F. Rossetto

Installation of a versatile multiaperture negative ion sourcea)

Neutral Beam Injectors (NBI), which need to be strongly optimized in the perspective of DEMO reactor, request a thorough understanding of the negative ion source used and of the multi-beamlet optics. A relatively compact RF ion source, named NIO1 (Negative Ion Optimization 1), with 9 beam apertures for a total H(-) current of 130 mA, 60 kV acceleration voltage, is being installed at Padua, in Consorzio RFX, to provide a test bench for source optimizations in the framework of the accompanying activities in support to the ITER NBI test facility. NIO1 construction and status of the overall installation, including a high voltage deck and an optical cavity ring down spectrometer are here summarized and reported. Plasma and low voltage beam operations are discussed. Development of a sampling beam calorimeter (with small sampling holes, and a segmented cooling circuit) is also discussed.

Development of versatile multiaperture negative ion sources

Enhancement of negative ion sources for production of large ion beams is a very active research field nowadays, driven from demand of plasma heating in nuclear fusion devices and accelerator applications. As a versatile test bench, the ion source NIO1 (Negative Ion Optimization 1) is being commissioned by Consorzio RFX and INFN. The nominal beam current of 135 mA at −60 kV is divided into 9 beamlets, with multiaperture extraction electrodes. The plasma is sustained by a 2 MHz radiofrequency power supply, with a standard matching box. A High Voltage Deck (HVD) placed inside the lead shielding surrounding NIO1 contains the radiofrequency generator, the gas control, electronics and power supplies for the ion source. An autonomous closed circuit water cooling system was installed for the whole system, with a branch towards the HVD, using carefully optimized helical tubing. Insulation transformer is installed in a nearby box. Tests of several magnetic configurations can be performed. Status of experiments, measured spectra and plasma luminosity are described. Upgrades of magnetic filter, beam calorimeter and extraction grid and related theoretical issues are reviewed.

Construction of a versatile negative ion source and related developments

The NIO1 project consisting of a 60 kV ion source (9 beamlets of 15 mA each of H−) is jointly developed by Consorzio RFX and INFN-LNL, with the purpose of providing a test ion source, capable of working in continuous mode and in condition similar to larger ion sources for Neutral Beam Injectors. The modular design allows for quick replacement and upgrading of parts. While the main body of the ion source construction is progressing at industry, some parts were separately developed at participating institution, as described in the following. A water free Carbon Fiber Composite (CFC) calorimeter is considered, together with more traditional water cooled calorimeters. A small rf plasma generator was installed at INFN-LNL and several rf matching boxes and a Cesium heater controller prototype were tested. Plasma generator (at ground) is followed by a puller and a positively biased Faraday cup, so that beam current can be measured. Plasma density estimated with a 4 wire Langmuir probe is consistent with plasma rf simulation, even if electron distribution deviation from Maxwellian seems large; new electronics with extended DC voltage sweep and a second Langmuir probe circuit are being tested. Finally preparation of the NIO1 site has begun at RFX and installation of source is expected to start in the end of 2012.

Design optimization of RF lines in vacuum environment for the MITICA experiment

This contribution regards the Radio Frequency (RF) transmission line of the Megavolt ITER Injector and Concept Advancement (MITICA) experiment. The original design considered copper coaxial lines of 1″ 5/8, but thermal simulations under operating conditions showed maximum temperatures of the lines at regime not compatible with the prescription of the component manufacturer. Hence, an optimization of the design was necessary. Enhancing thermal radiation and increasing the conductor size were considered for design optimization: thermal analyses were carried out to calculate the temperature of MITICA RF lines during operation, as a function of the emissivity value and of other geometrical parameters. Five coating products to increase the conductor surface emissivity were tested, measuring the outgassing behavior of the selected products and the obtained emissivity values.