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Characterization of the dielectric strength in vacuum of RF drivers for fusion neutral beam injectors
Full text linkThe 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.La stazione sperimentale Neutral Beam Test Facility (NBTF) dell'esperimento ITER [1], in costruzione a Padova presso il Consorzio RFX, ospita due esperimenti: MITICA, il prototipo in scala 1:1 del sistema di iniezione di particelle neutre per il riscaldamento del plasma in ITER (NBI) e SPIDER, il prototipo della sorgente ionica impiegata dal NBI.
Entrambi i progetti impiegano 8 "driver" a radiofrequenza (RF), ovvero sorgenti di plasma, per la generazione di ioni; ciascun driver è costituto da una camera da vuoto cilindrica su cui è avvolta una bobina che si accoppia induttivamente con il plasma.
Ogni bobina è alimentata da un'onda sinusoidale di tensione a 1 MHz, con una potenza fino a 100 kW alla quale corrisponde, con i parametri nominali di plasma, un valore efficace di tensione tra i terminali di circa 12 kV rms.
La soluzione progettuale della sorgente ionica deriva dall'attività di ricerca e sviluppo effettuata al Max-Planck-Institut für Plasmaphysik (IPP) negli scorsi anni [2] [3], ulteriormente studiata e sviluppata per raggiungere le prestazioni desiderate per ITER ed in particolare quelle legate al miglioramento dell'ottica del fascio e al funzionamento in vuoto con impulsi di durata prolungata fino ad un'ora [4] [5] [6] [7].
Tra le varie problematiche legate al soddisfacimento dei requisiti per ITER, particolare attenzione è rivolta alla tenuta della tensione in vuoto dei componenti e dei circuiti dell'iniettore, non solo per le griglie di accelerazione che sono soggette a tensioni dc fino a 1 MV, ma anche per i circuiti RF della sorgente e in particolare dei driver.
La consapevolezza della criticità di questo aspetto è maturata negli ultimi anni e di conseguenza è cresciuta l'attenzione al problema: anche i più recenti esperimenti presso IPP (RADI ed ELISE) prevedono la possibilità di mettere in vuoto (con pressione inferiore a 10-4 mbar [8]) il volume contenente i driver, per poter simulare meglio le condizioni operative di ITER [9] [10].
Per l'iniettore di neutri di ITER la preoccupazione è anche maggiore, poiché non vi potrà essere controllo diretto della pressione nella regione dei driver; al momento essa è stimata per mezzo di simulazioni numeriche. La tenuta di tensione della bobina dei driver è essenziale al fine di operare la sorgente alla piena potenza, requisito per il raggiungimento delle piene prestazioni dell'iniettore.
L'argomento del dottorato ricade nell'ambito della task "RF R&D" del programma di lavoro della NBTF ed è focalizzato allo sviluppo di un esperimento semplice, accessibile e flessibile chiamato "High Voltage RadioFrequency Test Facility" (HVRFTF), indirizzato allo studio delle problematiche legate alla tenuta di tensione in vuoto dei driver RF delle sorgenti di SPIDER e MITICA.
Il setup sperimentale di HVRFTF consente di ricreare le condizioni operative delle bobine dei driver e consiste in una camera da vuoto capace di ospitare diversi dispositivi in prova, chiamati Device Under Test (DUT) nella tesi, un sistema di pompaggio e immissione gas in grado di regolare la pressione e la specie di gas all'interno della camera e di un circuito a radiofrequenza in grado di produrre l'alta tensione.
HVRFTF permette la variazione delle grandezze fisiche che influenzano la tenuta di tensione, come ad esempio la pressione, la geometria e i materiali dei dispositivi in prova, al fine di poter effettuare analisi parametriche.
Questa flessibilità permette non solo di verificare il progetto dell'isolamento dei driver, ma anche di quantificarne i margini operativi e di identificare possibili miglioramenti o spunti per il progetto elettrico di nuovi driver.
Parte del lavoro di tesi è stato dedicato alla definizione dei requisiti di HVRFTF, a partire dallo studio della sorgente e delle condizioni operative dei driver che ne influenzano la tenuta di tensione.
Ho stimato la tensione applicata alla bobina RF dei driver a piena potenza e ricavato la relativa mappa di campo elettrico, che mi ha consentito di identificare la regione maggiormente stressata.
