Design of a beam emission spectroscopy diagnostic for negative ions radio frequency source SPIDER

First measurements of optical emission spectroscopy on SPIDER negative ion source

Optical Emission Spectroscopy (OES) on the SPIDER negative ion source has been collecting data since the beginning of operation in Summer 2018. The first few months were devoted to complete the diagnostic commissioning and its integration with the SPIDER control and data acquisition system. Consistent sets of spectroscopic data have been acquired under different experimental conditions, not only varying the plasma source filling pressure and injected power but also changing the RF generator frequencies and the strength of the magnetic field acting as a filter in front of the plasma grid. The main results of OES data analysis are presented in this work. SPIDER optical emission diagnostic comprises a set of 66 channels wavelength resolved and 36 single line channels by means of interference filtering. Some of them collect the photons along line of sight (LOS) perpendicular to the grids through the 8 RF drivers and others along LOS parallel to and near the grids, both horizontally and vertically. Since the starting of extraction experiments, 22 channels have been dedicated to collect the extracted beam emission. The LOS layout allows tracing two 9-point vertical profiles of the source plasma in the extraction region at 35 and 5 mm from the Plasma Grid (PG) and four 4-point horizontal profiles spanning the 65 mm region before the PG. It is also possible to collect spectra from LOS looking in between the grids. Both Balmer series and Fulcher band between 600 nm and 640 nm were routinely collected. Their intensities are very sensitive to the plasma parameters and when coupled to a collisional radiative model can give an estimation of the electron density and gas dissociation. It has been found that the Balmer emission and gas dissociation inside the drivers scale linearly with the RF power, the latter reaching a value up to 20% at high power and low pressure. Rotational gas temperature has also been evaluated; it ranged between 900 K and 1400 K, where higher values were reached for higher pressures and RF powers.

Modeling and simulation of a beam emission spectroscopy diagnostic for the ITER prototype neutral beam injector

A test facility for the development of the neutral beam injection system for ITER is under construction at Consorzio RFX. It will host two experiments: SPIDER, a 100 keV H(-)/D(-) ion RF source, and MITICA, a prototype of the full performance ITER injector (1 MV, 17 MW beam). A set of diagnostics will monitor the operation and allow to optimize the performance of the two prototypes. In particular, beam emission spectroscopy will measure the uniformity and the divergence of the fast particles beam exiting the ion source and travelling through the beam line components. This type of measurement is based on the collection of the Hα/Dα emission resulting from the interaction of the energetic particles with the background gas. A numerical model has been developed to simulate the spectrum of the collected emissions in order to design this diagnostic and to study its performance. The paper describes the model at the base of the simulations and presents the modeled Hα spectra in the case of MITICA experiment.

Modeling and design of a beam emission spectroscopy diagnostic for the negative ion source NIO1

Consorzio RFX and INFN-LNL are building a flexible small ion source (Negative Ion Optimization 1, NIO1) capable of producing about 130 mA of H(-) ions accelerated at 60 KeV. Aim of the experiment is to test and develop the instrumentation for SPIDER and MITICA, the prototypes, respectively, of the negative ion sources and of the whole neutral beam injectors which will operate in the ITER experiment. As SPIDER and MITICA, NIO1 will be monitored with beam emission spectroscopy (BES), a non-invasive diagnostic based on the analysis of the spectrum of the Hα emission produced by the interaction of the energetic ions with the background gas. Aim of BES is to monitor direction, divergence, and uniformity of the ion beam. The precision of these measurements depends on a number of factors related to the physics of production and acceleration of the negative ions, to the geometry of the beam, and to the collection optics. These elements were considered in a set of codes developed to identify the configuration of the diagnostic which minimizes the measurement errors. The model was already used to design the BES diagnostic for SPIDER and MITICA. The paper presents the model and describes its application to design the BES diagnostic in NIO1.