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This document presents the conceptual design of a Patch Panel and of a measuring tool called "V/I box" for the RF circuit of the HVRFTF.

The High Voltage RadioFrequency Test Facility (HVRFTF) [1] has been developed with the aim to be a simple, accessible and flexible test facility to characterize the dielectric strength in vacuum of the RF drivers of SPIDER and MITICA ion source and effectively address the issues related to their voltage hold off when subjected to their operating radiofrequency E-fields and pressure. HVRFTF makes use of a resonant circuit to produce the high voltage: at first components available at Consorzio RFX were used to verify the working principle, in view of an upgrade with more suitable components. This document presents the design criteria of the resonant circuit, with the final selection of suitable commercial products.

The inductively coupled radiofrequency plasma source of SPIDER and MITICA experiments for the ITER Neutral Beam Test Facility in Padova is designed with highly engineered device called "driver" responsible for the ignition and sustainment of the plasma in the source and delivering 100 kW power at 1 MHz. In order to provide high reliability of the source, such a high operating power requires accurate analyses of the potential issues related to breakdowns occurrence in the driver region due to the high electric field present on its components. In recent years particular effort has been devoted to supporting the study of the voltage hold off in radiofrequency regime in Consorzio RFX Padova and with F4E support, developing a dedicated experimental arrangement: the High Voltage RadioFrequency Test Facility (HVRFTF). This facility is designed to be a simple, accessible and flexible device and it aims in particular at the characterization of the voltage hold off of the RF components used for the plasma sources of SPIDER and MITICA. The facility is composed of a RF power amplifier (presently rated for 300 W but upgrade amplifier in the kW range is foreseen for the future) working at the frequency of 1 MHz, a resonant circuit to provide high RF voltage and a vacuum chamber to host the device under test (DUT). The full ratings of this facility comprises pressure range 10-3 - 105 Pa and a delivered testing voltage of up 17 kV rms (presently 10 kV rms are reached) at the frequency of 1 MHz. In order to provide an electrical qualification of the DUT in terms of maximum operative voltage it is necessary to test the voltage hold off up to the breakdown event. A set of breakdown events are identified being either operational foreseen DUT breakdowns or possible fault condition on RF components and also studied by numerical simulation of an electrical model of the HVRFTF RF circuit. The electrical stresses were found not tolerable by the circuit components, thus a proper passive protection circuit (PPC) based on spark gaps and semiconductor devices is identified and developed considering the ratings of the RF circuit components. The analyses and design of the protection system and of its integration in the HVRFTF RF circuit will be presented and discussed.

The High Voltage RadioFrequency Test Facility (HVRFTF) has been developed with the aim to be a simple, accessible and flexible test facility to characterize the dielectric strength in vacuum of the RF drivers of SPIDER and MITICA ion source and effectively address the issues related to their voltage hold off when subjected to their operating radiofrequency E-fields and pressure. This report presents the results of the test campaigns carried out in 2018, with a low pressure argon atmosphere, on driver mockups #1 and #3 having the same geometry (plane-disk-sphere) but with different dielectric disk material: alumina and quartz. The available RF amplifier and resonant circuit allowed the operation with voltage up to 9 kV at the frequency of 1.07 MHz. The breakdowns detected are associated to glow discharges, related to the operation in regimes described by the left branch of the Paschen curve.

This document presents the electrical and thermal design of the inductor rated for 3 kW and 20 kV for the resonant circuit of the High Voltage RadioFrequency Test Facility (HVRFTF).

