S. Petrenko

First experiments with the negative ion source NIO1

Neutral Beam Injectors (NBIs), 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 radio frequency (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, was installed at Consorzio RFX, including a high voltage deck and an X-ray shield, to provide a test bench for source optimizations for activities in support to the ITER NBI test facility. NIO1 status and plasma experiments both with air and with hydrogen as filling gas are described. Transition from a weak plasma to an inductively coupled plasma is clearly evident for the former gas and may be triggered by rising the rf power (over 0.5 kW) at low pressure (equal or below 2 Pa). Transition in hydrogen plasma requires more rf power (over 1.5 kW).

Development of a versatile multiaperture negative ion source

A 60 kV ion source (9 beamlets of 15 mA each of H(-)) and plasma generators are being developed at Consorzio RFX and INFN-LNL, for their versatility in experimental campaigns and for training. Unlike most experimental sources, the design aimed at continuous operation. Magnetic configuration can achieve a minimum ∣B∣ trap, smoothly merged with the extraction filter. Modular design allows for quick substitution and upgrading of parts such as the extraction and postacceleration grids or the electrodes in contact with plasma. Experiments with a radio frequency plasma generator and Faraday cage inside the plasma are also described.

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.

Design of a versatile multiaperture negative ion source

Negative ion sources are a key component of the neutral beam injector to be installed in the International Thermonuclear Experimental Reactor. At present research and development activities address several important issues related to beam extraction, optics, and optimization. Together with the design of real size devices and the accumulation of atomic cross section databases, a relatively small negative ion source [130 mA of H(-) at 60 kV, named Negative Ion Optimization phase 1 (NIO1)] is under construction at Consorzio RFX to contribute to benchmark numerical simulation tools and to test components, such as emittance scanners, beam dumps, and cesium ovens. NIO1 design, magnet configuration, and rf coupling simulations are described.

Development of Small Multiaperture Negative Ion Beam Sources and Related Simulation Tools

In the design of extraction systems for negative ion sources several fundamental questions still deserve further investigation, as the distribution of particles near the extraction sheath, the optimal magnetic structure and the space charge compensation length after acceleration. Large (and undesired) deflection differences may develop between beamlets of a multiaperture source, so that equalization of the magnetic field effect is necessary. To guarantee an uniform strength of filter field at extraction and in the acceleration, several configuration of arrays of permanent magnets were studied and fast simulation tools were developed. As an example of optimized magnetic configuration and as a possible experimental tool, the design of NIO1 (Negative Ion Optimization try 1) is here discussed. This project consists of a 3×3 matrix of 8 mm extraction holes, aimed at a total H− current about 130 mA with an extraction voltage Vs = −60 kV. A modular design is used, so several parts (the extraction grid, the accel...

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.

Status of NIO1 construction

The NIO1 (Negative Ion Optimization phase 1) project consists of a multiaperture negative ion source mounted on a 60 kV accelerating column; up to 9 beamlets (15 mA H− each one) arranged in 3×3 matrix with 14 mm spacing can be extracted. The moderate size and the modular concept make some relative rotation of the source magnetic filter and the electrodes possible, so that the effect of crossed and aligned field can be easily compared. Other goals of source experimental program are emittance (and beam profile) measurement at several distances (for simulation code validation), testing of diagnostic components and of radiofrequency coupling. A full set of construction drawing was completed; also the Fast Emittance Scanner (FES) and its vacuum chambers were built (four mounting positions are reserved to FES). Some low power rf matching boxes were developed for a test plasma, approximately half the source size. A cesium oven compatible with NIO1 is being also developed; by using some industrial standard 100 W ...

Experiments with rf ovens in ECR ion sources

A 34 mm diameter radio frequency (rf) oven system previously developed on bench was inserted and tested into the Electron Cyclotron Resonance (ECR) Ion Source Alice, producing beams from natural copper and silver samples; charge range was typically i=10–13 for copper and i=10–19 for silver, which well compares to previous source yield for xenon (charges 11–20). Some issues of oven design, including wire section effects, and circuit matching, are discussed; taps on the coupling transformer improved the flexibility of rf matching to different crucible materials (tantalum or steel). Details of operating experience (cleaning the oven and replacing sample) are reported; sample duration was more than 100 h and temperature Ts in excess of 1750 K were demonstrated. The ion source operation depends on both the oven distance Loe from ECR plasma and the bias voltage Vb of the sample. Best conditions were found for close (Loe≅70 mm) or preferably intermediate positions (Loe≅106 mm) and for sample negative respect to ...

Radio-frequency ovens for ECR ion sources

Radio-frequency (rf) ovens have perspective advantages over their Ohmic equivalent in terms of uniformity of heating, separation (extending lifetime and use of reactive sample) and insulation; on the other side, rf losses in the coil must be minimized. Several geometries, crucibles, and coils were tested to optimize efficiency (power in the sample/total power); the final axial geometry with a copper coil is described, discussing the optimization of thermal contacts. A script to simulate the two-dimensional geometry of a rf oven (in particular, the wire separation and shape) was written; optimal working frequencies were found to be about or over 1 MHz, as confirmed by experiments, while the coil should be slightly longer than the sample. A temperature of 1680 K was reached with an iron crucible and 80 W of total rf power; silver and copper evaporations were tested; and carbon crucibles can reach higher temperatures (1750 K in a preliminary version).