In seguito ho concepito diversi possibili modelli di driver da testare all'interno di HVRFTF: il migliore è basato su una coppia di elettrodi (un piano e una sfera) tra i quali è interposto un disco di materiale dielettrico. Tre sfere di diametro direttamente proporzionale al gap sono necessarie per riprodurre l'andamento del campo elettrico nell'intero intervallo di variazione del gap. Per le prime prove con HVRFTF ho deciso di testare degli elettrodi piani circolari con profilo di Rogowski, anche se non rappresentano un buon modello del driver, al fine di validare il setup sperimentale. L'uso di questo tipo di elettrodi è infatti ampiamente diffuso e documentato in letteratura, perché essi sono in grado di generare condizioni sperimentali riproducibili.
Per la generazione di alta tensione a radiofrequenza, tra possibili soluzioni ho adottato un circuito risonante adattato all'impedenza di uscita dell'amplificatore che lo alimenta, attraverso una rete a L rovesciato. In prima istanza, il carico da adattare potrebbe essere l'impedenza del DUT, che dal punto di vista elettrico risulta essere una capacità in serie ad una resistenza, entrambe dipendenti dalla geometria degli elettrodi, dalla loro distanza (gap) e dalle proprietà del materiale dielettrico interposto tra loro.
Tuttavia l'implementazione pratica di quest'approccio è complessa: l'impedenza del DUT durante la campagna sperimentale è variabile; inoltre i componenti del circuito di adattamento (almeno uno dei quali dovrebbe essere un induttore), introducono impedenze parassite non note, a loro volta da compensare.
Una soluzione ragionevole che ho infine elaborato consiste nel collegare in parallelo al DUT un induttore di caratteristiche opportune e di utilizzare l'impedenza equivalente come carico da adattare. Con questo approccio, una volta dimensionati i componenti in modo tale che la parte reale dell'impedenza di carico sia minore della parte reale dell'impedenza di uscita dell'amplificatore, la rete di adattamento a L rovesciato risulta composta da soli condensatori, le cui capacità si ricavano imponendo il vincolo di adattamento di impedenza e la frequenza di risonanza. L'utilizzo di condensatori aventi capacità regolabile permette infine di modificare la frequenza di risonanza in modo da poter operare in tutto l'intervallo di frequenze di interesse.
Per il progetto del circuito RF ho sviluppato modelli elettrici dettagliati per ogni componente impiegato, al fine di verificare e quantificare i requisiti di potenza attiva in funzione della tensione da raggiungere con HVRFTF.
La realizzazione preliminare di HVRFTF è stata completata nel 2016 con un circuito a radiofrequenza composto da condensatori aventi capacità fissa, alimentato da un amplificatore RF di potenza limitata; sia i condensatori che l'amplificatore erano già disponibili al Consorzio RFX.
La campagna di prove sperimentali con la coppia di elettrodi piani in acciaio ha dimostrato il corretto funzionamento dell'impianto sperimentale con il raggiungimento della tensione di 10 kV, ha consentito di ottenere i primi risultati sperimentali e di validare i modelli sviluppati durante la fase di progetto.
Il lavoro presentato in questa tesi è così organizzato:
- Capitolo 1: si presenta il contesto tematico all'interno del quale è stata sviluppata la tesi; a partire dal problema energetico, una possibile soluzione è un mix di fonti sostenibili tra cui la fusione nucleare. Si presentano in seguito ITER, il prossimo passo verso la fusione e "ITER Neutral Beam Test Facility", uno dei principali progetti a supporto di ITER con i suoi due esperimenti: SPIDER e MITICA.
- Capitolo 2: si descrive in dettaglio uno dei componenti delle sorgenti ioniche di SPIDER e MITICA, ritenuto critico dal punto di vista della tenuta di tensione: il driver. Si presentano le analisi eseguite per derivare le sue condizioni operative.
- Capitolo 3: si presenta l'esperimento "High Voltage Radio Frequency Test Facility" (HVRFTF), un piccolo impianto per la caratterizzazione sperimentale della rigidità dielettrica in vuoto dei driver.
- Capitolo 4: si presentano le analisi effettuate per la definizione dei dispositivi da testare con HVRFTF, con l'obiettivo che essi possano riprodurre condizioni operative simili a quelle del driver per lo studio della problematica di interesse .
- Capitolo 5: si riportano gli studi per la generazione di alta tensione a radiofrequenza e il progetto del circuito risonante adottato per HVRFTF.
- Capitolo 6: si presentano i risultati ottenuti con HVRFTF.
- Conclusioni
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