  Consorzio RFX and INFN-LNL have designed, built and operated the compact
radiofrequency negative ion source NIO1 (Negative Ion Optimization phase 1)
with the aim of studying the production and acceleration of H- ions. In
particular, NIO1 was designed to keep plasma generation and beam extraction
continuously active for several hours. Since 2020 the production of negative
ions at the plasma grid (the first grid of the acceleration system) has been
enhanced by a Cs layer, deposited though active Cs evaporation in the source
volume. For the negative ion sources applied to fusion neutral beam injectors,
it is essential to keep the beam current and the fraction of co-extracted
electrons stable for at least 1 h, against the consequences of Cs sputtering
and redistribution operated by the plasma. The paper presents the latest
results of the NIO1 source, in terms of caesiation process and beam
performances during continuous (6{\div}7 h) plasma pulses. Due to the small
dimensions of the NIO1 source (20 x (diam.)10 cm), the Cs density in the volume
is high (10^15 \div 10^16 m^-3) and dominated by plasma-wall interaction. The
maximum beam current density and minimum fraction of co-extracted electrons
were respectively about 30 A/m^2 and 2. Similarly to what done in other
negative ion sources, the plasma grid temperature in NIO1 was raised for the
first time, up to 80 {\deg}C, although this led to a minimal improvement of the
beam current and to an increase of the co-extracted electron current.
