Tyhjiövalokaarien mallintaminen : plasman syttymisestä pintavuorovaikutuksiin

A better understanding of vacuum arcs is desirable in many of today's 'big science' projects including linear colliders, fusion devices, and satellite systems. For the Compact Linear Collider (CLIC) design, radio-frequency (RF) breakdowns occurring in accelerating cavities influence efficiency optimisation and cost reduction issues. Studying vacuum arcs both theoretically as well as experimentally under well-defined and reproducible direct-current (DC) conditions is the first step towards exploring RF breakdowns. In this thesis, we have studied Cu DC vacuum arcs with a combination of experiments, a particle-in-cell (PIC) model of the arc plasma, and molecular dynamics (MD) simulations of the subsequent surface damaging mechanism. We have also developed the 2D Arc-PIC code and the physics model incorporated in it, especially for the purpose of modelling the plasma initiation in vacuum arcs. Assuming the presence of a field emitter at the cathode initially, we have identified the conditions for plasma formation and have studied the transitions from field emission stage to a fully developed arc. The 'footing' of the plasma is the cathode spot that supplies the arc continuously with particles; the high-density core of the plasma is located above this cathode spot. Our results have shown that once an arc plasma is initiated, and as long as energy is available, the arc is self-maintaining due to the plasma sheath that ensures enhanced field emission and sputtering. The plasma model can already give an estimate on how the time-to-breakdown changes with the neutral evaporation rate, which is yet to be determined by atomistic simulations. Due to the non-linearity of the problem, we have also performed a code-to-code comparison. The reproducibility of plasma behaviour and time-to-breakdown with independent codes increased confidence in the results presented here. Our MD simulations identified high-flux, high-energy ion bombardment as a possible mechanism forming the early-stage surface damage in vacuum arcs. In this mechanism, sputtering occurs mostly in clusters, as a consequence of overlapping heat spikes. Different-sized experimental and simulated craters were found to be self-similar with a crater depth-to-width ratio of about 0.23 (sim) - 0.26 (exp). Experiments, which we carried out to investigate the energy dependence of DC breakdown properties, point at an intrinsic connection between DC and RF scaling laws and suggest the possibility of accumulative effects influencing the field enhancement factor.Tulevaisuuden suuren kokoluokan fysiikan alan tietoa tuottavat ja hyödyntävät laitteet, kuten hiukkaskiihdyttimet, fuusiolaitteet ja satelliittijärjestelmät tullaan toteuttamaan yhä vaativammissa olosuhteissa. Näissä olosuhteissa sähköpurkauksen aiheuttama valokaari voi syntyä jopa tyhjiössä alentaen laitteiden suorituskykyä. Eräässä maailman suurimmista kiihdytinlaboratorioista, CERN:issä, suunnitellaan yhteistyössä kumppanuuslaitosten kanssa lineaarikiihdytintä, jota kutsutaan lyhenteellä CLIC (Compact Linear Collider). Sen avulla olisi mahdollista törmäyttää elektroneja ja niiden antihiukkasia, nk. positroneja, ennennäkemättömän suurella energialla, lähes tarkalleen valonnopeudella. Siitä huolimatta, että laitteen suunniteltu pituus on noin 50 km, laite olisi suhteellisen "kompakti" nykyisiin kiihdyttimiin verrattuna, sillä se käyttäisi tavallista paljon suurempaa sähkökenttää hiukkasten kiihdyttämiseen. Väitöskirjatyössä käsitellään näiden suurten sähkökenttien vaikutuksesta aiheutuvia valokaaria, jotka johtaisivat hiukkaskimpun menettämiseen CLIC:issä, ja näin ollen rajoittaisivat laitteen tehokkuutta. Vaikka väliaineessa läpilyövien valokaarien fysiikka ymmärretään nykyään jo varsin hyvin, tyhjiössä esiintyvien valokaarien syntyyn ja kehitykseen liittyvät seikat ovat edelleen hyvin epäselviä. Työssä annetaan aiempaa syvällisempi kuvaus tyhjiövalokaarien elinkaarista, eli siitä, miten plasma syttyy tyhjiövalokaaressa, miten se kehittyy, miten katodipinnalla kraatterit voivat muodostua plasman ja katodimateriaalin vuorovaikutuksesta johtuen, ja miten tyhjiövalokaari sammuu. Mallintamiseen käytetään kokeellisten ja simulaatiomenetelmien monipuolista yhdistelmää. CERN:issä suoritettujen kokeiden avulla määritettiin valokaarien ominaisuuksien riippuvuutta valokaaren käyttämästä energiasta. Tämä on CLIC:in kehityksen kannalta tärkeää sekä mahdollistaa teoreettisten ja kokeellisten tulosten vertailun. Plasman syntyä ja kehitystä tyhjiövalokaarissa mallinnettiin erityisesti tähän tarkoitukseen kehitetyn simulaatio-ohjelman avulla, joka käyttää nk. particle-in-cell-menetelmää. Simulaatiossa käytetty fysikaalinen malli perustuu oletukseen, että katodilla on aluksi emitteri, joka emittoi suurilla sähkökentillä elektroneja ja neutraaleja atomeja tietyllä tavalla. Simulaatiomallin erikoispiirteenä on materiaalifysiikan ja plasmafysiikan yhdistäminen, joka kytkee yhteen plasman kehityksen ja kraatterien muodostumisen. Kraatterit mallinnettiin materiaalifysiikassa usein käytetyn molekyylidynamiikkamenetelmän avulla. Näin saadut simulaatiotulokset olivat sopusoinnussa kraattereista tehtyjen kokeellisten havaintojen kanssa