The two projects of the ITER Neutral Beam Test Facility (NBTF) [1] in Padova are MITICA, the full scale prototype of the heating Neutral Beam Injector (NBI) and SPIDER, the full-size negative ion source of the NBI. Both include a Radio Frequency (RF) Ion Source where plasma is produced by the inductive coupling with coils wound around vacuum chambers called drivers. Each coil is fed at 1 MHz up to a power of 100 kW, which corresponds to a voltage of about 12 kV rms, with nominal plasma parameters. The ion source design derives from the R&D carried out at the Max-Planck-Institut für Plasmaphysik (IPP) during the past years [2] [3], with additional improvements to achieve the desired performance in long duration pulses (up to 1 h) on a full ITER-size device, in a vacuum environment and with optimized beamlet optics [4] [5] [6] [7]. Among the various issues connected to the fulfillment of the requirements for ITER, special attention should be paid to those related to the voltage hold off in vacuum of the beam source components; not only for the acceleration grids subjected to very high dc voltage but also for the RF circuits of the ion source and in particular the RF drivers. Some concern in this regard has arisen since several years ago and in fact, also in IPP, the last two test facilities RADI and ELISE have been realized in such a way the areas containing the drivers that can be put under vacuum (lower than 10 4 mbar [8]) to better simulate the ITER operating condition [9] [10]. For the ITER heating NBI the concern is deeper, since the rear side of the ion source, where the drivers are located, is not directly pumped and the pressure at the moment is only estimated by means of simulation. The voltage hold off of the driver coils is essential to operate the ion source at full power and thus to reach the full performance. The topic of the PhD activity belongs to the framework of the RF R&D task of the NBTF workprogramme, and was focused on the development of a simple, accessible and flexible device called "High Voltage Radio Frequency Test Facility" (HVRFTF) to characterize the dielectric strength in vacuum of the RF drivers of SPIDER and MITICA ion sources and to effectively address the issues related to their voltage hold off when subjected to radiofrequency E-fields at low pressure. The experimental arrangement worked out to reproduce the desired operating conditions consists in a vacuum vessel capable to host different types of driver mock-ups, called Devices Under Test (DUT) in the thesis, a gas injection and pumping system to supply the desired gas species up to the test pressure and a RF circuit designed to produce the high voltage. The HVRFTF allows the variation of the quantities which influence the voltage hold off, such as the pressure, geometry and materials of the DUTs, in order to perform parametric analyses. The idea behind this flexibility is not only to execute tests relevant for the verification of the driver insulation design, but also to quantify operative margins and to identify possible improvements or hints for the design of new drivers. Part of the thesis work was the identification of the requirements of the HVRFTF, consisting in analyses carried out to identify the driver operating conditions relevant to the voltage hold off (geometry, materials and pressure). I estimated the voltage applied to the RF coil of the drivers at full power, and the related E-field, with the identification of the most stressed area. I conceived several driver mockups to be tested within the HVRFTF: the best configuration worked out for the scope is based on a couple of electrodes (one plane and one spherical) with a dielectric material in between. However, the studies highlighted that a single sphere diameter is not sufficiently accurate to cover the entire gap range of interest; in particular the sphere diameter has to be increased as far as the gap increases. Nevertheless, three of these DUTs allow reproducing the desired E-field trend. I decided to test at first a planar circular electrode pair with Rogowski profile, even if it is not suitable as driver mock-up, since it is a test configuration widely treated in the literature and it generates the most reproducible experimental regime, thus allowing a validation of the basic test arrangement. As far as the RF high voltage generation is concerned, the feasibility study led me to work out a resonant circuit matched through a reversed L-type network, supplied by a low voltage amplifier. As a first design approach, the load of the circuit to be matched to the low voltage amplifier output impedance could be the DUT, but the practical implementation of this concept in the design of the RF circuit is complex due to the variation of the DUT impedance during the test campaign and the effect of stray impedances of circuit components. From the electrical point of view, the DUT represents a capacitance with an equivalent series resistance; both depend on the geometry of the electrode pair, on the gap between the two electrodes, and on the properties of the dielectric material in between. The selected approach was to design a suitable inductor to be connected in parallel to the DUT and to use their equivalent impedance as the load impedance to be matched. With this method and once verified that the real part of the load impedance is lower than the real part of the amplifier output impedance, the matching network can be composed by capacitors only, that were designed to assure the matching condition at the nominal frequency. Variable capacitors can be adopted in order to modify the resonance frequency and maintain the matching condition in the whole frequency range of interest. Another important phase of my design work was the development of the electrical model of the components to be used, in order to verify and quantify the real power requirements as a function of the voltage to be reached with the HVRFTF. The realization of the HVRFTF was completed in 2016 with a first RF circuit composed of fixed capacitors and supplied by a RF amplifier rated for a limited power, both already available at Consorzio RFX. The test campaigns on a stainless steel planar circular electrode pair proved the correct operation of the overall plant and allowed obtaining the first experimental results, including in particular the achievement of a voltage up to 10 kV rms. Moreover the tests gave the opportunity to improve the knowledge in this field, discover unexpected issues relevant to specific operating conditions and investigate on possible solutions. Another important fallout of the tests was the validation of the models developed during the design phase, essential for the continuation of the R&D work. The thesis is organized as follows: - Chapter 1 presents the thesis background: starting from the identification of the need for sustainable energy sources, nuclear fusion is identified as a suitable contributor. ITER is the next step toward nuclear fusion and PRIMA, the ITER neutral beam test facility is one of the main supporting R&D projects, with its two experiments SPIDER and MITICA. The experiments are introduced with a brief description. - Chapter 2 enters more in details in one of the components of SPIDER and MITICA beam sources which is considered critical as far as the voltage holding is concerned: the driver. Its operating conditions are described in this chapter. - Chapter 3 presents the High Voltage Radio Frequency Test Facility (HVRFTF), a small, accessible and flexible testbed to experimentally characterize the dielectric strength in vacuum of the driver. - Chapter 4 reports on the analyses carried out for the definition of the devices to be tested within the HVTFTF, relevant mockups of the drivers. - Chapter 5 reports on the studies and the design of the circuit used in the HVRFTF for the generation of high voltage at radiofrequency. - Chapter 6 presents the experimental results obtained so far with the HVRFTF. - Conclusions.

