The Transmission Line for the SPIDER experiment

The 100 keV Ion Source Test facility – Source for the Production of Ions of Deuterium Extracted from RF plasma (SPIDER) – is aimed to test the full scale prototype of the Ion Source for the ITER 1 MeV Neutral Beam Injector (NBI). The SPIDER facility requires the construction of a High Voltage Deck (HVD) and of a High Voltage Transmission Line (TL) respectively to host the Ion Source Power Supplies system polarized at 100 kV and to carry the power and signal conductors to the beam accelerator.In already existing NBI systems with beam energy above 100 keV, the TL is realized with the SF₆ Gas Insulated Line technology. In the SPIDER TL case, the presence of a large inner conductor (half meter diameter), would make the pressurized TL a complex and costly component; therefore a free air insulated solution has been proposed. The paper focuses on the design of this TL, which has to host inside the complex high potential (100 kV) inner electrode a number of power and measuring conductors and has to minimize the Electro Magnetic Interferences (EMI) produced by the frequent grids breakdowns.Finite Element (FE) analyses have been performed to verify the configuration from the electrostatic point of view, to evaluate EMI screening effectiveness and to assess the impact of the relatively high thermal dissipation of power conductors located inside the high potential electrode. Moreover, an experimental test campaign has been carried out on a TL mockup to validate the TL electrostatic configuration under DC voltage. Finally, the paper reports on the status of procurement activities for the Transmission Line.     

HVPTF—The high voltage laboratory for the ITER Neutral Beam test facility

In the MITICA research program for the construction of the ITER Neutral Beam Injector prototype, a Laboratory for the investigation on high voltage holding in vacuum has been set up. This Laboratory - HVPTF: High Voltage Padova Test Facility - is presently capable of experiments up to 300 kV dc, and planned for the upgrade to 800 kV. The specific mission for this ancillary lab is the support to the electrostatic design and construction of the MITICA accelerator and the development and testing of HV components to be installed inside the MITICA accelerator during its operation.The paper describes the structure of the lab, characterized by a high degree of automation and reports the results of the commissioning at 300 kV and the first results of voltage holding between test electrodes.     

A model for electrical fast transient analyses of the ITER NBI power supplies and the MAMuG accelerator

The design of ITER Neutral Beam Injector (NBI) is based on a five-stage electrostatic accelerator, known as Multi-Aperture Multi-Grid (MAMuG) and characterised by an overall acceleration voltage of ?1 MV. The MAMuG accelerator requires a five-stage power supply system under strict load protection requirements, being subjected in operation to breakdowns. In this paper a circuit model of ITER Neutral Beam Injector power supplies and MAMuG accelerator is illustrated, for the simulation of fast transients related to accelerator breakdowns in particular. Consideration of the high voltage involved and of the complex inductive and capacitive couplings implied careful assessment of stray parameters by calculations with finite element techniques. The circuit model, developed to address a number of design issues requiring simulations at system level, is now ready for use—the optimisation of passive protections being the most significant application.     

Conceptual design of a high-voltage compact bushing for application to future N-NBI systems of fusion reactors

The energy of future neutral beam injector (NBI) heating systems of fusion power plants ranges from 1 to 2 MeV. They are based on powerful (several tens of MW) hydrogen negative ion electrostatic accelerators where electrodes are polarized by DC high-voltage. The beam line under vacuum is supplied by HV power supplies via a transmission line pressured under SF₆ and a high voltage feedthrough called bushing. The paper presents results obtained over experimental campaigns dedicated to high voltage vacuum insulation for future NBI systems (ITER). It addresses the problematic of the electron field emission and the high voltage breakdown limit under vacuum between large electrode surfaces. The paper highlights the dependence of the electron emission (dark current) with the voltage and the background tank pressure: at low pressure (～1E?3 Pa in hydrogen), an important dark current of I ～ 100 mA has been measured at 500 kV, while at higher pressure (～0.3 Pa in helium), the dark current has been nearly suppressed (less than 3 mA of dark current at 970 kV). The paper shows that a field induced gas adsorption process could occur on the emitting surfaces (cathode), and this process tends to lower the electron field emission current by increasing the work function of the electrode surface. The Fowler–Nordheim law applied to the measured dark current indicates about 70% of work function increase at 0.3 Pa in helium. Finally, a new high-voltage bushing concept relevant to the future NBI systems is presented; it is based on these experimental findings in high voltage vacuum insulation; the main feature of the new bushing concept is to take benefit of the field induced adsorption effect, i.e., the suppression of the dark current with helium gas, in the inner part of the bushing where the electric field intensity is highest.     

