compensation. A target made of monoaxial carbon-carbon fibre composite and an infrared camera were used for accurate beam optics measurement. The results obtained have confirmed the effectiveness of the ADCM concept for the compensation of the criss-cross ion deflection. Some discrepancies in the scaling of ion deflection with respect to 3D simulations were also evidenced and investigated. The paper presents the main results obtained during the experimental campaign on NITS at QST, which lasted in total six weeks, together with a discussion of the first data analysis results.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
