Spatially resolved diagnostics for optimization of large ion beam sources.

Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

CRISP: A compact RF ion source prototype for emittance scanner testing.

A movable Allison type emittance scanner is being developed to characterize the phase-space distribution of the beamlets of spectral phase interferometry for direct electric-field reconstruction, the prototype RF negative ion source of the ITER heating neutral beam injector. To test the electronics and verify the capability of the device to resolve nearby beamlets, a compact RF ion source prototype has been set up, capable of accelerating 1 mA of helium ions up to a voltage of 2 kV. A commercial 100 W RF generator creates a plasma inside a Pyrex tube, with a density between 1015 and 1016 m-3 and an electron temperature up to 15 eV. Three multi-aperture grids in accel-decel configuration extract and accelerate the ions, which are measured with a Faraday cup. We present in this paper the characterization of the ion source and its first operation, showing that it is suitable for the commissioning of the Allison scanner.

First operation in SPIDER and the path to complete MITICA.

The requirements of ITER neutral beam injectors (1 MeV, 40 A negative deuterium ion current for 1 h) have never been simultaneously attained; therefore, a dedicated Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Padova, Italy). The NBTF includes two experiments: SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma), the full-scale prototype of the source of ITER injectors, with a 100 keV accelerator, to investigate and optimize the properties of the ion source; and MITICA, the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. The present paper gives an account of the status of the procurements, of the timeline, and of the voltage holding tests and experiments for MITICA. As for SPIDER, the first year of operation is described, regarding the solution of some issues connected with the radiofrequency power, the source operation, and the characterization of the first negative ion beam.

Interpreting the dynamic equilibrium during evaporation in a cesium environment.

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained.