title: Simulation of space charge compensation in a multibeamlet negative ion beam.
abstract: Ion beam space charge compensation occurs by cumulating in the beam potential well charges having opposite polarity, usually generated by collisional processes. In this paper we investigate the case of a H(-) ion beam drift, in a bi-dimensional approximation of the NIO1 (Negative Ion Optimization phase 1) negative ion source. H(-) beam ion transport and plasma formation are studied via particle-in-cell simulations. Differential cross sections are sampled to determine the velocity distribution of secondary particles generated by ionization of the residual gas (electrons and slow H2 (+) ions) or by stripping of the beam ions (electrons, H, and H(+)). The simulations include three beamlets of a horizontal section, so that multibeamlet space charge and secondary particle diffusion between separate generation regions are considered, and include a repeller grid biased at various potentials. Results show that after the beam space charge is effectively screened by the secondary plasma in about 3 μs (in agreement with theoretical expectations), a plasma grows across the beamlets with a characteristic time three times longer, and a slight overcompensation of the electric potential is verified as expected in the case of negative ions.
authors.affiliation: Consorzio RFX (CNR, ENEA, INFN, UNIPD, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Sartori
authors.affiliation: Consorzio RFX (CNR, ENEA, INFN, UNIPD, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy.
authors.forename: T J
authors.initials: TJ
authors.lastname: Maceina
authors.affiliation: Consorzio RFX (CNR, ENEA, INFN, UNIPD, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Veltri
authors.affiliation: INFN-LNL, Viale dell'Università n. 2, 35020 Legnaro, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Cavenago
authors.affiliation: Consorzio RFX (CNR, ENEA, INFN, UNIPD, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Serianni