Beam Longitudinal Dynamics Simulation Suite BLonD

The beam longitudinal dynamics code BLonD has been developed at CERN since 2014 and has become a central tool for longitudinal beam dynamics simulations. In this paper, we present this modular simulation suite and the various physics models that can be included and combined by the user. We detail the reference frame, the equations of motion, the BLonD-specific options for radio-frequency parameters such as phase noise, fixed-field acceleration, and feedback models for the CERN accelerators, as well as the modeling of collective effects and synchrotron radiation. We also present various methods of generating multi-bunch distributions matched to a given impedance model. BLonD is furthermore a well-tested and optimized simulation suite, which is demonstrated through examples, too

CAS course on "RF for Accelerators", 18 June - 01 July 2023, Berlin Germany

The lectures treat longitudinal beam tracking in synchrotrons. After introducing the basics of creating an accelerator model and discretisation, the lecture discusses the choice of the time frame and coordinates for longitudinal tracking. Concerning tracking without intensity effects, it touches on symplectic, periodic boundary conditions, and RF gymnastics. Concerning collective effects, the course details how to include and discretise induced voltage in simulations and how to choose simulation parameters to well resolve beam and impedance. It briefly describes beam instabilities, multi-turn wakes, and synchrotron radiation. Thereafter, RF modelling is treated, including cavity-beam-transmitter interaction, phase and frequency modulation, as well as modelling global and local control loops. Then, the lecture walks through the generation of particle distributions and mentions some six-dimensional effects. Finally, code optimisation and benchmarking are treated, touching on code-design aspects, good practises runtime and memory considerations, as well as how to test, compare, and benchmark simulation code

CAS course on "RF for Accelerators", 18 June - 01 July 2023, Berlin Germany

The lectures treat longitudinal beam tracking in synchrotrons. After introducing the basics of creating an accelerator model and discretisation, the lecture discusses the choice of the time frame and coordinates for longitudinal tracking. Concerning tracking without intensity effects, it touches on symplectic, periodic boundary conditions, and RF gymnastics. Concerning collective effects, the course details how to include and discretise induced voltage in simulations and how to choose simulation parameters to well resolve beam and impedance. It briefly describes beam instabilities, multi-turn wakes, and synchrotron radiation. Thereafter, RF modelling is treated, including cavity-beam-transmitter interaction, phase and frequency modulation, as well as modelling global and local control loops. Then, the lecture walks through the generation of particle distributions and mentions some six-dimensional effects. Finally, code optimisation and benchmarking are treated, touching on code-design aspects, good practises runtime and memory considerations, as well as how to test, compare, and benchmark simulation code

Compensation of Synchrotron Radiation at Top Energy in the FCC-hh

To counteract the significant synchrotron radiation damping in the FCC-hh, a continuous longitudinal emittance blow-up is foreseen at top energy. In this paper, we study the compensation of synchrotron radiation emittance damping via RF phase noise injection. Adapting the noise bandwidth to the targeted bunch length, a good regulation of the bunch length can be achieved in the long run, while on short timescales the fluctuations remain relatively large. The steady-state bunch profile is shown and characterised with a binomial distribution. Finally, possible future developements are being discussed

2D ArcPIC Code Description: Description of Methods and User / Developer Manual (second edition)

Vacuum discharges are one of the main limiting factors for future linear collider designs such as that of the Compact LInear Collider (CLIC). To optimize machine efficiency, maintaining the highest feasible accelerating gradient below a certain breakdown rate is desirable; understanding breakdowns can therefore help us to achieve this goal. As a part of ongoing theoretical research on vacuum discharges at the Helsinki Institute of Physics, the build-up of plasma can be investigated through the particle-in-cell method. For this purpose, we have developed the 2D ArcPIC code introduced here. We present an exhaustive description of the 2D ArcPIC code in several parts. In the first chapter, we introduce the particle-in-cell method in general and detail the techniques used in the code. In the second chapter, we describe the code and provide a documentation and derivation of the key equations occurring in it. In the third chapter, we describe utilities for running the code and analyzing the results. The last chapter contains suggestions for viable and useful avenues for further work on the code. The code and documentation are original work of the authors, written in 2010–2014, and is therefore under the copyright of the authors

Estimated LHC RF system performance reach at injection during Run III and beyond

This note describes the performance limitations of the present LHC RF system on the injection plateau, in view of increased intensities in Run III and in the HL-LHC era. The minimum required injection voltage is derived from beam dynamics constraints and operational experience. The maximum available voltage is given by beam measurements with the half-detuning beam-loading compensation scheme. From these two constraints, the performance reach with increased intensities is evaluated. Various studies, such as a potential RF upgrade, or alternative beam-loading compensation scenarios, are recommended

LHC MD 652: Coupled-Bunch Instability with Smaller Emittance (all HOMs)

The aim of the MD was to measure the coupled-bunch stability from all HOM impedances, with a reduced longitudinal emittance in order to explore the HL-LHC conditions. The acceleration ramp was performed with the nominal beams of 2016, but a reduced target bunch length and RF voltage. With this reduced emittance, the beam remained close but above the single-bunch stability threshold. No coupled-bunch oscillations were observed, so we can conclude that the stability threshold for coupled-bunch instability is not lower than the single-bunch threshold. An interesting observation in the MD was the long-lasting injection oscillations, whose traces can still be seen at arrival to flat top; in agreement with observations in earlier MDs. The measurements took place between 28th October 20:00 and 29th October 05:10