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

Characterization of cesium and H-/D- density in the negative ion source SPIDER

The Heating Neutral Beam Injectors (HNBs) for ITER will have to deliver 16.7 MW beams of H/D particles at 1 MeV energy. The beams will be produced from H-/D- ions, generated by a radiofrequency plasma source coupled to an ion acceleration system. A prototype of the ITER HNB ion source is being tested in the SPIDER experiment, part of the ITER Neutral Beam Test Facility at Consorzio RFX. Reaching the design targets for beam current density and fraction of coextracted electrons is only possible by evaporating cesium in the source, in particular on the plasma facing grid (PG) of the acceleration system. In this way the work function of the surfaces decreases, significantly increasing the amount of surface reactions that convert neutrals and positive ions into H-/D-. It is then of paramount importance to monitor the density of negative ions and the density of Cs in the proximity of the PG. Monitoring the Cs spatial distribution along the PG is also essential to guarantee the uniformity of the beam current. In SPIDER, this is possible thanks to the Cavity Ringdown Spectroscopy (CRDS) and the Laser absorption Spectroscopy diagnostics (LAS), which provide line-integrated measurements of negative ion density and neutral, ground state Cs density, respectively. The paper discusses the CRDS and LAS measurements as a function of input power and of the magnetic and electric field used to reduce the coextraction of electrons. Negative ion density data are in qualitative agreement with the results in Cs-free conditions. In agreement with simulations, Cs density is peaked in the center of the source; a top/bottom non uniformity is however present. Several effects of plasma on Cs deposition are presented.Comment: 17 pages, 9 figures. Paper (Preprint) following the poster contribution at the SOFT 2022 conference. The destination journal is Fusion Engineering and Desig

Radiocesio nei mieli nel Friuli-Venezia Giulia dopo l'incidente di Chernobyl

ATTI CONVEGNO"10 ANNI DA CHERNOBYL: RICERCHE IN RADIOECOLOGIA, MONITORAGGIO AMBIENTAL

Influence of plasma grid-masking on the results of early SPIDER operation

SPIDER is the prototype negative ion source of the heating neutral beam injectors for ITER. As required by the ITER injector design, the SPIDER beam source is entirely contained inside the vacuum vessel, so that voltage holding depends on the background gas pressure. During the recent operation, the number of extraction apertures was limited in order to routinely operate the ion source with its nominal filling pressure while minimizing the vessel pressure. To close the apertures, a machined molybdenum sheet was installed downstream the plasma-facing electrode of the accelerator. In this contribution, we discuss the implications of this configuration, highlighting possible influences on the experimental results obtained so far, and the differences that can be expected in the future high-current operation (with all 1280 apertures). Among other topics, the possible effects on the use of caesium, the negative-ion density at the extraction region, the per-veance of the extractor and the profile of neutrals along the accelerator will be discussed

Study of positive ion transport to the plasma electrode in giant RF negative ion sources

Negative ion sources are fundamental components of neutral beam injectors (NBI), one of the main heating systems for fusion reactors. SPIDER is the full-scale prototype negative ion source for ITER NBIs. It is hosted in Padua as part of the Neutral Beam Test Facility (NBTF). It aims to extract up to 330 A m−2 of negative hydrogen ions from an inductively coupled plasma, generated inside 8 cylindrical drivers. The negative ion production is enhanced by caesium evaporation inside the source. In caesium-seeded negative ion sources, negative ions are produced close to the extraction apertures, and they are mainly generated by surface conversion of neutral atoms and positive ions impinging on the ion source walls, particularly on the plasma grid. The conversion yields depend on the energy distribution of these precursors, and so does the energy of those particles which are reflected as negative ions. The positive ion flow in the extraction region may also impact on the extraction probability of negative ions, via momentum transfer. Besides, in giant multi-driver RF sources such as SPIDER, a gradient of plasma potential is present in the expansion region Sartori et al. (2021), affecting the positive ion transport towards the caesiated plasma electrode and their energy. To approach this complex problem, a 3D test-particle Monte Carlo code for tracing plasma motion in SPIDER was developed. Positive ions species are generated in different positions within the plasma source volume and are tracked under the influence of electric and magnetic fields. Then, Monte Carlo collisions are used to simulate the interaction with predetermined backgrounds of plasma and neutrals, with profiles derived from experimental data. The particles are traced until they hit the ion source walls. Finally, the energy distribution of the different particle species impinging on the plasma grid (PG) are determined, and used to assess the generation and the energy distribution of the produced H

Analytical study of caesium-wall interaction parameters within a hydrogen plasma

Understanding the distribution of caesium by plasma within the expansion region of the SPIDER beam source is essential to maximise the efficiency of negative ion surface-production. The caesium is redistributed by plasma diffusion and is greatly affected by the plasma-wall interactions with the molybdenum walls. This work considers the processes which make up the plasma wall interaction and the transmission of generated particles across the plasma sheath. An analytic model is set-up for each of the four main processes that occur at the wall: thermal desorption (evaporation), physical desorption (sputtering), backscattering and adsorption, with the latter being most significant. These processes occur in response not only to the wall temperature but also to the influx of particles to the wall, and generate atoms/ions outwards to the plasma bulk which must first cross the collisional plasma sheath and pre-sheath. The probability of re-entering the bulk is quantified by a transmission factor, which is calculated specifically for each charge state and process, as it considers the energies at which the process occurs. The transmission factor considers the mean free path of ionising collisions, along with comparing the potential and kinetic energy of each ion with the threshold energy to leave the sheath. The combination of the probability of transmission and the fluxes generated at the wall allows for the study of the redistribution from the plasma discharge within the beam source

Development of a Triple Langmuir Probe for Plasma Characterization in SPIDER

This contribution describes the commissioning and first operation of a fixed triple Langmuir probe in source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER), the full-scale prototype of the negative-ion source for ITER neutral beam injectors. This diagnostic is capable of measuring the time evolution of the main plasma features for the entire duration of the plasma discharge, allowing to correlate local plasma properties, such as density, electron temperature, and plasma potential, to control parameters, i.e., filter field intensity, radio frequency (RF) power, and bias voltages. The probe was installed on the rear wall of the expansion chamber, and this being a region of specific interest for the operation with cesium since it may act as a reservoir, contributing to the cesium recycling toward the extraction region. Furthermore, the capability of the triple probe arrangement of measuring the instantaneous plasma density and electron temperature (also at high frequency) has been exploited to investigate the modulation of plasma parameters

Interpreting the dynamic equilibrium during evaporation in a cesium environment

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained