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Padua, Italy

Cavenago M.,Viale Delluniversita N2 | Veltri P.,Consorzio RFX
Plasma Sources Science and Technology

Deflection of negative ion beamlets due to the magnets embedded in the first extraction electrode for the purpose of dumping the co-extracted electrons is a serious issue for multiaperture ion accelerators of neutral beam injectors. Several kinds of magnet arrays which offer the possibility of cancelling ion deflection, employing crossed rows of magnets or even more compact parallel row arrangements, are discussed. A general equation for beamlet deflection is presented here, and the interference of the magnetic deflection and the electrostatic lens steering is carefully calculated; this equation may also include beamlet-beamlet interactions and image charge effects. Analytical expressions are given for the field and the line integrals for the magnet arrays, and these are simplified for beam optics calculations, but still retain an excellent agreement with numerical values. Optimization formulas for the filling fraction xy of the magnets are given, for cancellation of deflection both after the first electrode or after the second accelerating electrode. The latter case is of direct interest for the design of small accelerators (e.g., NIO1), for which compact solutions are proposed, while the former case may approximate well the design of a large accelerator such as MITICA, with a predicted xy = 0.1015 against a numerical optimized value of 0.0975 ±0.005 in normal conditions. The detailed comparison between simulation results and theory shows that thin lens models are suitable approximations for calculating beam steering. Stability of optimal xy prediction with respect to the first accelerating gap length is shown, and the variation of xy with the voltage is discussed. © 2014 IOP Publishing Ltd. Source

Pasqualotto R.,Consorzio RFX
Journal of Instrumentation

ITER nuclear fusion experiment requires additional heating via neutral beams by means of two injectors, delivering 16.5 MW each, up to one hour. This power level results from the neutralization of negative deuterium ions generated by an RF source and accelerated to 1 MeV. Such specifications have never been simultaneously achieved so far and therefore a test facility is being constructed at Consorzio RFX, to demonstrate the feasibility of a prototype neutral beam injector. The facility will host two experimental devices: SPIDER, a 100 kV negative hydrogen/deuterium RF source, full size prototype of the ITER source, and MITICA, a prototype of the full ITER injector. SPIDER will be devoted to optimize the extracted negative ion current density and its spatial uniformity and to minimize the co-extracted electron current. Negative hydrogen is mainly produced by conversion of hydrogen particles at the cesium coated surface of the plasma grid. The interplay of these two species is fundamental to understand and optimize the source performance. Two laser-aided diagnostics play an important role in measuring the negative hydrogen and cesium density: cavity ring down spectroscopy and laser absorption spectroscopy. Cavity ring down spectroscopy will use the photo-detachment process to measure the absolute line-of-sight integrated negative ion density in the extraction region of the source. Laser absorption spectroscopy will be employed to measure the line integrated neutral cesium density, allowing to study the cesium distribution in the source volume, during both the plasma and the vacuum phases. In this paper, the design of the laser-aided diagnostic systems on SPIDER is presented, supported by a review of results obtained in other operating experiments. © 2012 IOP Publishing Ltd and Sissa Medialab srl. Source

In the framework of innovative feedback schemes for control of dynamo tearing modes (TMs) in reversed field pinch (RFP) devices, the possibility of placing active coils between a non-conducting first wall and the vacuum vessel is investigated with a MHD based model. In this formulation the vacuum vessel plays the role of a stabilizing shell. With active coils placed outside the vacuum vessel and magnetic sensors located inside it, a previous study (Zanca 2009 Plasma Phys. Control. Fusion 51 015006) has shown that the ratio between the TM radial field amplitudes at the sensors' radius and at the resonant surface can be made close to but not smaller than the ideal-shell limit. This analysis considered a continuous-time modelling of the feedback. The same model admits a very appealing solution when applied to the in-vessel coil configuration: For high gains the radial field measured by sensors located between the first wall and the coils is reduced virtually to zero and the TM rotation frequency approaches the unperturbed natural value. In this case the feedback would mimic the stabilizing action of an ideal shell placed at the sensor radius. An improvement which makes the model closer to a realistic digital feedback is also presented. This is realized by introducing a discretetime feedback action which includes a non-zero latency time. Unfortunately, the nice solution for in-vessel coils becomes unstable for realistic TM amplitudes when passing to the discrete-time feedback. In contrast the feedback of outvessel coils is robust to time discretization, except when the vacuum vessel time constant becomes very small. This analysis indicates that future RFPs should rely on feedback systems of out-vessel coils. © 2010 IOP Publishing Ltd. Source

Consorzio RFX | Date: 2013-05-31

A method for making junctions between a first body (A) made of a first material includes a tube (A

Wang Z.R.,Consorzio RFX | Guo S.C.,Consorzio RFX | Liu Y.Q.,EURATOM
Physics of Plasmas

The physics of kinetic effects on the resistive wall mode (RWM) stability is studied, and a comparison between reversed field pinch (RFP) and Tokamak configurations is made. The toroidal, magnetohydrodynamic (MHD)-kinetic hybrid stability code MARS-K, in which the drift kinetic effects are self-consistently incorporated into the MHD formulation, is upgraded with an extensive energy analysis module. In the tokamak configuration, the kinetic effect can stabilize the mode with very slow, or vanishing plasma rotation, due to the mode resonance with the toroidal precession drift of thermal trapped particles. In RFP, instead, stabilization of the RWM comes mainly from the ion acoustic Landau damping (i.e., the transit resonance of passing particles). In the high beta region, the critical flow rotation frequency required for the mode stabilization is predicted to be in the ion acoustic range. Detailed physical analyses, based on the perturbed potential energy components, have been performed to gain understanding of the stabilizing mechanism in the two different systems. © 2012 Euratom. Source

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