In-vacuum sensors for the beamline components of the ITER neutral beam test facility

Simulation of the beamline thermal measurements to derive particle beam parameters in the ITER neutral beam test facility

Injection of high energy neutral beam particles will be used in the ITER experiment for plasma heating and current drive. In a ITER heating beam injector, a 40 MW electrostatically accelerated negative beam will be neutralised and filtered along the beamline, obtaining a nominal 16.5 MW neutral beam power to be injected in the tokamak plasma or intercepted during conditioning and commissioning. The beam will heat the actively cooled panels of the beamline components with up to 13 MW/m² surface power density and 18 MW power. These extreme conditions require testing in a ITER full scale neutral beam test facility under construction in Padova where the temperature of the beamline components will be monitored by 610 embedded thermocouples for protection against critical conditions, for recognising beam conditioning, and for deriving beam parameters. Power density maps of the expected beam-component interactions are applied on a parametric non-linear finite element model to simulate fields of expected temperatures. Such thermal maps are analyzed to derive the beam parameters during operation: divergence of 3-7 mrad and misalignment of 0-3 mrad. The sensibility of the temperature measurements is discussed considering a minimum 10% fraction of the nominal beam power.

Study of a high power hydrogen beam diagnostic based on secondary electron emission

In high power neutral beams for fusion, beam uniformity is an important figure of merit. Knowing the transverse power profile is essential during the initial phases of beam source operation, such as those expected for the ITER heating neutral beam (HNB) test facility. To measure it a diagnostic technique is proposed, based on the collection of secondary electrons generated by beam-surface and beam-gas interactions, by an array of positively biased collectors placed behind the calorimeter tubes. This measurement showed in the IREK test stand good proportionality to the primary beam current. To investigate the diagnostic performances in different conditions, we developed a numerical model of secondary electron emission, induced by beam particle impact on the copper tubes, and reproducing the cascade of secondary emission caused by successive electron impacts. The model is first validated against IREK measurements. It is then applied to the HNB case, to assess the locality of the measurement, the proportionality to the beam current density, and the influence of beam plasma.