PRIMA (Padova Research on ITER Megavolt Accelerator) is the ITER Neutral Beam Test Facility in Padova, Italy, for the development of the ITER Neutral Beam Injectors (NBI). It comprises two experiments: MITICA, the full-scale prototype of the NBI, designed to produce and neutralize a 40 A negative ions beam, accelerated up to the energy of 1 MeV, and SPIDER, the full-size negative ions source of the NBI. ITER NBI includes a Radio Frequency (RF) plasma source where plasma is produced by the inductive coupling with coils wound around complex vacuum chambers called drivers. Each coil is fed at 1 MHz up to a power of 100 kW, which corresponds to a voltage of about 12 kV rms, with nominal plasma parameters. The voltage hold off in vacuum of the beam source components is one of the most critical issues connected to the fulfillment of the requirements for ITER, not only for the particles acceleration system subjected to very high dc voltage (up to 1 MV) but also for the RF circuits of the plasma source and in particular the RF drivers. The development of a simple, accessible and flexible device called "High Voltage Radio Frequency Test Facility" (HVRFTF) was launched to effectively characterize the voltage hold off of the RF drivers of SPIDER and MITICA, subjected to radiofrequency E-fields at low pressure. The experimental arrangement worked out to reproduce the driver operating conditions includes a vacuum vessel capable to host different types of Devices Under Test (DUT), a gas injection and pumping system to supply the desired gas species up to the test pressure and a RF circuit designed to produce the high voltage. The first DUT tested with the HVRFTF is composed of planar circular electrodes, a configuration not directly relevant for the driver but widely treated in literature and significant for the validation of the basic test assessment. This paper presents the results obtained during the first operations of the HVRFTF with the DUT in an Argon atmosphere in the pressure range 10-3 - 102 Pa. A voltage up to 10 kV rms, at the frequency of 1 MHz was applied to the electrodes, with gaps of 0.1 mm - 40 mm. Breakdowns were observed for pressures higher than 0.2 Pa, associated with the appearance of a glow discharge between the high voltage electrode and the grounded vacuum vessel, as expected according to the Paschen's law. For lower pressures the measuring equipment did not detect any breakdown, although sparks were noticed between the electrodes.

The report summarizes the activities carried out in 2016 under the RF R&D subtask (5.8) of the 2016 Work Programme. F4E-RFX-PMS_A-WP-2016 - Agreement on the Neutral Beam Test Facility Project. Task deliverable: T5.8/P1.1, RF R&D

The High Voltage RadioFrequency Test Facility (HVRFTF) has been developed with the aim to be a simple, accessible and flexible test facility to characterize the dielectric strength in vacuum of the RF drivers of SPIDER and MITICA ion source and effectively address the issues related to the ir voltage hold off when subjected to their operating radio frequency E - fields and pressure . This report presents the results of the test campaign s carried out in 2017 on stainless steel planar circular electrodes with Argon injection. Currently the HVRFTF is able to supply RF voltage of more than 10 kV, producing E - fields up to 55 MV/m for a gap of 0.2 mm. The experimental activity, besides the collection of data relevant for the main HVRFTF scope, allowed gaining experience and addressing specific issues related to the setu p and operation of the facility.

