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The conceptual designing of the vacuum system for the new tokamak DTT (Divertor Tokamak Test) is hereafter presented.

The main purpose of the Divertor Tokamak Test facility (DTT) is to study alternative solutions to mitigate the issue of power exhaust under integrated physics and technical conditions relevant for ITER and DEMO. In this framework, the conceptual design of the beamline for the DTT Neutral Beam Heating system is here summarized, with a particular focus on the technical solutions adopted to fulfill the requirements and maximize beamline performances. The proposed system features a beamline providing deuterium neutrals (D0) with an energy of 510 keV and an injected power of 10 MW. Various design options were considered, and a comprehensive set of simulations was carried out using several physics and engineering codes to drive the choice of the most suitable design options and optimize them, aiming at finding a good compromise among different design requirements. These simulations mainly regard the efficiency of the main processes, the optics of the beam, the physics reactions along the beamline (stripping, charge-exchange and ionization), the thermo-mechanical behaviour of the acceleration grids and the coupling between the beam and the plasma in the tokamak chamber. This paper describes the design of the main components of the injector for the DTT NBI system, i.e. ion source, accelerator, beam line components and vacuum vessel, explaining the motivations for the main design choices.

The high-Q operation of the ITER tokamak will require two Neutral Beam Injectors (NBIs) for plasma heating and current drive. Each beam will be generated by a 40 A current of Deuterium negative ions, accelerated up to the specific energy of 1 MeV and then neutralized. The power delivered to the plasma by each NBI shall reach 16 MW with duration up to 1 h. The beam source will be constituted by an RF-driven negative ion source at -1 MV potential, by a Multi-Aperture, Multi-Grid (MAMuG) electrostatic accelerator (consisting of 5 stages at intermediate potentials), and a gas-box neutralizer at ground potential. All components will be installed in a vacuum vessel (also at ground potential), together with a high-capacity cryo-pumping system which controls of the background gas pressure. In order to validate the ITER NBI design and address all the outstanding issues related to these demanding requirements, a full scale prototype called MITICA (Megavolt ITER Injector & Concept Advancement), is under construction in Padova at Consorzio RFX. Voltage insulation in vacuum and/or very low-pressure gas on a single gap is indeed one of the expected issues which MITICA will have to deal with. For this reason, before the installation of the MITICA Beam source, an experimental program for anticipating the full-scale voltage holding tests is studied and proposed, using the real MITICA vacuum vessel (on-site) together with a beam source mock-up without and with an intermediate electrostatic shield, in order to get a first voltage holding assessment of the expected MITICA voltage holding capability, to evaluate the effect of the intermediate shield and to assess the effect of the background pressure on the voltage holding capability of the system.

A dedicated Neutral Beam Test Facility (NBTF) has been set up to verify ITER requirements for the neutral beam injectors. The facility, located at Consorzio RFX in Padua (Italy), hosts two experiments: SPIDER and MITICA. SPIDER (Source for the Production of Ions of Deuterium Extracted from Rf plasma) is the full-scale prototype of the ITER source injectors. Its accelerator can be set up to 100 kV and its aim is to investigate and optimize the properties of the ion source. MITICA (Megavolt ITER Injector and Concept Advancement) is the full-scale prototype of the entire injector, devoted to the issues related to the accelerator, including voltage holding at low gas pressure. SPIDER has started operations in 2019 and the plasma without caesium has been characterized. Caesium evaporation, aimed at enhancing the production of negative ions, has recently started during SPIDER experimental campaigns. Understanding the behaviour of caesium during plasma operation is necessary to optimize the generation of negative Hydrogen ions. There are several kinds of contribution to the caesium evaporation in a plasma source. To determine the relative contribution of these fluxes, it is necessary to know the desorption energy of the Cs compounds deposited over the source walls. The Temperature Programmed Desorption (TPD) diagnostic allows to to estimate these energies thanks to a pair of tungsten filaments. The design and development of the device are presented. The diagnostic has been duplicated so it was possible to test its reliability while installing the twin one in SPIDER. First SPIDER experimental results, both using the diagnostic as a TPD and as a Langmuir-Taylor Detector, are shown. Finally, first estimations of the desorption energy are determined in order to finalize plasma-Cs model for SPIDER.

The MITICA experiment at Consorzio RFX Padova, within the framework of the European Magnetic Confinement Fusion effort, is a full-scale prototype of the Neutral Beam Injector (NBI) heating system envisioned for ITER, which will be required to produce 1MeV beams of deuterium with an ion current of 40 A for up to 1 hour. These specifications far exceed any current NBI devices with a compact electrostatic accelerator to operate at 1MV being double the voltage reached up to now. To achieve these conditions the entire beam source and accelerator of MITICA are insulated by a vacuum instead of the more commonly used insulating gas. This is to avoid radiation induced conductivity in the gas by radiation produced from fusion reactions. However, this serves to be an overall more complex system and vacuum insulation has its own substantial hurdles regards stability and voltage holding. The High Voltage Padova Test Facility (HVPTF) is a concurrent R&D project focused on understanding the physical processes behind breakdown and micro-discharges in high-voltage insulation, as well as developing new diagnostics, models, and operational modes to address these challenges and aid in the stable operation of MITICA. The highresolution charac-terization of the X-ray energy spectrum during the high voltage conditioning of a multi electrode vacuum insulated system such as HVPTF could be vital to understand the fundamentals behind voltage breakdowns and which stage is causing the micro discharge onset. This technique seems particularly promising for measuring small dark currents (< 10?A), which are generally difficult to be measured with standard techniques. The X-rays produced during HV experiments are mainly due to bremsstrahlung interaction between the electrons escaping from the HV electrodes (field emission effect) and the background gas or the chamber walls. The X-rays spectrum extends from low energy ( ? keV) up to several hundreds of keV depending on the maximum potential difference applied to the electrodes (up to 800 kV). During these events high photon flux (>106 photons/cm2s) is produced. This paper shows the development and preliminary results of a newly designed X-Ray diagnostic based on Gas Electron Multiplier (XR-GEM). This detector is able to stand very high rate (> MHz) in single photon counting mode and can cover the energy range from 3 - 50 keV. The XR GEM detector is equipped with anodic pads (256 pads 6x6 mm2) readout with a new data acquisition system called GEMINI, which gives the possibility to obtain a counting rate of several MHz and, thus, sub-ms time resolution together with mm spatial resolution.

