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A negative ion beam source is a key component of a neutral beam injector such as that under development for ITER. Its efficient and reliable operation depends on a network of processes affecting generation, extraction and acceleration of the negative ions. Such processes interact with each other in a complex fashion. Controllability of complex networks [1] can offer interesting new tools to both identify the most crucial processes in the negative ion beam source NIO1 [2] [3] and to verify its controllability. In this contribution a model based on this theory has been developed and tested. NIO1 processes have been mapped into a network of 40 nodes and 292 links to highlight the most important processes (the driver nodes). Predictions obtained with the model have then been compared with the observed behaviour of NIO1. The test has been performed by numerically perturbing the model with input signals that mimic the ones used in NIO1 experimental campaigns. Experimental data and outputs from the model have then been compared. Preliminary results show a good agreement between model predictions and experimental behaviour of the source so that a subset of processes which in principle allow the system to be driven towards any state has been identified. As an outcome of the analysis the most crucial processes have been identified and discussed in terms of emerging diagnostic and modelling necessities required to improve the understanding of the physics behind them.

ITER operation relies on heating neutral beam injectors (HNB) based on negative ion deuterium beams accelerated at 1 MeV, with up to one hour 17 MW power, uniform intensity and low divergence. A neutral beam test facility is being built at Consorzio RFX to demonstrate the feasibility of a prototype injector with such demanding specifications and to optimize its performances. It comprises SPIDER, a 100 kV negative hydrogen/deuterium RF source, full size prototype of the HNB source, and MITICA, a prototype of the full HNB. The diagnostics for SPIDER and MITICA are essential to qualify and optimize the HNB for ITER and to assess the information on source and beam obtainable with the reduced set of the HNB diagnostics. Main parameters are measured with different complementary techniques to exploit the combination of their specific features. In the source the still open issues of cesium dynamics, negative ion generation and uniformity are investigated with a set of electrostatic probes, with optical emission spectroscopy, laser absorption for neutral cesium density and cavity ring down spectroscopy for negative ion density. The beam profile uniformity and its divergence will be studied with beam emission spectroscopy, visible tomography, neutron imaging and a special calorimeter made of carbon tiles which can achieve a spatial resolution of few millimetres. All components heated by beam power load will be monitored with a large set of thermocouples behind the heated surfaces and on the cooling water circuits. The test facility will also be the natural choice for hosting experiments on innovative concepts for DEMO HNBs, e.g. more efficient neutralization techniques and cesium free source operation.