Start of SPIDER operation towards ITER neutral beams

Heating Neutral Beam (HNB) Injectors will constitute the main plasma heating and current drive tool both in ITER and JT60-SA, which are the next major experimental steps for demonstrating nuclear fusion as viable energy source. In ITER, in order to achieve the required thermonuclear fusion power gain Q=10 for short pulse operation and Q=5 for long pulse operation (up to 3600s), two HNB injectors will be needed [1], each delivering a total power of about 16.5 MW into the magnetically-confined plasma, by means of neutral hydrogen or deuterium particles having a specific energy of about 1 MeV. Since only negatively charged particles can be efficiently neutralized at such energy, the ITER HNB injectors [2] will be based on negative ions, generated by caesium-catalysed surface conversion of atoms in a radio-frequency driven plasma source. A negative deuterium ion current of more than 40 A will be extracted, accelerated and focused in a multi-aperture, multi-stage electrostatic accelerator, having 1280 apertures (~ 14 mm diam.) and 5 acceleration stages (~200 kV each) [3]. After passing through a narrow gas-cell neutralizer, the residual ions will be deflected and discarded, whereas the neutralized particles will continue their trajectory through a duct into the tokamak vessels to deliver the required heating power to the ITER plasma for a pulse duration of about 3600 s. Although the operating principles and the implementation of the most critical parts of the injector have been tested in different experiments, the ITER NBI requirements have never been simultaneously attained. In order to reduce the risks and to optimize the design and operating procedures of the HNB for ITER, a dedicated Neutral Beam Test Facility (NBTF) [4] has been promoted by the ITER Organization with the contribution of the European Union\u2019s Joint Undertaking for ITER and of the Italian Government, with the participation of the Japanese and Indian Domestic Agencies (JADA and INDA) and of several European laboratories, such as IPP-Garching, KIT-Karlsruhe, CCFE-Culham, CEA-Cadarache. The NBTF, nicknamed PRIMA, has been set up at Consorzio RFX in Padova, Italy [5]. The planned experiments will verify continuous HNB operation for one hour, under stringent requirements for beam divergence (< 7 mrad) and aiming (within 2 mrad). To study and optimise HNB performances, the NBTF includes two experiments: MITICA, full-scale NBI prototype with 1 MeV particle energy and SPIDER, with 100 keV particle energy and 40 A current, aiming at testing and optimizing the full-scale ion source. SPIDER will focus on source uniformity, negative ion current density and beam optics. In June 2018 the experimental operation of SPIDER has started

Evidences of accumulation points in cascade regenerative phenomena observed in high voltage dc devices insulated by long vacuum gaps

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FIRST TEST RESULTS OF RF GUN FOR THE RACE-TRACK MICROTRON RECUPERATOR OF BINP SB RAS*

Abstract A new electron source for the Race-Track Microtron Recuperator is being developed by BINP SB RAS. It will increase average beam current and brightness of synchrotron radiation. Instead of the static 300kV electron gun operated now we are developing RF gun with the same energy of electrons. This RF gun consists of RF cavity with a gridded thermo cathode mounted on the back wall. RF cavity is driven by a 60 kW generator with last stage equipped by GU101A tetrode tube. Operational frequency of the cavity is 90.2 MHz. It is equal to the second subharmonic of the Microtron RF system frequency. A set of low power electronics controls amplitude of the cavity voltage and its tuner. This system, including a diagnostics beam line, has been installed to serve as a test bench to test the RF cavity and for beam dynamics studies. In continuous regime the designed 300 kV voltages at the acceleration gap is obtained. This paper summarizes the first test results of the cavity in this configuration

A novel tool for breakdown probability predictions on multi-electrode multi-voltage systems

An innovative approach for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps, is presented. It is based on the correlation between the clump mechanism and a statistical approach to the breakdown probability. The aim of this paper is twofold. First, the numerical solution of 3-D electrostatic problems by a couple of complementary formulations is presented. Second, an efficient post-processing tool is introduced, based on the analytical solution of the equations of motion in a domain covered by a tetrahedral mesh, to estimate the breakdown probability associated to the electrically charged microparticles leaving one electrode and clashing to the electrode with opposite polarity with sufficient energy to get vaporization. This approach has been benchmarked on a reference configuration (sphere/plane) problem and applied to calculate the particle trajectories in a very complex multi-electrode multi-voltage system

Effect of magnetic field on voltage holding in the MITICA electrostatic accelerator

MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for ITER. This experimental device, presently under construction in Padova, includes a negative Ion Source (H- or D-), and an electrostatic Accelerator (1 MV, 40 A, 3600 s). Voltage holding is recognized to be one of the most critical issues for the 1 MV accelerator operations, not only due to the complex multi-stage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the co-extracted and secondary electrons as early as possible, before they are accelerated. The required magnetic field is produced by a combination of several sources, such as permanent magnets and current-carrying conductors. In order to avoid breakdowns, the design of the accelerator shall guarantee that electrostatic configuration and the pressure distribution correspond to an operating point located on the left branch of the Paschen curve, i.e. where the H2 gas pressure multiplied by the largest electrode distance (p.d) is always below a certain limit, which is 0.1-0.3 Pa.m. Actually, the presence of the magnetic field might reduce this limit, moving the left branch of the Paschen curve towards the operating point thus affecting voltage holding even during the conditioning phase. In order to support the design of MITICA at low gas pressure and in the presence of magnetic field, an experimental campaign has been carried out at the High Voltage Padova Test Facility

Voltage holding optimization of the MITICA electrostatic accelerator

Two Heating Neutral Beam Injectors (H-NBI) are planned to be installed in ITER with a total delivered heating power of 33 MW. The main parameters are: 870 kV acceleration voltage with 46 A beam current for Hydrogen beam, and 1 MV voltage with 40 A current for Deuterium beam. The required pulse duration is 3600 s. The combination of these design requirements constitutes a large leap with respect to the performances of the Heating NBI systems presently in operation. For this reason the construction of a specific test facility (PRIMA) has been launched in order to provide the design and operational experience necessary for the successful implementation of the ITER H-NBI system. PRIMA will include a full-size Negative Ion Source with 100 keV extractor and accelerator (SPIDER) and a complete prototype of the ITER 1 MeV Injector accelerator and neutralizer (MITICA). The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long pulse (3600s) beam operation, due to the complex electrostatic structure formed by differently shaped electrodes polarized at different potentials. Furthermore, the system works in the p.d range at the left of the Paschen curve where the classical Townsend breakdown criterion is no longer valid. The voltage holding is governed by the mechanism of the long gap insulation in high vacuum, not yet well understood and consolidated. This paper is aimed to describe the optimization of the voltage holding capability for MITICA electrostatic accelerator. As a first step an accurate 3D solution is obtained by a pair of complementary formulations which provide rigorous upper and lower bounds of the numerical solution (fundamental especially in problems that are big enough to prevent error convergence study. The results of this analysis will constitute the input for the Probabilistic Model which is adopted to predict the breakdown probability by means of 2D analyses of the multi electrode \u2013 multi voltage syste

Goal-oriented adaptivity for voltage breakdown prediction

An innovative approach has been recently proposed for the voltage breakdown prediction in high-voltage systems, insulated by large vacuum gaps. This approach is based on complementary geometric formulations for electrostatics coupled to the analytical solution of the equations of motion for charged particles. In this paper, a goal-oriented local mesh refinement technique is introduced, which allows to increase the rate of convergence of the solution, enabling an effective voltage breakdown prediction also in large-scale systems with complex geometries. \ua9 1965-2012 IEEE

Simulation, Code Benchmarking and Optimization of the Magnetic Field Configuration in a Negative Ion Accelerator

SPIDER is a full-size Negative Ion Source and a multi-beamlet 100 kV accelerator, presently under con- struction in Padova and is part of PRIMA, the test bed for the full development of the Heating Neutral Beam system for ITER. During the \ufb01nal design, the SPIDER extractor and accelerator system has been improved with respect to the original con\ufb01guration. In particular, the magnetic con\ufb01guration has been optimized in order to improve the performances in terms of ion beam optics and aiming, and to obtain an ef\ufb01cient \ufb01lter for the extracted electrons

Voltage holding optimization of the MITICA electrostatic accelerator

Two Heating Neutral Beam Injectors (H-NBI) are planned to be installed in ITER with a total delivered heating power of 33 MW [1]. The main parameters are: 870 kV acceleration voltage with 46 A beam current for hydrogen beam, and 1 MV voltage with 40 A current for deuterium beam. The voltage holding in the 1 MV ITER Neutral Beam Accelerator is recognized to be one of the most critical issues for long pulse (3600 s) beam operation, due to the complex electrostatic structure formed by electrodes polarized at different potentials immersed in vacuum or low-pressure gas. As a matter of fact, the system shall work in a p × d range at the left of the Paschen curve where the classical Townsend breakdown criterion is no longer valid. The voltage holding is governed by the mechanism of the long gap insulation in high vacuum, not yet well consolidated from the physical point of view. This paper is aimed to describe the optimization of the voltage holding capability for MITICA electrostatic accelerator. The results of this analysis will constitute the input for the probabilistic model [3] which is adopted to predict the breakdown probability by means of 2D analyses of the multi electrode – multi voltage system

Prediction of lightning impulse voltage induced breakdown in vacuum interrupters

A concept to predict the Lightning Impulse Voltage (LIV) breakdown probability of medium voltage Vacuum Interrupter (VI) tubes has been developed and tested for the first time employing the Voltage Holding Prediction Model (VHPM) originally formulated at Consorzio RFX. The VHPM is capable to calculate the Weibull breakdown probability curve of any multi-electrode multi-voltage system insulated in vacuum under dc voltage. Even though the possibility to employ the VHPM in the prediction of voltage breakdown under LIV conditions is not straightforward, the potential benefits of the VHPM usage in VI tube technology with regard to development, design and testing at medium and high voltage levels, strongly recommend the assessment of VHPM under LIV conditions. The paper aims to describe the methodology adopted to identify experimentally the VHPM parameters to be employed in such a novel application to assess its prediction capability. The measurements have been done in the Siemens Berlin VI tube factory test stand for LIV, compliant with the IEC standard. Different vacuum tube types manufactured by Siemens were investigated. The main difficulty was to obtain, with such a test bed, voltage breakdown distributions well fitted by a Weibull Distribution - upon which the VHPM is based - primarily due to the difficulty to reach a well-defined end of the conditioning test sequence preceding the relevant voltage breakdown test sequence suitable to be analyzed by the VHPM. Nevertheless, the VHPM prediction resulted in reasonable agreement with the measured probability curves. Finally, investigating VI tubes with spiral-type Radial Magnetic Field (RMF) contacts, characterized by a geometry strongly deviating from axial 2-D symmetry, the first implementation of a full 3-D version of the VHPM has been tested. \ua9 1994-2012 IEEE