A new approach to passive protection against high energy and high current breakdowns in the ITER NBI accelerator

The energy stored in the 1 MV ITER Neutral Beam Injector power supply system will exceed by far the energy stored in the existing largest NB Injectors; as a consequence, the limitation of the grid breakdown effects–grids damage and Electro Magnetic Interference emission–are critical issues. In the present ITER NBI reference design the mitigation system is based on the concept of the concentrated core snubber which, due to the large amount of stored energy, is a huge component. Furthermore, in the NBI a relatively large part of HV capacitance to ground remains downstream the core snubber, so neither the arc peak current nor the high-frequency oscillations can be effectively limited. Moreover, the concentrated core snubber is ineffective in limiting the voltage reversal caused by internal insulation fault, increasing the risk of cascade failures in components like HV bushing and transmission line. The paper proposes an alternative approach to limit the grid breakdown effects, based on the concepts of Damper Resistor- substituting the direct connection to ground of the zero-potential accelerating grid – and of Distributed Core Snubber (DCS) – installed along the whole length of the transmission lines. The DCS concept has been subjected also to experimental validation by a small scale setup supported by electrical modelling.     

Mechanical tensile test of the full-size ITER TF conductor jacket materials at 4.2 K

316LN stainless steel is selected as a material for toroidal-field (TF) conductor jacket of International Thermonuclear Experimental Reactor (ITER). In order to evaluate the true mechanical performance of the jacket material at 4.2 K and its suitability as the ITER TF conductor jacket, the mechanical properties of the full-size TF conductor jacket tube and sub-size specimens at 4.2 K and 300 K were investigated according to ASTM standards. The measured yield strength and elongation at 4.2 K for sub-size specimens and full-size tubes are more than 950 MPa and 20%, respectively. In addition, the fractographies of all fractured specimens were observed using scanning electron microscope (SEM). These results suggest that the TF conductor jacket can satisfy ITER requirements and the result of the full-size tube at 4.2 K is more representative and important for practical applications.     

5 MW CW supply system for the ITER gyrotrons Test Facility

ECH (Electron Cyclotron Heating) for ITER will deliver into the plasma 20 MW of RF power. The procurement of the RF sources will be shared equally between the three following partners: Europe, Japan and Russia. Moreover, Europe decided to develop a RF source capable of 2 MW CW of RF power, based on the design of a coaxial gyrotron with a depressed collector. In order to be able to develop and test these RF sources, a Test Facility (TF) has been built at the CRPP premises in Lausanne (CH).The present paper will first remind the main operation conditions considered to test safely a gyrotron. The power supplies parameters allowing to fulfill these conditions will be reviewed. The core of the paper content will describe the newly installed Main High Voltage Power Supply (MHVPS), to be connected to the gyrotron cathode and capable of ?60 kV/80 A-CW. The principle, the characteristics, the on-site test results will be described at the light of the requirements imposed by the gyrotron testing. Particular aspects of the installation and commissioning on-site will be highlighted in comparison with the ITER environment. The synchronized operation of the MHVPS and the BPS (Body Power Supply) on dummy load, piloted through the TF remote control, will be presented and commented.Since the TF supply structure has been built integrating the particular conditions and requirements expected for ITER, a conclusion will summarize the performances obtained at the light of these criteria.     

Design optimization of the 100 kV HV bushing for ITER-DNB

The 100 kV bushing of the Diagnostic Neutral Beam (DNB) injector is a cylindrical feedthrough which forms the interface between the gas insulated transmission line and the torus primary vacuum and provides all necessary services to the beam source. All conventions for safety, voltage holding requirement, vacuum compatibility and the choice of materials have been addressed in the design. Finite Element Analyses (FEAs) for the electrostatic and the structural configuration is carried out to validate the design of the High Voltage Bushing (HVB). Several iterations and optimizations of the stress shields are carried out to meet electrostatic criteria, especially at the triple point (the ceramic, metal and vacuum joint), which is critical for good voltage holding. Structural analyses is carried out to assess the stress distribution in the fiber reinforced plastic insulator, the alumina insulator and the integrated HVB for different load cases like operational orientation (horizontal), normal operation and accidental case. Design is further validated for seismic conditions for Seismic Loading-2 (SL-2).     