The Neutral Beam Test Facility (NBTF, Padova, Italy) is a facility devoted to the test and optimization of the Neautral Beam Injector for ITER. A full-scale prototype, called MITICA, is under construction at NBTF and has to demonstrate all the design parameters for ITER, among which a stable beam operation at the enrgy of 1MeV. In order to test and optimize the stable high voltage holding of 1MV, a dedicated campaign is foreseen for 2022, utilizing realistic mock-ups of MITICA ion source and accelerator. During the campaign, the mock-up of the ion source will be biased with an increasing potential up to -1MV and the discharge towards the grounded electrodes will be observed. In this way, possible weak points of the geometry will be detected and possibly mitigated. As diagnostics for discharge position and intensity, a system of cameras was designed, including visible cameras, IR camera and fast camera. The results of this optical diagnostics will be then compared with the electric measuments of the discharges taken on a dedicated ground electrode, for a cross check. This paper describes the motivations of the choice of the type and number of diagnostic cameras adopted during the experiments, the optimization of their layout, which includes also the use of a series of mirrors, the design of supports and protection systems, and the procedure to interpret the experimental data and cross check them with the electric measurements.

Questa è la Specifica Tecnica per la realizzazione della linea di immissione gas per gli HV tests che verranno condotti su MITICA prima dell'installazione della Beam Source.

Il documento contiene le specifiche tecniche per la realizzazione del sistema di automazione dell'impianto denominato impianto ausiliario di immissione gas e pompaggio di MITICA. Il sistema sarà utilizzato per le prove in alta tensione previste in MITICA.

An important diagnostic for the high voltage test campaign on MITICA mockup is the visual investigation of the electrode surface during glow discharges and breakdowns. A system of five or more cameras is foreseen for the tests, including visible cameras, a fast camera and an infrared camera. In order to set up the camera system, the suitable ports on beam source vessel shall be chosen and the supports for the cameras shall be designed. This document describes the scope statement of the camera system, the specification of the surfaces to be monitored during the experiment, the determination of the suitable ports and the technical requirements for the support system to be procured.

The development of MITICA, the prototype for a neutral beam injector for ITER, drives the interest in investigating HV insulation in vacuum. The High Voltage Padova Test Facility (HVPTF) is an experimental device which has the double aim of studying the physical phenomena underlying the voltage holding in vacuum and testing technical solutions to increase the breakdown threshold. HVPTF features a vacuum chamber containing two stainless steel electrodes separated by an adjustable gap of few centimeters. Electrodes are available in different shapes and can achieve an HV difference up to 800 kV. Both the current and the voltage of the electrodes are sampled at a 100 Hz rate along with the vacuum pressure and the gas composition. Two scintillating crystals, a LYSO and a LaBr3, are installed to detect the hard X-ray bremsstrahlung radiation produced by the interaction of the free charges accelerated by the HV difference on the electrode surfaces. Both scintillators are coupled to photomultipliers and have small active volumes and fast electronics, resulting in very fast signals (40-100 ns); this minimizes the pile-up effect and enhances time resolution, allowing for the measurement of X-ray emission spectra to up to 500 keV with a time-width of few hundreds of ?s. The electrodes are subject to a conditioning process through which the breakdown voltage is gradually increased until the system reaches a saturation value. Between major breakdown discharges a series of current micro-discharges are observed, during which the number of bremsstrahlung photons drops almost to zero. A global increase in gas emission is measured in correspondence of such events, likely due to degassing induced by the discharges. The aim of this contribution is to expand the knowledge around the micro-discharge dynamics focusing on the bremsstrahlung spectra obtained through the scintillators. Different micro-discharge types will be characterized and put in relation with the different electrodes and the conditioning phase.

High voltage insulation across a single gap in vacuum and low-pressure gas is a critical issue in relation to the development and realization of the electrostatic accelerator for the ITER Neutral Beam Injector (NBI). The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the Beam Source of MITICA, the prototype of the ITER NBI now at the beginning of the commissioning phase, in Padova The high voltage conditioning procedure is an in situ treatment based on progressively increase of the voltage with frequent breakdown aimed at reaching the ultimate voltage holding, by eliminating the sources of breakdown located on the surface of the electrodes (microtips, particles, adsorbed gas) insulated by high vacuum (

Questo documento rappresenta la specifica tecnica per la fornitura dei due adattatori ISO F DN500 per il collegamento di due criopompe al Vessel da vuoto dell'esperimento MITICA.