PRIMA (Padova Research on ITER Megavolt Accelerator) is a large experimental facility under construction in Padova, Italy, aimed at the development and test of the full scale prototype of Neutral Beam Injectors (NBI), called MITICA, for ITER. MITICA is designed to accelerate a beam of 40 A of negative deuterium ions up to 1 MV, in order to deliver a power of about 17 MW to the plasma with a pulse length of one hour; requirements that have never been reached before all together. The negative ions are produced by means of an ion source composed of 8 radio frequency (RF) drivers working at 1 MHz, each generating a cold plasma at a pressure of 0.3 Pa with a power of 100 kW. A set of grids at different electrical potentials, extracts and accelerates the negative ions producing a negative ion beam which is then neutralized in order to enter and heat the plasma. To prove the possibility to achieve these requirements a second experiment will be hosted in PRIMA, called SPIDER: the full scale prototype of the ITER NBI negative ion source. To gain experience on the RF voltage holding in vacuum, a dedicated experimental investigation is needed. Thus, the High Voltage Radio Frequency Test Facility (HVRFTF) is being built in Padova, at Consorzio RFX. The HVRFTF scope is to reproduce operating conditions of RF components in the ITER NBI ion source, in particular the voltage up to 15 kV at 1 MHz and the operating pressure in the range of 0.001 - 0.3 Pa. In HVRFTF a vacuum vessel is used to contain a low pressure atmosphere of the desired gas species produced by a gas injection and pumping system. The devices to be tested are placed inside and insulated from the vacuum vessel, and are supplied with RF voltage by means of a couple of feedthroughs. At first, circular planar stainless steel electrodes are used to derive Paschen curves with RF and dc voltage. The distance between the electrodes is adjustable, since one feedthrough is mounted on a bellow which is axially moved by a linear translator. Both the pressure and distance can independently adjusted, to derive the breakdown voltage threshold for the specific gas and for a specific configuration of electrodes. The RF high voltage is generated exploiting an LC resonance of a circuit supplied by a low voltage broadband amplifier. The operating frequency is adapted to look for the resonance frequency of the circuit, which is influenced by the parasitic capacitance of the device to be tested. The paper will present the specific features and issues related to the design and setup of the HVRFTF and how they have been faced and solved.

This document describes the design of the High Voltage Radio Frequency Test Facility (HVRFTF) to experimentally verify the insulation design of the ion source RF circuits adopted for SPIDER. Starting from the geometry and the operating condition of the ion source of SPIDER the requirements for HVRFTF are derived. Then the detailed description of the design of all the HVRFTF systems is reported. The design verifications are finally presented.

Inside the new laboratory for the development of the ITER neutral beam injector prototype PRIMA (Padova Research on Injector Megavolt Accelerated), whose construction was entrusted to the Consorzio RFX Padova, two experimental plants will be installed to study new systems for the production of neutral particles accelerated to high energy to heat the plasma, called MITICA (Megavolt ITER Injector & Concept Advancement) and SPIDER (Source for Production of Ion of Deuterium Extracted from Rf plasma). The ions for the plasma heating are produced inside SPIDER and MITICA negative ion sources by 8 Radio Frequency (RF) drivers, supplied by 4 independent and identical circuits, each composed of a RF oscillator working at 1 MHz, Coaxial cables with characteristic impedance, Matching network for load adaptation and two RF coils connected in series. To study the voltage holding capability of the RF drivers, a test bed called HVRFTF (High Voltage Radio Frequency Test Facility) has been installed at Consorzio RFX. The RF drivers working conditions in terms of voltage and pressure are reproduced for that purpose and to identify possible critical issues and improvements. This is practically made using a cylindrical vacuum chamber where two electrodes are installed: a proper pumping and gas injection system allow the achievement of the desired working pressure, while the high voltage is generated by means of a resonant circuit supplied with a RF low voltage power amplifier. The paper will describe the functional specifications, criteria, and setup of the plants designed for HVRFTF.

The High Voltage RadioFrequency Test Facility (HVRFTF) [1] has been developed with the aim to be a simple, accessible and flexible test facility to characterize the dielectric strength in vacuum of the RF drivers of SPIDER and MITICA ion source and effectively address the issues related to the ir voltage hold off when subjected to their operating radiofrequency E - fields and pressure . This report presen ts the studies for the definition of driver mockups to be used as Device Under Test (DUT) within HVRFTF. Different geometries are assumed as possible mockups and their E - fields are calculated by means of electrostatic analyses. The best configuration worke d out for the scope is based on a couple of electrodes (one plane and one spherical) with a dielectric material in between. However, the studies highlighted that a single sphere diameter is not sufficiently accurate to cover the entire gap range of interes t; in particular the sphere diameter has to be increased as far as the gap increases. Nevertheless, three of these DUTs can allow reproducing the E - field trend of the driver.

The report summarizes the activities carried out in 2015 under the RF R&D subtask (5.8) of the 2015 Wo rk Programme, focused to: o The improvement of the estimation via simulation o f the real impedance of inductors at high frequency, and in particular on their effective res istance; o The evaluation of the EM field that will be genera ted during the HVRFTF tests and to define relevant safety measures to be adopted; o The setup of the laboratory and the HVRFTF.