Improvements of the Jena AMS system

A new slit stabilization circuit and a modified generator drive were installed on an AMS system based on a 3 MV Tandetron?, produced by High Voltage Engineering Europa (HVEE). Furthermore our test procedure used at the Jena AMS system for the rectifiers of the Cockroft–Walton generator is presented.     

Study of large gap 200 kV breakdowns in vacuum with high stored energy for ITER neutral beam accelerator

In the context of the ITER contract “ITER/CT/07/219–200 kV Stored Energy Tests”, electrical breakdown tests have been performed in vacuum with a stored energy of up to 425 J. The experiments have been conceived and performed with the collaboration of Consorzio RFX. The tests are being performed in the 1 MV test facility at IRFM, CEA-Cadarache. They should simulate the conditions that will be found in the ITER Neutral Beam accelerator, at 200 kV. This paper presents the set-up of the test bed, the choice of critical components, the diagnostic equipments and the results obtained with 200 kV applied on the anode electrode.     

The negative ion source test facility ELISE

The ITER neutral beam system is using inductively coupled radio frequency (RF) ion sources, that have demonstrated the required ITER parameters on (small) sources with extraction areas up to 200 cm². As a next step towards the full size ITER source IPP is presently constructing the test facility ELISE (“Extraction from a Large Ion Source Experiment”) operating with a “half-size” source which has approximately the width but only half the height of the ITER source. The modular driver concept is expected to allow a further extrapolation to the full size in one direction to be made. The main aim of this experiment is to demonstrate the production of a large uniform negative ion beam with ITER relevant parameters in stable conditions up to one hour.Plasma operation of the source is foreseen to be performed continuously for 1 h; extraction and acceleration of negative ions up to 60 kV is only possible in pulsed mode (10 s every 180 s) due to limitations of the existing IPP HV system. The design of the source and extraction system implements a high experimental flexibility and a good diagnostic access while still staying as close as possible to the ITER design. The main differences are the source operating in air and the use of a large gate valve between the source and the target chamber.ELISE is expected to start operation at the end of 2011 and is an important step for the development of the ITER NBI system; the experience gained early will support the design as well as the commissioning and operating phases of the PRIMA NBI test facilities and the ITER neutral beam system.     

Preliminary design of the ITER ECH Upper Launcher

The design of the ITER electron cyclotron launchers recently reached the preliminary design level - the last major milestone before design finalization. The ITER ECH system contains 24 installed gyrotrons providing a maximum ECH injected power of 20 MW through transmission lines towards the tokamak. There are two EC launcher types both using a front steering mirror; one equatorial launcher (EL) for plasma heating and four upper launchers (UL) for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). A wide steering angle range of the ULs allows focusing of the beam on magnetic islands which are expected on the rational magnetic flux surfaces q = 1 (sawtooth instability), q = 3/2 and q = 2 (NTMs).In this paper the preliminary design of the ITER ECH UL is presented, including the optical system and the structural components. Highlights of the design include the torus CVD-diamond windows, the frictionless, front steering mechanism and the plasma facing blanket shield module (BSM). Numerical simulations as well as prototype tests are used to verify the design     

Near-infrared detectors for ITER LIDAR Thomson scattering

In this paper we discuss strategies for the development of fast photodetectors suitable for operation in the λ > 850 nm near-infrared (NIR) spectral region in the ITER core LIDAR Thomson scattering (TS) system. Detection of this spectral range is necessary if a Nd:YAG laser operating at the fundamental wavelength (λ = 1.06 μm) will be used as the input laser source. Different types of NIR photodetectors are potentially suitable for use in ITER LIDAR TS: the transferred electron (TE) InGaAsP/InP hybrid photodiodes and microchannel plate photomultipliers (MCP PMTs), the In_xGa_1?xAs MCP image intensifiers and PMTs, and the detectors based on transmission Si photocathodes. But their characteristics of either sensitivity, active area or speed of response, do not match the ITER specifications and all devices require some developmental work. For each of these detector types we review the characteristics of devices presently available and suggest a realistic development strategy suitable to extend their performances to meet the ITER specifications. Finally the expected performance of the ITER LIDAR TS system for different detector choices are compared by calculating the expected signal-to-noise ratio of the measured plasma temperature and density.     

Infrared thermography inspection of the ITER vertical target qualification prototypes manufactured by European industry using SATIR

High heat flux loaded components which will be installed in the ITER Divertor require a heat flux removal capability in the range 5–10 MW/m² at steady-state and up to 20 MW/m² in transients. Within the ITER plasma facing components procurement context, each party should demonstrate its technical capability to carry out the manufacturing with the required quality. This is achieved through the successful manufacturing and testing of medium-size qualification prototypes. Each Qualification Prototype consists of three high heat flux units mounted onto an actively cooled supporting structure. Currently, the SATIR method has been identified by the ITER Organization as the basic test to decide upon the final acceptance of the ITER Divertor components. SATIR testing was performed on each CFC part of European HHF units prior to the insertion of the twisted tape and prior to assembling the units onto the steel support structure. The paper deals with SATIR results of all qualification prototypes manufactured by European industry.     

Mechanical test on the ITER CC and Feeder jacket

This paper focuses on mechanical tests on the ITER correction coils (CC) and Feeder jacket 316L stainless steel material. During manufacture, the conductor will be compacted and spooled after cable insertion. Therefore, sample jackets were prepared under compaction in order to simulate the status of conductor during manufacturing. Yield strength (0.2% offset), ultimate tensile strength, Young's modulus and elongation at failure shall be reported. The mechanical properties of materials were measured at 300 K and low temperature (<7 K). The cryogenic test results show that the present jackets have very high properties. It is concluded that the results meet the ITER requirement.     

High power millimeter wave experiment of torus diamond window prototype for ITER EC H&CD system

The design of the torus diamond window for the ITER electron cyclotron heating and current drive (EC H&CD) system has advanced considering a reliable and manufacturable structure. The diamond window prototype was fabricated based on the design and the high power experiment was carried out to verify the millimeter wave transmission capability. Transmission of 740 kW-100 s was demonstrated and no significant temperature increase of the window structure and no damage on the diamond disk were obtained. The temperature saturation of the cooling water for the window was observed and loss tangent of 7.8 × 10^?6, which was the lowest value that we had ever obtained at JAEA, was evaluated. This result indicates that the diamond window design is feasible and promising the high power more than 1 MW transmission.     

Test of piezo-ceramic motor technology in ITER relevant high magnetic fields

In the framework of a Fusion for Energy (F4E) grant, a test campaign started in 2012 in order to assess the performance of the in-vessel viewing system (IVVS) probe concept and to verify its compatibility when exposed to ITER typical working conditions. ENEA laboratories went through with several tests simulating high magnetic fields, high temperature, high vacuum, gamma radiation and neutron radiation.A customized motor has been adopted to study the performances of ultrasonic piezo motors technology in high magnetic field conditions. This paper reports on the testing activity performed on the motor in a multi Tesla magnetic field. The job was carried out in a test facility of ENEA laboratories able to achieve 14 T. A maximum field of 10 T, fully compliant with ITER requirements (8 T), was applied. A specific mechanical assembly has been designed and manufactured to hold the motor in the region with high homogeneity of the field. Results obtained so far indicate that the motor is compatible with high magnetic fields, and are presented in the paper.     

The 70 kA pyrobreaker for ITER magnet back-up protection

The paper presents the results of development and testing of an explosively actuated circuit-breaker (the so-called pyrobreaker) designed and manufactured at the Efremov Institute [1]. In accordance with the ITER specifications this switch will be used for continuous operation with DC currents up to 70 kA and shall be capable, on command, to transfer this current to a resistive load under a voltage up to 10 kV in less than 1 ms.A number of current commutation tests have been carried out on several prototypes [2]. The last experimental campaign has demonstrated reliable operation of the pyrobreaker with 20% safety margin for the interrupted current and 100% margin for the recovery voltage relative to the ITER requirements.Besides, peak current withstand tests have been performed with pulse currents up to 420 kA generated by the unipolar current generator available at the Efremov Institute.